Neural activity in the primate superior colliculus and saccadic reaction times in double-step experiments

Although primates including humans can do 2-3 saccades per second while observing their environment, this seems to be more complicated when the same visual target is displaced twice in brief succession. When the subject has to follow this target with its gaze, the reaction time of the second saccade is longer than that of the first. We present data from electrophysiological recordings in the superior colliculus of a monkey that is performing a double-step saccade task. Analysis of the neuronal activity shows that the fixation neurons and the saccadic neurons respond differently in single- and double-step tasks. Fixation neurons are not as active between the two saccades as could be expected from single-step trials. Therefore, the fixation neurons are not likely to cause the increase in reaction time. The recorded saccadic neurons usually showed a presumably visual activation about 70 ms after target appearance and a motor burst starting briefly before the saccade. A target-aligned response was encountered in half of the neurons about 150 ms after the second target appearance. The early visual target-aligned response is often lost before the second saccade in a double-step task with short stimulus delay. The rise of activity was slower before the second than before the first saccade. The neural latency was therefore longer before the second saccade. The motor burst coincides with the second saccade although it is delayed. Thus, the motor burst was always predictive of the occurrence of the saccade. We conclude that the fixation neurons in the superior colliculus are not likely to cause the delay of the second saccade, and that the activity in the saccadic neurons in the superior colliculus encodes the timing of the second saccade even if it is delayed.