We examined the contribution of area 17 to visual function in two cats whose fixation was monitored by means of scleral search coils. Ibotenic acid lesions were made within the physiologically identified representation of the lower left visual field of area 17. In a detection task in which the cats simply indicated the presence or absence of a vertical grating, contrast sensitivity loss was greatest at middle spatial frequencies with no loss in spatial resolution. However, when cats were required to discriminate between vertical and horizontal gratings, sensitivity loss was profound at both middle and high spatial frequencies with an octave loss of spatial resolution. This greater loss was not due to disrupted orientation discrimination since sensitivity to the orientation of coarse gratings was unaffected in the lesioned hemifield. We also found deficits in the ability to discriminate the direction of grating motion, but only at higher spatial and lower temporal frequencies. The role of area 17 in perceiving the global motion of complex patterns was also studied with high contrast, dynamic random dots drifting at high speeds. Paradoxically, area 17 lesion improved the perception of global motion. This improvement was eliminated by spatially filtering the dot patterns to remove high spatial frequencies, suggesting that the lesion has enhanced performance by interfering with masking by high spatial frequencies. Our results demonstrate that the performance of traditional detection tasks may be insensitive to the effects of area 17 lesions. Discrimination tasks, on the other hand, revealed that area 17 neurons play a major role in the perception of higher spatial frequency stimuli as long as they move or flicker at low rates, but contribute little to these functions when the stimuli are coarse and move at high speeds.