How our brain decodes sound

In an interesting discovery, scientists have found that when we hear a sound, neurons fire in sync with the rhythmic structure of the sound, exactly encoding its original structure in the timing of spikes.

When people hear the sound of footsteps or the drilling of a woodpecker, the rhythmic structure of the sounds is striking.

Even when the temporal structure of a sound is less obvious, as with human speech, the timing still conveys a variety of important information, said Michael Wehr from the University of Oregon.

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.

"As the information progresses towards the auditory cortex, however, the representation of sound undergoes a transformation.

"There is a gradual shift towards neurons that use an entirely different system for encoding information," said Wehr.

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.

Neuroscientists previously have speculated that the transformation from temporal coding to rate coding may explain the perceptual boundary experienced between rhythm and pitch.

Slow trains of clicks sound rhythmic, but fast trains of clicks sound like a buzzy tone.

It could be that these two very different experiences of sound are produced by the two different kinds of neurons, Wehr said.

The study was published in the journal Neuron.