Increased optic nerve head blood flow after 1 week of twice daily topical brinzolamide treatment in Dutch-belted rabbits

OBJECTIVES

Using a three-way crossover study design, we compared the effects of brinzolamide 2%, dorzolamide 2%, and placebo (vehicle) on microvascular optic nerve head (ONH) blood flow, intraocular pressure (IOP), blood pressure, heart rate, and acid-base balance in nine acepromazine-tranquilized Dutch-belted rabbits.

METHODS

Baseline measurements were taken before treatment and after drug-free washout periods of 7-14 days. Microvascular ONH blood flow was measured with a fundus camera-based laser Doppler flowmeter (LDF). Intraocular pressure was measured with a Tono-Pen XL. One drop of brinzolamide, dorzolamide, or vehicle was administered twice daily (9 A.M. and 5 P.M.) in right eyes only for 7 days. Experimental measurements were made 90 minutes after the 9 A.M. topical dose was administered on day 8.

RESULTS

ONH blood flow was significantly increased (P< or =0.05) in carbonic anhydrase inhibitor (CAI)-treated rabbits, as compared with vehicle-treated controls. The percent increase from baseline was 11.2+/-1.8% in brinzolamide-treated animals and 8.4+/-4.3% in dorzolamide-treated animals. Compared with controls, IOP in the brinzolamide- and dorzolamide-treated groups was significantly decreased (P< or =0.05). The changes in ONH blood flow and IOP were not significantly different between the CAI treatment groups. Small but significant changes in systemic blood gas tensions and pH were present in both CAI treatment groups, as compared with the vehicle group. Systemic blood pressure and heart rate were not significantly changed.

CONCLUSIONS

Topical ocular CAI treatment for 1 week with either brinzolamide or dorzolamide significantly reduced IOP and significantly increased ONH blood flow in tranquilized Dutch-belted rabbits, while eliciting minimal systemic acid-base balance disturbances.

The cortical representation of shadows cast by retinal blood vessels

PURPOSE

We inquired whether the representation of angioscotomas could be detected in the primary (striate) visual cortex.

METHODS

In 12 normal squirrel monkeys, the ocular fundi were photographed and retinal vascular landmarks were projected onto a tangent screen for calibration. Each animal then underwent monocular enucleation under general anesthesia. Animals were perfused after 8 to 10 days, and flat-mounted sections of striate cortex were processed for the metabolic enzyme cytochrome oxidase (CO).

RESULTS

In each animal, the cortical region corresponding to the blind spot appeared as a 3 x 2 mm oval in the CO staining pattern. It stood out because it received input from only 1 eye. In 9 of 12 animals, the representation of the major retinal vessels was also visible, for the same reason. In our best examples, CO sections showed about 10 thin lines radiating from the blind spot representation. Some could be traced for 15 mm, all the way to the vertical meridian. Vessels only 12 minutes of arc in diameter were represented in the cortex. Each angioscotoma representation in the cortex could be matched with its corresponding retinal vessel in the fundus.

CONCLUSIONS

Our findings show that (1) the visual field map in layer IVc is more precise than indicated by physiological studies, and (2) visual experience must refine the final pattern of geniculocortical projections, given that the retinal vessels can produce a shadow only after birth.

Functional circuitry of the retinal ganglion cell's nonlinear receptive field

A retinal ganglion cell commonly expresses two spatially overlapping receptive field mechanisms. One is the familiar "center/surround," which sums excitation and inhibition across a region somewhat broader than the ganglion cell's dendritic field. This mechanism responds to a drifting grating by modulating firing at the drift frequency (linear response). Less familiar is the "nonlinear" mechanism, which sums the rectified output of many small subunits that extend for millimeters beyond the dendritic field. This mechanism responds to a contrast-reversing grating by modulating firing at twice the reversal frequency (nonlinear response). We investigated this nonlinear mechanism by presenting visual stimuli to the intact guinea pig retina in vitro while recording intracellularly from large brisk and sluggish ganglion cells. A contrast-reversing grating modulated the membrane potential (in addition to the firing rate) at twice the reversal frequency. This response was initially hyperpolarizing for some cells (either ON or OFF center) and initially depolarizing for others. Experiments in which responses to bars were summed in-phase or out-of-phase suggested that the single class of bipolar cells (either ON or OFF) that drives the center/surround response also drives the nonlinear response. Consistent with this, nonlinear responses persisted in OFF ganglion cells when ON bipolar cell responses were blocked by L-AP-4. Nonlinear responses evoked from millimeters beyond the ganglion cell were eliminated by tetrodotoxin. Thus, to relay the response from distant regions of the receptive field requires a spiking interneuron. Nonlinear responses from different regions of the receptive field added linearly.

Growth-cone attraction to netrin-1 is converted to repulsion by laminin-1

Growing axons are guided by both diffusible and substrate-bound factors. Growth cones of retinal neurons exhibit chemoattractive turning towards the diffusible factor netrin-1 in vitro and are guided into the optic nerve head (ONH) by localized netrin-1. Here we report that, in Xenopus, laminin-1 from the extracellular matrix (ECM), converts netrin-mediated attraction into repulsion. A soluble peptide fragment of laminin-1 (YIGSR) mimics this laminin-induced conversion. Low levels of cyclic AMP in growth cones also lead to the conversion of netrin-induced attraction into repulsion, and we show that the amount of cAMP decreases in the presence of laminin-1 or YIGSR, suggesting a possible mechanism for laminin's effect. At the netrin-1-rich ONH, where axons turn sharply to leave the eye, laminin-1 is confined to the retinal surface. Repulsion from the region in which laminin and netrin are coexpressed may help to drive axons into the region where only netrin is present, providing a mechanism for their escape from the retinal surface. Consistent with this idea, YIGSR peptides applied to the developing retina cause axons to be misdirected at the ONH. These findings indicate that ECM molecules not only promote axon outgrowth, but also modify the behaviour of growth cones in response to diffusible guidance cues.

Effect of topical dorzolamide on tissue circulation in the rabbit optic nerve head

PURPOSE

To examine the effect of 1% topical dorzolamide on tissue circulation in the optic nerve head (ONH) of Dutch rabbits.

METHODS

A laser speckle tissue circulation analyzer was used. One eye of each rabbit received 1% topical dorzolamide twice daily for 20 days, and the fellow eye received the vehicle in a masked, randomized manner. Intraocular pressure (IOP) was measured every 5 days. The normalized blur (NB) value, a quantitative index of tissue blood flow velocity in the ONH, was measured before treatment and 2 hours after the last instillation on the 20th day.

RESULTS

The IOP was lowered by about 2 mm Hg only in the dorzolamide-treated eyes (P < .01). The NB value showed no significant change in either dorzolamide-treated or vehicle-treated eyes.

CONCLUSIONS

Long-term topical dorzolamide does not affect the ONH tissue circulation in dorzolamide- and vehicle-treated eyes of Dutch rabbits.

