Poster D101, Monday, March 27, 8:00 – 10:00 am, Pacific Concourse
Associations between neurochemistry and oscillatory speech coding
Roeland Hancock1, Srikantan Nagarajan1, Fumiko Hoeft1; 1University of California, San Francisco
A hierarchy of oscillatory neural processing can provide a mechanism for de-multiplexing and coding auditory signals. In particular, gamma band oscillations, nested within theta oscillations, may provide a basis for discretizing the speech signal. These oscillations are dependent on cortical excitation-inhibition balance, partially regulated by gamma aminobutyric acid (GABA). The present study examined how neural synchronization to the speech signal is modulated by resting GABA levels. We hypothesized that higher levels of GABA, reflecting greater cortical inhibition, would be associated with greater information processing at higher gamma band (>40 Hz) frequencies. Magnetoencephalographic recordings were obtained from 28 healthy right-handed adults while they listened to conversational speech and GABA was measured from the left superior temporal gyrus (STG) using magnetic resonance spectroscopy (MRS). Mutual information analyses revealed coupling between the phase of theta (4-7 Hz) oscillations in left primary auditory cortex (PAC) and the speech envelope. Consistent with previous research, the amplitude of gamma oscillations in left PAC provided additional, synergistic information about speech theta phase. This effect was examined in two low-gamma frequency bands, 25-35 Hz and 35-45Hz. As predicted, the amount of information synergy provided by gamma amplitude at 35-45Hz relative to 25-35Hz was positively correlated with GABA concentration (r=.58, p<.005). Synergy at 35-45Hz was also positively correlated with Word Attack scores, a measure of phonological processing (r=.51, p < .01). These results suggest that levels of cortical inhibition in the STG, as measured by MRS, are associated with the frequency at which the speech signal is encoded.
Topic Area: PERCEPTION & ACTION: Audition