Age, Experience and the Changing Brain
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.
Section snippets
Assumptions
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
Historical context
Although the idea that experience can modify brain structure can probably be traced back at least to Ramon y Cajal [70], it was Hebb who made this a central feature of his neuropsychological theory [25]. Hebb did the first experiments on the consequences of enriched rearing on the behavior of the rat [24], but it was not until the group at Berkeley began to demonstrate changes in brain weight, cortical thickness, acetylcholine levels, and dendritic structure that there was any structural
Analysis of dendritic material
We indicated earlier that there is a high correlation between the extent of dendritic arborization and the number of synapses. Although synapses can only be studied (and counted) directly with EM procedures, an estimate of synapse number can be made by calculating the total dendritic length. Cells can be stained using one of two different procedures. The oldest procedure was devised by Golgi in the late 19th century and it involves depositing a heavy metal, such as gold or silver, on cell
Analysis of glia
There are three main types of glia in the cortex: astrocytes, microglia, and oligodendroctyes. The oligodendroctyes form the myelin and have not been studied with respect to experience. The astrocytes are large glia that are found both in the white matter as well as the grey matter of the cortex. Many astrocytes have processes that resemble dendrites and these processes expand in response to various events, including experience. Microglia are normally visible only when the brain is injured and
Experience-dependent change in the intact brain
As we began our studies of experience-dependent changes in the brain, we used the logic of those before us who had compared animals in laboratory cages to others in “enriched environments” (e.g. 25, 18, 74). Thus, we placed animals in same-sex groups of 4–6 in rat condominiums, which are 1 m×0.6 m×1.8 m high enclosures (Fig. 2). The condominiums feature sawdust floors to allow digging and three hardware cloth walls to allow climbing. The enclosures contain numerous objects, tree branches, and
Plasticity in the injured adult brain
When the cortex is damaged there are changes in the remaining cortex that are correlated with functional outcome. For example, when we removed the frontal cortex in adult rats, we found an initial drop in dendritic arborization in proximal cortical regions such as parietal cortex. This atrophy slowly resolved and 4 months later there was a significant increase in dendritic morphology, which was correlated with partial restitution of function (e.g. 39, 40, 77). This morphological plasticity may
Plasticity in injured developing brain
One of our consistent findings over the past decade has been that the anatomical sequelae of cortical injury vary with precise developmental stage. In brief, when the brain of rats is damaged in the first few days of life, which corresponds to a time just after neural proliferation is complete but neural migration and differentiation are still ongoing, there is a marked generalized atrophy of dendritic arborization and a decrease in spine density in neurons throughout the cortical mantle 43, 49
Sex and experience-dependent changes in the brain
There is accumulating evidence that the male and female brain differ in their structure, respond differently to environmental events, and respond differently to injury. We consider each aspect in turn.
Conclusions
One of the most intriguing questions in behavioral neuroscience concerns the manner in which the brain, and especially the neocortex, can modify its structure and ultimately its function throughout one's lifetime. As the review has suggested, the cortex can be changed dramatically by experience and this change is modulated by various factors. Several basic conclusions can be extracted regarding the nature of the relationship between experience, brain plasticity, and behavior.
1. Experience
Acknowledgements
This research was supported by an NSERC of Canada grant to Bryan Kolb.
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