To investigate the possibility that rhythmic activity originating in the ce
ntral nervous system may modulate human eye movements, anticipatory eye mov
ements were generated by tracking an intermittently obscured sinusoidally m
oving target. Eight subjects tracked intermittently obscured sinusoids of t
hree different frequencies and of two different amplitudes. Eye movements w
ere 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 stagger
ed series of anticipatory saccades with intervening smooth anticipatory eye
movements or relatively stationary periods. In sections where the interven
ing 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 oscilla
tion of smooth anticipatory movement was not derived from head tremor and t
hat 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 osci
llation or "tremor" superimposed upon smooth movements that might in other
circumstances be inhibited by direct visual feedback. This smooth eye movem
ent oscillation is thought to originate from the central nervous system and
may reflect a widespread frequency modulation of motor commands. (C) 1999
IBRO. Published by Elsevier Science Ltd.