RESPONSE PROPERTIES OF PRETECTAL OMNIDIRECTIONAL PAUSE NEURONS IN THEBEHAVING PRIMATE

We have identified a region in the pretectum of rhesus monkeys (Macaca mulatta) that contains units that evince a complete cessation in firi ng immediately after saccades. The pause occurs for saccades to target steps and catch up saccades during smooth pursuit, spontaneously in c omplete darkness or after quick phases of nystagmus. Because the pause in unit firing always follows saccade onset, we call these neurons fo llowing omnidirectional pause neurons (FOPNs). Because the pause also occurs with saccades in the dark, it is related to the saccade per se and is not a visually contingent response. The duration of the pause i n firing exceeded the duration of all saccades up to 40 deg. For targe ting saccades, the start of the pause was locked rather tightly to the beginning of the saccade but began an average of 51 ms after the sacc ade did. The end of the pause was linked only loosely to either the be ginning or end of the saccade. About half (54%) of our 59 FOPNs also d ischarged a distinct burst of firing that preceded the pause. In diffe rent units, the burst preceded saccade onset by from 0 to 20 ms with a n average of 11 ms and therefore could signal the occurrence of an imp ending saccade. The presaccadic burst was not correlated with any para meter of the saccade. Most FOPNs were found 278 mu m, on average, dors al to the direction-selective units characteristic of the pretectal nu cleus of the optic tract (NOT) and occasionally slightly beyond the an terior-posterior and medial-lateral borders of the NOT. The FOPN regio n does not coincide with any known anatomically or functionally deline ated pretectal nucleus. Because the characteristics of the FOPN pause are not reflected in the characteristics of the saccade and the FOPN p ause occurs well after the saccade is over, it is unlikely that the pa use in pretectal FOPNs is involved with saccade generation. On the oth er hand, the leading burst exhibited by the majority of FOPNs reliably signals that a saccade is occurring but neither its size nor directio n. Perhaps this signal indicating the occurrence of all saccades is ro uted 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 pregenicul ate cells are discussed in the context of this suggestion.