Predicting perimetric defects from en face maps of retinal nerve fibre layer reflectance

PURPOSE

To develop criteria to predict visual hemifields with deep perimetric defects based on retinal nerve fibre layer (RNFL) reflectance, in a transparent process whose components can be assessed by independent laboratories analysing data from their own small groups.

METHODS

The analysis was carried out in four stages, using three independent groups of patients-30, 33 and 62 participants-with glaucoma and age-similar controls. The first stage used Group 1 to develop a criterion for RNFL reflectance images at 24, 36 or 48 μm below the inner limiting membrane (ILM). The second stage evaluated the criterion using Group 2. The third stage developed a second criterion to improve performance for Groups 1 and 2 combined. The fourth stage evaluated the second criterion with Group 3. Confidence intervals for sensitivity and specificity were then computed by combining results from all three groups.

RESULTS

The first criterion identified all hemifields with deep defects and no hemifields from controls, using a within-eye reference for healthy RNFL. For Group 2, specificity remained high but sensitivity was reduced. The second criterion improved sensitivity by using location-specific reference values. For Group 3, sensitivity remained high but reduced specificity was found. Confidence intervals showed substantial overlap for the two criteria.

CONCLUSIONS

We developed two criteria to identify patients with deep perimetric defects with high specificity and sensitivity. Several improvements are warranted: automated identification of the fovea-disc angle and optic disc locations, evaluation of normal variation in patterns of RNFL thickness, improved segmentation of ILM and major vasculature, reduction of within-eye variability in RNFL reflectance of healthy eyes, assessment of effects of image quality, assessment of effects of comorbidity and effectiveness of other devices.

Using high-density perimetry to explore new approaches for characterizing visual field defects

High-density threshold perimetry has found that conventional static threshold perimetry misses defects due to undersampling. However, high-density testing can be both slow and limited by normal fixational eye movements. We explored alternatives by studying displays of high-density perimetry results for angioscotomas in healthy eyes-areas of reduced sensitivity in the shadows of blood vessels. The right eyes of four healthy adults were tested with a Digital Light Ophthalmoscope that gathered retinal images while presenting visual stimuli. The images were used to infer stimulus location on each trial. Contrast thresholds for a Goldmann size III stimulus were measured at 247 locations of a 13°×19° rectangular grid, with separation 0.5°, extending from 11° to 17° horizontally and -3° to +6° vertically, covering a portion of the optic nerve head and several major blood vessels. Maps of perimetric sensitivity identified diffuse regions of reduced sensitivity near the blood vessels, but these showed moderate structure-function agreement that was only modestly improved when effects of eye position were accounted for. An innovative method termed slice display was used to locate regions of reduced sensitivity. Slice display demonstrated that many fewer trials could yield similar structure-function agreement. These results are an indication that test duration might be reduced dramatically by focusing on location of defects rather than maps of sensitivity. Such alternatives to conventional threshold perimetry have the potential to map the shape of defects without the extensive time demands of high-density threshold perimetry. Simulations illustrate how such an algorithm could operate.

Structure-function assessment in glaucoma based on perimetric sensitivity and en face optical coherence tomography images of retinal nerve fiber bundles

Many studies have assessed structure-function relations in glaucoma, but most without topographical comparison across the central 30°. We present a method for assessing structure-function relations with en face images of retinal nerve fiber layer (RNFL) bundles allowing topographical comparison across much of this retinal area. Forty-four patients with glaucoma (median age 61 years) were recruited and tested with Optical Coherence Tomography (OCT) and perimetry. Six rectangular volume scans were gathered, and then montaged to provide en face views of the RNFL bundles. We calculated the proportion of locations showing a perimetric defect that also showed an en face RNFL defect; and the proportion of locations falling on an RNFL defect that also showed a perimetric defect. A perimetric defect for a location was defined as a total deviation (TD) value equal to or deeper than -4 dB. We found that the median (IQR) number of locations with abnormal RNFL bundle reflectance that also had abnormal TD was 78% (60%) and for locations with abnormal TD that also had abnormal RNFL bundle reflectance was 75% (44%). We demonstrated a potential approach for structure-function assessment in glaucoma by presenting a topographic reflectance map, confirming results of previous studies and including larger retinal regions.

Within-eye and between-subject variability for reflectance of the retinal nerve fibre layer

PURPOSE

Reflectance of retinal nerve fibre layer (RNFL) can contribute to detecting the presence of glaucomatous damage and defining its extent. As a step towards developing a normative database for RNFL reflectance, we assessed within-eye and between-subject variability for RNFL reflectance in healthy eyes.

METHODS

Vertical 30° × 15° volume scans at the optic disc were gathered using SD-OCT (Spectralis OCT) from people free of eye disease. Scans were gathered for both eyes of 30 younger adults (mean ± SD = 27 ± 3 years) and for one eye of 30 older adults (68 ± 8 years). Reflectance was quantified for each voxel as the depth-resolved attenuation coefficient (AC). Values for AC were extracted for four slabs (0-52, 24-52, 24-36 and 36-60 μm) and at depths from 24 to 60 μm below the inner limiting membrane (ILM) in 4 μm steps.

RESULTS

Between-subject and within-eye standard deviations (SDs) for the logarithm of AC were similar; median differences were 0.02-0.03 log unit across all four slabs and depths from 24 to 48 μm. Means for the logarithm of AC were higher for younger than older eyes by ~0.1 log unit; this age effect was not due to differences in the raw reflectance of the RNFL, but rather to age-related changes in reflectance of deeper retina affecting the calculation of AC.

CONCLUSIONS

In both groups, within-eye variability in RNFL reflectance near the optic disc was similar to between-subject variability. A better understanding of within-eye variability would be useful for developing normative databases.

Long- and Short-Term Variability of Perimetry in Glaucoma

Purpose

Test–retest variability in perimetry consists of short-term and long-term components, both of which impede assessment of progression. By minimizing and quantifying the algorithm-dependent short-term variability, we can quantify the algorithm-independent long-term variability that reflects true fluctuations in sensitivity between visits. We do this at locations with sensitivity both < 28 dB (when the stimulus is smaller than Ricco's area and complete spatial summation can be assumed) and > 28 dB (when partial summation occurs).

Methods

Frequency-of-seeing curves were measured at four locations of 35 participants with glaucoma. The standard deviation of cumulative Gaussian fits to those curves was modeled for a given sensitivity and used to simulate the expected short-term variability of a 30-presentation algorithm. A separate group of 137 participants was tested twice with that algorithm, 6 months apart. Long-term variance at different sensitivities was calculated as the LOESS fit of observed test–retest variance minus the LOESS fit of simulated short-term variance.

Results

Below 28 dB, short-term variability increased approximately linearly with increasing loss. Long-term variability also increased with damage below this point, attaining a maximum standard deviation of 2.4 dB at sensitivity 21 dB, before decreasing due to the floor effect of the algorithm. Above 30 dB, the observed test–retest variance was slightly smaller than the simulated short-term variance.

Conclusions

Long-term and short-term variability both increase with damage for perimetric stimuli smaller than Ricco's area. Above 28 dB, long-term variability constitutes a negligible proportion of test–retest variability.

Translational Relevance

Fluctuations in true sensitivity increase in glaucoma, even after accounting for increased short-term variability. This long-term variability cannot be reduced by altering testing algorithms alone.

Detection and discrimination of achromatic contrast: A ganglion cell perspective

The magnocellular (MC) pathway in the primate has much higher achromatic contrast sensitivity than the parvocellular (PC) pathway, and is implicated in luminance contrast detection. But MC pathway responses tend to saturate at lower achromatic contrast than do PC pathway responses. It has been proposed that the PC pathway plays a major role in discriminating suprathreshold achromatic contrast, because the MC pathway is in saturation. This has been termed the pulsed-pedestal protocol. To test this hypothesis, responses of MC and PC pathway ganglion cells have been examined under suprathreshold conditions with stimulus configurations similar to those in psychophysical tests. For MC cells, response saturation was much less for flashed or moving edges than for sinusoidal modulation, and MC cell thresholds predicted for these stimuli were similar to psychophysical discrimination (and detection) data. Results suggest the protocol is not effective in segregating MC and PC function.

Interpreting Retinal Nerve Fiber Layer Reflectance Defects Based on Presence of Retinal Nerve Fiber Bundles

SIGNIFICANCE

Adaptive-optics scanning-laser-ophthalmoscopy (AOSLO) retinal imaging of the retinal nerve fiber layer (RNFL) helps predict the severity of perimetric damage based on absence of fibers and projection of the defects in en face images of the RNFL from spectral-domain optical coherence tomography (SD-OCT).

PURPOSE

En face images of the RNFL reveal reflectance defects in patients with glaucoma and predict locations of perimetric defects. These defects could arise from either loss of retinal nerve fiber bundles or reduced bundle reflectance. This study used AOSLO to assess presence of bundles in areas with RNFL reflectance defects on SD-OCT.

