Direction and distance deficits in path integration after unilateral vestibular loss depend on task complexity.

The effects of peripheral vestibular disorders on the direction and distance components of the internal spatial representation were investigated. The ability of Menière's patients to perform path integration was assessed in different situations aimed at differentiating the level of spatial processing (simple versus complex tasks), the available sensory cues (proprioceptive, vestibular, or visual conditions), and the side of the path (towards the healthy versus the lesioned side). After exploring two legs of a triangle, participants were required either to reproduce the exploration path, to follow the reverse path, or to take a shortcut to the starting point of the path (triangle completion). Patients' performances were recorded before unilateral vestibular neurotomy (UVN) and during the time-course of recovery (1 week and 1 month) and were compared to those of matched control subjects tested at similar time intervals. Both the angular and linear path components of the trajectory were impaired for patients compared to controls. However, deficits were restricted to the complex tasks, which required a higher level of spatial processing. Most deficits were maximal 1 week after UVN, and some remained up to the first post-operative month. Spatial representation was differentially impaired according to the available sensory cues: deficits were absent in active locomotor blindfolded condition, appeared in conditions involving visual and vestibular information, and were maximal when visual cues alone were available. Finally, concerning the side of the path, unilateral vestibular loss led to global impairment of the internal spatial representation, yet some asymmetrical spatial performances were observed 1 week after UVN. On the whole, results suggest that the environment experienced by the patients is different after UVN and that a different internal spatial representation is constructed, especially for tasks requiring high levels of spatial processing.