The influence of visual and auditory information on the perception of speech and non-speech oral movements in patients with left hemisphere lesions.

Patients with lesions of the left hemisphere often suffer from oral-facial apraxia, apraxia of speech, and aphasia. In these patients, visual features often play a critical role in speech and language therapy, when pictured lip shapes or the therapist's visible mouth movements are used to facilitate speech production and articulation. This demands audiovisual processing both in speech and language treatment and in the diagnosis of oral-facial apraxia. The purpose of this study was to investigate differences in audiovisual perception of speech as compared to non-speech oral gestures. Bimodal and unimodal speech and non-speech items were used and additionally discordant stimuli constructed, which were presented for imitation. This study examined a group of healthy volunteers and a group of patients with lesions of the left hemisphere. Patients made substantially more errors than controls, but the factors influencing imitation accuracy were more or less the same in both groups. Error analyses in both groups suggested different types of representations for speech as compared to the non-speech domain, with speech having a stronger weight on the auditory modality and non-speech processing on the visual modality. Additionally, this study was able to show that the McGurk effect is not limited to speech.