DISCHARGE OF SUPERIOR COLLICULAR NEURONS DURING SACCADES MADE TO MOVING TARGETS

1. The discharge of neurons in the deeper layers of the monkey superio r colliculus was recorded during saccades made to stationary and to sm oothly moving visual targets. 2. All neurons that discharged for sacca des made to stationary targets also discharged during saccades made to moving targets, but there was a systematic shift in the saccade vecto r yielding maximal activity (i.e., center of the movement field) of co llicular neurons for the latter class of movements. The shift moved th e center of the movement fields toward larger-amplitude pursuit saccad es for target motion away from the fovea, in comparison with saccades made to stationary targets. However, the discharge at the center of th e movement field for pursuit saccades was 14% lower when averaged over the sample of recorded cells. 3. The saccades made during pursuit tra cking of moving visual stimuli have different dynamics than saccades m ade to stationary targets. At similar amplitudes pursuit saccades are slower, and their velocity profiles often show secondary velocity peak s or inflection points and have longer-duration decelerating phases. 4 . The combined experimental observations of a change in saccade dynami cs and the shift in movement fields in collicular neurons for pursuit saccades are compatible with the hypothesis that saccades made to movi ng targets are controlled by neural processing in two partially separa te pathways. In this theory, one path is concerned with correction of a presaccadic retinal position error (a path that includes the collicu lus) and another path is concerned with position extrapolations based on the velocity of the moving target (a path that does not include the colliculus).