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We investigated how a short-term luminance reduction in one eye can influence temporal processing of that eye after luminance is restored by measuring the relative delay between the eyes.A paradigm based on the Pulfrich effect, which is a visual illusion of depth when no depth cue is present, was used to measure relative delay in visual processing between the eyes. We deprived the monocular luminance in adults with normal vision across different intensities. In the first experiment, the ratio of the light level between the eyes stayed constant, whereas the absolute value was allowed to vary. In the second experiment, both the ratio and the absolute light level stayed constant, by controlling the environmental light level. In both experiments, we measured the changes in relative delay before and after 60 min of light deprivation.Our results indicated that short-term monocular deprivation of luminance slows the processing in the previously dimmed eye and that the magnitude of the delay is correlated with the degree of luminance reduction. In addition, we observed that the absolute luminance difference, rather than the absolute luminance levels seen by the dimmed eye, is important in determining the magnitude of delay in the previously dimmed eye. These findings differ from what has been reported previously for the monocular deprivation of contrast.Taken together, these findings support the view that short-term deprivation of visual information could affect two distinct mechanisms (contrast gain and temporal dynamics) of neural plasticity.

The adult human visual system maintains the ability to be altered by sensory deprivation. What has not been considered is whether the internal neural states modulate visual sensitivity to short-term monocular deprivation. In this study we manipulated the internal neural state and reported changes in intrinsic neural oscillations with a patched eye open or closed. We investigated the influence of eye open/eye closure on the unpatched eye’s contrast sensitivity and ocular dominance (OD) shifts induced by short-term monocular deprivation. The results demonstrate that internal neural states influence not only baseline contrast sensitivity but also the extent to which the adult visual system can undergo changes in ocular dominance.

The adult human visual system can exhibit a degree of neuroplastic change under the right conditions which has implications for future treatments to recover vision loss that have occurred in infancy. The exogenous factors that promote neuroplastic change have been the focus of human psychophysical, electrophysiological and neuroimaging research over the last decade. What has not been considered is the importance of endogenous factors. In this study we modulate the neural oscillations that determines the internal neural state to demonstrate that endogenous factors play a critical role in not only baseline contrast sensitivity but also the extent to which the adult visual system can undergo neuroplastic change in binocular balance.

An interlude of dark exposure for about one week is known to shift the excitatory/inhibitory (E/I) balance of the mammalian visual cortex, promoting cortical plasticity across species and accelerating visual recovery in animals that have experienced cortical lesions during development. However, the translational impact of our understanding of dark exposure from animal studies on humans remains elusive. Here, we use magnetic resonance spectroscopy as a probe for E/I balance in the primary visual cortex (V1) to determine the effect of 60-min dark exposure, and measure binocular combination as a behavioral assay to assess visual plasticity in 18 normally sighted human adults (13 females). To induce neuroplastic changes in the observers, 60-min monocular deprivation was performed, which is known to shift sensory eye balance in favor of the previously deprived eye. We report that prior dark exposure strengthens cortical excitability in V1 and boosts visually plasticity in normal adults. Our findings are surprising, given the fact that the interlude is very brief. We present direct evidence that an environmental manipulation that reduces intracortical inhibition can act as a metaplastic facilitator for visual plasticity in adult humans.

Recent studies indicate that short-term monocular deprivation increases the deprived eye's contribution to binocular fusion in both adults with normal vision and amblyopia. In this study, we investigated whether the changes in visual plasticity depended on the duration of deprivation in normal and amblyopic adults.Twelve anisometropia amblyopic observers (aged 24.8 ± 2.3 years) and 12 age-matched normal observers (aged 23.9 ± 1.2 years) participated in the study. The non-dominant eye of normal observers or amblyopic eye of amblyopic observers was deprived for 30, 120, and 300 min in a randomized order. Their eye balance was measured with a phase combination task, which is a psychophysical test, before and after the deprivation. This design enabled us to measure changes induced in binocular balance as an index visual plasticity due to monocular deprivations.By comparing the ocular dominance changes as a result of monocular deprivation with different deprivation durations, we found evidence that the ocular dominance changes are slightly larger after longer deprivations in both normal and amblyopic observers, albeit with a statistical significance. The changes from 120-min were significantly greater than those from 30-min deprivation in both groups. The magnitude of changes in sensory eye balance was significantly larger in normal observers than that in the amblyopic observers; however, the longevity of changes in visual plasticity was found to be more long-lasting in amblyopic observers than the normal counterparts.The duration of deprivation matters in both normal and amblyopic observers. Ocular dominance imbalance that is typically observed in amblyopia can be more ameliorated with a longer duration of deprivation.

