The Pathways for Intelligible Speech: Multivariate and Univariate Perspectives

An anterior pathway, concerned with extracting meaning from
sound, has been identified in nonhuman primates. An analogous
pathway has been suggested in humans, but controversy exists
concerning the degree of lateralization and the precise location
where responses to intelligible speech emerge. We have demonstrated
that the left anterior superior temporal sulcus (STS) responds
preferentially to intelligible speech (Scott SK, Blank CC,
Rosen S, Wise RJS. 2000. Identification of a pathway for intelligible
speech in the left temporal lobe. Brain. 123:2400–2406.). A functional
magnetic resonance imaging study in Cerebral Cortex used
equivalent stimuli and univariate and multivariate analyses to argue
for the greater importance of bilateral posterior when compared
with the left anterior STS in responding to intelligible speech
(Okada K, Rong F, Venezia J, Matchin W, Hsieh IH, Saberi K, Serences
JT,Hickok G. 2010. Hierarchical organization of human auditory
cortex: evidence from acoustic invariance in the response to
intelligible speech. 20: 2486–2495.). Here, we also replicate our
original study, demonstrating that the left anterior STS exhibits the
strongest univariate response and, in decoding using the bilateral
temporal cortex, contains the most informative voxels showing an
increased response to intelligible speech. In contrast, in classifications
using local “searchlights” and a whole brain analysis, we
find greater classification accuracy in posterior rather than anterior
temporal regions. Thus, we show that the precise nature of the
multivariate analysis used will emphasize different response profiles
associated with complex sound to speech processing.