Behavioral Neuroscience Laboratory
The broad objective of our research is to extend the data and theory about the role of the hippocampus in learning and memory processes in light of the new ideas about how inhibitory learning occurs and what inhibitory learning entails. Currently, we are focused on systematically testing predictions from the Inhibition Hypothesisproposed by Chan, Morell, Jarrard, & Davidson (2001). The proposed studies will use the ibotenic acid lesion technique to selectively ablate neurons of the rat hippocampus. These rats will be compared to intact controls in associative conditioning tasks specifically designed to involve the learning of certain types of inhibitory associations.
Our lab employs the methodological and theoretical framework of Pavlovian conditioning. By clearly describing the circumstances that enable a conditioned stimulus (CS) to gain the capacity to evoke a conditioned response (CR), this framework yields testable predictions about how the subtlest changes in the learning situation can produce marked changes in conditioned responding. Within a Pavlovian conditioning framework, inhibition and excitation are often viewed as the most fundamental learning processes (e.g., see Pavlov, 1927; Rescorla & Wagner, 1972; Wagner & Rescorla, 1972). Excitatory associations are thought to be formed when a conditioned stimulus (CS), such as a tone or a light, signal the presentation of a Pavlovian unconditioned stimulus (US), such as food. Inhibitory learning is often said to occur when a stimulus acquires the capacity to suppress or prevent the performance of excitatory conditioned responses. In other words, associative inhibition involves learning that the presentation of a US, which had been delivered previously, will not be forthcoming. In recent years, the conceptualization of associative inhibition has undergone some important changes.
A number of earlier theories have proposed a link between hippocampal-lesions and impairment in inhibitory learning (e.g., Douglas, 1967; Douglas & Pribram, 1966; Kimble, 1968; 1969). In fact, the idea that the hippocampus is important for inhibition was the dominant view in the scientific literature in the 1960's and early 70's. These theories fell out of favor when Solomon (1977) reported that aspiration lesion of the dorsal hippocampus had no effect on one particular conditioned inhibition task involving the eye-blink response in rabbits. However, a number of more recent findings suggest there may be more than one type of inhibition, and at least some training procedures provide the opportunity for subjects to acquire conditioned excitation and conditioned inhibtion within a single stimulus (e.g., Matzel, Gladstein, & Miller, 1988; Williams & Overmier, 1988). The recent theoretical advances in our understanding of the different types of inhibitory association together with the development of techniques to produce more selective and more complete lesions of the hippocampus provide a basis to re-evaluate the idea that the hippocampus might be involved in some types or parameters of associative inhibition.