The hippocampus is widely believed to be essential for learning about the context in which conditioning occurs. This view is based primarily on evidence that lesions of the dorsal hippocampus disrupt freezing to contextual cues after fear conditioning. However, lesions that disrupt freezing produce no effect on fear-potentiated startle, a second measure of contextual fear. Moreover, hippocampal lesions also do not disrupt the contextual 'blocking' phenomenon, which provides an indirect measure of contextual fear. In these paradigms, at least, it appears that hippocampal lesions disrupt the expression of freezing, rather than contextual fear itself. This interpretation is supported by the finding that rats showing preserved contextual blocking after hippocampal lesions show deficits not only in contextual freezing, but also in unconditioned freezing. These findings are consistent with a growing body of data from other conditioning paradigms that contextual learning is spared after lesions of the dorsal hippocampus. Nonetheless, there remain some reports of impaired contextual fear conditioning after hippocampal lesions that cannot be attributed easily to a disruption of freezing. Thus, it is concluded that the hippocampus may be involved in contextual learning under certain - as yet, unspecified - circumstances, but is not critical for contextual learning in general. (C) 2000 Elsevier Science B.V.
Bibliographical noteFunding Information:
This work was supported by the National Institute of Mental Health Grants MH-47840, MH-19951, and MH-11370, Research Scientist Development Award MH-00004, and Air Force Office of Scientific Research Grant F49620. Jonathan Gewirtz is now in the Department of Psychology, University of Minnesota. Kenneth McNish is now at Glaxo Wellcome Pharmaceutical, Research Triangle Park, NC 27709-3398. Correspondence should be addressed to: Jonathan C. Gewirtz, Department of Psychology, University of Minnesota, Elliot Hall, 75 East River Road, Minneapolis, MN 55455-03 44.
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