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Response of the brain to enrichment

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Before 1960, the brain was considered by scientists to be immutable, subject only to genetic control. In the early sixties, however, investigators were seriously speculating that environmental influences might be capable of altering brain structure. By 1964, two research laboratories proved that the morphology and chemistry or physiology of the brain could be experientially altered (Bennett et al. 1964, Hubel and Wiesel 1965). Since then, the capacity of the brain to respond to environmental input, specifically "enrichment," has become an accepted fact among neuroscientists, educators and others. In fact, the demonstration that environmental enrichment can modify structural components of the rat brain at any age altered prevailing presumptions about the brain's plasticity (Diamond et al. 1964, Diamond 1988). The cerebral cortex, the area associated with higher cognitive processing, is more receptive than other parts of the brain to environmental enrichment. The message is clear: Although the brain possesses a relatively constant macro structural organization, the ever-changing cerebral cortex, with its complex microarchitecture of unknown potential, is powerfully shaped by experiences before birth, during youth and, in fact, throughout life. It is essential to note that enrichment effects on the brain have consequences on behavior. Parents, educators, policy makers, and individuals can all benefit from such knowledge.
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Response of the Brain to Enrichment
MARIAN C. DIAMOND
Department of Integrative Biology, 3060 Valley Life Sciences Building
University of California, Berkeley, CA94720, USA
Manuscript received on March 5, 2001; accepted for publication on March 12, 2001;
presented by Leny A. Cavalcante
ABSTRACT
Before 1960, the brain was considered by scientists to be immutable, subject only to genetic control. In
the early sixties, however, investigators were seriously speculating that environmental influences might be
capable of altering brain structure. By 1964, two research laboratories proved that the morphology and
chemistry or physiology of the brain could be experientially altered (Bennett et al. 1964, Hubel and Wiesel
1965). Since then, the capacity of the brain to respond to environmental input, specifically “enrichment,
has become an accepted fact among neuroscientists, educators and others. In fact, the demonstration that
environmental enrichment can modify structural components of the rat brain at any age altered prevailing
presumptions about the brain’s plasticity (Diamond et al. 1964, Diamond 1988).
The cerebral cortex, the area associated with higher cognitive processing, is more receptive than other parts
of the brain to environmental enrichment. The message is clear: Although the brain possesses a relatively
constant macrostructural organization, the ever-changing cerebral cortex, with its complex microarchitecture
of unknown potential, ispowerfully shaped by experiences before birth, during youth and, in fact, throughout
life. It is essential to note that enrichment effects on the brain have consequences on behavior. Parents,
educators, policy makers, and individuals can all benefit from such knowledge.
Key words: enrichment, cerebral cortex, hippocampus, aging, adult neurogenesis, dendrites.
INTRODUCTION
Can experience produce measurable changes in the
brain? The hypothesis that changes occur in brain
morphology as a result of experience is an old one.
In 1815 Spurzheim asked whether organ size could
be increased by exercise. He reported that the brain
as well as muscles could increase with exercise “be-
causetheblood is carriedingreaterabundancetothe
parts which are excited and nutrition is performed
by the blood.” In 1874 Charles Darwin mentioned
that the brains of domestic rabbits were consider-
Invited paper
E-mail: diamond@socrates.Berkeley.EDU
ably reduced in bulk in comparison with those from
the wild because, as he concluded, these animals
did not exert their intellect, instincts, and senses as
much as did animals in the wild. However, it was
not until the 1960s, that the first controlled studies
in animals demonstrated that enriching the environ-
mental condition in which they were confined could
alter both the chemistry and anatomy of the cerebral
cortex and, in turn, improve the animals’ memory
and learning ability.
In these early experiments only the brains of
young animals were studied. Although many were
impressed to learn that the cerebral cortex could in-
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An. Acad. Bras. Cienc.,(2001)73 (2)
212 MARIAN C. DIAMOND
crease its thickness in response to enriched living
conditions, they raised the question about whether
enrichment might similarly affect older animals.
