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Copyright © 2003 American Psychological Society
1
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
Abstract
Although the brain was once
seen as a rather static organ, it is
now clear that the organization
of brain circuitry is constantly
changing as a function of expe-
rience. These changes are re-
ferred to as brain plasticity,
and they are associated with
functional changes that include
phenomena such as memory,
addiction, and recovery of
function. Recent research has
shown that brain plasticity and
behavior can be influenced by
a myriad of factors, including
both pre- and postnatal experi-
ence, drugs, hormones, matu-
ration, aging, diet, disease, and
stress. Understanding how
these factors influence brain
organization and function is
important not only for under-
standing both normal and ab-
normal behavior, but also for
designing treatments for be-
havioral and psychological dis-
orders ranging from addiction
to stroke.
Keywords
addiction; recovery; experi-
ence; brain plasticity
One of the most intriguing ques-
tions in behavioral neuroscience
concerns the manner in which the
nervous system can modify its or-
ganization and ultimately its func-
tion throughout an individual’s
lifetime, a property that is often re-
ferred to as
plasticity.
The capacity
to change is a fundamental charac-
teristic of nervous systems and can
be seen in even the simplest of or-
ganisms, such as the tiny worm
C.
elegans
, whose nervous system has
only 302 cells. When the nervous
system changes, there is often a
correlated change in behavior or
psychological function. This behav-
ioral change is known by names
such as learning, memory, addiction,
maturation, and recovery. Thus, for
example, when people learn new
motor skills, such as in playing a
musical instrument, there are plas-
tic changes in the structure of cells
in the nervous system that underlie
the motor skills. If the plastic
changes are somehow prevented
from occurring, the motor learning
does not occur. Although psychol-
ogists have assumed that the ner-
vous system is especially sensitive
to experience during develop-
ment, it is only recently that they
have begun to appreciate the po-
tential for plastic changes in the
adult brain. Understanding brain
plasticity is obviously of consider-
able interest both because it pro-
vides a window to understanding
the development of the brain and
behavior and because it allows in-
sight into the causes of normal and
abnormal behavior.
THE NATURE OF BRAIN
PLASTICITY
The underlying assumption of
studies of brain and behavioral plas-
ticity is that if behavior changes,
there must be some change in orga-
nization or properties of the neural
circuitry that produces the behav-
ior. Conversely, if neural networks
are changed by experience, there
must be some corresponding change
in the functions mediated by those
networks. For the investigator inter-
ested in understanding the factors
that can change brain circuits, and
ultimately behavior, a major chal-
lenge is to find and to quantify the
changes. In principle, plastic changes
in neuronal circuits are likely to re-
flect either modifications of exist-
ing circuits or the generation of
new circuits. But how can research-
ers measure changes in neural cir-
cuitry? Because neural networks
are composed of individual neu-
rons, each of which connects with a
subset of other neurons to form in-
terconnected networks, the logical
place to look for plastic changes is
at the junctions between neurons,
that is, at synapses. However, it is a
daunting task to determine if syn-
apses have been added or lost in a
particular region, given that the
human brain has something like
100 billion neurons and each neuron
makes on average several thousand
synapses. It is clearly impractical to
scan the brain looking for altered
synapses, so a small subset must be
identified and examined in detail.
But which synapses should be
studied? Given that neuroscientists
have a pretty good idea of what re-
gions of the brain are involved in
particular behaviors, they can nar-
row their search to the likely areas,
but are still left with an extraordi-
narily complex system to examine.
There is, however, a procedure that
makes the job easier.
In the late 1800s, Camillo Golgi
invented a technique for staining a
random subset of neurons (1–5%)
so that the cell bodies and the den-
dritic trees of individual cells can
be visualized (Fig. 1). The den-
drites of a cell function as the scaf-
folding for synapses, much as tree
branches provide a location for
leaves to grow and be exposed to
sunlight. The usefulness of Golgi’s
technique can be understood by
pursuing this arboreal metaphor.
