Studies of the vestibulo-ocular reflex and visual-vestibular interactions during active head movements

The ratio of slow phase eye speed to head speed (Gain) during voluntary sinusoidal head rotations at 0.33, 0.67 and 1.0 Hz was studied in normal subjects under various target presentations. With mental arithmetic in the dark, the mean value of the Gain was 0.8 irrespective of turning frequencies. When subjects looked at or imagined (in the dark) a spatially fixed target, the Gain was always maintained at unity. The Gain measured with a head-fixed target was 0.22, 0.33 and 0.54 at 0.33, 0.67 and 1.0 Hz, respectively. However, imagining a target in this condition failed to suppress the Gain obtained with mental arithmetic in the dark. When a target moved twice as fast as the head (pursuit of the reflected image in a head-fixed mirror), the Gain was -0.60, -0.11 and 0.38 at 0.33, 0.67 and 1.0 Hz, respectively.