The contribution of acoustic complexity to speech-specific processing

Speech segments are delivered in an acoustically complex signal. This paper examines the role of acoustic complexity in speech-specific processing in the brain. Mismatch Negativity (MMN) is a deviance detection event-related response elicited by both speech and non-speech stimuli. Speech-specific MMN can further be sensitive to segment-driven categorisation processes (Näätänen et al., 1997). Existing research on the neural basis of speech processing often employs only pure tones as a non-speech control (Schofield et al, 2009). We investigate the specific contribution of acoustic complexity to speech-specific processing using a passive oddball task to elicit MMN in three conditions: speech, spectrally rotated speech, and pure tones. EEG event-related potentials were examined for scalp-based observations and source connectivity; source activity at the level of pyramidal cells in neuronal groups was modeled as dynamic states (Kiebel et al., 2007). Results indicate left lateralization only for acoustically complex sounds; both speech and rotated speech show within-region cortical coupling in left Heschl’s gyrus. However, an increase in feedforward activity towards left superior temporal gyrus is only observed in speech but not rotated speech. Pure tone stimuli do not elicit left lateralization. We conclude that acoustic complexity is a necessary but insufficient condition for speech-specific processing.