We will evaluate the ear-canal pressure sensitivity of subjects with superior canal dehiscence (SCD) with echo planar BOLD functional magnetic resonance imaging (fMRI) in order to address vestibular cortical areas (parieto-insular and possibly others) active during cognitive tasks requiring an estimate of verticality and/or self-motion. This unique, unilateral vestibular stimulus can provide (with the appropriate individual thresholds) a precise vertical canal-related functional window into the organization of vestibular inputs. By pre-training subjects to calibrate their own perceptual responses to pressures against the affected canal, their in-magnet responses (via joystick) can be correlated to cortical regions associated with different spatial orientation tasks. Previous vestibular stimulation experiments (caloric, galvanic) using functional imaging relied on only passive conditions and likely failed to evoke internal spatial reference frame activity. Pre-training of eligible SCD subjects will utilize EEG recordings to confirm that our ear canal pressure or tone device both creates perceptual tilts and cortical activity. Subjects with strong effects and good reproducibility will then be analyzed with fMRI during steady-state epochs of vestibular and visual (static or optokinetic) stimulus combinations which combine or conflict the two modalities. We will test the reciprocal inhibition hypothesis between the occipital and parieto-insular (presumed vestibular) cortices in the same subjects under conditions where visual or vestibular sensory inputs are dominant or attended. In addition, the data could provide novel functional and anatomical information about pathway symmetry (from the unilateral stimulus) and serve as a comparison to earlier studies where primarily horizontal canal, and/or otolith end-organs were stimulated (caloric or galvanic techniques, respectively).
