The ability to rapidly encode the direction of frequency contour in frequency-modulated (FM) sweeps is essential for speech processing. Psychophysical evidence points to a common temporal window threshold for human listeners in processing rapid changes in frequency glides. No neural evidence has been provided for the existence of a cortical temporal window threshold underlying the encoding of rapid transitions in frequency glides. The present magnetoencephalography study used the cortical mismatch negativity activity (MMNm) to investigate the minimum temporal window required for detecting different magnitudes of directional changes in frequency sweeps. An oddball paradigm was used in which upward or downward frequency sweep serves as the standard and the same type of sweep with the opposite direction serves as its deviant. Stimuli consisted of unidirectional frequency-sweep complexes that swept across speech-relevant frequency bands in durations of 10, 20, 40, 80, 160, and 320 ms. Data revealed significant MMNm responses across all durations, with slower-rate sweeps eliciting larger MMNm responses. A greater temporally-related enhancement in MMNm response was obtained for rising but not falling sweeps. A hemispheric asymmetry in MMNm response pattern was observed corresponding to sweep direction. Contrary to psychophysical findings, we report a temporal window as short as 10 ms sufficient to elicit a robust MMNm response to a directional change in frequency sweeps. The results suggest that the cortex requires extremely brief temporal window to implicitly differentiate a dynamic change in frequency of linguistically-relevant contours. Findings provide cortical evidence for the biological relevance of FM sweeps in speech processing.
I-Hui Hsieh, Institute of Cognitive Neuroscience, Taiwan
Wan-Ting Yeh, Institute of Cognitive Neuroscience, Taiwan
Jia-Wei Liu, Institute of Cognitive Neuroscience, Taiwan