Neural processing along the ascending auditory pathway is often associated with a progressive reduction in characteristic processing rates. For instance, the well-known frequency-following response (FFR) of the auditory midbrain, as measured with electroencephalography (EEG), is dominated by frequencies from ~100 Hz to several hundred Hz, phase-locking to the stimulus waveform at those frequencies. In contrast, cortical responses, whether measured by EEG or magnetoencephalography (MEG), are typically characterized by frequencies of a few Hz to a few tens of Hz, time-locking to acoustic envelope features. In this study we investigated a crossover, cortically generated responses time-locked to continuous speech features at FFR-like rates. Using MEG, we analyzed high-frequency responses (70-300 Hz) to continuous speech using neural source-localized reverse correlation and its corresponding temporal response functions (TRFs). Continuous speech stimuli were presented to 40 subjects (17 younger, 23 older adults) with clinically normal hearing and their MEG responses were analyzed in the 70-300 Hz band. Consistent with the insensitivity of MEG to many subcortical structures, the spatiotemporal profile of these response components indicated a purely cortical origin with ~40 ms peak latency and a right hemisphere bias. TRF analysis was performed using two separate aspects of the speech stimuli: a) the 70-300 Hz band of the speech waveform itself, and b) the 70-300 Hz temporal modulations in the high frequency envelope (300-4000 Hz) of the speech stimulus. The response was dominantly driven by the high frequency envelope, with a much weaker contribution from the waveform (carrier) itself. Age-related differences were also analyzed to investigate a reversal previously seen along the ascending auditory pathway, whereby older listeners show weaker midbrain FFR responses than younger listeners, but, paradoxically, have stronger cortical low frequency responses. In contrast to both these earlier results, this study does not find clear age- related differences in high frequency cortical responses. Finally, these results suggest that EEG high (FFR-like) frequency responses have distinct and separable contributions from both subcortical and cortical sources. Cortical responses at FFR-like frequencies share some properties with midbrain responses at the same frequencies and with cortical responses at much lower frequencies.

High Frequency Cortical Processing of Continuous Speech in Younger and Older Listeners