INVESTIGATION OF A PATIENT WITH SEVERELY IMPAIRED DIRECTION DISCRIMINATION - EVIDENCE AGAINST THE INTERSECTION-OF-CONSTRAINTS MODEL

A man with presumed posterior cortical atrophy had a markedly elevated threshold for orientation discrimination (approx. 25 deg) and selecti ve impairment of ''pop-out'' tasks based on orientation. Direction dis crimination for moving plaids was superior to direction discrimination for their component gratings. The superior performance for plaids dis appeared when the spatial frequencies of the component gratings were a ltered to eliminate coherence. This finding implies that extraction of plaid motion is not dependent on pre-processing within narrow spatial frequency bands. It is inconsistent with simulations based on the ''i ntersection of constraints'' model, which predict that the error rate for plaids would be larger than the error rate for gratings, particula rly for the plaids composed of gratings moving at nearly opposing angl es. It is consistent with models such as the Heeger [(1987) Journal of the Optical Society of America A, 4, 1455-1471] model, which extract direction from the pattern of activity across broadly-tuned spatiotemp oral filters.