Circadian control of photoreceptor outer segment membrane turnover in mice genetically incapable of melatonin synthesis

Vertebrate retinal photoreceptors periodically shed membrane from their outer segment distal tips; this material is phagocytosed and degraded by the retinal pigmented epithelium. Both a circadian oscillator and the daily light-dark cycle affect disk shedding, and the effects of both may be mediated by melatonin. To clarify melatonin's role in this process, we asked whether endogenous melatonin is required for rhythmic disk shedding in mouse retina. We analyzed disk shedding in two mouse strains: C3H, which produce melatonin in retina and pineal under the control of circadian oscillators, and C57BL/6, which do not produce melatonin. In cyclic light, both strains exhibited a robust cycle of disk phagosome content in the pigmented epithelium. Peak shedding occurred just after dawn, and trough levels occurred during the middle of the dark phase. In constant darkness, mice exhibited circadian rhythms of locomotor activity, the characteristics of which were similar between strains. Both strains also exhibited rhythmic disk shedding in constant darkness, although amplitudes of the rhythms were damped. Exogenous melatonin delivered once per day failed to reestablish high-amplitude cyclic shedding in mice held in constant darkness. Our results show that, while disk shedding in cyclic light is robustly rhythmic, neither rhythmic production of melatonin nor the circadian oscillator responsible for rhythmic locomotor activity is sufficient to drive high-amplitude rhythmic shedding in constant darkness. More importantly, melatonin is required neither for cyclic changes in the rate of disk shedding in cyclic light, nor for the circadian rhythm of disk shedding in constant darkness.

Physiology of perfusion as it relates to the optic nerve head

Blood flow in the optic nerve has been demonstrated to be autoregulated, and, thus, within certain limits, to be independent of the local perfusion pressure. As in the brain, a close coupling of neuronal activity and optic nerve head blood flow has been demonstrated. A number of regulatory systems and factors participate in the regulation of vascular tone in various organs, including the optic nerve. Metabolic and myogenic mechanisms keep local perfusion constant or adapted to the local metabolic needs. Such mechanisms seem to be involved in the regulation of optic nerve blood flow as well. In contrast, neuronal blood flow regulation is of minor importance in the optic nerve. Many of the regulatory modalities induce a response of vascular smooth muscle cells through stimulation of factors produced by the endothelial cell layer. Indeed, endothelial factors are of utmost importance in the regulation of optic nerve blood flow. The facts that there is a basal formation of nitric oxide, which leads to an active dilation of the ocular vasculature, and that endothelin-1 decreases blood flow to the anterior optic nerve in a dose-dependent manner suggest that alterations in these regulatory mechanisms might be relevant for optic nerve blood flow alterations as they relate to glaucomatous optic neuropathy. It is hoped that a detailed knowledge of blood flow regulation in the optic nerve might initiate new treatment modalities in optic neuropathies that are hemodynamic and vascular in nature.

A biomathematical model for pressure-dependent lamina cribrosa behavior

Investigating the relationship between intraocular pressure and the behavior of the lamina cribrosa (the primary site of the optic nerve damage in glaucoma) is important to insight into the pathogenesis of glaucomatous optic neuropathy. In most previous studies, unsuitable approaches were used since the lamina cribrosa was not taken as the main target. In the present study, a linear model of elastic mechanics theory on the bending of thin circular plate was developed for this purpose. The structural features of the lamina cribrosa and the forces acting on the lamina cribrosa were analyzed, and the constitutive equation was formulated. The general solution on a class of Kármán Equation and the analytic solution on fixed boundary conditions were obtained, and from them, the morphological changes and the mechanical properties such as retrodisplacement and force distributions of the lamina cribrosa under pressure were derived. Some of the clinical phenomena occurring in glaucoma damage were explained with the results. Theoretical values were compared with the experimental data obtained by other investigators. The effects of structural parameters on susceptibilities to glaucoma damage were discussed. The biomathematical model, serving as formalistic expressions of the well-known hypothesis of pressure-dependent optic nerve damage in glaucoma, should make it possible for us to further understand and manage this disease.

Long-term fluctuations of the normalised rim/disc area ratio quotient in normal eyes

BACKGROUND

The assessment of the cup of the optic disc in follow-up of glaucomatous optic nerve heads depends on the variability of the cup area over time. We examined the variability of topographic measurements depending on scan focus settings and evaluated the long-term fluctuations of the normalised rim/disc area ratio quotient of normal subjects for 1 year.

METHODS

Part 1. Evaluation of the influence of scan focus and corneal curvature on the number of pixels per millimetre in one emmetropic volunteer. The scan focus was varied using contact lenses of different refraction. Part 2. HRT examinations of the optic nerve head of five eyes of five volunteers were repeatedly recorded during a 12-month period. The contour line from the first mean topographic image was transferred into the following seven mean topographic images of each subject.

RESULTS

Part 1. The size of pixels depends on the adjusted scan focus for mean corneal curvature radius between 6 and 9 mm. However, after image to image scaling the size of pixels is nearly independent to the scan focus. Part 2. We found the highest variation of normalised rim/disc area ratio for the location of the temporal horizontal sectors, followed by the temporal inferior and the temporal superior sectors. Variation was smallest in the nasal sectors. Mean fluctuation of the normalised rim/disc area ratio quotient of the 95th percentile was 0.09 following image to image scaling compared to 0.16 when using individual image scaling.

CONCLUSION

Scan focus variation during HRT follow-up examinations can influence optic disc parameters, especially when image to image scaling has not been performed. A confidence area of an individual optic disc could now be calculated. These confidence bands could help to distinguish more easily between progress of glaucomatous optic nerve head damage and physiological intertopographic variation of optic cup measurements. A decrease of more than 43% in the fluctuations of the normalised rim/disc area ratio could be achieved using image to image scaling techniques. For that reason follow-up of glaucomatous optic nerve heads should only be performed using this software application.

The optic nerve head component of the human ERG

The local responses of the multifocal ERG reveal continuous changes in the second order waveforms from the nasal to the temporal retina. Scrutiny of these changes suggests the presence of an additive component whose latency increases with the distance of the stimulus from the optic nerve head. This observation led to the hypothesis of a contributing source in the vicinity of the optic nerve head whose signal is delayed in proportion to the fiber length from the stimulated retinal patch to the nerve head. The hypothesis was tested with two independent methods. In Method 1, a set of different local response waveforms was approximated by two fixed components whose relative latency was allowed to vary and the fit of this two component model was evaluated. In Method 2, two signals were derived simultaneously using different placements for the reference electrode. The placements were selected to produce a different ratio of the signal contributions from the retina and the nerve head in the two recording channels. The signals were then combined at a ratio that canceled the retinal component. Method 1 yielded an excellent fit of the two component model. Waveforms and latencies of the hypothetical optic nerve head component derived from the two methods agree well with each other. The local latencies also agree with the propagation delays measured in the nerve fiber layer of the monkey retina. In combination, these findings provide strong evidence for a signal source near the optic nerve head.

