Visual display lowers detection threshold of angular, but not linear, whole-body motion stimuli

The influence of a visual display, fixed relative to the subject, on thresholds for detection (at 75% correct) of discrete Y-axis linear movements and of discrete Z-axis angular movements, was determined in a group of 12 subjects. Both the linear and the angular, whole-body, motion stimuli had a cosine bell velocity trajectory with a duration of 2.6 s. Thresholds for the detection of the linear motion stimuli in darkness were not significantly different from those obtained when either a simple LED display or an instrument dial were illuminated; the mean threshold was 0.039 m.s-2. None of the subjects reported apparent movement of the visual display. In contrast, the presence of either visual display lowered angular motion thresholds on average by a factor of 2.7 from that obtained in darkness (1.61 degrees.s-1). All subjects reported apparent movement of the visual display, an oculogyral illusion, at stimulus intensities close to the dark threshold. These findings imply that otolithic afferents, unlike those from the semicircular canals, do not interact with neural centres mediating visual localization.