Top row: Transgenic mice in which the expression of a green reporter construct (tau-LacZ) was under the control of doxycycline (dox) were exposed to a highly familiar environment. Dox was withdrawn during exposure, such that any cells that were active during the experience were labeled green. Middle row: Dox was then re-administered prior to exposure to the same environment again with or without the addition of an electric shock between exposures (A); to a very different environment (B); or to a similar environment (C). Any cells that were activated during the second experience were labeled with the immediate early gene cfos (red). Bottom row: Summary figure characterizing the numbers of cells activated in the first (green) or second (red) experience, or in both experiences (yellow). Repeated exposures to the same environment (with or without an electric shock) activated largely overlapping cell populations in the CA1 subregion, but had no effect on which cells were activated in the dentate gyrus (A). Exposure to two very different environments activated largely distinct cell populations in the dentate gyrus, but did not affect which cells were activated in CA1 (B). Exposure to two similar environments activated largely overlapping populations of CA1 neurons, but largely distinct populations of dentate gyrus granule cells (C). In all three conditions, most of the cells that were activated in the dentate gyrus were mature granule cells (locations of inactive mature cells indicated in blue) rather than immature adult-born neurons (locations of inactive adult-born cells indicated in gray).