Cortical processing in vestibular navigation

: Visual and vestibular perceptual processes are intimately related. Previous data suggest a reciprocal visual-vestibular inhibition with regard to head motion (velocity) perception with each sensory modality mediated by distinct cerebral cortical loci. The relationship between visual and vestibular perceptual processes with regard to displacement perception is unknown. In a path-reversal vestibular navigation task, we investigated the effects of early visual deprivation on vestibular perception by comparing performance and strategies employed (i.e., displacement vs. velocity matching) by a group of congenitally blind subjects to that of a sighted control group. In a second experiment in a sighted group, we attempted to disrupt firstly displacement encoding and then velocity encoding, during a path-reversal vestibular navigation task, by applying repetitive transcranial magnetic stimulation (rTMS) to the right posterior parietal cortex (PPC), vs. a left motor cortex control. Our data show that for a path-reversal vestibular navigation task, when given free choice, sighted and blind subjects can utilize both displacement- and velocity-matching strategies, and overall, both groups display equivalent performance in performing the path-reversal task. In sighted subjects, when rTMS was applied during encoding in the path-reversal task, displacement but not velocity encoding was disrupted for leftward rotations. In summary, our data suggest that early visual deprivation does not degrade the perception of vestibular signals of head velocity or of derived percepts of angular displacement. The rTMS results in the sighted group show that leftward vestibular-derived displacement perception is encoded in the right PPC, an area prominent in visuo-spatial perception. Thus, in contrast to current theories of vestibular head velocity perception which involve reciprocal inhibition between separate and competing visual and vestibular cortical areas, we suggest that at least for displacement perception, visual and vestibular-derived signals are encoded in a common cortical locus.