Task-dependent Modulation Masking of 4 Hz Envelope Following Responses.

The perception and recognition of natural sounds, like speech, rely on the processing of slow amplitude modulations. Perception can be hindered by interfering modulations at similar rates, a phenomenon known as modulation masking. Cortical envelope following responses (EFRs) are highly sensitive to these slow modulations, but it is unclear how modulation masking impacts these cortical envelope responses. To dissociate stimulus-driven and attention-driven effects, we recorded EEG responses to 4 Hz modulated noise in a two-way factorial design, varying the level of modulation masking and intermodal attention. Auditory stimuli contained one of three random masking bands in the stimulus envelope, at various proximities in modulation frequency to the 4 Hz target, or an unmasked reference condition. During EEG recordings, the same stimuli were presented while participants performed either an auditory or a visual change detection task. Attention to the auditory modality resulted in a general enhancement of sustained EFR responses to the 4 Hz target. In the visual task condition only, EFR 4 Hz power systematically decreased with increasing modulation masking, consistent with psychophysical masking patterns. However, during the auditory task, the 4 Hz EFRs were unaffected by masking and remained strong even with the highest degrees of masking. Rather than indicating a general bottom-up modulation selective process, these results indicate that the masking of cortical envelope responses interacts with attention. We propose that auditory attention allows robust tracking of masked envelopes, possibly through a form of glimpsing of the target, whereas envelope responses to task-irrelevant auditory stimuli reflect stimulus salience.