ISOLATION AND CHARACTERISTICS OF A STEADY-STATE VISUALLY-EVOKED POTENTIAL IN HUMANS RELATED TO THE MOTION OF A STIMULUS

We have examined the visual potential evoked by two motion stimuli. In the first stimulus (termed coherent motion) a random-dot pattern osci llated between phases of coherent and incoherent (''snowstorm'') motio n, and in the second a random-dot pattern alternated in direction of m otion (termed direction change). We found that the response to the coh erent motion stimulus is low-pass with respect to speed, has low contr ast sensitivity and increases steadily with the contrast of the stimul i. The direction change visually-evoked potential (VEP) is band-pass w ith respect to speed, has high contrast sensitivity but then saturates and even reduces as the stimulus contrast is raised above 0.1. The be haviour of the direction change VEP is similar in nature to results fr om psychophysical experiments of motion perception and to the known pr operties of directionally selective cells of the cortex. On the other hand the behaviour of the coherent motion VEP suggests this may not be mediated by a mechanism specific to motion.