Perceptual adaptation to degraded speech: Tuning in cortical and subcortical brain structures

Listeners show a remarkable ability to adjust to degraded speech input. One such example is noise-vocoded speech, where the temporal envelope of the auditory signal is preserved, while the spectral information is highly degraded. However, the functional neuroanatomy of such short-term perceptual adjustments is not understood. Here, we conducted a functional MRI study of vocoded-speech learning. In a sparse-sampling, cardiac-gated fMRI acquisition, 30 normal-hearing participants listened to 100 4-band-vocoded sentences and repeated back what they had understood. Clear-speech sentences and silent trials served as baseline conditions. As adaptation to vocoded speech is correlated with individual sensitivity to temporal envelope rate discrimination, participants additionally performed an amplitude modulation (AM) rate discrimination task (where AM rates were centered on 4 Hz) in the scanner. Vocoded more than clear speech activated a network comprising the anterior insula and caudate bilaterally, left anterior cingulate, and left inferior frontal gyrus (IFG), whereas clear speech activated more the superior temporal cortices bilaterally. This is consistent with “effortful” listening. Interestingly, we observed a very similar pattern of activation in AM discrimination when we contrasted indiscriminable (i.e., standard and comparison were modulated at the same rate) against easily discriminable AM rate stimuli. Trial-by-trial fluctuations in vocoded speech recognition were positively correlated with hemodynamic signal change in the temporal cortices, left IFG, thalamus and globus pallidus bilaterally, reflecting intelligibility. When we modeled individual learning curves as linear fits to speech recognition over time, we found that activity in the anterior cingulate, the putamen, medial geniculate body (MGB) and thalamic reticular nuclei decreased with adaptation. In conclusion, we have shown that trial-by-trial fluctuations in speech recognition are reflected in activity changes in a network comprising the auditory cortices, left IFG and the thalamus. Adaptation-related changes of the fMRI signal were in particular observed in subcortical structures such as the MGB, which is likely to reflect re-tuning of an acoustic-perceptual network, with faster adapters down-regulating this network more