Modification of smooth pursuit initiation by a nonvisual, afferent feedback signal

PURPOSE. To investigate the role of extraocular muscle afferent signals in the initiation and early maintenance of smooth-pursuit eye movements. METHODS. A suction scleral contact lens was used to impede the movements of the right eye while subjects tracked small targets in a step-ramp pursuit paradigm. Movements of the left eye were measured by infrared oculography. Pursuit latency, eye acceleration, and velocity were analyzed trial-by-tria l and compared before, while, and after the right eye was impeded. RESULTS. When the right eye was impeded, initial acceleration and eye veloc ity were reduced. Pursuit latency was unchanged. The velocity effect had a rapid onset and offset; there was no evidence that the effects built up ove r a number of trials. Detailed analysis suggested that the reduction in vel ocity occurred approximately 40 msec after pursuit was initiated. CONCLUSIONS. These results are consistent with the hypothesis that extraocu lar muscle afferent signals provide a feedback signal of the movements of t he eyes that may be used to modify the initiation and early maintenance of smooth pursuit on-line. It appears that for pursuit, as with saccades, the priority in these conditions is to maintain conjugacy.