Environmental Enrichment and Hebbian Pathways

Research in environmental enrichment began in 1949 with Dr. Hebb’s investigation with two groups of rats. Rats raised in his “impoverished” laboratory setting performed worse in detour and maze problems than rats raised as pets by his two daughters. While this initial investigation was very crude, it opened the door for more controlled studies in environmental enrichment and impoverishment and its impact on learning and the brain.

By 1964 Bennett et al. securely established that rats reared in an enriched environment are faster learners than their littermates raised in relative isolation. The early “enriched” environments usually were large cages that featured toys, running wheels, and other objects, and they were cohabitated by other rats. The control rats were raised in smaller cages, alone, with no objects. Rosenzweig and Bennett reviewed literature in the area in 1996, again confirming their earlier hypothesis. It was not clear then, or now, what the particular elements of the environment made it enriched, and it was not clear weather the greater physical activity of the rats in the enriched environment contributed to their higher performance in problem solving and learning.

As late as 2000 van Praag et al. in a comprehensive review could not find any isolated variables that contributed to making an environment “enriched” for animals ranging from drosophila, to rats, to humans; though they did confirm that neither observing and enriched environment without being able to participate in it (TV rat) nor social interaction alone could explain the effects of enriched environments. They did discover a confounding variable. The interaction between the environment and neural development goes both ways since neural developments effect the animal’s perception of the environment. It is not simply the environment impressing itself upon the brain. In higher animals, it is certainly possible that factors like motivation play a substantial role in the effect of an enriched environment, and the effects of voluntary physical activity are still unknown. Clearly, isolating the specific factors in an enriched environment would be greatly beneficial to the advancement of research in the area and clear up confusion.

Diamond and Hopson at the University of California (the same university that Bennet et al. made their landmark discoveries) have suggested some factors that create an enriched environment for children in their book, Magic Trees of the Mind (1999), though none of these factors have been experimentally verified. They come simply from clinical experience, surveys, and interviews (mostly anecdotal and poorly controlled).

According to Diamond and Hopson, enriched environments…

∑ Include a steady source of positive emotional support

∑ Provide a nutritious diet with enough protein, vitamins, minerals and calories

∑ Stimulate all the senses (but not all at once)

∑ Have an atmosphere free of undue pressure and stress but suffused with a degree of
pleasurable intensity

∑ Present a series of novel challenges that are neither too easy nor too difficult for the stage of development

∑ Allow for social interaction for a significant percentage of activities

∑ Promote the development of a broad range of skills and interests that are mental, aesthetic, social and emotional

∑ Give the child an opportunity to choose many of his or her own activities

∑ Give the child a change to assess the results of his or her efforts and to modify them

∑ Offer an enjoyable atmosphere that promotes exploration and the fun of learning

∑ Above all, allow the child to be an active participant rather than a passive observer.

These factors echo the observations posed in the review article. Motivation, choosing activities, being active rather than passive, social interaction, and stimulation all seem important. Nutrition is obviously important and will not be discussed further. A point that seems to be overlooked is the assessment of physical effort. Diamond and Hopson do not discus physical activity’s impact on learning.

Empirically verifying the particular factors that contribute to an enriched environment would surely be helpful. It could advise parents on weather or not to purchase Baby Einstein videos for their children, or which particular educational video would be best (though it has already been established that participation is necessary in an enriched environment. TV babies will probably do no better than TV rats). This information would be very valuable to educators, too. Arranging classrooms and curriculums may be enhanced by knowledge of enrichment’s impact on learning. Perhaps simple changes in the classroom could elicit great improvements in learning.

The best way to understand the improved performance of individuals in enhanced environments is to decipher a biological mechanism, rather than simply correlating performance and factors in an enriched environment because correlation will never be able to distinguish the interaction between the environment and neural development.

In 1949 Hebb proposed that learning and memory are based on the strengthening of synapses that occurs when pre- and postsynaptic neurons are simultaneously active. Either the pre synaptic neuron or the postsynaptic neuron (or both) changes in such a way that the activation of one cell becomes more likely to cause the other to fire. Recently, neuroscientists have amassed data showing that Hebb was somewhat correct, and the maxim “cells the fire together, wire together” has become a common colloquial term (Schwartz and Begley, 2002). This can provide the biological framework for understanding enrichment’s impact on the individual.

Dr. Kandel, the 2000 Nobel Prize winner in Physiology or Medicine, earned his share of the medal by investigating the “cells that fire together, wire together” maxim as it relates to learning in the humble Aplysia californica, the sea snail. He found that sensitized neurons had undergone long-lasting change: when excited (by touch), they discharge more neurotransmitter than do neurons that have not undergone sensitization. They also found that after periods of stimulation, certain reflex actions could be enhanced for a period of time. These stimuli modulate secondary messenger molecules like cAMP, which has been found to stimulate the formation of neural connections (Schwartz and Begley, 2002).

