Process of Discovery Mark R. Rosenzweig Psychology Professor of Graduate Studies at the University of California at Berkeley How experience molds the plastic brain How did you first become interested in psychology?

I became interested when taking undergraduate courses at the University of Rochester, because psychology offers scientific ways to investigate and understand behavior and its mechanisms. From there, I went to Harvard, where I obtained my Ph.D. in psychology.

How did you come up with your important discovery?

The discovery is that learning or enriched experience alter the chemistry and anatomy of the brain, and these changes are probably involved in storing memories. My colleagues and I found this while looking for something else. David Krech, a psychologist, Edward L. Bennett, a neurochemist, and I were interested in whether individual differences in learning ability among laboratory rats were related to differences in brain chemistry.

We tested groups of rats in spatial learning apparatuses and then measured brain samples for acetylcholinesterase (AChE)-the enzyme that inactivates the synaptic transmitter acetylcholine. We did find significant correlations, but we also noticed that rats tested on the more difficult problems had higher levels of AChE activity than those tested on easier problems. Thus, it appeared that training and testing could modify brain chemistry, something that had never before been demonstrated. We also found that simply housing rats in an enriched environment (a large cage with varied stimulus objects) raised AChE activity compared to that of rats kept in standard colony cages. Then we had another surprise: We were measuring enzymatic activity per unit of tissue weight and noticed that the weights of standard samples of cerebral cortex also varied with experience!

We were then joined by neuroanatomist Marian C. Diamond, and we and others found that differential experience produces detailed changes in neuroanatomy, including size of neuron cell bodies, numbers of synaptic contacts, and branching of the dendritic tree. Other changes were also found. Hebb and his students had already discovered that experience in an enriched environment favors development of problem-solving ability, so we and others helped explain that behavioral effect in terms of brain plasticity.

How has the field you inspired developed over the years?

It has developed in several directions. For example, many investigators have come to realize that animals-and people-raised without sufficient stimulation do not develop full growth of brain or full behavioral capacities. We now know that the effects of differential experience can be found throughout the life span, although they are larger earlier in life. Effects of this sort have also been found in several species of mammals, and in birds, fish, and insects-from fly to philosopher. Further effects are still being found, as in the recent finding that enriched experience favors the retention of new nerve cells even in the adult brain.

What is your prediction on where the field is heading?

Research and applications are heading in several directions. The discovery of brain plasticity in response to experience has encouraged increasingly detailed studies of the neural processes involved in learning and memory formation. Some investigators are trying to find what aspects of the environment, or what combinations of factors, are important for development of different aspects of behavior and ability. Others are using enriched environments, sometimes in combination with drugs, to improve recovery of function after injury to the nervous system.