Quantification of tracking eye movements in normal subjects.

Voluntary tracking eye movements were analysed in 20 normal subjects whose gaze was fixed on a visual target moving at six different constant velocities between 10 degrees s-1 and 60 degrees s-1. Tracking ability was quantified according to four parameters. The mean value and dispersion of each parameter at each velocity were determined. The maximum velocity gain of smooth pursuit was, on average, 0.98-0.75, gradually diminishing with increasing target velocities of 10-60 degrees s-1. Amplitude of smooth pursuit decreased gradually and was replaced by superimposed saccades at increasing target velocities. Saccades with amplitudes of 3-10 degrees were present at all target velocities, while those greater than 10 degrees occurred mostly at target velocities above 30 degrees s-1. Square waves were rare but equally frequent at all target velocities and seemed to occur randomly during tracking eye movements. An additional group of 9 subjects was investigated twice. Mean values of maximum velocity gain of amplitude of smooth pursuit and of frequency of superimposed saccades were higher on the second occasion, probably reflecting the effect of learning. Co-operation and interaction of the smooth pursuit and the saccadic subsystems to produce the voluntary tracking were discussed.