High-frequency neural activity predicts word parsing in ambiguous speech streams

During speech listening, the brain parses a continuous acoustic stream of information into computational units (e.g., syllables or words) necessary for speech comprehension. Recent neuroscientific hypotheses have proposed that neural oscillations contribute to speech parsing, but whether they do so on the basis of acoustic cues (bottom-up acoustic parsing) or as a function of available linguistic representations (top-down linguistic parsing) is unknown. In this magnetoencephalography study, we contrasted acoustic and linguistic parsing using bistable speech sequences. While listening to the speech sequences, participants were asked to maintain one of the two possible speech percepts through volitional control. We predicted that the tracking of speech dynamics by neural oscillations would not only follow the acoustic properties but also shift in time according to the participant's conscious speech percept. Our results show that the latency of high-frequency activity (specifically, beta and gamma bands) varied as a function of the perceptual report. In contrast, the phase of low-frequency oscillations was not strongly affected by top-down control. Whereas changes in low-frequency neural oscillations were compatible with the encoding of prelexical segmentation cues, high-frequency activity specifically informed on an individual's conscious speech percept.

Organization in the Perception of Speech by Young Infants

Evidence is presented that 3- and 4-month-old infants are able to integrate two sounds with different sources and locations to form a coherent speech percept. Synthetic speech patterns were presented dichotically so that one ear received the third-formant transition appropriate for the syllable [da] or [ga], and the other car received the base, that is, the remaining acoustic information necessary for syllabic perception. Adults typically perceive these stimuli as a birdlike chirp at the ear receiving the transition and. depending on which transition is presented, as the syllable [da] or [ga] at the ear receiving the base. Infants discriminated the two dichotic patterns. They also discriminated them when the third-formant transitions were attenuated to the extent that infant listeners could not discriminate them when they were presented in isolation. The results support the contention that the infants integrated the two disparate sources of acoustic information into a coherent percept that is presumably phonetic in nature, and they are also consistent with the view that this organization arises from a specialized system for the perception of speech.