Once middle-aged rats brains showed positive re-
sponses to enrichment, the next step was to experi-
ment with very old animals. Once again, increases
in corticalthickness were found. It then becameim-
portant to discover what was responsible for these
changes.
One step at a time, the level of morphologi-
cal changes from neuronal soma size, to num-
ber and length of dendrites, to types and numbers
of dendritic spines, to synaptic thickening, to capil-
lary diameter, and to glial types and numbers – was
examined. Age, gender, duration of exposure, etc.
were critical variables that had to be tested in new
experiments.
Most of the basic data reported on the enrich-
ment paradigm and its impact on brain and behavior
have accumulated through studies on the rat. Ef-
fects of enriched and impoverished environments
on the nerve cells and their neurotransmitters in the
cerebral cortex have now been generalized to sev-
eral mammalianand avianspecies (Rosenzweigand
Bennett 1996). Some corroborating studies men-
tioned herein involved cats and monkeys, as well
as isolated studies in human subjects. For example,
Jacobs et al. (1993) using an isolated portion of the
human cerebral cortex responsible for word under-
standing, Wernicke’s area, compared the effects of
enrichment in tissue from deceased individuals who
had had a college education and from those who
had had only a high school education. They demon-
strated that the nerve cells in the college-educated
showedmore dendrites than those in the latter. (Tis-
suewasobtainedfromtheVeteran’sHospitalin west
Los Angeles.) Experiments on human tissue fre-
quently support the data obtained from studies in
the rat, and, in turn, benefit from these animal stud-
ies. We can now safely say that the basic concept of
brain changes in response to enrichment hold true
for a wide variety of animals and for humans.
THE EFFECTS OF ENRICHMENT
ON THE CEREBRAL CORTEX
What do we mean by “enrichment” for the rats who
haveserved asthe animalof choicefor mostof these
studies? Thirty six Long-Evansrats were sortedinto
three experimental conditions using 12 animals in
each group: 1) enriched 2) standard or 3) impov-
erished environments. All animals had free access
to food and water and similar lighting conditions.
Eventually, it was determined that animals main-
tained in their respective environments from the age
of 30 days to 60 days developed the most extensive
cerebral cortical changes. For the enriched environ-
ment, the 12 animals lived together in a large cage
(70× 70× 46 cm) and were provided 5-6 objects to
explore and climb upon (e.g., wheels, ladders, small
mazes). Theobjectswerechangedtwotothreetimes
a week to provide newness and challenge; the fre-
quent replacement of objects is an essential com-
ponent of the enriched condition. The combination
of “friends” and “toys” was established early on by
Krech as vital to qualify the experiential environ-
ment as “enriched.” (Krech et al. 1960). For the
standard environment, the animals were housed 3 to
a small cage (20× 20 × 32 cm) with no exploratory
objects. For the impoverished environment, one an-
imal remained alone in a small cage with no ex-
ploratory objects.
The numbers of animals placed in these sepa-
rate conditions were based on the manner in which
theroutinehousingwasestablishedin therat colony.
Three rats in a cage has been considered standard
for all experimental work over the decades. Since
priorto theseexperimentsnoone haddesignedstud-
ies to examine brain changes in response to differ-
ent environmental conditions, the decisions about
what represented “impoverishment” and what rep-
resented“enrichment”wasmorearbitrarily than sci-
entifically reasoned.
After 30 days in their respective environments,
all animalswere anesthetized beforethe brains were
removed for comparison among the three groups.
