RESPONSES OF PRIMATE AREA MT DURING THE EXECUTION OF OPTOKINETIC NYSTAGMUS AND AFTERNYSTAGMUS

The directional selectivity of the visual response properties was dete rmined in 148 neurons, all located in area MT of three hemispheres of two macaque monkeys. The perferred direction of every neuron was obtai ned by analyzing the response obtained by a circular movement of the b ackground while the monkeys fixated a stationary target. The distribut ion of the preferred directions was isotropic and showed no ipsiversiv e bias. MT neurons were excited in a directionally selective manner du ring the execution of optokinetic nystagmus, in a similar way to that produced by visual stimulation during fixation. The majority of neuron s showed a sensitivity to the velocity of retinal image slip. Activity during the execution of optokinetic nystagmus could be traced back to residual retinal image slip in the direction of optokinetic stimulati on. No dynamic effects of the neuronal activity during the build-up of eye velocity in early optokinetic nystagmus were observed. Obviously, the activity in area MT did not reflect the charging of the velocity storage mechanism. Accordingly, following the cessation of stimulation , the activity dropped to the level of spontaneous activity and did no t parallel the execution of optokinetic afternystagmus. These results suggest that area MT is not part of the velocity storage mechanism and , furthermore, that the storage mechanism must be downstream of area M T in the processing of visual motion for the generation of the optokin etic nystagmus and afternystagmus.