Oblique effect

Abstract: Visual perception critically depends on orientation-specific signals that arise early in visual processing Humans show greater behavioral sensitivity to gratings with horizontal or vertical (0 degrees /90 degrees; 'cardinal') orientations than to other, 'oblique' orientations Here we used functional magnetic resonance imaging (fMRI) to measure an asymmetry in the responses of human primary visual cortex (V1) to oriented stimuli We found that neural responses in V1 were larger for cardinal stimuli than for oblique (45 degrees /135 degrees ) stimuli Thus the fMRI pattern in V1 closely resembled subjects' behavioral judgments; responses in V1 were greater for those orientations that yielded better perceptual performance

Abstract: The details of oriented visual stimuli are better resolved when they are horizontal or vertical rather than oblique. This "oblique effect" has been confirmed in numerous behavioral studies in humans and to some extent in animals. However, investigations of its neural basis have produced mixed and inconclusive results, presumably due in part to limited sample sizes. We have used a database to analyze a population of 4,418 cells in the cat's striate cortex to determine possible differences as a function of orientation. We find that both the numbers of cells and the widths of orientation tuning vary as a function of preferred orientation. Specifically, more cells prefer horizontal and vertical orientations compared with oblique angles. The largest population of cells is activated by orientations close to horizontal. In addition, orientation tuning widths are most narrow for cells preferring horizontal orientations. These findings are most prominent for simple cells tuned to high spatial frequencies. Complex cells and simple cells tuned to low spatial frequencies do not exhibit these anisotropies. For a subset of simple cells from our population (n = 104), we examined the relative contributions of linear and nonlinear mechanisms in shaping orientation tuning curves. We find that linear contributions alone do not account for the narrower tuning widths at horizontal orientations. By modeling simple cells as linear filters followed by static expansive nonlinearities, our analysis indicates that horizontally tuned cells have a greater nonlinear component than those tuned to other orientations. This suggests that intracortical mechanisms play a major role in shaping the oblique effect.