Twenty micrometer frozen sections were cut and
An. Acad. Bras. Cienc.,(2001)73 (2)
AABC 73 2 b 2
BRAIN AND ENRICHMENT 213
stained, and the thickness of the frontal, parietal
and occipital cortices were measured. Results in-
dicated clearly that the cortex from the enriched
grouphad increasedinthickness compared withthat
living in standard conditions, whereas, the brains
from the impoverished group decreased compared
to thestandard. Because the nerve cellswere farther
apart in the enriched vs. the impoverished brains, it
was thought that the major component of the brain
changes due to enrichment had to do with alter-
ations in the dendritic branching. With more de-
tailed studies, the cortical thickness increases were
found to be due to several factors, including in-
creased nerve cell size, number and length of den-
drites, dendritic spines, and length of postsynaptic
thickeningasmeasuredonelectronmicroscopicpic-
tures of synapses. (Diamond et al. 1964 and 1988).
In the initial experiments designed to explore
the impact of an enriched environment on the brain
of post-weaned rats, only enriched and impover-
ished groups were used. Rats were maintained in
theirrespectiveenvironmentsfrom25 to105 daysof
agebecausetherewerenoavailabledataonhowlong
itwouldtaketocreatechemicalorstructuralchanges
in the cortex. Chemical and anatomical measure-
ments taken from these animals showed significant
differences between the two groups in cortical
thickness, cortical weight, acetylcholinesterase,
cholinesterase, protein and hexokinase levels, (Ben-
nett et al. 1964, Diamond et al. 1964). In these
initial experiments, however, it was not clear if the
changes were due to enrichment or impoverishment
because there were no standard conditions estab-
lished as controls.
Nonetheless, the differences in cortical thick-
ness with this 80-day exposure to the two environ-
mental conditions were not as great as during the
30-day exposure. Consequently, in subsequent ex-
periments, the period of exposure to the experimen-
tal conditions was reduced from 80 days to 30 days,
then 15 days, 7 days and finally to 4 days. At each
of these intervals, animals from the enriched envi-
ronment showed increases in cerebral cortical thick-
ness in some areas but not in others. For example,
in the male animals exposed for 80 days to enriched
conditions, the somatosensory cortex did not show
significant changes, whereas male animals exposed
for 30days did developsignificant differences inthe
somatosensory cortex. The occipital cortex showed
significant changes for both the 80- and the 30-day
experiments,but, again, the differenceswere greater
at 30 days than at 80 days. It is possible that the
longerexposureservedtoincreasecorticalthickness
inthe early days of enrichment butthat overtime the
environmental condition became monotonous and
this effect decreased. In later experiments the ex-
perimental conditions were modified to try to es-
tablish what the major factors were that created the
observed cortical changes. For example, wasthe ef-
fect associated with the number of rats exposed or
to the presence of stimulus objects?
The new conditions included one rat living
alone in the large enrichment cage with the objects
that were changed several times each week. The
cortex of these rats did not show a significant effect
of enrichment. Twelve rats living together in the
largecage without thestimulus objects did notshow
as great an effect as 12 rats living with the stimulus
objects. In other words, the combination of social
conditions and frequent exposure to new stimulus
objects were necessary for the animals to gain the
full effect of enrichment.
Establishing what constitutes “enrichment” for
human beings is more problematic. Not only are
controlled experiments not feasible, but no two hu-
man brains are identical. Individuals differ in their
geneticbackgroundsandenvironmentalinputs. Fur-
thermore, what is considered enrichment for one in-
dividual may be quite different for another. Yet, as
mentioned earlier, the enrichment effect was evi-
dent in Wernicke’s area from measurements of the
amount of dendritic branching in brain tissue from
college-educated individuals versus that from high
school-educated people. The basic finding of den-
dritic growth in response to environmental stimula-
tion appears in all brains studied to date. It would
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An. Acad. Bras. Cienc.,(2001) 73 (2)
214 MARIAN C. DIAMOND
appear that newness and challenge are important for
the human cortex as well as for that of animals.
