Vestibulo-Spatial Navigation: Pathways and Sense of Direction.

Aims of the present article are: 1) assessing vestibular contribution to spatial navigation, 2) exploring how age, global positioning systems (GPS) use and vestibular navigation contribute to subjective sense of direction (SOD), 3) evaluating vestibular navigation in patients with lesions of the vestibular-cerebellum (downbeat nystagmus patients, DBN) which could inform on the signals carried by vestibulo-cerebellar-cortical pathways. We applied two navigation tasks on a rotating chair in the dark: return-to-start(RTS), where subjects drive the chair back to the origin after discrete angular displacement stimuli (path reversal), and complete-the-circle(CTC) where subjects drive the chair on, all the way round to origin (path completion). We examined 24 normal controls (20-83 year old), 5 DBN patients (62-77 year old) and, as proof of principle, 2 patients with early dementia (84 and 76 year old). We found a relationship between SOD, assessed by Santa-Barbara-Sense-Of-Direction-scale, and subject's age (positive), GPS use (negative) and CTC-vestibular-navigation-task (positive). Age-related decline in vestibular navigation was observed with the RTS-task but not with the complex CTC-task. Vestibular navigation was normal in vestibulo-cerebellar patients but abnormal, particularly CTC, in the demented patients. We conclude that vestibular navigation skills contribute to the build-up of our SOD. Unexpectedly, perceived SOD in the elderly is not inferior, possibly explained by increased GPS use by the young. Preserved vestibular navigation in cerebellar patients suggests that ascending vestibular-cerebellar projections carry velocity (not position) signals. The abnormalities in the cognitively impaired patients suggest that their vestibulo-spatial navigation is disrupted.