Neural Mechanisms Controlling Velocity and Position Guided Orienting

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Cats were trained to look at a light spot and then to orient to the light spot which jumped to or moved in a ramp-hold manner to a new target from the fixation point. The two stimulus evoked position guided and velocity guided orienting, respectively, since the former use positional and the latter velocity information of the light spot.1. Velocity guided orienting is characterized by short latency (50-80ms) and large velocity (500°/s). Optimal visual stimulus velocity is 300-600°/s and their latencies, velocities and amplitudes decreased with other stimulus velocities.2. Position guided orienting is characterized by long latencies (150 -300 ms) and slower velocities of about half of the velocity guided orienting.3. Neurons in the superior colliculus activated with latencies of about 30 ms from onset of the visual stimulus and lasted during stimulus. Firing frequencies were maximal with the same optimal velocities for orienting.4. Two major types of neurons in the pontomedullary reticular formation were observed. One type activated only with moving stimulus, with latencies of about 40 ms and active up to middle of head movement in time. The other is activated during head movements irrespective of stimulus types. The differenced were well explained by these neural activities.