At the finest bit of sound from, a cat’s ear is already precisely directed towards the sound source: In contrast to humans, many animals can move their ears impressively. But as a study shows, we too still possess remnants of this ability: humans unconsciously make tiny ear movements that are connected to the direction to which they are currently drawing their attention. These reactions are not only a curious rudiment of our evolutionary history, they could also serve the development of hearing aids, say the scientists.
According to a new research, it seems clear that if one goes back only far enough in the history of evolution, humans also had ancestors, who had similarly mobile ears as many primate species do today. They use this fine motor capability to precisely align the earcups to noise sources to better capture them and locate them precisely in space. Why this ability has been lost in the course of the evolutionary history of man and his closest relatives remains unclear.
Rudimentary ear muscles
However, it is well known that humans still have rudimentary muscles in the area of the ears – some can make their ears wobble with them. The researchers, led by Daniel Strauss from the University of Saarland in Saarbrücken, have now investigated whether this rudimentary motor system of the ear still shows reactions in hearing impressions. To detect potentially invisible ear movements during hearing, the researchers captured the control signals of the nervous system using surface electromyograms. To do this, they have glued some volunteer sensors to the skin that record electrical impulses in the area of the ear muscles.
Equipped in this way, the subjects took part in auditory tests. Then, researchers examined two types of attention: To assess those who automatically experience unexpected sounds, they surprised study participants by making sudden noises from different lateral positions while they were busy reading. In the second test, the scientists examined the experimental question in goal-oriented attention, which is typical for active listening. The subjects were supposed to capture a short story from a lateral speaker, while ignoring a “competing” story from the opposite side.
Tiny alignment movements
As the researchers report, the evaluations of electrical activity in both experiments showed that fine movements of the rudimentary muscles in the human ear reflect the direction of the sounds that a person pays attention to. Although they elude our ability to observe them, the subtle movements are even visually detectable, further research has shown. During the experiments, the researchers made high-resolution video recordings of the subjects’ ears. Evaluations on the computer then made the tiny movements visible.
Depending on the type of stimulus, subtle upward movements of the ear and varying degrees of reverse movements of the side edge of the earcup occur, the scientists report. “Humans have most likely maintained a rudimentary orientation system that attempts to control the movement of their earcups, which has persisted as a ‘neural fossil’ in the brain for about 25 million years,” Strauss says.
According to him and his colleagues, the significance of the study may go beyond biology: “Our results show that with the electromyogram of the ear muscles, there is an easy way to capture auditory attention. It can not only be used in basic research, but also interesting applications,” says Strauss. For example, the system could be used to develop hearing aids. Special functions could detect the electrical activity of the ear muscles at lightning speed. The hearing aid could then detect the direction in which the ears are trying to orient themselves and adjust the amplification of the directional microphones accordingly. “This could amplify the sounds that the wearer is trying to hear, while the system suppresses the sounds that he or she trying to ignore,” Strauss says.