Development of many psychological functions is known to occur within a limited time window and beyond such a critical period, environmental stimulation has little or no effect. With a split-litter 2×2 factorial design in which neonatal and early adulthood novelty exposure were two within-litter factors, we examined whether infancy is a critical period for novelty exposure experience to induce a long-lasting functional enhancement in spatial memory and whether the known enhancement effect induced by early life experience can be further augmented by later adult experience. Spatial memory performance was evaluated in a three-day moving platform Morris water maze task. We found that the animals deprived of novelty exposure during infancy could nevertheless benefit from a delayed adult exposure and that the magnitude of this enhancement was comparable to the enhancement induced by neonatal exposure. This finding indicates that infancy is not a critical period beyond which novelty exposure will have little effect on enhancing spatial memory. Surprisingly, although both neonatal and adult experience alone produced enhancement in spatial memory performance, animals that experienced both neonatal and adult exposures showed no statistically significant difference from those who had none. This finding indicates that the cumulative effect of neonatal and early adulthood experience of novelty is not additive but a result of non-linear interaction and that knowing the effect of a single-stage-intervention does not allow one to predict the final developmental outcome without taking into consideration events occurring before or after.
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