Figure 2.

Formation of trace memories requires an intact hippocampus. (a) Eye-blinks that were detected during the temporal gap were considered conditioned responses and are represented as a percentage of responses during training. As shown, animals with excitotoxic lesions of the hippocampus emitted virtually no conditioned responses even after > 1000 trials of trace eyeblink training [14]. (b) However, animals with lesions to the hippocampus can learn the conditioned response when a delay training protocol is used. In this experiment, the intervals between the conditioned stimulus (CS) and the unconditioned stimulus (US) were the same except that the CS was temporally contiguous and overlapped slightly with the US. After training with delay conditioning (dashed line), animals with hippocampal lesions could perform the trace conditioning task. (c) As the stimulus relationships become more difficult to learn, the hippocampus becomes involved. Using a long-delay paradigm in which the CS and the US are continuous in time but the time between stimuli onsets is extended, animals with lesions to the hippocampus are learning-impaired. These animals eventually learned and, moreover, were able to perform the trace conditioned response by moving the response to accommodate the new onset time of the US (data not shown) [14]. All three panels were generated using data from Ref. [14].