Brain regions recruited for the effortful comprehension of noise-vocoded words

Abstract

We used functional magnetic resonance imaging (fMRI) to investigate the neural basis of comprehension and perceptual learning of artificially degraded [noise vocoded (NV)] speech. Fifteen participants were scanned while listening to 6-channel vocoded words, which are difficult for naïve listeners to comprehend, but can be readily learned with appropriate feedback presentations. During three test blocks, we compared responses to potentially intelligible NV words, incomprehensible distorted words and clear speech. Training sessions were interleaved with the test sessions and included paired presentation of clear then noise-vocoded words: a type of feedback that enhances perceptual learning. Listeners' comprehension of NV words improved significantly as a consequence of training. Listening to NV compared to clear speech activated left insula, and prefrontal and motor cortices. These areas, which are implicated in speech production, may play an active role in supporting the comprehension of degraded speech. Elevated activation in the precentral gyrus during paired clear-then-distorted presentations that enhance learning further suggests a role for articulatory representations of speech in perceptual learning of degraded speech.

Keywords:

• Vocoded speech
• Neuroimaging
• Motor system
• Left inferior frontal cortex

Acknowledgements

This work was supported by a United Kingdom Medical Research Council PhD studentship to Alexis Hervais-Adelman, and by Medical Research Council funding of the Cognition and Brain Sciences Unit (MC_US_A060_0038) and by the Canada Research Chairs Programme (Ingrid S. Johnsrude). We would also like to thank two anonymous reviewers for their helpful comments and suggestions.