Interactions between flicker thresholds and luminance pedestals

Evaluating the Consistency of Online Circular Contrast Perimetry Across Different Computer Monitors: A Cross-sectional Study

Aim and background

The aim of this study is to evaluate the agreement between perimetric findings of a novel 24°, 52-loci online circular contrast perimetry (OCCP) application on three different computer monitors to determine its stability of testing across varying displays.

Materials and methods

Sixty-one participants (19 healthy controls, 42 with glaucoma) underwent SAP testing followed by OCCP testing on three uncalibrated computer monitors in randomized order: a large-screen (24-inch) desktop personal computer (DPC) (Dell, Texas, US), a 17-inch laptop (LPC) (Dell), and a 14-inch MacBook Pro (MP) (Apple, California, US).

Results

Agreement of mean deviation (MD), pattern standard deviation (PSD), and visual field index (VFI)/visual index (VI) values between MP, DPC, and LPC OCCP were strong, with intraclass correlations and Deming's coefficients ranging from 0.96 to 1.00 and 0.93 to 1.03, respectively. When OCCP tests were compared to SAP, ICCs and Deming's coefficients were less strong, ranging from 0.89 to 0.95 and 0.72 to 0.89. Bland-Altman analyses revealed higher biases (2.90 to 3.59 dB) and wider limits of agreement when comparing OCCP to SAP than when comparing OCCP on different monitors. Bland-Altman bias of contrast sensitivities for each 24-2 testing location revealed stronger relationships between OCCP tests on different monitors (-0.82 to 0.78) than between OCCP and SAP tests (-1.53 to 1.32).

Conclusion

OCCP demonstrates strong levels of test-retest agreement when performed on computer monitors of varying display and moderate to strong levels of correlation to SAP perimetric indices.

Clinical significance

With further enhancements, OCCP could potentially be used on different personal computers, which could help address current challenges in glaucoma care, such as limited access to traditional perimetric testing. This has the potential to expand the scope of glaucoma detection and monitoring, particularly in remote and underserved areas of our community.

How to cite this article

Gong A, Busija L, Skalicky SE. Evaluating the Consistency of Online Circular Contrast Perimetry Across Different Computer Monitors: A Cross-sectional Study. J Curr Glaucoma Pract 2025;19(1):15-27.

Visual Field Asymmetries in Responses to ON and OFF Pathway Biasing Stimuli

Recent reports suggest the ON and OFF pathways are differentially susceptible to selective vision loss in glaucoma. Thus, perimetric assessment of ON- and OFF-pathway function may serve as a useful diagnostic. However, this necessitates a developed understanding of normal ON/OFF pathway function around the visual field and as a function of input intensity. Here, using electroencephalography, we measured ON- and OFF-pathway biased contrast response functions in the upper and lower visual fields. Using the steady-state visually evoked potential paradigm, we flickered achromatic luminance probes according to a saw-tooth waveform, the fast phase of which biased responses towards the ON or OFF pathways. Neural responses from the upper and lower visual fields were simultaneously measured using frequency tagging - probes in the upper visual field modulated at 3.75 Hz, while those in the lower visual field modulated at 3 Hz. We find that responses to OFF/decrements are larger than ON/increments, especially in the lower visual field. In the lower visual field, both ON and OFF responses were well described by a sigmoidal non-linearity. In the upper visual field, the ON pathway function was very similar to that of the lower, but the OFF pathway function showed reduced saturation and more cross-subject variability. Overall, this demonstrates that the relationship between the ON and OFF pathways depends on the visual field location and contrast level, potentially reflective of natural scene statistics.

Glaucoma Clinic Monitoring Over 6 Months Using Online Circular Contrast Perimetry in Comparison with Standard Automatic Perimetry: The Developing-World Setting

Purpose

Online circular contrast perimetry provides visual field testing on any computer or tablet without additional hardware. This study compared outcomes of online circular contrast perimetry (OCCP) and standard automated perimetry (SAP) in a developing world setting.

Methods

The longitudinal and observation study was conducted on patients sampled during 2023 at Hanoi Medical University Hospital. Participants were either healthy volunteers as controls or stable glaucoma patients with either primary angle closure or primary open-angle glaucoma. They underwent a comprehensive ocular examination, retinal nerve fiber layer optical coherence tomography scan, and visual field tests performed at baseline and after 3 months and 6 months, using OCCP and SAP in clinic.

