Visual recalibration of auditory spatial perception: two separate neural circuits for perceptual learning.

A remarkable example of rapid perceptual learning is the visual recalibration of auditory spatial perception, which can result in either a bias (ventriloquism after-effect) or an improvement (multisensory enhancement) in auditory localization. Here, we examine the possibility that these after-effects might depend on two distinct neural pathways (geniculostriate vs. collicular-extrastriate). To this end, patients with a lesion of the striate cortex (hemianopic patients) or temporoparietal cortex (neglect patients) were asked to localize weak sounds, before and after a brief exposure to repetitive auditory-visual stimulation which was given either in the normal or in the affected field. Adaptation comprised spatially disparate (Experiment 1) or spatially coincident (Experiment 2) auditory-visual stimuli. After exposure to spatially disparate stimuli in the normal field, all patients exhibited the usual shifts toward the visual attractor, at each sound location. In contrast, when the same kind of adaptation was given in the affected field, a consistent shift was still evident in neglect patients but not in patients with hemianopia. After adaptation to spatially coincident stimuli, and regardless of the adaptation hemifield, all patients exhibited a significant improvement in auditory localization, which was largest for sounds presented at the adapted location. The findings suggest the presence of two distinct recalibration mechanisms. Adapting to spatially conflicting stimuli invokes a corrective mechanism implemented within the geniculostriate circuit, which tries to reduce the registered discrepancy. Adapting to spatially aligned inputs invokes a mechanism implemented along a collicular-extrastriate circuit, which tries to reduce the localization error.