Burst activity of identified tecto-reticulo-spinal neurons in the alert cat

Activity of tecto-reticulo-spinal neurons (TRSN), identified electrophysiologically and/or by intra-axonal HRP injections, was studied in alert cats during presentation of moving visual stimuli. A majority of TRSNs showed complex visuomotor properties: directionally selective visual responses in the absence of motor counterparts of orienting, enhanced bursting when stimuli triggered saccades, and no activation for spontaneous saccades. Highest intraburst frequencies were observed during active orienting towards novel, "interesting" objects. The more vigorous bursts usually contained repetitive grouped discharges attaining instantaneous frequencies up to 700 imp/s but average firing rates remained in the range of 120-300 imp/s. Intra-axonal HRP injections confirmed terminations of TRSN collaterals in the premotor areas of the lower brain stem, including the abducens nucleus, but also disclosed differences in the details of collateralization between neurons showing different types of visuo-motor activity.