Results

The current study was carried out in 168 eyes of 87 patients at baseline, 133 eyes of 69 patients at 3 months, and 121 eyes of 63 patients at 6 months. At baseline, OCCP mean deviation (MD) (R2 = 0.804, p < 0.001) and visual index (VI) (R2 = 0.892, p < 0.001) were strongly correlated with SAP MD and visual field index (VFI) respectively. There was strong agreement and correlation between MD and VI/VFI for SAP and OCCP on repeated testing after 6 months. At 6 months AUC of SAP VFI (0.79) was superior to AUC of OCCP VI (0.67, p = 0.036); otherwise there was no difference in AUC of MD or VI/VFI at baseline, 3 and 6 months, when comparing OCCP and SAP.

Conclusion

OCCP parameters are significantly correlated with those of SAP. OCCP has the potential to provide a complementary role to SAP in glaucoma screening and monitoring.

Repeatability of Online Circular Contrast Perimetry Compared to Standard Automated Perimetry

PRCIS

Online circular contrast perimetry provides visual field assessment on any computer or tablet with no extra hardware. It has good test repeatability and reliability that is comparable with standard automated perimetry. It holds promise for use in disease screening and surveillance to expand the provision of glaucoma care.

PURPOSE

To evaluate the repeatability of online circular contrast perimetry (OCCP) compared to standard automated perimetry (SAP) in normal participants and patients with stable glaucoma over 18 weeks.

METHODS

Thirty-six participants (13 normal controls and 23 patients with open angle glaucoma) were recruited. OCCP and SAP perimetry tests were performed twice at baseline, then at 6, 12, and 18 weeks. Global perimetric indices were compared between perimetry types and analyzed for short-term and intermediate-term repeatability.

RESULTS

There were no statistically significant changes over time for both OCCP and SAP across all groups for mean deviation (MD), pattern standard deviation, and visual index/visual field index ( P >0.05). Test-retest intraclass correlation coefficients (ICCs) for OCCP MD were excellent at baseline (0.98, 95% CI: 0.89-0.99) and good at 18 weeks (0.88, 95% CI: 0.51-0.98). SAP test-retest ICCs were excellent at baseline (0.94, 95% CI: 0.70-0.99) and 18 weeks (0.97, 95% CI: 0.84-0.99). Inter-test ICCs were good, ranging from 0.84 to 0.87. OCCP testing time was shorter than SAP (5:29 ± 1:24 vs. 6:00 ± 1:05, P <0.001). OCCP had similar false-positive (3.84 ± 3.32 vs. 3.66 ± 4.53, P =0.48) but lower false-negative (0.73 ± 1.52 vs. 4.48 ± 5.00, P <0.001) and fixation loss responses (0.91 ± 1.32 vs. 2.02 ± 2.17, P <0.001).

CONCLUSIONS

OCCP allows visual field assessment on any computer screen with no additional hardware. It demonstrated good repeatability and reliability with similar performance indices to SAP in both the short term and intermediate term. OCCP has the potential to be utilized as a glaucoma screening and surveillance tool for in-clinic and at-home testing, expanding the provision of care.

Online Circular Contrast Perimetry via a Web-Application: Establishing a Normative Database for Central 10-Degree Perimetry

Purpose

To establish a normative database using a central 10-degree grid pattern for the online circular contrast perimetry (OCCP) application.

Participants

Fifty participants with mean age 65 ± 13 years were selected for this study. One eye from each participant that met inclusion criteria was randomly included in the cohort.

Methods

The web-application delivered online 52-loci perimetry in a central 10-degree pattern using circular flickering targets. These targets consist of concentric sinusoidal alternating contrast rings. Users were guided by the application to the correct viewing distance and head position using in-built blind spot localization and webcam monitoring. A spinning golden star was used as the fixation target and patients performed the test in a darkened room following standard automated perimetry (SAP).

Results

The reliability rates and global indices for OCCP were similar to SAP. OCCP mean sensitivity reduced with age at a similar rate to SAP. Mean sensitivity per loci of 10-degree OCCP was greater than SAP by 1.24 log units (95% CI 1.23 to 1.26) and obeyed a physiological hill of vision. Small differences existed in mean sensitivities between OCCP and SAP which increased with increasing spot eccentricity. Mean deviation (MD) displayed good agreement between the two tests.

