Objective evaluation of binocular function using the pattern reversal visual evoked response. II. Effect of mean luminosity

Delayed Correction for Extrapolation in Amblyopia

Purpose

It has been suggested that amblyopes present impaired motion extrapolation mechanisms. In this study, we used the flash grab effect (FGE), the illusory mislocalization of a briefly flashed stimulus in the direction of a reversing moving background, to investigate whether the amblyopic visual system can correct overextrapolation.

Methods

Thirteen amblyopes and 13 control subjects participated in the experiment. We measured the monocular FGE magnitude for each subject. Two spatial frequency (2 and 8 cycles), two texture configurations (square wave or sine wave), and two speed conditions (270 degrees/s and 67.5 degrees/s) were tested. In addition, control subjects were further tested in reduced luminance conditions.

Results

Compared with controls, amblyopes exhibited a larger FGE magnitude both in their fellow eye (FE) and amblyopic eye (AE). The FGE magnitude of their AE was significantly larger than that of the FE. In a control experiment, we observed that the FGE magnitude increases with the decreasing of the luminance. The FGE magnitude of amblyopes fall into the same range as that of controls under reduced luminance conditions.

Conclusions

We observed a lager FGE in patients with amblyopia, which indicates that the amblyopic visual system does not accurately correct the overextrapolation when a moving object abruptly reverses its direction. This spatiotemporal processing deficit could be ascribed to delayed visual processing in the amblyopic visual system.

Reduced Monocular Luminance Increases Monocular Temporal Synchrony Threshold in Human Adults

Purpose

The purpose of this study was to present our investigation of the influence of reduced monocular luminance on monocular and dichoptic temporal synchrony processing in healthy adults.

Methods

Ten adults with normal or corrected to normal visual acuity participated in our psychophysical study. The temporal synchrony threshold in dichoptic (experiment 1), monocular (experiment 2), and binocular (experiment 3) viewing configurations was obtained from each observer. Four flickering Gaussian dots (one synchronous and one asynchronous pair of two dots) were displayed, from which the observers were asked to identify the asynchronous pair. The temporal phase lag in the signal pair (asynchronous) but not in the reference pair (synchronous) was varied. In addition, a neutral density (ND) filter of various intensities (1.3 and 2.0 log units) was placed before the dominant eye throughout the behavioral measurement. In the end, dichoptic, monocular, and binocular thresholds were measured for each observer.

Results

With decreasing monocular luminance, the dichoptic threshold (2 ND vs. 0 ND, P < 0.001; 2 ND vs. 1.3 ND P = 0.001) and monocular threshold (2 ND vs. 0 ND, P < 0.001; 2 ND vs. 1.3 ND, P = 0.003) increased; however, the bincoular threshold remained unaffected (P = 0.576).

Conclusions

Reduced luminance induces delay and disturbs the discrimination of temporal synchrony. Our findings have clinical implications in visual disorders.

Binocular vision adaptively suppresses delayed monocular signals

A neutral density filter placed before one eye will produce a dichoptic imbalance in luminance, which attenuates responses to visual stimuli and lags neural signals from retina to cortex in the filtered eye. When stimuli are presented to both the filtered and unfiltered eye (i.e., binocularly), neural responses show little attenuation and no lag compared with their baseline counterpart. This suggests that binocular visual mechanisms must suppress the attenuated and delayed input from the filtered eye; however, the mechanisms involved remain unclear. Here, we used a Steady-State Visual Evoked Potential (SSVEP) technique to measure neural responses to monocularly and binocularly presented stimuli while observers wore an ND filter in front of their dominant eye. These data were well-described by a binocular summation model, which received the sinusoidal contrast modulation of the stimulus as input. We incorporated the influence of the ND filter with an impulse response function, which adjusted the input magnitude and phase in a biophysically plausible manner. The model captured the increase in attenuation and lag of neural signals for stimuli presented to the filtered eye as a function of filter strength, while also generating the filter phase-invariant responses from binocular presentation for EEG and psychophysical data. These results clarify how binocular visual mechanisms-specifically interocular suppression-can suppress the delayed and attenuated signals from the filtered eye and maintain normal neural signals under imbalanced luminance conditions.

Interocular contrast difference drives illusory 3D percept

Any processing delay between the two eyes can result in illusory 3D percepts for moving objects because of either changes in the pure disparities over time for disparity sensors or by changes to sensors that encode motion/disparity conjointly. This is demonstrated by viewing a fronto-parallel pendulum through a neutral density (ND) filter placed over one eye, resulting in the illusory 3D percept of the pendulum following an elliptical orbit in depth, the so-called Pulfrich phenomenon. Here we use a paradigm where a cylinder rotating in depth, defined by moving Gabor patches is presented at different interocular phases, generating strong to ambiguous depth percepts. This paradigm allows one to manipulate independently the contrast and the luminance of the patches to determine their influence on perceived motion-in-depth. Thus we show psychophysically that an interocular contrast difference can itself result in a similar illusory 3D percept of motion-in-depth. We argue that contrast, like luminance (ND filter) can modify the dynamics of visual neurons resulting in an interocular processing delay or an interocular velocity difference.

