Fig. 2.

Nine exposures to an environmental context separated by 25 min, but not 24 h, decrease the proportion of Arc⁺ CA1 neurons. Confocal projection images from the CA1 region for each experimental condition: CAGED group (a); NOVEL group (b); SPACED group (c); MASSED-I group (d); MASSED–NOVEL group (e). The arrow in b indicates a cell containing two Arc transcription foci (yellow color) within a cell nucleus (blue color). (Scale bar, 50 μm.) (f) The percentage of Arc⁺ CA1 neurons (mean ± SEM) for the five behavioral groups is indicated. *, Different from all other behavioral groups (P < 0.02). Δ, different from CAGED, MASSED, and MASSED–NOVEL groups (P < 0.02). n = 4–6 rats per group. (g) Illustration explaining results from the MASSED–NOVEL group. Repeated exposure to environment B decreases the probability of Arc transcription by ≈63% in the “MASSED-B” cell population, these refractory cells are indicated with an “X” over a closed circle. The open circles indicate cells capable of Arc transcription. Based on the principle of random selection with replacement, 16% of the 40% of CA1 cells active in environment A would have also been active in environment B. At 63% failure rate, however, only 6% of the “AB overlap” population would be capable of activating Arc transcription. Therefore, the percentage of cells activating Arc transcription in the final “NOVEL-A” exposure is predicted to be 30% (24% + 6%), essentially identical to the observed value for the MASSED–NOVEL group.