Humans can no longer orient their ears towards a specific sound, unlike dogs or cats. However, this capacity has not completely disappeared. Researchers from the University of Saarland in Germany, supported by neuroscientist Steven Hackley of the University of Missouri, have shown that certain human auricular muscles still react when a person is concentrated to listen in a noisy environment.
An evolutionary vestige which still comes alive
The three main human auricular muscles – superior, posterior and anterior – are the vestiges of a system of sound orientation formerly essential to the survival of our ancestors. These muscles allowed the ears to rotate to locate the source of sound with precision. A reflex always present in many animals such as cats or horses. But over the course of evolution, with the development of binocular vision and more sophisticated vocal communication, the need to move the ears has faded. Deprived of their functional usefulness, these muscles have athoged without completely disappearing. They keep a residual connection with certain regions of the brain involved in the orientation of auditory attention. These are these connections, described as “neuronal fossil” by several researchers, that scientists track today to better understand the persistence of these mechanisms.
This ghost muscle activity, imperceptible to the naked eye, testifies to a deeply buried evolving memory in our nervous system. When a sound arises in a noisy environment or attention must focus on a specific voice, these muscles are active briefly. In other words, they still seek to direct the ear towards the sound source. Electromyographic recordings show that this response remains automatic, independent of any conscious will. And it is proportional to the listening effort perceived by the listener. This archaic reflex, unusable in practice, but always wired in our cerebral circuits, reveals the way in which our brain mobilizes ancient resources to meet contemporary sensory challenges. It is the tangible trace of a disappeared auditory adaptation which continues, at low noise, to influence our perception of the sound world.
Listening under muscle surveillance
To precisely measure the activity of the auricular muscles, the researchers used surface electromyography (EMG). This technique records the electrical signals produced by the muscles when they contract. Electrodes have been placed on the skin, around the ears of the participants, so as to capture the variations in the activity of the upper and posterior muscles. These electrodes, connected to a computer, made it possible to analyze in real time the smallest muscle activations in response to the sounds perceived. The experimental protocol has been designed to avoid interference. The volunteers were installed in an soundproofed cabin. They fixed a visual point to prevent any head movement that can distort the measurements. Thus, any detected activity was directly attributable to the involuntary muscle response of the ears in the face of listening effort.
© A. Schroeer et al., 2025
The experience was structured in several five -minute sessions, with 3 difficulty levels. Participants had to listen to an audio book while ignoring parasitic noises. In the simplest condition, the audio book was stronger than background noise. And the narrator's voice had a distinct stamp. In the intermediate condition, the noise volume increased and approached the narrator's stamp. Finally, in the most difficult condition, several parasitic voices almost completely covered the audio book.
This forced participants to redouble attention. After each session, volunteers had to subjectively assess their level of effort to follow history. They also indicated how many times they had lost the thread. By crossing this data with EMG signals, the researchers were able to establish a direct link between the intensity of muscle activation and the perceived difficulty of listening. They thus confirmed that these vestigial muscles are always sensitive to variations in auditory attention.
Between cognitive effort and auditory frustration
This discovery opens up important perspectives to better understand what it really means to “listen” to difficult environments. So far, the cognitive charge linked to listening was difficult to measure objectively. By identifying a direct link between atrial muscle activity and listening effort, researchers have a measurable physiological marker. And it does not depend on memory or the subjective perception of the person. This indicator could make it possible to better diagnose hearing fatigue. This underestimated phenomenon pushes many people to avoid complex communication situations, for lack of energy to decipher sounds. Integrated into hearing aids or portable devices, these sensors could automatically report when the effort becomes too large. It would trigger real -time adaptation of amplification or sound filtering.
But this promising track raises a crucial question. Does this muscular activity only reflect the effort of concentration? Or does it also reflect a form of frustration in the face of an overly aggressive sound environment? As Matthew Winn points out, to NPRthere is a risk of confusing an increasing annoyance – linked to the ambient cacophony – with the pure effort of attentive listening. If the muscles react to both attention and annoyance, it becomes essential to distinguish these two states to avoid distorting the interpretation of signals. This implies better understanding how the brain orchestrates this muscular response. We must also understand how it varies according to motivation, the interest in conversation or personal tolerance to noise. Ultimately, refining this distinction could on the one hand improve the personalization of hearing aids. On the other hand, it would help to shed light on the way in which the listening effort influences cognitive fatigue on a daily basis.
Source: Andreas Schroeer, et al., “Andreas Schroeerelectromyographic Correlates of Listening in the Vestigial Auricuromotor System”, Frontiers in Neuroscience (2025)
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