Abstract
Single cell recording studies of motion-processing neurons in nonhuman primates provide important data with which to develop models of motion processing in the human visual system. Bearing this in mind, it is important to establish whether activity of motion-processing neurons in nonhuman primates mirrors that of human motion-processing neurons. Previous research (Snowden et al, 1991) has mapped out direction tuning of suppressive effects in macaque MT neurons. Specifically, a neuron's spiking rate in the presence of its preferred motion is suppressed when an additional direction is added to the stimulus; despite the fact that the additional motion direction causes the neuron to fire when presented in isolation. We used a motion adaptation phenomenon, the direction aftereffect (DAE), to test whether this pattern of suppression applies to human motion-sensitive neurons. Motion adapters that evoke a stronger response in neurons usually result in greater changes in the neurons' direction tuning functions, which are thought to impact on DAE magnitude. We measured DAE magnitude following adaptation to random dot kinematograms, in which either all dots moved in the same direction (45 deg clockwise from vertical up) or half had a direction of 45 deg and the other half moved in one of several other directions clockwise from 45 deg. We then measured DAE magnitude following adaptation to each of the individual directions used in the first experiment. If macaque MT is an accurate model of human motion-processing, it would predict that 1) DAE magnitude will drop off with increasing direction difference in experiment 1, and 2) additional directions causing DAE suppression will induce a measurable DAE when presented in isolation. This is precisely the pattern of results we obtained; supporting the view that the response properties of nonhuman motion-processing neurons mirror those of human motion-processing neurons.