Conclusion

Central 10-degree online circular contrast perimetry via a computer-based application has comparable perimetric results to standard automated perimetry in a normal cohort.

Comparing a head-mounted virtual reality perimeter and the Humphrey Field Analyzer for visual field testing in healthy and glaucoma patients

PURPOSE

To compare clinical visual field outputs in glaucoma and healthy patients returned by the Humphrey Field Analyzer (HFA) and virtual reality (Virtual Field, VF) perimetry.

METHODS

One eye of 54 glaucoma patients and 41 healthy subjects was prospectively tested (three times each in random order) using the HFA and VF perimeters (24-2 test grids). We extracted and compared global indices (mean deviation [MD] and pattern standard deviation [PSD]), pointwise sensitivity (and calculated 'equivalent' sensitivity after accounting for differences in background luminance) and pointwise defects. Bland-Altman (mean difference [Mdiff ] and 95% limits of agreement [LoA]) and intraclass correlation analyses were performed.

RESULTS

The VF test was shorter (by 76 s) and had lower fixation losses (by 0.08) and false-positive rate (by 0.01) compared to the HFA (all p < 0.0001). Intraclass correlations were 0.86, 0.82 and 0.47 for MD, PSD and pointwise sensitivity between devices, respectively. Test-retest variability was higher for VF (Mdiff 0.3 dB, LoA -7.6 to 8.2 dB) compared to the HFA (Mdiff -0.3 dB, LoA -6.4 to 5.9 dB), indicating greater test-retest variability. When using each device's underlying normative database, the HFA detected, on average, 7 more defects (at the p < 0.05 level) out of the 52 test locations compared to this iteration of VF in the glaucoma cohort.

CONCLUSIONS

Virtual Field returns global results that are correlated with the HFA, but pointwise sensitivities were more variable. Differences in test-retest variability and defect detection by its current normative database raise questions about the widespread adoption of VF in lieu of the HFA.

Circular Contrast Perimetry via Web Application: A Patient Appraisal and Comparison to Standard Automated Perimetry

Purpose

The purpose of the study was to compare a novel, 24°, 52-locus online circular contrast perimetry (OCCP) application against standard automated perimetry (SAP) in terms of both diagnostic accuracy and patient attitudes.

Design

This was a cross-sectional study.

Subjects

Ninety-five participants (42 controls and 53 open-angle glaucoma patients) were included.

Methods

Participants performed both perimetry tests and then completed an online survey. Subjective feedback responses were collected.

Main Outcome Measures

Agreement, sensitivity, specificity, and area under receiver operating curves (AUCs) were compared for the parameters of OCCP, SAP, and OCT for the retinal nerve fiber layer (RNFL) and macular ganglion cell complex inner plexiform layer (GCC + IPL). Participant attitudes toward the OCCP test versus the SAP test, in both glaucoma patients and controls, were compared. Rasch analysis assessed the psychometric properties of the survey and intergroup variability.

Results

The AUC for OCCP mean deviation (MD) was 0.959 ± 0.02. Compared with other instruments' parameters with the highest AUC, it was superior to SAP MD (0.871 ± 0.04, P = 0.03) and OCT GCC + IPL (0.871 ± 0.04, P = 0.03) and similar to OCT RNFL inferior thickness (IT) (0.917 ± 0.03, no significance). Online circular contrast perimetry pointwise sensitivity was less than SAP by 4.30 dB (95% confidence interval = 4.02-4.59); 95% limits of agreement ranged from -6.28 to -2.33 dB. At the best cutoff, the OCCP MD had a sensitivity of 98% and specificity of 85% for detecting glaucoma. Cohen's kappa demonstrated good agreement with SAP MD (0.69) and OCT RNFL IT (0.62) and moderate agreement with OCT GCC + IPL IT (0.57). Participants preferred OCCP across most survey parameters (P < 0.0001). Rasch analysis demonstrated no differential item functioning for clinical group, gender, or age.

Conclusions

With similar diagnostic metrics to SAP, OCCP offers an improved user experience with the potential to increase the provision of care and improve disease surveillance outcomes.

