Study Notes

Rosenzweig, Bennett and Diamond (1972)

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Last updated 22 Mar 2021

Environmental effects on brain growth and brain chemistry.

Background information: The researchers conducted a series of 16 experiments over a period of 10 years to try to address the issue of the effect of experience on the brain. Because they wanted to study the brain post-mortem they did not use humans in their studies, but rather, as in many classic psychological experiments, their subjects were rats.

Aim: To investigate the effects on rats’ brain growth and chemistry of a highly stimulating environment when compared with the effects of an impoverished environment.

Method: Three male rats were chosen from each litter. They were then randomly assigned to one of three conditions. One rat remained in the laboratory cage with the rest of the colony; another was assigned to the "enriched" environment cage; and the third was assigned to the "impoverished" cage. There were 12 rats in each of these conditions for each of the 16 experiments.

The three different environments were:

  • The standard laboratory colony cage - several rats in an adequate space with food and water always available.
  • The impoverished environment - a slightly smaller cage isolated in a separate room in which the rat was placed alone with adequate food and water.
  • The enriched environment - 6-8 rats in a large cage furnished with a variety of objects with which they could play.

The rats were allowed to live in these different environments for various periods of time, ranging from four to 10 weeks. Following this differential treatment period, they were humanely sacrificed so that autopsies could be carried out on their brains to determine if any differences had developed.

Results: The brains of the enriched rats were different from the impoverished rats in many ways. The cerebral cortex of the enriched rats was significantly heavier and thicker. The cortex is the part of the brain that responds to experience and is responsible for movement, memory, learning, and all sensory input (vision, hearing, touch, taste, smell). While there were no significant differences found in the number of brain cells (called neurons), the enriched environment produced larger neurons. Under great magnification using the electron microscope, it was found that the synapses themselves of the enriched rats' brains were 50% larger than those of the impoverished rats.

Conclusion: The enriched environment had significantly increased the cerebral cortex of the rats’ brains, by enlarging the neurons and synapses and increasing the chemical activity of the brain.


Could the changes be due to increased handling, rather than enriched environment? In response to this possible criticism, further experiments were carried out, wherein the impoverished rats were handled as much as the other two groups, and the differences were still found. Also, sometimes, the rats in the enriched environments were not handled, but still they retained the increased brain growth.

Can the results be generalised to humans? It has been found that learning itself is enhanced by enriched environmental experiences and that even the brains of adult animals raised in impoverished conditions can improve when placed in an enriched environment (see Bennett, 1976).

Some evidence exists to indicate that experience does indeed alter brain development in humans. Through careful autopsies of humans who have died naturally, it appears that as a person develops a greater number of skills and abilities, the brain actually becomes more complex and heavier. Other findings come from examinations during autopsies of the brains of people who were unable to have certain experiences. For example, in a blind person's brain, the portion of the cortex used for vision is significantly less developed, less convoluted, and thinner than in the brain of a person with normal sight.

Marian Diamond, one of the authors of the original article, has applied the results of work in this area to the process of human intellectual development throughout life. She says, "For people's lives, I think we can take a more optimistic view of the aging brain .... The main factor is stimulation. The nerve cells are designed for stimulation. And I think curiosity is a key factor. If one maintains curiosity for a lifetime, that will surely stimulate neural tissue and the cortex may in turn respond .... I looked for people who were extremely active after 88 years of age. I found that the people who use their brains don't lose them. It was that simple" (Hopson, 1984, p. 70).

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