Independent Variables: Age and Gender
Among the many variables researchers must con-
sider as they seek to understand and accurately in-
terpret the effects of enrichment on the brain, age
and gender are important considerations. Enrich-
ment has been shown to enhance many aspects of
cortical structure at any age from prenatal to ex-
tremely old rats (904 days of age). The amount of
change varies with the age of the animal. For ex-
ample, when a 30-day-old rat is put in an enriched
environment for fourdays, the effectsare notas pro-
nouncedastheyareinthe60-day-old-ratmaintained
in enriched conditions for four days. Is four days
too short a time for the very young animal to ad-
just and benefit from enrichment? A young animal
maintained for 30 days in an impoverished environ-
ment shows reduced morphological development of
its cortex when compared to that of an adult animal
maintained in impoverished conditions for 30 days.
In further age-related experiments, another compo-
nent was added to the enrichment conditions of old
rats. Despite significant increases in the length of
the dendrites in the brains of 600-day-old rats that
had been placed in an enriched environment for 30
days (600 to 630 days), several of the old rats in this
population died.
To determine whether the enrichment condi-
tions could be modified to extend the animals’ life
span, the investigators added a new component:
hand-holding the rats each day for several minutes
while the cages were cleaned. In an attempt to in-
crease the life spanof therats, rats wereplaced three
to a cage after weaning at 25 days of age, and main-
tainedinthesestandardconditionsuntiltheyreached
766 days,at which timehalf went into enriched con-
ditions until they reached 904 days of age and half
stayed in the standard conditions. The only variable
added was the daily hand-holding of the rats as they
aged. Is it possible that handling the rats had ex-
tended their life span? Indeed, many investigators
have been amazed that these rats survived to 904
days of age. The 904 day-old rats in enriched con-
ditions developed a cortex significantly thicker than
the cortex of rats living in the standard conditions
(Diamond 1988). These experiments offered sup-
port to the thesis that the cerebral cortex is capable
of respondingpositively toan enriched environment
at any age (See Fig. 1).
Experiments comparing the effects of enrich-
ment on male and female brains are few. Most en-
richmentstudieshavebeencarriedout on malebrain
toavoidthecompoundingfactorsassociatedwiththe
estrous cycle. In one study focused on gender, the
female neocortex was found to respond differently
fromthe maleneocortexexposedtothe sametypeof
enrichment conditions (Diamond 1988). The male
showed significant changes in cortical thickness in
theoccipital cortex,butno significantchangesin the
somatosensory cortex. (Although the right cerebral
cortex in the brain of the male rat is thicker than the
left, especially in the visual or occipital region, an
enriched environment appears to alter both the right
and left cortex similarly.) In the female, the thick-
ness of the occipitalcortexincreased significantlyin
response to enrichment, although not as much as in
themale,butthethicknessofthesomatosensorycor-
tex increased significantly more in the female than
in the male. In a follow-up experiment, however,
in which obstacles were piled up in front of the fe-
male food cup to provide a greater challenge to her
already enriched environment, the thickness of the
occipital cortex increased as much as did that of the
male without the additional challenge.
Inratswhosetesteswereremovedeitheratbirth
or at 30 days of age before the rats were placed in
an enriched environment for 30 days, the increases
observed in cortical thickness were similar to those
of their littermates with intact testes. (Diamond
1988) These findings suggested that testosterone is
not implicated in the increases in cortical thickness
observedin thebrains of rats livingin enrichedenvi-
ronments. Since sex differences were evident in the
responses of the animals to enrichment, interest was
An. Acad. Bras. Cienc.,(2001)73 (2)
AABC 73 2 b 2
BRAIN AND ENRICHMENT 215
Fig. 1 Two possible patterns of age-related alterations in cortical pyramidal
cells. The normal mature neuron (A) may show regressive dendritic changes
characterized by loss of basilar dendritic branches and eventual loss of the entire
dendritic tree (D, E, F). Other neurons (B, C) may show progressive increase in
dendritic branching. Drawing based on Golgi impregnations.
nowfocused on the brains of pregnant rats, in which
the concentrations of sex steroid hormonal concen-
trations are greatly altered. The brains of female
rats living in the enriched environment from 60 to
90 days and then becoming pregnant and returning
to enrichment until 116 days of age were compared
betweennonpregnantand pregnantanimals livingin
animpoverishedenvironmentforthesametimeperi-
ods. Whenanimalsfromthe twogroupswere autop-
siedat116days,nosignificantdifferencesincortical
thickness were found. Evidently, pregnancy has an
effect on the cerebral cortex regardless of whether
the environment is impoverished or enriched.
