Abstract
Natural auditory stimuli are characterized by slow fluctuations in amplitude and frequency. However, the degree to which the neural responses to slow amplitude modulation (AM) and frequency modulation (FM) are capable of conveying independent time-varying information, particularly with respect to speech communication, is unclear. In the current electroencephalography (EEG) study, participants listened to amplitude- and frequency-modulated narrow-band noises with a 3-Hz modulation rate, and the resulting neural responses were compared. Spectral analyses revealed similar spectral amplitude peaks for AM and FM at the stimulation frequency (3 Hz), but amplitude at the second harmonic frequency (6 Hz) was much higher for FM than for AM. Moreover, the phase delay of neural responses with respect to the full-band stimulus envelope was shorter for FM than for AM. Finally, the critical analysis involved classification of single trials as being in response to either AM or FM based on either phase or amplitude information. Time-varying phase, but not amplitude, was sufficient to accurately classify AM and FM stimuli based on single-trial neural responses. Taken together, the current results support the dissociable nature of cortical signatures of slow AM and FM. These cortical signatures potentially provide an efficient means to dissect simultaneously communicated slow temporal and spectral information in acoustic communication signals.
Highlights
Natural auditory stimuli, including speech and non-human animal vocalizations, are characterized by slow fluctuations in amplitude and frequency
The current study focuses on the time-varying cortical representations of amplitude modulation (AM) and frequency modulation (FM), in the context of slow, speech-relevant modulation rates
In order to test for different patterns of results for AM versus FM, a 2 (Frequency: 3 Hz, 6 Hz) × 2 (Stimulus Type: FM, AM) repeated-measures ANOVA was conducted on spectral amplitude values for phase-aligned trials
Summary
Natural auditory stimuli, including speech and non-human animal vocalizations, are characterized by slow fluctuations in amplitude and frequency. An important research question concerns the degree to which the timevarying neural signatures of amplitude modulation (AM) and frequency modulation (FM) differ, and the extent to which the two modulation types are capable of communicating independent “streams” of information. In this respect, there are (at least) two levels of analysis that can be considered with respect to the nature of AM and FM processing. We directly compared the amplitude and phase characteristics of EEG responses to slow (3-Hz) AM and FM in order to characterize the features of the cortical response that would afford potential perceptual separation of the two modulation types. We used a single-trial classification approach that involved categorization of neural responses based on phase or amplitude information
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