Levels of Processing in the Pathways for Intelligible Speech
- Author(s): Maddox, Christopher Dale
- Advisor(s): Hickok, Gregory S
- et al.
There are divergent claims concerning the broad cortical organization of speechrecognition. One model holds that speech perception and comprehension is governed by a left lateralized anterior temporal lobe (ATL) pathway. Another model argues that bilateral superior temporal regions are critically important, and, in fact, represent a lower level of processing that drives ATL activation in a bottom up fashion. These models were tested in a series of auditory fMRI experiments that gradually investigated lower levels of speech analysis. The experiments contrasted listening to clear monosyllabic words, pseudowords, sentences, and word lists with unintelligible spectrally rotated and time-reversed speech. In the first experiment, posterior temporal regions did not respond differentially to sentence versus word list stimuli, consistent with the idea that bilateral regions of the superior temporal plane support speech recognition at a lower (perhaps phonological) level. An area of the ATL centered around the superior temporal sulcus (STS) was activated more for sentences than word lists, indicating that the region may be involved in sentence-level operations. In the second experiment, this same region in the left hemisphere was activated more by monosyllabic words than rotated words. This suggests that the anterior focus is not exclusively attributable to sentence-level operations. In the third experiment, lexical status was found to differentially modulate anterior and posterior STS regions. There was more activation in the aSTS bilaterally for words than pseudowords, but these conditions did not lead to activation differences in the posterior region. It appears that anterior temporal speech-selective regions respond to lexical-semantic aspects of speech, whereas posterior temporal speech-selective areas are coding lower level phonemic information.