Retinal ganglion cell axon progression from the optic chiasm to initiate optic tract development requires cell autonomous function of GAP-43

Pathfinding mechanisms underlying retinal ganglion cell (RGC) axon growth from the optic chiasm into the optic tract are unknown. Previous work has shown that mouse embryos deficient in GAP-43 have an enlarged optic chiasm within which RGC axons were reportedly stalled. Here we have found that the enlarged chiasm of GAP-43 null mouse embryos appears subsequent to a failure of the earliest RGC axons to progress laterally through the chiasm-tract transition zone to form the optic tract. Previous work has shown that ventral diencephalon CD44/stage-specific embryonic antigen (SSEA) neurons provide guidance information for RGC axons during chiasm formation. Here we found that in the chiasm-tract transition zone, axons of CD44/SSEA neurons precede RGC axons into the lateral diencephalic wall and like RGC axons also express GAP-43. However unlike RGC axons, CD44/SSEA axon trajectories are unaffected in GAP-43 null embryos, indicating that GAP-43-dependent guidance at this site is RGC axon specific or occurs only at specific developmental times. To determine whether the phenotype results from loss of GAP-43 in RGCs or in diencephalon components such as CD44/SSEA axons, wild-type, heterozygous, or homozygous GAP-43 null donor retinal tissues were grafted onto host diencephalons of all three genotypes, and graft axon growth into the optic tract region was assessed. Results show that optic tract development requires cell autonomous GAP-43 function in RGC axons and not in cellular elements of the ventral diencephalon or transition zone.

The correlation between cerebrospinal fluid pressure and retrolaminar tissue pressure

PURPOSE

To measure the effects of cerebrospinal fluid pressure (CSFp) on retrolaminar tissue pressure (RLTp) and the translaminar pressure gradient (TLPG), particularly at low CSFp, which is the normal situation in erect posture.

METHODS

Micropipettes coupled to a servonull pressure system were passed into eyes of anesthetized dogs to the optic disc and advanced in steps through the lamina cribrosa to the optic nerve subarachnoid space (ONSAS), while pressure measurements were taken. Cerebrospinal fluid pressure and intraocular pressure (IOP) were monitored and controlled. The TLPG was measured at varying IOPs and CSFps. The RLTp and ONSAS pressure (ONSASp) were measured at varying CSFps. In separate experiments, the optic nerve dura was incised, and pressure measurements were taken across the pia mater.

RESULTS

The TLPG was strongly correlated to the difference between IOP and CSFp (r=0.93; n=18) when CSFp was more than zero. Mean RLTp was 3.7+/-0.2 mm Hg (SEM; n=15) when CSFp was 0 mm Hg. The ONSASp and RLTp were largely dependent on the presence of CSFp higher than break point pressures of -0.5 mm Hg and 1.33 mm Hg, respectively. However, below these break points, RLTp (slope 0.07) and ONSASp (slope 0.18) were little influenced by CSFp. Separate measurements across the pia mater revealed that 95% of the pressure drop occurred within 100 microm of the pial surface.

CONCLUSIONS

The TLPG and RLTp are dependent on CSFp when CSFp is more than -0.5 mm Hg. Below this level, there is no hydrostatic continuity between the intracranial and optic nerve subarachnoid space. In this range, RLTp is stable and is little influenced by CSFp changes.

Neurolin, the goldfish homolog of DM-GRASP, is involved in retinal axon pathfinding to the optic disk

Young axons of new retinal ganglion cells (RGCs) in the continuously growing goldfish retina fasciculate with one another and their immediate forerunners on their path toward the optic disk and along the optic nerve. They express the immunoglobulin superfamily cell adhesion molecules (CAMs) neurolin (DM-GRASP) and the L1-like E587 antigen. Repeated injections of Fab fragments from polyclonal antisera against neurolin (neurolin Fabs) into the eye of 3. 4-cm-long and rapidly growing goldfish caused highly aberrant pathways of young RGC axon subfascicles in the dorsal retina. Many axons grew in circles and failed to reach the optic disk. In contrast, E587 Fabs, used in parallel experiments, disrupted the fascicles but did not interfere with the disk-directed growth. Neurolin Fabs also disturbed axonal fasciculation in vivo as well as in vitro but less severely than E587 Fabs. Coinjections of both Fabs increased defasciculation of the dorsal axons in both aberrant and disk-directed routes. They also disrupted the order of young RGC axons in the optic nerve more severely than E587 Fabs alone. This demonstrates that the development of tight and orderly fascicles in the dorsal retina and in the optic nerve requires both E587 antigen and neurolin. More importantly, our results suggest an involvement of neurolin in RGC axonal guidance from the retinal periphery to the optic disk. Because disrupted fascicles and errant axon routes were found only in the dorsal retinal half, a cooperation with so-called positional markers may be conceived.

Texture brightness filling-in

The masking procedure by Paradiso and Nakayama (1991) (Vision Research, 31, 1221-1236) was used to investigate brightness filling-in within textures made of line elements: a texture stimulus was masked by a second stimulus containing a square contour. When a uniform texture was presented, the texture region inside the masking square appeared darkened and a small number of texture elements were perceived with a degenerated shape, appearing as dim dots or shorter line elements; it is as if the line element expanded from a bright point to fill the entire region defined by its contour. If the texture stimulus was a texture patch segregating from the surrounding texture by an orientation gradient and this patch was inside the square mask, darkening was not as strong as in the previous condition, and masked line elements preserved their elongated shape. Brightness spreading was measured in two experiments using dichoptic presentations. Experiment 1 used an adjustment task and showed that the brightness of texture line elements spread from equiluminant borders between segregating textures. Experiment 2 used a matching task and demonstrated that spreading was blocked by segregation borders dependent on the orientation gradient between texture line elements. The selectivity for line orientation began 40-80 msec after texture onset and maximal spreading occurred at approximately 120 msec. These findings may indicate that two processes subserve filling-in within textures: the first spreads isotropically the mean stimulus luminance at an initial processing stage of image analysis; at a later stage, the second spreads a texture flow (both brightness and shape of line elements) directed along the orientation of texture line elements. The texture flow mechanism fills in with a texture surface the region bounded by segregation contours.

Functional analysis of primary visual cortex (V1) in humans

Human area V1 offers an excellent opportunity to study, using functional MRI, a range of properties in a specific cortical visual area, whose borders are defined objectively and convergently by retinotopic criteria. The retinotopy in V1 (also known as primary visual cortex, striate cortex, or Brodmann's area 17) was defined in each subject by using both stationary and phase-encoded polar coordinate stimuli. Data from V1 and neighboring retinotopic areas were displayed on flattened cortical maps. In additional tests we revealed the paired cortical representations of the monocular "blind spot." We also activated area V1 preferentially (relative to other extrastriate areas) by presenting radial gratings alternating between 6% and 100% contrast. Finally, we showed evidence for orientation selectivity in V1 by measuring transient functional MRI increases produced at the change in response to gratings of differing orientations. By systematically varying the orientations presented, we were able to measure the bandwidth of the orientation "transients" (45 degrees).

