Activation in auditory cortex by speechreading in hearing people: FMRI studies

Which brain networks support simple silent speechreading (identifying spoken numbers by eye alone)? We summarize recent fMRI findings that show auditory cortex is reliably activated by seen speech. Activation of these regions is robust under numerous testing conditions, and parts of the network are specific to speechreading rather than viewing faces performing rhythmic lower face movements that cannot be construed as speech (facial gurning). In one adult with a longstanding malfunction of auditory cortex, speechreading was both functionally impaired and cortically anomalous. Areas traditionally described as auditory cortex are probably specialized for the perception of natural segmented language, rather than for the perception of heard signals. 1. AUDITORY CORTEX (BA 41, 40, 42 and 22) IS ACTIVATED BY SPEECHREADING Classical neurological studies of lesions in human cerebral cortex have indicated the speechprocessing regions of the brain [1]. Many of these insights have been confirmed and refined by recent brain imaging techniques [2]. The primary reception area for heard speech (primary auditory cortex, BA 41) is located in a restricted area, lying near the middle of the lower lip of the major lateral “fold” in the human cortex, the sylvian fissure. Patients with bilateral destruction of this region show cortical deafness: they are unable reliably to report the identity of a spoken utterance, or of a musical or environmental sound [3]. Secondary (association) cortex surrounds this restricted cortical area, and receives projections from it, and is on the lateral surface of the temporal lobe (BA 22,42 – superior temporal gyrus). These regions occupy the length of the superior temporal gyrus (ridge) along the lower lip of the sylvian fissure, extending up to the supramarginal gyrus (SMG BA 40) Figure 1: A schematic view of the brain (LH), showing secondary auditory cortex (BA 22 and BA 42) and primary auditory cortex (BA 41) – all on the superior temporal gyrus. The approximate positions of the sectional scans in Figures 2 and 3 are also shown. BA 40 occupies the supramarginal gyrus (SMGlateral parietal-temporal junction) and is also considered part of secondary auditory cortex. In patients with destruction of this region after language has been normally acquired, even when damage is confined to the left hemisphere, the perception of heard speech is impaired, although intact speaking skills (fluency) can often be demonstrated. The linguistic function of the superior temporal gyrus (STG) may not be determined solely by auditory experience, for there are suggestions that this region is implicated in the perception of sign language by native deaf signers [4-6]. STG also has a role in “reading-for-sound”, in tasks such as detecting rhyme in word pairs of different spellings, and in verbal working memory tasks such as digit or word span [7-10]. One common feature of tasks that specifically activate STG (BA 42) is that they appear to involve the segmental structure (phonology) of the language. In the first fMRI study of speechreading [11], STG (including both BA 22 and BA 42) was bilaterally activated by silent speechreading, both for meaningful words (Experiment 1, see figure 2 for images and task description) and meaningless (pseudoword) utterances (Experiment 2). ISCA Archive