These initial experiments, all of which were
replicated, clearly indicate gender differences in the
brain’s response to enrichment. Having dealt with
the independent variables, we turn to the impact
of dependent variables in the enrichment paradigm.
For these studies, one must look at: duration of ex-
posure, brain anatomy and chemistry, presence of
lesions or fetal neocortical grafts, negative air ions,
stress, physical activity and nutrition, as well as be-
havioral effects. These are discussed in turn below.
Duration: The duration of exposureto the enriched
environment is clearly a significant dependent vari-
able that must be factored into research in this area.
As short a period as 40 minutes of enrichment has
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An. Acad. Bras. Cienc.,(2001) 73 (2)
216 MARIAN C. DIAMOND
been found to produce significant changes in RNA
andin thewet weightof cerebral corticaltissue sam-
pled. One day of enrichment was insufficient to
produce measurable changes in cortical thickness,
whereas four consecutive days of exposure (from
60 to 64 days of age) to an enriched environment
did produce significant increases in cortical thick-
ness, but only in the visual association cortex (area
18) (Diamond 1988).
Whenyoung adult ratswereexposedto 30days
of enrichment, however, the entire dorsal cortex, in-
cluding frontal, parietal and occipital cortices, in-
creased in thickness. Extending the duration of the
stay in enriched conditions to 80 days did not pro-
duce any greater increase in cortical thickness than
that seen at 30 days (in fact, it was often even less);
however, the longer the rat remained in the enriched
conditions, the longer the cortex retained its in-
creased dimensions following return to the standard
environment (Bennettetal. 1974). Whenwe looked
at age-related differences in the context of duration
of stay in the enriched environment, we found that
old rats ( 766 days of age) placed in enriched con-
ditions for 138 days showed an increase in cortical
thickness that was quite similar to that observed in
young adult rats (60 days of age) that had lived in
enriched conditions for 30 days.
Anatomical and chemical components: Early ex-
periments, and those to follow in subsequent years,
again demonstrated significant differences in brain
chemistryandanatomyassociatedwithenrichedliv-
ing conditions. Anatomical increases include all
of the structural constituents measured in the cere-
bral cortex to date, such as cortical thickness (Di-
amond et al. 1964), nerve cell soma size, nerve
cell nuclear size (Diamond 1988), dendritic dimen-
sions(Holloway1966, Greenoughet al. 1973), den-
dritic spines, synaptic size and number (Mollgaard
et al. 1971, Black et al. 1990), number of glia, cap-
illary diameter (Diamond 1988), dendritic number
after lesions (McKenzie et al. 1990), and success-
ful tissue grafts, (Mattsson et al. 1997). Chemi-
cal increases include: total protein, RNA-to-DNA
ratio, cholinesterase-to-acetylcholine ratio, Nerve
Growth Factor mRNA, cyclic AMP, choline acetyl-
transferase, cortical polyamines, NMDA (N Methyl
D Aspartate) receptors, and hexokinase, etc.
Lesions: Another variable has to do with the im-
pactofenrichedconditionson purposefully incurred
brain lesions. In a 1990 study, 60-day-old rodents
were exposed for 30 days to either an enriched or
standardenvironmenttwodaysafterhavingreceived
a lesion inthe left frontal cortexthat created a motor
dysfunction in the right forepaw. Animals living in
the enriched condition showed significant increases
in cortical dendritic branching in both hemispheres,
the lesioned and the non-lesioned sides, along with
a significant return of motor function in the right
forepaw compared to those animals living in stan-
dard conditions (McKenzie et al. 1990).
