Human anticipatory eye movements may reflect rhythmic central nervous activity

To investigate the possibility that rhythmic activity originating in the central nervous system may modulate human eye movements, anticipatory eye movements were generated by tracking an intermittently obscured sinusoidally moving target. Eight subjects tracked intermittently obscured sinusoids of three different frequencies and of two different amplitudes. Eye movements were recorded by an infra-red reflection technique. The eye velocity records were analysed in the frequency domain by power spectral estimates. During periods where the target was obscured, eye movements consisted of a staggered series of anticipatory saccades with intervening smooth anticipatory eye movements or relatively stationary periods. In sections where the intervening smooth components of anticipatory tracking were of high velocity (above 15 deg/s), a superimposed smooth tremulous oscillation at around 10 Hz was sometimes present. Coherence analysis showed that this 10 Hz range oscillation of smooth anticipatory movement was not derived from head tremor and that the same oscillation was present in both eyes. This oscillation was not generally observed during smooth tracking of pseudorandom waveforms. Investigation of anticipatory eye movements has revealed a 10-Hz range oscillation or "tremor" superimposed upon smooth movements that might in other circumstances be inhibited by direct visual feedback. This smooth eye movement oscillation is thought to originate from the central nervous system and may reflect a widespread frequency modulation of motor commands.