Study reveals how brain distinguishes speech from noise

Scientists have given physiological evidence that a pervasive neuromodulation system - a group of neurons that regulate the functioning of more specialized neurons - strongly influences sound processing in an important auditory region of the brain.

The neuromodulator -- acetylcholine -- may even help the main auditory brain circuitry distinguish speech from noise.

"While the phenomenon of these modulators' influence has been studied at the level of the neocortex, where the brain's most complex computations occur, it has rarely been studied at the more fundamental levels of the brain," said study author R Michael Burger from the Lehigh University in the US.

The study, published in the JNeurosci: The Journal of Neuroscience, will likely bring new attention in the field to the ways in which circuits like this, widely considered a 'simple' one, are in fact highly complex and subject to modulatory influence like higher regions of the brain.

The team conducted electrophysiological experiments and data analysis to demonstrate that the input of the neurotransmitter acetylcholine, a pervasive neuromodulator in the brain, influences the encoding of acoustic information by the medial nucleus of the trapezoid body (MNTB), the most prominent source of inhibition to several key nuclei in the lower auditory system.

MNTB neurons have previously been considered computationally simple, driven by a single large excitatory synapse and influenced by local inhibitory inputs.

The team demonstrated that in addition to these inputs, acetylcholine modulation enhances neural discrimination of tones from noise stimuli, which may contribute to processing important acoustic signals such as speech.

Additionally, they describe novel anatomical projections that provide acetylcholine input to the MNTB.

Burger studies the circuit of neurons that are "wired together" in order to carry out the specialized function of computing the locations from which sounds emanate in space.

He described neuromodulators as broader, less specific circuits that overlay the more highly-specialized ones.

"This modulation appears to help these neurons detect faint signals in noise. You can think of this modulation as akin to shifting an antenna's position to eliminate static for your favourite radio station," Burger said.

"In this paper, we show that modulatory circuits have a profound effect on neurons in the sound localization circuitry, at the very low foundational level of the auditory system," the authors wrote.