Diamond, M.C., Krech, D., & Rosenzweig, M.R. (1964). The Effects of an Enriched Environment on the Histology of the Rat Cerebral Cortex
Arhe Vaninetti
Research Methods and Statistics, PSY 301, Pacific University, Oregon
February 6, 2023
Diamond et al.’s (1964) study on cognitive enrichment processes was a simple yet groundbreaking study for both the study of psychology and for the development of animal welfare. Specifically regarding brain plasticity and habitat enrichment, Diamond et al. (1964) were able to conclude that the environment can shape the brain’s structure and capabilities. The inspiration for this study originated from the notion that animals raised in enriched environments were able to have increased cognitive capabilities, including improved learning and memory performance. Diamond et al. (1964) used randomized sampling from a University of California laboratory rat colony to select 11 pairs of male rats in the first experiment, and 9 pairs of male rats in the second experiment.
Half of the rats were placed in a plain laboratory cage (the control condition), and half were placed in a larger, enriched cage that included toys that they could play with and manipulate (the experimental condition). For 30 minutes each day, each experimental rat was allowed to play and explore an ever-changing maze before it was placed back in with its group mates in the enriched environment. The control group rats were not introduced to this maze. The researchers housed the control group rats separately and did not allow them to see the other rats to limit social interaction, which may have caused stress on the animals. After the same system was followed for 80 days, the rats were ‘sacrificed,’ and the researchers were randomly assigned rats to dissect. The rats’ brains were cross-sectioned and studied for effects on the cerebral cortex and in the capillaries. Diamond et al. (1964) found that rats in the enriched environment had thicker cerebral cortex’ and a higher density of capillaries in certain regions of the brain, compared to rats kept in standard laboratory cages. This study suggested that an enriched environment can have a positive effect on brain structures.
This study was found to be quite impactful on cognitive science and on animal rights and welfare. Cognitively, Diamond et al. (1964) confirmed a common observation at the time and placed it into empirical research, opening the doors for further research on the impacts of the environment on beings’ cognitive structures. One of the initial critiques of the study was that the effects of isolation on the control group rats may have prevented the study from reaching its intended goal of understanding environmental impacts on cognitive function. Similarly, the socialization in the experimental rat group may have created the same confounds. With issues regarding confounds, it is hard to detect if the researchers were actually testing their hypothesis.
Regarding animal welfare, much is to be said of the research. Rats are inherently social creatures, and by placing them in barren laboratory cages, this may have been increasing the stress on the creatures, which may have impacted the results of the study. Because this study was conducted in 1964, before the creation of the 1966 US Welfare Act, it may have been lacking in experimental animal welfare techniques. Because the experiment lasted 80 days, the laboratory had been cycling through rats for each replication of the study which, while standard at the time, would be objectionable in the current age. Similarly, due to the 80 day duration of the experiment, the notion that it did not last long enough could be made to argue against the study’s validity.
Lastly, because the study utilized male rats within the enriched enclosure, it suggests that there may have been fighting, power dynamics, or territory issues with the male rats. The study did not mention neutering the rats, so conflict among the rats may have been possible and may have led to enhanced stress and the creation of hierarchies and power dynamics. Why the researchers did not use female rats is unknown, but it may have increased the study’s validity across the board, and it is what a contemporary researcher may suggest.