Betahistine treatment improves the recovery of static symptoms in patients with unilateral vestibular loss.

Vestibular loss induces a combination of postural, oculomotor, and perceptive symptoms that are compensated over time. The aim of this study was to analyze the influence of betahistine dihydrochloride on vestibular compensation. A randomized, double-blind, placebo-controlled study was performed in Menière's disease patients who underwent a curative unilateral vestibular neurotomy (UVN). The effects of betahistine treatment were investigated on a broad spectrum of vestibular-induced changes resulting from vestibular loss: body sway, head orientation, ocular cyclotorsion, spontaneous nystagmus, verticality perception, and self-evaluation of the postural stability. The time course of the recovery was compared in 16 patients who received either a placebo or betahistine (24 mg b.i.d.) from 3 days up to 3 months after UVN. Patients were examined before (day -1) and after UVN (days 7, 30, and 90). Results indicate that betahistine reduces the time to recovery by 1 month or more depending on the tested functions. Betahistine was effective as soon as 4 days after treatment administration, and the effect remained during the whole compensation period (up to 3 months). The observed clinical effects may be attributed to an action of betahistine in rebalancing the neuronal activity between contralateral vestibular nuclei.

Response delay and spatial representation in pointing movements.

Pointing movements decrease in accuracy when target information is removed before movement onset. This time effect was analyzed in relation with the spatial representation of the target location, which can be egocentric (i.e. in relation to the body) or exocentric (i.e. in relation to the external world) depending on the visual environment of the target. The accuracy of pointing movements performed without visual feedback was measured in two delay conditions: 0 and 5-s delay between target removal and movement onset. In each delay condition, targets were presented either in the darkness (egocentric localization) or within a structured visual background (exocentric localization). The results show that pointing was more accurate when targets were presented within a visual background than in the darkness. The time-related decrease in accuracy was observed in the darkness condition, whereas no delay effect was found in the presence of a visual background. Therefore, contextual factors applied to a simple pointing action might induce different spatial representations: a short-lived sensorimotor egocentric representation used in immediate action control, or a long-lived perceptual exocentric representation which drives perception and delayed action.

Role of visual context and oculomotor conditions in pointing accuracy.

Localizing a target in the extrapersonal space may rely on two types of spatial coordinate systems: egocentric or exocentric. Two experiments investigated the role of these systems in the accuracy of goal-directed movements. The accuracy of pointing movements performed without visual feedback of the hand was measured in two conditions of target presentation (darkness or within a visually structured background), and in two conditions of eye-hand coordination (eye fixed on a fixation point, or with a foveation saccade). The results showed (1) that pointing accuracy increased in the presence of a visual background, and (2) enhancement of this beneficial effect by a steady retinal image of target and background, that is without a foveation saccade. An object that has to be reached in the prehension space can be localized in two ways: (1) in relation to the body (egocentric), induced by the absence of a structured visual environment, (2) in relation to the external space (exocentric), favored by the presence of a visually structured background. We investigated the importance of the localizing mode in the accuracy of reaching movements, and how the two modes can be optimized. We found better reaching accuracy with exocentric than with egocentric localization, particularly when the retinal image of the goal and its environment was stabilized in the absence of eye movements.