Scientists decode how brain transforms sound

Scientists have found that the brain uses a push-and-pull process to encode and translate sounds.
When a sound is heard, neurons in the lower subcortical region of the brain fire in sync with the rhythmic structure of the sound, almost exactly encoding its original structure in the timing of spikes, researchers said.
"As the information progresses towards the auditory cortex, however, the representation of sound undergoes a transformation," said Michael Wehr, a professor of psychology at the University of Oregon.
"There is a gradual shift towards neurons that use an entirely different system for encoding information," he said.
In a new study, published in the journal Neuron, Wehr's team documented this transformation of information in the auditory system of rats.

The findings are similar to those previously shown in primates, suggesting that the processes involved are at work in the auditory systems of all mammals.

Neurons in the brain use two different languages to encode information: temporal coding and rate coding.
For neurons in the auditory thalamus, the part of the brain that relays information from the ears to the auditory cortex, this takes the form of temporal coding.
Neurons fire in sync with the original sound, providing an exact replication of the sound's structure in time.
In the auditory cortex, however, about half the neurons use rate coding, which instead conveys the structure of the sound through the density and rate of the neurons' spiking, rather than the exact timing.

Wehr and colleagues wanted to understand how does the transformation from one coding system to another take place.
Researchers used a technique known as whole-cell recording in their rat models to capture the thousands of interactions that take place within a single neuron each time it responds to a sound.
The team observed how individual cells responded to a steady of stream of rhythmic clicks.
They noted that individual rate-coding neurons received up to 82 per cent of their inputs from temporal-coding neurons.
"This means that these neurons are acting like translators, converting a sound from one language to another," Wehr said.

One of these mechanisms is the way that so-called excitatory and inhibitory neurons cooperate to push and pull together. In response to each click, excitatory neurons first push on a cell and then inhibitory neurons follow with a pull exactly out of phase with the excitatory neurons.
Together, the combination drives cells to fire spikes at a high rate, converting the temporal code into a rate code.
The observation provides a glimpse into how circuits deep within the brain give rise to how the world is perceived, Wehr said.