Effects of acute vestibular lesions on visual orientation and spatial memory, shown for the visual straight ahead.

The aim of this study was to investigate the relation between the vestibular system and orientation in space and the role of acute vestibular lesions in spatial memory. In a first step it could be shown that determining the visual straight ahead is systematically influenced by caloric irrigation: deviations up to 10 degrees appeared, which are directed to the slow phase of the simultaneously provoked nystagmus. In a second step, 20 healthy subjects and 20 patients suffering from an acute vestibular disorder were asked to adjust a luminous point visual straight ahead. Whereas the healthy subjects' visual straight ahead was in a range of +/-2 degrees, the vestibular patients showed significantly greater deviations. If healthy subjects and patients were asked to replace a luminous point, which was exposed for a short time, to its original position, young and older healthy subjects showed good performance, with an error not greater than 3 degrees, whereas the vestibular patients showed significantly greater inaccuracy. We conclude that the visual straight ahead is a vestibular-dependent parameter of visual orientation and that spatial memory is influenced by acute vestibular lesions. For the first time it is shown that acute vestibular lesions impair spatial memory. This can be explained by the close relationship between the vestibular system and the hippocampus.