Neural response latency of smooth pursuit responsive neurons in cortical area MSTd

Abstract

Event Abstract Back to Event Neural response latency of smooth pursuit responsive neurons in cortical area MSTd Lukas Brostek1*, Seiji Ono2, Michael J. Mustari2, Ulrich Buttner1 and Stefan Glasauer1 1 Bernstein Center for Computational Neuroscience, Germany 2 Yerkes National Primate Research Center, United States Lesion and microstimulation studies in primate cortices have shown that the medial superior temporal (MST) area is involved in the control of smooth pursuit (SP) eye movements. The lateral part of MST (MSTl) has been implicated in the coding of visual target motion [1] used to drive pursuit responses. The role of the dorsal part of MST (MSTd) in the generation of SP eye movements is highly disputed, even though about one third of MSTd neurons show strong neuronal responses to visual pursuit stimuli. Experimental evidence, for example by blanking of the target, suggested that these responses contain an extraretinal component. It has therefore been suggested that the pursuit-related neurons in MSTd may code an estimate of gaze velocity [2].Computational models of SP control posit that an efference copy of the ocular motor command is used to generate an estimate of eye velocity via an internal plant model. The estimate of target motion is constructed by adding the retinal error velocity to this signal. Simulations of our dual pathway SP control model [3] show that for stability reasons the delay of the estimated eye velocity signal with respect to the eye motor command should approach the sum of latencies in the primary retinal feedback loop, i.e., the latency between target motion and eye movement, which exhibits multi-trial mean values between 100 and 150 ms. Indeed, we recently showed that on average eye velocity related neuronal signals in MSTd lag behind eye motion with a remarkably similar latency [4]. Thus, SP-related neurons in MSTd may code an estimate of eye velocity suited to reconstruct target motion in MSTl.Based on these observations, we hypothesized that if SP-related neurons carry a signal derived from an efference copy, then the delay of SP-related neurons must be related to the eye movement latency on a trial-to-trial basis. This relation could either be a constant delay or a linear relation reflecting the actual variation of the eye movement latency. We examined the responses of pursuit-sensitive MSTd neurons to step-ramp pursuit (laser spot, 4 target velocities, max. 30°/s) recorded in two awake macaque monkeys. The latency of eye movement and the delay of neuronal response with respect to target motion onset were determined for each trial.The neuronal latency with respect to target onset correlated significantly with eye movement latency, thus supporting our hypothesis of an efferent copy origin of the MSTd signal. Further analysis showed that the neuronal latency lagged behind eye movement onset by a constant value of 100 to 150 ms, and did not reflect trial-to-trial variations in eye movement latency. Thus, the delay mechanism between the efference copy of the eye motor command and the estimate of eye velocity works independently of the variable latency in pursuit eye movement onset.