Most neurons in the brain’s auditory cortex detect where a sound is coming from relative to the head, but some are tuned to a sound source’s actual physical location, a new study found.
“Our brains can represent sound location in either an egocentric manner – for example, when I can tell that a phone is ringing to my left – or in an allocentric manner – hearing that the phone is on the table,” said the study’s first author, Stephen Town of University College London.
When a person moves their head, neurons with an egocentric focus will respond differently, as the phone’s position relative to the ears has changed. But the allocentric neurons will not change their response, Town said.
Researchers monitored ferrets while they moved around a small arena surrounded by speakers that emitted clicking sounds. Electrodes monitored the firing rates of neurons in the ferrets’ auditory cortex, while LEDs were used to track the animals’ movement.
Among the neurons under investigation that picked up sound location, the study showed that most displayed egocentric orientations by tracking where a sound source was relative to the animal’s head. Butt approximately 20 percent of the spatially tuned neurons instead tracked a sound source’s actual location in the world, independent of the ferret’s head movements.
The researchers also found that neurons were more sensitive to sound location when the ferret’s head was moving quickly.
“Most previous research into how we determine where a sound is coming from used participants with fixed head positions, which failed to differentiate between egocentric and allocentric tuning. Here we found that both types coexist in the auditory cortex,” said the study’s senior author, Jennifer Bizley of University College London.
The findings could help in the design of technologies involving augmented or virtual reality, the researchers said.
“We often hear sounds presented through earphones as being inside our heads, but our findings suggest sound sources could be created to appear externally, in the world, if designers incorporate information about body and head movements,” Town said.
The study is published in the journal PLOS Biology.
Source: University College London