Circularvection: psychophysics and single-unit recordings in the monkey.

In psychophysical experiments, human subjects indicated the amount of circularvection (CV) that experienced during sinusoidal rotation (0.01-5 Hz) of the visual surround. Accelerations varied between 5 and 160 degrees/second2; maximal velocities did not exceed 160 degrees/second. Below 0.1 Hz and 20 degrees/second2, most subjects experienced full CV; above, CV was only partial. Subjects then perceived a combination of CV and object motion. All subjects still had some CV at 2 Hz. The upper frequency limit seemed to occur around 5 Hz. In related neurophysiological studies, single units were investigated in the vestibular cortex (area 2v) of the alert monkey. Neurons responded to animal rotation in the dark as well as to sinusoidal rotation of the visual surround (0.01-1 Hz). Units responded to the visual stimulus in the high-frequency range with a gain increase. These experiments demonstrate the prominent influence of the visual system on vestibular neurons even at high frequencies.