Brain Cells Pause to Determine What We Hear
According to scientists at the Laboratory for Auditory Neurophysiology at KU Leuven in Belgium, cells in the brainstem that govern sound localization can compare signals at both ears and appear to pause while doing so. In a February 9, 2015, article in the online edition of Nature Neuroscience, researchers explain how they obtained both in- and outgoing electrical signals of these brain cells in a study using Mongolian gerbils in order to understand how we know where a sound source is located. Through their study, the researchers discovered that our brain takes a moment to determine the location of sound by computing differences between the signals that reach our two ears.
“The sound of a source to your right reaches both ears, but the sound at your right ear arrives a tiny bit earlier and is slightly more intense than the sound at your left ear,” said Philip X. Joris, PhD, of the Laboratory of Auditory Neurophysiology, KU Leuven. “Our brain computes and interprets such differences in intensity and arrival time between the two ears. Humans are particularly sensitive to the time differences–we can detect differences 100 times smaller than a thousandth of a second.”
In an announcement from KU Leuven, Joris explained that sound stimulates our cochlea, or inner ear, which transmits electrical pulses via the auditory nerve to cells in the brainstem, which in turn compare the sounds at the two ears.
“The brainstem contains an array of hyper-specialized cells which each prefer a certain time difference,” said Joris. “For example, one cell may respond to sounds right in front of us which reach both ears at the same time, while another cell may respond to sounds to our side, which reach the ears with a time difference of half a millisecond. Depending on which cell is active, we know where the sound source is in space. But how cells compute this time difference has been a matter of conjecture because it is exceedingly difficult to study these cells in the brainstem.”
Until now, the researchers say, it was thought that cells in the brainstem function as coincidence detectors. It was generally believed that for sound localization, somewhere along the path from ear to brainstem cells, any time difference was compensated for by slowing the signal from one ear, so that the signals from both ears would arrive coincidentally at a brainstem cell, which would then fire off an electrical pulse.
According to the announcement from KU Leuven, Tom P. Franken, PhD, conducted doctoral research during which he tested this hypothesis by studying brain cells in gerbils. He reportedly inserted a fine electrode into cells of the brainstem to record both their in- and outgoing signals, and observed that, when stimulated with their preferred time difference, the incoming signals were not necessarily coincident. The cells could receive a signal from one side, a little later from the other side, and only then fire an electrical pulse.
“These cells pause and remember being activated by one ear, and can wait for the signals coming from the other ear before firing off an electrical pulse,” said Franken. “In other words, these cells have a more active role in time comparison than was thought.”
The research team at KU Leuven believes that this research is important for the development of hearing aids and cochlear implants. “Auditory prostheses brought a revolution for patients with hearing impairments, but they are far from perfect,” said Joris. “Patients experience difficulties in localizing sound sources and in filtering out background sounds, tasks in which the detection of tiny time differences between the ears is a key element. This research on gerbils helps us understand how the brain accomplishes these tasks.”
Source: KU Leuven (Katholieke Universiteit Leuven)