Audiovisual integration of speech in a patient with Broca's Aphasia

Abstract

Lesions to Broca's area cause aphasia characterized by a severe impairment of the ability to speak, with comparatively intact speech perception. However, some studies have found effects on speech perception under adverse listening conditions, indicating that Broca's area is also involved in speech perception. While these studies have focused on auditory speech perception other studies have shown that Broca's area is activated by visual speech perception. Furthermore, one preliminary report found that a patient with Broca's aphasia did not experience the McGurk illusion suggesting that an intact Broca's area is necessary for audiovisual integration of speech. Here we describe a patient with Broca's aphasia who experienced the McGurk illusion. This indicates that an intact Broca's area is not necessary for audiovisual integration of speech. The McGurk illusions this patient experienced were atypical, which could be due to Broca's area having a more subtle role in audiovisual integration of speech. The McGurk illusions of a control subject with Wernicke's aphasia were, however, also atypical. This indicates that the atypical McGurk illusions were due to deficits in speech processing that are not specific to Broca's aphasia.

Keywords: Broca's area; aphasia; audiovisual; multisensory integration; speech perception.

Figure 1

CT-scans of the lesions of patients ML (Broca's aphasic) and MK (Wernicke's aphasic). Arrows indicate the approximate location of the inferior frontal gyrus (IFG) in Patient ML and the superior temporal sulcus (STS) in Patient MK. See main text for a description of the patients' lesions.

Figure 2

Response proportions for all participants. Plots are arranged in rows and columns according to the visual and auditory component of the stimulus respectively. The response category “X” corresponds to no answer. Error bars show the multinomial standard deviation.

Figure 3

The proportion correct (according to the auditory stimulus) as a function of stimulus type. For the incongruent audiovisual stimuli, a low proportion correct reflects a strong McGurk illusion. Only stimuli expected to elicit visual dominance, fusion or combination illusions are included in the incongruent audiovisual stimulus category (see text for details). Within subject comparisons are based on Fisher's exact one-sided test. Error bars show the binomial standard deviation.