Feedback loops and localization errors

October 24, 2013, Ludwig Maximilian University of Munich
Feedback loops and localization errors

Neurobiologists at Ludwig Maximilian University of Munich describe a feedback loop that modulates the processing of auditory signals in the brainstem in a frequency-dependent manner, and can lead to systematic errors in the subjective localization of sound sources.

Perception of acoustic signals depends on the transmission of nerve impulses from receptor cells in the inner ear via the to processing centers in the brain. One crucial function of the auditory system is the of sound sources in space. This is computed from differences in the intensity and time of arrival of the signals perceived by the two ears. The ipsilateral ear, being closer to the source, is stimulated more strongly, and perceives the signal sooner, than the contralateral ear. The latter parameter, the interaural time difference (ITD), is the dominant cue for localization.

In a study just published in Nature Neuroscience, a team of neurobiologists at LMU Munich, led by Professor Benedikt Grothe, describes a new neuronal circuit in the auditory cortex in the brainstem. The circuit originates in a bilateral nerve center known as the medial superior olive (MSO), where initial processing of ITDs takes place. The new work demonstrates that the sensitivity of MSO neurons to ITDs is modulated via a feedback loop that involves receptors for the inhibitory neurotransmitter gamma-amino butyric acid (GABA).

Localization is relative, not absolute

The auditory system localizes sound sources by comparing the levels of firing activity in the left and right MSOs. These in turn depend on differences in the arrival times of excitatory and inhibitory inputs from both . Moreover, both types of input are modulated by activation of receptors for GABA. It turns out that the degree of modulation is determined by the overall level of activity induced by preceding stimuli (over a period of a few hundred milliseconds to a few seconds duration) and is highly dependent on sound frequency. If a sound emanates from a source located in the midline, the MSOs in both brain hemispheres are activated to the same degree. If the signal comes from the right, the left MSO is stimulated more strongly than the right. In response to a second signal, the preferentially reduces the activity level in the dominant (left) MSO, thus briefly altering the ratio of the activities in left and right MSOs.

The researchers deduced that this effect should lead to a shift in the subjective perception of the location of the second signal, and that test subjects should make systematic errors in under these conditions. Subsequent experiments with human subjects confirmed that the expected errors indeed occur in precisely the manner predicted.

"The new circuit reveals that we make context-dependent and predictable errors in the absolute localization of sounds. This is because we can determine the locations of sounds only in relative sense," says Grothe. And that is true of all mammals, not just humans. The latest results confirm similar findings in experimental animals previously reported by the LMU team.

The study was supported, in part, by funding provided for Collaborative Research Center (SFB) 870 on "Formation and Function of Neuronal Circuits in Sensory Systems".

Explore further: Study aims to understand how, when the auditory system registers complex auditory-visual synchrony

More information: www.nature.com/neuro/journal/v … nt/full/nn.3548.html

Related Stories

Study aims to understand how, when the auditory system registers complex auditory-visual synchrony

October 23, 2013
Imagine the brain's delight when experiencing the sounds of Beethoven's "Moonlight Sonata" while simultaneously taking in a light show produced by a visualizer.

Brain wiring quiets the voice inside your head

September 3, 2013
During a normal conversation, your brain is constantly adjusting the volume to soften the sound of your own voice and boost the voices of others in the room.

Interaction between auditory cortex and amygdala responsible for our response to unpleasant sounds, research finds

October 10, 2012
(Medical Xpress)—Heightened activity between the emotional and auditory parts of the brain explains why the sound of chalk on a blackboard or a knife on a bottle is so unpleasant.

Look at what I'm saying: Engineers show brain depends on vision to hear

September 4, 2013
University of Utah bioengineers discovered our understanding of language may depend more heavily on vision than previously thought: under the right conditions, what you see can override what you hear. These findings suggest ...

Decoding sound's source: Researchers unravel part of the mystery

October 1, 2013
As Baby Boomers age, many experience difficulty in hearing and understanding conversations in noisy environments such as restaurants. People who are hearing-impaired and who wear hearing aids or cochlear implants are even ...

Recommended for you

New neuron-like cells allow investigation into synthesis of vital cellular components

January 22, 2018
Neuron-like cells created from a readily available cell line have allowed researchers to investigate how the human brain makes a metabolic building block essential for the survival of all living organisms. A team led by researchers ...

Finding unravels nature of cognitive inflexibility in fragile X syndrome

January 22, 2018
Mice with the genetic defect that causes fragile X syndrome (FXS) learn and remember normally, but show an inability to learn new information that contradicts what they initially learned, shows a new study by a team of neuroscientists. ...

Epilepsy linked to brain volume and thickness differences

January 22, 2018
Epilepsy is associated with thickness and volume differences in the grey matter of several brain regions, according to new research led by UCL and the Keck School of Medicine of USC.

Research reveals atomic-level changes in ALS-linked protein

January 18, 2018
For the first time, researchers have described atom-by-atom changes in a family of proteins linked to amyotrophic lateral sclerosis (ALS), a group of brain disorders known as frontotemporal dementia and degenerative diseases ...

Fragile X finding shows normal neurons that interact poorly

January 18, 2018
Neurons in mice afflicted with the genetic defect that causes Fragile X syndrome (FXS) appear similar to those in healthy mice, but these neurons fail to interact normally, resulting in the long-known cognitive impairments, ...

How your brain remembers what you had for dinner last night

January 17, 2018
Confirming earlier computational models, researchers at University of California San Diego and UC San Diego School of Medicine, with colleagues in Arizona and Louisiana, report that episodic memories are encoded in the hippocampus ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.