Figure 1.

Optimal trial-spacing permits long-term memory formation using two training trials. A, Rats received a single pairing of light that predicted shock (n = 16) or two trials separated by intertrial intervals ranging from 4 min to 30 d (n's = 8–30/group) and memory was tested 2 d following the last trial. B, Spacing the trials by 60 min to 3 d resulted in a very robust memory. C, D, Superior memory in rats trained with a 60 min interval versus those trained with a single trial or two trials spaced by 4 min is still observed when time in the training chamber is equated. E, Levels of contextual fear did not differ between groups in rats left in during the ITI. F, Rats trained with a 60 min ITI and removed during the ITI showed higher levels of contextual fear compared with the group trained with a single trial. G, Rats were trained with a single trial and tested for short term memory 1 h later and long-term memory 2 d later. Another group was only tested for long term memory. H, Rats showed no evidence of short or long term memory. I, Rats pre-exposed to the light showed similar levels of fear as those not pre-exposed to the light cue (*p < 0.05 vs 1 trial; #p < 0.05 vs 4 min). In all graphs, error bars indicate ±SEM.