Online circular contrast perimetry via a web-application: optimising parameters and establishing a normative database

PURPOSE

To establish a normative database and optimise parameters for personal-computer based perimetry via a web-application using circular contrast targets.

METHODS

Online 24-degree 52-loci perimetry was delivered through a web-application using circular flickering contrast targets. Embedding contrast differentials within targets allows calculation of relative decibel (rdB) per 256-bit greyscale level differential. Target light-band maximum brightness colour was fixed, while the dark-band varied to achieve the desired rdB level. A staircase system was used with two reversals ranging from 0 to 36 rdB levels. Blind spot localisation at the start of the test was used to optimise viewing distance and subsequently count fixation losses. Gaze was maintained on a spinning golden star which moves mid-test to maximise sampling area. Patients performed the test to each eye separately using a computer in a darkened room at 40-45 cm.

RESULTS

158 eyes of 101 patients completed the Online Circular Contrast Perimetry (OCCP) test. Mean age was 62.9 ± 14.3 years old. Mean sensitivity reduced with age, at 1.0 relative decibel per decade. Mean sensitivity per locus correlated with standard automated perimetry (SAP) in a physiological hill of vision, with an average difference of 4.02 decibels (95% confidence interval (CI) = 3.77-4.27, p < 0.001) and good agreement between tests.

CONCLUSIONS

Online circular contract perimetry provides accurate perimetric testing with comparable results to standard automated perimetry.

Recent developments in perimetry: test stimuli and procedures

Automated perimetry has evolved substantially in recent years, in part due to modern computer technology that enables more complex visual stimuli and test procedures to be realised than those incorporated in traditional white-on-white luminance increment perimetry. This paper reviews briefly a number of advances in automated perimetry. The review includes discussion of new test types: frequency doubling technology perimetry, short wavelength automated perimetry, flicker perimetry, high-pass resolution perimetry and rarebit perimetry. Test algorithms applied to perimetry such as zippy estimation of sequential thresholds (ZEST), Swedish interactive thresholding algorithm (SITA), tendency-oriented perimetry (TOP) and multi-sampling supra-threshold perimetry are also discussed.

Responsive Neural Activities in the Primary Visual Cortex of Retina-Degenerated Rats

To study the responsive neural activities in the primary visual cortex (V1) of retinal degeneration (RD) models, experiments involving the wild-type (WT) and RD rats were conducted. The neural responses in the V1 were recorded extracellularly, while a visual stimulus with varied light intensity was given to the subjects. First, the firing rate and its relationship with light intensity were compared between the WT and RD groups. Second, the mutual information (MI) between the visual stimulus and neural response was determined for every isolated unit to quantify the amount and efficiency of information transmission in the V1 for both the control and experimental groups. Third, the local field potential (LFP) signal was characterized and its power used to compute the MI and further evaluate the function change in the RD model regarding information transmission. Analysis of spiking activity showed that the RD group exhibited a relatively decreased firing rate, information amount and efficiency compared with the control group. However, the information transmission performance of the RD model was similar to that of the WT group in the context of LFP activity. Therefore, for the RD rats, the early stage of the visual system was impaired, while the later stage of the visual system, V1, was able to capture the information about the visual stimulus, especially at the population level. Thus, this pathway could be used to restore visual ability, such as by visual prostheses.

An Analysis of the Factors Associated with Visual Field Deficits Measured with Flickering Stimuli in-between Migraine

We have previously demonstrated that perimetric performance measured with flickering stimuli is not normal in some individuals who experience migraine with aura in the period between their attacks. In this study, flicker perimetric performance is measured in a broad group of migraineurs to determine whether the existence of such visual field deficits is dependent on the presence of visual aura, is correlated with the duration of migraine history, or frequency of attacks. Twenty-eight migraine with aura, 25 migraine without aura, and 24 non-headache control subjects participated. The performance of the migraine groups was not significantly different from each other. The migraine groups showed significantly lower general sensitivity across the visual field and higher incidence of localized visual field deficits relative to controls. Both length of migraine history and frequency of migraine occurrence over the past 12 months were significantly correlated with lower general sensitivity to flickering visual stimuli.