Bilateral symmetry in vision and influence of ocular surgical procedures on binocular vision: A topical review

We analyze the role of bilateral symmetry in enhancing binocular visual ability in human eyes, and further explore how efficiently bilateral symmetry is preserved in different ocular surgical procedures. The inclusion criterion for this review was strict relevance to the clinical questions under research. Enantiomorphism has been reported in lower order aberrations, higher order aberrations and cone directionality. When contrast differs in the two eyes, binocular acuity is better than monocular acuity of the eye that receives higher contrast. Anisometropia has an uncommon occurrence in large populations. Anisometropia seen in infancy and childhood is transitory and of little consequence for the visual acuity. Binocular summation of contrast signals declines with age, independent of inter-ocular differences. The symmetric associations between the right and left eye could be explained by the symmetry in pupil offset and visual axis which is always nasal in both eyes. Binocular summation mitigates poor visual performance under low luminance conditions and strong inter-ocular disparity detrimentally affects binocular summation. Considerable symmetry of response exists in fellow eyes of patients undergoing myopic PRK and LASIK, however the method to determine whether or not symmetry is maintained consist of comparing individual terms in a variety of ad hoc ways both before and after the refractive surgery, ignoring the fact that retinal image quality for any individual is based on the sum of all terms. The analysis of bilateral symmetry should be related to the patients' binocular vision status. The role of aberrations in monocular and binocular vision needs further investigation.

Assessing visual pathway function in multiple sclerosis patients with multifocal visual evoked potentials

Multifocal visual evoked potentials provide a topographic measure of visual response amplitude and latency. The objective of this study was to evaluate the sensitivity and specificity of the multifocal visual evoked potential technique in detecting visual abnormalities in patients with multiple sclerosis. Multifocal visual evoked potentials were recorded from 74 patients with multiple sclerosis with history of optic neuritis (MS-ON, n = 74 eyes) or without (MS-no-ON, n = 71 eyes), and 50 normal subjects (controls, n = 100 eyes) using a 60-sector pattern reversal dartboard stimulus (VERIS). Amplitude and latency for each sector were compared with normative data and assigned probabilities. Size and location of clusters of adjacent abnormal sectors (p < 0.05) were examined. Mean response amplitudes were (+/- SE) 0.39 +/- 0.02, 0.53 +/- 0.02, and 0.60 +/- 0.01 for MS-ON, MS-no-ON, and control groups, respectively, with significant differences between all groups (p < 0.0001). Mean latencies (ms; +/-SE relative to normative data) were 12.7 +/- 1.3 (MS-ON), 4.3 +/- 1.1 (MS-no-ON), and 0.3 +/- 0.4 (controls); group differences again significant (p < 0.0001). Half the MS-ON eyes had clusters larger than five sectors compared with 13% in MS-no-ON and 2% in controls. Abnormal sectors were distributed diffusely, although the largest cluster was smaller than 15 sectors in two-thirds of MS-ON eyes. Cluster criteria combining amplitude and latency showed an area of 0.96 under the receiver operating characteristic curve, yielding a criterion with 91% sensitivity and 95% specificity. We conclude that the multifocal visual evoked potential provides high sensitivity and specificity in detecting abnormalities in visual function in multiple sclerosis patients.

Mechanism of binocular interaction in refraction errors: study using pattern-reversal visual evoked potentials

In this study we sought to determine whether a natural condition involving fine discrimination, for example moderately severe myopia, might yield interesting information regarding the binocular interaction expressed by visual evoked potentials (VEPs). We studied ten normal subjects with a mild refraction deficits. Transient VEPs were elicited by monocular and binocular stimulation under conditions of natural and lens-corrected vision. The visual stimulus was a pattern-reversal checkerboard consisting of 15' and 40' checks. VEPs in response to binocular stimulation were compared with monocular VEPs. We plotted the monocular 'better-VEP' and 'worse-VEP' response, since significant differences between individual eye stimulations were present. We found no significant difference between the mean N75 and P100 latencies of the binocular VEP and the better monocular VEP, regardless of the check size used and of natural or corrected vision. Under all stimulus conditions, the mean amplitude of the N75-P100 of the binocular VEPs was also larger than the better monocular VEP response. The difference proved more significant when we stimulated our subjects with smaller squares and left vision uncorrected. The mean P100-N145 amplitude obtained with binocular stimulation was larger than the better monocular VEP response only when using small checks (15') and uncorrected vision. Overlapping latencies are consistent with an earlier hypothesis that monocular and binocular VEPs originate postsynaptically from the binocular neurons in the primary visual cortex. The gain in amplitude achieved by binocular stimulation may depend upon the removal of 'tonic interocular inhibition' and/or on a cortical modulatory mechanism.

