Anti-correlation between natural scene orientation structure and activity in visual cortex

Abstract

Representing the orientation of features in the visual image is a fundamental operation of the early cortical visual system. Using functional magnetic resonance imaging (fMRI), we have previously shown that the magnitude of activity evoked by stimuli of differing orientation is anisotropic in human visual cortex (Mannion, McDonald, & Clifford, 2010). Oblique grating orientations evoked greater magnitudes of activity than horizontal and, to a lesser extent, vertical orientations as early as V1. Furthermore, sections of the visual image in which the orientation was radial to the point of fixation evoked greater magnitudes of activity than sections in which it was tangential. To investigate the relationship between such anisotropies and the spatial structure of the natural environment, we analysed a sequence of natural images aligned to the gaze of a freely-moving human observer (Schumann, Einhäuser, Vockeroth, Bartl, Schneider, & König, 2008). We find an over-representation of orientations that are horizontal and, to a lesser extent, vertical and orientations that are tangential to the point of fixation. Hence, the prevalence of orientations in the natural environment is anti-correlated with the magnitude of evoked V1 activity. This relationship between visual ecology and visual sensitivity suggests that processing in V1 serves to equalize the output of orientation-selective mechanisms by anticipating a diet of retinal image structure characteristic of free-viewing of the natural environment.

Acknowledgments

Supported by an Australian Postgraduate Award to DM, an Australian Research Fellowship to CC, and by grants from the Australian Research Council and the National Health and Medical Research Council.