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In October 2024, we published a study evaluating the reliability of the Dichoptic Contrast Ordering Test (DiCOT) as a measure of imbalances in binocular vision, comparing its results with those of the Dichoptic Letter Test (DLT).

In binocular vision, the relative strength of the input from the two eyes can have significant functional impact. These inputs are typically balanced; however, in some conditions (e.g., amblyopia), one eye will dominate over the other. To quantify imbalances in binocular vision, we have developed the Dichoptic Contrast Ordering Test (DiCOT). Implemented on a tablet device, the program uses rankings of perceived contrast (of dichoptically presented stimuli) to find a scaling factor that balances the two eyes. We measured how physical interventions (applied to one eye) affect the DiCOT measurements, including neutral density (ND) filters, Bangerter filters, and optical blur introduced by a +3-diopter (D) lens. The DiCOT results were compared to those from the Dichoptic Letter Test (DLT). Both the DiCOT and the DLT showed excellent test–retest reliability; however, the magnitude of the imbalances introduced by the interventions was greater in the DLT. To find consistency between the methods, rescaling the DiCOT results from individual conditions gave good results. However, the adjustments required for the +3-D lens condition were quite different from those for the ND and Bangerter filters. Our results indicate that the DiCOT and DLT measures partially separate aspects of binocular imbalance. This supports the simultaneous use of both measures in future studies.

In September 2024, we published a study in which we evaluated a contrast-rebalanced dichoptic cartoon treatment for childhood amblyopia.

We developed and tested a dichoptic treatment designed for younger children that can be viewed freely and involves a dichoptic manipulation of a popular animation series that enables contrast-rebalancing without disrupting fusion. Our aim was to assess whether this novel amblyopia treatment is superior to patching in children aged 3-5 years

I am very pleased to welcome Carson Cajka who joined the Baldwin Vision Lab as a graduate student in Fall 2024.

My name is Carson Cajka, and I am a first year MSc student in the Integrated Program in Neuroscience at McGill University. I graduated from Wake Forest University with a Bachelors of Science in Biophysics with a minor in neuroscience. I am interested in applying computational approaches to neural mechanics. My current project uses psychophysical methods to better understand coherence texture discrimination. This should shed some light on the murkier areas of texture perception, as coherence texture is relatively understudied. I am also an avid outdoorsman, so I like to recharge my mental batteries by escaping the city. In the future I’d like to apply my talents to solving biological problems using biophysical methods and computational analytics.

In June 2024, we published a study investigating stereo-anomalies in binocular vision, finding that many participants who could detect binocular disparity had difficulty discriminating its direction.

Within the population of humans with otherwise normal vision, there exists some proportion whose ability to perceive depth from binocular disparity is poor or absent. The prevalence of this “stereo-anomaly” has been reported to be as small as 2%, or as great as 30%. We set out to investigate this discrepancy. We used a digital tool to measure stereoacuity in tasks requiring either the detection of disparity or the discrimination of the direction of disparity. In a cohort of 228 participants, we found that 98% were able to consistently perform the detection task. Of these, only 69% consistently performed the discrimination task. The 31% of participants who had difficulty with the discrimination task could further be divided into 17% who were consistently unable to perform the task and 14% who showed limited ability. This suggests that identification of the direction of disparity requires further processing beyond merely detecting its presence.

We have just published a pre-print in bioRxiv on the work from my graduate student Raffles Zhu.

In human vision, the retinal input is transformed into internal representations through a series of stages. In earlier stages, the signals from a particular visual field locus are passed in parallel from one visual processing area to the next. The connections at each stage may therefore introduce “error”, where incorrect or convergent projections result in a loss of spatial precision. Psychophysical and physiological studies have implicated spatial scrambling of this sort as a cause of the visual deficits in amblyopia. Several methods to measure scrambling (both in amblyopia and in healthy vision) have been developed in recent decades. In this work, we introduce a new approach. We consider two stages of visual processing where scrambling may occur: either at the input to or the output from the simple cell stage in V1. We refer to these as “subcortical” and “cortical” scrambling respectively. We investigated the impact of these two types of scrambling on a letter identification task. A physiologically-inspired decomposition and resynthesis algorithm was used to generate letter stimuli that simulate scrambling at each of these two stages. To establish a performance benchmark, we trained separate Convolutional Neural Networks (CNNs) to perform the task with each scrambling type. Comparing CNN performance against that of eight humans with normal healthy vision, we found humans exhibited greater resilience to subcortical scrambling compared to cortical scrambling. We further investigated performance by comparing confusion matrices. Compared to a simple template matching model, we found the human strategy to be more consistent with our CNNs. We conclude: i) the human resilience for subcortical scrambling suggests this may be the stage at which a greater degree of scrambling is introduced in the visual hierarchy, and ii) humans employ flexible strategies for identifying scrambled stimuli, more sophisticated than a simple template match to the expected target.