Binocular interaction in normal vision studied by pattern-reversal visual evoked potential (PR-VEPS)

Monocular and binocular visual evoked potentials (VEPs) in response to different check size (15-21-38-84 minutes or arc) were studied in 14 subjects with normal visual acuity and stereopsis. The binocular VEP amplitude is slightly higher than the VEP amplitude on stimulation of the "better eye" and significantly higher than the VEP amplitude on stimulation of the "worse eye"; this effect is observed using small checks and almost exclusively involved N75-P100. Both the N75 and P100 peaks occur earlier after binocular than monocular stimulation. The shortening of the N75 mean latency is significantly greater than that of the P100 mean latency when larger check sizes are used. The mean latency of the N145 potential is not significantly different in monocular and binocular stimulus conditions. The slight summation effect and latency shortening in the binocular VEPs are not consistent with the hypothesis that it is the sum of separate monocular signals originating from the visual cortex that gives rise to the response. The early components of both monocular and binocular VEPs are thought to be of post-synaptic origin (outside layer 4c of area 17), where the inputs become mixed so that most cells receive information from both eyes. The amplitude enhancement of binocular VEPs, which mainly occurs when using small checks, may be related to the increase in the total amount of cortical activity representing the macular region; this may account for binocular superiority in fine spatial resolution. The latency shortening in binocular conditions can be explained by considering that the critical determinant of the latency is the fundamental spatial frequency of the pattern. When coarse patterns are used, their effectiveness in parafoveal stimulation may affect the VEPs, with a significant contribution coming from the more peripheral retina. The enlargement of the visual field when the eyes see simultaneously may therefore further reduce the latency of the response when using the larger checks suitable for eccentric stimulation.

The effect of binocular stimulation on each component of transient and steady-state VEPs

We recorded the monocular and binocular VEPs to the alternation of sinusoidal gratings in order to evaluate the binocular interaction in each component of transient and steady-state VEPs in 13 normal subjects. Three spatial frequencies (1.3, 2.6 and 5.3 c/deg) with a 90% contrast were used as visual stimuli. The latencies and amplitudes of N70 and P100 of the transient VEPs were measured. The steady-state VEPs were Fourier analyzed, and both the phase and amplitude of the second (2F) and fourth (4F) harmonic responses were obtained. Binocular interaction was influenced by spatial frequency such that a binocular summation or even an inhibition occurred. For the transient VEPs, a binocular summation was more pronounced in the amplitude of N70 than in that of P100 at all spatial frequencies. There were no significant effects of binocular stimulation on latencies of N70 or P100. However, the latencies of N70 and P100 showed different spatial frequency characteristics. For the steady-state VEPs, the amplitude of 2F revealed a binocular summation that was more pronounced at 5.3 c/deg, whereas the 4F amplitude showed binocular inhibition at 2.6 and 5.3 c/deg. The 2F phase showed binocular inhibition at all spatial frequencies, whereas no such inhibition was observed in the 4F phase. These results suggest that individual components of transient and steady-state VEPs are physiologically distinct and may therefore be generated from different neuronal populations in striate cortex.

Effects of light scatter, defocusing, mean luminosity, contrast, and central scotoma on the PVER amplitude-check size function curve

We analyzed the effects of various stimulus parameters, ie, light scatter, defocusing, mean luminosity, contrast, and central scotoma, on the normal pattern reversal visual evoked response (PVER), amplitude-check size function curve in six normal subjects. The steady-state PVER was recorded with five check sizes ranging from 160 to 10 min in 1-octave increments. The PVER amplitude, especially with the smaller check sizes, was markedly decreased by light scatter induced by acrylic sheets. The function curve quickly changed to a low pass filter shape when +2.0 diopters of defocus were added, with the decrease most marked in the small check sizes. When the mean luminosity was decreased, the function curve maintained its normal inverted-U shape up to 5 cd/m2, but the shape flattened with lower luminosity. Amplitude decreases were seen with all check sizes in low luminosity. With contrast changes from 95 to 24%, the function curve maintained its normal shape, but with slightly reduced amplitudes. The amplitude decrease was moderate even with the lowest contrast. With a two-degree central scotoma, the PVER amplitude was reduced more with the smaller than the larger check sizes. The function curve became somewhat flatter, with its peak shifting to the larger check sizes. Results indicated that the shape of the PVER amplitude-check size function curve changes in response to different modes of stimulation.