Fetalneocorticalgraft: Similarly, providinganen-
riched environment to rats that had undergone fe-
tal neocortical grafts one week after lesioning was
found to improve behaviorally and to reduce the at-
rophy in the thalamus, a major structure beneath the
cortexthatsuppliesneuralinputtothecortex(Matts-
son et al. 1997). The fact that the fetal neocortical
graft when placed in the lesioned cerebral cortex
could prevent atrophy in the underlying thalamus
as a consequence of enrichment is of great interest
to researchers considering the future possibility of
using such grafts for brain-damaged individuals.
Air ions: The possibility that physical environmen-
tal stimuli other than those classically regarded as
“sensory” could have an effect on the brain was
tested experimentally by exposing rats living in en-
riched or standard environments to high concentra-
tions of negative air ions. The experiments were un-
dertaken to determine whether the effect of negative
ions on serotonin, the putative second messenger
cyclic-AMP, and on cyclic GMP in the cerebral cor-
tex, differ depending on whether the animals lived
in enriched or standard conditions. Studies demon-
strated that rats placed in the enriched environment
An. Acad. Bras. Cienc.,(2001)73 (2)
AABC 73 2 b 2
BRAIN AND ENRICHMENT 217
in the presence of enhanced negative air ions (ion
density of 1× 10
5
) showed a significant decrease in
serotonin, an effect not found in the brains of ani-
mals living in standard conditions (Diamond et al.
1980). Measurements of cyclic AMP decreased as
well in the brains of the animals living in the en-
riched conditions, but cyclic GMP did not. These
results indicate the importance of considering air
quality and atmospheric conditions in determining
the brain’s response to enrichment.
Stress: The presence or absenceof stress represents
yet another variable to be taken into consideration
in such studies, certainly so in any extrapolation of
these findings to humans. Stress is a major factor in
contemporary, fast-moving urban life. Crowding,
for example, is deemed stressful under conditions
where competition for space or food is likely. Ex-
periments were set up to assess the effect of crowd-
ing on the brains of rats maintained in an enriched
environment. To create a condition in which crowd-
ing would be experienced as stressful, 36 rats were
placed in an enrichment cage usually housing only
12 rats, and kept there for 30 days. The results indi-
catedthat, comparedwithratslivinginstandardcon-
ditions, the thickness of the medial occipital cortex
increased significantly whether the enrichment cage
housed 12 or 36 animals, (Diamond et al. 1987).
One hypothesis to come from this study was that the
animals’ interaction with the toys might be divert-
ing their attention or entertaining them sufficiently
to mitigate the stress of the crowded condition.
Chronic stress has been reported by Meaney
et al. (1988) to produce excess glucocorticoids,
which are toxic to neurons – especially those of the
hippocampus. Aged rats are particularly vulnera-
ble to chronic stress. The investigations of Meaney
showed that enriching the living conditions of old
rats, or handling them in their infancy, helps to pre-
vent stress-related hippocampal damage.
It is possible that stress can be produced by
increasing the frequency with which the various ob-
jects in the enrichment cage are changed. In all pre-
vious studies, objects had been replaced daily or at
leastseveraltimeseachweek. Thenthequestionwas
asked whether increasing the frequency of chang-
ing the objects would further increase the growth
of the cortical thickness, or, alternatively, would it
be experienced as a stress factor, given that the an-
imals were inhibited from interacting with them in
themoreleisurely manner towhichtheywereaccus-
tomed. For these experiments, rats 60 to 90 days of
age found their objects changedeveryhour for three
hours on four nights of each week for four consec-
utive weeks. Under this regime, the cerebral cor-
tical thickness did not grow significantly compared
to cortices from rats whose objects were changed
several times each week for four weeks (Diamond,
unpublished.)
