Selective Listening to Unpredictable Sound Sequences Increases Tonic Muscle Activity in the Human Vestigial Auriculomotor System.

Recent investigations have revealed that selective attention to lateralized speech increases ipsilateral tonic electromyographic activity in the vestigial human auriculomotor system. However, it has yet to be determined whether this modulation depends upon predictive cues that are inherent in continuous speech or whether it is a general concomitant of selective attention to sounds in the auditory periphery. The present study addressed this question by replacing speech with randomized, unpredictable sequences of brief tonal stimuli in a dichotic listening task that necessitated a sustained anticipatory focus of attention. Participants (8 female, 23 male) were presented with sequences of brief tone bursts in one ear and frequency-modulated "chirps" in the other ear and were instructed to focus on sounds in one ear and report attenuated deviant stimuli in that ear. Posterior auricular muscle (PAM) activity was recorded behind both ears, and non-rectified stimulus-locked responses were assessed to ensure the reliability of PAM activity. Recordings of non-stimulus-locked rectified activity indicated that ipsilateral tonic PAM amplitudes were elevated when same-side sounds were attended, and follow-up analyses demonstrated that these modulations were independent of sound-evoked PAM reflexes. These findings provide evidence that this ipsilateral tonic increase in PAM activity is generally present in scenarios of lateralized selective listening and not reliant on predictive linguistic cues that may facilitate tracking of the attended stream. Due to its accessibility and capability of decoding the spatial focus of attention, this PAM modulation could support the development of intelligent hearing devices that maximize sensitivity towards a user's listening goals.Significance Statement This study demonstrates that ipsilateral tonic activity of the posterior auricular muscle (PAM) is elevated when lateralized sounds are selectively attended, even when stimulation sequences have an intermittent and unpredictable structure that requires a sustained, non predictive mode of attending. This represents an important extension of previous studies that revealed similar modulations during listening to continuous speech and suggests that the tonic modulation of PAM activity is generalizable across different scenarios of selective listening. Additionally, the present findings support the emerging view that the human auriculomotor system, despite its vestigial expression, operates in a remarkably nuanced manner. Integrating the observed modulations into intelligent hearing devices could provide an easily accessible means for decoding the lateralized focus of endogenously directed auditory attention.