DISCHARGE CHARACTERISTICS OF VESTIBULAR SACCADE NEURONS IN ALERT MONKEYS

We previously described a class of neurons, located in and around the interstitial nucleus of Cajal of the cat, that discharged during vesti bular stimulation and before saccades. We called these neurons vestibu lar saccade neurons (VSNs). Ln the present study, we characterized sim ilar neurons in the monkey. These neurons discharged before vertical s accades and during vertical vestibular stimulation as well as vertical smooth pursuit. Like cat VSNs, the discharge metrics of these VSNs we re poorly related to saccade metrics and showed only occasional, weak sensitivity to eye position. They discharged most intensely (on-direct ion) for movements that were either upward or downward, and their on-d irections were consistent during pitch and pursuit but not for eye pos ition. For saccades, the correlation coefficient of number of spikes a nd vertical saccade size varied from 0.08 to 0.90 with a mean of simil ar to 0.6. The average sensitivity (i.e., slope) of the number of spik es and vertical saccade size linear regression was 0.3 +/- 0.2 spike/d eg. Average correlations between peak discharge rate and peak saccade velocity and between burst duration and saccade duration were 0.5 and 0.4; sensitivities were 0.2 +/- 0.2 spike per s per deg/s and 0.6 +/- 0.5 ms/ms, respectively. Average vestibular sensitivities during 0.5 H z, +/- 10 degrees sinusoidal pitch while the animals suppressed their vestibular ocular reflex were 0.97 spike/s per deg/s for up VSNs and 0 .66 spike/s per deg/s for down VSNs. The average static position sensi tivity for the population of 39 VSNs tested was 0.55 spike/s per deg. The average gain for VSNs tested during 0.5 Hz, +/- 10 degrees sinusoi dal smooth pursuit tracking was 1.4 spike/s per deg/s. As we could not identify analogous neurons in the region of the monkey pontomedullary junction, we conclude that horizontal on-direction VSNs do not exist in the monkey. We discuss a possible functional role for VSNs and simi lar neurons described in previous studies and conclude that these neur ons are most likely involved with the process of neural integration (i n a mathematical sense) of velocity-coded inputs from a variety of ocu lomotor subsystems and are not a pivotal element in saccade generation .