Neural Oscillations Carry Speech Rhythm through to Comprehension

Abstract

A key feature of speech is the quasi-regular rhythmic information contained in its slow amplitude modulations. In this article we review the information conveyed by speech rhythm, and the role of ongoing brain oscillations in listeners’ processing of this content. Our starting point is the fact that speech is inherently temporal, and that rhythmic information conveyed by the amplitude envelope contains important markers for place and manner of articulation, segmental information, and speech rate. Behavioral studies demonstrate that amplitude envelope information is relied upon by listeners and plays a key role in speech intelligibility. Extending behavioral findings, data from neuroimaging – particularly electroencephalography (EEG) and magnetoencephalography (MEG) – point to phase locking by ongoing cortical oscillations to low-frequency information (~4–8 Hz) in the speech envelope. This phase modulation effectively encodes a prediction of when important events (such as stressed syllables) are likely to occur, and acts to increase sensitivity to these relevant acoustic cues. We suggest a framework through which such neural entrainment to speech rhythm can explain effects of speech rate on word and segment perception (i.e., that the perception of phonemes and words in connected speech is influenced by preceding speech rate). Neuroanatomically, acoustic amplitude modulations are processed largely bilaterally in auditory cortex, with intelligible speech resulting in differential recruitment of left-hemisphere regions. Notable among these is lateral anterior temporal cortex, which we propose functions in a domain-general fashion to support ongoing memory and integration of meaningful input. Together, the reviewed evidence suggests that low-frequency oscillations in the acoustic speech signal form the foundation of a rhythmic hierarchy supporting spoken language, mirrored by phase-locked oscillations in the human brain.

Is there a common oscillatory brain mechanism for producing and predicting language?

“…Patterns of brain connectivity (not necessarily reflected in amplitude modulations, but for example in synchronous phase oscillations) represent the coupling between brain regions that could support top-down communication to primary sensory regions (Sensory Predictions). Such long-range connectivity has been reported for low-frequency phase oscillations (in the theta range ~4-7 Hz) in reading (Molinaro et al, 2013a) and speech perception (Peelle & Davis, 2012). In addition, cross-frequency coupling between lowfrequency phase entrainment (delta, <4 Hz) and the amplitude of high-frequency neural activity (gamma, 30-80 Hz) has been associated with enhanced perceptual attention (Lakatos, Karmos, Mehta, Ulbert & Schroeder, 2008).…”

Entrained neural oscillations in multiple frequency bands comodulate behavior

“…Hence, natural rhythms have the potential to act as pacemakers for neural oscillations in a number of frequency bands simultaneously (30). Consider, for example, speech processing, which is suggested to be supported by entrainment of theta-band neural oscillations by amplitude fluctuations corresponding to the syllable envelope (26,31). Notably, however, speech is a rhythmically complex stimulus possessing not only theta-band amplitude fluctuations but also slower delta-band frequency fluctuations corresponding to prosodic contour (32,33).…”

Oscillatory phase shapes syllable perception

“…The apparent relevant oscillation range is therefore theta, with periods ranging between 111 and 250 ms (4-9 Hz). This oscillation range has already been proposed as a candidate to encode information, and seems specifically important for speech perception (13,14).…”

“…The apparent relevant oscillation range is therefore theta, with periods ranging between 111 and 250 ms (4-9 Hz). This oscillation range has already been proposed as a candidate to encode information, and seems specifically important for speech perception (13,14).To test this hypothesis of oscillatory phase biasing auditory syllable perception in the absence of visual signals, we presented ambiguous auditory syllables that could be interpreted as /da/ or /ga/ while recording EEG. In a second experiment, we used sensory entrainment (thereby externally enforcing oscillatory patterns) to demonstrate that entrained phase indeed determines whether participants identify the presented ambiguous syllable as being either /da/ or /ga/.…”