METHODS

Adaptive-optics scanning laser ophthalmoscopy was used to image a vertical strip of RNFL measuring approximately 30 × 3° between the optic disc and the fovea. Fifteen patients with glaucoma who had SD-OCT reflectance defects that passed through this region were chosen. Four patients had reflectance defects in both superior and inferior hemifields, so presence of bundles on AOSLO was assessed for 19 hemifields. Where bundles were present, the hemifield was scored for whether bundles seemed unusual (low contrast and/or low density). Perimetric defects were considered deep when sensitivity was below 15 dB.

RESULTS

Ten hemifields had a region with no fibers present on AOSLO; all had a corresponding deep perimetric defect. The other nine hemifields had no region in the AOSLO image without fibers: four with normal fibers and five with unusual fibers. The only one of these nine hemifields with a deep perimetric defect was one with low-contrast fibers and overall thin RNFL.

CONCLUSIONS

Retinal nerve fiber layer reflectance defects, which were associated with deep perimetric defects, usually had a region with absence of fibers on AOSLO images of RNFL. Ability to predict severity of perimetric damage from en face SD-OCT RNFL reflectance images could benefit from quantification that differentiated between absence of fibers and unusual fibers.

A Novel Stimulus to Improve Perimetric Sampling within the Macula in Patients with Glaucoma

SIGNIFICANCE

Identifying glaucomatous damage to the macula has become important for diagnosing and managing patients with glaucoma. In this study, we presented an approach that provides better perimetric sampling for the macular region, by testing four locations, with a good structure-function agreement.

PURPOSE

We previously presented a basis for customizing perimetric locations within the macula. In this study, we aimed to improve perimetric sampling within the macula by presenting a stimulus at four locations, with maintaining a good structure-function agreement.

METHODS

We tested one eye each of 30 patients (aged 50 to 88 years). Patients were selected based on observed structural damage to the macula, whereas perimetric defect (using 24-2) did not reflect the locations and extent of this damage. We used en face images to visualize retinal nerve fiber bundle defects. To measure perimetric sensitivities, we used a blob stimulus (standard deviation of 0.25°) at the 10-2 locations. A perimetric defect for a location was defined as any value equal to or deeper than -4, -5, and -6 dB below the mean sensitivity for 37 age-similar controls (aged 47 to 78 years). We also presented an elongated sinusoidal stimulus for 20 patients at four locations within the macula, in which we defined a perimetric defect as any value below the 2.5th percentile from controls.

RESULTS

The -4, -5, and -6 dB criteria identified perimetric defects in 14, 13, and 11 patients, respectively. When testing with the elongated stimulus, 18 patients were identified with perimetric defect. The perimetric defects were consistent with the structural damage.

CONCLUSIONS

The elongated stimulus showed a good structure-function agreement with only four testing locations as compared with 68 locations used with the blob stimulus. This demonstrates a clinical potential for this new stimulus in the next generation of perimetry.

Functional characteristics of glaucoma related arcuate defects seen on OCT en face visualisation of the retinal nerve fibre layer

Purpose

To assess continuity of perimetric defects corresponding to arcuate defects seen on optical coherence tomography (OCT) en face reflectance images of the retinal nerve fibre layer (RNFL) in patients with glaucoma.

Methods

Seven patients with glaucoma who had arcuate structural defects on OCT RNFL en face images were recruited. Static suprathreshold stimuli were presented along different meridians to localise perimetric defects in the corresponding hemifield. Then two contrasts, one 6 dB greater than the other, were used with kinetic perimetry to assess the slope of the defect. Findings with kinetic and 24‐2 perimetry were compared.

Results

Static perimetry found that regions of perimetric abnormality spatially corresponded with the regions of en face RNFL hyporeflectivity. Kinetic perimetry found that the slopes of the edges of the defects ranged from 3–12 dB degree⁻¹, and that the functional abnormalities were continuous with the physiologic blind spot even when the 24‐2 protocol only showed paracentral defects.

Conclusions

Perimetric abnormalities and arcuate RNFL en face defects were spatially correspondent. Perimetric testing guided by OCT en face reflectance images can reveal greater functional detail of glaucomatous abnormality than 24‐2 testing.

Within-subject variability in human retinal nerve fiber bundle width

With the growing availability of high-resolution imaging there has been increased interest in developing new metrics for integrity of the retinal nerve fiber layer. In particular, it has been suggested that measurement of width of retinal nerve fiber bundles (RNFBs) may be useful in glaucoma, due to low between-subject variability in mean RNFB width. However, there have also been reports of substantial within-subject variability in the width of individual RNFBs. To assess within-subject variability as a potential source of selection bias in measurements of RNFB width, we used an adaptive optics scanning laser ophthalmoscope (AOSLO) to measure widths of individual RNFBs in one eye each of 11 young adults in good ocular health. In a pilot study we analyzed a large AOSLO image of RNFL in one participant then, based on those findings, in the main study we used AOSLO to image a smaller region in 10 additional healthy young adults. The pilot study of one eye found RNFB widths ranging from 10 μm to 44 μm. This suggested that biological variability was too high for measuring small changes arising from disease processes. This was confirmed in measurements of 10 eyes in the main study, RNFB widths ranged from 9 μm to 55 μm and every eye had large within-subject variability (exceeding 19 μm in all eyes) in RNFB width for nearby bundles. The within-subject variability in RNFB width, as well as variation in the width of single RNFBs over relatively short distances (<300 um) depending on the precise location of measurement, suggests that bundle width measurements would be highly susceptible to selection bias and therefore of limited clinical use.

Comparison of defect depths for sinusoidal and circular perimetric stimuli in patients with glaucoma

Purpose

Clinical use of perimetric testing in patients with glaucoma typically assumes that perimetric defects will be less deep for larger than smaller stimuli. However, studies have shown that very large sinusoidal stimuli can yield similar defects as small circular stimuli. In order to provide guidelines for new perimetric stimuli, we tested patients with glaucoma using five different stimuli and compared defects to their patterns of retinal nerve fibre layer (RNFL) damage.

Methods

Twenty subjects with glaucoma were imaged with optical coherence tomography (OCT) volume scans to allow for en face RNFL images and were also tested on a custom perimetry station with five stimuli: Goldmann sizes III and V, a two‐dimensional Gaussian blob (standard deviation 0.5°) and a 0.5 cycle degree⁻¹ sinusoidal grating presented two ways: flickered at 5 Hz, and pulsed for 200 ms instead of flickered. En face RNFL images were reviewed with the visual field locations overlaid, and each location was labelled for a patient as either no visible RNFL defect or as wedge, slit, edge, or diffuse defect. Nineteen age‐similar controls were tested with the same stimuli to define depth of defect as difference from mean normal. Bland‐Altman analysis was used to test three predictions of neural modelling by making five comparisons.

Results

Bland‐Altman analysis confirmed the three predictions. The flickered sinusoid gave deeper defects in damaged areas than the pulsed sinusoid (r = 0.25, p < 0.0001). When comparing data for sizes III and V there was increased spread of the data in deeper defects in the direction of size III having deeper defect (r = 0.35, p < 0.0001). The size V stimulus yielded shallower defects than a stimulus of similar size but with blurred edges (r = 0.20, p = 0.0004).

Conclusions

On average, all stimuli produced similar results comparing across type of RNFL damage. However, there were systematic patterns consistent with predictions of neural modelling: in damaged areas, depth of defect tended to be greater for the flickered sinusoid than the pulsed sinusoid, greater for the size III stimulus than the size V stimulus, and greater for the Gaussian blob than for the size V stimulus.

Evaluating glaucomatous abnormality in peripapillary optical coherence tomography enface visualisation of the retinal nerve fibre layer reflectance

Purpose

Optical coherence tomography (OCT) enface visualisation of the retinal nerve fibre layer (RNFL) reflectance has been found to have some advantages over retinal thickness measures. However, it is not yet clear how abnormalities on enface images relate to findings of abnormalities from other clinical measures such as the circumpapillary retinal nerve fibre layer thickness (cRNFLT). We developed a technique to analyse the RNFL reflectance on the OCT enface images, and to investigate its relation with the cRNFLT.

Methods

Spectralis (http://www.heidelbergengineering.com) OCT scans of the central retinal ±24° were analysed in the study eye of 31 controls and 33 patients, ages 61 (±9) and 69 (±8) years respectively. Enface slab‐images were extracted at 16–24, 24–36, and 24–52 μm from the inner limiting membrane in the temporal raphe, perifoveal and disc regions respectively. Reflectance probability maps were generated for the patients based on the control data. Glaucomatous abnormality was defined on the slab‐images when the slab‐area with reflectance abnormality was greater than the 95th percentile, and on the cRNFLT when the thickness measure was less than the fifth percentile, of that found in controls. The fraction of slab‐image showing reflectance abnormality was compared to cRNFLT in the patient group, using Spearman's rho. Agreement between the findings of abnormality based on cRNFLT and slab‐image reflectance was assessed using Cohen's kappa.