Enriched environments should have particular features, which can be directly linked to neural sensitization. In the case of the Aplysia, this was accomplished by simply spraying a jet of water onto its soft body tissues. Obviously different things in an enriched environment will simulate humans, but since both humans and Aplysia share similar neuronal and secondary messenger systems, it is likely that the Hebian conception of neural learning will apply to both. The task is now to demonstrate what environmental elements sensitize human neurons.

Enrichment has been linked to an increase in brain weight and dendritic branching (Pacteau et al., 1989) and improving cortical synaptic plasticity (Wainwright et al., 1993) in rats, both of which are conceptually Hebbian mechanisms. If it could be demonstrated that particular elements in an enriched environment influence these quantifiable physiological measures, then hard, empirical evidence would exist for developing enriched environments for humans.

Physiological measures of neuronal neuronal activity used on rats (which involved destruction of the animal) cannot be carried out on humans. Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) uses alterations in brain hemodynamics to infer changes in neural activity by measuring small changes in deoxyhemoglobin within the brain’s vasculature, allowing non-invasive analysis of single-neuronal activity. The relation between fMRI and physiological measurements (such as Clark-style polographic microelectrodes inserted into the brain) is well substantiated in cats (Thompson et al., 2003), which are more closely related to humans than rats, though BOLD has not been substantiated in primates to date.

An experiment could test this by correlating specific features of the environment (for example, social interaction) to BOLD fMRI measurements and problem solving measures. It is known that the effects of an impoverished environment can be demonstrated rapidly (within a day or two) in humans, and normal humans make a rapid recovery once they are reintroduced to a normal environment (Heron 1957), so these experiments can be conducted rather safely.
An experiment is therefore proposed. A group of human subjects will be placed in environments with defined, controllable elements of enrichment for short periods of time (perhaps a day or two). They will then be compared to a group of human subjects (control 1), which were simply allowed to go about their daily lives normally with BOLD fMRI and pencil and paper problem solving tests. Another group would be placed in an impoverished environment similar to Heron’s 1957 experiment and compared in the same way. By varying the elements of enrichment, particular elements could be isolated to give an accurate picture of what really results in enrichments. Also, pedometers or heart-rate monitors could be placed on the subjects to approximate physical activity to determine if it is a variable.

A plausible hypothesis would be that elements in the enriched environment that causes “arousal,” would provide stimulation for the Hebbian pathway (which can be monitored using BOLD fMRI) and result in better problem solving scores. Schwartz and Begley suggested this “arousal hypothesis” in 2002. A contrary finding would be if no evidence for the Hebbian pathways was found, or if the neuronal activity was highly irregular in different individuals.

References:

Bennett, E. L., Diamond, M. C., Krech, D. and Rosenzweig, M. R. (1964) Chemical and Anatomical Plasticity in the Brain. Science. 146: 610-619

Diamond, M. C. and Hopson, J. L. (1999) Magic Trees of the Mind: How to Nurture Your Child's Intelligence, Creativity, and Healthy Emotions from Birth Through Adolescence. New York. Penguin.

Hebb, D. O. (1949) The Organization of Behavior. New York. Wiley.

Heron, W. (1957, January) Pathology of Boredom. Scientific American. 52-56

Pacteau, C., Einon, D. and Sinden, J. (1989) Early Rearing Environment and Dorsal Hippocampal Ibotenic Acid Lesions: Long-Term Influences on Spatial Learning and Alteration in the Rat. Behavioral Brain Research. 34: 79-96

Rozenzweig, M. R. and Bennett, E. L. (1996) Psychobiology of plasticity: effects of training and experience on brain and behavior. Behavioral Brain Research. 78: 57-65

Schwartz, J. M. and Begley, S. (2002) The Mind and the Brain: Neuroplasticity and the Power of Mental Force. New York. HarperCollins.

Thompson, J. K., Peterson, M. R. and Freeman, R. D. (2003) Single-Neuron Activity and Tissue Oxygenation in the Cerebral Cortex. Science. 299: 1070-1072

Van Praag, H., Kempermann, G. and Gage, F. H. (2000) Neural Consequences of Environmental Enrichment. Nature Reviews Neuroscience. 1:191-198

Wainwright, P. E. et al. (1993) Effects of Environmental Enrichment on Cortical Depth and Morris-maze Performance in B6D2F2 Mice Exposed Prenatally to Ethanol. Neurobehavioral Toxicology and Teratology. 15: 11-20