Activity of mesencephalic vertical burst neurons during saccades and smooth pursuit

Activity of mesencephalic vertical burst neurons during saccades and smooth pursuit. J. Neurophysiol. 83: 2080-2092, 2000. The activity of vertical bu rst neurons (BNs) was recorded in the rostral interstitial nucleus of the m edial longitudinal fasciculus (riMLF-BNs) and in the interstitial nucleus o f Cajal (NIC-BNs) in head-restrained cats while performing saccades or smoo th pursuit. BNs emitted a high-frequency burst of action potentials before and during vertical saccades. On average, these bursts led saccade onset by 14 +/-: 4 ms (mean +/- SD, n = 23), and this value was in the range of lat encies (similar to 5-15 ms) of medium-lead burst neurons (MLBNs). All NIC-B Ns (n = 15) had a downward preferred direction, whereas riMLF-BNs showed ei ther a downward (n = 3) or an upward (n = 5) preferred direction. We found significant correlations between saccade and burst parameters in all BNs: v ertical amplitude was correlated with the number of spikes, maximum vertica l velocity with maximum of the spike density, and saccade duration with bur st duration. A correlation was also found between instantaneous vertical ve locity and neuronal activity during saccades. During fixation, all riMLF-BN s and similar to 50% of NIC-BNs (7/15) were silent. Among NIC-BNs active du ring fixation (8/ 15), only two cells had an activity correlated with the e ye position in the orbit. During smooth pursuit, most riMLF-BNs were silent (7/8), but all NIC-BNs showed an activity that was significantly correlate d with the eye velocity. This activity was unaltered during temporary disap pearance of the visual target, demonstrating that it was not visual in orig in. For a given neuron, its on-direction during smooth pursuit and saccades remained identical. The activity of NIC-BNs during both saccades and smoot h pursuit can be described by a nonlinear exponential function using the ve locity of the eye as independent variable. We suggest that riMLF-BNs, which were not active during smooth pursuit, are vertical MLBNs responsible for the generation of vertical saccades. Because NIC-BNs discharged during both saccades and pursuit, they cannot be regarded as MLBNs as usually defined. NIC-BNs could, however, be the site of convergence of both the saccadic an d smooth pursuit signals at the premotoneuronal level. Alternatively, NIC-B Ns could participate in the integration of eye velocity to eye position sig nals and represent input neurons to a common integrator.