Results

Slab‐image and cRNFLT findings were in agreement for 26/33 eyes; four subjects showed cRNFLT abnormality but not slab‐image abnormality, and three subjects showed slab‐image abnormality but not cRNFLT abnormality. Spearman's rho found r_s(31) = −0.82. The reflectance findings and cRNFLT findings were consistent in 27/33 for both the superior temporal (ST) and inferior temporal (IT) sectors, and Cohen's kappa found 0.53 and 0.61 respectively.

Conclusion

The surface area of enface slab‐images showing RNFL reflectance were strongly related to the cRNFLT measures, and the classification of a subject with glaucoma based on enface reflectance findings and cRNFLT findings had a generally good agreement. The larger retinal area assessed by the enface method preserves the spatial location of the RNFL abnormalities, and makes the technique a useful approach for identifying regions of potential RNFL abnormality for targeted perimetry.

Customizing Perimetric Locations Based on En Face Images of Retinal Nerve Fiber Bundles With Glaucomatous Damage

Purpose

Prior studies suggested the use of customized perimetric locations in glaucoma; these studies were limited by imaging only the superficial depths of the retinal nerve fiber layer (RNFL) and by prolonged perimetric testing. We aimed to develop a rapid perimetric test guided by high-resolution images of RNFL bundles.

Methods

We recruited 10 patients with glaucoma, ages 56 to 80 years, median 68 years, and 10 controls, ages 55 to 77 years, median 68 years. The patients were selected based on discrepancies between locations of glaucomatous damage for perimetric and structural measures. Montaging was used to produce optical coherence tomography en face images of the RNFL covering much of the 24-2 grid locations. In experiment 1, we presented the Goldmann size III stimulus at preselected retinal locations of glaucomatous damage, using just two contrasts. In experiment 2, we developed an elongated sinusoidal stimulus, aligned within the defect, to measure contrast sensitivities; abnormalities were defined based on lower 95% reference limits derived from the controls.

Results

The percentage of predicted locations where size III was not seen at 28 dB ranged from 16% to 80%, with a median of 48%. Contrast sensitivity for the sinusoidal stimulus was below the 95% reference range for 37 of 44 stimuli aligned within the defects.

Conclusions

We developed methods for rapid perimetric testing guided by en face images of the RNFL bundles in patients with glaucoma. Results indicated ganglion cell damage under all of the visible RNFL defects.

Translational Relevance

Customized perimetric locations have potential to improve clinical assessment of glaucoma.

A basis for customising perimetric locations within the macula in glaucoma

PURPOSE

It has been recognised that the 24-2 grid used for perimetry may poorly sample the macula, which has been recently identified as a critical region for diagnosing and managing patients with glaucoma. We compared data derived from patients and controls to investigate the efficacy of a basis for customising perimetric locations within the macula, guided by en face images of retinal nerve fibre layer (RNFL) bundles.

METHODS

We used SD-OCT en face montages (www.heidelbergengineering.com) of the RNFL in 10 patients with glaucoma (ages 56-80 years, median 67.5 years) and 30 age-similar controls (ages 47-77, median 58). These patients were selected because of either the absence of perimetric defect while glaucomatous damage to the RNFL bundles was observed, or because of perimetric defect that did not reflect the extent and locations of the glaucomatous damage that appeared in the RNFL images. We used a customised blob stimulus for perimetric testing (a Gaussian blob with 0.25° standard deviation) at 10-2 grid locations, to assess the correspondence between perimetric defects and damaged RNFL bundles observed on en face images and perimetric defects. Data from the age-similar controls were used to compute total deviation (TD) and pattern deviation (PD) values at each location; a perimetric defect for a location was defined as a TD or PD value of -0.5 log unit or deeper. A McNemar's test was used to compare the proportions of locations with perimetric defects that fell outside the damaged RNFL bundles, with and without accounting for displacement of ganglion cell bodies.

RESULTS

All patients but one had perimetric defects that were consistent with the patterns of damaged RNFL bundles observed on the en face images. We found six abnormal perimetric locations of 2040 tested in controls and 132 abnormal perimetric locations of 680 tested in patients. The proportions of abnormal locations that fell outside the damaged RNFL bundles, with and without accounting for displacement of the ganglion cell bodies were 0.08 and 0.07, respectively. The difference between the two proportions did not reach statistical significance (p = 0.5 for a one-tailed test).

CONCLUSIONS

We demonstrated that it is effective to customise perimetric locations within the macula, guided by en face images of the RNFL bundles. The perimetric losses found with a 10-2 grid demonstrated similar patterns as the damaged RNFL bundles observed on the en face images.

Retinal putative glial alterations: implication for glaucoma care

PURPOSE

Gliosis-like retinal alterations, presumed to be activated retinal astrocytes and Müller cells (ARAM), have been reported to occur frequently in patients with glaucoma but rarely in controls. We investigated the association between glaucomatous abnormality and the presence, the extent of retinal region, and the spatial distribution, of hyperreflective retinal alterations on optical coherence tomography (OCT) en-face images, presumed to be ARAM.

METHODS

Findings of hyperreflective structures, presumed to be ARAM, in the central retinal ±24 degrees of OCT en-face images (acquired with the SPECTRALIS^® OCT) were compared between 35 younger controls, 42 older controls and 38 patients with glaucoma. Presumed ARAM was defined as reflective structures on the en-face images other than retinal vasculature and retinal nerve fibre bundles. Chi-square tests were used to compare the proportion of younger controls vs older controls with presumed ARAM to investigate the effect of ageing, and the proportion of patients vs age-similar older controls with presumed ARAM to investigate the effect of disease. We also investigated the effect of glaucoma on the retinal area with presumed ARAM when it was present; we used an analysis of covariance (ancova) to compare the retinal area with hyperreflectivity in patients vs controls, adjusting for the effects of age and axial length.

RESULTS

The mean (S.D.) age of the younger controls, older controls, and patients with glaucoma was 26 (3), 62 (10) and 69 (8) years, respectively. The median (25th quartile, 75th quartile) of the retinal region with the hyperreflective structures, presumed to be ARAM, was zero (0,0), 1 (0,6), and 11 (0,43) degrees square in the younger controls, older controls and patients with glaucoma respectively. The chi-square test investigating the effect of ageing found χ² (1, N = 77) = 24.8, p < 0.001, and that investigating the effect of disease found χ² (1, N = 80) = 2.3, p = 0.1. The ancova found F(1, 46) = 10.32, P = 0.02.

CONCLUSIONS

There was an effect of ageing on the presence of the hyperreflective structures, presumed to be ARAM, on OCT images. Compared to the presence of hyperreflective structures, the extent of retinal region with the hyperreflective structures has a greater potential of being an indicator of glaucomatous degeneration. Further study is needed to investigate the nature of the relation between glaucomatous abnormality and the extent of the retina with the hyperreflective structures, presumed to be ARAM.

Using perimetric data to estimate ganglion cell loss for detecting progression of glaucoma: a comparison of models

PURPOSE

Models relating perimetric sensitivities to ganglion cell numbers have been proposed for combining structural and functional measures from patients with glaucoma. Here we compared seven models for ability to differentiate progressing and stable patients, testing the hypothesis that the model incorporating local spatial scale would have the best performance.

METHODS

The models were compared for the United Kingdom Glaucoma Treatment Study (UKGTS) data for the right eyes of 489 patients recently diagnosed with glaucoma. The SITA 24-2 program was utilised for perimetry and Stratus OCT fast scanning protocol for thickness of circumpapillary retinal nerve fibre layer (RNFL). The first analysis defined progression in terms of decline in RNFL thickness. The highest and lowest quintiles (22 subjects per group) were identified for change in thickness of inferior temporal (IT), superior temporal (ST), and global RNFL (μm year^-1 ); a two-way anova was used to look for differences between the models in ability to discriminate the two quintiles. The second analysis defined a 'progression group' as those who were flagged by the UKGTS criteria as having progressive loss in perimetric sensitivity, and a 'no progression' group as those with rate of change in Mean Deviation (MD) closest to 0 dB year^-1 (87 subjects per group). The third analysis characterised variability of retinal ganglion cell (RGC) models for the two groups in the second analysis, using the standard deviation of residuals from linear regression of ganglion cell number over time to compute Coefficient of Variation (CoV).

RESULTS

The first analysis produced a negative result because the three anovas found no effect of model or interaction of model and group (F_6,294 < 3.1, p > 0.08). There was an effect of group only for the anova with the ST sector (F_6,294 = 12.2, p < 0.001). The second analysis also produced a negative result, because ROC areas were in the range 0.69-0.72 for all models. The third analysis found that even when variability in MD was low, the CoV was so large that test-retest variation could include 100% loss of ganglion cells.

