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BBB Seminar: Karsten Specht

When a sound becomes a word
Tracing the ventral stream for speech perception with functional imaging: a meta-analysis


Karsten Specht
Department of Biological and Medical Psychology, Bergen fMRI-Group, University of Bergen

The temporal lobe is the region of the brain that is considered to be of most crucial importance for converting an acoustic signal into a speech signal. Here, a hierarchical processing network is assumed that extends from the primary auditory cortex in the upper posterior part of the temporal lobe, towards the anterior part of the temporal lobe. This flow of information along the posterior-anterior axes of the temporal lobe is also known as the “ventral stream”. In this meta-analysis, this information flow was tracked with a series of successive functional imaging studies on auditory, phonetic, phonological, lexical, and semantic processing, in combination with new analytical approaches, such as dynamic causal modelling (DCM), independent component analysis (ICA), as well as support vector machine (SVM). The results demonstrate that the gradual flow of information, originating from the primary auditory cortex towards anterior portions of the temporal lobe, in particular involves the superior temporal gyrus and sulcus. This was seen in both hemispheres, but it became also evident that the left temporal lobe is of increasing importance as the sound becomes more and more a clear, intelligible, and meaningful speech signal. In this respect, a lateralisation gradient is emerging with an increasing lateralisation towards the left. While the primary auditory processing did not show any lateralisation, the activity became increasingly left-lateralised when the phonetic, sub-lexical and, finally, the lexico-semantic information is processed. In addition, a contribution of the “dorsal stream” was observed, but only during the processing of distorted speech signals.

Chair: Helge Nordby , Department of Biological and Medical Psychology