Failure to update the egocentric representation of the visual space through labyrinthine signal.

The present study evaluated the capacity to compare retinal and vestibular signals in a heterosensorial matching task. In the first experiment, subjects evaluated the magnitude of passive whole-body rotations in relation to the eccentricity of a visual target briefly presented before rotation. Such a task elicits multimodal sensory stimulations experienced by a subject during normal goal-oriented head movements, i.e., retinal and vestibular stimulations. A good capacity to evaluate vestibular signals in relation with retinal inputs might suggest that the labyrinthine output is part of a cognitive feedback-loop controlling active head movements oriented toward a visual target and/or that the labyrinthine signal might make a major contribution to judging the position of the target in space after goal-directed head movements. Results showed that body rotation magnitudes had to exceed the amplitude of the visual target by about 120 and 89% to be perceived as having a similar magnitude to a 10 degree and an 18 degree visual target, respectively. A second experiment was designed to test whether this major discrepancy originated either from (a) an overestimation of the peripheral visual target locations, (b) an underestimation of the labyrinthine signal, (c) a deficiency in matching sensory signals from different modalities, or (d) any combination of (a), (b), and (c). In the second experiment, the actual perception of retinal and labyrinthine signals, as indicated by verbal responses, was quantified. Results from this experiment showed that most of the large underestimation of the vestibular stimulation found in Experiment I ought to emerge from a poor capacity to integrate heterogeneous sensory signals by the perceptual system rather than from pure misperception of the retinal and/or labyrinthine signals. Overall, results from Experiments I and II argue for a deficiency of the CNS to integrate labyrinthine signals for updating the egocentric representation of the peripheral visual target during passive body (head) rotations.