Differentiation of speech and nonspeech processing within primary auditory cortex

Abstract

Primary auditory cortex (PAC), located in Heschl's gyrus (HG), is the earliest cortical level at which sounds are processed. Standard theories of speech perception assume that signal components are given a representation in PAC which are then matched to speech templates in auditory association cortex. An alternative holds that speech activates a specialized system in cortex that does not use the primitives of PAC. Functional magnetic resonance imaging revealed different brain activation patterns in listening to speech and nonspeech sounds across different levels of complexity. Sensitivity to speech was observed in association cortex, as expected. Further, activation in HG increased with increasing levels of complexity with added fundamentals for both nonspeech and speech stimuli, but only for nonspeech when separate sources (release bursts/fricative noises or their nonspeech analogs) were added. These results are consistent with the existence of a specialized speech system which bypasses more typical processes at the earliest cortical level.

The motor theory of speech perception reviewed

“…At the moment, such evidence is difficult to obtain. Research conducted under a weaker criterion-that is, a finding that some circuit of the nervous system is active if speech is either perceived or produced-suggests that speech is processed by a neural circuit different from the circuit that processes nonvocal sounds (Belin, Zatorre, Lafaille, Ahad, & Pike, 2000;Whalen et al, 2006). However, the neural circuit that processes speech seems to be the same as the one that processes other nonphonetic vocal sounds (e.g., laughs or coughs).…”

An investigation of prototypical and atypical within-category vowels and non-speech analogues on cortical auditory evoked related potentials (AERPs) in 9year old children

“…P1 in response to incoming sounds and has been described as a stage dedicated to mere sensory processing of sounds (Martin et al, 2008) and recent functional magnetic resonance imaging (fMRI) results support a "low-level" processing interpretation of primary auditory cortex function (Whalen et al, 2006). However, as Young (2008) reviews, numerous studies using finer measurements such as direct recordings from auditory cortical neurons in animals show that these neurons react to more than simply the spectro-temporal aspects of auditory stimuli if a stimulus contains relevant information for the behaviour of the organism, a finding which also received support from recent fMRI studies in humans (Staeren et al, 2009;Woods and Alain, 2009).…”

Phoneme and word recognition in the auditory ventral stream

“…42, 45, 47-50, but see refs. [51][52][53]. The hierarchical model predicts an increase in neural receptive field size and complexity along the ventral stream.…”