Directional sensitivity of the auriculomotor system to transient central and lateralized auditory stimuli.

Background: Electromyographic (EMG) activity of auricular muscles in humans has been shown to be sensitive to the direction of auditory stimuli. Specifically, transient auricular EMG responses are significantly influenced by the laterality (left or right) and anteriority (in or outside the visual field) of auditory stimuli. As these factors co-occurred in previous research, this study aimed to isolate their influence, specifically of anteriority.

Methods: EMG signals of several auricular muscles were recorded from 11 participants. Transient auditory stimuli were presented from central (0° and 180°) and lateralized positions behind the participants (±150°).

Results: As previously reported, ipsilateral responses were significantly larger than contralateral responses. Surprisingly, however, responses recorded when stimuli were presented from 180° were also significantly smaller than ipsilateral responses, and approximately as large as contralateral responses. Responses generated by stimuli originating from 0° were extremely small, or almost nonexistent.

Conclusion: The implication of these results is that the main driving force of these responses is the lateralization of the auditory stimuli, while the anteriority appears to be only a secondary, minor contributor in the absence of lateralization. This behavior of the vestigial pinna-orienting system could be interpreted as an attempt to aid sound localization when visual information is not available, by, for example, changing the frequency filter properties of the pinna or by introducing interaural level differences by reorienting one pinna. Future studies investigating the auriculomotor system should be aware that presenting transient stimuli from central locations at 0° or 180° could generate only minor responses.