Saccade-related neurons in cat superior colliculus: pandirectional movement cells with postsaccadic responses

The superior colliculus is known to contain cells discharging before saccadic eye movements as well as cells responding to sensory stimuli. In this study extracellular single unit recordings were made in the alert trained cat with the head fixed. A novel type of eye movement-related response was found in 9% (32/344) of the cells recorded. These cells differ from previously reported eye movement-related neurons in the timing of their discharge, which accompanies but does not precede saccades. The timing of discharge varies across units from less than 10 ms after the onset of eye movement to as much as 80 ms. Comparable latencies were found regardless of whether saccades were directed contralateral or ipsilateral to the recording site. Most units have an abrupt onset of discharge, but some show a very gradual increase in discharge rate. Most cells (69% or 22/32) discharged with equal vigor for all saccades, regardless of direction. The remainder tended to show higher-frequency bursts when saccades were directed contralaterally, but even these units were not encoding saccade direction by their pattern of discharge. Thus the discharge pattern could be summarized as an omnidirectional burst. For the vast majority of cells (81% or 26/32) the duration of discharge did not correlate with the duration of eye movement. The same pattern of firing was seen with saccades in light and in complete darkness. Thus the saccade-related discharge was not due to changes in visual stimulation during saccades. A minority of the units (15% or 5 of 32) that discharged with but not before saccades also responded to visual stimuli in the absence of eye movements. Saccade-related activity was dependent on alertness of the cat, as measured by behavioral performance and EEG. The close temporal association between saccades and unit discharge disappeared during drowsiness. These units could be reflecting either proprioceptive or corollary discharge signals to the superior colliculus. However, their response properties differ somewhat from those found in previous studies of proprioceptive inputs to the colliculus. Such differences could be due to the effects of the anesthetics that were used in studies of proprioceptive responses. Alternatively, the cells reported here could be conveying a corollary discharge signal. These cells occurred in patches or clusters. This is consistent with a wealth of anatomic data indicating a modular organization of the colliculus.