Investigating neural processing of syllabic and phonemic timescales in spoken language

Recent experimental and theoretical advances in neuroscience support the idea that both low (theta) and high (gamma) frequency cortical oscillations play an important role in speech processing (Giraud and Poeppel, 2012). However, experimental evidence has better established that theta activity in the auditory cortex tracks the syllabic rhythm during speech perception. This work aims at testing the hypothesis that speech is sampled in parallel at both syllabic and phonemic timescales. A new behavioural paradigm was developed, in which independent manipulations of the number of syllabic and phonemic units were performed. Intracranial recordings from ten epileptic patients with electrodes implanted primarily in auditory regions were acquired while patients listened to these sentences. Using cerebro-acoustic coherence analyses (Peelle et al., 2012), we show that theta neural activity tracks the speech envelope and allows decoding of syllabic rhythm - while low-gamma activity tracks spectral flux allowing decoding of the rate of phonemic units.