Pathways mediating orienting and visual discrimination and their roles in information processing in cats

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Effects of moving visual stimulus (100-4500*/s) and of interrupted light stimulus (gap) on orienting and neuronal activities in the pontomedullary reticular formation were examined. Cats were trained to fixate a light spot on center of a flat panel and then to pursuit the moving light spot with the head and eyes. For visual stimuli of 450-1000*/s, cats oriented to a trarget with a single saccade and a saccade-like head movement (head saccade), but with 2-4 eye and head saccades for other stimulus seed. Gaze and head velocities were maximal between 300-600*/s of visual stimuli but decreased for the other range of stimuli, while eye velocities changed little with these stimuli. Head movements preceded eye saccades by 10-40 ms and their latencies were shortest (about 60ms) for the stimulus range 450-1000*/s but prolonged in the other range of stimulus speed. The gap (30ms) inserted in the early part of the visual stimuli prolonged latencies of orienting with little effects on velocities, while a gap inserted in the later part of the stimuli reduced the head velocity without affecting latencies. Eye velocities were little affected by gap stimulation. These results suggested that head and eye movements are controlled differentially and circuitry controlling head movements is very sensitive to the velocity and gap of visual stimuli. In the pontomedullary reticular formation, two major types of neurons have been differentiated, phasic sustained neurons (PSNs) and phasic neurons (PNs). PSNs fired maximally (best tuned) around 450-700*/s and decreased in the other range. Firing of PNs were a little affected by these stimuli. These results suggested that the velocity tuning of orienting movements are chiefly mediated by PSNs while velocity independent head movements are controlled by PNs.