Abstract
Modulation of a neuron’s responses by the stimuli presented outside of its classical receptive field is ubiquitous in the visual system. This “surround modulation” mechanism is believed to be critical for efficient processing and leads to many well-known perceptual effects. The details of surround modulation, however, are still not fully understood. One of the open questions is related to the differences in surround modulation mechanisms in different cortical areas, and their interactions. Here we study patterns of surround modulation in primary visual cortex (V1) and middle temporal complex (hMT+) utilizing a well-studied effect in motion perception, where human observers’ ability to discriminate the drift direction of a grating improves as its size gets bigger if the grating has a low contrast, and deteriorates if it has a high contrast. We first replicated the findings in the literature with a behavioral experiment using small and large (1.67 and 8.05 degrees of visual angle) drifting gratings with either low (2%) or high (99%) contrast presented at the periphery. Next, using functional MRI, we found that in V1 with increasing size cortical responses increased at both contrast levels. Whereas in hMT+ with increasing size cortical responses remained unchanged or decreased at high contrast, and increased at low contrast, reflecting the perceptual effect. We also show that the divisive normalization model successfully predicts these activity patterns, and establishes a link between the behavioral results and hMT+ activity. We conclude that surround modulation patterns in V1 and hMT+ are different, and that the size-contrast interaction in motion perception is likely to originate in hMT+.
Highlights
Visual neurons respond to stimuli only within their classical receptive fields (RF) when these stimuli are presented in isolation
We compared the magnitudes of BOLD responses between small and large gratings at two contrast levels within predefined region of interest (ROI) that correspond to the location and size of small stimuli (i.e. “center”)
Because our goal here is to measure the modulatory effect of surround stimulation on the responses of the neurons whose classical RF centers are inside the visual space that correspond to the small grating, BOLD response differences evoked by presenting the large and small-sized stimuli would highlight the suppressive or facilitative influence of the surround on the center
Summary
Visual neurons respond to stimuli only within their classical receptive fields (RF) when these stimuli are presented in isolation. The RF and its surround are stimulated together, the response patterns of the neurons alter This kind of surround modulation is found in many levels of the visual hierarchy (Angelucci et al, 2017). Even though surround modulation is heavily studied in primary visual cortex (V1), it is not clear whether the basic principles of the mechanism remains the same for a variety of stimuli in other visual areas. To tackle this question we used a well-known perceptual effect in motion perception and investigated surround modulation in V1 and human middle temporal complex (hMTþ)
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Published Version
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