BRYAN KOLB*, MARGARET FORGIE, ROBBIN GIBB, GRAZYNA GORNY AND SHARON ROWNTREE Department of Psychology, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada.
In principle, there are two ways that experience could alter the brain: either by modifying existing circuitry or by creating novel circuitry (63). It is reasonable to suppose that the brain makes use of both strategies, although the details of the particular strategy will likely vary with the age of the animal. Indeed, during development of the brain all circuitry is, by definition, novel. One way to examine the experience dependent changes in the brain is to look at the effects of different experiences on neuronal structure and function. For psychologists, this rationale usually means adopting one of two approaches: either animals are placed in differential environments such as so-called ‘‘enriched environments’’ versus ‘‘impoverished environments’’; or animals are trained in specific types of tasks, such as mazes. In either paradigm, the experience is correlated with some measure of structure such as brain weight or dendritic extent (e.g. (17)). These experiments generally show that particular experiences embellish circuitry relative to the absence of experience, which fails to do so. Although this type of experimental psychological approach would appear to have considerable appeal in understanding experience-dependent changes in the brain, the impact of this type of research has been surprisingly limited. Indeed, Purves (63) noted that for reasons that are as much sociological as scientific, the experimental neuropsychological perspective has not been embraced generally by most neurobiologists and that these psychological experiments are rarely referred to in the mainstream literature. Oddly, again for reasons that are both sociological as much as scientific, the importance of studies of enriched experience also have had limited impact in mainstream psychology where there has been a longstanding bias against structural interpretations of psychological phenomena. Nevertheless, the study of experience dependent changes in experiments that manipulate external experiences has provided a rich broth of information that is relevant both to basic neurobiological theories of brain function as well as to general theories of behavioral organization. The goal of the current review is to illustrate some of principles that have emerged. The review will begin with a summary of some of experience-dependent changes in the intact brain followed by a consideration of the effects of manipulating factors such as gonadal hormones or neurotrophins and the effects of cortical injury.
As we begin, we must first admit to several biases. Firstly, we assume that the structural properties of the brain are important in understanding its function. Although such an assumption is self-evident to most neuroscientists, it is not as ubiquitously assumed by psychologists who do not study the brain (e.g. (69,83)). An important corollary of this assumption is that changes in the structural properties of the brain reflect changes in the function of neural circuits. Secondly, we assume both that the mechanisms of cortical plasticity are most likely to be found at the synapse and that synaptic changes can be measured by analysis of either pre- or post-synaptic structure. Traditionally, the emphasis in the literature on synaptic plasticity has been upon the presynaptic, or axonal terminal side. For example, in the studies of the effects of unilateral entorhinal cortex in rats, various investigators have shown a major reorganization of the remaining hippocampal afferents (e.g. (85)). Similar inferences have been made in other models, such as in studies of cholinergic outgrowth after cortical injury (e.g. (6)), collateral sprouting after peripheral nerve crush (7), and terminal sprouting after various types of central injuries (e.g. (12)). One difficulty with studying presynaptic changes is that they are very difficult to locate unless one knows apriori where to look. In addition, once found, they are difficult to quantify. The ability to quantify specific morphological features is critical if one is to correlate structural change with behavior.