The role of the hippocampus in trace conditioning: Temporal discontinuity or task difficulty?

It is well established that the hippocampal formation is critically involve d in the acquisition of trace memories, a paradigm in which the conditioned (CS) and unconditioned stimuli (US) are separated by a temporal gap (Solom on et al., 1986). The structure is reportedly not critical for the acquisit ion of delay memories, where the CS and the US overlap in time (Berger & Or r. 1983; Schmaltz & Theios, 1972). Based on these results, it is often stat ed that the hippocampus is involved in "filling the gap" or otherwise assoc iating the two stimuli in time. However, in addition to the presence of a t emporal gap, there are other differences between trace and delay conditioni ng. The most apparent difference is that animals require many more trials t o learn the trace task, and thus it is inherently more difficult than the d elay task. Here, we tested whether the hippocampus was critically involved in delay conditioning, if it was rendered more difficult such that the rate of acquisition was shifted to be analogous to trace conditioning. Groups o f rats received excitotoxic lesions to the hippocampus, sham lesions or wer e left intact. Using the same interstimulus intervals (ISI), control animal s required more trials to acquire the trace than the delay task. As predict ed, animals with hippocampal lesions were impaired during trace conditionin g but not delay conditioning. However, when the delay task was rendered mor e difficult by extending the ISI (a long delay task), animals with hippocam pal lesions were impaired. In addition, once the lesioned animal learned th e association between the CS and the US during delay conditioning, it could learn and perforin the trace CR. Thus, the role of the hippocampus in clas sical conditioning is not limited to learning about discontiguous events in time and spaced rather the structure can become engaged simply as a functi on of task difficulty. (C) 2001 Academic Press.