CONCLUSIONS

Two very different approaches for testing the hypothesis both gave a negative result. For all seven ganglion cell models, rates of ganglion cell loss were highly affected by fluctuations in height of the hill of vision. Methods for reducing effects of between-visit variability are needed in order to assess progression by relating perimetric sensitivities and ganglion cell numbers.

Individual differences in the shape of the nasal visual field

Between-subject differences in the shape of the nasal visual field were assessed for 103 volunteers 21-85years of age and free of visual disorder. Perimetry was conducted with a stimulus for which contrast sensitivity is minimally affected by peripheral defocus and decreased retinal illumination. One eye each was tested for 103 volunteers free of eye disease in a multi-center prospective longitudinal study. A peripheral deviation index was computed as the difference in log contrast sensitivity at outer (25-29° nasal) and inner (8° from fixation) locations. Values for this index ranged from 0.01 (outer sensitivity slightly greater than inner sensitivity) to -0.7 log unit (outer sensitivity much lower than inner sensitivity). Mean sensitivity for the inner locations was independent of the deviation index (R²<1%), while mean sensitivity for the outer locations was not (R²=38%, p<0.0005). Age was only modestly related to the index, with a decline by 0.017 log unit per decade (R²=10%). Test-retest data for 21 volunteers who completed 7-10 visits yielded standard deviations for the index from 0.04 to 0.17 log unit, with a mean of 0.09 log unit. Between-subject differences in peripheral deviation persisted over two years of longitudinal testing. Peripheral deviation indices were correlated with indices for three other perimetric stimuli used in a subset of 24 volunteers (R² from 20% to 49%). Between-subject variability in shape of the visual field raises concerns about current clinical visual field indices, and further studies are needed to develop improved indices.

Between-Subject Variability in Healthy Eyes as a Primary Source of Structural-Functional Discordance in Patients With Glaucoma

Purpose

To test with an independent data set the finding that between-subject variability in healthy eyes is the primary source of structural–functional discordance in patients with glaucoma.

Methods

Neuroretinal rim area, retinal nerve fiber layer thickness, and perimetric data were analyzed for one eye in each of 55 control subjects and for 245 right eyes of patients in the United Kingdom Glaucoma Treatment Study. Data were gathered with the Heidelberg Retina Tomograph (HRT), Stratus Optical Coherence Tomograph (OCT), and Humphrey Field Analyzer (HFA). Discordance was quantified as width of the limits of agreement from a Bland-Altman analysis of depth of defect. The ratio of variances (F test) for the patient and control groups was computed for comparisons of HFA-OCT, HFA-HRT, and OCT-HRT. Bonferroni adjustment required P less than 0.017 for statistical significance. The discordance in the patients was also quantified as the 95% prediction interval computed from the discordance in controls using the Hood-Kardon model for the HFA-OCT comparison.

Results

The F ratio comparing discordance in patients and controls was 0.77, 1.43, and 1.32 for the HFA-OCT, HFA-HRT, and OCT-HRT comparisons with P values 0.88, 0.06, and 0.11, respectively. For the Hood-Kardon model, 4.7% of the patients had discordance outside the 95% prediction interval computed from the discordance in controls. Similar results were obtained when all comparisons were repeated for left eyes of patients.

Conclusions

These results confirm previous findings that between-subject variability in healthy eyes is the primary source of structural–functional discordance in patients with glaucoma, and extends this finding to a structural–structural comparison.

The Effect of Limiting the Range of Perimetric Sensitivities on Pointwise Assessment of Visual Field Progression in Glaucoma

Purpose

Automated perimetry does not produce reliable estimates of true psychophysical threshold in glaucomatous visual fields when the perimetric threshold falls below 15 to 19 dB. It may be possible to truncate testing at such locations and not use stimuli with very high contrast. However, this can only be recommended if it does not harm the ability to monitor change. This study examined the effect of applying such a cutoff by censoring sensitivities in two existing longitudinal datasets.

Methods

Series of six visual fields were taken from participants with glaucoma or high-risk ocular hypertension in the Portland Progression Project (P3) and Rotterdam Eye Study (RES). Pointwise linear regression was used to find “progressing” locations, defined as a slope ≤ −1 dB/y with P < 1%. An eye was labeled progressing if ≥3 locations were progressing. This was repeated after setting any sensitivities below the cutoff value C (CdB) to instead equal that value for different integer values of CdB.

Results

In the P3 cohort tested using Swedish Interactive Testing Algorithm (SITA) Standard, censoring below 15 to 19 dB did not reduce the number of eyes flagged as progressing. For the RES cohort tested using the Full Threshold algorithm, censoring below 10 dB did not reduce the number of eyes flagged as progressing, but a modest reduction was seen for CdB between 10 dB and 15 to 19 dB.

Conclusions

The proportion of eyes flagged as progressing was not decreased by censoring unreliable sensitivities. Restricting the range of contrast used in clinical perimetry may be possible without hampering the ability to monitor glaucomatous visual field progression.

Imaging Glaucomatous Damage Across the Temporal Raphe

PURPOSE

To image and analyze anatomical differences at the temporal raphe between normal and glaucomatous eyes using adaptive optics scanning laser ophthalmoscopy (AOSLO) and optical coherence tomography (OCT), and to relate these differences to visual field measurements.

METHODS

Nine glaucomatous eyes of 9 patients (age 54-78 years, mean deviation of visual field [MD] -5.03 to -0.20 dB) and 10 normal eyes of 10 controls (age 54-81, MD -1.13 to +1.39 dB) were enrolled. All the participants were imaged in a region that was centered approximately 9° temporal to the fovea. The size of imaging region was at least 10° vertically by 4° horizontally. The raphe gap, defined as the distance between the superior and inferior retinal nerve fiber layer (RNFL) bundles, was measured. A bundle index was computed to quantify the relative reflectivity and density of the nerve fiber bundles. We also measured thickness of the ganglion cell complex (GCC) and RNFL.

RESULTS

The raphe gap was larger in glaucomatous eyes than control eyes. Specifically, eight glaucomatous eyes with local averaged field loss no worse than -3.5 dB had larger raphe gaps than all control eyes. The bundle index, GCC thickness, and RNFL thickness were on average reduced in glaucomatous eyes, with the first two showing statistically significant differences between the two groups.

CONCLUSIONS

Structural changes in the temporal raphe were observed and quantified even when local functional loss was mild. These techniques open the possibility of using the raphe as a site for glaucoma research and clinical assessment.

Assessing assumptions of a combined structure-function index

Purpose

Medeiros et al. developed a combined structure-function index for glaucoma by combining two ganglion cell models developed by Harwerth et al. The current study assessed assumptions of the Medeiros combined structure-function index by evaluating whether the two Harwerth models gave similar distributions of ganglion cells in an independent dataset.

Methods

The Harwerth models were applied to our previously published data for retinal nerve fibre layer (RNFL) thickness (Stratus OCT 3.4) and visual field sensitivities (24-2 SITA Standard) from one eye each of 51 patients with glaucoma and 62 age-similar control subjects free of eye disease. RNFL thicknesses and perimetric sensitivities were converted to ganglion cell numbers using the Harwerth model for perimetry and the Harwerth model for RNFL. These two estimates of ganglion cell number were compared for the inferior temporal (IT) and superior temporal (ST) sectors of the optic disc and the corresponding visual field locations. Comparisons were made with 14 visual field locations per sector (including a point in the macula for the inferior temporal sector) and with 13 locations (no point in the macula). Data for controls and patients were analysed separately, comparing mean values for RNFL perimetry models. Bonferroni correction was applied to control for repeated tests of significance. The difference between mean values for the RNFL and perimetry models was quantified by equating the means for controls through reduction of the assumed axon diameter used by the RNFL model.

Results

For the control group, the Harwerth RNFL model yielded smaller mean number of retinal ganglion cells than the Harwerth perimetry model, 23–47% lower (t > 13, p < 0.0001). This corresponded to mean axon diameters from 0.48 to 0.69 μm, with the smallest axons when the 14th location in the macula was included. With these new axon diameters, estimates of ganglion cell numbers for patients were still lower than for the RNFL model, by 19–28% (t > 6.5, p < 0.0001).

Conclusions

The Harwerth RNFL model consistently gave lower ganglion cell numbers than the Harwerth perimetry model, and this discordance persisted in patients even after reducing assumed axon diameter for controls. This finding contradicts the assumptions of the Medeiros structure-function index.

Choice of Stimulus Range and Size Can Reduce Test-Retest Variability in Glaucomatous Visual Field Defects

PURPOSE

To develop guidelines for engineering perimetric stimuli to reduce test-retest variability in glaucomatous defects.

METHODS

Perimetric testing was performed on one eye for 62 patients with glaucoma and 41 age-similar controls on size III and frequency-doubling perimetry and three custom tests with Gaussian blob and Gabor sinusoid stimuli. Stimulus range was controlled by values for ceiling (maximum sensitivity) and floor (minimum sensitivity). Bland-Altman analysis was used to derive 95% limits of agreement on test and retest, and bootstrap analysis was used to test the hypotheses about peak variability.