We aimed to study the effect of stimulus contrast on the orientation selectivity of interocular interaction in amblyopia using a dichoptic masking paradigm.Eight adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. The contrast threshold in discriminating a target Gabor in the tested eye was measured with mean luminance in the untested eye, as well as with a bandpass oriented filtered noise in the other eye at low spatial frequency (0.25 c/d). Threshold elevation, which represents interocular suppression, was assessed using a the dichoptic masking paradigm (i.e. the contrast threshold difference between the target only and masked conditions), for each eye. Orientation selectivity of the interocular suppression as reflected by dichoptic masking was quantified by the difference between the parallel and orthogonal masking configurations. Two levels of mask's contrast (3 times or 10 times that of an individual's contrast threshold) were tested in this study.The strength of dichoptic masking suppression was stronger at high, rather than low mask contrast in both amblyopic and control subjects. Normal controls showed orientation-dependent dichoptic masking suppression both under high and low contrast levels. However, amblyopes showed orientation-tuned dichoptic masking suppression only under the high contrast level, but untuned under the low contrast level.We demonstrate that interocular suppression assessed by dichoptic masking is contrast-dependent in amblyopia, being orientation-tuned only at high suprathreshold contrast levels of the mask.

Boundaries in the visual world can be defined by changes in luminance and texture in the input image. A “contour integration” process joins together local changes into percepts of lines or edges. A previous study tested the integration of contours defined by second-order contrast-modulation. Their contours were placed in a background of random wavelets. Subjects performed near chance. We re-visited second-order contour integration with a different task. Subjects distinguished contours with “good continuation” from distractors. We measured thresholds in different amounts of external orientation or position noise. This gave two noise-masking functions. We also measured thresholds for contours with a baseline curvature to assess performance with more curvy targets. Our subjects were able to discriminate the good continuation of second-order contours. Thresholds were higher than for first-order contours. In our modelling, we found this was due to multiple factors. There was a doubling of equivalent internal noise between first- and second-order contour integration. There was also a reduction in efficiency. The efficiency difference was only significant in our orientation noise condition. For both first- and second-order stimuli, subjects were also able to perform our task with more curved contours. We conclude that humans can integrate second-order contours, even when they are curved. There is however reduced performance compared to first-order contours. We find both an impaired input to the integrating mechanism, and reduced efficiency seem responsible. Second-order contour integration may be more affected by the noise background used in the previous study. Difficulty segregating that background may explain their result.

Purpose: To determine whether monocularly- and binocularly-induced spherical and meridional blur and aniseikonia had similar effects on stereopsis thresholds. Methods: Twelve participants with normal binocular vision viewed McGill modified random dot stereograms to determine stereoacuities in a four-alternative forced-choice procedure. Astigmatism was induced by placing trial lenses in front of the eyes. Twenty-three conditions were used, consisting of zero (no lens), +1 D and +2 D spheres and cylinders at axes 180, 45 and 90 in front of the right eye, and the following binocular combinations of both lens powers: R × 180/L × 180, R × 45/L × 45, R × 90/L × 90, R sphere/L sphere, R × 180/L × 90, R × 45/L × 135, R × 90/L × 180. Aniseikonia was induced by placing magnifying lenses in front of the eyes. Twenty-three conditions were used, consisting of zero, 6% and 12% overall magnification and both magnifications at axes 180, 45 and 90 in front of the right eye only, and the following binocular combinations using 3% and 6% lenses: R × 90/L × 90, R × 45/L × 45, R × 180/L × 180, R overall/L overall, R × 90/L × 180, R × 45/L × 135, and R × 180/L × 90. Results: Stereopsis losses for binocular blur effects with parallel axes (non-anisometropic) were the same as for monocular blur effects of the same axes, and these were strongly dependent on axis (spherical blur and ×90 had the greatest effects). Binocular blur effects with orthogonal axes had greater effects than with parallel axes, with the axis combination of the former having no effect (e.g. R × 90/L × 180 was similar to R × 45/L × 135). For induced aniseikonia, splitting the magnifications between the eyes improved stereopsis slightly, and the effects were not dependent on axis. Conclusion: Binocular blur affects stereopsis similarly to monocular meridional blur if axes in the two eyes are parallel, whereas the effect is greater if the axes are orthogonal. In meridional aniseikonia, splitting magnification between the right and left lenses produces a small improvement in stereopsis that is independent of axis direction and right/left combination.

