Fearful faces in peripheral vision: The vestigial auriculomotor system as an indicator of affective processing

The human auriculomotor system responsible for orienting the pinna has been named a "neural fossil" considering its vestigial nature. However, it remains a subject of interest in cognitive research and has been shown to be modulated by attention, motivation, and affection. This study explores audiovisual integration in an emotion recognition paradigm by examining this pinna-orienting system, focusing on peripheral vision. 23 participants were evaluated in an experimental paradigm requiring discrimination between fearful and neutral facial expressions at three eccentricities (22.5°, 45°, and 67.5°) in the peripheral visual field. Post-auricular muscle (PAM) activity was measured in response to an auditory chirp presented at 260 ms after visual stimulation, the analysis considering eccentricity, facial emotion, and recognition accuracy. We adopt a broader perspective on the post-auricular muscle function by examining both the PAM reflex elicited by a sound and PAM's ongoing activity outside the audio stimulus. The recognition rates in the emotion discrimination task exceeded chance level for the 22.5° and 45° eccentricities. Electromyographic spike activity of the post-auricular muscle was measured and revealed reduced myographic spiking activity in the interval between face stimulus presentation and onset of auditory stimuli. Correctly identified fearful faces elicited a significantly higher PAM reflex compared to neutral faces at the most outer eccentricity (67.5°). In regard to previous studies reporting attenuated activity for stimuli of negative valence, specifically angry faces, these findings raise questions about the origin of the modulation. We propose that the PAM reflex may not be exclusively modulated by emotional valence, but be part of a broader threat-avoidance strategy. We further discuss the possibility that reduced spiking activity after visual stimulation is indicative of increased receptivity to subsequent auditory cues. The observations offer a novel perspective on the pinna-orienting system in emotional processing and attentional mechanisms, providing an exploratory account that encourages further investigation of the underlying causes.