The contribution of cat area 18 to spatiotemporal sensitivity and to motion processing was assessed in cats with unilateral ibotenic acid lesions placed in physiologically identified portions of area 18. The lesions were centered in the representation of the lower right visual field, about 10 degrees from the vertical meridian. In one of the animals, the lesion invaded a small portion of area 19. We measured detectability of various spatiotemporal stimuli placed within the lesioned and intact portions of the visual field, while monitoring eye position with a scleral search coil. We found a loss of sensitivity to gratings of low and intermediate spatial frequency, within the ablated portion of the visual field. The sensitivity loss was 0.6–1.0 log units at low and intermediate spatial frequencies, and decreased at higher frequencies with the resolution limits remaining intact. The loss extended over a range of temporal frequencies for both drifting gratings and grating modulated in counterphase. We also found that within the lesioned hemifield, the cats were unable to discriminate between rightward and leftward motion even at the highest contrasts. These results demonstrate that area 18 plays an important role in detecting drifting low- and intermediate-spatial-frequency targets and is likely to represent a critical stage in the cortical processing of motion signals.