Experiments on rats carried out at the University of California provided physiological evidence that the brain expands because of activity and stimulation. The researchers divided the rats into pairs. One pair was kept in an impoverished environment until their age was equivalent to the human age of 75 (ten days in a rat’s life is roughly equivalent to a year in a human life); the other pair was placed in an enriching environment. Subsequent anatomical studies revealed a far greater number of connections between the brain cells of the stimulated rats than in the impoverished rats. Their brain cells had also grown in size by 15 percent. Interestingly, in a further experiment, rats who were kept in an impoverished environment and then transferred to an enriched one until their age was equivalent to a human age of 90, showed considerable growth in neural connections. The term coined to describe this sort of growth is structural plasticity.