RESPONSE PROPERTIES OF PRETECTAL omnidirectional PAUSE NEURONS IN THE BEHAVING PRIMATE. mustari, michael j., albert f. fuchs and milton pong. Department of Anatomy and Neuroscience, University of Texas Medical Branch, Galveston TX 77550, Department of Physiology and Biophysics and Regional Primate Research Center, University of Washington, Seattle, WA 98195.
APStracts 3:0217N, 1996.
1. We have identified a region in the primate pretectum, which contains units that cease firing for a period of time immediately following saccades. A complete cessation in firing follows saccades in all directions whether they are made to a jumping target, as catch up movements during smooth pursuit, spontaneously in complete darkness or as quick phases of nystagmus. Because the pause in unit firing always follows saccade onset, we call these neurons following omnidirectional pause neurons (FOPNs). Since the pause also occurs with saccades in the dark, it is related to the saccade per se and is not a visually contingent response. 2. The duration of the pause in firing exceeded the duration of all saccades up to 40. For targeting saccades, the start of the pause was rather tightly locked to the beginning of the saccade but began an average of 51 ms after the saccade did. The end of the pause was only loosely linked to either the beginning or end of the saccade. Apparently, the pause of FOPN firing can have little to do with specifying the characteristics of the saccade itself. 3. About half (54%) of our 59 FOPNs also discharged a distinct burst of firing that preceded the pause. In different units, the burst preceded saccade onset by from 0 to 20 ms with an average of 11 ms and therefore, could signal the occurrence of an impending saccade. The presaccadic burst was not correlated with any parameter of the saccade. 4. Most FOPNs were found 278 m, on average, dorsal to the direction-selective units characteristic of the pretectal nucleus of the optic tract (NOT) and occasionally slightly beyond the anterior-posterior and medial-lateral borders of the NOT. The FOPN region is a thin sheet of cells, which does not coincide with any known anatomically or functionally-delineated pretectal nucleus. 5. Because the characteristics of the FOPN pause are not reflected in the characteristics of the saccade and the FOPN pause occurs well after the saccade is over, it is unlikely that the pause in pretectal FOPNs is involved with saccade generation. On the other hand, the leading burst exhibited by the majority of FOPNs reliably signals that a saccade is occurring but neither its size nor direction. Perhaps this signal indicating the occurrence of all saccades is routed to visual relay neurons to effect saccadic modification of visual pathways. The substantial efferent connections of the FOPN/NOT region to the pregeniculate nucleus and the saccadic discharge of pregeniculate cells are discussed in the context of this suggestion.

Received 15 March 1996; accepted in final form 5 September 1996.
APS Manuscript Number J217-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996