Transitions between pursuit eye movements and fixation in the monkey: dependence on context. Krauzlis, R.J. and F.A. Miles. Laboratory of Sensorimotor Research, National Institutes of Health, Bethesda, MD.
APStracts 3:0092N, 1996.
SUMMARY AND CONCLUSIONS
1. We compared the visuomotor processing underlying the onset and offset of pursuit by recording the eye movements of three monkeys as they smoothly tracked a target that was initially at rest, started to move suddenly at a constant velocity along the horizontal meridian, and then stopped. We presented this sequence of target motions in two different contexts. In the first, the target sometimes stopped after 500 ms, but on other interleaved trials, the target either continued moving at a constant velocity, slowed down, speeded up, or reversed direction. In the second, the target always stopped, but the duration of the preceding constant velocity was randomized from 500 to 700 ms. 2. The dynamics of the eye velocity during the offset of pursuit were markedly different in the two experiments. When the target stopped only sometimes, the decrease in eye velocity at the offset of pursuit often overshot zero, producing a brief, small reversal in the direction of pursuit before eye speed settled to zero. When the target always stopped, the decrease in eye velocity at the offset of pursuit followed a more gradual transition toward zero with no overshoot. Thus, the eye velocity profiles obtained in the first experiment contradict, whereas those obtained in the second experiment confirm, previous characterizations of the offset of pursuit as an exponential decay toward zero eye speed. 3. To investigate the basis of the different eye velocity profiles obtained in the two experiments, we probed the state of transmission along the visuomotor pathways for pursuit using small perturbations in the motion of the target. We used perturbations consisting of 1 o step changes in target position superimposed on the constant velocity motion of the target, based on previous findings that such perturbations elicit saccades during fixation, but smooth changes in eye speed during maintained pursuit. Single perturbations were imposed at regularly spaced intervals on separate interleaved trials during the onset, maintenance and offset of pursuit. 4. Perturbations imposed during the onset and maintenance of pursuit had similar effects regardless of whether the target stopped only sometimes or always. In both experiments, perturbations which stepped the target in the direction opposite to the constant velocity of the target produced decreases in eye speed; perturbations in the same direction produced neglible or inconsistent changes in eye speed. The changes in eye speed caused by perturbations were largest for those perturbations introduced within the first 100 ms after the start of target motion, before the onset of the smooth eye movement, and became progressively smaller as target motion continued. The largest changes in eye speed were therefore caused by those perturbations imposed during periods of large retinal slip and by those perturbations whose direction opposed that slip. 5. Perturbations imposed during the offset of pursuit had different effects depending on whether the target stopped only sometimes or always. When the target stopped only sometimes, forward perturbations produced large increases in eye speed, whereas backward perturbations produced negligible or inconsistent changes in eye speed. Thus, the visuomotor processing underlying the offset of pursuit in this experiment strongly resembled that underlying the onset of pursuit: in both cases, those perturbations in the direction opposing large retinal slip produced the largest effects. In contrast, when the target always stopped, neither forward nor backward perturbations imposed during the offset of pursuit produced large changes in eye speed. This indicates that the visuomotor processing underlying the offset of pursuit in this experiment was different from the processing underlying the onset of pursuit. 6. Perturbations also produced changes in the frequency of saccades, although these effects were less consistent than the changes in pursuit eye speed. During the onset of pursuit, such effects as did occur were similar in the two experiments: perturbations in the same direction as the motion of the target increased the frequency of saccades, whereas perturbations in the opposite direction either decreased the frequency of saccades or had no effect. In contrast, during the offset of pursuit, perturbations had different effects on saccadic frequency, dependent on whether the target stopped only sometimes or always. When the target stopped only sometimes, backward perturbations increased the frequency of saccades, whereas forward perturbations decreased the frequency of saccades. However, when the target always stopped, there was little difference between the effects of forward and backward perturbations on saccades. Thus, the effects of the perturbations on the programming of saccades complemented the effects on smooth eye velocity. 7. Our data indicate that the visuomotor processing underlying the offset of pursuit can be either very similar to or very different from that underlying the onset of pursuit. These results raise doubts about the previous proposal that the onset and offset of pursuit are mediated by different mechanisms. For parsimony, we suggest that the onset and offset of pursuit are mediated by a single mechanism, but that the transmission of visual and motor signals through this mechanism can vary during pursuit. We present simulations to demonstrate that a model incorporating this suggestion can account for the variety of eye velocity profiles observed during the onset and offset of pursuit. 

Received 12 February 1995; accepted in final form 10 May 1996.
APS Manuscript Number J107-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96