Neural oscillations in speech: Don't be enslaved by the envelope

In a recent “Perspective” article (Giraud and Poeppel, 2012), Giraud and Poeppel lay out in admirable clarity how neural oscillations and, in particular, nested oscillations at different time scales, might enable the human brain to understand speech. They provide compelling evidence for “enslaving” of ongoing neural oscillations by slow fluctuations in the amplitude envelope of the speech signal, and propose potential mechanisms for how slow theta and faster gamma oscillatory networks might work together to enable a concerted neural coding of speech. This model is unparalleled in its fruitful incorporation of state-of-the-art computational models and neurophysiology (e.g., the intriguing pyramidal–interneuron gamma loops, PING – which will unfortunately not be observable in healthy, speech-processing humans within the near future). The authors propose a scenario focused on theta and gamma, where problems in speech comprehension are sorted out if (and only if) the brain syncs well enough to the amplitude fluctuations of the incoming signal.