Ventriloquism aftereffects occur in the rear hemisphere

Abstract

After exposure to a consistent spatial disparity of auditory and visual stimuli, subjective localization of sound sources is usually shifted in the direction of the visual stimuli. This study investigates whether such aftereffects can be observed in humans after exposure to a conflicting bimodal stimulation in virtual reality and whether these aftereffects are confined to the trained locations. Fourteen subjects participated in an adaptation experiment, in which auditory stimuli were convolved with non-individual head-related transfer functions, delivered via headphones. First, we assessed the auditory localization of subjects in darkness. They indicated the perceived direction of a sound using an angular pointer. We then immersed the subjects in a virtual environment by means of a head-mounted display. They were asked to reproduce sequences of movements of virtual objects with a mouse click on the objects. However, we introduced a spatial disparity of 15° between the visual event and the concurrent auditory stimulation. After 20 min of exposure, we tested the subjects again in total darkness to determine whether their auditory localization system had been modified by the conflicting visual signals. We observed a shift of subjective localization towards the left in both dorsal and frontal hemifields of the subject, mainly for auditory stimuli located in the right hemispace. This result suggests that interaural difference cues and monaural spectral cues were not equally adapted, and that visual stimuli mainly influence the processing of binaural directional cues of sound localization.