Latent Inhibition of Conditioned Odor Potentiation of Startle: A Developmental Analysis Rick Richardson Michelle Fan A. Sophie Parnas School of Psychology The University of New South Wales Sydney 2052 Australia E-mail: R.Richardson@unsw.edu.au ABSTRACT: We conducted a two-part study of age and latent inhibition in the rat. In the first part of the study, rats given odor– shock pairings at 23 or 75 days of age exhibited a potentiated startle response in the presence of the odor the following day. This effect did not occur in rats trained at 16 or 20 days of age. Odor pre- exposure on the day prior to conditioning markedly reduced the odor potentiation of startle effect in 23- and 75-day-old rats but had no effect in 16 and 20-day-olds. In the second part of the study, rats were pre-exposed to the odor at 16 or 20 days of age and then conditioned at 23 days of age. When tested the day after conditioning, these pre-exposed rats exhibited a disruption in the odor potentiation of startle effect. We compare our results with other studies of latent inhibition, and with recent studies on whether conditioned responses are appropriate to the animal’s age at training or their age at test. ß 2003 Wiley Periodicals, Inc. Dev Psychobiol 42: 261–268, 2003. Keywords: latent inhibition; development; potentiated startle; olfactory con- ditioning; rats Pairings of an initially neutral conditioned stimulus (CS) with a biologically relevant unconditioned stimulus (US) leads to acquisition of an association between these two stimuli. Because of this association, the animal’s response to subsequent presentations of the CS is altered. However, the learning, or at least the expression, of this association is impaired if rats receive non-reinforced exposures to the CS prior to conditioning, an effect known as latent in- hibition (Lubow & Moore, 1959). The latent inhibition effect is well established in adult rats (e.g., Aguado, Symonds, & Hall, 1994; Escobar, Arecediano, & Miller, 2002; Killcross, Kiernan, Dwyer, & Westbrook, 1998), but there has been considerably less attention paid to this effect in the developing rat. A developmental analysis of basic associative phe- nomena, such as latent inhibition, will contribute to our understanding of the differences and similarities in how information is encoded at different points of development. Such research will also potentially provide converging evidence for the neural analysis of these processes in adult animals with experimentally induced brain lesions (e.g., Fanselow & Rudy, 1998). The limited literature on latent inhibition (LI) during development is not consistent. That is, some investigators have reported that LI does not occur in preweanling rats (e.g., Nicolle, Barry, Veronesi, & Stanton, 1989; Wilson, Phinney, & Brennan, 1974), while others have reported that it does (e.g., Kraemer, Hoffman, & Spear, 1988; Rudy & Cheatle, 1979). Nicolle et al. used a conditioned taste aversion procedure and found LI in 32- day-old rats, but not in 18- or 25-day-olds, while Rudy and Chealte used an odor-avoidance procedure and found LI in rats as young as 8 days of age. Clearly, more research is needed in this area. In a recent review, Stanton (2000) reported some very interesting results concerning LI in the developing rat. Stanton used the eyeblink conditioning procedure in this work. Earlier research from his laboratory had shown that conditioned eyeblink responses to an auditory CS emerge Received 14 June 2002; Accepted 22 August 2002 Correspondence to: Rick Richardson Contract grant sponsor: Australian Research Council Contract grant number: A79800074 Contract grant sponsor: UNSW Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/dev.10099 ß 2003 Wiley Periodicals, Inc.