Frequency dependence of human thresholds - both perceptual and vestibulo-ocular reflex (VOR) thresholds.

While perceptual thresholds have been widely studied, VOR thresholds have received less attention, so the relationship between vestibulo-ocular reflex (VOR) and perceptual thresholds remains unclear. We compared the frequency dependence of human VOR thresholds to human perceptual thresholds for yaw head rotation in both upright ("yaw rotation") and supine ("yaw tilt") positions using the same human subjects and motion device. VOR thresholds were generally a little smaller than perceptual thresholds. We also found that horizontal VOR thresholds for both yaw rotation about an earth-vertical axis and yaw tilt (yaw rotation about an earth-horizontal axis) were relatively constant across 4 frequencies (0.2, 0.5, 1 and 2 Hz) with little difference between yaw rotation and yaw tilt VOR thresholds. For yaw tilt stimuli, perceptual thresholds were slightly lower at the lowest frequency and nearly constant at all other (higher) frequencies. However, for yaw rotation, perceptual thresholds increased significantly at the lowest frequency (0.2Hz). We conclude: (i) that VOR thresholds were relatively constant across frequency for both yaw rotation and yaw tilt, (ii) that the known contributions of velocity storage to the VOR likely yielded these VOR thresholds that were similar for yaw rotation and yaw tilt for all frequencies tested, and (iii) that the integration of otolith and horizontal canal signals during yaw tilt when supine contributes to stable perceptual thresholds, especially relative to the low frequency perceptual thresholds recorded during yaw rotation.