Auditory Cortex Encodes the Perceptual Interpretation of Ambiguous Sound

Abstract

The confounding of physical stimulus characteristics and perceptual interpretations of stimuli poses a problem for most neuroscientific studies of perception. In the auditory domain, this pertains to the entanglement of acoustics and percept. Traditionally, most study designs have relied on cognitive subtraction logic, which demands the use of one or more comparisons between stimulus types. This does not allow for a differentiation between effects due to acoustic differences (i.e., sensation) and those due to conscious perception. To overcome this problem, we used functional magnetic resonance imaging (fMRI) in humans and pattern-recognition analysis to identify activation patterns that encode the perceptual interpretation of physically identical, ambiguous sounds. We show that it is possible to retrieve the perceptual interpretation of ambiguous phonemes-information that is fully subjective to the listener-from fMRI measurements of brain activity in auditory areas in the superior temporal cortex, most prominently on the posterior bank of the left Heschl's gyrus and sulcus and in the adjoining left planum temporale. These findings suggest that, beyond the basic acoustic analysis of sounds, constructive perceptual processes take place in these relatively early cortical auditory networks. This disagrees with hierarchical models of auditory processing, which generally conceive of these areas as sets of feature detectors, whose task is restricted to the analysis of physical characteristics and the structure of sounds.

Categorical Speech Processing in Broca's Area: An fMRI Study Using Multivariate Pattern-Based Analysis

“…Thus, the findings here may indicate that Broca's area and the SMG use different spatial scales (intervoxel vs intravoxel, respectively) to process speech inputs. Recently, MVPA studies have shown distributed neural patterns for speech perception within large expanses of the temporal lobes (Formisano et al, 2008;Kilian-Hütten et al, 2011). One possible reason for the discrepancy between our main findings and those of previous studies is that auditory representations of phonemes are coded by patterns of activity distributed across large expanses of temporal cortex, while frontal representations are coded in patterns across smaller expanses.…”

“…P, posterior; A, anterior; L, left; R, right. Poldrack, 2007;Desai et al, 2008;Hutchison et al, 2008;Chang et al, 2010;Zevin et al, 2010;Kilian-Hütten et al, 2011). Nevertheless, theories implicating sensorimotor mapping in speech sound categorization (Devlin and Aydelott, 2009) suggest frontal regions should be involved as well.…”

“…This technique, based on multivariate patternanalysis (MVPA) (Haxby et al, 2001;Pereira et al, 2009), probes local information of neural pattern differences across different conditions by moving a searchlight unit on a voxel-by-voxel basis. Some fMRI studies have used MVPA to examine speech perception (Formisano et al, 2008;Okada et al, 2010;Raizada et al, 2010;Kilian-Hütten et al, 2011). Most, however, examined the neural activity patterns exclusively within superior temporal regions.…”

Learning of New Sound Categories Shapes Neural Response Patterns in Human Auditory Cortex

“…The lateral posterior part of HG and the posteriorly adjacent areas have previously been shown to code perceptual states rather than purely acoustic differences of sounds (Kilian-Hütten et al, 2011). Furthermore, these areas have been used to reliably decode speaker information from natural and variable speech sounds .…”

“…It has been proposed that multivoxel pattern analysis (MVPA) is sensitive to changes in distributed activation patterns in absence of changes in overall activation level (Haxby et al, 2001). This method has been successfully used to reveal subtle differences in overlapping sound representations Staeren et al, 2009) and purely perceptual processes in the visual (Li et al, 2007(Li et al, , 2009) and auditory (Kilian-Hütten et al, 2011) domain.…”