Voluntary ocular tracking movements in response to a sinusoidally moving intermittently active sound source.

Voluntary eye movements in response to a sinusoidally moving, intermittently active sound source were investigated in 40 young healthy human subjects. Visual afferent input was prevented during the experiments. With increasing burst repetition rates of the sound source, different forms of eye movements were observed: a) so-called single and repetitive adversive movements consisting of saccades which occur with certain latencies in response to single bursts; b) so-called saccadic eye tracking movements which are characterized by numerous small saccades occurring in accordance with the target (sound source) movement but lacking a time correlation between the saccades and single bursts. It was found that saccadic eye tracking movements were performed instead of adversive movements when at least 4 to 10 bursts per movement period of the sound source was transmitted (with movement frequencies of the sound source from 0.1 to 0.5 Hz). Slight intersaccadic drifts, which could be regularly observed in all types of eye movements were interpreted as instability of fixation in darkness.