Simultaneous EEG and MEG recordings reveal vocal pitch elicited cortical gamma oscillations in young and older adults

Abstract

The frequency-following response with origin in the auditory brainstem represents the pitch contour of voice and can be recorded with electrodes from the scalp. MEG studies also revealed a cortical contribution to the high gamma oscillations at the fundamental frequency (f0) of a vowel stimulus. Therefore, studying the cortical component of the frequency-following response could provide insights into how pitch information is encoded at the cortical level. Comparing how aging affects the different responses may help to uncover the neural mechanisms underlying speech understanding deficits in older age. We simultaneously recorded EEG and MEG responses to the syllable /ba/. MEG beamformer analysis localized sources in bilateral auditory cortices and the midbrain. Time-frequency analysis showed a faithful representation of the pitch contour between 106 Hz and 138 Hz in the cortical activity. A cross-correlation revealed a latency of 20 ms. Furthermore, stimulus onsets elicited cortical 40-Hz responses. Both the 40-Hz and the f0 response amplitudes increased in older age and were larger in the right hemisphere. The effects of aging and laterality of the f0 response were evident in the MEG only, suggesting that both effects were characteristics of the cortical response. After comparing f0 and N1 responses in EEG and MEG, we estimated that approximately one-third of the scalp-recorded f0 response could be cortical in origin. We attributed the significance of the cortical f0 response to the precise timing of cortical neurons that serve as a time-sensitive code for pitch.