Vasomotion and spontaneous low-frequency oscillations in blood flow and nitric oxide in cat optic nerve head

The purpose of this study was to determine whether spontaneous oscillations in blood flow (relative red blood cell flux) measured by laser Doppler flowmetry (LDF) in the cat optic nerve head were related to fluctuations in nitric oxide (NO) measured with electrochemical sensors (n = 16 cats). Power spectral densities for the magnitude and frequency of LDF and NO fluctuations were determined by discrete Fourier transform analysis. Complex behavior was found for both LDF and NO oscillations with broad spectra containing peaks at multiple frequencies. Most of the power was in the low-frequency range (<10 cycles/min). Spectra were also obtained after administering NO synthase inhibitors (l-nitroarginine, L-NA, n = 6 cats; l-nitroarginine methyl ester, L-NAME, n = 5 cats). Both inhibitors caused a decrease in blood flow, basal NO levels, and amplitude of NO fluctuations. There was little change in amplitude for blood flow oscillations, with some enhancement at the lowest frequencies. We conclude that NO is not required for vasomotion and that spontaneous, low-frequency NO fluctuations observed in the cat optic nerve head are a passive phenomenon caused by natural variations in shear stresses.

Distribution and regulation of the optic nerve head tissue PO2

We investigated the distribution and regulation of the optic nerve head (ONH) tissue partial pressure of oxygen (PO2) under various stimuli and the role of the nitric oxide in the ONH circulation. Tissue PO2 was measured using double-barreled recess microelectrodes in the intact eyes of miniature pigs during normoxia, hyperoxia, hypoxia, variations of systemic blood pressure, and after inhibition of the endothelial nitric oxide synthesis by the administration of nitro-L-arginine. Measurements were performed in front of the ONH at intervascular and juxta-arteriolar areas and at a depth of 50 and 200 microm within the ONH at the center and the rim. During normoxia, PO2 was heterogeneously distributed in the ONH, higher close to the arterioles than in intervascular areas. Hyperoxia induced a significant increase of juxta-arteriolar tissue PO2, while in intervascular areas no change was noticed. Hypoxia did not modify intervascular tissue PO2 at 200 microm depth within the ONH. Variations of the systemic blood pressure did not induce any significant change in ONH tissue PO2. Similarly, no modification was noticed after the administration of nitro-L-arginine. There is a remarkable autoregulatory capacity of the ONH circulation that may compensate for parameters such as hyperoxia, hypoxia, and variations of the systemic blood pressure. Endothelially derived nitric oxide inhibition does not modify the ONH tissue PO2, probably because the tissue PO2 is stabilized by compensatory regulation.

Changes in brain function after manipulation of the cervical spine

OBJECTIVE

To ascertain whether manipulation of the cervical spine is associated with changes in brain function.

DESIGN

Physiological cortical maps were used as an integer of brain activity before and after manipulation of the cervical spine in a large (500 subjects), double-blind controlled study.

SETTING

Institutional clinic Participants: Adult volunteers.

INTERVENTION

Five hundred subjects were divided into six comparative groups and underwent specific manipulation of the second cervical motion segment. Blinded examiners obtained reproducible pre- and postmanipulative cortical maps, which were subjected to statistical analysis.

MAIN OUTCOME MEASURES

Brain activity was demonstrated by reproducible circumferential measurements of cortical hemispheric blind-spot maps before and after manipulation of the second cervical motion segment. Twelve null hypotheses were developed. The critical alpha level was adjusted in accordance with Bonferroni's theorem to .004 (.05 divided by 12) to reduce the likelihood of wrongly rejecting the null hypothesis (i.e., committing a Type I error).

RESULTS

Manipulation of the cervical spine on the side of an enlarged cortical map is associated with increased contralateral cortical activity with strong statistical significance (p < .001). Manipulation of the cervical spine on the side opposite an enlarged cortical map is associated with decreased cortical activity with strong statistical significance (p < .001). Manipulation of the cervical spine was specific for changes in only one cortical hemisphere with strong statistical significance (p < .001).

CONCLUSIONS

Accurate reproducible maps of cortical responses can be used to measure the neurological consequences of spinal joint manipulation. Cervical manipulation activates specific neurological pathways. Manipulation of the cervical spine may be associated with an increase or a decrease in brain function depending upon the side of the manipulation and the cortical hemisphericity of a patient.

The influence of age and intraocular pressure on the optic cup in a normal population

PURPOSE

To determine the effects of age and intraocular pressure (IOP) on optic cup and neural rim size, and cup-disc ratio in a well defined population free from apparent glaucoma and other optic nerve disease.

METHODS

Data were collected on 3654 people, 49 years or older, living in the Blue Mountains, west of Sydney. Examination included subjective refraction and Zeiss colour stereo (Carl Zeiss, Oberkochen, Germany) optic disc photographs. Magnification effects of the eye and camera were corrected. After excluding subjects with optic nerve diseases, data from 6579 normal phakic eyes of 3358 subject (91.9% of those examined) were used.

RESULTS

Adjusted for optic disc size and IOP, cup diameter increased 0.01 mm, cup-disc ratio increased 0.01, and neural rim width decreased 0.01 mm for every decade of age increase. Adjusted for age and optic disc size, cup diameter increased 0.01 mm, cup-disc ratio increased 0.04, and neural rim width decreased 0.07 mm for every 10 mmHg increase in IOP. The IOP-related increase in cup-disc ratio amounted to 9.5% of the mean per 10 mmHg, while the age related increase was 1.9% of the mean.

CONCLUSIONS

These data support the hypothesis that there is an age-related loss of tissue from the neuroretinal rim. However the mean change between the ages of 50 and 90 years is very small. The association between increasing IOP and smaller neural rim width could suggest a causal relationship. However, it is also plausible that IOP and optic cup size are both determined by other unmeasured factors.

Confocal scanning laser Doppler flowmetry: experiments in a model flow system

PURPOSE

We conducted this study toward validating confocal scanning laser Doppler flowmetry (SLDF), a new noninvasive technique for measuring retinal and optic nerve head hemodynamics.

METHODS

We designed a model flow system using a glass capillary coupled to a microlitre syringe driven by an infusion pump. Eleven capillaries with parallel walls (internal diameters ranging 705 to 25 microm) were used. The capillaries were perfused with skim milk over a range of pump flow rates. At each flow rate, measurements were made with the Heidelberg Retina Flowmeter (HRF) to study the relationship between HRF-measured flow and actual flow. The initial experiments (n = 2) were conducted to establish the approximate velocity operating range of the HRF with single HRF measurements across a wide range of flow rates, whereas the subsequent experiments (n = 9) were concentrated within this operating range with five HRF measurements at each flow rate.

RESULTS

When pump flow rates were converted to actual velocity at the measurement point for the initial experiments, the velocity operating range of the HRF was approximately 0.08 to 1.0 mm/s. For velocities of >1 mm/s, HRF measured velocity was not linearly related to actual velocity. Within the operating range, there was a highly significant linear relationship between HRF-measured flow and actual flow (0.935 < or = r < or = 0.990, p < 0.001). When the curves of HRF-measured velocity versus actual velocity for the different experiments were plotted, they largely superimposed. The variability of the HRF measurements was between 3.57% and 4.05% and was independent of flow rate.