Measuring visual function in age-related macular degeneration with frequency-doubling (matrix) perimetry

PURPOSE

To determine the agreement between the Humphrey Matrix perimeter 10-2 test and the 10-2 Humphrey Field Analyzer (HFA) test when assessing visual function in patients with age-related macular degeneration (AMD).

METHODS

Forty-two eyes of 42 subjects with AMD (average 75.0 years, SD=6.2: median visual acuity in logarithm of the minimum angle of resolution of 0.26, range, -0.12 to 1.04) were evaluated with the Matrix and HFA 10-2 visual field tests. Mean deviation (MD), pattern standard deviation, and test time were recorded. We calculated spatial concordance of individual test locations, being the proportion of spatially agreeing locations with identical classification (normal vs. abnormal, p < 5%) on the pattern deviation plot. As multiple HFA stimuli overlapped with some Matrix locations, several criteria for grouping HFA data into locations were investigated.

RESULTS

Both MD and pattern standard deviation were significantly correlated for the two devices (r(2)=0.79 and r(2)=0.80, respectively, p<0.0001). Using our standard criterion for abnormal HFA locations (≥50% stimuli abnormal), the median spatial concordance was 0.76, with 95% of tests giving a concordance of ≥0.59. A small, but significant, increase in concordance occurred when a stricter criterion (all stimuli abnormal at a location) was applied. Median fixation loss percentages were 7 and 0% for the HFA and Matrix, respectively. Visual acuity in logarithm of the minimum angle of resolution showed modest correlations with both defect depth (HFA MD: r(2)=0.39, p<0.0001) and size of defect (number of abnormal points on the HFA: r(2)=0.24, p<0.0001).

CONCLUSIONS

Using a simple metric to calculate spatial concordance, the Matrix 10-2 test quantifies the spatial extent of significant depression of the central visual fields in AMD in a manner similar to the HFA 10-2. The spatial extent and depth of central visual field loss in AMD are only modestly predicted by visual acuity measurements.

Detection and discrimination of flicker contrast in migraine

AIMS

Flickering light is strongly aversive to many individuals with migraine. This study was designed to evaluate other abnormalities in the processing of temporally modulating visual stimulation.

METHODS

We measured psychophysical thresholds for detection of a flickering target and for the discrimination of suprathreshold flicker contrasts (increment thresholds) in 14 migraineurs and 14 healthy controls with and without prior adaptation to high-contrast flicker. Visual discomfort (aversion) thresholds were also assessed.

RESULTS

In the baseline (no adaptation) conditions, detection and discrimination thresholds did not differ significantly between groups. Following adaptation, flicker detection thresholds were elevated equivalently in both groups; however, discrimination thresholds were more strongly affected in migraineurs than in controls, showing greater elevation at moderate contrasts and greater threshold reduction (sensitisation) at high contrast (70%). Migraineurs also had significantly elevated discomfort scores, and these were significantly correlated with number of years with migraine.

DISCUSSION

We conclude that visual flicker not only causes discomfort but also exerts measurable effects on contrast processing in the visual pathways in migraine. The findings are discussed in the context of the existing literature on habituation, adaptation and contrast-gain control.

A new desktop instrument for measuring macular pigment optical density based on a novel technique for setting flicker thresholds

A rapid portable technique for estimating macular pigment optical density (MPOD) in large populations is described. The new instrument utilises a novel method for setting flicker thresholds which is undemanding for naïve and elderly observers and easily operated by a non-technical person. The method has good repeatability (r = 0.97) and the data are comparable with an optical method based on retinal reflectometry (r = 0.78). MPOD spatial profiles are presented for seven normal observers and these are well described (r = 0.99) by a decaying exponential function consistent with previous reports. MPOD values are presented from 5581 (2435 females and 3146 males) individuals measured in 48 optometric practices. The mean MPOD of this population was 0.33 (S.D. +/- 0.187) which is similar to previous large scale studies of MP.