The endogenous neurotransmitter acetylcholine (ACh) is known to affect the excitatory/inhibitory (E/I) balance of primate visual cortex, enhancing feedforward thalamocortical gain while suppressing corticocortical synapses. Recent advances in the study of the human visual system suggest that ACh is a likely component underlying interocular interactions. However, our understanding of its precise role in binocular processes is currently lacking. Here we use binocular rivalry as a probe of interocular dynamics to determine ACh's effects, via the acetylcholinesterase inhibitor (AChEI) donepezil, on the binocular visual system. A total of 23 subjects (13 male) completed two crossover experimental sessions where binocular rivalry measurements were obtained before and after taking either donepezil (5 mg) or a placebo (lactose) pill. We report that enhanced cholinergic potentiation attenuates perceptual suppression during binocular rivalry, reducing the overall rate of interocular competition while enhancing the visibility of superimposition mixed percepts. Considering recent evidence that perceptual suppression during binocular rivalry is causally modulated by the inhibitory neurotransmitter GABA, our results suggest that cholinergic activity counteracts the effect of GABA with regards to interocular dynamics and may modulate the inhibitory drive within the visual cortex. SIGNIFICANCE STATEMENT Our research demonstrates that the cholinergic system is implicated in modulating binocular interactions in the human visual cortex. Potentiating the transmission of acetylcholine (ACh) via the cholinergic drug donepezil reduces the extent to which the eyes compete for perceptual dominance when presented two separate, incongruent images.

Purpose: Stereopsis depends on horizontally disparate retinal images but otherwise concordance between eyes. Here we investigate the effect of spherical and meridional simulated anisometropia and aniseikonia on stereopsis thresholds. The aims were to determine effects of meridian, magnitude and the relative effects of the two conditions. Methods: Ten participants with normal binocular vision viewed McGill modified random dot stereograms through synchronised shutter glasses. Stereoacuities were determined using a four-alternative forced-choice procedure. To induce anisometropia, trial lenses of varying power and axes were placed in front of right eyes. Seventeen combinations were used: zero (no lens) and both positive and negative, 1 and 2 D powers, at 45, 90 and 180 axes; spherical lenses were also tested. To induce aniseikonia 17 magnification power and axis combinations were used. This included zero (no lens), and 3%, 6%, 9% and 12% at axes 45, 90 and 180; overall magnifications were also tested. Results: For induced anisometropia, stereopsis loss increased as cylindrical axis rotated from 180° to 90°, at which the loss was similar to that for spherical blur. For example, for 2 D meridional anisometropia threshold increased from 1.53 log sec arc (i.e. 34 sec arc) for x 180 to 1.89 log sec arc (78 sec arc) for x 90. Anisometropia induced with either positive or negative lenses had similar detrimental effects on stereopsis. Unlike anisometropia, the stereopsis loss with induced meridional aniseikonia was not affected by axis and was about 64% of that for overall aniseikonia of the same amount. Approximately, each 1 D of induced anisometropia had the same effect on threshold as did each 6% of induced aniseikonia. Conclusion: The axes of meridional anisometropia but not aniseikonia affected stereopsis. This suggests differences in the way that monocular blur (anisometropia) and interocular shape differences (aniseikonia) are processed during the production of stereopsis.

