Abstract
A neurological patient (L.M.) suffering a specific loss of visual motion perception (Zihl et al., 1983) due to extrastriate cortical damage was studied using random dot “limited-lifetime” stimuli with a direction discrimination task. With a stimulus like that of Newsome and Pare (1988), the patient exhibited a severe deficit for motion perception, only being able to perform well for very high values of coherence. Different versions of the stimulus were employed to separate out the effects of limited lifetime versus the effects of additive noise as coherence was lowered. When all “signal” dots had a fixed, specified value of lifetime, and varying percentages of “noise” dots were added, the patient showed a profound deficit. In contrast, a stimulus consisting of no noise dots at all, and signal dots having fixed values of lifetime, revealed relatively good performance for surprisingly brief dot lifetimes. Thus, it is the presence of noisy, incoherent dot motion, rather than brief lifetimes, that causes such poor performance on the stimulus of Newsome and Pare (1988). Most surprising was the finding that the presence of even very small percentages of stationary noise dots was sufficient to disrupt totally direction discrimination of moving signal dots. The findings reported here suggest that one major role of extrastriate cortical processing might be the interpretation of stimuli that suffer from an impaired signal-to-noise ratio; the most commonly encountered form of “noise” would presumably be contamination by irrelevant directional spatio- temporal frequency components.
