Electrical responses to short-range kinematogram displays: an occipital lobe global motion process in humans?

Abstract

In a series of studies Manning et al. have investigated visual evoked potentials (VEPs) in response to different “classes” of apparent motion (AM) displays; shortrange (random-dot kinematograms, Manning, Finlay 8z Fulham, 1991), long-range (simple two-element displays, Manning, Finlay & Fenelon, 1988; Manning 8z Finlay, 1989a; Finlay & Manning, 1991), and rotary (the “waggon-wheel effect”, Manning & Finlay, 1989b). Short-range AM was investigated using dynamic kinematogram displays very similar to those used by Braddick (1974). VEPs were recorded in a number of experimental conditions in which the magnitude of the lateral displacement of a central rectangular region was varied (Manning et al., 1991). A number of VEP components were identified, and each of these was focussed over midline occipital cortex. For one of the components, amplitudes were significantly lower in response to lateral displacements within the limits of “short-range AM” than in response to lateral displacements beyond the limits (beyond D,,). In addition to recording VEPs from kinematograms in which the subjective percept of motion was manipulated by varying the magnitude of displacement, Manning ef al. (1991) also made use of the bistable nature of AM effects to record VEPs associated with different perceptual states. For most AM displays, an identical set of stimulus parameters may produce either the strong impression of a single object in motion, or alternatively may fail to produce such a percept. In the case of random dot kinematograms an identical stimulus arrangement may produce either the impression of a single moving object (the rectangular displacement region of dots), or may produce the impression of incoherent movement of individual stimulus elements in all directions within the displacement region. The loss in identity of the single object in motion (for long-range AM) has been termed the “AM breakdown effect” (Finlay & von Griinau, 1987). Using this breakdown effect Manning et al. (1991) presented a single subject with a random dot kinematogram display and recorded VEPs from a single