Figure 1.

Diagram of the experimental procedure. Experimental animals first received long-term sensitization training (solid black bars). Memory expression (hatched bars) was then monitored with two post-tests taken every 24 h (Phase I). On the second consecutive day without expression of the original memory (open bars), animals were retrained with two spaced tail shocks (Phase II, solid arrows). The duration of LTM expression in individual animals varied; thus, each experimental animal was retrained with a matched no-shock control (open arrows). Memory was assessed following retraining at 10 min, 2 h, and 24 h. Examples are given of individual cases where original memory expression lasted 1 d (top bar), 2 d (middle bar), or 3 d (bottom bar) and retraining occurred on the third, fourth, or fifth day, respectively, after original training.