Target selection and saccade generation in monkey superior colliculus.

The superior colliculus (SC) of the monkey has been shown to be involved in not only initiation of saccades but in the selection of the target to which the saccade can be directed. The present experiments examine whether SC neuronal activity related to target selection is also related to saccade generation. In an asynchronous target task, the monkey was required to make a saccade to the first of two spots of light to appear. Using choice probability analysis over multiple trials, we determined the earliest time at which neurons in the SC intermediate layers indicated target selection. We then determined how closely the neuronal selection was correlated to saccade onset by using our asynchronous reaction time task, which allowed the monkey to make a saccade to the target as soon as the selection had been made. We found that the selection became evident at widely differing times for different neurons. Some neurons indicated target selection just before the saccade (close to the pre-saccadic burst of activity), others did so at the time of the visual response, and some showed an increase in their activity even before the target appeared. A fraction of this pre-stimulus bias resulted from a priming effect of the previous trial; a saccade to the target in the movement field on the previous trial produced both a higher level of neuronal activity and a higher probability for a saccade to that target on the current trial. We found that most of the neurons (76%) showed a correlation between selection time and reaction time. Furthermore, within this 76% of neurons, many indicated a selection very early during the visual response. There was no evidence of a sequence from target selection first and saccade selection later, but rather that target selection and saccade initiation are intertwined and are probably inseparable.