There are a number of ways one
Brain Plasticity and Behavior
Bryan Kolb,
1
Robbin Gibb, and Terry E. Robinson
Canadian Centre for Behavioural Neuroscience, University of Lethbridge, Lethbridge,
Alberta, Canada (B.K., RG.), and Department of Psychology, University of Michigan,
Ann Arbor, Michigan (T.E.R.)
2 VOLUME 12, NUMBER 1, FEBRUARY 2003
Published by Blackwell Publishing Inc.
could estimate how many leaves are
on a tree without counting every leaf.
Thus, one could measure the total
length of the tree’s branches as well
as the density of the leaves on a
representative branch. Then, by sim-
ply multiplying branch length by
leaf density, one could estimate to-
tal leafage. A similar procedure is
used to estimate synapse number.
About 95% of a cell’s synapses are
on its dendrites (the neuron’s
branches). Furthermore, there is a
roughly linear relationship be-
tween the space available for syn-
apses (dendritic surface) and the
number of synapses, so researchers
can presume that increases or de-
creases in dendritic surface reflect
changes in synaptic organization.
FACTORS AFFECTING BRAIN
PLASTICITY
By using Golgi-staining proce-
dures, various investigators have
shown that housing animals in
complex versus simple environ-
ments produces widespread differ-
ences in the number of synapses in
specific brain regions. In general,
such experiments show that partic-
ular experiences embellish cir-
cuitry, whereas the absence of
those experiences fails to do so
(e.g., Greenough & Chang, 1989).
Until recently, the impact of these
neuropsychological experiments
was surprisingly limited, in part
because the environmental treat-
ments were perceived as extreme
and thus not characteristic of
events experienced by the normal
brain. It has become clear, how-
ever, not only that synaptic organi-
zation is changed by experience,
but also that the scope of factors
that can do this is much more ex-
tensive than anyone had antici-
pated. Factors that are now known
to affect neuronal structure and be-
havior include the following:
experience (both leading pre-
and post-natal)
psychoactive drugs (e.g., amphet-
amine, morphine)
gonadal hormones (e.g., estrogen,
testosterone)
anti-inflammatory agents (e.g.,
COX-2 inhibitors)
growth factors (e.g., nerve growth
factor)
dietary factors (e.g., vitamin and
mineral supplements)
genetic factors (e.g., strain differ-
ences, genetically modified mice)
disease (e.g., Parkinson’s disease,
schizophrenia, epilepsy, stroke)
stress
brain injury and leading disease
We discuss two examples to illus-
trate.
Early Experience
It is generally assumed that ex-
periences early in life have differ-
ent effects on behavior than similar
experiences later in life. The reason
for this difference is not under-
stood, however. To investigate this
question, we placed animals in
complex environments either as ju-
veniles, in adulthood, or in senes-
cence (Kolb, Gibb, & Gorny, 2003).
It was our expectation that there
would be quantitative differences in
the effects of experience on synaptic
organization, but to our surprise, we
also found
qualitative
differences.
Thus, like many investigators be-
fore us, we found that the length of
dendrites and the density of syn-
•
•
•
•
•
•
•
•
•
•
Fig. 1. Photograph of a neuron. In the view on the left, the dendritic field with the
extensive dendritic network is visible. On the right are higher-power views of den-
dritic branches showing the spines, where most synapses are located. If there is an
increase in dendritic length, spine density, or both, there are presumed to be more
synapses in the neuron.
Copyright © 2003 American Psychological Society
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 3
apses were increased in neurons in
the motor and sensory cortical re-
gions in adult and aged animals
housed in a complex environment
(relative to a standard lab cage). In
contrast, animals placed in the
same environment as juveniles
showed an increase in dendritic
length but a decrease in spine den-
sity. In other words, the same envi-
ronmental manipulation had quali-
tatively different effects on the
organization of neuronal circuitry
in juveniles than in adults.
To pursue this finding, we later
gave infant animals 45 min of daily
tactile stimulation with a little
paintbrush (15 min three times per
day) for the first 3 weeks of life.
