Anne-Lise Giraud

A recent opinion article (Neural oscillations in speech: do not be enslaved by the envelope. Obleser et al., 2012) questions the validity of a class of speech perception models inspired by the possible role of neuronal oscillations in decoding speech (e.g., Ghitza, 2011; Giraud and Poeppel, 2012). The authors criticize, in particular, what they see as an over-emphasis of the role of temporal speech envelope information, and an over-emphasis of entrainment to the input rhythm while neglecting the role of top-down processes in modulating the entrainment of neuronal oscillations. Here we respond to these arguments, referring to the phenomenological model of Ghitza (2011), taken as a representative of the criticized approach.

This perspective describes how neural oscillations are likely to play an important role in speech processing, particularly in creating a discrete neural code. Neuronal oscillations are ubiquitous in the brain and may contribute to cognition in several ways: for example, by segregating information and organizing spike timing. Recent data show that delta, theta and gamma oscillations are specifically engaged by the multi-timescale, quasi-rhythmic properties of speech and can track its dynamics. We argue that they are foundational in speech and language processing, 'packaging' incoming information into units of the appropriate temporal granularity. Such stimulus-brain alignment arguably results from auditory and motor tuning throughout the evolution of speech and language and constitutes a natural model system allowing auditory research to make a unique contribution to the issue of how neural oscillatory activity affects human cognition.

How human language has evolved remains enigmatic. It is plausible to assume that mutual constraints of speech perception and production mechanisms shaped spoken communication. Speaking and understanding (speech) depend on performance limits of the related sensory and motor systems. Hence, the auditory apparatus evolved to understand speech as it is produced by the motor system, and the motor system such as to produce a signal that is decodable by the auditory system. Two theories attempt to bridge the functional properties of perceptual and motor language systems to account for the physical characteristics of the speech signal.