DETECTION AND DISCRIMINATION OF FIRST-ORDER AND 2ND-ORDER MOTION IN PATIENTS WITH UNILATERAL BRAIN-DAMAGE

The present investigation explored the extent to which extrastriate co rtex is necessary for various aspects of motion processing and whether the processing of first-order (Fourier) and second-order (non-Fourier ) motion involves the same extrastriate cortical regions. Orientation, direction, and speed discrimination thresholds were measured in 21 pa tients with unilateral damage to the lateral occipital, temporal, or p osterior parietal cortex. Their results were compared with those of 14 age-matched control subjects. The stimuli were static random-dot nois e patterns, the luminance of which (first-order) or contrast (second-o rder) was modulated by a drifting sinusoid. Each image was presented a t an eccentricity of 5.6 deg in one of the four visual quadrants. The contrasts required to identify orientation and direction were measured in a forced-choice paradigm for three speeds (1.5, 3, and 6 deg/sec). Speed discrimination performance was measured for stimuli presented s imultaneously in two of the four quadrants. The results indicate the f ollowing: (1) orientation thresholds were increased only slightly in t he patients; (2) direction thresholds were modestly elevated, and this effect was more pronounced for second-order stimuli than for first-or der stimuli; (3) speed discrimination thresholds were elevated signifi cantly in the patients with lesions in the region bordering superior-t emporal and lateral-occipital cortex; and (4) speed discrimination thr esholds for first-order stimuli were more elevated than those for seco nd-order stimuli. The results suggest that there is substantial overla p in the cortical areas involved in first- and second-order speed disc rimination.