Our behavioral studies showed
that this seemingly benign early
experience enhanced motor and
cognitive skills in adulthood. The
anatomical studies showed, in ad-
dition, that in these animals there
was a decrease in spine density but
no change in dendritic length in
cortical neurons—yet another pat-
tern of experience-dependent neu-
ronal change. (Parallel studies have
shown other changes, too, including
neurochemical changes, but these
are beyond the current discussion.)
Armed with these findings, we then
asked whether prenatal experience
might also change the structure of
the brain months later in adulthood.
Indeed, it does. For example, the off-
spring of a rat housed in a complex
environment during the term of her
pregnancy have increased synaptic
space on neurons in the cerebral cor-
tex in adulthood. Although we do
not know how prenatal experiences
alter the brain, it seems likely that
some chemical response by the
mother, be it hormonal or otherwise,
can cross the placental barrier and al-
ter the genetic signals in the develop-
ing brain.
Our studies showing that expe-
rience can uniquely affect the de-
veloping brain led us to wonder if
the injured infant brain might be
repaired by environmental treat-
ments. We were not surprised to
find that postinjury experience,
such as tactile stroking, could mod-
ify both brain plasticity and behav-
ior because we had come to believe
that such experiences were power-
ful modulators of brain develop-
ment (Kolb, Gibb, & Gorny, 2000).
What was surprising, however,
was that prenatal experience, such
as housing the pregnant mother in
a complex environment, could af-
fect how the brain responded to an
injury that it would not receive un-
til after birth. In other words, pre-
natal experience altered the brain’s
response to injury later in life. This
type of study has profound impli-
cations for preemptive treatments
of children at risk for a variety of
neurological disorders.
Psychoactive Drugs
Many people who take stimu-
lant drugs like nicotine, amphet-
amine, or cocaine do so for their
potent psychoactive effects. The
long-term behavioral consequences
of abusing such psychoactive
drugs are now well documented,
but much less is known about how
repeated exposure to these drugs
alters the nervous system. One ex-
perimental demonstration of a very
persistent form of drug experience-
dependent plasticity is known as
behavioral sensitization. For exam-
ple, if a rat is given a small dose of
amphetamine, it initially will show
a small increase in motor activity
(e.g., locomotion, rearing). When
the rat is given the same dose on
subsequent occasions, however,
the increase in motor activity in-
creases, or sensitizes, and the ani-
mal may remain sensitized for
weeks, months, or even years, even
if drug treatment is discontinued.
Changes in behavior that occur as
a consequence of past experience,
and can persist for months or years,
like memories, are thought to be due
to changes in patterns of synaptic or-
ganization. The parallels between
drug-induced sensitization and
memory led us to ask whether the
neurons of animals sensitized to
drugs of abuse exhibit long-lasting
changes similar to those associated
with memory (e.g., Robinson &
Kolb, 1999). A comparison of the ef-
fects of amphetamine and saline
treatments on the structure of neu-
rons showed that neurons in am-
phetamine-treated brains had greater
dendritic material, as well as more
densely organized spines. These
plastic changes were not found
throughout the brain, however, but
rather were localized to regions such
as the prefrontal cortex and nu-
cleus accumbens, both of which are
thought to play a role in the reward-
ing properties of these drugs. Later
studies have shown that these drug-
induced changes are found not only
when animals are given injections by
an experimenter, but also when ani-
mals are trained to self-administer
drugs, leading us to speculate that
similar changes in synaptic organi-
zation will be found in human drug
addicts.
Other Factors
All of the factors we listed earlier
have effects that are conceptually
similar to the two examples that
we just discussed. For instance,
brain injury disrupts the synaptic
organization of the brain, and
when there is functional improve-
ment after the injury, there is a cor-
related reorganization of neural
circuits (e.g., Kolb, 1995). But not
all factors act the same way across
the brain. For instance, estrogen
stimulates synapse formation in
some structures but reduces syn-
apse number in other structures
(e.g., Kolb, Forgie, Gibb, Gorny, &
Rowntree, 1998), a pattern of
change that can also be seen with
some psychoactive drugs, such as
morphine. In sum, it now appears
that virtually any manipulation
4 VOLUME 12, NUMBER 1, FEBRUARY 2003
Published by Blackwell Publishing Inc.
that produces an enduring change
in behavior leaves an anatomical
footprint in the brain.