RESULTS

Limits of agreement for the three custom stimuli were similar in width (0.72 to 0.79 log units) and peak variability (0.22 to 0.29 log units) for a stimulus range of 1.7 log units. The width of the limits of agreement for size III decreased from 1.78 to 1.37 to 0.99 log units for stimulus ranges of 3.9, 2.7, and 1.7 log units, respectively (F = 3.23, P < 0.001); peak variability was 0.99, 0.54, and 0.34 log units, respectively (P < 0.01). For a stimulus range of 1.3 log units, limits of agreement were narrowest with Gabor and widest with size III stimuli, and peak variability was lower (P < 0.01) with Gabor (0.18 log units) and frequency-doubling perimetry (0.24 log units) than with size III stimuli (0.38 log units).

CONCLUSIONS

Test-retest variability in glaucomatous visual field defects was substantially reduced by engineering the stimuli.

TRANSLATIONAL RELEVANCE

The guidelines should allow developers to choose from a wide range of stimuli.

Assessment of the reliability of standard automated perimetry in regions of glaucomatous damage

PURPOSE

Visual field testing uses high-contrast stimuli in areas of severe visual field loss. However, retinal ganglion cells saturate with high-contrast stimuli, suggesting that the probability of detecting perimetric stimuli may not increase indefinitely as contrast increases. Driven by this concept, this study examines the lower limit of perimetric sensitivity for reliable testing by standard automated perimetry.

DESIGN

Evaluation of a diagnostic test.

PARTICIPANTS

A total of 34 participants with moderate to severe glaucoma; mean deviation at their last clinic visit averaged -10.90 dB (range, -20.94 to -3.38 dB). A total of 75 of the 136 locations tested had a perimetric sensitivity of ≤ 19 dB.

METHODS

Frequency-of-seeing curves were constructed at 4 nonadjacent visual field locations by the Method of Constant Stimuli (MOCS), using 35 stimulus presentations at each of 7 contrasts. Locations were chosen a priori and included at least 2 with glaucomatous damage but a sensitivity of ≥ 6 dB. Cumulative Gaussian curves were fit to the data, first assuming a 5% false-negative rate and subsequently allowing the asymptotic maximum response probability to be a free parameter.

MAIN OUTCOME MEASURES

The strength of the relation (R(2)) between perimetric sensitivity (mean of last 2 clinic visits) and MOCS sensitivity (from the experiment) for all locations with perimetric sensitivity within ± 4 dB of each selected value, at 0.5 dB intervals.

RESULTS

Bins centered at sensitivities ≥ 19 dB always had R(2) >0.1. All bins centered at sensitivities ≤ 15 dB had R(2) <0.1, an indication that sensitivities are unreliable. No consistent conclusions could be drawn between 15 and 19 dB. At 57 of the 81 locations with perimetric sensitivity <19 dB, including 49 of the 63 locations ≤ 15 dB, the fitted asymptotic maximum response probability was <80%, consistent with the hypothesis of response saturation. At 29 of these locations the asymptotic maximum was <50%, and so contrast sensitivity (50% response rate) is undefined.

CONCLUSIONS

Clinical visual field testing may be unreliable when visual field locations have sensitivity below approximately 15 to 19 dB because of a reduction in the asymptotic maximum response probability. Researchers and clinicians may have difficulty detecting worsening sensitivity in these visual field locations, and this difficulty may occur commonly in patients with glaucoma with moderate to severe glaucomatous visual field loss.

Assessing spatial and temporal properties of perimetric stimuli for resistance to clinical variations in retinal illumination

PURPOSE

To develop perimetric stimuli for which sensitivities are more resistant to reduced retinal illumination than current clinical perimeters.

METHODS

Fifty-four people free of eye disease were dilated and tested monocularly. For each test, retinal illumination was attenuated with neutral density (ND) filters, and a standard adaptation model was fit to derive mean and SEM for the adaptation parameter (NDhalf). For different stimuli, t-tests on NDhalf were used to assess significance of differences in consistency with Weber's law. Three experiments used custom Gaussian-windowed contrast sensitivity perimetry (CSP). Experiment 1 used CSP-1, with a Gaussian temporal pulse, a spatial frequency of 0.375 cyc/deg (cpd), and SD of 1.5°. Experiment 1 also used the Humphrey Matrix perimeter, with the N-30 test using 0.25 cpd and 25 Hz flicker. Experiment 2 used a rectangular temporal pulse, SDs of 0.25° and 0.5°, and spatial frequencies of 0.0 and 1.0 cpd. Experiment 3 used CSP-2, with 5-Hz flicker, SDs from 0.5° to 1.8°, and spatial frequencies from 0.14 to 0.50 cpd.

RESULTS

In Experiment 1, CSP-1 was more consistent with Weber's law (NDhalf ± SEM = 1.86 ± 0.08 log unit) than N-30 (NDhalf = 1.03 ± 0.03 log unit; t > 9, P < 0.0001). All stimuli used in Experiments 2 and 3 had comparable consistency with Weber's law (NDhalf = 1.49-1.69 log unit; t < 2).

CONCLUSIONS

Perimetric sensitivities were consistent with Weber's law when higher temporal frequencies were avoided.

Choice of statistical method influences apparent association between structure and function in glaucoma

PURPOSE

The aim of this study was to explore how different statistical methods may lead to inconsistent inferences about the association between structure and function in glaucoma.

METHODS

Two datasets from published studies were selected for their illustrative value. The first consisted of measurements of neuroretinal rim area in the superior-temporal sector paired with the corresponding visual field sensitivity. The second consisted of measurements of average retinal nerve fiber layer thickness over all sectors paired with the corresponding visual field sensitivity. Statistical methods included linear and segmented regression, and a nonparametric local-linear fit known as loess. The analyses were repeated with all measurements expressed as percent of mean normal.

RESULTS

Slopes from linear fits to the data changed by a factor of 10 depending on the linear regression method applied. Inferences about whether structural abnormality precedes functional abnormality varied with the statistical design and the units of measure used.

CONCLUSIONS

The apparent association between structure and function in glaucoma, and consequent interpretation, varies with the statistical method and units of measure. Awareness of the limitations of any statistical analysis is necessary to avoid finding spurious results that ultimately may lead to inadequate clinical recommendations.

The visualFields package: a tool for analysis and visualization of visual fields

This paper introduces the R package visualFields, a contributed, open-source software for the analysis of the visual field. The package aims to provide a framework for collaborative research, including data sharing and conventional and novel methods. Single visual field and progression analyses, such as Permutation of Pointwise Linear Regression can be performed with visualFields using simple scripts. The package can be easily customized and it allows the inclusion of custom test locations and different normative values. Here, we demonstrate how to use the visualFields package and discuss its capabilities. The analyses presented here are easy to replicate upon installation of the package, which is freely available for download from the Comprehensive R Archive Network. The relevant R code is shown and commented on. A shift from proprietary to an open-source research platform is an important step towards more direct collaborative research. The visualFields package is part of the Open Perimetry Initiative, which is expected to grow as researchers contribute new routines and datasets.

'Structure-function relationship' in glaucoma: past thinking and current concepts

An understanding of the relationship between functional and structural measures in primary open-angle glaucoma is necessary for both grading the severity of disease and for understanding the natural history of the condition. This article outlines the current evidence for the nature of this relationship and highlights the current mathematical models linking structure and function. Large clinical trials demonstrate that both structural and functional change are apparent in advanced stages of disease, and at an individual level, detectable structural abnormality may precede functional abnormality in some patients, whereas the converse is true in other patients. Although the exact nature of the 'structure-function' relationship in primary open-angle glaucoma is still the topic of scientific debate and the subject of continuing research, this article aims to provide the clinician with an understanding of the past concepts and contemporary thinking in relation to the structure-function relationship in primary open-angle glaucoma.

Estimation of spatial scale across the visual field using sinusoidal stimuli

PURPOSE

To characterize contrast sensitivity for sinusoidal stimuli across the central visual field and help bridge the gap between perimetry and visual psychophysics by developing a contrast-sensitivity template for spatial scale (experiment 1) and testing it on a new dataset (experiment 2).

METHODS

In experiment 1, 40 subjects free of eye disease, ages 43 to 84 years, had one eye tested. Twenty-three locations along the horizontal and vertical meridians were tested with sinusoidal stimuli having peak spatial frequencies of 0.5, 1.0, and 2.0 cpd and a spatial bandwidth of 1.0 octave. Contrast sensitivity functions were fit with a low-pass template slid horizontally on a log-log plot by a spatial scale factor. In experiment 2, 29 of the original subjects had one eye tested. Twenty-six locations in grid form were tested with sinusoidal stimuli having peak spatial frequencies of 0.375, 0.53, 0.75, and 1.5 cpd. Spatial scale values were predicted using the 0.375 cpd data and template and compared to empirical values determined from the remaining data.