Corticosteroids, released under stress, have
been shown to reduce cortical thickness and future
experiments would be necessary to compare differ-
ences in corticosteroid levels in animals exposed to
these differing conditions.
Behavior: Psychologists have known for a long
time that early experience influences the adult per-
formance of an animal. In experiments in the 1950s
(Bingham and Griffiths 1952 and Forgays and For-
gays 1952) investigators were interested in de-
termining how much experience in complex envi-
ronments was necessary to produce a highly intel-
ligent adult animal and when, specifically, during
early life these experiences had to occur. These
studies showed that all of the animals maintained
in enriched conditions were better problem solvers
than those with no enrichment; however, in some
other occasions, using other tests, enriched rats did
not perform significantly better than controls.
One of the most robust effects of environmen-
tal enrichment on the behavior of rats appears in the
areas of learning and memory. Investigators (York
et al. 1989 and Kempermann et al 1997) study-
ing the effects of enrichment in the rat brain have
reported that new nerve cells develop in the adult
dentate gyrus, an area dealing with recent memory
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An. Acad. Bras. Cienc.,(2001) 73 (2)
218 MARIAN C. DIAMOND
processing. IntheYorkexperimentsthe ratswere 60
to 90days of age(truly adult animals) during the en-
richment experience, whereas in the Kempermann
experiments the mice were 21 to 40 days of age.
These finding are significant because neurogenesis
had not previously been found in the cerebral cortex
of the mammalian adult. Earlier studies had found
that enriched environments stimulate the growth of
dendrites in the dentate gyrus, and only in female
rats. (Juraska et al. 1985).
PhysicalActivity: Onecomponent ofenrichmentis
the physical exercise involved in the animals’ hav-
ing to move about the cage, interacting with and
climbing upon the novel objects. These activities
appear to influence the motor cortex as well as the
hippocampus. Olsson et al. (1994) showed that
rats living in enriched environments at 50 days of
age showed higher expression of the gene-encoding
glucocorticoid receptors and induction of genes for
Nerve Growth Factors in the hippocampus.
Nutrition: Nutritionisclearlyanimportantvariable
to consider in all studies dealing with brain and be-
havior. Environmental enrichment and impoverish-
ment have pronounced effects on nutritionally defi-
cientanimals. Onestudycomparedtheeffectsofen-
vironmental enrichment on the offspring of mother
rats living on protein-rich or protein-deficient di-
ets during pregnancy (Carughi et al. 1990). The
protein-richdietprovedbeneficialforthehealthyde-
velopmentofthecerebralcorticaldendritesinyoung
rats and even more so when combined with an en-
riched environment.
The cerebral cortical dendrites in rat pups from
mothers with a protein-deficient diet were signifi-
cantly less well developed than those of their coun-
terparts, but, of greater importance, the cortex from
theprotein-deficientanimalsdidnotsignificantlyin-
crease with enrichment. On the other hand, when
protein-deficient pups were fed a protein-rich diet
and maintained in an enriched environment during
their early postnatal life, cortical development im-
proved almost to the level seen in rat pups from
mothersona high-proteindietduringpregnancyfol-
lowed by postnatal enrichment. These data are very
encouraging, because they suggest the possibility of
making up for lost brain growth during pregnancy
by enriching both the diet and the environmental
conditions during the postnatal period.