CONCLUSIONS AND ISSUES
There are several conclusions to
draw from our studies. First, expe-
rience alters the brain, and it does
so in an age-related manner. Second,
both pre- and postnatal experience
have such effects, and these effects
are long-lasting and can influence
not only brain structure but also
adult behavior. Third, seemingly
similar experiences can alter neu-
ronal circuits in different ways, al-
though each of the alterations is
manifest in behavioral change.
Fourth, a variety of behavioral con-
ditions, ranging from addiction to
neurological and psychiatric disor-
ders, are correlated with localized
changes in neural circuits. Finally,
therapies that are intended to alter
behavior, such as treatment for ad-
diction, stroke, or schizophrenia,
are likely to be most effective if
they are able to further reorganize
relevant brain circuitry. Further-
more, studies of neuronal structure
provide a simple method of screen-
ing for treatments that are likely to
be effective in treating disorders
such as dementia. Indeed, our
studies show that the new genera-
tion of antiarthritic drugs (known
as COX-2 inhibitors), which act to re-
duce inflammation, can reverse age-
related synaptic loss and thus ought
to be considered as useful treatments
for age-related cognitive loss.
Although much is now known
about brain plasticity and behav-
ior, many theoretical issues re-
main. Knowing that a wide vari-
ety of experiences and agents can
alter synaptic organization and
behavior is important, but leads to
a new question: How does this
happen? This is not an easy ques-
tion to answer, and it is certain
that there is more than one an-
swer. We provide a single exam-
ple to illustrate.
Neurotrophic factors are a class
of chemicals that are known to af-
fect synaptic organization. An ex-
ample is fibroblast growth factor-2
(FGF-2). The production of FGF-2 is
increased by various experiences,
such as complex housing and tactile
stroking, as well as by drugs such as
amphetamine. Thus, it is possible
that experience stimulates the pro-
duction of FGF-2 and this, in turn,
increases synapse production. But
again, the question is how. One
hypothesis is that FGF-2 somehow
alters the way different genes are
expressed by specific neurons and
this, in turn, affects the way synapses
are generated or lost. In other words,
factors that alter behavior, including
experience, can do so by altering gene
expression, a result that renders the
traditional gene-versus-environment
discussions meaningless.
Other issues revolve around the
limits and permanence of plastic
changes. After all, people encounter
and learn new information daily. Is
there some limit to how much cells
can change? It seems unlikely that
cells could continue to enlarge and
add synapses indefinitely, but what
controls this? We saw in our studies
of experience-dependent changes in
infants, juveniles, and adults that ex-
perience both adds and prunes syn-
apses, but what are the rules govern-
ing when one or the other might
occur? This question leads to another,
which is whether plastic changes in
response to different experiences
might interact. For example, does ex-
posure to a drug like nicotine affect
how the brain changes in learning a
motor skill like playing the piano?
Consider, too, the issue of the perma-
nence of plastic changes. If a person
stops smoking, how long do the nico-
tine-induced plastic changes persist,
and do they affect later changes?
One additional issue surrounds
the role of plastic changes in disor-
dered behavior. Thus, although
most studies of plasticity imply
that remodeling neural circuitry is
a good thing, it is reasonable to
wonder if plastic changes might
also be the basis of pathological be-
havior. Less is known about this
possibility, but it does seem likely.
For example, drug addicts often
show cognitive deficits, and it
seems reasonable to propose that at
least some of these deficits could
arise from abnormal circuitry, es-
pecially in the frontal lobe.
In sum, the structure of the brain
is constantly changing in response
to an unexpectedly wide range of
experiential factors. Understanding
how the brain changes and the rules
governing these changes is impor-
tant not only for understanding
both normal and abnormal behav-
ior, but also for designing treat-
ments for behavioral and psycho-
logical disorders ranging from
addiction to stroke.
Recommended Reading
Kolb, B., & Whishaw, I.Q. (1998). Brain
plasticity and behavior.
Annual Re-
view of Psychology, 49
, 43–64.
