Magnetic response of human extrastriate cortex in the detection of coherent and incoherent motion

Although direction selectivity is a cardinal property of neurons in the vis ual motion detection system, movement of numerous elements without global d irection (incoherent motion) has been shown to activate human and monkey vi sual systems, as does coherent motion which has global direction. We used m agnetoencephalography to investigate the neural process underlying response s to these types of motions in the human extrastriate cortex. Both motions were created using a random dot kinematogram and four speeds (0, 0.6, 9.6 a nd 25 degrees/s). The visual stimuli were composed of two successive motion s at different speeds; a coherent motion at a certain speed that changed to incoherent motion at another speed or vice versa. Magnetic responses to th e change in motion consisted of a few components, the first of which was al ways largest. The peak latency of the first component was inversely related to the speed of the preceding motion, but for both motions it was not affe cted by the speed of the subsequent motion. For each subject, the estimated origin of the first component was always in the extrastriate cortex, and t his changed with the speed of the preceding motion. For both motions, the l ocation for the slower preceding motion was lateral to that for the faster preceding motion. Although the latency changes of the two motions differed, their overall response properties were markedly similar. These findings show that the speed of incoherent motion is represented in t he human extrastriate cortex neurons to the same degree as coherent motion. We consider that the human visual system has a distinct neural mechanism t o perceive random dots' motion even though they do not move in a specific d irection as a whole. (C) 2000 IBRO. Published by Elsevier Science Ltd.