Effects of chemical and physical treatments on learning and memory.

The search for the engram h as played a prominent role in physiological psychology since the days of Franz & Lashley (53). There has been a grow­ ing optimism in recent years that subcellular and particularly molecular mechanisms involved in and responsible for memory storage will be revealed and controlled (64). The problem can be stated very simply. For some organisms certain experiences leave an aftereffect which manifests itself in changed behavior at a later time. This change in behavior indicates that learning has occurred and has left a memory trace somewhere, presumably in the brain but not necessarily. Examples of experiences with long-lasting aftereffects are visible trauma, immunological reactions (23) , drug tolerance and dependence (87), adaptive enzyme formation (110), sensory adaptation, habituation (170), and associative learning, including conditioning and perceptual learning (10(1). All of these phenomena involve plastic changes in various tissues. This review will concern itself mainly with associative learning where two sets of stimuli become interdependent because of spatial and temporal contiguity. The possibility that similar cellular and biochemical mechanisms, such as protein synthesis or modifications of neurohumoral transmission, underlie all of these phenomena is an important guiding principle in present-day drug research. Newer histological and neurochemical techniques, such as histochemical fluorescence (54), offer great promise of contributing to the resolution of memory problems. Only when all the anatomical and chemical links in the information chain are specified will experimenters be certain that an engram