HUMAN NEUROSCIENCE GENERAL COMMENTARY Hierarchical processing for speech in human auditory cortex and beyond A commentary on Hierarchical organization of human auditory cortex: evidence from acoustic invariance in the response to intelligible speech

The anatomical connectivity of the primate auditory system suggests that sound perception involves several hierarchical stages of analysis , raising the question of how the processes required for human speech comprehension might map onto such a system. One intriguing possibility is that earlier areas of auditory cortex respond to acoustic differences in speech stimuli, but that later areas are insensitive to such features. Providing a consistent neural response to speech content despite variation in the acoustic signal is a critical feature of "higher level" speech processing regions because it indicates they respond to categorical speech information, such as phonemes and words, rather than idiosyncratic acoustic tokens. In a recent fMRI study, Okada et al. (in press) used multi-voxel pattern analysis (MVPA) to investigate neural responses to spoken sentences in canonical auditory cortex (i.e., superior temporal cortex), using a design modeled after In a standard whole-brain univariate analysis, Okada et al. found intelligibility-related responses (i.e., intelligible activity > unintelligible activity) in large portions of the superior temporal lobes bilaterally, as well as smaller activations in left inferior frontal gyrus, posterior fusiform gyrus, and premotor cortex. The authors then chose maxima for each participant within anatomically defined regions (bilateral posterior, middle, and anterior superior temporal sulcus [STS], as well as Heschl's gyrus) and performed MVPA analyses to assess the ability of these regions to discriminate among the four acoustic conditions. They found that Heschl's gyrus could reliably distinguish all conditions, despite showing a similar average hemodynamic response in the traditional mass univariate analysis. Regions of the STS showed varying degrees of sensitivity to acoustic information. In the left hemisphere, posterior STS was the most acoustically insensitive, followed by anterior STS and Heschl's gyrus (no reliable middle STS activation was identified). In the right hemisphere the greatest acoustic insensitivity was observed in middle STS, close to Heschl's gyrus, followed by anterior and posterior STS respectively. The authors interpret these findings as generally consistent with a hierarchical structure for speech processing in the temporal lobe, with regions of STS in both hemispheres playing a critical role in abstract phonological processes as indicated by their high acoustic insensitivity. With these results, Okada et al. partially replicate previous univariate fMRI results reported by Moving beyond the temporal lobe, the results o

Neural networks supporting audiovisual integration for speech: A large-scale lesion study

Auditory and visual speech information are often strongly integrated resulting in perceptual enhancements for audiovisual (AV) speech over audio alone and sometimes yielding compelling illusory fusion percepts when AV cues are mismatched, the McGurk-MacDonald effect. Previous research has identified three candidate regions thought to be critical for AV speech integration: the posterior superior temporal sulcus (STS), early auditory cortex, and the posterior inferior frontal gyrus. We assess the causal involvement of these regions (and others) in the first large-scale (N = 100) lesion-based study of AV speech integration. Two primary findings emerged. First, behavioral performance and lesion maps for AV enhancement and illusory fusion measures indicate that classic metrics of AV speech integration are not necessarily measuring the same process. Second, lesions involving superior temporal auditory, lateral occipital visual, and multisensory zones in the STS are the most disruptive to AV speech integration. Further, when AV speech integration fails, the nature of the failure-auditory vs visual capture-can be predicted from the location of the lesions. These findings show that AV speech processing is supported by unimodal auditory and visual cortices as well as multimodal regions such as the STS at their boundary. Motor related frontal regions do not appear to play a role in AV speech integration

A SENtence Supramodal Areas AtlaS (SENSAAS) based on multiple task-induced activation mapping and graph analysis of intrinsic connectivity in 144 healthy right-handers

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