Abstract
Low-gamma (25–45 Hz) and theta (4–8 Hz) oscillations are proposed to underpin the integration of phonemic and syllabic information, respectively. How these two scales of analysis split functions across hemispheres is unclear. We analyzed cortical responses from an epileptic patient with a rare bilateral electrode implantation (stereotactic EEG) in primary (A1/BA41 and A2/BA42) and association auditory cortices (BA22). Using time-frequency analyses, we confirmed the dominance of a 5–6 Hz theta activity in right and of a low-gamma (25–45 Hz) activity in left primary auditory cortices (A1/A2), during both resting state and syllable processing. We further detected high-theta (7–8 Hz) resting activity in left primary, but also associative auditory regions. In left BA22, its phase correlated with high-gamma induced power. Such a hierarchical relationship across theta and gamma frequency bands (theta/gamma phase-amplitude coupling) could index the process by which the neural code shifts from stimulus feature- to phonological-encoding, and is associated with the transition from evoked to induced power responses. These data suggest that theta and gamma activity in right and left auditory cortices bear different functions. They support a scheme where slow parsing of the acoustic information dominates in right hemisphere at a syllabic (5–6 Hz) rate, and left auditory cortex exhibits a more complex cascade of oscillations, reflecting the possible extraction of transient acoustic cues at a fast (~25–45 Hz) rate, subsequently integrated at a slower, e.g., syllabic one. Slow oscillations could functionally participate to speech processing by structuring gamma activity in left BA22, where abstract percepts emerge.
Highlights
Low-gamma (25–45 Hz) and theta (4–8 Hz) oscillations could under pin the chunking of continuous speech into phonemic and syllabic segments, respectively (Poeppel, 2003; Giraud and Poeppel, 2012)
Oscillatory activity was present in all three-regions (A1, A2, and BA22) and peaked at 7.5 Hz, with an additional peak at 15 Hz
SPECIFIC ROLES OF EVOKED THETA/GAMMA ACTIVITY To address the function of theta and gamma activity in right and left auditory cortices, we examined TF activity in response to syllables
Summary
Low-gamma (25–45 Hz) and theta (4–8 Hz) oscillations could under pin the chunking of continuous speech into phonemic and syllabic segments, respectively (Poeppel, 2003; Giraud and Poeppel, 2012). Asymmetric endogenous oscillatory activity is compatible with distinct integration properties in right and left auditory cortices, because they could drive alternating low- and high-neuronal excitability states at different time-scales (Schroeder et al, 2008). This functional asymmetry is a plausible neurophysiological substrate of the greater sensitivity of the left auditory cortex to short sound segments and brief speech features (Jamison et al, 2006; Obleser et al, 2008), and of the greater sensitivity of the right one to slower acoustic fluctuations and larger steady speech signals such as vowels and syllables (Boemio et al, 2005; Abrams et al, 2008; Telkemeyer et al, 2009).
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