Abstract
It is important to detect meaningful objects rapidly and accurately in a natural scene, which is typically cluttered with many distracting stimuli. Here we focus on image statistics that concentrically orientated contours become prevalent as a meaningful object enters the foveal region of the visual field during visual search. This is so because typical meaningful objects (e.g., fruits in foraging context or human faces in social context) are likely to have edges consisting of concentric contours. One possible strategy of exploiting this natural prior at the low level of visual system is to enhance the salience of concentric contours either by facilitating neural responses to those contours, or by suppressing responses to distractors, or both. To explore these neural implications of concentric contour processing, we acquired fMRI response timeseries of human early visual cortex (V1–3, n=29) to concentrically and radially oriented Gabor patches that drift slowly along the polar axes over the visual field (8° in eccentricity). We then compared the amounts of surround suppression between the ‘concentric’ and ‘radial’ conditions by fitting a difference-of-Gaussian receptive-field model (Dumoulin and Wandell, 2012) to the fMRI timeseries separately to concentric and radial Gabor patches. Surround suppression effects were greater in the concentric than in the radial condition. The multiple regression analysis further showed that the concentric bias in suppression was quite robust yet substantively modulated by the retinotopic position of local cortical sites: the concentric bias was pronounced around the foveal region and along the oblique meridian in the lower visual field. Our results suggest that the visual cortical system forms salient representations of concentrically oriented edges at its earliest stage by augmenting its suppressive responses to the surround.
Acknowledgement: Supported by the National Research Foundation of Korea (NRF-2015M3C7A1031969, NRF-2018R1A4A1025891)