University of Oregon neuroscientists have shown that a person's hearing can be assessed by measuring dilation of the pupils in eyes, a method that is as sensitive as traditional methods of testing hearing.
The approach is being developed as a potential way to test hearing in babies, young adults with developmental disabilities and adults suffering from a stroke or illness -- populations where direct responses are not possible.
In the experiments, changes in pupil size of 31 adults were monitored with eye-tracking technology for about three seconds as they performed a traditional tone-based hearing test while also staring at an object on a monitor. Dilation in all subjects matched their subsequent push-button responses when prompted by a question mark on the screen, signifying whether or not a tone was heard.
The project, detailed in an open-access paper published online last month in the Journal of the Association for Research in Otolaryngology, was inspired more than a decade ago when the study's lead author, Avinash Singh Bala, noticed changes in the pupils of barn owls in response to unexpected noises in their environment.
In the interim, Bala and co-author Terry T. Takahashi -- whose lab studies how owls hear and process sounds -- researched how an eye-focused hearing exam could produce results with the same sensitivity as traditional tone-based exams.
"This study is a proof of concept that this is possible," said Bala, a researcher in Takahashi's lab in the UO Institute of Neuroscience. "The first time we tested a human subject's pupil response was in 1999. We knew it could work, but we had to optimize the approach for capturing the detection of the quietest sounds."
In the research, a traditional hearing exam and eye-tracking methods were done simultaneously to allow for comparison. A dot appeared on the screen, and tones at 1, 2, 4, and 8 kilohertz were played at randomized delays to make sure subjects couldn't predict when the sound would appear.
Pupil size was tracked for at least one second before the sound, and two seconds after the sound. Then, the dot on the screen changed to a question mark, cuing participants to push one of two buttons to indicate whether or not they heard the sound. Since just pushing a button can change pupil size, the button press task was delayed until after pupil size had been tracked for two seconds.
"In this project, we randomized the timing of the tone's pulsing in relation to the dots, which also helped us avoid the expectation of a tone within a pattern," said Takahashi, a professor of biology and member of the Institute of Neuroscience.
Pupils began to change within 250 milliseconds, about one-quarter of a second, of the sound stimulus. The swiftness of the response, Bala said, allowed the team "to see and establish causality."
"What we found was that pupil dilation was as sensitive as the button-press method," Bala said. "We had presented early data analyses at conferences, and there was a lot of resistance to the idea that by using an involuntary response we could get results as good as button-press data."
Internal grants from the UO supported the research. Elizabeth A. Whitchurch, a former doctoral student now at Humboldt State University, was a co-author.
"A pupil dilation test is not as useful in adults, who can communicate with the tester," Takahashi said. "The utility of the method is in testing people who can't tell us whether they heard a sound -- for example, babies."
Takahashi and Bala are now part of a university-supported collaboration with Dare Baldwin, a UO psychology professor, to test the approach in babies. The two neuroscientists also have formed a UO spinout, Perceptivo LLC, to pursue development of an infant-hearing assessment.
Article first appeared on Science Daily