Activity in the parabigeminal nucleus during eye movements directed at moving and stationary targets.

The parabigeminal nucleus (PBN) is a small satellite of the superior colliculus located on the edge of the midbrain. To identify activity related to visuomotor behavior, we recorded from PBN cells in cats trained to fixate moving and stationary targets. Cats tracked moving targets primarily with small catch-up saccades, and for target speeds of 2-6 degrees /s, they did so with sufficient accuracy to keep targets within 2.5 degrees of the visual axis most of the time. During intersaccade intervals of such close-order tracking, PBN cells fired at rates related to retinal position error (RPE), the distance between the center of the retina and the saccade target. Each cell was characterized by a best direction of RPE. Most commonly, activity rose rapidly with increasing RPE, peaked at a small RPE within the area centralis, and dropped off gradually with increasing target distance. For some cells, the range over which activity was monotonically related to RPE was considerably larger, but because the PBN was not systematically sampled, the maximum range of RPE encoded is presently unknown. During saccades, activity began to change at about peak saccade velocity and then rapidly reached a level appropriate to the RPE achieved at saccade end. Most response fields were large, and stationary saccade targets presented anywhere within them evoked brisk responses that terminated abruptly on saccade offset. Spontaneous saccades in the dark had little effect on PBN activity. These data suggest that the PBN is an integral part of a midbrain circuit generating target location information.