To simulate both lens-induced and screen-induced aniseikonia, and to assess its influence on stereopsis. Additionally, to determine if screen-based size differences could neutralise the effects of lens-induced aniseikonia.A four-circle (4-C) paradigm was developed, where one circle appears in front or behind the others because of crossed or uncrossed disparity. This stereotest was used for three investigations: (1) Comparison with the McGill modified random dot stereogram (RDS), with anisometropia introduced with +2 D spheres and cylinders, and with aniseikonia introduced with 6% overall and 6% meridional (×180, ×90) magnifiers before the right eye; (2) Comparison of lens-induced and screen-induced 6% overall and meridional magnifications and (3) Determining if lens and screen effects neutralised, by opposing 6% lens-induced magnification to the right eye with screen-inducements of either 6% left eye magnification or 6% right eye minification. A pilot study of the effect of masking versus not masking the surround was also conducted.The 4-C test gave higher stereo-thresholds than the RDS test by 0.5 ± 0.2 log units across both anisometropic and aniseikonic conditions. However, variations in power, meridian and magnification affected the two tests similarly. The pilot study indicated that surround masking improved neutralisation of screen and lens effects. With masking, lens-induced and screen-induced magnifications increased stereo-thresholds similarly. With lens and screen effects opposed, for most participants stereo-thresholds returned to baseline for overall and ×180 magnifications, but not for ×90 magnification. Only three of seven participants showed good compensation for ×90 magnification.Effects of lens-induced aniseikonia on stereopsis cannot always be successfully simulated with a screen-based method. The ability to neutralise refractive aniseikonia using a computer-based method, which is the basis of digital clinical measurement, was reasonably successful for overall and ×180 meridional aniseikonia, but not very successful for ×90 aniseikonia.

Purpose We developed a stereo task that is based on a motion direction discrimination to examine the role that depth can play in disambiguating motion direction. Methods In this study, we quantified normal adults’ static and dynamic (i.e., laterally moving) stereoscopic performance using a psychophysical task, where we dichoptically presented randomly arranged, limited lifetime Gabor elements at two depth planes (one plane was at the fixation plane and the other at an uncrossed disparity relative to the fixation plane). Each plane contained half of the elements. For the dynamic condition, all elements were vertically oriented and moved to the left in one plane and to the right in another plane; for the static condition, the elements were horizontally oriented in one plane and vertically oriented in another plane. Results For the range of motion speed that we measured (from 0.17°/s to 5.33°/s), we observed clear speed tuning of the stereo sensitivity (P = 3.0 × 10−5). The shape of this tuning did not significantly change with different spatial frequencies. We also found a significant difference in stereo sensitivity between stereopsis with static and laterally moving stimuli (speed = 0.67°/s; P = 0.004). Such difference was not evident when we matched the task between the static and moving stimuli. Conclusions We report that lateral motion modulates human global depth perception. This motion/stereo constraint is related to motion velocity not stimulus temporal frequency. We speculate that the processing of motion-based stereopsis of the kind reported here occurs in dorsal extrastriate cortex.

Recent evidence suggests that the primary deficit in amblyopia is loss of binocular vision and that the loss of monocular acuity is a secondary consequence. This new understanding derived from recent laboratory studies questions the present therapy and its primary acuity-based endpoint, which have been the gold standard in the treatment of this condition for the past 200 years.

Purpose Our visual system compares the inputs received from the two eyes to estimate the relative depths of features in the retinal image. We investigated how an imbalance in the strength of the input received from the two eyes affects stereopsis. We also explored the level of agreement between different measurements of sensory eye imbalance. Methods We measured the sensory eye imbalance and stereoacuity of 30 normally sighted participants. We made our measurements using a modified amblyoscope. The sensory eye imbalance was assessed through three methods: the difference between monocular contrast thresholds, the difference in dichoptic masking weight, and the contribution of each eye to a fused binocular percept. We referred them as the "threshold imbalance," "masking imbalance," and "fusion imbalance," respectively. The stereoacuity threshold was measured by having subjects discriminate which of four circles were displaced in depth. All of our tests were performed using stimuli of the same spatial frequency (2.5 cycles/degree). Results We found a relationship between stereoacuity and sensory eye imbalance. However, this was only the case for fusion imbalance measurement (ρ = 0.52; P = 0.003). Neither the threshold imbalance nor the masking imbalance was significantly correlated with stereoacuity. We also found the threshold imbalance was correlated with both the fusion and masking imbalances (r = 0.46, P = 0.011 and r = 0.49, P = 0.005, respectively). However, a nonsignificant correlation was found between the fusion and masking imbalances. Conclusions Our findings suggest that there exist multiple types of sensory eye dominance that can be assessed by different tasks. We find only imbalances in dominance that result in biases to fused percepts are correlated with stereoacuity.

