Age, Experience and the Changing Brain

Abstract

KOLB, B., M. FORGIE, R. GIBB, G. GORNY, S. ROWNTREE. Age, experience and the changing brain. NEUROSCI BIOBEHAV REV 22(2), 143-159, 1998.—In this review, various aspects of how environmental experience affects the structure of the cortex at different times in the age of the animal are summarized. The interactions of brain injury and sex on the age-dependent plastic changes in the cortex are also considered. Finally, we have attempted to reach some general conclusions that describe the effects of age, experience, sex, and injury on the cortex.

Introduction

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 long-standing 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.