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I was honoured to give a talk recently in the McGill University Quantitative Life Sciences seminar series. A recording is embedded below.

In visual psychophysics, we aim to characterise and explain human behaviour when performing a task involving a visual stimulus. One of the simplest abilities we can investigate is the detection of a low contrast target. For example, an experiment might involve asking a participant whether they can see a small pattern shown on a display. In general, when the contrast is very low the target will be too faint to be seen. By measuring detection performance as a function of contrast, one can find the contrast required for reliable detection of that target. It is generally the case that larger targets can be detected at lower contrast. The nature of the relationship between size and detectability has been investigated through studies of spatial summation. One central question has been whether the “summation” that occurs reflects an additive pooling of local responses to parts of the target. This would benefit performance by increasing the signal-to-noise ratio at the decision stage. Other “probability summation” models attribute the performance benefit to the participant having more opportunities to detect a larger target’s individual parts. In this seminar, I will discuss various simple models of summation and their implications for visual processing.

We have just published a study in Ophthalmic & Physiological Optics investigating how image distortions from aniseikonia impact stereoacuity.

Purpose: 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.

Method: 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.

Results: 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.

Conclusions: 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.

We have just published a study in PLoS ONE introducing a psychophysical method for assessing the quality of processed visual images. This publication also presents the Cartesian-Separable log-Gabor in a quite useful manner.

Image processing algorithms are used to improve digital image representations in either their appearance or storage efficiency. The merit of these algorithms depends, in part, on visual perception by human observers. However, in practice, most are assessed numerically, and the perceptual metrics that do exist are criterion sensitive with several shortcomings. Here we propose an objective performance-based perceptual measure of image quality and demonstrate this by comparing the efficacy of a denoising algorithm for a variety of filters. For baseline, we measured detection thresholds for a white noise signal added to one of a pair of natural images in a two-alternative forced-choice (2AFC) paradigm where each image was selected randomly from a set of n = 308 on each trial. In a series of experimental conditions, the stimulus image pairs were passed through various configurations of a denoising algorithm. The differences in noise detection thresholds with and without denoising are objective perceptual measures of the ability of the algorithm to render noise invisible. This was a factor of two (6dB) in our experiment and consistent across a range of filter bandwidths and types. We also found that thresholds in all conditions converged on a common value of PSNR, offering support for this metric. We discuss how the 2AFC approach might be used for other algorithms including compression, deblurring and edge-detection. Finally, we provide a derivation for our Cartesian-separable log-Gabor filters, with polar parameters. For the biological vision community this has some advantages over the more typical (i) polar-separable variety and (ii) Cartesian-separable variety with Cartesian parameters.

We are pleased to announced that the lab’s basic science research on visual perception will be supported by a recently-awarded NSERC Discovery Grant “Changes in visual processing strategy: effects of context, task, and training” from 2022-2027.

Our project in collaboration with Robert Hess and Alexandre Reynaud (PI on this grant) has received Ignite Grant funding from Healthy Brains Healthy Lives. The project will explore the development and evaluation of “Dichoptic presentation of ebooks for the treatment of amblyopia and other visual conditions”.

I am very happy to welcome Kiana Zanetti joining the Baldwin Vision Lab as an undergraduate project student in McGill’s PSYC-494 program. Her project concerns how the visual system uses “good continuation” to integrate contours.

We have just published a study in Investigative Ophthalmology & Visual Science looking at the relationship between measures of different aspects of binocular vision.

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.

I am very happy to have had Annabel Fan joining the Baldwin Vision Lab as a PhD student as of Fall 2021.

I have a deep appreciation for visual perception and the neural mechanisms underlying our conscious experiences. This interest has led me to pursue doctoral studies at McGill University where I am investigating how visual information processing changes as we get older. My research aims to help older adults drive more safely and uses behavioural methods (including having older adults use driving simulators!). I am also a firm believer in knowledge mobilisation and have experience in translating research into web and mobile applications. I hope to combine my training in neuroscience and my passion for video games and virtual reality to improve human health. Outside of work, I enjoy various artistic hobbies (illustration, painting, sewing), and am working my way through a very long book list (currently on “Atlas of AI” by Kate Crawford).

I am very pleased to welcome Jeong Ung Song who joined the Baldwin Vision Lab as a graduate student in Fall 2021.

Hello, my name is Jeong Ung Song and I am a graduate student pursuing thesis-based Master’s Degree in Neuroscience at McGill University. I completed my undergraduate degree in Psychology at the University of Alberta, and worked as a lab manager for a computational neuroscience lab. Currently, I am using psychophysics method to study a rare visual disorder called visual snow, which causes people to see numerous particles in their vision. We believe that studying visual snow will get us closer to understanding the process in our visual system which modulates internal visual noise. In my spare time, I direct theatre and recently put on a production called CHANZO at the Edmonton Fringe Festival.

I am excited to announce that Raffles Zhu has joined the Baldwin Vision Lab as a graduate student as of Fall 2021. Raffles will be co-supervised by the MVR director Robert Hess.

My name is Raffles Xingqi Zhu and I am a first-year Masters student in the Integrated Program in Neuroscience (IPN). Prior to joining Dr. Baldwin’s lab, I completed a bachelor of science in physics with a concentration in biophysics at McGill. In my undergraduate years, I was fortunate to have worked on various research projects at the interface of biological and physical sciences, ranging from constructing phylogenetic trees to studying diffusion of nanoparticles. After I stumbled upon a neuroscience course, I grew interested in the field and decided I want to pursue graduate studies in visual neurosciences. My masters project will have a basic component involving the study of blur perception and a clinical component that aims to unravel the basis of visual deficits experienced by patients with amblyopia. In my free time, I enjoy doing photography and hiking outdoors.