Eye movements induced by linear acceleration are modified by visualisation of imaginary targets

Lateral eye movement responses to linear acceleration in the lateromedial axis of the head have been examined in normal human subjects who were seated within a cabin, free to move on a horizontal linear track. The motion stimulus was either sinusoidal (0.2-0.8 Hz) or pseudo-random (0.11-1.25 Hz) in form, with a peak acceleration of 1.5 m.s-2. In darkness, while carrying out mental arithmetic, the mean ratio of slow-phase eye velocity to linear cart velocity increased from 2.8 degrees/m at 0.2 Hz to 10.5 degrees/m at 0.8 Hz during sinusoidal stimulation. When subjects were instructed to imagine a near head-fixed target in darkness, eye velocity decreased by 25%-48% during both sinusoidal and pseudo-random stimulation. When subjects were instructed to visualise an earth-fixed target during sinusoidal stimulation eye velocity was augmented by 47% when imagining a target 3 m distant and by 175% when visualising a target 0.6 m distant. Response augmentation was not as great during pseudo-random stimulation. The results indicate that the otolith-ocular response is highly modifiable by mental set.