Human face-to-face communication is essentially audiovisual. Typically, people talk to us face-to-face, providing concurrent auditory and visual input. Understanding someone is easier when there is visual input, because visual cues like mouth and tongue movements provide complementary information about speech content. Here, we hypothesized that, even in the absence of visual input, the brain optimizes both auditory-only speech and speaker recognition by harvesting speaker-specific predictions and constraints from distinct visual face-processing areas. To test this hypothesis, we performed behavioral and neuroimaging experiments in two groups: subjects with a face recognition deficit (prosopagnosia) and matched controls. The results show that observing a specific person talking for 2 min improves subsequent auditory-only speech and speaker recognition for this person. In both prosopagnosics and controls, behavioral improvement in auditory-only speech recognition was based on an area typically involved in face-movement processing. Improvement in speaker recognition was only present in controls and was based on an area involved in face-identity processing. These findings challenge current unisensory models of speech processing, because they show that, in auditory-only speech, the brain exploits previously encoded audiovisual correlations to optimize communication. We suggest that this optimization is based on speaker-specific audiovisual internal models, which are used to simulate a talking face. fMRI ͉ multisensory ͉ predictive coding ͉ prosopagnosia H uman face-to-face communication works best when one can watch the speaker's face (1). This becomes obvious when someone speaks to us in a noisy environment, in which the auditory speech signal is degraded. Visual cues place constraints on what our brain expects to perceive in the auditory channel. These visual constraints improve the recognition rate for audiovisual speech, compared with auditory speech alone (2). Similarly, speaker identity recognition by voice can be improved by concurrent visual information (3). Accordingly, audiovisual models of human voice and face perception posit that there are interactions between auditory and visual processing streams (Fig. 1A) (4, 5) . Based on prior experimental (6-8) and theoretical work (9-12) we hypothesized that, even in the absence of visual input, the brain optimizes auditory-only speech and speaker recognition by harvesting predictions and constraints from distinct visual face areas (Fig. 1B) . Experimental studies (6, 8) demonstrated that the identification of a speaker by voice is improved after a brief audiovisual experience with that speaker (in contrast to a matched control condition). The improvement effect was paralleled by an interaction of voice and face-identity sensitive areas (8). This finding suggested that the associative representation of a particular face facilitates the recognition of that person by voice. However, it is unclear whether this effect also extends to other audiovisual dependencies in human communication. Such a finding, for example in the case of speech recognition, would indicate that the brain fills-in missing information routinely to make auditory communication more robust. To test this hypothesis, we asked the following question: What does the brain do when we listen to someone whom we have previously seen talking? Classical speech processing models (the ''auditory-only'' model) predict that the brain uses auditory-only processing capabilities to recognize speech and speaker (13, 14) . Under the ''audiovisual'' model, we posit that the brain uses previously learned audiovisual speaker-specific information to improve recognition of both speech and speaker (Fig. 1B) . Even without visual input, face-processing areas could use encoded knowledge about the visual orofacial kinetics of talking and simulate a specific speaker to make predictions about the trajectory of what is heard. This visual online simulation would place useful constraints on auditory perception to improve speech recognition by resolving auditory ambiguities. This constructivist view of perception has proved useful in understanding human vision (15, 16) and may be even more powerful in the context of integration of prior multimodal information. To identify such a mechanism in speech perception would not only have immediate implications for the ecological validity of auditory-only models of speech perception but would also point to a general principle of how the brain copes with noisy and missing information in human communication. Speech and speaker recognition largely rest on two different sets of audiovisual correlations. Speech recognition is based predominantly on fast time-varying acoustic cues produced by the varying vocal tract shape, i.e., orofacial movements (17, 18). Conversely, speaker recognition uses predominantly timeinvariant properties of the speech signal, such as the acoustic properties of the vocal tract length (19) . If the brain uses stored visual information for processing auditory-only speech, the relative improvement in speech and speaker recognition could, therefore, be behaviorally and neuroanatomically dissociable. To investigate this potential dissociation, we recruited prosopagnosics who have impaired perception of face identity but seem to have intact perception of orofacial movements (20) . Neurophysiological face processing studies indicate that distinct brain areas are specialized for processing time-varying information [facial movements, superior temporal sulcus (STS)
Similarly, in the neuroscience community, the authors of REF investigated the influence of the facebenefit in speech and speaker recognition.
Simulation of talking faces in the human brain improves auditory speech recognition.
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Medicine Computer Science