A deficit perceiving slow motion after brain damage and a parallel deficit induced by crowding.

Motion perception is known to involve at least 2 kinds of mechanisms-lower level signal detectors and higher level algorithms for comparing object positions over time. When stimulus motion is modal (continuously visible), it is generally assumed that processing via lower level mechanisms is sufficient to make accurate motion judgments. We investigated the possibility that higher level mechanisms may also be involved in the processing of slow motion, even when it is smooth and continuous. This possibility was suggested by results from a brain-damaged patient, JKI, who showed left visual field deficits in both the explicit representation of object position and judgments concerning the direction of slow, but not fast, smooth motion. We investigated the possibility further by using crowding to induce a behaviorally similar motion-perception deficit in healthy observers. Crowding, which is known to impair object-position representation, impaired direction judgments for slow, but not for faster, smooth motion. The results suggest an everyday role for higher level mechanisms in the perception of slow motion, and they reinforce the taxonomy of motion perception in terms of underlying processing mechanisms as opposed to stimulus properties.