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In naturalistic situations, sounds are often perceived in conjunction with matching visual impressions. For example, we see and hear the neighbor’s dog barking in the garden. Still, there is a good chance that we recognize the neighbor’s dog even when we only hear it barking, but do not see it behind the fence. Previous studies with normal-hearing (NH) listeners have shown that the audio-visual presentation of a perceptual object (like an animal) increases the probability to recognize this object later on, even if the repeated presentation of this object occurs in a purely auditory condition. In patients with a cochlear implant (CI), however, the electrical hearing of sounds is impoverished, and the ability to recognize perceptual objects in auditory conditions is significantly limited. It is currently not well understood whether CI users – as NH listeners – show a multisensory facilitation for auditory recognition. The present study used event-related potentials (ERPs) and a continuous recognition paradigm with auditory and audio-visual stimuli to test the prediction that CI users show a benefit from audio-visual perception. Indeed, the congruent audio-visual context resulted in an improved recognition ability of objects in an auditory-only condition, both in the NH listeners and the CI users. The ERPs revealed a group-specific pattern of voltage topographies and correlations between these ERP maps and the auditory recognition ability, indicating a different processing of congruent audio-visual stimuli in CI users when compared to NH listeners. Taken together, our results point to distinct cortical processing of naturalistic audio-visual objects in CI users and NH listeners, which however allows both groups to improve the recognition ability of these objects in a purely auditory context. Our findings are of relevance for future clinical research since audio-visual perception might also improve the auditory rehabilitation after cochlear implantation.

A cochlear implant (CI) is an auditory prosthesis which can partially restore the auditory function in patients with severe to profound hearing loss. However, this bionic device provides only limited auditory information, and CI patients may compensate for this limitation by means of a stronger interaction between the auditory and visual system. To better understand the electrophysiological correlates of audiovisual speech perception, the present study used electroencephalography (EEG) and a redundant target paradigm. Postlingually deafened CI users and normal-hearing (NH) listeners were compared in auditory, visual and audiovisual speech conditions. The behavioural results revealed multisensory integration for both groups, as indicated by shortened response times for the audiovisual as compared to the two unisensory conditions. The analysis of the N1 and P2 event-related potentials (ERPs), including topographic and source analyses, confirmed a multisensory effect for both groups and showed a cortical auditory response which was modulated by the simultaneous processing of the visual stimulus. Nevertheless, the CI users in particular revealed a distinct pattern of N1 topography, pointing to a strong visual impact on auditory speech processing. Apart from these condition effects, the results revealed ERP differences between CI users and NH listeners, not only in N1/P2 ERP topographies, but also in the cortical source configuration. When compared to the NH listeners, the CI users showed an additional activation in the visual cortex at N1 latency, which was positively correlated with CI experience, and a delayed auditory-cortex activation with a reversed, rightward functional lateralisation. In sum, our behavioural and ERP findings demonstrate a clear audiovisual benefit for both groups, and a CI-specific alteration in cortical activation at N1 latency when auditory and visual input is combined. These cortical alterations may reflect a compensatory strategy to overcome the limited CI input, which allows the CI users to improve the lip-reading skills and to approximate the behavioural performance of NH listeners in audiovisual speech conditions. Our results are clinically relevant, as they highlight the importance of assessing the CI outcome not only in auditory-only, but also in audiovisual speech conditions.