RESULTS

In experiment 1, the change in spatial scale alone fit the mean sensitivities well (residual sum of squares = 0.01 log unit). Spatial scale increased with eccentricity except for horizontal nasal displacements between 3° and 15°. In experiment 2, differences between empirical and predicted spatial scale values were within ±0.1 log unit (mean and SEM: 0.00 ± 0.01 log unit).

CONCLUSIONS

Spatial scale characterized the visual field tested in perimetry well and can contribute to further linkage between clinical perimetry and basic vision science.

Assessment of linear-scale indices for perimetry in terms of progression in early glaucoma

Currently, global indices that summarize the visual field combine sensitivities on a logarithmic (decibel) scale. Recent structure-function models for glaucoma suggest that contrast sensitivity should be converted to a linear scale before averaging across visual field locations, to better relate sensitivity with the number of surviving retinal ganglion cells (RGCs). New indices designed to represent the number of RGCs already lost are described. At least one was found to be a significantly better predictor of subsequent rate of change than traditional Mean Deviation (p=0.014) in participants with glaucomatous optic neuropathy. Issues concerning the creation of optimal global indices are discussed.

Responses of primate retinal ganglion cells to perimetric stimuli

PURPOSE

Perimetry is used clinically to assess glaucomatous ganglion cell loss. It has been proposed that frequency-doubling stimuli are better than the conventional size III perimetric stimulus in preferentially stimulating magnocellular (M) versus parvocellular (P) ganglion cells. However, little is known about how primate ganglion cells respond to perimetric stimuli. The authors recorded contrast responses of M and P ganglion cells to size III and frequency-doubling stimuli and compared contrast gain of M and P cells to these stimuli to assess the ability of these stimuli to preferentially stimulate M versus P cells.

METHODS

Data were recorded from 69 macaque retinal ganglion cells, by an in vivo preparation, at eccentricities of 5° to 15°. The size III stimulus was a circular luminance increment 26 min arc in diameter, 200 ms in duration. The frequency-doubling stimulus was a sinusoidal grating (0.5 cyc/deg) temporally modulated in counterphase at 13 Hz. A Michaelis-Menten function was fit to each cell's contrast responses to assess contrast gain.

RESULTS

For both size III and frequency-doubling stimuli, ganglion cell responses increased linearly at low contrasts, and then the increase slowed at high contrasts (saturation). The mean (± SE) difference in estimated log contrast gain between M and P cells for the size III stimulus was significantly higher than that for the frequency-doubling stimulus (1.24 ± 0.09 vs. 0.89 ± 0.13; P < 0.01).

CONCLUSIONS

The size III stimulus was superior to the frequency-doubling stimulus in preferentially stimulating M cells versus P cells.

Differential vertical visual latency as determined with a simultaneity paradigm

Behavioral studies, as well as anatomical and physiological data, suggest differences in functionality for inferior and superior visual fields. Previous investigations comparing latencies of the two fields have employed motor reaction times. This approach is of limited usefulness in elderly clinical populations where various degrees of motor impairment may be present. In this report, we describe a simultaneity paradigm that allows the determination of relative latencies without dependence on motor reaction times. A slightly, but statistically significant, shorter latency (3.9+/-5.9ms) was found for the superior visual field. The results are not affected by age, and both within- and between-session variability are low.

Potential signal to accommodation from the Stiles-Crawford effect and ocular monochromatic aberrations

The purpose of this study is to determine if cues within the blurred retinal image due to the Stiles-Crawford (SC) effect and the eye's monochromatic aberrations can drive accommodation with a small pupil (3 mm) that is typical of bright photopic conditions.The foveal, psychophysical SC function (17 min arc) and ocular monochromatic aberrations were measured in 21 visually normal adults. The retinal image of a 10.2 min arc disc was simulated for spherical defocus levels of -1 D, 0 D and +1 D in each of four conditions consisting of combinations of the presence or absence of the individual SC function and monochromatic aberrations with a 3 mm pupil. Accommodation was recorded in eleven participants as each viewed the simulations through a 0.75-mm pinhole.The SC effect alone did not provide a significant cue to accommodation. Monochromatic aberrations provided a statistically significant but rather small cue to monocular accommodation.

A two-stage neural spiking model of visual contrast detection in perimetry

Perimetry is a commonly used clinical test for visual function, limited by high variability. The sources of this variability need to be better understood. In this paper, we investigate whether noise intrinsic to neural firing could explain the variability in normal subjects. We present the most physiologically accurate model to date for stimulus detection in perimetry combining knowledge of the physiology of components of the visual system with signal detection theory, and show that it requires that detection be mediated by multiple cortical cells in order to give predictions consistent with psychometric functions measured in human observers.

Assessment of contrast gain signature in inferred magnocellular and parvocellular pathways in patients with glaucoma

PURPOSE

Contrast gain signatures of inferred magnocellular and parvocellular postreceptoral pathways were assessed for patients with glaucoma using a contrast discrimination paradigm developed by Pokorny and Smith. The potential causes for changes in contrast gain signature were investigated using model simulations of ganglion cell contrast responses.

METHODS

Foveal contrast discrimination thresholds were measured with a pedestal-Delta-pedestal paradigm developed by Pokorny and Smith [Pokorny, J., & Smith, V. C. (1997). Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America A, 14(9), 2477-2486]. Stimuli were 27 ms luminance increments superimposed on 227 ms pulsed Delta-pedestals. Contrast thresholds and contrast gain signatures mediated by the inferred magnocellular (MC) and parvocellular (PC) pathways were assessed using linear fits to contrast discrimination thresholds at either lower or higher Delta-pedestal contrasts, respectively. Twenty-seven patients with glaucoma were tested, as well as 16 age-similar control subjects free of eye disease.

RESULTS

Contrast sensitivity and contrast gain signature mediated by the inferred MC pathway were lower for the glaucoma group, and reduced contrast gain signature was correlated with reduced contrast sensitivity (r(2)=45%, p<.0005). These two parameters mediated by the inferred PC pathway were little affected for the glaucoma group. Model simulations suggest that the reduced contrast sensitivity and contrast gain signature were consistent with the hypothesis that reduced MC ganglion cell dendritic complexity can lead to reduced effective retinal illuminance, and hence increased semi-saturation contrast of the ganglion cell contrast response functions.

CONCLUSIONS

The contrast sensitivity and contrast gain signature of the inferred MC pathway were reduced in patients with glaucoma. The results were consistent with a model of ganglion cell dysfunction due to reduced synaptic density.

Chromatic temporal integration and retinal eccentricity: psychophysics, neurometric analysis and cortical pooling

Psychophysical chromatic sensitivity deteriorates in peripheral retina, even after appropriate size scaling of targets. This decrease is more marked for stimuli targeted at the long- (L) to middle-wavelength (M) cone opponent system than for stimuli targeted at short-wavelength (S) pathways. Foveal chromatic mechanisms integrate over several hundred milliseconds for pulse detection. If the time course for integration were shorter in the periphery, this might account for sensitivity loss. Psychophysical chromatic temporal integration (critical duration) for human observers was estimated as a function of eccentricity. Critical duration decreased by a factor of 2 (from approximately 200 to approximately 100 ms) from the fovea to 20 degrees eccentricity. This partly (but not completely) accounts for the decrease in /L-M/ sensitivity in the periphery, but almost completely accounts for the decrease in S-cone sensitivity. Some loss of /L-M/I sensitivity thus has a cortical locus. In a physiological analysis, we consider how the /L-M/ cone parvocellular pathway integrates chromatic signals. Neurometric contrast sensitivities of individual retinal ganglion cells decreased with the square-root of stimulus duration (as expected from Poisson statistics of ganglion cell firing). In contrast, psychophysical data followed an inverse linear relationship (Bloch's law). Models of cortical pooling mechanisms incorporating uncertainty as to stimulus onset and duration can at least partially account for this discrepancy.

Development and evaluation of a contrast sensitivity perimetry test for patients with glaucoma

PURPOSE

To design a contrast sensitivity perimetry (CSP) protocol that decreases variability in glaucomatous defects while maintaining good sensitivity to glaucomatous loss.

METHODS

Twenty patients with glaucoma and 20 control subjects were tested with a CSP protocol implemented on a monitor-based testing station. In the protocol 26 locations were tested over the central visual field with Gabor patches with a peak spatial frequency of 0.4 cyc/deg and a two-dimensional spatial Gaussian envelope, with most of the energy concentrated within a 4 degrees circular region. Threshold was estimated by a staircase method: Patients and 10 age-similar control subjects were also tested on conventional automated perimetry (CAP), with the 24-2 pattern with the SITA Standard testing strategy. The neuroretinal rim area of the patients was measured with a retinal tomograph (Retina Tomograph II [HRT]; Heidelberg Engineering, Heidelberg, Germany). A Bland-Altman analysis of agreement was used to assess test-retest variability, compare depth of defect shown by the two perimetric tests, and investigate the relations between contrast sensitivity and neuroretinal rim area.

