Our distant ancestors had several body parts that have become fairly useless over the course of evolution–our wisdom teeth, the tails that are present during the sixth week of gestation, and the auricular muscles in our ears. The auricular muscles helped change the shape of the shell–or pinna–of the ear and helped humans living millions of years ago listen closely. They are still used by roughly 10 to 20 percent of people with the fun ability to wiggle their ears.
However, these vestigial muscles might be used more than scientists once believed. The auricular muscles appear to activate when humans are trying to listen to competing sounds, not just when wiggling the ears. The findings are detailed in a study published January 31 in the journal Frontiers in Neuroscience.
“There are three large muscles which connect the auricle to the skull and scalp and are important for ear wiggling,” Andreas Schröeer, a study co-author and neuroscientist from Saarland University in Germany, said in a statement. “These muscles, particularly the superior auricular muscle, exhibit increased activity during effortful listening tasks. This suggests that these muscles are engaged not merely as a reflex but potentially as part of an attentional effort mechanism, especially in challenging auditory environments.”
Sounds vying for attention
This new study used a technique called electromyography to try to determine how hard a person is listening without asking individuals to report it. Electromyography measures the electrical activity in a muscle and can help scientists pinpoint activity in the auricular muscles that is already linked to listening closely. Similar research found the largest muscles–posterior and superior auricular muscles–react during attentive listening. These muscles pull the ears up and back and scientists believe that they have historically been involved in moving the pinna in order to capture sounds.
“The exact reason these became vestigial is difficult to tell, as our ancestors lost this ability about 25 million years ago,” Schröeer said. “One possible explanation could be that the evolutionary pressure to move the ears ceased because we became much more proficient with our visual and vocal systems.”
To see if these muscles are more active during more difficult listening tasks, the team studied 20 people without any documented hearing problems. They first put electrodes on the participants’ auricular muscles. Then, they played an audiobook and some distracting podcasts from speakers positioned in front of or behind the listeners. All 20 participants underwent 12 five-minute trials, covering three different levels of difficulty.
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During easy mode, the podcast was quieter than the audiobook, while the speaker’s voice was a stronger contrast to the audiobook. In the medium and difficult modes, the researchers added a podcast that sounded more like the audiobook and made the distractors louder. However, they had to make even the most difficult situations achievable because if the participants gave up, a physiological effort would not register on the machines.
The team then asked the study participants to rate their effort levels and to estimate how often they lost what was going on during the audiobook in each trial. Participants were also quizzed on the material in the audiobook.
Just trying its best
The team found that two auricular muscles reacted differently to the various conditions. The posterior auricular muscles reacted to changes in direction. By comparison, the superior auricular muscles reacted to the difficulty level of the task. The participants’ self-reported measures of how hard it was to listen and how often they lost track of the audiobook also increased in line with the difficulty of the task. The accuracy of their responses to questions about the audiobook also dropped significantly between the medium and the difficult mode.
According to the authors, this correlated with the superior auricular muscles’ activity levels. These muscles did not activate more during the medium mode than they did during the easy mode, but were very active during the difficult mode. This suggests that the activity of these muscles could provide an objective measure of listening effort. However, it remains unclear if the muscle activity actually helps people hear better.
“The ear movements that could be generated by the signals we have recorded are so minuscule that there is probably no perceivable benefit,” said Schröeer. “However, the auricle itself does contribute to our ability to localize sounds. So, our auriculomotor system probably tries its best after being vestigial for 25 million years, but does not achieve much.”
Additional research is necessary to confirm the results and develop any practical applications. Like many hearing studies, the sample size was relatively small and made up of younger individuals without hearing problems. Larger and more diverse participant groups tested in more real-world conditions are needed in order to confidently say that these muscles are doing more than just sitting in our ears.
“Investigating the possible effects of muscle strain itself or the ear’s miniscule movements on the transmission of sound is something we want to do in the future,” said Schröer. “The effect of these factors in people with hearing impairments would also be interesting to investigate.”
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