Orientation selectivity for sinusoidal gratings; evidence for an optical component

The human visual system exhibits lower contrast thresholds for detecting vertical and horizontal gratings compared with those orientated obliquely. This is known as the oblique effect and has been thought to be due wholly to neural factors. In this study, sensitivity to gratings of different orientation is compared when viewing with small and large pupils. Optical aberrations increase with pupil size. In some subjects, we find a statistically significant change in sensitivity to some orientations for the large pupil condition. This suggests that there is an optical component to orientation selectivity induced by the higher-order aberrations of the eye.