RESULTS

Variability showed less dependence on defect depth for CSP than for CAP (z = 9.3, P < 0.001). Defect depth was similar for CAP and CSP when averaged by quadrant (r = 0.26, P > 0.13). The relation between defect depth and rim area was more consistent with CSP than with CAP (z = 9, P < 0.001).

CONCLUSIONS

The implementation of CSP was successful in reducing test-retest variability in glaucomatous defects. CSP was in general agreement with CAP in terms of depth of defect and was in better agreement than CAP with HRT-determined rim area.

A new pattern electroretinogram paradigm evaluated in terms of user friendliness and agreement with perimetry

PURPOSE

To assess ease of use of the pattern electroretinogram optimized for glaucoma screening (PERGLA) paradigm by a novice operator; to study test-retest variability of the PERGLA parameters; and to compare results from the PERGLA to those from perimetry.

DESIGN

Cohort study.

PARTICIPANTS

Twelve healthy control subjects and 16 patients with moderate to advanced glaucoma in at least 1 eye.

METHODS

Pattern electroretinograms were recorded simultaneously from both eyes using a commercially available testing station. Each participant underwent PERGLA procedures in 2 sessions. One eye of each subject was tested on contrast sensitivity perimetry (CSP) in which a 0.4 cycles/degree Gabor patch served as a stimulus. Central visual fields results from conventional automated perimetry (CAP) were obtained from patients' records. Bland-Altman analysis was performed on PERGLA results to assess normal test-retest variability. Differences from mean normal (in decibels [dB]) were compared for PERGLA versus CSP and CAP.

MAIN OUTCOME MEASURES

Pattern electroretinogram amplitude, noise, phase, and test-retest variability (coefficient of variation); contrast sensitivity from CSP; perimetric sensitivity from CAP; and differences from mean normal for PERGLA, CSP, and CAP.

RESULTS

The mean log amplitude (0.08+/-0.12 log muV) and the mean phase (1.92+/-0.07 pi rad) for the control group were consistent with published PERGLA norms, as was test-retest variability for both amplitude (coefficient of variation [CV] = 8.2+/-7.0%) and phase (CV = 1.1+/-0.9%). The mean signal-to-noise ratio (8.7+/-4.5) was lower than published norms. The test-retest variability increased as PERGLA log amplitude decreased (R2>0.12, P<0.05). On average, differences from mean normal were similar for PERGLA versus CSP and for PERGLA versus CAP (mean differences<0.5 dB) with 95% confidence intervals near +/-4 dB in both comparisons.

CONCLUSIONS

A novice operator successfully replicated published PERGLA norms in a young control group for amplitude, phase, and repeatability. Higher test-retest variability was found in eyes with smaller signals. On average, PERGLA results were in reasonable agreement with those from perimetry, although there existed large individual differences which may limit the usage of PERGLA in screening or in following progression of glaucoma.

Variability of visual field measurements is correlated with the gradient of visual sensitivity

Conventional static automated perimetry provides important clinical information, but its utility is limited by considerable test-retest variability. Fixational eye movements during testing could contribute to variability. To assess this possibility, it is important to know how much sensitivity change would be caused by a given eye movement. To investigate this, we have evaluated the gradient, the rate at which sensitivity changes with location. We tested one eye each, twice within 3 weeks, of 29 patients with glaucoma, 17 young normal subjects and 13 older normal subjects. The 10-2 test pattern with the SITA Standard algorithm was used to assess sensitivity at locations with 2 degrees spacing. Variability and gradient were calculated at individual test locations. Matrix correlations were determined between variability and gradient, and were substantial for the patients with glaucoma. The results were consistent with a substantial contribution to test-retest variability from small fixational eye movements interacting with visual field gradient. Successful characterization of the gradient of sensitivity appears to require sampling at relatively close spacing, as in the 10-2 test pattern.

A clinical evaluation of proview pressure phosphene tonometry in children

PURPOSE

The Proview tonometer measures intraocular pressure by inducing a pressure phosphene through the eyelid and, if reliable and valid, may offer a quick, nonthreatening and noninvasive alternative method of obtaining intraocular pressures (IOPs) without the use of eye drops. This study compares the IOP measurements obtained in children using Proview pressure phosphene tonometry (PPPT) and Goldmann tonometry (GT).

METHODS

One hundred four 5- to 12-year-old patients of the University Optometric Center/SUNY College of Optometry participated in the study. Subjects were randomized to receive, by different investigators, either PPPT or GT first. Two measurements with each instrument were attempted on each eye of all subjects. A subgroup of 41 subjects was asked which of the two methods was preferred.

RESULTS

Seven percent of the subjects did not report a pressure phosphene response compared with 12% of the subjects on whom the investigators were unable to perform GT. The remaining 85 subjects completed the subject protocol. Of the 41 subjects asked, 56% preferred PPPT, 24% had no preference, and 20% preferred GT. The coefficient of repeatability between the two readings was higher for PPPT (3-4 mm Hg) than for GT (1 mm Hg). Mean IOP was 4 mm Hg higher for PPPT than GT with the difference in readings between the two instruments increasing with higher IOPs (r >19%, p < 0.005).

CONCLUSIONS

In our study of healthy young subjects, PPPT measurements of IOP appear to be repeatable within a few millimeters of mercury in most children, but for some children, variability in repeat measurements can be substantial. Our data showed a mean difference in readings of 4 mm Hg with a 95% confidence interval that the PPPT reading was between 12 mm Hg above GT and 4 mm Hg below GT. This wide range of values indicates that PPPT is not comparable to GT. However, because our study found that children can appreciate pressure phosphenes and most prefer PPPT over GT, the Proview monitor may have value as a noninvasive, portable screener in pediatric patients. To fully evaluate this potential, further studies are needed that include patients with high IOPs.

A cortical pooling model of spatial summation for perimetric stimuli

Contemporary models of perimetric sensitivity assume probability summation of retinal ganglion cell sensitivities, ignoring cortical processing. To assess the role of cortical processing in perimetric spatial summation, we used a common form of multiple-mechanism spatial vision model in which the stimulus is sampled by receptive fields analogous to those of simple cells in primary visual cortex. Psychophysical threshold was computed by probability summation across the receptive fields. When the receptive fields were nonoriented (like ganglion cells), the spatial summation function had a large nonmonotonic transitional region that was inconsistent with perimetric spatial summation data. When the receptive fields were orientation tuned (like cortical cells), the model was able to give good fits to perimetric spatial summation data. The predictions of the model were evaluated with a masking study, in which noise masks either enlarged the critical area or changed the shape of the spatial summation functions. We conclude that cortical pooling by multiple spatial mechanisms can account for perimetric spatial summation, whereas probability summation across ganglion cells cannot.

Ganglion cell loss and age-related visual loss: a cortical pooling analysis

PURPOSE

To evaluate the ability of the cortical pooling model to predict the effects of random, mild ganglion cell loss, we compared the predictions of the model with the age-related loss and variability in achromatic and chromatic contrast sensitivity.

METHODS

The relative sensitivity to small (0.5 degrees ) and large (3.0 degrees ) stimuli was compared in older (mean = 67 years, n = 27) and younger (mean = 23 years, n = 32) adults. Contrast sensitivity for modulations along the luminance, equiluminant L-cone, and equiluminant S-cone axes was assessed at the fovea and at four peripheral locations (12 degrees ).

RESULTS

When the stimuli were large, threshold measurements obtained from all participants were reliable and well within the range of modulations along the chromatic axes that could be produced by the phosphors of the CRT. For the large stimuli, neither long- nor short-term variability increased as a function of age. Increasing the size of the stimulus did not decrease the magnitude of the age-related losses when the stimulus was chromatic, and visual losses observed with large chromatic stimuli were not different from those obtained with small achromatic stimuli. Moreover, chromatic contrast sensitivity assessments identified significant visual losses in four individuals who were not identified by achromatic contrast sensitivity assessments and only missed identifying one individual with significant losses in achromatic contrast sensitivity.

CONCLUSIONS

The declines in achromatic and chromatic sensitivity as a function of age (0.4-0.7 dB per decade) were similar to those obtained in previous studies of achromatic and chromatic perimetry and are consistent with the loss of retinal ganglion cells reported in histologic studies. The results of this study are consistent with the predictions the cortical pooling model makes for both variability and contrast sensitivity. These findings emphasize that selective visual impairments do not necessarily reflect preferential damage to a single ganglion cell class and that it is important to include the influence of higher cortical processing when quantifying the relation between ganglion cells and visual function.

Evaluation of a two-stage neural model of glaucomatous defect: an approach to reduce test-retest variability

PURPOSE

The purpose of this study is to model perimetric defect and variability and identify stimulus conditions that can reduce variability while retaining good ability to detect glaucomatous defects.

