Acoustic cue selection and discrimination under degradation: differential contributions of the inferior parietal and posterior temporal cortices

Task-General and Acoustic-Invariant Neural Representation of Speech Categories in the Human Brain

A significant neural challenge in speech perception includes extracting discrete phonetic categories from continuous and multidimensional signals despite varying task demands and surface-acoustic variability. While neural representations of speech categories have been previously identified in frontal and posterior temporal-parietal regions, the task dependency and dimensional specificity of these neural representations are still unclear. Here, we asked native Mandarin participants to listen to speech syllables carrying 4 distinct lexical tone categories across passive listening, repetition, and categorization tasks while they underwent functional magnetic resonance imaging (fMRI). We used searchlight classification and representational similarity analysis (RSA) to identify the dimensional structure underlying neural representation across tasks and surface-acoustic properties. Searchlight classification analyses revealed significant "cross-task" lexical tone decoding within the bilateral superior temporal gyrus (STG) and left inferior parietal lobule (LIPL). RSA revealed that the LIPL and LSTG, in contrast to the RSTG, relate to 2 critical dimensions (pitch height, pitch direction) underlying tone perception. Outside this core representational network, we found greater activation in the inferior frontal and parietal regions for stimuli that are more perceptually similar during tone categorization. Our findings reveal the specific characteristics of fronto-tempo-parietal regions that support speech representation and categorization processing.

Neural mechanisms for coping with acoustically reduced speech

In spoken language, reductions of word forms occur regularly and need to be accommodated by the listener. Intriguingly, this accommodation is usually achieved without any apparent effort. The neural bases of this cognitive skill are not yet fully understood. We here presented participants with reduced words that were either preceded by a related or an unrelated visual prime and compared electric brain responses to reduced words with those to their full counterparts. In time-domain, we found a positivity between 400 and 600 ms differing between reduced and full forms. A later positivity distinguished primed and unprimed words and was modulated by reduction. In frequency-domain, alpha suppression was stronger for reduced than for full words. The time- and frequency-domain reduction effects converge towards the view that reduced words draw on attention and memory mechanisms. Our data demonstrate the importance of interactive processing of bottom-up and top-down information for the comprehension of reduced words.