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Hierarchical Processing for Speech in Human Auditory Cortex and Beyond

J. Peelle,I. Johnsrude,Matthew H. Davis

Published 2010 in Frontiers in Human Neuroscience

ABSTRACT

The anatomical connectivity of the primate auditory system suggests that sound perception involves several hierarchical stages of analysis (Kaas et al., 1999), 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. (2010) 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 Scott et al. (2000). Okada et al. (2010) used a factorial design that crossed speech clarity (clear speech vs. intelligible noise vocoded speech) with frequency order (normal vs. spectrally rotated). Noise vocoding reduces the amount of spectral detail in the speech signal but faithfully preserves temporal information. Depending on the reduction in spectral resolution (i.e., the number of bands used in vocoding), noise vocoded speech can be highly intelligible, especially following training. By contrast, spectral rotation of the speech signal renders it almost entirely unintelligible without any change in overall level of spectral detail. Thus, the clear and vocoded sentences used by Okada et al. (2010) provided two physically dissimilar presentations of intelligible speech that the authors could use to identify acoustically insensitive neural responses; spectrally rotated stimuli allowed the authors to look for response changes due to intelligibility, independent of reductions in spectral detail.

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