Characteristics and functional identification of saccadic inhibitory burst neurons in the alert monkey

With the use of single-unit recording, the reticular formation immediately caudal to the abducens nucleus was searched for saccadic burst neurons in alert, trained rhesus monkeys. We recorded 80 short- and long-lead burst neurons, investigated their connections, and quantitatively analyzed their discharge characteristics. 2. Like excitatory burst neurons located rostral to the abducens, these caudal burst neurons fire optimally for ipsilaterally directed saccades, fire less for vertical saccades, and fire minimally, if at all, for contralateral saccades. The direction associated with the maximum number of spikes was approximately along the horizontal axis (1 +/- 12 degrees (SD); n = 33). 3. The first spike of the burst led the saccade by 2-120 ms, depending on the unit. Neurons were divided into short lead (45%) and long lead (55%) using a burst-lead criterion of 15 ms. In the on-direction, the discharges of both types exhibited strong correlations between number of spikes in the burst and size of the horizontal saccade component; duration of the burst and duration of the saccade; and peak frequency of the burst and peak velocity of the saccade. These relations were looser for long-lead neurons than for short-lead neurons. 4. Horseradish peroxidase injected into the abducens nucleus retrogradely labeled cells in the contralateral reticular formation where burst neurons were recorded, showing that cells in this region make crossed monosynaptic connections. There was good agreement between the limits of this region, as determined physiologically and anatomically. 5. Microstimulation at the locus of recorded burst neurons elicited EMG potentials in the contralateral lateral rectus muscle of the appropriate sign and latency for a monosynaptic inhibitory projection to abducens motoneurons. Stimulation also elicited eye movements consistent with inhibition of the contralateral lateral rectus. 6. It is argued that these characteristics make it likely that the short-lead neurons are the source of the afference which generate the pause in contralateral abducens motoneuron firing during adducting saccades. These neurons are therefore analogous to the inhibitory burst neurons (IBNs) found in the cat. The characteristics of long-lead burst neurons, particularly their lead, make them less likely to subserve this function. These cells might be better suited to providing input to omnipause neurons or to the short-lead IBNs.