595 Intern. J. Neuroscience, 115:595–611, 2005 Copyright  2005 Taylor & Francis Inc. ISSN: 0020-7454 / 1543-5245 online DOI: 10.1080/00207450590523440 HIPPOCAMPAL CYTOCHROME OXIDASE ACTIVITY OF RATS IN EASY AND DIFFICULT VISUAL DISCRIMINATION LEARNING DAN HU McGovern Institute for Brain Research, MIT Cambridge, Massachusetts, USA XIAOJUAN XU Department of Psychology Grand Valley State University Allendale, Michigan, USA FRANCISCO GONZALEZ-LIMA Department of Psychology and Institute for Neuroscience University of Texas Austin, Texas, USA The present study investigated the functional involvement of the rat hippocam- pal formation in easy and difficult visual discrimination learning by measuring regional brain cytochrome oxidase (C.O.) activity, an index of neuronal meta- bolic activity. The results showed that learning was related to C.O. activity in the CA field and the dentate gyrus of the hippocampal formation of rats in the difficult discrimination, whereas learning was related to C.O. activity only in the dentate gyrus of rats in the easy discrimination. The results suggest that difficult visual discrimination learning required greater involvement of the hip- pocampal formation than easy visual discrimination learning. Keywords CA1, CA2, cytochrome oxidase, dentate gyrus, discrimination learning, vicarious trial-and-error (VTE) Received 17 June 2004. The authors gratefully acknowledge! the advice of Professor Emeritus Abram Amsel in the initial conception of this study. Dr. FGL is partially supported by NIH grant R01N537755. Address correspondence to Dr. Dan Hu, McGovern Institute for Brain Research, MIT, E25–618, 45 Carleton Street, Cambridge, MA 02139, USA. E-mail: hudan@mit.edu