Putting Plasticity in Its Place

activity corresponding to a series of shuttle runs across a square arena, heteroassociative connections between activated place cells become potentiated, cells with overlapping place fields develop strong bidirectional connections and asymmetric connections between cells against the direction of motion become depressed. The elevated connectivity of the activated ensemble resembles the broad development of a “cognitive map” with bidirectional connections providing detailed information about the relative proximity of individual place fields. The firing patterns of cells with overlapping place fields satisfy the STDP rule and enable connections between such cells to become potentiated, as demonstrated in slices (Isaac et al., 2009). The biasing of asymmetric connection weights toward the last direction of motion enables the encoding of recent trajectories or episodes. Thus the requirements for spatial encoding and sequence learning can be fulfilled simultaneously. The authors note that if there was only a single direction of motion, the connections between the place cells would become purely asymmetric and the cognitive map would thus remain undeveloped. During simulated random exploration of an arena, a similar pattern of bidirectional connectivity develops between place cells encoding proximal place fields, with local asymmetric connections reflecting fragments of recent trajectories. Repeated traversals of a one dimensional route in a single direction through such a learned environment does not alter the gross structure of the map, but profoundly alters the connections of place cells that lie on the route. As before, connections following the direction of travel are significantly potentiated but those against it are depressed, suggesting information concerning recent trajectories can be embedded in the cognitive map. A commentary on