After 50 years of latent inhibition (LI) research, it may come as a surprise that there is, as yet, no generally accepted theoretical account of this apparently simple, robust effect (i.e., a deficit in conditioned responding as a result of prior exposure to the conditioned stimulus). A variety of explanations have been proposed, and many of them, such as conditioned inhibition and habituation, discarded (for a review, see Lubow,1989, pp. 141–189; but see Honey, Iordanova & Good, this volume; Schmajuk, this volume). Currently, one can identify three basic types of LI theories. A-theories, which provided the first accounts of LI (e.g., Lubow, Weiner & Schnur, 1981; Mackintosh, 1975; Pearce & Hall, 1980; Wagner, 1976, 1981; more recently, McLaren & Mackintosh, 2000), accept that passive, inconsequential stimulus preexposures reduce the ability of that stimulus to enter into new associations. In addition, most A-theories explicitly acknowledge the role of attention and attribute the reduction of associability to the decline in stimulus-specific attention or salience.
In contrast to A-theories, R-theories eschew attentional concepts and maintain that stimulus preexposure has no effect on stimulus associability; the stimulus preexposed (PE) and non-preexposed (NPE) groups enter the acquisition stage with the same capability for forming new associations with the unconditioned stimulus. According to R-theories, in the stage-three test, the associations formed during the stage-one preexposure compete for expression with the association formed in the stage-two acquisition (e.g., Bouton, 1993; Miller, Kasprow & Schachtman, 1986; Weiner, 1990).