THE ROLE OF STRIATE CORTEX IN VISUAL FUNCTION OF THE CAT

We examined the contribution of area 17 to visual function in two cats whose fixation was monitored by means of scleral search coils. Iboten ic acid lesions were made within the physiologically identified repres entation of the lower left visual field of area 17. In a detection tas k in which the cats simply indicated the presence or absence of a vert ical grating, contrast sensitivity loss was greatest at middle spatial frequencies with no loss in spatial resolution. However, when cats we re required to discriminate between vertical and horizontal gratings, sensitivity loss was profound at both middle and high spatial frequenc ies with an octave loss of spatial resolution. This greater loss was n ot due to disrupted orientation discrimination since sensitivity to th e orientation of coarse gratings was unaffected in the lesioned hemifi eld. We also found deficits in the ability to discriminate the directi on of grating motion, but only at higher spatial and lower temporal fr equencies. The role of area 17 in perceiving the global motion of comp lex patterns was also studied with high contrast, dynamic random dots drifting at high speeds. Paradoxically, area 17 lesion improved. the p erception of global motion. This improvement was eliminated by spatial ly filtering the dot patterns to remove high spatial frequencies, sugg esting that the lesion has enhanced performance by interfering with ma sking by high spatial frequencies. Our results demonstrate that the pe rformance of traditional detection tasks may be insensitive to the eff ects of area 17 lesions. Discrimination tasks, on the other hand, reve aled that area 17 neurons play a major role in the perception of highe r spatial frequency stimuli as long as they move or flicker at low rat es, but contribute little to these functions when the stimuli are coar se and move at high speeds.