Abstract
Speech perception requires that sounds be transformed into speech-related objects with lexical and semantic meaning. It is unclear at what level in the auditory pathways this transformation emerges. Primary auditory cortex has been implicated in both representation of acoustic sound attributes and sound objects. While non-primary auditory cortex located on the posterolateral superior temporal gyrus (PLST) is clearly involved in acoustic-to-phonetic pre-lexical representations, it is unclear what role this region plays in auditory object formation. Additional data support the importance of prefrontal cortex in the formation of auditory objects, while other data would implicate this region in auditory object selection. To help clarify the respective roles of auditory and auditory-related cortex in the formation and selection of auditory objects, we examined high gamma activity simultaneously recorded directly from Heschl's gyrus (HG), PLST and prefrontal cortex, while subjects performed auditory semantic detection tasks. Subjects were patients undergoing evaluation for treatment of medically intractable epilepsy. We found that activity in posteromedial HG and early activity on PLST was robust to sound stimuli regardless of their context, and minimally modulated by tasks. Later activity on PLST could be strongly modulated by semantic context, but not by behavioral performance. Activity within prefrontal cortex also was related to semantic context, and did co-vary with behavior. We propose that activity in posteromedial HG and early activity on PLST primarily reflect the representation of spectrotemporal sound attributes. Later activity on PLST represents a pre-lexical processing stage and is an intermediate step in the formation of word objects. Activity in prefrontal cortex appears directly involved in word object selection. The roles of other auditory and auditory-related cortical areas in the formation of word objects remain to be explored.
Highlights
Speech perception requires that incoming sounds be transformed into word objects
We propose that activity in posteromedial Heschl’s gyrus (HG) and early activity on posterolateral superior temporal gyrus (PLST) primarily reflect the representation of spectrotemporal sound attributes
These differences represent a contribution in the high gamma responses of phase locking to the lower fundamental frequency of the male talkers within posteromedial HG [sites (a) through (d)]
Summary
Speech perception requires that incoming sounds be transformed into word objects. It is unclear at what level in the auditory pathways this transformation occurs. Other data suggest that primary auditory cortex is more directly involved in sound object representation (Nelken, 2008; Nelken and Bar-Yosef, 2008). It is unclear what role nonprimary auditory cortex, located on the posterolateral superior. PLST is critical for acoustic-to-phonetic transformations (Boatman, 2004; Poeppel et al, 2008; Chang et al, 2010; Steinschneider et al, 2011; Mesgarani et al, 2014) This process could be interpreted as a remapping of the speech signal from one encoding acoustic attributes to one representing its phonemic components. Word object formation would be expected to take place at higher levels in auditory and auditory-related cortex (Griffiths and Warren, 2004; Griffiths et al, 2012)
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