IMPAIRED VISUAL-MOTION PERCEPTION IN THE CONTRALATERAL HEMIFIELD FOLLOWING UNILATERAL POSTERIOR CEREBRAL-LESIONS IN HUMANS

Contrast thresholds for a number of tasks were measured in the contral ateral and ipsilateral upper quadrants of the visual field (eccentrici ty = 10 degrees) before and after an occipito-parietal surgical resect ion, in one patient, carried out for intractable epilepsy. Postoperati vely the contrast thresholds for discriminating the speed of movement of drifting sine-wave gratings were elevated by greater than a log uni t in the contralateral field with little or no change in the detection thresholds for the same stimuli. Contrast thresholds for opposite dir ection-of-motion (DOM) discrimination of a contrast modulated (CMod) g rating (a 'non-Fourier' motion stimulus) were also elevated by about a log unit in the contralateral hemifield but thresholds for DOM discri mination of a sine-wave (luminance modulated, LMod) grating were unaff ected. Contrast thresholds for orientation discrimination of stationar y gratings (a non-motion task) were unaffected. This general pattern o f results was found in two other patients following lateral occipital surgical resections. Eight other patients with occipito-temporal (two cases), parietal (three cases) and medial occipital lobe lesions (thre e cases) showed no difference between the two hemifields on any of the tasks. Comparison of the location of the lesions leads to the conclus ion that damage to the lateral occipital gyri is responsible for the p attern of visual deficit observed. Damage to an extra-striate visual a rea concerned with motion perception (the human homologue of primate V S-MT) may have occurred. There has been no previous description of imp airment of motion perception localized to a hemifield in humans. The c haracteristics of the residual motion perception in these cases is des cribed further in the accompanying article [Plant and Nakayama (1993), Brain, 116, 1337-1353].