The vestigial pinna-orienting system in humans briefly suppresses superior auricular muscle activity during reflexive orienting toward auditory stimuli

Abstract

The vestigial pinna-orienting system in humans is capable of increasing the activity of several auricular muscles in response to lateralized transient auditory stimuli. For example, transient increases in electromyographic activity in the posterior auricular muscle (PAM) to an attention-capturing stimulus have been documented. For the current study, surface electromyograms (EMGs) were recorded from the PAMs and superior auricular muscles (SAMs) of ten normal-hearing participants. During the experiments, lateralized transient auditory stimuli, such as, a crying baby, a shattering vase, or the participant's first names, were presented. These transient stimuli were either presented in silence, or when participants actively listened to a podcast. While ipsilateral PAM activity increased in response to transient stimuli, the SAM displayed the opposite behaviour, i.e., a brief, ipsilateral suppression of activity. This suppression of ipsilateral SAM activity was more frequent on the right (75%) than left side (35%), whereas an ipsilateral PAM increase was roughly equal in prevalence on the two sides (left: 90%, right: 95%). During the active listening task, SAM suppression on the right ear was significantly larger in response to ipsilateral stimuli, compared to contralateral ones (p = 0.002), whereas PAM activity increased significantly (p = 0.002). Overall, this study provides evidence of a systematic transient suppression of the SAM during exogenous attention. This could suggest a more complex system than previously assumed, as the presence of synchronized excitatory and inhibitory components in different auricular muscles points towards a coordinated attempt at reflexively orienting the pinna towards a sound.