Defining the detection mechanisms for symmetric and rectified flicker stimuli

Symmetric flicker modulates about a background light level and effects no change in the time-average luminance. Rectified flicker is achieved by modulating a luminance-increment and results in both a flickering component and an increase in the time-averaged luminance (luminance-pedestal) above the adapting background light level. We studied the effect that changes in adapting light level and local luminance (within the area of the flickering target) have on thresholds. We measured thresholds for single and multiple cycles of flicker over a range of adapting light levels (Threshold versus Intensity paradigm) and defined their gain as a function of luminance-pedestal amplitude (Threshold versus Amplitude paradigm). The dynamics of symmetric and rectified flicker responses were determined using a Stimulus Onset Asynchrony paradigm. The data show rectified flicker thresholds differ from symmetric flicker thresholds due to two factors that can be contrast-dependent or contrast-independent: (1) local adaptation, which varies with stimulus duration and (2) surround interactions that depend on adapting light level. The dynamics of the thresholds for symmetric and rectified flicker stimuli suggest the detection mechanisms occur early in the visual pathways, involving the magnocellular pathway.

Visual function and dysfunction in early and late age-related maculopathy

Late age-related maculopathy (ARM) is responsible for the majority of blind registrations in the Western world among persons over 50 years of age. It has devastating effects on quality of life and independence and is becoming a major public health concern. Current treatment options are limited and most aim to slow progression rather than restore vision; therefore, early detection to identify those patients most suitable for these interventions is essential. In this work, we review the literature encompassing the investigation of visual function in ARM in order to highlight those visual function parameters which are affected very early in the disease process. We pay particular attention to measures of acuity, contrast sensitivity (CS), cone function, electrophysiology, visual adaptation, central visual field sensitivity and metamorphopsia. We also consider the impact of bilateral late ARM on visual function as well as the relationship between measures of vision function and self-reported visual functioning. Much interest has centred on the identification of functional changes which may predict progression to neovascular disease; therefore, we outline the longitudinal studies, which to date have reported dark-adaptation time, short-wavelength cone sensitivity, colour-match area effect, dark-adapted foveal sensitivity, foveal flicker sensitivity, slow recovery from glare and slower foveal electroretinogram implicit time as functional risk factors for the development of neovascular disease. Despite progress in this area, we emphasise the need for longitudinal studies designed in light of developments in disease classification and retinal imaging, which would ensure the correct classification of cases and controls, and provide increased understanding of the natural course and progression of the disease and further elucidate the structure-function relationships in this devastating disorder.

A noisy transform predicts saccadic and manual reaction times to changes in contrast

One of the most important factors affecting the time taken to respond to a visual stimulus is contrast, and studies of reaction time can provide precise, quantitative information about the underlying signal processing. In this study we measured both saccadic and manual reaction times to step increments in target contrast. Our results over a range of initial contrasts are consistent with a simple model consisting of a noisy logarithmic transducer followed by a rise-to-threshold accumulator. A systematic comparison with previous contrast-processing models also shows that the commonly used method of linear regression may not be a particularly sensitive tool in deciding between them. We found similar parameters for the contrast processor in both saccadic and manual reaction times, as might be expected if a common target detection stage precedes each type of reaction.

Comparison of the ASA, MOBS, and ZEST threshold methods

The modified binary search, or MOBS, technique is an adaptive, non-parametric procedure for estimating thresholds [Tyrrell, R. A, & Owens, D. A. (1988). A rapid technique to assess the resting states of eyes and other threshold phenomena: the modified binary search (MOBS). Behavior Research Methods, Instruments, and Computers, 20, 137-141.], which has recently been incorporated into a commercially available perimeter. Information regarding the performance of this technique is limited, however. We performed Monte Carlos simulations on the MOBS procedure, as well as on a Zippy Estimation by Sequential Testing, or ZEST, procedure (King-Smith, Grigsby, Vingrys, Benes & Supowit, 1994) and an Accelerated Stochastic Approximation, or ASA, procedure (Kesten, 1958) for comparison. The efficiency, convergence probability, and robustness to false positive and false negative responses were determined. Differences between the three procedures typically were small, the most prominent being the number of presentations required to estimate threshold.

Processing of global form and motion in migraineurs

Previous studies have identified anomalies of cortical visual processing in migraineurs that appear to extend beyond V1. Migraineurs respond differently than controls to transcranial magnetic stimulation of V5, and can demonstrate impairments of global motion processing. This study was designed to assess the integrity of intermediate stages of both motion and form processing in people with migraine. We measured the ability to integrate local orientation information into a global form percept, and to integrate local motion information into a global motion percept. Control subjects performed significantly better than migraineurs on both tasks, suggesting a diffuse visual cortical processing anomaly in migraine.

