Abstract
Despite well-established anatomical differences between primary and non-primary auditory cortex, the associated representational transformations have remained elusive. Here we show that primary and non-primary auditory cortex are differentiated by their invariance to real-world background noise. We measured fMRI responses to natural sounds presented in isolation and in real-world noise, quantifying invariance as the correlation between the two responses for individual voxels. Non-primary areas were substantially more noise-invariant than primary areas. This primary-nonprimary difference occurred both for speech and non-speech sounds and was unaffected by a concurrent demanding visual task, suggesting that the observed invariance is not specific to speech processing and is robust to inattention. The difference was most pronounced for real-world background noise—both primary and non-primary areas were relatively robust to simple types of synthetic noise. Our results suggest a general representational transformation between auditory cortical stages, illustrating a representational consequence of hierarchical organization in the auditory system.
Highlights
Despite well-established anatomical differences between primary and non-primary auditory cortex, the associated representational transformations have remained elusive
To select realworld background “noise”, we measured the stability of sound properties of hundreds of natural sounds, assigning sounds with the most stationary statistics to a background noise set (Fig. 1a)
We divided the cochleagram into small temporal segments, and in each segment separately measured three sets of perceptually relevant statistics[41,42,43]: (i) the mean of each frequency channel, (ii) the correlation across frequency channels, and (iii) the power in a set of temporal modulation filters applied to each frequency channel
Summary
Despite well-established anatomical differences between primary and non-primary auditory cortex, the associated representational transformations have remained elusive. The difference was most pronounced for real-world background noise—both primary and nonprimary areas were relatively robust to simple types of synthetic noise. A cascade of sensory processing is thought to be required to transform the incoming waveforms into a representational format where behaviorally relevant information is made explicit[1,2,3] This cascade begins with the cochlea, continues through the auditory midbrain and thalamus, and culminates in what are believed to be multiple stages of cortical processing[3,4,5,6,7,8,9]. We probe for a general sensory transformation that might differentiate stages of representation, measuring the invariance of sound-evoked responses throughout the auditory cortex to the presence of background noise. We hypothesized that robustness to these more structured sources of everyday noise might necessitate mechanisms situated later in the putative cortical hierarchy
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