Sensory-motor factors triggering the suppression of post-rotary vestibular responses in different gravitoinertial force backgrounds.

We studied suppression of oculomotor and perceptual after-responses by post-rotary head movements in high (1.8 G), low (0 G), and normal (1 G) gravitoinertial force backgrounds in parabolic flight. Our aim was to identify what sensory and motor signals are critical for triggering suppression. In a prior experiment (DiZio and Lackner 1988), we found suppression using 40 degrees post-rotary head tilts in 1 G and 1.8 G but not 0 G force backgrounds. However, in free fall even without head tilts there was a significant suppression of nystagmus relative to 1 G and 1.8 G force backgrounds, thus potentially masking an effect of head tilt on suppression in 0 G. We have retested four of the original subjects with 90 degrees head tilts to maximize the likelihood of detecting suppression in 0 G. Although nystagmus and illusory after-rotation were suppressed by post-rotary head tilts in normal and high gravitoinertial force environments, there was still no evidence of suppression in free fall. We present evidence that the lack of suppression in 0 G is not attributable to post-rotary responses already being at a "basement" level, but rather that suppression depends on the registration of a change in head position relative to a significant level of gravitoinertial force.