RECOGNITION MEMORY IN RATSÐI. CONCEPTS AND CLASSIFICATION T. STECKLER* k, W. H. I. M. DRINKENBURG$, A. SAHGAL% and J. P. AGGLETON} *Max Planck Institute of Psychiatry, Clinical Institute, Kraepelinstr. 2-10, D-80804, Munich, Germany, $Scienti®c Development Group, Organon Laboratories Ltd., Newhouse, Lanarkshire ML1 5SH, U.K, %MRC Neurochemical Pathology Unit, Newcastle General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE, U.K. and }School of Psychology, University of Wales, Cardi, PO Box 901, Cardi CF1 3YG, U.K. (Received 29 July 1997) AbstractÐRecognition is the process by which a subject is aware that a stimulus has been previously ex- perienced. It requires that the characteristics of events are perceived, discriminated, identi®ed and then compared (matched) against a memory of the characteristics of previously experienced events. Under- standing recognition memory, its underlying neuronal mechanisms, its dysfunction and alleviation of the latter by putative cognition enhancing drugs is a major research target and has triggered a wealth of ani- mal studies. One of the most widely used animals for this purpose is the rat, and it is the rat's recog- nition memory which is the focus of this review. In this ®rst part, concepts of recognition memory, stages of mnemonic processing and paradigms for the measurement of the rat's recognition memory will be discussed. In two subsequent articles (parts II and III) we will focus on the neuronal mechanisms underlying recognition memory in rats. Three major points arise from the comparison of paradigms that have in the past been used to assess recognition memory in rats. First, it should be realized that some tasks which, at face value, can all be considered to measure recognition memory in rats, may not assess recognition memory at all but may, for example, be based on recall rather than recognition. Second, it is evident that dierent types of recognition memory can be distinguished and that tasks dier in the type of recognition memory taxed. Some paradigms, for example, measure familiarity, whereas others assess recency. Furthermore, paradigms dier as to whether spatial stimuli or items are employed. Third, dier- ent processes, ranging from stimulus±response learning to the formation of concepts, may be involved to varying extent in dierent tasks. These are important considerations and question the predictive validity of the results obtained from studies examining, for example, the eects of putative cognition enhancing drugs. # 1998 Elsevier Science Ltd CONTENTS 1. Introduction 290 2. Concepts of recognition memory 290 2.1. Delayed response and delayed comparison 291 2.2. Memory for items and memory for spatial context 291 2.3. Memory for familiarity and memory for recency 293 2.4. Recognition involves dierent levels of cognitive abilities 294 3. Stages of mnemonic processing involved in recognition memory 295 3.1. Acquisition/Encoding vs storage and retrieval 295 3.2. Retention 295 3.3. Proactive interference 296 3.4. Retroactive interference 296 4. Distinguishing mnemonic from non-mnemonic processes 296 5. Paradigms for the measurement of recognition memory 297 5.1. Delayed response paradigms 297 5.1.1. Spatial delayed alternation (DA) 297 5.1.2. Delayed matching to position (DMTP)/Delayed non-matching to position (DNMTP) 297 5.1.3. Delayed conditional discrimination (DCD) 301 5.2. Delayed comparison paradigms 301 5.2.1. Delayed paired matching to sample (DMTS)/Delayed paired non-matching to sample (DNMTS) 301 5.2.2. Delayed continuous non-matching to sample (CNMTS) 304 5.2.3. Delayed paired comparison (DPC) 304 5.3. Delayed continuous comparison (DCC) 307 6. Conclusion 307 References 309 Progress in Neurobiology Vol. 54, pp. 289 to 311, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301-0082/98/$19.00 PII: S0301-0082(97)00060-9 k Author for correspondence. Fax: 0049-089-30-62-2200. 289