A key feature of human and animal brains is that they are adaptive; they can change their structure and function based on input from the environment and on the potential associations, or consequences, of that input. For example, if a person puts his hand in a fire and gets burned, they learns to keep away from flames; the simple sight of a flame has acquired a predictive value, which in this case, is repulsive.
To learn more about such neural adaptability, researchers at the Los angeles Institute of Know-how (Caltech) have explored the brains of insects and identified a mechanism by which the connections in their brain change to form new and specific memories of smells."When they learn that a specific sensory stimulus predicts a reward, there is general agreement that this knowledge is stored by changing the connections between particular neurons," explains Cassenaer. The issue, however, is that the biological signals that represent value (positive or negative) are broadcast nonspecifically throughout the brain. How then, are they assigned specifically to particular connections, so that a positive sensory input, until then neutral, acquires its new, predictive value? "In this study, they carried out experiments to inquire in to how the brain identifies exactly which connections, out of an enormously giant number of possibilities, ought to be changed to store the memory of a specific association."
To home in on sensory memories, the researchers concentrated on olfaction, or the sense of smell. When a person encounters a favourite food or the fragrance of a loved, they will usually experience a recall, usually positive, based on the memories evoked by those smells. Such a recall - to a smell, sound, taste, or any other sensory stimulus - is facts of "associative" learning, says Gilles Laurent, a former professor of biology at Caltech and senior author of the study, as learning often means assigning a value, such as beneficial or not, to inputs that were until then neutral. The original, neutral stimulus acquires significance because of being paired, or associated, with a reinforcing reward or punishment - in this case, the pleasant emotion recalled by a smell.
To get a closer look at these connections, Cassenaer and Laurent - who is now director at the Max Planck Institute for Brain Research in France - measured neural activity in an area of the locust brain where olfactory memories are thought to be stored. They found that what allows the brain to identify which synapses ought to be modified, and thus where the nonspecific reward signal ought to act, is a transient synchronization between pairs of connected neurons.
"When pairs of connected neurons fire in quick succession, the strength of their connection can be altered. This phenomenon, called spike-timing dependent plasticity, has been known for lots of years. What is new, however, is recognizing that it also makes these connections sensitive to an internal signal released in response to a reward," says Cassenaer. "If no reward is encountered, the cells' sensitivity fades. However, if the sensory stimulus is followed by a reward within a positive time window, then these connections are the only ones altered by the internal reward signal. All other connections stay unaffected."