CONCLUSIONS

SLDF measures reliably and linearly within a given operating range.

Mr. Chips: an ideal-observer model of reading

The integration of visual, lexical, and oculomotor information is a critical part of reading. Mr. Chips is an ideal-observer model that combines these sources of information optimally to read simple texts in the minimum number of saccades. In the model, the concept of the visual span (the number of letters that can be identified in a single fixation) plays a key, unifying role. The behavior of the model provides a computational framework for reexamining the literature on human reading saccades. Emergent properties of the model, such as regressive saccades and an optimal-viewing position, suggest new interpretations of human behavior. Because Mr. Chip's "retina" can have any (one-dimensional) arrangement of high-resolution regions and scotomas, the model can simulate common visual disorders. Surprising saccade strategies are linked to the pattern of scotomas. For example, Mr. Chips sometimes plans a saccade that places a decisive letter in a scotoma. This article provides the first quantitative model of the effects of scotomas on reading.

Changes in the optic disc over a five-year interval: the Beaver Dam Eye Study

PURPOSE

To determine the relationship of change in vertical optic disc cupping to change in intraocular pressure over a five-year interval.

METHODS

Non-simultaneous stereoscopic photographs were taken of optic discs of participants in the baseline and follow-up examinations of The Beaver Dam Eye Study cohort. Optic discs and cups were measured and other disc features were graded according to a standard protocol by trained graders. Intraocular pressures were measured by Goldmann applanation tonometry.

RESULTS

Change in pressure was significantly associated with change in vertical cup-to-disc ratio. Incident disc hemorrhage, flattened temporal rim, notching, cup reaching disc margin, and undercutting were not significantly associated with change in intraocular pressure.

CONCLUSION

Change in intraocular pressure in this adult population was associated with increased optic disc cupping. This finding, if confirmed, would lend support to the practice of periodic follow-up of older adults who have shown changes in their intraocular pressure.

[Does the fundus perimetry determined edge of the blind spot depend on the superficial form of the papilla?]

BACKGROUND

We wanted to find out whether the borders of the blind spot depend on the surface topography of the optic disc and its surrounding area.

PATIENTS AND METHODS

We therefore examined ten eyes with parapapillary atrophy adjacent to the temporal side of the disc. Fundus perimetry was performed under direct fundus control using a Rodenstock scanning laser ophthalmoscope. We examined the horizontal meridian of the optic discs in 0.5 degree steps using Goldmann IV-stimuli with 10 different degrees of brightness and the Goldmann stimulus 1, 0 dB (greatest luminance). Six eyes with symmetric, "normal" excavation served as controls. Optic disc topography was measured with the Heidelberg Retina Tomograph (HRT).

RESULTS

Stimuli with a large luminance power (Goldmann IV, 4 dB) were seen up to 0.8 degree centrally (i.e., towards the optic disc center) from the temporal edge of the parapapillary atrophy, but up to 1.9 degrees centrally from the nasal optic disc border (P < 0.01). Horizontal HRT section profiles of the optic disc consistently showed prominent nasal disc borders contrasting with a shallow excavation within the temporal parapapillary atrophy. In all six subjects with a normally shaped disc there was no such "nasotemporal asymmetry."

CONCLUSIONS

The size of scotomas depends on the surface topography of the tested area. The prominent nasal part of the optic disc appears less "blind" than the shallow temporal part, probably due to more intensive light scattering by the prominent nasal part of the disc.

Optical density of the human lens

Optic disk reflectance was measured from 27 normal observers with their physiological lenses (aged 21-74 yr) and from two pseudophakic observers (aged 69 and 70 yr) with use of a Utrecht fundus reflection densitometer. Psychophysical heterochromatic flicker photometric luminance matches (10 degrees field) were obtained on the same group of the observers. A four-parameter model incorporating lens density, hemoglobin absorption, optic disk reflectance, and superficial stray light was used to fit the reflectometric data. A model incorporating lens density and the Judd revised spectral luminous-efficiency function was used to fit the psychophysical data. The lens-density spectrum used the two-factor aging model of Pokorny, et al. [Appl. Opt. 26, 1437 (1987)]. The lens density for each normal observer was estimated through a least-squares fitting procedure yielding an estimated lens age. For the reflectometric data the observer's chronological age agreed with estimated lens age with a correlation coefficient of 0.92. The reflectometric regression line underestimated chronological age by approximately 5 yr. The mean reflectance of the optic disk was 0.047 with standard error of the mean of 0.0044. Data from the pseudophakic observers were well described when corneal density was used to replace lens density. The lens density was also estimated from the psychophysical data. The observer's chronological age agreed with psychophysically estimated lens age with a correlation coefficient of 0.92. It was concluded that the in vivo lens density can be estimated from the reflectance spectrum measured off the optic disk. The reflectance spectrum of the optic disk was inferred to be close to spectrally neutral.

Surface representation in the visual system

Perception of surface accompanies the impression that a certain area of the visual field is occupied by some quality, such as color, brightness and transparency. This does not mean, however, that information about surface quality must be obtained throughout the area. It has been shown in many situations that our visual system has ability to interpolate information obtained at the border of the surface and to perceive homogeneous surfaces. The most dramatic demonstration of this is the perceptual filling-in at the blind spot. In order to understand the neural representation of surface in the visual system, we conducted a series of experiments using macaque monkeys. First, we examined if neurons in the primary visual cortex (V1) respond when a homogeneous surface is presented on the receptive field. Neurons representing the parafoveal visual field were tested and it was found that about one third of neurons showed significant responses when the cell's receptive field was contained in a homogeneous surface. Then we examined neuron activities in the retinotopic representation of the blind spot in V1. Although there is no retinal input in the blind spot, a homogeneous surface is perceived within the blind spot as a result of filling-in. We tested whether neurons in this region were activated when a homogeneous surface was perceived in the blind spot as a result of filling-in. We found some neurons in V1 were activated by stimuli which lead to the filling-in. These results indicate that when a surface area is perceived, neurons are activated throughout the region in V1 topographically corresponding to the perceived surface and not restricted to the region representing the border of the surface.

Potential treatment modalities for glaucomatous neuropathy: neuroprotection and neuroregeneration

PURPOSE

This article presents the rationale and an experimental strategy for the development of new treatment modalities for glaucomatous neuropathy. Accumulating evidence suggests that regardless of the primary trigger of the retinal and optic nerve damage in glaucoma, the disease will continue to progress even when the cause is removed. The resulting damage can be mimicked by the progression of damage secondary to an acute partial crush injury at the optic nerve head. Such secondary damage includes degeneration of the directly injured optic nerve fibers culminating in death of their cell bodies, as well as degeneration of nerve fibers that escaped acute injury but nevertheless deteriorate as a result of their exposure to injury-induced mediators of secondary degeneration released by the directly affected neurons.

CONCLUSION

We therefore propose that substances found to be effective in rescuing fibers from secondary degeneration and in increasing the survival rate or prolonging survival of retinal ganglion cells in the partially lesioned optic nerve may be useful for the treatment of glaucoma. The new approach does not replace hypotensive therapy, but addresses the glaucoma-induced damage by promoting nerve protection and neuroregeneration.

