Modern brain imaging methods have contributed enormously to contemporary knowledge about the particular brain systems involved in specific cognitive functions. Unfortunately, mechanistic insights have lagged behind, largely because brain mapping in humans lacks the spatial resolution required to study brain activity dynamically at the level of individual neurons and neuronal circuits. The aim of the present project is to use state-of-the-art viral transfection technology to image brain function at the res olution of microcircuits, cell types and individual neurons in behaving animals during encoding and retrieval of memory. We shall use recently developed viral vectors for introducing molecular markers into specific cell types in the hippocampus and entorh inal cortex in vitro and in vivo. This approach will allow us to determine how newborn neurons are integrated into the cellular network of the dentate gyrus, and has the potential to disclose the locus and function of synaptic plasticity in remapping and coordinate transformations in grid cells of the entorhinal cortex and place cells of the hippocampus.