Human DC scalp potentials during vestibular and optokinetic stimulation: non-specific responses?

In normal subjects transient horizontal body rotation in the dark (vestibular stimulation) elicited a DC potential change with the maximum at Cz. The response appeared to covary with stimulus velocity, which is the most relevant stimulus parameter transferred by the horizontal semicircular canal system. A similar response was obtained during optokinetic stimulation. However, the following findings suggested that the responses are related to 'high' perceptual functions rather than representing a visuo-vestibular evoked cortical potential: (a) the visual response was found independent of whether the subjects perceived, as a task, self-motion in a stationary environment or 'object' motion about the stationary body; its amplitude depended on the subjects' subjective compliance to the task; (b) when presenting various combinations of optokinetic and vestibular stimuli, the response amplitude depended on the subjective and objective intersensory conflict in the combinations; (c) sinusoidal stimulation yielded a negative shift of the DC potential, but the potential was not modulated along with the waxing and waning of stimulus velocity or of the self-motion sensation evoked; (d) patients with loss of vestibular functions showed a similar Cz response during body rotation in the dark.