Early subcortical response at the fundamental frequency of continuous speech measured with MEG

Abstract

Most parts of speech are voiced, exhibiting a degree of periodicity with a fundamental frequency and many higher harmonics. Some neural populations respond to this temporal fine structure, in particular at the fundamental frequency. This frequency-following response to speech (speech-FFR) consists of both subcortical and cortical contributions and can be measured through electroen-cephalography (EEG) as well as through magnetoencephalography (MEG), although both differ in the aspects of neural activity that they capture: EEG is sensitive to both radial and tangential sources as well as to deep sources, while MEG is more restrained to the measurement of tangential and superficial neural activity. EEG responses to continuous speech have shown an early subcortical contribution, at a latency of around 9 ms, in agreement with MEG measurements in response to short speech tokens, whereas MEG responses to continuous speech have not yet revealed such an early component. Here we analyze MEG responses to long segments of continuous speech. We find an early subcortical response at a latency of 9 ms, followed by later right-lateralized cortical activities at delays of 20 - 57 ms as well as potential subcortical activities. Our results show that the early subcortical component of the FFR to continuous speech can be measured from MEG, and that its latency agrees with that measured with EEG. They furthermore show that the early subcortical component is temporally well separated from later cortical contributions, enabling an independent assessment of both components towards further aspects of speech processing.