The influence of target distance on perceptual self-motion thresholds and the vestibulo-ocular reflex during interaural translation.

An elegant and influential mathematical model of eye movements is the geometric compensation required for visual fixation location in the translational vestibulo-ocular reflex (VOR). Compensatory eye velocity scales with the inverse of fixation distance during head translation because larger angular eye movements are required to minimize retinal slip during head translation when targets are closer. This model has been extensively verified in experiments. Since the VOR and vestibular perception have shared anatomic pathways, we asked whether the same scaling may affect motion perception. Since perception does not require the linear-to-angular transformation required for the translational VOR, we hypothesized that perception would not scale with target distance. Subjects were tested with a motion direction-recognition threshold task in which they reported their perception of small translations of their body. Thresholds were measured in three conditions: (1) with a near target (0.20m) that extinguished just before each motion; (2) with a far target (0.47m); 3) with no target. The subject was always in darkness during motion. Thresholds were 0.59, 0.61 and 0.61cm/s, respectively. Translational VOR sensitivity (eye angular velocity divided by head translation velocity) was also measured and modulated with target distance. The scaling ratio of responses for the near vs. far target was 0.97 for perceptual thresholds, which was significantly different from the compensatory ratio (2.35; P<0.001) and the translational VOR scaling ratio (1.59; P=0.007) but not from no compensation (1.00; P=0.93). Thus, we conclude that despite shared anatomy for the VOR and perception, the brain processes signals according to the geometric functional constraints of each task.