Effects of phonological and lexical processing on speech segmentation
During speech comprehension continuous speech needs to be segmented to yield chunks that correspond to linguistic units for further processing. The phase-locking of cortical theta oscillations to the speech acoustics provides a mechanism for this temporal segmentation. Linguistic processing may affect the temporal segmentation of the speech acoustics possibly due to a top-down modulation. Specific temporal dynamics might underlie this hierarchical processing, involving connectivity between frontal, motor areas and auditory cortex in the delta- and theta-band. It is unclear, however, at which linguistic level top-down effects occur and which mechanisms underlie this interaction. Here, we probe the temporal dynamics of hierarchical language processing. We recorded Magnetoencephalography during a frequency-tagging paradigm to investigate effects of lexical access and sub-lexical contingencies on the temporal segmentation at the syllabic scale. Two experiments were conducted: Experiment 1, with sequences of German (native) and Turkish (foreign) words, and Experiment 2, with sequences of German and Non-Turkish words (without sub-lexical contingencies). Syllable rate was 4 syllables/sec and word rate was 2 words/sec. Acoustic cues and sub-lexical contingencies for word grouping were removed and controlled between languages. In Experiment 1, we hypothesized brain-wave spectral peaks at 2 Hz due to lexical access, for German stimuli but not for Turkish stimuli. In Experiment 2, the effect of sub-lexical statistics was measured. In both experiments we expected top-down effects to increase connectivity between higher order processing areas and the auditory cortex. Our findings provide evidence for lexical segmentation at 2 Hz in frontal and temporal brain areas. Interestingly, participants were sensitive to sub-lexical contingencies even when listening to a non-native language. Sub-lexical contingencies resulted in broad activation increases in frontal, temporal and motor areas at 2 Hz. The findings provide new insights into the temporal dynamics and localization of hierarchical lexical-related processes.