ix, 51 leaves : ill. ; 29 cmThis study sought to determine whether the DG, CA3, and CA1 regions contain
uniformly excitable populations and test the hypothesis that rapid addition of new, more
excitable, granule cells in prepubescence results in a low activation probability (P1) in the
DG. The immediate-early gene Homer1a was used as a neural activity marker to quantify
activation in juvenile (P28) and adult (~5 mo) rats during track running. The main finding
was that P1 in juveniles was substantially lower not only the DG, but also CA3 and CA1.
The P1 for a DG granule cell was close to 0 in juveniles, versus 0.58 in adults. The low P1
in juveniles indicates that sparse, but non-overlapping, subpopulations participate in
encoding events. Since sparse, orthogonal coding enhances a network’s ability to
decorrelate input patterns (Marr, 1971; McNaughton & Morris, 1987), the findings
suggest that juveniles likely possess greatly enhanced pattern separation ability
