Rapid associative learning: conditioned bradycardia and its central nervous system substrates.

It has become clear from the study of different response systems during classical conditioning that some responses are acquired quite rapidly and others show a much slower rate of acquisition. The most often studied rapidly acquired responses have been classically conditioned autonomic changes (e.g., heart rate); the slowly acquired responses most often studied are skeletal responses, such as the eyeblink or leg flexion response. Although there are various other differences between rapidly acquired and slowly acquired responses, we have suggested that the most important difference is the possibility that they represent different stages of the learning process. In the present review I describe research in our laboratory that has focused on conditioned bradycardia as a model system of a rapidly acquired associative system and contrast it with the more slowly acquired Pavlovian conditioned eyeblink response. I also describe the generality of conditioned bradycardia and discuss the differential role of subdivisions of the prefrontal cortex as a substrate for mediating this response. Finally, I briefly discuss the other brain areas involved in conditioned bradycardia, and its functional significance as it relates to the learning process.