Sound in the Brain: An Orderly Orchestra of Synapses
Unlike the mapping of the sense of smell, which has odourant maps all over the olfactory bulb in no particular order, the sense of sound is remarkably well ordered. Since the earlier study on the neural mapping of the sense of smell, revealing that the internal wiring of the olfactory system was like a series of sliding scales blending into one another. A lot like an orchestra but in three dimensions, there was some concern that the other systems might be similarly ordered.
Instead of finding distinct neural codes for each scent, each scent is a combination of multiple codes added together in a specific sequence. This is what made neural mapping of the sense of of smell so difficult. If the other senses were the same way in internal wiring, it would make truly mapping the senses into a virtual environment a herculean task.
A University at Buffalo study on the neural mapping of the auditory system has calmed those fears somewhat. The research focuses on a section of the brain called the cochlear nucleus, the first way-station in the brain for information coming from the ear. In particular, the study examined tiny biological structures called synapses that transmit signals from the auditory nerve to the cochlear nucleus.
The major finding: The synapses in question are not grouped randomly. Instead, they are also arranged like orchestra, but in a different way. Synapses line up like musicians sitting in their own sections, with the synapses bundled together by a key trait: plasticity.
Plasticity relates to how quickly a synapse runs down the supply of neurotransmitter it uses to send signals, and plasticity can affect a synapse's sensitivity to different qualities of a sound. So whilst one synapse will concentrate on how exactly how a sound starts, concentrating on every nuance; very quickly it's supply of neurotransmitter is exhausted, and it switches off. Meanwhile another neuron has not sampled the sound to the same degree, but because of this decreased fidelity, it is transmitting more slowly, and the same supply of neurotransmitter lasts longer.
So, there is overlap in the processing of sounds, but a sound is not a combination of this code plus this code, this code and this code, when transmitted together, produce this sound, but rather each synapse samples the sound in a different way, adding it's voice to the collective. Individual neural codes that are still processed individually, but overlap to create a richer experience. Miss some stages out, and you still get the same sound, but maybe with diminished richness or complexity. There is no one single synapse that is necessary to pick a sound up; rather they all detect it at the same time.
UB Associate Professor Matthew Xu-Friedman, who led the study, said the findings raise new questions about the physiology of hearing. The research shows that synapses in the cochlear nucleus are arranged by plasticity, but doesn't yet explain why this arrangement is beneficial, he said.
"It's clearly important, because the synapses are sorted based on this. What we don't know is why," said Xu-Friedman, a member of UB's Department of Biological Sciences. "If you look inside a file cabinet and find all these pieces of paper together, you know it's important that they're together, but you may not know why."
In the study, Xu-Friedman and Research Assistant Professor Hua Yang used brain slices from mice to study about 20 cells in the cochlear nucleus called bushy cells, which receive information from synapses attached to auditory nerve fibres.
The experiments revealed that each bushy cell was linked to a network of synapses with similar plasticity. This means that bushy cells themselves may become specialized, developing unique sensitivities to particular characteristics of a sound, Xu-Friedman said.
"One reason this may not have been noticed before is that measuring the plasticity of two different synapses onto one cell is technically quite difficult," Xu-Friedman said.