Another dietary factor significant to optimal
brain function is glucose. The brain depends almost
exclusively on glucose for its energy. Synapses use
a great deal of energy and glucose supplies this en-
ergy. Although we know that different parts of the
brain useglucose at different rates, to learn which of
30 discrete brain regions were most active in adult
rats placed in enriched living conditions from 57 to
87 days of age, we studied their radioactive glucose
uptake during this 30-day period and compared it
with that of rats raised in standard conditions (Dia-
mond 1988). Again, the cerebral cortex showed the
greatest differences between enriched and nonen-
riched groups, but, surprisingly, of the two groups,
glucose uptake was lower in rats maintained in en-
riched conditions. We concluded from this finding
that glucose uptake is more efficient in the brain of
animals living in enriched environments. Out of the
30 areasof thebrain measured, including thecortex,
only one area showed significantly greater glucose
uptakeintheenrichedanimals: thecorpuscallosum,
specifically, the large mass of axons connecting the
nerve cells between the two cerebral hemispheres.
Could the axons forming the corpus callosum from
the nerve cells in the cerebral cortex be more active
than the nerve cell bodies from which they arise?
Yet the right and left cerebral cortices show compa-
rable cortical thickness increases with enrichment
due to the effects on dendritic branching, but now
the data show that the rates of glucose utilization in
boththefrontalandparietalcorticeswere13%lower
in the enriched rats than in the standard control rats,
a paradox to be untangled in the future.
Methodological issues associated with enrich-
ment research in humans: Of the vast number of
animal studies that yield results of interest to human
An. Acad. Bras. Cienc.,(2001) 73 (2)
AABC 73 2 b 2
BRAIN AND ENRICHMENT 219
research, studies on the impact of an enriched envi-
ronment on brain development and behavior can be
ofenormousinteresttohumans. Despitesimilarities
insome key respects betweenthe brainof therat and
other mammals, replicating or extrapolating from
anatomical and chemical studies conducted in ani-
mals is fraught with difficulty, for obvious reasons.
Not only is it not presently possible to control all of
the experimental variables at work in humans, but
the diversity and complexity of human experience
militates against designing experiences comparable
to those used with lower animals.
Nevertheless, these studies and what few hu-
man studies have been done, suggest that there are
measurable benefits to enriching an individual’s en-
vironment in whatever terms that individual per-
ceives his immediate environment as enriched. At
theveryleast, thisworkindicatesthattherearemany
opportunities for enhancing brain activity and be-
havioratall ages, and thattheycanhavepronounced
effects throughout the life span.
RESUMO
Antes de 1960, os cientistas consideravam o encé-
falo como imutável, sujeito apenas ao controle genético.
Entretanto, no início dos anos 60, alguns pesquisadores
especulavam seriamente que influências ambientais po-
diam ser capazes de alterar a estrutura cerebral. Por volta
de 1964, dois laboratórios de pesquisa demonstraram que
a morfologia e a química ou a fisiologia do cérebro pode-
ria ser modificada pela experiência (Bennett et al. 1964,
Hubel e Wiesel 1965). Desde então, a capacidade do
cérebro a responderpara responder ainsumos ambientais,
especificamente ao “enriquecimento”, tornou-se um fato
aceito por neurocientistas, educadores e outros. De fato,
a demonstração de que o enriquecimento ambiente pode
modificar componentes estruturais do cérebro de rato, em
qualquer idade, alterou suposições prevalentes a respeito
da plasticidade cerebral (Diamond et al. 1964, Diamond
1988).
O córtex cerebral, a área associada com o processamento
cognitivo superior, é mais receptivo do que outras partes
do encéfalo ao enriquecimento ambiental. A mensagem é
clara: embora o encéfalo possua uma organização macro-
estrutural relativamente constante, o sempre-mutável cór-
tex cerebral, com sua microarquitetura complexa de po-
tencial desconhecido, é fortemente moldado pelas expe-
riências antes do nascimento, durante a juventude e, em
verdade, ao longo de toda a vida. É essencial que se note
que os efeitos do enriquecimento sobre o encéfalo têm
consequências no comportamento. Pais, educadores, ge-
radores de políticas e quaisquer indivíduos podem todos
se beneficiar de tal conhecimento.
Palavras-chave: enriquecimento, córtex cerebral, hipo-
campo, envelhecimento, neurogênese em adultos, den-
dritos.
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