METHODS

The two-stage neural model of Swanson et al. was extended to explore relations among perimetric defect, response variability, and heterogeneous glaucomatous ganglion cell damage. Predictions of the model were evaluated by testing patients with glaucoma using a standard luminance increment 0.43 degrees in diameter and two innovative stimuli designed to tap cortical mechanisms tuned to low spatial frequencies. The innovative stimuli were a luminance-modulated Gabor stimulus (0.5 c/deg) and circular equiluminant red-green chromatic stimuli whose sizes were close to normal Ricco's areas for the chromatic mechanism. Seventeen patients with glaucoma were each tested twice within a 2-week period. Sensitivities were measured at eight locations at eccentricities from 10 degrees to 21 degrees selected in terms of the retinal nerve fiber bundle patterns. Defect depth and response (test-retest) variability were compared for the innovative stimuli and the standard stimulus.

RESULTS

The model predicted that response variability in defective areas would be lower for our innovative stimuli than for the conventional perimetric stimulus with similar defect depths if detection of the chromatic and Gabor stimuli was mediated by spatial mechanisms tuned to low spatial frequencies. Experimental data were consistent with these predictions. Depth of defect was similar for all three stimuli (F = 1.67, p > 0.19). Mean response variability was lower for the chromatic stimulus than for the other stimuli (F = 5.58, p < 0.005) and was lower for the Gabor stimulus than for the standard stimulus in areas with more severe defects (t = 2.68, p < 0.005). Variability increased with defect depth for the standard and Gabor stimuli (p < 0.005) but not for the chromatic stimulus (slope less than zero).

CONCLUSIONS

Use of large perimetric stimuli detected by cortical mechanisms tuned to low spatial frequencies can make it possible to lower response variability without comprising the ability to detect glaucomatous defect.

Linearity can account for the similarity among conventional, frequency-doubling, and gabor-based perimetric tests in the glaucomatous macula

PURPOSES

The purposes of this study are to compare macular perimetric sensitivities for conventional size III, frequency-doubling, and Gabor stimuli in terms of Weber contrast and to provide a theoretical interpretation of the results.

METHODS

Twenty-two patients with glaucoma performed four perimetric tests: a conventional Swedish Interactive Threshold Algorithm (SITA) 10-2 test with Goldmann size III stimuli, two frequency-doubling tests (FDT 10-2, FDT Macula) with counterphase-modulated grating stimuli, and a laboratory-designed test with Gabor stimuli. Perimetric sensitivities were converted to the reciprocal of Weber contrast and sensitivities from different tests were compared using the Bland-Altman method. Effects of ganglion cell loss on perimetric sensitivities were then simulated with a two-stage neural model.

RESULTS

The average perimetric loss was similar for all stimuli until advanced stages of ganglion cell loss, in which perimetric loss tended to be greater for size III stimuli than for frequency-doubling and Gabor stimuli. Comparison of the experimental data and model simulation suggests that, in the macula, linear relations between ganglion cell loss and perimetric sensitivity loss hold for all three stimuli.

CONCLUSIONS

Linear relations between perimetric loss and ganglion cell loss for all three stimuli can account for the similarity in perimetric loss until advanced stages. The results do not support the hypothesis that redundancy for frequency-doubling stimuli is lower than redundancy for size III stimuli.

Development and evaluation of a linear staircase strategy for the measurement of perimetric sensitivity

Perimetric sensitivity of patients with glaucoma has traditionally been measured in logarithmic (dB) units, but linear sensitivity correlates better with conventional structural measures of glaucomatous damage. Monte Carlo simulations of perimetric algorithms were used to assess potential effects of logarithmic steps on bias and variability when perimetric sensitivity was represented in linear units, and to assess the potential benefits of algorithms using linear steps. Simulations predicted that linear staircases could reduce the sensitivity-dependence of bias, variability and efficiency. These predictions were supported by a perimetric study of 21 patients with glaucoma and 20 age-similar controls who made repeat visits over several weeks.

Pupillary evaluation of retinal asymmetry: development and initial testing of a technique

Glaucomatous damage to upper and lower retina is often unequal. We have developed a rapid, objective, quantitative measure of asymmetry of retinal sensitivity, using infrared pupillometry and pairs of large stimuli that are symmetric about the horizontal meridian. Results for a group of 11 young subjects free of eye disease indicate that the distribution of asymmetry is close to a normal distribution centered near upper/lower symmetry. Some subjects showed modest amounts of asymmetry, which was relatively uniform within each eye, and between the two eyes, of the subject. This approach to determination of asymmetry within an eye is potentially applicable to testing patients with glaucoma. The narrowness of the distribution should make it possible to detect asymmetries caused by disease.

Empiric Determination of Corrected Visual Acuity Standards for Train Crews

PURPOSE

Probably the most common visual standard for employment in the transportation industry is best-corrected, high-contrast visual acuity. Because such standards were often established absent empiric linkage to job performance, it is possible that a job applicant or employee who has visual acuity less than the standard may be able to satisfactorily perform the required job activities. For the transportation system that we examined, the train crew is required to inspect visually the length of the train before and during the time it leaves the station. The purpose of the inspection is to determine if an individual is in a hazardous position with respect to the train. In this article, we determine the extent to which high-contrast visual acuity can predict performance on a simulated task.

METHODS

Performance at discriminating hazardous from safe conditions, as depicted in projected photographic slides, was determined as a function of visual acuity. For different levels of visual acuity, which was varied through the use of optical defocus, a subject was required to label scenes as hazardous or safe.

RESULTS

Task performance was highly correlated with visual acuity as measured under conditions normally used for vision screenings (high-illumination and high-contrast): as the acuity decreases, performance at discriminating hazardous from safe scenes worsens.

CONCLUSIONS

This empirically based methodology can be used to establish a corrected high-contrast visual acuity standard for safety-sensitive work in transportation that is linked to the performance of a job-critical task.

Quantifying effects of retinal illuminance on frequency doubling perimetry

PURPOSE

To measure and quantify effects of variation in retinal illuminance on frequency doubling technology (FDT) perimetry.

METHODS

A Zeiss-Humphrey/Welch Allyn FDT perimeter was used with the threshold N-30 strategy. Study 1, quantifying adaptation: 11 eyes of 11 subjects (24-46 years old) were tested with natural pupils, and then retested after stable pupillary dilation with neutral density filters of 0.0, 0.6, 1.2, and 1.6 log unit in front of the subject's eye. Study 2, predicting effect of reduced illuminance: 17 eyes of 17 subjects (26-61 years old) were tested with natural pupils, and then retested after stable pupillary miosis (assessed with an infrared camera). A quantitative adaptation model was fit to results of Study 1; the mean adaptation parameter was used to predict change in Study 2.

RESULTS

Study 1: Mean defect (MD) decreased by 10 dB over a 1.6 log unit range of retinal illuminances; model fits for all subjects had r2> 95%. Study 2: Change in MD (DeltaMD) ranged from -7.3 dB to +0.8 dB. The mean adaptation parameter from Study 1 accounted for 69% of the variance in DeltaMD (P <0.0005), and accuracy of the model was independent of the magnitude of DeltaMD (r2< 1%, P >0.75).

CONCLUSIONS

The results confirmed previous findings that FDT perimetry can be dramatically affected by variations in retinal illuminance. Application of a quantitative adaptation model provided guidelines for estimating effects of pupil diameter and lens density on FDT perimetry.

Perimetric defects and ganglion cell damage: interpreting linear relations using a two-stage neural model

PURPOSE

To better understand the relations between glaucomatous perimetric defects and ganglion cell damage, a neural model was developed to interpret empiric findings on linear relations between perimetric defects and measures of ganglion cell loss.

METHODS

A two-stage model computed responses of ganglion cell mosaics (first stage), then computed perimetric sensitivity in terms of processing by spatial filters (second stage) that pool the ganglion cell responses. Cell death and dysfunction were introduced in a local patch of the first-stage ganglion cell mosaic, and perimetric defect depth was computed for the corresponding region of the visual field. Calculations were performed for both sparse and dense ganglion cell mosaics and for spatial filters with peak frequencies from 0.5 to 4.0 cyc/deg.

RESULTS

The model yielded nonlinear functions for perimetric defect depth in decibel versus the percentage of ganglion cell damage, but functions for lower spatial frequencies became linear when perimetric defect was expressed as a percentage of normal. The relations between perimetric defects and percentage of ganglion cell loss were determined primarily by spatial tuning of the second-stage spatial filters. For averaging sensitivities across different visual field locations, linear units (arithmetic mean) can more closely approximate mean ganglion cell loss than decibel units (geometric mean). Fits to data from experimental glaucoma required ganglion cell dysfunction in addition to ganglion cell loss.

CONCLUSIONS

Pooling by second-stage spatial filters can account for empiric findings of linear relations between perimetric defects and measures of ganglion cell loss.