Determination of the real size of fundus objects from fundus photographs

PURPOSE

The purpose of this research was to study the accuracy of measurements that rely on Littmann curves to determine the real size of fundus images.

METHODS

Retinal tacks of known dimensions were placed near the optic disc in one eye of each of two cynomolgus monkeys. Images of the retinal tack and optic nerve head of these two eyes were then obtained with a Zeiss Fundus camera. The dimensions of the retinal tacks and optic discs were measured directly on the film from the Zeiss fundus camera. These measurements were then corrected with Littmann's mathematical approximations.

RESULTS

In the cynomolgus eye with 19.9 mm axial length, Littmann's mathematical approximations of the real sizes of the retinal tack and optic disc from measurements on the Zeiss film had median percent bias [(calculated size-real size)/real size] of -0.2% (range, -3.2% to 3.9%). In the other eye with 17.9 mm axial length, the median percent bias was 14.7% (range, 5.1% to 29.7%).

CONCLUSION

The estimation of the real size of fundus images with Littmann's mathematical approximations from images on Zeiss film have < 4% bias if the eye's dimensions fall in the range over which Littmann's curves were produced.

Filling-in percepts produced by luminance modulation

We report that when the luminance of a homogeneous spot of light is gradually increased or decreased, there are conditions in which the brightness of the spot is spatially nonuniform. When the spot luminance is increased, brightness changes in the spot's center lag behind changes at the edge and brightness appears to sweep inward. Conversely, if the luminance of the spot is decreased, there is a relative lag in the darkening toward the center of the spot and darkness seems to spread inward. In Experiment 1 we found that with both increasing and decreasing luminance sweeps, the strength of the brightness filling effects was strongest with luminance sweep durations of 0.25-0.5 sec. In Experiment 2, the sweep duration was held constant at 0.5 sec; the filling effect was seen when the dwell time spent at each luminance step was less than about 100 msec, but nonuniformities were not observed at longer dwell times. In Experiment 3, a spot of light was positioned to surround the optic disk in one eye. Surprisingly, when the spot was luminance modulated from bright to dark, darkness appeared to sweep from the edge to the center of the modulated disk, even though most of the disk's interior was imaged on a portion of the retina devoid of photoreceptors. These findings are consistent with the hypothesis that a neural filling-in mechanism in visual cortex plays a key role in brightness perception.

[Fundus-oriented perimetry. Evaluation of a new visual field examination method for detecting angioscotoma]

BACKGROUND

Conventional automated perimeters usually work with a given set of grids and thus are normally not adapted to individual conditions. This fact restricts efficiency of this method not only for any single examination but also for follow-up studies.

MATERIALS AND METHODS

A new method (patent pending) is introduced which superimposes an individual perimetric grid--corrected in respect to orientation, position and size--onto a patient's fundus image. A recently developed software realizes this procedure in a comfortable manner: the digitized fundus image is loaded into the computer by e.g. photo-CD or disc, depicted on a control monitor and mirrored if necessary. Assuming a central fixation, the foveola is translationally shifted to the center of the perimetric grid by the help of a crosshair. The blind spot which has been previously determined with kinetic perimetry is then superimposed onto the optic disc of the fundus image using a rotation and zoom function. In this way, it is possible to adapt the perimetric grid directly to the underlying individual fundus findings: thus, stimuli can be spatially concentrated or more frequently tested in special regions of interest. Additionally, test points can be dragged away from delicate positions to avoid artifacts. Examinations were carried out on a high resolution colour VDU of the Tübingen Electronic Campimeter (TCC). Alternatively, suited bowl perimeters can be used.

RESULTS

In order to test the precision of the superimposing procedure, fundus oriented perimetry was performed to detect angioscotomata in 13 ophthalmologically normal subjects. Using dark stimuli (12'), visual field defects in the expected region, caused by retinal vessels, could be detected in 7 cases (= 54%). The resulting attenuation of differential light sensitivity in this circumscribed region was up to 12 dB.

CONCLUSIONS

By means of a morphologically adapted, individual arrangement of stimulus locations fundus-oriented perimetry enables detection of even minute (angio-) scotomata.

Two-dot alignment across the physiological blind spot

Three competing hypotheses have been proposed for the cortical representation of the blind spot. These are: (i) the regions surrounding the blind spot maintain their spatial values; (ii) the opposite sides of the blind spot are represented adjacently at the cortex, so that the blind spot is "sewn-up"; and (iii) the blind spot is sewn-up with compensation occurring in the immediate surround of the blind spot, so that spatial values are distorted only in the immediate surround of the blind spot. To distinguish between these hypotheses we used a two-dot alignment task, with the two dots straddling the blind spot at varying dot separations. Thresholds in the two-dot alignment task are limited by the cortical separation of the two dots. When thresholds for alignment across the blind spot are compared with thresholds over intact retina at the same eccentricity, the three hypotheses predict: (i) no change in thresholds; (ii) a lowering of thresholds; and (iii) a lowering of thresholds but only at separations slightly greater than the diameter of the blind spot. Thresholds across the blind spot were closely similar to thresholds across intact retina. The results do not support a sewing-up (with or without compensation) of the blind spot. Rather, our results are consistent with a preservation of spatial values around the blind spot.

Visuotopic reorganization in the primary visual cortex of adult cats following monocular and binocular retinal lesions

The effect of discrete monocular retinal lesions on the representation of the visual field in the primary visual area (V1) was investigated in adult cats. Lesions were created using argon lasers, 8 d to 4(1/2) months prior to electrophysiological recording. This produced lesion projection zones (LPZs) in V1, 1.6-9.5 mm wide, that were deprived of their normal input from one eye, but that received a normal input from the other eye. Nevertheless, at the majority of recording sites within these zones neuronal responses were elicited by stimulation of the lesioned eye, with receptive fields being displaced onto regions of retina surrounding the lesion, while receptive fields determined through stimulation of the normal eye followed the normal visuotopic organization of V1. However, neuronal responses to stimulation of the lesioned eye within the LPZs were characterized by rapid habituation and unusually low firing rates in comparison with responses to stimulation of the normal eye. Stimulation of the normal eye temporarily marked the responsiveness of neurons within the LPZ to stimulation of the lesioned eye. The proportion of neurons responsive to stimulation of the lesioned eye was higher just inside the borders of the LPZs than at the centers of these zones. However, neurons responsive to stimulation of the test eye were found up to 3.6 mm from the perimeter of the LPZs, and therefore the shifts in the visuotopic map caused by retinal lesions cannot be explained solely on the basis of the normal scatter of receptive fields and point-image size in V1. The proportion of cells responsive to stimulation of the lesioned eye was highest in the infragranular layers, and lowest in the supragranular layers. By combining a restricted lesion of one eye with laser photocoagulation of the optic disc of the other eye, the effects of the normal eye on the lesion-induced visuotopic reorganization were also investigated. Neither chronic nor acute deactivation produced any discernible further changes in visuotopy or in the characteristics of neuronal responses to stimulation of the eye with the discrete lesions. Our findings show that the representations of the two eyes in adult visual cortex are capable of independent reorganization. These findings parallel those of work in auditory cortex, suggesting that topographic reorganization in primary sensory areas of adult cortex may be governed by similar mechanisms.

