Contrast independence of cardinal preference: stable oblique effect
in orientation maps of the ferret visual cortex
A. Grabska-Barwinska, C. Distler, K.-P. Hoffmann & D. Jancke
Eur.J.Neurosci. 29: 1258-1270, 2009
- The oblique effect was first described as enhanced detection and discrimination of cardinal orientations compared
with oblique orientations. Such biases in visual processing are believed to originate from a functional adaptation
to environmental statistics dominated by cardinal contours. At the neuronal level, the oblique orientation effect
corresponds to the numerical overrepresentation and narrower tuning bandwidths of cortical neurons representing
the cardinal axes. The anisotropic distribution of orientation preferences over large cortical regions was revealed
with optical imaging, providing further evidence for the cortical oblique effect in several mammalian species.
Our present study explores whether the dominant representation of cardinal contours persists at different stimulus
contrasts. Performing intrinsic optical imaging in the ferret visual cortex and presenting drifting gratings at
various orientations and contrasts (100%, 30% and 10%), we found that the overrepresentation of vertical and horizontal
contours was invariant across stimulus contrasts. In addition, the responses to cardinal orientations were also
more robust and evoked larger modulation depths than responses to oblique orientations. We conclude that orientation
maps remain constant across the full range of contrast levels down to detection thresholds. Thus, a stable layout
of the functional architecture dedicated to processing oriented edges seems to reflect a fundamental coding strategy
of the early visual cortex.
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