Researcher observes active role of auditory neurons

This is a group of neurons. Credit: EPFL/Human Brain Project

Cells in the brainstem that underlie sound localization, compare signals at the two ears and can pause while doing so. This was shown by researchers at the Laboratory for Auditory Neurophysiology in Leuven, who were the first to obtain both in- and outgoing electrical signals of these cells in Mongolian gerbils.

How do we know where in space a source is located? Our brain computes its location by computing differences between the signals that reach our two . Professor Philip Joris of the Laboratory of Auditory Neurophysiology: "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. 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."

Sound stimulates our snail-shaped cochlea in the 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. 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."

Up until now, it was thought that these cells function as coincidence detectors. Imagine again a sound on your right: your right ear receives the sound first, and the left ear a little later. It was generally thought that somewhere along the path from ear to brainstem cells, this time difference was compensated for, e.g. by slowing the signal from the right ear, so that the signals from the two ears would arrive coincidentally at a brainstem cell, which would then fire off an electrical pulse.

During his doctoral research, Dr. Tom Franken managed, for the first time, to test this hypothesis by inserting a fine electrode into cells of the to record both their in- and outgoing signals. He observed that, when stimulated with their preferred , 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 . In other words, these have a more active role in time comparison than was thought."

This fundamental research is important for the development of hearing aids and cochlear implants. According to Joris: "These auditory prostheses brought a revolution for patients with hearing impairments, but they are far from perfect. Patients experience severe 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."

Explore further

Researchers reveal how hearing evolved

More information: "In vivo coincidence detection in mammalian sound localization generates phase delays," Nature Neuroscience: … n3/full/nn.3948.html.
Journal information: Nature Neuroscience

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Apr 01, 2015
In the human, amplitude differences are detectedable only at 3db steps (twice the intensity) and therefore can not be the source of directional information where detection of differences as small as 0.1 db are required.

Detection of variations in frequency response is vastly superior and the ears have evolved to exploit this ability.

The ability to detect absolute position of a sound is very poor and the ability to locate pure sine wave sound at almost any frequency and location is extremely poor.

The ability to locate sound has been explored by the audio industry and this research has clearly been ignored by the authors of this article who have entirely failed to understand human hearing.

Some basic research would have soon shown the shortcomings of relying on intensity for direction, an ability that humans do not have (turn the balance on headphones to 3db difference between left and right ear, few, if any, people can detect the difference).

Apr 01, 2015
Frequency response changes with the height of a sound due to the shape of the ear but only for sound in the upper octaves, easily testable with pink noise and a tunable notch filter. The head acts as a baffle for sounds to the left and right so cutting higher frequencies to the further ear at much greater than 3db although the average intensity of the sound is within 3db and therefore undetectable.

Having built numerous sound systems and had ordinary people listen to them I found it astonishing how far out a speaker must be before a person can tell and always the detection of differences, such as a midrange speaker being unplugged, is detected as the quality (eg, for the example given, "vocals should be more forward") and almost as never the intensity of the sound, bass region being an exception (but only if it is out by more than 6db!!)

Human hearing treats general frequency response anomalies as acoustic properties of the listening environment and ignores them.

Apr 02, 2015
Excuse me for interrupting your incomprehensible meandering.

This is about phase shift detection.

I envy all researchers no longer forced or exposed to read textbooks upholding a phase shift insensitive ear.

Now readers know there is no excuse for not interrupting your nonsense.
Of course no one will interrupt you.
Your self indulgence babysits you and your pretend play of understanding.

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