Amblyopia is a relatively common (incidence 3%) developmental disorder in which there is loss of vision as a consequence of a disruption to normal visual development. Although the deficit is monocular and known to be of cortical origin, the nature of the processing deficit is controversial. Human behavioral studies have identified two main deficits - a loss of contrast sensitivity and perceived spatial distortions. Here we use a multifocal fMRI approach to ascertain, in a group of anisometropic amblyopes, whether these two deficits have a single common cause or whether they are the result of two underlying independent cortical disorders. We found that fMRI magnitudes were attenuated in amblyopic eye stimulation, and that there was poor fidelity for co-localization of the activity clusters between the amblyopic and fellow-fixing eye stimulation. These effects varied across eccentricities and correlate with the degree of amblyopia but not with one another, suggesting two independent cortical deficits: a reduced responsiveness as well as reduced fidelity of spatial representation. These deficits are independent of eccentricity within the central field and consistent across early cortical visual areas.

The mammalian brain can take into account the neural delays in visual information transmission from the retina to the cortex when accurately localizing the instantaneous position of moving objects by motion extrapolation. In this study, we wanted to investigate whether such extrapolation mechanism operates in a comparable fashion between the eyes in normally sighted and amblyopic observers.To measure interocular extrapolation, we adapted a dichoptic version of the flash-lag effect (FLE) paradigm, in which a flashed bar is perceived to lag behind a moving bar when their two positions are physically aligned. Twelve adult subjects with amblyopia and 12 healthy controls participated in the experiment. We measured the FLE magnitude of the subjects under binocular, monocular, and dichoptic conditions.In controls, the FLE magnitude of binocular condition was significantly smaller than that of monocular conditions (P ≤ 0.023), but there was no difference between monocular and dichoptic conditions. Subject with amblyopia exhibited a smaller FLE magnitude in the dichoptic condition when the moving bar was presented to the amblyopic eye and the flash to the fellow eye (DA condition) compared to the opposite way around (DF condition), consistent with a delay in the processing of the amblyopic eye (P = 0.041).Our observations confirm that trajectory extrapolation mechanisms transfer between the eyes of normal observers. However, such transfer may be impaired in amblyopia. The smaller FLE magnitude in DA compared to DF in patients with amblyopia could be due to an interocular delay in the amblyopic visual system. The observation that normal controls present a smaller FLE in binocular conditions raises the question whether a larger FLE is or is not an indicator of better motion processing and extrapolation.

Purpose Amblyopes suffer a defect in temporal processing, presumably because of a neural delay in their visual processing. By measuring flash-lag effect (FLE), we investigate whether the amblyopic visual system could compensate for the intrinsic neural delay due to visual information transmissions from the retina to the cortex. Methods Eleven adults with amblyopia and 11 controls with normal vision participated in this study. We assessed the monocular FLE magnitude for each subject by using a typical FLE paradigm: a bar moved horizontally, while a flashed bar briefly appeared above or below it. Three luminance contrasts of the flashed bar were tested: 0.2, 0.6, and 1. Results All participants, controls and those with amblyopia, showed a typical FLE. However, the FLE magnitude of participants with amblyopia was significantly shorter than that of the control participants, for both their amblyopic eye (AE) and fellow eye (FE). A nonsignificant difference was found in FLE magnitude between the AE and the FE. Conclusions We demonstrate a reduced FLE both in the AE as well as the FE of patients with amblyopia, suggesting a global visual processing deficit. We suggest it may be attributed to a more limited spatiotemporal extent of facilitatory anticipatory activity within the amblyopic primary visual cortex.

