Dependence of short-latency ocular following and associated activity in the medial superior temporal area (MST) on ocular vergence.

Abstract

Motion of a large-field pattern elicits short-latency ocular following responses (OFR) in the monkey, which are mediated at least in part by the medial superior temporal area of the cortex (MST). The magnitude of the OFR is known to be inversely related to viewing distance, and we investigated the dependence of OFR and the associated neuronal activity in the MST on a major cue to viewing distance, ocular vergence, in alert monkeys (Macaca fuscata). The vergence angle, expressed in terms of the apparent viewing distance, ranged from infinity to 16.6 cm (0-6 m(-1)). The magnitude of the initial OFR increased monotonically with increases in convergence at a mean (+/-SD) rate of 19.6+/-4.5%/m(-1) in four monkeys (over the range 0-4 m(-1)). In two monkeys, we recorded the single unit activity of 160 MST neurons that responded to motion of a large-field pattern with directional selectivity. The mean latency (+/-SD) of the MST discharges elicited by large-field motion was 50+/-7.5 ms (n=115), which preceded the onset of OFR by an average of 10+/-9.9 ms. The discharge modulation elicited by large-field motion showed a significant dependence on vergence in 91/160 neurons (57%), 72 of which (79%) increased their firing rate with increasing convergence ("near" neurons), and the remainder increasing their firing rate with decreasing convergence ("far" neurons). However, on average, the sensitivity of these MST neurons to vergence was only about 30% of that shown by the OFR. It could be that only those neurons that are very sensitive to vergence angle contribute to the OFR, but it is also possible that much of the modulation of OFR with vergence occurs downstream from the MST or in alternative pathways (yet to be discovered) that contribute to OFR.