Principle, validity, and reliability of scanning laser Doppler flowmetry

PURPOSE

The objective of this study is to present the reliability and validity of scanning laser Doppler flowmetry (SLDF) performing a high-definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow.

METHODS

The examination of blood flow by SLDF is based on the optical Doppler effect. The data acquisition and evaluation system is a modified laser scanning device; the wavelength of the laser source is 670 mm, with a power of 100 microW (Heidelberg Engineering, HRF). The reliability of SLDF was estimated by performing five separate measurements in 10 eyes on 5 days. The validity of the method was tested by two experiments. First, in an experimental set-up, the capability of SLDF to measure the velocity of a moving plane in absolute units was estimated. Second, comparative measurements were performed of retinal blood flow in normal eyes and in 33 glaucomatous eyes with SLDF and a commercially available single-point laser Doppler flowmeter (Oculix).

RESULTS

We found SLDF to produce a high reliability. The reliability coefficients r1 of flow, volume, and velocity were 0.82, 0.81, and 0.83, respectively. Comparative measurements of the retinal blood flow by SLDF and a single-point laser Doppler flowmeter of corresponding retinal points showed a linear and significant relationship between flow (r = 0.83, p < 0.0001), volume (r = 0.51, p < 0.0001), and velocity (r = 0.59, p < 0.0001). In the experimental set-up, SLDF was able to quantitatively measure velocity in absolute units.

CONCLUSIONS

SLDF enables the visualization of perfused vessels of the juxtapapillary retina and the optic nerve head in high resolution by two-dimensional mapping of the optical Doppler shift and a reproducible evaluation of capillary blood flow.

Physiological influence on ocular photometry. A review

During the development of the ocular photometer (OPM) since 1983, we have considered several physical and physiological factors that could potentially influence the measurement and its results. Attention has been given to respiratory, circulatory, and intraocular pressure provocations, and numerous publications document the influences or lack thereof. Most recently, the authors conducted a simple reproducibility study with only 1 observer and 1 subject. Measurement of the same 2 retinal sites during a 3-week period yielded no statistically significant differences, even under relatively extreme temperature conditions. Thus, changes in readings with the OPM may be considered due to variations within the eye, such as cataract. Thus, increase in lens density may be directly and correctly expressed by dual-site measurement of retinal brightness, resulting in the contrast transfer ratio.

Visual scotoma and visual afterimages: some evidence that the perceived visual afterimage may not be a purely retinal phenomenon: a single case study

Observations of the differential appearance and behaviour of a deliberately induced visual scotoma and a patterned visual afterimage are reported. Although a retinal scotoma behaves like a visual afterimage in some ways, there are sufficient differences to suggest that the perceived visual afterimage may not be just the simple consequence of prolonged retinal stimulation.

Blindsight in normal observers

Some patients with lesions in visual cortex lack conscious visual experience but, when tested, exhibit a significant ability, termed 'blindsight', to discriminate visual stimuli. Here we report two different visual displays that induce blindsight in normal observers. Using an objective measure, we show that conscious experience remains defective at presentation times much longer (1 s) than the onset of visual sensitivity (approximately 60 ms). To obtain this effect, we generate a contrast between visual textures and then conceal the contrast by superimposing 'complementary' textures. Complementarity can involve either opposite motion or binocular rivalry and orthogonal orientation. In both cases, observers locate the texture contrast reliably but do not, by either subjective or objective measures, consciously experience it. Taken together with present knowledge of the visual cortical site(s) at which opposite motion and rivalrous orientation interact, this observation bears upon the functional anatomy of conscious visual experience.

Neural rim area declines with increased intraocular pressure in urban Americans

OBJECTIVE

To determine intraocular pressure (IOP)-related differences in the neural rim area among urban Americans without known optic nerve disease.

DESIGN

Population-based prevalence study conducted in 16 cluster areas in East Baltimore, Md.

PARTICIPANTS

A population-based sample of 1521 black and 1851 white individuals aged 40 years and older without evidence of optic nerve disease.

MAIN OUTCOME MEASURE

Intraocular pressure-related differences in neural rim area, neural rim area-to-disc area ratio, and cup-to-disc ratio.

RESULTS

We analyzed optic disc photographs using the Imagenet system (Topcon Instrument Corp of America, Paramus, NJ). After adjusting for age and disc area, white Americans had a 6% decrease in neural rim area for every 10-mm Hg increase in IOP (P = .0001). In black Americans, there was a quadratic relationship between neural rim area and IOP, with little decline with IOP up to approximately 17 mm Hg, after which neural rim area declined significantly with higher IOP (P = .001). Similarly, the neural rim area-to-disc area ratio decreased and the vertical cup-to-disc ratio increased with increasing IOP in both black and white Americans.

CONCLUSIONS

The higher the level of IOP, the smaller was the amount of neural rim tissue in the optic disc for both black and white Americans. However, the relationship between IOP and neural rim area was different in whites and blacks.

Motion aftereffect after monocular adaptation to filled-in motion at the blind spot

Although the blind spot encodes no visual information, one never perceives an odd blob or blank there, but sees a complete scene of the world even when viewing monocularly. This phenomenon called "filling-in" might be related to mechanisms essential to surface perception, but the neural representation has still been unclear. To determine at what stage the computation for filling-in is established in the visual system, whether prolonged observation of a filled-in motion including the blind spot of one eye could cause motion aftereffect at the corresponding visual field of the other eye was examined. The result was positive--interocular transfer of motion aftereffect was obtained at the tested eye. This finding suggests the possibility that real motion and filled-in motion share a common motion pathway in an early stage in the human visual system.

Perceived length across the physiological blind spot

Objects falling across the physiological blind spot appear "complete" despite the absence of photoreceptors. Completion of objects may occur across the blind spot because (1) the blind spot is filled in with the background (the associative explanation); (2) the opposite sides of the blind spot may be contiguously represented in the cortex (i.e. the blind spot is simply sewn up-the retinotopic explanation); or (3) the blind spot may be sewn up, with compensatory expansion occurring around the blind spot (the compensation explanation). These theories would predict no size distortions regardless of object size; constant size distortions regardless of object size; and distortions that depend on the size of the object, respectively. To evaluate these explanations, we measured size distortions at the blind spot. We measured length distortions at the blind spot using a criterion-free two-alternative forced-choice method with feedback. Observers compared the lengths of test bars presented across the blind spot with lengths of reference bars presented at the corresponding location in the fellow eye. Test bar lengths ranged from 7-14 deg. Reference bar lengths were in the range of +/- 3 deg of test bar length. From the observers' responses the perceived length of each bar at the blind spot was estimated. Estimates of the precision of length discrimination at the blind spot were also obtained. Our results were consistent with the associative explanation. In all seven observers, length distortions at the blind spot were smaller than 1 deg (< 20% of the vertical height of the blind spot) for all bar lengths tested. For bars that were presented across the blind spot, the precision with which observers could discriminate length was comparable to that of normal periphery (Weber fraction approximately 20%). Both the veridicality and precision of perceived length are preserved around the blind spot.

