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This review summarizes our current knowledge of multisensory vestibular structures and their functions in humans. Most of it derives from brain activation studies with PET and fMRI conducted over the last decade. The patterns of activations and deactivations during caloric and galvanic vestibular stimulations in healthy subjects have been compared with those in patients with acute and chronic peripheral and central vestibular disorders. Major findings are the following: (1) In patients with vestibular neuritis the central vestibular system exhibits a spontaneous visual-vestibular activation-deactivation pattern similar to that described in healthy volunteers during unilateral vestibular stimulation. In the acute stage of the disease regional cerebral glucose metabolism (rCGM) increases in the multisensory vestibular cortical and subcortical areas, but simultaneously it significantly decreases in the visual and somatosensory cortex areas. (2) In patients with bilateral vestibular failure the activation-deactivation pattern during vestibular caloric stimulation shows a decrease of activations and deactivations. (3) Patients with lesions of the vestibular nuclei due to Wallenberg's syndrome show no activation or significantly reduced activation in the contralateral hemisphere during caloric irrigation of the ear ipsilateral to the lesioned side, but the activation pattern in the ipsilateral hemisphere appears 'normal'. These findings indicate that there are bilateral ascending vestibular pathways from the vestibular nuclei to the vestibular cortex areas, and the contralateral tract crossing them is predominantly affected. (4) Patients with posterolateral thalamic infarctions exhibit significantly reduced activation of the multisensory vestibular cortex in the ipsilateral hemisphere, if the ear ipsilateral to the thalamic lesion is stimulated. Activation of similar areas in the contralateral hemisphere is also diminished but to a lesser extent. These data demonstrate the functional importance of the posterolateral thalamus as a vestibular gatekeeper. (5) In patients with vestibulocerebellar lesions due to a bilateral floccular deficiency, which causes downbeat nystagmus (DBN), PET scans reveal that rCGM is reduced in the region of the cerebellar tonsil and flocculus/paraflocculus bilaterally. Treatment with 4-aminopyridine lessens this hypometabolism and significantly improves DBN. These findings support the hypothesis that the (para-) flocculus and tonsil play a crucial role in DBN. Although we can now for the first time attribute particular activations and deactivations to functional deficits in distinct vestibular disorders, the complex puzzle of the various multisensory and sensorimotor functions of the phylogenetically ancient vestibular system is only slowly being unraveled.
Analysis of vestibulo-ocular reflex experiments requires knowledge of the absolute orientations (with respect to skull landmarks) of semicircular canals (SCC). Data relating SCC orientations to accessible skull landmarks in humans are sparse, apart from a classic study of 10 skulls, which concluded that the horizontal and anterior SCC are not mutually orthogonal (111 +/- 7.6 degrees). Multiple studies of isolated labyrinths have shown the inter-SCC angles are close to 90 degrees. We hypothesized that a larger sample would yield mean absolute SCC orientations closer to the mutual orthogonality demonstrated for isolated labyrinths. We measured canal orientations with respect to accessible skull landmarks using 3-D multiplanar reconstructions of computerized tomography scans of the temporal bones of 22 human subjects. Images were acquired with 0.5-mm thickness and reconstructed with in-plane resolution of 234 microm. There was no significant difference between the left and a mirror image of the right (p > 0.57 on multiway ANOVA of orientation vector coefficients), so data were pooled for the 44 labyrinths. The angle between the anterior and posterior SCC was 94.0 +/- 4.0 degrees (mean +/- SD). The angle between the anterior and horizontal SCC was 90.6 +/- 6.2 degrees. The angle between the horizontal and posterior SCC was 90.4 +/- 4.9 degrees. The direction angles between a vector normal to the left horizontal SCC and the positive Reid's stereotaxic X (+nasal), Y (+left), and Z (+superior) axes were 108.7 +/- 7.5 degrees, 92.2 +/- 5.7 degrees, and 19.9 +/- 7.0 degrees, respectively. The angles between a vector normal to the left anterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 125.9 +/- 5.2 degrees, 38.4 +/- 5.1 degrees, and 100.1 +/- 6.2 degrees, respectively. The angles between a vector normal to the left posterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 133.6 +/- 5.3 degrees, 131.5 +/- 5.1 degrees, and 105.6 +/- 6.6 degrees, respectively. The mean anterior SCC-contralateral posterior SCC angle was 15.3 +/- 7.2 degrees. The absolute orientations of human SCC are more nearly orthogonal than previously reported.
