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The Journal of Neuroscience, August 23, 2006, 26(34):8702-8706; doi:10.1523/JNEUROSCI.1742-06.2006

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Brief Communications
Trace Conditioning and the Hippocampus: The Importance of Contiguity

Debra A. Bangasser, David E. Waxler, Jessica Santollo, and Tracey J. Shors

Department of Psychology and Center for Collaborative Neuroscience, Rutgers University, Piscataway, New Jersey 08854

Correspondence should be addressed to Tracey J. Shors, Department of Psychology, Rutgers University, 152 Frelinghuysen Road, Piscataway, NJ 08854. Email: shors{at}rutgers.edu

Trace conditioning, a form of classical conditioning in which the presentation of the conditioned stimulus (CS) and the unconditioned stimulus (US) is separated in time by an interstimulus interval, requires an intact hippocampus. In contrast, classical conditioning procedures in which the CS and US are not separated by an interstimulus interval (i.e., delay conditioning procedures) typically do not (Solomon et al., 1986). However, why trace conditioning is dependent on the hippocampus is unknown. Several theories suggest that it is specifically the discontiguity between the CS and US in trace conditioning that critically engages the hippocampus. However, there are other explanations that do not depend on discontiguity. To determine whether the lack of contiguity renders trace conditioning hippocampal dependent, we designed a "contiguous trace conditioning" (CTC) paradigm in which CS–US contiguity is restored by re-presenting the CS simultaneously with the US. Although rats with excitotoxic lesions of the hippocampus could not learn a standard trace fear-conditioning paradigm, lesioned rats trained on CTC showed significant conditioning, at levels similar to those with sham surgeries. Importantly, lesioned rats trained solely with simultaneous CS–US presentations did not demonstrate conditioning. Together, these data suggest that rats with hippocampal lesions can form a memory of a trace CS–US association when contiguity is restored. Therefore, the dependence of traditional trace paradigms on the hippocampus can be attributed to the absence of temporal contiguity.

Key words: associative learning; fear conditioning; learning; memory; trace conditioning; time

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Received April 24, 2006; revised June 12, 2006; accepted June 16, 2006.

Correspondence should be addressed to Tracey J. Shors, Department of Psychology, Rutgers University, 152 Frelinghuysen Road, Piscataway, NJ 08854. Email: shors{at}rutgers.edu

This article has been cited by other articles:

				D. R. Smith, M. Gallagher, and M. E. Stanton Genetic background differences and nonassociative effects in mouse trace fear conditioning Learn. Mem., September 5, 2007; 14(9): 597 - 605. [Abstract] [Full Text] [PDF]

				B. Leuner, J. Waddell, E. Gould, and T. J. Shors Temporal Discontiguity Is neither Necessary nor Sufficient for Learning-Induced Effects on Adult Neurogenesis J. Neurosci., December 27, 2006; 26(52): 13437 - 13442. [Abstract] [Full Text] [PDF]

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