Nonlinearities in binocular visual evoked potentials in children

Evidence is controversial in respect to the optimal conditions in which visual evoked potentials provide an objective measure of binocular visual function, related and unrelated to stereopsis, and there is little emphasis on the type of stimulus that produces facilitation in binocular recording. We investigated the effects of stimulus type (flicker or pattern), contrast, and temporal modulation on facilitation, which was defined as a binocular response greater than sum of monocular responses. Monocular and binocular responses to sinusoidally modulated flicker and grating patterns were recorded in children and Fourier analyzed. The relationship of the fundamental Fourier component for flicker and the second harmonic component for pattern were each examined as function of temporal modulation at two levels of contrast for monocular and binocular visual evoked potentials. Binocular facilitation was found across all conditions for flicker. Data suggest that processing of pattern and flicker has different sites of origin within the visual system. Facilitation in binocular visual evoked potentials also indicates that they are not a result of simple summation of monocular responses, since there appears a nonlinear component to such interaction.

Effect of stimulus field size and localization on the binocular pattern reversal visual evoked response

The role of the central and peripheral stimulus fields on monocular and binocular amplitude and binocular summation of the pattern reversal visual evoked response were investigated. When the central stimulus field size was smaller than 2.4 degrees, there was no significant difference between the amplitude of the monocular and the binocular responses, but when it was equal to or larger than 3.2 degrees x 3.2 degrees, the binocular amplitude was significantly larger than the monocular. The value of binocular summation was highest at the central stimulus field of 4.0 degrees x 4.0 degrees; at larger sizes, there were no significant changes in the value. Use of a central stimulus field size larger than 3.2 degrees x 3.2 degrees was therefore considered a prerequisite for the effective assessment of visual function, especially binocular function, by means of the pattern reversal visual evoked response. With regard to the role of peripheral stimulus field on pattern reversal response, both the monocular and binocular responses, but particularly the latter, were found to be sensitive to a scotoma produced by covering the center of a full-field stimulus. The value of the binocular summation showed a significant reduction with a small central scotoma. We concluded that the pattern reversal visual evoked response is very sensitive to a central scotoma and that binocular function is mediated mainly through the central stimulus field.

Objective evaluation of binocular function with pattern reversal VER. IV. Effect of spatial and temporal frequency

The effects of changing spatial and temporal frequencies on the amplitude of pattern reversal monocular and binocular visual evoked response (VER) were investigated. The pattern reversal VER and the degree of binocular summation (binocular VER amplitude/monocular VER amplitude) were lowest at the spatial frequency (check size) of 0.3 cycles per degree (CPD) and highest at the spatial frequency of 4.0 CPD. The largest VER amplitudes were observed at 1.2 CPD under both the binocular and monocular recording conditions. Regarding the effects of changing temporal frequency (alteration or reversal rate) on the pattern reversal VER, the transient condition (1.5 to 3.0 Hz) did not produce significant binocular summation. At the higher temporal frequencies (6 to 12 Hz), significant binocular summation was produced compared with the transient stimulus condition. At very high temporal frequencies, the degree of the binocular summation showed a decrease. From these results, we selected a pattern with an element around 1.2 CPD with a relatively swift temporal frequency for evaluating binocular function with the pattern reversal VER.

Objective evaluation of binocular function with pattern reversal VER. III. Effect of stimulus size and localization

The effect of central and peripheral stimulus field on monocular and binocular amplitude and binocular summation of the pattern reversal VER were investigated. When the central stimulus field size was smaller than 2.0 degrees X 2.0 degrees, there was no significant difference between the amplitudes of the monocular and binocular VER, but when it was equal to or larger than 2.4 degrees, the binocular VER amplitude was significantly larger than the monocular. The value of the binocular summation was highest at the central stimulus field size of 4.0 degrees X 4.0 degrees; at larger sizes, there were no significant changes in the value. Use of a central stimulus field size larger than 2.4 degrees X 2.4 degrees was therefore considered a prerequisite for the effective assessment of visual function, especially binocular function using the pattern reversal VER. Regarding the effect of peripheral stimulus field, both the monocular and binocular VER, but particularly the latter, were found to be sensitive to the central scotoma produced by covering the center of the full-field stimulus. The value of the binocular summation showed a significant reduction with the small central scotoma. We concluded that the pattern reversal VER is very sensitive to a central scotoma and that binocular function is mediated mainly through the central part of the stimulus field.