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

To investigate the role of extraocular muscle afferent signals in the initiation and early maintenance of smooth-pursuit eye movements.

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-trial and compared before, while, and after the right eye was impeded.

When the right eye was impeded, initial acceleration and eye velocity were reduced. Pursuit latency was unchanged. The velocity effect had a rapid onset and offset; there was no evidence that the effects built up over a number of trials. Detailed analysis suggested that the reduction in velocity occurred approximately 40 msec after pursuit was initiated.

These results are consistent with the hypothesis that extraocular muscle afferent signals provide a feedback signal of the movements of the 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.