Learning how the brain does its coding

Most organisms with brains can store and process a staggering range of information. The fundamental unit of the brain, a single neuron, however, can only communicate in the simplest of manners, by sending a simple electrical pulse. The challenge of understanding how information is contained in the pattern of these pulses has been bothering neurobiologists for decades, and has been given its own name: neural coding. 

In principle, there are two ways coding could be handled. In dense coding, a single neuron would convey lots of information through a complex series of voltage spikes. To a degree, however, this creates as many problems as it solves, since the neuron on the receiving end will have to be able to interpret this complex series properly, and separate it from operating noise. 

The alternative, sparse coding, tends to be used for memory recall and sensory representations. Here, a single neuron only conveys a limited amount of information (i.e., there's something moving horizontally in the field of vision) through a simple pulse of activity. Detailed information is then constructed by aggregating the inputs of lots of these neurons. 

A study released in yesterday's Science provides some perspective on just how flexible this sort of system can be. Researchers worked with the olfactory system of insects, where structures in the brain called mushroom bodies integrate the inputs from sensory neurons. (they're called mushroom bodies for the highly technical reason that they're shaped kind of like a mushroom.) The mushroom bodies use sparse coding to interpret and recall odors, with most neurons only firing a few times in response to a scent. 

The authors of the paper traced the connections among the neurons in the mushroom body, and found that most were contacted by a single, giant interneuron that sent them inhibitory signals. By toning all the other neurons down, this giant cell enforces sparse coding by limiting the amount of activity that is elicited by a new odor. It also allows the fine tuning of activity for the entire mushroom body. Increasing its activity is sufficient to shut the entire system down, essentially making the insect blind to smells, while decreasing its activity will make the insect hypersensitive to scents. 

Although us mammals don't have neurons of this sort—they appear to be an innovation exclusive to the insects—the authors predict that a system that functions similarly may be found in vertebrates, simply because it's so simple and functional. 

By John Timmer

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