SACCADE-RELATED BURST NEURONS WITH TORSIONAL AND VERTICAL ON-DIRECTIONS IN THE INTERSTITIAL NUCLEUS OF CAJAL OF THE ALERT MONKEY

The interstitial nucleus of Cajal (iC) is known to be the neural integ rator for vertical and torsional eye movements. Burst-tonic neurons an thought to be the neural substrate for this function. Until now, the iC has not been specifically considered to play a part in saccade gene ration. The aim of this study was to characterize saccade-related burs t neurons in the iC during torsional and vertical eye movements. Sacca de-related burst neurons were recorded in the iC of macaque monkeys du ring fast phases of torsional and vertical vestibular nystagmus, spont aneous and visually guided eye movements, and in light and darkness. B urst neurons in the iC (n=85) were found intermingled between burst-to nic and tonic neurons. They were not spontaneously active, showed no e ye position sensitivity, and responded during saccades and quick phase s of nystagmus with a burst of activity whose duration was closely cor related with saccade amplitude and hence saccade duration (correlation coefficients up to 0.9). Latency in the on-direction was, on average, 10.4 ms (range 5-23 ms); it decreased with different saccade directio ns and became negative in the off-direction. In a horizontal-vertical coordinate system, on-direction of the majority of neurons was either upward (n=52) Or downward (n=33). There was no horizontal on-direction . Burst neurons of different vertical on-directions were found intermi ngled throughout the iC. In the vertical-torsional plane, on-direction always showed an ipsiversive torsional component, i.e., a clockwise ( positive) torsion for neurons in the right iC and a counter-clockwise (negative) torsional component when recorded in the left iC. The findi ngs indicate that saccade-related burst neurons in the iC control coor dinate axes for vertical and torsional quick eye rotations. As in the rostral interstitial nucleus of the medial longitudinal fasciculus (ri MLF), burst neurons in the iC encode vertical saccades with an ipsitor sional direction with similar burst characteristics. It is suggested t hat iC burst neurons play a part in the local feedback loop of the rec iprocal iC-riMLF projections.