Biology subjects

Harrison, W.

Publications and source records attributed to Harrison, W..

3 recordsLinked to original sources

bioRxiv2023

Neural tuning instantiates prior expectations in the human visual system

Perception is often modelled as a process of active inference, whereby prior expectations are combined with noisy sensory measurements to estimate the structure of the world. This mathematical framework has proven critical to understanding perception, cognition, motor control, and social interaction. While theoretical work has shown how priors can be computed from environmental statistics, their neural instantiation could be realised through multiple competing encoding schemes. Using a data-driven approach, here we extract the brains representation of visual orientation and compare this with simulations from different sensory coding schemes. We found that the tuning of the human visual system is highly conditional on stimulus-specific variations in a way that is not predicted by previous proposals. We further show that the adopted encoding scheme effectively embeds an environmental prior for natural image statistics within the sensory measurement, providing the functional architecture necessary for optimal inference in the earliest stages of cortical processing.

neuroscience↗

bioRxiv2018

Border-ownership-dependent tilt aftereffect for shape defined by binocular disparity and motion parallax

Discerning objects from their surrounds (i.e., figure-ground segmentation) in a way that guides adaptive behaviours is a fundamental task of the brain. Neurophysiological work has revealed a class of cells in the macaque visual cortex that may be ideally suited to support this neural computation: border-ownership cells (Zhou, Friedman, & von der Heydt, 2000). These orientation-tuned cells appear to respond conditionally to the borders of objects. A behavioural correlate supporting the existence of these cells in humans was demonstrated using two-dimensional luminance defined objects (von der Heydt, Macuda, & Qiu, 2005). However, objects in our natural visual environments are often signalled by complex cues, such as motion and depth order. Thus, for border-ownership systems to effectively support figure-ground segmentation and object depth ordering, they must have access to information from multiple depth cues with strict depth order selectivity. Here we measure in humans (of both sexes) border-ownership-dependent tilt aftereffects after adapting to figures defined by either motion parallax or binocular disparity. We find that both depth cues produce a tilt aftereffect that is selective for figure-ground depth order. Further, we find the effects of adaptation are transferable between cues, suggesting that these systems may combine depth cues to reduce uncertainty (Bulthoff & Mallot, 1988). These results suggest that border-ownership mechanisms have strict depth order selectivity and access to multiple depth cues that are jointly encoded, providing compelling psychophysical support for their role in figure-ground segmentation in natural visual environments.\n\nSIGNIFICANCE STATEMENTSegmenting a visual object from its surrounds is a critical function that may be supported by \"border-ownership\" neural systems that conditionally respond to object borders. Psychophysical work indicates these systems are sensitive to objects defined by luminance contrast. To effectively support figure-ground segmentation, however, neural systems supporting border-ownership must have access to information from multiple depth cues and depth order selectivity. We measured border-ownership-dependent tilt aftereffects to figures defined by either motion parallax or binocular disparity and found aftereffects for both depth cues. These effects were transferable between cues, but selective for figure-ground depth order. Our results suggest that the neural systems supporting figure-ground segmentation have strict depth order selectivity and access to multiple depth cues that are jointly encoded.

neuroscience↗

bioRxiv2017

Voluntary control of illusory contour formation

The extent to which visual inference is shaped by attentional goals is unclear. Voluntary attention may simply modulate the priority with which information is accessed by higher cognitive functions involved in perceptual decision making. Alternatively, voluntary attention may influence fundamental visual processes, such as those involved in segmenting an incoming retinal signal into a structured scene of coherent objects, thereby determining perceptual organisation. Here we tested whether the segmentation and integration of visual form can be determined by an observers goals by exploiting a novel variant of the classical Kanizsa figure. We generated predictions about the influence of attention with a machine classifier, and tested these predictions with a psychophysical response classification technique. Despite seeing the same image on each trial, observers perception of illusory spatial structure depended on their attentional goals. These attention-contingent illusory contours directly conflicted with equally plausible visual form implied by the geometry of the stimulus, revealing that attentional selection can determine the perceived layout of a fragmented scene. Attentional goals, therefore, not only select pre-computed features or regions of space for prioritised processing, but, under certain conditions, also greatly influence perceptual organisation and thus visual appearance.\n\nSIGNIFICANCE STATEMENTThe extent to which higher cognitive functions can influence perceptual organisation is debated. The role of voluntary spatial attention, the ability to focus on only some parts of a scene, has been particularly controversial among neuroscientists and psychologists who aim to uncover the basic neural computations involved in grouping image features into coherent objects. To address this issue, we repeatedly presented the same novel ambiguous image to observers and changed their attentional goals by having them make fine spatial judgements about only some elements of the image. We found that observers attentional goals determine the perceived organisation of multiple illusory shapes. We thus reveal that voluntary spatial attention can control the fundamental processes that determine perceptual organisation.

neuroscience↗