Robinson, T.E., & Berridge, K.C. (in
press). Addiction.
Annual Review
of Psychology.
Shaw, C.A., & McEachern, J.C.
(2001).
Toward a theory of neuro-
plasticity.
New York: Taylor and
Francis.
Acknowledgments—
This research was
supported by a Natural Sciences and
Engineering Research Council grant to
B.K. and a National Institute on Drug
Abuse grant to T.E.R.
Note
1. Address correspondence to Bryan
Kolb, CCBN, University of Lethbridge,
Lethbridge, AB, Canada T1K 3M4.
References
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Cere-
Copyright © 2003 American Psychological Society
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The Malicious Serpent: Snakes as a
Prototypical Stimulus for an Evolved
Module of Fear
Arne Öhman
1
and Susan Mineka
Department of Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden (A.Ö.),
and Department of Psychology, Northwestern University, Evanston, Illinois (S.M.)
Abstract
As reptiles, snakes may
have signified deadly threats
in the environment of early
mammals. We review findings
suggesting that snakes remain
special stimuli for humans. In-
tense snake fear is prevalent in
both humans and other pri-
mates. Humans and monkeys
learn snake fear more easily
than fear of most other stimuli
through direct or vicarious
conditioning. Neither the elici-
tation nor the conditioning of
snake fear in humans requires
that snakes be consciously per-
ceived; rather, both processes
can occur with masked stimuli.
Humans tend to perceive illu-
sory correlations between
snakes and aversive stimuli,
and their attention is automati-
cally captured by snakes in
complex visual displays. To-
gether, these and other findings
delineate an evolved fear mod-
ule in the brain. This module is
selectively and automatically
activated by once-threatening
stimuli, is relatively encapsu-
lated from cognition, and de-
rives from specialized neural
circuitry.
Keywords
evolution; snake fear; fear
module
Snakes are commonly regarded
as slimy, slithering creatures worthy
of fear and disgust. If one were to be-
lieve the Book of Genesis, humans’
dislike for snakes resulted from a
divine intervention: To avenge the
snake’s luring of Eve to taste the fruit
of knowledge, God instituted eternal
enmity between their descendants.
Alternatively, the human dislike of
snakes and the common appear-
ances of reptiles as the embodiment
of evil in myths and art might reflect
an evolutionary heritage. Indeed,
Sagan (1977) speculated that human
fear of snakes and other reptiles
may be a distant effect of the condi-
tions under which early mammals
evolved. In the world they inhabited,
the animal kingdom was dominated
by awesome reptiles, the dinosaurs,
and so a prerequisite for early mam-
mals to deliver genes to future gen-
erations was to avoid getting caught
in the fangs of Tyrannosaurus rex
and its relatives. Thus, fear and re-
spect for reptiles is a likely core
mammalian heritage. From this
perspective, snakes and other rep-
tiles may continue to have a special
psychological significance even for
humans, and considerable evi-
dence suggests this is indeed true.
Furthermore, the pattern of find-
ings appears consistent with the
evolutionary premise.
THE PREVALENCE OF SNAKE
FEARS IN PRIMATES
Snakes are obviously fearsome
creatures to many humans. Agras,
Sylvester, and Oliveau (1969) inter-
viewed a sample of New England-
ers about fears, and found snakes
to be clearly the most prevalent ob-
ject of intense fear, reported by
38% of females and 12% of males.
Fear of snakes is also common
among other primates. According
to an exhaustive review of field
data (King, 1997), 11 genera of pri-
mates showed fear-related responses
(alarm calls, avoidance, mobbing) in
virtually all instances in which they
were observed confronting large
snakes. For studies of captive pri-
mates, King did not find consistent
evidence of snake fear. However,
in direct comparisons, rhesus (and
squirrel) monkeys reared in the
wild were far more likely than lab-
reared monkeys to show strong
phobiclike fear responses to snakes
(e.g., Mineka, Keir, & Price, 1980).
That this fear is adaptive in the
wild is further supported by inde-
pendent field reports of large
snakes attacking primates (M.
Cook & Mineka, 1991).
This high prevalence of snake
fear in humans as well as in our
