Support for the memory index theory The link between contextual memory representations and locations or routes represented by hippocampal place cells during exploration remains unknown. Tanaka et al. examined spatial firing properties of neurons in hippocampal area CA1 on the basis of whether they had recently expressed the immediate-early activity-induced gene c-Fos in response to a novel context. The c-Fos–positive neurons displayed a more on-off firing pattern than the c-Fos–negative cells during context discrimination. In a contextual recognition paradigm, these results support the index theory of hippocampal function over a cognitive mapping theory. Science , this issue p. 392