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 integrator for vertical and torsional eye movements. Burst-tonic neurons are thought to be the neural substrate for this function. Until now, the iC has not been specifically considered to play a part in saccade generation. The aim of this study was to characterize saccade-related burst neurons in the iC during torsional and vertical eye movements. Saccade-related burst neurons were recorded in the iC of macaque monkeys during fast phases of torsional and vertical vestibular nystagmus, spontaneous and visually guided eye movements, and in light and darkness. Burst neurons in the iC (n = 85) were found intermingled between burst-tonic and tonic neurons. They were not spontaneously active, showed no eye position sensitivity, and responded during saccades and quick phases of nystagmus with a burst of activity whose duration was closely correlated 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 directions 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 intermingled 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 counterclockwise (negative) torsional component when recorded in the left iC. The findings indicate that saccade-related burst neurons in the iC control coordinate axes for vertical and torsional quick eye rotations. As in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), burst neurons in the iC encode vertical saccades with an ipsitorsional direction with similar burst characteristics. It is suggested that iC burst neurons play a part in the local feedback loop of the reciprocal iC-riMLF projections.