Figure 1.

Performance during Experiment 1 measured by N (A) and MAX (B). Avoidance of the room-defined NE sector was rapidly learned with both hippocampi intact (ne1–ne5). Performance was asymptotic by day 3. Retrieval of the avoidance was disrupted by unilateral hippocampal inactivation (ne6-TTX). Good avoidance was observed when on the next day, only saline was injected (ne7-SAL). The room frame track and shocks (open circles in the shaded punished sector) during experiments from a representative rat are shown as Insets. Asymptotic performance (ne5) is shown in the Left Inset. Performance on the next day (ne6-TTX) during TTX inactivation is shown in the Right Inset. Notice that the rat entered only once during the intact session (at 298 s), but entered 18 times (from 42 to 948 seconds) under TTX. During the TTX session it often reacted to the shock not by escaping, but by actively moving deeper into the shock area. On day 8 (EXT), the shock was turned off to extinguish the avoidance. The following three days (sw1-TTX–sw3-TTX), avoidance of the opposite, SW sector was trained during TTX inactivation of the left hippocampus. The rats were unable to learn this new avoidance. Performance was worse than when under TTX a previously learned avoidance could be remembered (ne6-TTX). The number of entrances increased to >50% more than what would occur if the rat sat immobile because after escaping a short distance from shock, the rats often sat motionless and were thus soon transported into the punished area again. Labels: Sessions during which the hippocampi were both functional are marked in the plots by open circles. Black and gray circles mark the sessions during TTX inactivation and following saline injection, respectively. Asterisks mark the P < 0.001 significance of the decrement in performance caused by TTX inactivation relative to the asymptotic performance on the previous day.