Effects of intrastimulus modality change on audiovisual time-to-arrival judgments

In this research, anticipatory perception of an approaching vehicle was examined. By varying the availability of visual and acoustic media, conditions occurred in which the input to the modalities was repetitively interrupted while the presentation of the approach event continued. In these conditions, the audio and the visual signals were presented either in short and concurrent segments at regular intervals or in alternation (interleaved) at regular intervals. It was found that interrupting the signal within a single modality did not affect performance if the approach information was available in the alternate modality. These results are consistent with a modally flexible detection mechanism for the perception of approaching objects. This modal flexibility may provide some evidence that information is detected using a modality-neutral strategy.

Utility of a dynamic termination criterion in the ZEST adaptive threshold method

Bayesian adaptive threshold procedures may be run for a fixed number of trials, or may be stopped when the calculated confidence interval for the threshold reaches a selected limit (a dynamic termination criterion). This study used Monte-Carlo simulations to determine whether the confidence interval is a useful predictor of errors in the estimated threshold. No difference was found between the distribution of errors in a fixed trial procedure versus a dynamically terminated procedure of the same average number of trials. In addition, the width of the confidence interval failed to usefully predict observer variability arising from a shallow psychometric function slope or increased false positive response probabilities. This study suggests that dynamic termination criteria are of little use in Bayesian adaptive threshold procedures.

Adaptation of spatiotemporal mechanisms by increment and decrement stimuli

Sawtooth modulation has been used in the past to examine visual sensitivity to luminance increments and decrements. The threshold elevation caused by adaptation depends on the spatial profile of the stimulus field and the polarities of the adaptation and test stimuli. We hypothesized that the adaptation effects reflect a change in the sensitivity of the spatiotemporal channels that detect the stimuli. We used a 2-deg disk centered in a larger surround field. Five levels of contrast between the test field and surround were investigated: equiluminant, three intermediate levels, and dark. At each contrast, observers adapted for 5 s to 2-Hz sawtooth modulation (rapid-on or rapid-off). Immediately after adaptation, thresholds were measured for detection of a single cycle of either a rapid-on or a rapid-off waveform. Varying the contrast of the surround affected observers' sensitivity to the polarity of the sawtooth stimulus to the extent that the pattern of sensitivity with the equiluminant surround was the opposite of that with the dark surround. To examine temporal factors, we measured thresholds for slow (500-ms ramps) and fast (8.3-ms pulses) test stimuli. The adaptation effect was preserved with the ramp stimuli but not with the pulse stimuli. Blurring the edge between the test and surround fields in the equiluminant surround condition raised thresholds for all sawtooth test stimuli, suggesting that spatiotemporal channels sensitive to high spatial frequencies and low temporal frequencies facilitate detection in that condition. These findings suggest that adaptation to sawtooth modulation can differentially effect the sensitivity of ON and OFF pathways, but the relative desensitization of each pathway depends on an interaction with the adaptation state of spatiotemporal channels that are involved in detection.

Effect of eccentricity on luminance-pedestal flicker thresholds

We investigated the effect that spatially coincident luminance increments (luminance pedestals) have on flicker thresholds at several eccentricities and target sizes. Luminance pedestals elevated flicker amplitude-thresholds more when stimuli were presented eccentrically, both at low (4 Hz) and high (20 Hz) temporal frequencies. Altering the size of the eccentric stimulus failed to equate central and eccentric thresholds at all pedestal amplitudes. Comparisons with flicker thresholds at various background luminances suggests that the increase in luminance-pedestal flicker thresholds peripherally is due to increased suppressive rod-cone interactions, increased effectiveness of luminous contrast on edge-sensitive flicker mechanisms, as well as increased gain in the light adaptation response.

Multiple processes mediate flicker sensitivity

By systematically manipulating the luminance of a flickering spot and the area immediately surrounding it, we investigated why thresholds from flickering stimuli that cause a change in average luminance are elevated relative to those from stimuli with no luminance change. Threshold elevation resulted from local light adaptation and from temporal-frequency-specific interactions between the spot and its surround: at low frequencies, the contrast between the spot and the surround elevated thresholds, whereas at high frequencies, dark adaptation within the surround elevated thresholds. Our findings suggest that two common ways of determining temporal sensitivity may give markedly different outcomes.