Rosenzweig and Bennett's experiments in the 1960s were some of the first to show that our brain can change as a result of our environment (which is an example of neuroplasticity).

Key Study: Animal research on neuroplasticity (Rosenzweig and Bennett, 1961)

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Background Info

For a long time it was widely believed that our brain’s growth happened at a fixed rate. Many people thought that by the time we were about 4 – 6 years old, our brains had stopped developing and we were either going to be smart, dumb or average and that this wouldn’t change in our lives. However, research studies have demonstrated our brain’s impressive ability to grow and adapt throughout our lives as a result of environmental stimulation and even our thought processes – this is called neuroplasticity.

lab rat cage

Study rats in a laboratory allows researchers to conduct carefully controlled environments so they can deduce causal relationships.

During the process of learning something new, our neurons undergo a process called dendritic branching. This is when the dendrites of neurons make new connections to other neurons. It is these connections that enable us to learn and master new skills. Each area of the brain has a particular function, this is known as localization of brain function. However, this isn’t fixed and can change. People can lose parts of their brain and due to dendritic branching and the establishment of new neural connections they can shift the function to a new part of the brain. This is another example of neuroplasticity. Even more impressive is the fact that the brain may be capable of growing new neurons, which is called neurogenesis (research is very new on this). It was believed for a long time that this wasn’t possible. So what are the factors that can cause our brain to grow and develop in new ways? This was the area of research for Mark Rosenzweig and his colleagues.

This study is a good example of the value of animal models in providing insight into human behaviour.

The Environments Influence on the Brain (Rosenzweig and Bennett)

To investigate the effects of the environment on brain growth and development (neuroplasticity), Rosenzweig and his colleagues designed an experiment where rats were placed in different cages and lived for 30-60 days before they were euthanized and a post-mortem study was conducted to measure the thickness and heaviness of the brain cortex as well as the amount of acetylcholine receptors and synapses.  

In Rosenzweig’s rat experiment, male rats were chosen from different litters to be randomly allocated to two different conditions: enriched condition (EC), and the deprived condition (DC). In the EC, there were about 10-12 rats and there were a range of toys that the rats could play with. This group also received “maze training”. The DC was a cage that was slightly smaller and the rat was alone and the cage was isolated in a separate room from the other cages. Both conditions had adequate food and water. The rats lived in these different conditions for four to ten week periods (30-60 days). After these treatment periods, the rats were autopsied in order to determine if any differences had developed. The scientist doing the autopsy did not know which cages the rats had been in. The rat’s brains were dissected and various sections were measured, weighed and analyzed to determine the amount of cell growth and levels of neurotransmitter activity.

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This study showed the effects of different environments on the brain.

Rosenzweig found that rats living in the EC developed a heavier and thicker brain cortex. More specifically, the frontal lobes of the rats were heavier and they had developed more acetylcholine receptors. Further studies found that the brain weight of the rats can increase 7 – 10% and synapses increase by about 20%. The results were quite groundbreaking at the time and Rosenzweig was so surprised by the results that he replicated the research numerous times. With each replication the same results were obtained.

Follow up experiments even investigated if researchers could determine which rats had lived in which cages based on watching videos of the rats. Depending on their curiosity and activity, the researchers could determine the enriched and deprived rats.

While this study has been performed on rats, can we make conclusions regarding enrichment and stimulation and the human brain? One study shows that pre-mature babies (of both rats and humans) put on weight more quickly and develop more neurologically (i.e. their brain develops) more when they are handled (e.g. touched and cuddled) and when they are not. Perry also performed MRI scans of 3 year old children who had been extremely neglected and those who had not. The MRI scans reveal that the neglected children had smaller brains than the “normal” children.

Rosenzweig’s experiments have had a significant impact on psychology as they clearly show that there is a cause and effect between the environment and brain development. The work by Rosenzweig and Bennet served as a catalyst for continued research in this area.

Critical Thinking Questions

  • What are the ethical considerations related to this study?
  • Why might these results not apply to humanas?
  • How did Rosenzweig control for possible confounding variables?
  • What are the strengths and limitations of these studies?
  • How can this knowledge be applied to everyday life and society?

References

Psychobiology of plasticity: effects of training and experience on brain and behavior. Mark R. Rosenzweig * and Edward L. Bennett Department of Psychology-1650, 3210 Tolman Hall, University of California, Berkeley, CA 94720-1650, USA

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