After a period of exposure to auditory-visual spatial disparity, sound localization is systematically shifted in the direction of the visual stimulus. In order to investigate the locus of the underlying processes, we asked patients with parietal (i.e.,neglect) and occipital lesion (i.e.,hemianopic) to localize weak sounds by laser pointing, before and after a 4 min adaptation to repetitive auditory-visual stimulation. During the adaptation phase, bimodal stimuli could be presented spatially displaced (8°) or spatially coincident (0°), in either hemifield. After adaptation to spatial disparity in the normal field, both groups showed significant shifts toward the visual stimulus. In contrast, after adaptation in the affected field, there was a significant shift in neglect patients, but not in the hemianopic patients. This reveals the importance of visual cortex for the remapping of auditory space: a lesion of this area prevents any form of visual bias. Interestingly, after exposure to coincident stimuli, all patients showed a reduction in localization error in both hemifields, with greater improvement for sounds presented at the adapted location. Such an effect may be explained by a covert processing of visual stimuli, possibly mediated by spared retino-tectal pathway.
The present results suggest the independence of mechanisms underlying visual bias and audio-visual enhancement in the recalibration of auditory space.