Summary Amblyopia is a visual disorder that originates from the brain. It exhibits no pathology in the eye. Studies have shown that measuring both visual acuity and binocular balance for assessing amblyopia could be more helpful. However, tests that measure binocular balance are time-consuming, often exceeding 30 min. Their long test durations prevent them from being used in the clinic. For this reason, we have developed a quick (i.e., about 7 min) and precise tool that quantitatively measures binocular balance of patients with amblyopia. The new test can capture binocular imbalance that is typically exhibited at high spatial frequency in amblyopes. In addition, it has an excellent test-retest reliability and repeatability between two experimental sessions. We hope that our newly developed test can pave the road for physicians and researchers to better assess and diagnose amblyopia and other visual disorders that disrupt binocular balance beyond the laboratory., Graphical abstract, Highlights • Measuring binocular balance of amblyopes is difficult and time-consuming (>30 min) • We introduce a psychophysical test that is reliable and quick (7 min), Health sciences; Medical tests; Medicine

Purpose We investigate the orientation tuning of interocular suppression using a dichoptic masking paradigm in adult controls and amblyopes. Methods Fourteen adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. Contrast sensitivity was measured by presenting a target Gabor in the tested eye and mean luminance in the untested eye (monocular) and by presenting a target in the tested eye and a bandpass oriented filtered noise in the other eye (masked). Interocular suppression was defined as the thresholds difference between the monocular and masked conditions for each eye. Interocular suppression was measured under parallel and orthogonal suppression configurations. The peak spatial frequency of the target and mask was 0.25 c/d in experiment 1 (low), 1.31 c/d in experiment 2 (mid), and 6.87 c/d in experiment 3 (high). Results The masking suppression induced by the amblyopic eye was less strong than that induced by the fellow eye. The suppression from the fellow eye was similar to that observed in the controls. Interocular suppression under parallel configuration was less strong than under orthogonal configuration in amblyopes at low and mid spatial frequency, but not at high spatial frequency. Conclusions We demonstrate that the abnormal interocular masking in amblyopia displays the expected characteristic of orientation selectivity expected of normal controls at low and mid spatial frequency, but not at high spatial frequency. The dichoptic masking imbalance between the eyes of amblyopes results in a net suppression of the amblyopic eye during binocular viewing, modeling clinical suppression.

It is now accepted that short-term deprivation of one eye in adults results in not only a post-deprivation strengthening of the vision in the previously deprived eye but also a deterioration in the vision of the previously non-patched eye. Such monocular deprivation of 1-2 hours induces changes that last approximately 30-90 minutes. There is some support for this neuroplastic effect being the consequence of a change in the contrast gain within the binocular circuity. What is not known is when these changes in gain are initiated. One possibility is that they are initiated only once the patch is removed. The other possibility is that they are the result of a slow build up from the moment the patch is first applied.In this study, we measure monocular contrast detection thresholds of the non-deprived eye over time during the deprivation of the other eye. We show that contrast threshold increases over time during the deprivation of the other eye. This observation suggest that the patching effect is mediated by a slow build up over the deprivation period: reducing the vision of the non-deprived eye and enhancing the vision of the deprived eye reflecting reciprocal changes in sensitivity. These results highlight a hitherto unknown feature of human vision, namely that monocular vision per se is intrinsically unstable which is a consequence of the reciprocal inhibitory circuits that homeostatically regulate binocular vision. This questions a whole corpus of studies of visual function that rely on the assumption that monocular vision is intrinsically stable.

The input from the two eyes is combined in the brain. In this combination, the relative strength of the input from each eye is determined by the ocular dominance. Recent work has shown that this dominance can be temporarily shifted. Covering one eye with an eye patch for a few hours makes its contribution stronger. It has been proposed that this shift can be enhanced by exercise. Here, we test this hypothesis using a dichoptic surround suppression task, and with exercise performed according to American College of Sport Medicine guidelines. We measured detection thresholds for patches of sinusoidal grating shown to one eye. When an annular mask grating was shown simultaneously to the other eye, thresholds were elevated. The difference in the elevation found in each eye is our measure of relative eye dominance. We made these measurements before and after 120 minutes of monocular deprivation (with an eye patch). In the control condition, subjects rested during this time. For the exercise condition, 30 minutes of exercise were performed at the beginning of the patching period. This was followed by 90 minutes of rest. We find that patching results in a shift in ocular dominance that can be measured using dichoptic surround suppression. However, we find no effect of exercise on the magnitude of this shift. We further performed a meta-analysis on the four studies that have examined the effects of exercise on the dominance shift. Looking across these studies, we find no evidence for such an effect.