E NVIRONMENTAL COMPLEXITY, CEREBRAL CHANGE,
AND BEHAVIOR '
MARK R . ROSENZWEIG
University of California, Berkeley
ELATING b rain processes to behavior in
animals is the s ubject of this paper. Addressing an audience not f rom, my own
special field reminds me of the animal psychologist
who p resented a paper at a psychoanalytic meeting. At the end of the paper, one of thepsychoanalysts grumbled, "We knew all this already."
"Yes," a colleague placated him, "but we d idn't
know t hat it was t rue of rats I" In my own case, I
hope t hat you will n either reject this research as
irrelevant to your own n or, on the other hand,
apply it u ncritically to work in h uman behavior
and development. Therefore, a fter p resenting the
main findings, I will w ant to discuss with youthe
possible significance of s uch work for the s tudy of
The research has been done by a team in which
Edward L. Bennett, a biochemist, David Krech, a
psychologist, and I have collaborated for a dozen
years. M ore recently we have been joined by
Marian C. Diamond, a n euroanatomist. The overall scope of our p rogram concerns both h ereditary
and e nvironmental factors that have been demonstrated to a ffect l earning ability. We have been
attempting to determine whether they do so by
affecting the a natomy and chemistry of the b rain.
Today I will c oncentrate on these questions: Can
differential experience m odify the b rain in measurable anatomical and chemical terms? If so, can
these cerebral changes be related to the e ffects of
experience on learningability?
The suggestion that t hinking might induce
growth of the b rain is an old one, dating back at
least to the late eighteenth century (Sommering,
1791). In the early nineteenth century, the phrenologist Spurzheim (1815) held it highly probable
A bbreviated version of an address given to t he Division o f D evelopmental Psychology a t A merican P sychological A ssociation, Chicago, September 196S. This research h as been s upported b y g rants o r c ontracts f rom t he
National I nstitute o f M ental Health, U nited S tates Public
Health S ervice; t he Surgeon General's O ffice; a nd the
National A eronautics and Space A gency. It has also
received support f rom the United States Atomic Energy
that "the organs [ of the b rain] increase b y exercise." Showingmore caution than usual, Spurzheim added, "In order, however, to be able to
answer this question positively, we ought to observe the same persons when exercised and when
not exercised; or at least observe many persons
who are, and many others who are not, exercised
during all periods of l ife [ pp. 554-555]." This
idea has r ecurred f rom time to t ime; essentially the
same researchproposal was made a c entury and a
half later by the anatomist, J. Z. Young (1964, p.
In our research on this question starting in
1958, we followed essentially the design suggested
by Spurzheim. However, we benefited from many
intervening investigations demonstrating t hat t he
brain is relatively stable and not likely to show
large changes—a f act ofwhich Sommering ( 1785)
was already aware. Because the cerebral e ffects
that we hoped to induce might be small at best, we
employed p rocedures t hat we hoped would maximize these effects of experience and that would
minimize v ariability f rom extraneous sources.
In an a ttempt to maximize the e ffects of d ifferential experience on the b rain, we decided to set up
two markedly d ifferentexperimental situations, to
put the r ats in one or the other at an early age
when their brains might be most plastic, and to
maintain the animals in these situations for a
prolonged p eriod. Animals are t herefore assigned
at weaning (about 25 clays of age) and kept for
80 days in e ither an enriched environment—environmental complexity and training (ECT)—or
in an impoverished condition (...
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