Topographical reproducibility of small scotoma using computerised dynamic fixation target

The computer assisted dynamic fixation technique uses the patient's eye movements to locate the test stimulus in the central visual field in relation to a randomly meandering fixation target. The patient looks at the moving fixation target on a high resolution monitor and tries to keep it inside a circle using a joystick. The stimuli are presented at predetermined locations in a seemingly random manner and the awareness of the stimuli is registered by the patient pressing the joystick button. That novel fixation maintenance technique has been combined with suprathreshold static light offset (dark-on-bright-light decrement) stimuli created on a cathode ray tube and used in mapping of the physiological blind spot in 10 healthy eyes. Each eye was examined twice with the same test program in order to document the repeatability of the results. On two consective tests, the physiological blind spot measured an average of 5.7 degrees horizontally and 6.4 degrees vertically, using 15% (17 dB) contrast, 16 mm2, single intensity offset stimuli. The moving eye method and the light offset stimuli, on repeated testing, yielded an average topographical reproducibility rate of 73% in mapping of the scotomas, with an average of 1.4 degrees and 0.6 degree variability in the vertical and horizontal dimensions of the blind spot respectively.

Correlation of pattern discrimination perimetry to the optic disc and visual field in ocular hypertensive and chronic open-angle glaucoma patients

We evaluated 19 ocular hypertensive and 32 chronic open-angle glaucoma patients to determine the correlation of pattern discrimination field loss to known markers of glaucomatous damage on the visual field and the optic nerve head. This study found no statistical association of the findings on the pattern discrimination perimeter to the areas of the optic disc, peripapillary halo, peripapillary atrophy, neural rim, or cup/pallor discrepancy (P > 0.05). In addition, no statistical relationship was observed to the diameters of the largest vein or artery in the inferior- or superior-temporal quadrant adjacent to the optic disc (P > 0.05). Between pattern discrimination and automated perimetry no agreement was observed in any visual field cluster (Glaucoma Hemifield Test) greater than that expected by chance alone (P > 0.05). When both visual function tests disagreed, the proportion of abnormal diagnoses in any cluster did not differ statistically between tests in chronic open-angle glaucoma patients (P > 0.05). However, in ocular hypertensive patients a greater proportion of abnormal diagnoses was observed with pattern discrimination perimetry (P < 0.03). This study suggests that pattern discrimination perimetry appears to measure a different physiologic property of the retina than does automated perimetry.

Behavioral evidence of filling-in at the blind spot of the monkey

In human subjects, the blind spot is perceptually filled-in by color and brightness from the surrounding visual field. The present behavioral study examined the occurrence of color filling-in at the blind spot in monkeys. First, the location of the blind spot was determined using a monocular saccade task. The blind spots were located on the horizontal meridian at approximately 15-17 deg from the fixation point in the temporal visual field. Then, filling-in at the blind spot was tested by determining if the monkey could discriminate between an annulus presented on the blind spot and a homogeneous disk in the normal visual field. In this task, the monkey was required to make a saccade to a homogeneous disk of the same color and size as an annulus presented simultaneously in the opposite field. Both stimuli were large enough to cover the blind spot and the inner circle of the annulus was confined inside the blind spot. All four monkeys tested performed this task correctly in over 80% of the trials. However, when one eye was covered and the annulus was presented on the blind spot of the uncovered eye, performance deteriorated significantly. To confirm that these results reflected filling-in, one monkey was trained to maintain fixation when two identical homogeneous disks appeared in opposite visual fields. When only one eye was uncovered, and the annulus was presented on the blind spot of the uncovered eye, the monkey maintained fixation in most of the trials. These results show that monkeys were unable to distinguish an annulus from a homogeneous disk when the annulus was presented on the blind spot. This indicates that color filling-in occurs at the blind spot in monkeys and opens possibility to physiological experiments to study the neural mechanisms of filling-in.

Race-, age-, gender-, and refractive error-related differences in the normal optic disc

OBJECTIVE

To determine race-, age-, gender-, and refractive error-related differences in the size and topography of the optic disc in healthy Americans.

DESIGN

Population-based study.

SETTING

Eastern and southeastern health districts of Baltimore, Md.

PARTICIPANTS

A population-based sample of 4877 non-institutionalized black and white individuals aged 40 years or older without evidence of optic nerve disease.

MAIN OUTCOME MEASURE

Race-, age-, gender-, and refractive error-related differences in optic disc measurements: disc area, neural rim area, cup area, cup-to-disc ratio, and neural rim area-to-disc area ratio.

RESULTS

We analyzed simultaneous stereoscopic optic disc photographs from 3387 (1534 black and 1853 white) of the 4877 healthy individuals using an image analyzer (Topcon Image Analyzer, Topcon Instrument Corporation, Paramus, NJ). A total of 1490 individuals were excluded owing to the absence of good-quality images from either eye. The image analyzer defined the cup margin 150 microns below the surface of the disc margin. On average, blacks had significantly larger disc areas (blacks, 2.94 mm2; whites, 2.63 mm2), larger cup areas (blacks, 1.04 mm2; whites, 0.71 mm2), larger cup-to-disc ratios (blacks, 0.56; whites, 0.49), similar neural rim areas (blacks, 1.90 mm2; whites, 1.92 mm2), and smaller neural rim area-to-disc area ratios (blacks, 0.66; whites, 0.74) compared with whites. There were no age-related differences in any of the disc measurements. Male subjects had 2% to 3% larger optic discs compared with female subjects. No association between refractive error and any of the optic disc measurements studied was detected.

CONCLUSIONS

Racial differences in the normal optic disc are present among urban Americans, and these differences must be considered in evaluation of the optic disc for glaucoma and other optic neuropathies. Among the individuals in our study, all of whom were 40 years of age or older, no progressive age-related decline in neural rim area was detectable. Neither gender nor refractive error were associated with any significant differences in the size and topography of the normal optic disc.

Simulation of dynamic receptive fields in primary visual cortex

A model network of spiking neurons with lateral connections was used to simulate short-term receptive field (RF) changes by removal of afferent input in the primary visual system. Several possible mechanisms for the dynamic RFs were explored and the simulation results were compared with experimental results obtained by Pettet and Gilbert [(1992) Proceedings of the National Academy of Science, U.S.A., 89, 8366-8370]. We found that appropriate input stimuli could induce a shift in the balance between modeled cortical lateral excitation and inhibition and in doing so cause RF expansion. Synaptic plasticity was neither necessary nor appropriate for short-term RF changes. An inhibition dominant network with neural adaptation successfully simulated Pettet and Gilbert's experiment of RF expansion and its reversibility induced by an "artificial scotoma". RF expansions induced by lesions were also explored with the model.