"When we learn that a particular sensory stimulus predicts a reward, there is general agreement that this knowledge is stored by changing the connections between particular neurons," explains Cassenaer. The problem, 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 certain sensory input, until then neutral, acquires its new, predictive value? "In this study, we carried out experiments to investigate how the brain identifies exactly which connections, out of an enormously large number of possibilities, should be changed to store the memory of a specific association."
To get a closer look at these connections, Cassenaer and Laurentwho is now director at the Max Planck Institute for Brain Research in Germanymeasured 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 should be modified, and thus where the nonspecific reward signal should act, is a very 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 many 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 certain time window, then these connections are the only ones altered by the internal reward signal. All other connections remain unaffected."
Laurent says that the molecular underpinnings of this phenomenon,
|Contact: Deborah Williams-Hedges|
California Institute of Technology