Mapping phonemic processing zones along human perisylvian cortex: an electro-corticographic investigation

Abstract

The auditory system is organized such that progressively more complex features are represented across successive cortical hierarchical stages. Just when and where the processing of phonemes, fundamental elements of the speech signal, is achieved in this hierarchy remains a matter of vigorous debate. Non-invasive measures of phonemic representation have been somewhat equivocal. While some studies point to a primary role for middle/anterior regions of the superior temporal gyrus (STG), others implicate the posterior STG. Differences in stimulation, task and inter-individual anatomical/functional variability may account for these discrepant findings. Here, we sought to clarify this issue by mapping phonemic representation across left perisylvian cortex, taking advantage of the excellent sampling density afforded by intracranial recordings in humans. We asked whether one or both major divisions of the STG were sensitive to phonemic transitions. The high signal-to-noise characteristics of direct intracranial recordings allowed for analysis at the individual participant level, circumventing issues of inter-individual anatomic and functional variability that may have obscured previous findings at the group level of analysis. The mismatch negativity (MMN), an electro-physiological response elicited by changes in repetitive streams of stimulation, served as our primary dependent measure. Oddball configurations of pairs of phonemes, spectro-temporally matched non-phonemes, and simple tones were presented. The loci of the MMN clearly differed as a function of stimulus type. Phoneme representation was most robust over middle/anterior STG/STS, but was also observed over posterior STG/SMG. These data point to multiple phonemic processing zones along perisylvian cortex, both anterior and posterior to primary auditory cortex. This finding is considered within the context of a dual stream model of auditory processing in which functionally distinct ventral and dorsal auditory processing pathways may be engaged by speech stimuli.

Decoding of Chinese phoneme clusters using ECoG

“…discriminative to Chinese phonemes, were mostly located around the posterior superior temporal and inferior parietal areas, consistent with previous results [21]. However, SI also picks some electrodes outside these areas.…”

Automatic cortical representation of auditory pitch changes in Rett syndrome

“…While the current work does suggest that at least the early stages of the cortical auditory hierarchy show intact processing capacities for pitch differences, this study represents only a beginning step in the characterization of cortical processing abilities in this population. It is also important to note that the neural measure we used to determine that frequency processing was intact in the auditory cortex of Rett girls, the MMN component of the ERP, does not require explicit engagement on the part of the listener for its evocation [ 25 , 29 , 34 – 36 ]. That is, it is considered a largely pre-attentive measure of cortical processing and, although in healthy individuals it has been strongly linked to discrimination abilities [ 21 , 22 ], it cannot with certainty be taken to indicate that the individual can actively manipulate the information in conscious awareness [ 37 ].…”

Emergence of category-level sensitivities in non-native speech sound learning

“…Larger MMN responses are seen to deviant tokens which cross a phonetic category boundary than those that change within the category (Phillips, 2001 ). Of interest, the MMN source is thought to arise from bilateral temporal cortex, shows greater left-lateralization for native language contrasts (see Naatanen et al, 2007 for review; Zevin et al, 2010 ), and MMN responses over the left temporal lobe are larger to phonetic than non-phonetic contrasts when employing direct cortical recording (Molholm et al, 2014 ), particularly in or near the STS. This MMN response is not restricted to temporal lobes however; the MMN response is thought to have a secondary source in left prefrontal cortex (Paavilainen et al, 2003 , see further discussion of frontal contributions in section “Left inferior frontal involvement in categorical responses to native-language contrasts”).…”