Weakened feedback abolishes neural oblique effect evoked by pseudo-natural visual stimuli in area 17 of the cat

The psychological oblique effect, a well-known phenomenon that humans and some mammals are more visually sensitive to cardinal (vertical and horizontal) contours than to oblique ones, has commonly been associated with the overrepresentation of cardinal orientations in the visual cortex. In contrast to the oblique effect, however, Essock et al. [E.A. Essock, J.K. DeFord, B.C. Hansen, M.J. Sinai, Oblique stimuli are seen best (not worst!) broad-band stimuli: a horizontal effect, Vision Res. 43 (2003) 1329–1335] reported a psychological ‘horizontal effect’, in which visual stimuli dominated by oblique orientations were best perceived by human subjects when tested with unique natural broad-band stimuli. In this study, using optical imaging and the similar visual stimuli, we found an overrepresentation of cardinal orientations, i.e. the neural oblique effect, but not ‘horizontal effect’, in area 17 of the cat. In addition, the oblique effect was abolished by GABA administration in area 21a due to the preferred orientation shifting (6.0%) and decrease of orientation selectivity strength of neurons (26.9%) in area 17. These results indicate a neuronal basis of the oblique effect when animals watch a more natural scene, whereas no evidence was found for the ‘horizontal effect’.