Early auditory sensory processing is facilitated by visual mechanisms

There is compelling evidence that low-level sensory areas are sensitive to more than one modality. For example, auditory cortices respond to visual-only stimuli (Calvert et al., 1997; Meyer et al., 2010; Pekkola et al., 2005) and conversely, visual sensory areas respond to sound sources even in auditory-only conditions (Poirier et al., 2005; von Kriegstein et al., 2008; von Kriegstein and Giraud, 2006). Currently, it is unknown what makes the brain activate modality-specific, sensory areas solely in response to input of a different modality. One reason may be that such activations are instrumental for early sensory processing of the input modality — a hypothesis that is contrary to current text book knowledge. Here we test this hypothesis by harnessing a temporally highly resolved method, i.e., magnetoencephalography (MEG), to identify the temporal response profile of visual regions in response to auditory-only voice recognition. Participants ( n = 19 ) briefly learned a set of voices audio–visually, i.e., together with a talking face in an ecologically valid situation, as in daily life. Once subjects were able to recognize these now familiar voices, we measured their brain responses using MEG. The results revealed two key mechanisms that characterize the sensory processing of familiar speakers’ voices: (i) activation in the visual face-sensitive fusiform gyrus at very early auditory processing stages, i.e., only 100 ms after auditory onset and (ii) a temporal facilitation of auditory processing (M200) that was directly associated with improved recognition performance. These findings suggest that visual areas are instrumental already during very early auditory-only processing stages and indicate that the brain uses visual mechanisms to optimize sensory processing and recognition of auditory stimuli.