Neurogenetic mechanisms of learning: a phylogenetic perspective.

Common psychiatric disorders often involve abnormal patterns of adaptive response to environmental stimuli. Thus, an understanding of the pathophysiology and inheritance of such maladaptation requires specification of the underlying neural mechanisms involved in learning, which is broadly defined as the modification of behavior as a result of individual experience. The phylogenetic development of learning ability in animals is shown to proceed by a sequence of discrete steps in which non-associative learning is augmented successively by classical conditioning to aversive stimuli, followed by classical conditioning to food and other rewarding stimuli, then exploratory learning about novel habitats and operant conditioning of behavioral responses, and finally conceptual learning ability. As a result of this complex phylogenetic history, adaptive responses to the environment in mammals including human beings is multidimensional. A model of the structure of stimulus-response and cognitive-processing characteristics in human beings is described. The model is discussed in terms of its evolutionary advantages for survival and reproductive fitness as well as its importance in understanding susceptibility to psychiatric disorders, including personality and anxiety disorders.