We have just published a study in Frontiers in Neuroscience looking at how the relationship between different measures of binocular function changes with aging.

Introduction: Changes in vision that occur in normal healthy aging can be seen in fundamental measures of monocular vision. However, the nature of the changes in binocular vision with age remain unclear.

Methods: A total of 28 older (53–66 years) and 28 younger adults (20–31 years) were enrolled in this study. We performed a battery of tests to assess differences in monocular contrast thresholds and various binocular visual functions including dichoptic masking weight and strength, the binocular balance point for fused stimuli, and stereoacuity in the aging and control groups.

Results: Aging significantly increased monocular contrast thresholds (p < 0.001). Although this suggests that aging reduces the effective “input gain” to vision, we also found a significantly elevated contribution of those weaker signals to interocular suppression (p < 0.001). Consequently, there was no significant net difference in the strength of interocular suppression (p = 0.065). We did not find a significant difference of absolute balance point between the two groups (p = 0.090). Lastly, the mean stereoacuity was worse in the older group compared to the younger group (p = 0.002).

Discussion: Our findings confirm previous results showing differences in contrast sensitivity and stereoacuity with aging. Furthermore, we find a change in interocular suppression that is a possible consequence of the change in contrast sensitivity. It is suggestive of a cortical system that maintains a homeostatic balance in interocular suppression across the lifespan.

We are happy to announce Savannah Dunberry is joining the lab as an undergraduate project student in McGill’s PSYC-396 course. Her project concerns mapping sensitivity to both achromatic luminance and isoluminant colour contrast across the visual field.

We have just published a pre-print in bioRxiv in collaboration with colleagues from Wenzhou Medical University.

Within the population of humans with otherwise normal vision, there exists some proportion whose ability to perceive depth from binocular disparity is poor or absent. The prevalence of this “stereoanomaly” has been investigated in previous studies, some finding the proportion to be as small as 2%, others finding it to be as great as 30%. In this study, we set out to investigate the possible reason for the wide range of results found in these studies. We used a digital stereoacuity measurement tool that could measure performance in tasks requiring either the detection of disparity or the discrimination of the sign of disparity. The stimulus design was otherwise similar between the two tasks. In a cohort of 228 participants, we found that 98% were able to consistently perform the detection task. In contrast, only 69% consistently performed the discrimination task. The 31% of participants who had difficulty with the discrimination task could further be divided into 17% who were consistently unable to perform the task (seeming to behave at chance), and 14% who showed some ability to perform the task. We propose that the greater prevalence of stereo-anomaly is revealed when tasks require the judgement of the direction of disparity.

We have just published a study in Vision Research using a noise-masking approach to investigate stereopsis in amblyopia.

People with amblyopia demonstrate a reduced ability to judge depth using stereopsis. Our understanding of this deficit is limited, as standard clinical stereo tests may not be suited to give a quantitative account of the residual stereo ability in amblyopia. In this study we used a stereo test designed specifically for that purpose. Participants identified the location of a disparity-defined odd-one-out target within a random-dot display. We tested 29 amblyopic (3 strabismic, 17 anisometropic, 9 mixed) participants and 17 control participants. We obtained stereoacuity thresholds from 59% of our amblyopic participants. There was a factor of two difference between the median stereoacuity of our amblyopic (103 arcsec) and control (56 arcsec) groups. We used the equivalent noise method to evaluate the role of equivalent internal noise and processing efficiency in amblyopic stereopsis. Using the linear amplifier model (LAM), we determined the threshold difference was due to a greater equivalent internal noise in the amblyopic group (238 vs 135 arcsec), with no significant difference in processing efficiency. A multiple linear regression determined 56% of the stereoacuity variance within the amblyopic group was predicted by the two LAM parameters, with equivalent internal noise predicting 46% alone. Analysis of control group data aligned with our previous work, finding that trade-offs between equivalent internal noise and efficiency play a greater role. Our results allow a better understanding of what is limiting amblyopic performance in our task. We find this to be a reduced quality of disparity signals in the input to the task-specific processing.

We are happy to have Linda Wang joining us as an undergraduate project student in McGill’s COGS-444 program. Her project concerns contour integration, specifically the ability of the visual system to integrate contours formed from isoluminant variations in colour.