Ancient Human Ear-Orienting System Helps Find Clues To Hearing Deficits In Infants

Researchers may have detected the clues to hearing deficits in infants, by studying ancient vestigial muscles which are found behind the human ear.

Steven Hackley, a psychologist at the University of Missouri, found that vestigial muscles behind the ears in humans have determined that ancient neural circuits, responsible for moving the ears, may still be responsive to sounds that attract our attention, according to a news release.

These ancient muscles could be used to study positive emotions and infant hearing deficits.

"There is a 'cognitive fossil' that lies more or less intact in the human brain and could be more than 25 million years old. Significant changes in the human auditory system began soon after the evolution of dry-nosed primates more than 30 million years ago. Ear size decreased and the associated musculature changed," Hackley said.

In the study, Hackley examined over 60 published studies on vestigial ear muscles and he found that research on the muscles dates back more than a century. Scientists discovered that human subjects who shifted their gaze to the left or right weakly activated a muscle within the posterior wall of the outer ear, or pinna. In later studies, scientists found that there was weak electrical activity triggered within vestigial muscles when either interesting or intense sounds were introduced.

The pinna musculature activates during standard laboratory tests of attention and that makes it useful to psychologists and neuroscientists studying evolution and the brain. When the subjects were presented with relevant sounds, reflexes moved the eyes and attempted to move the ears toward the sounds they wanted to hear and see. The reflexes were too weak to cause actual movement of the ears, according to Hackley's study.

"Understanding pinna orientation and the ancient muscles that caused it could help researchers who study auditory attention in humans," said Hackley said. "Finally, gaining insight into how the relevant vestigial systems work may assist ongoing efforts to develop tests of hearing deficits in infants and, perhaps surprisingly, objective measures of positive emotions in adults."

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