Auditory evoked fields differentially encode speech features: an MEG investigation of the P50m and N100m time courses during syllable processing

The functional organization of speech sound processing in the human brain and its unfolding over time are still not well understood. While the N100/N100m is a comparatively well-studied, and quite late, component of the auditory evoked field elicited by speech, earlier processes such as those reflected in the P50m remain to be resolved. Using magnetoencephalography, the present study follows up on previous reports of N100m-centred spatiotemporal encoding of phonological features and coarticulatory processes in the auditory cortex during consonant-vowel syllable perception. Our results indicate that the time course and response strength of the P50m and N100m components of evoked magnetic fields are differentially influenced by mutually exclusive place-of-articulation features of a syllable's stop consonant and vowel segments. Topographical differences in P50m generators were driven by place contrasts between consonants in syllables, with spatial gradients orthogonal to the ones previously reported for N100m. Peak latency results replicated previous findings for the N100m and revealed a reverse pattern for the earlier P50m (shorter latencies depending on the presence of a back vowel [o]). Our findings allow attribution of a role in basic feature extraction to the comparatively early P50m time window. Moreover, the observations substantiate the assumption that the N100m response reflects a more abstract phonological representational stage during speech perception.