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42 VOLUME 8, NUMBER 2, APRIL 1999
Neural plasticity can best be
thought of as the subtle but orches-
trated dance that occurs between
the brain and the environment;
specifically, it is the ability of the
brain to be shaped by experience
and, in turn, for this newly remold-
ed brain to facilitate the embrace of
new experiences, which leads to
further neural changes, ad infini-
tum. As a rule, there are three
mechanisms by which experience
induces changes in the brain. An
anatomical change might be the
ability of existing synapses (i.e.,
connections between nerve cells) to
modify their activity by sprouting
new axons or by expanding the
dendritic surface. A neurochemical
change might be reflected in the
ability of an existing synapse to
modify its activity by increasing
synthesis and release of chemicals
that transmit nerve impulses (i.e.,
neurotransmitters). Finally, an ex-
ample of a metabolic change might
be the fluctuations in metabolic ac-
tivity (e.g., use of glucose or oxy-
gen) in the brain in response to ex-
perience.
All of these changes can occur at
virtually any point in the life cycle,
although to varying degrees of suc-
cess. For example, some domains
of behavior can be acquired only
during a sensitive or critical period.
Examples include song learning in
the Zebra finch, social imprinting
2
in some mammals, and the devel-
opment of binocular vision (for dis-
cussion, see Knudsen, 1999). Other
behaviors, such as learning and
memory, depend less on experience
occurring at a particular point in
development, and thus occur
throughout the life span (see
Nelson, in press). Regardless of
whether there is or is not a critical
period, experience is responsible
for the changes that occur in the
brain, which in turn determines the
behavioral profile and develop-
ment of the organism. Alas, this
malleability is a two-edged sword,
in that such changes can be both
adaptive and maladaptive for the
organism.
Let us begin with the bad news,
which is that the wrong experi-
ences can have deleterious effects
on the brain. Perhaps the clearest
example concerns the effects of
stress on the developing and devel-
oped brain. Rats exposed to stress
pre- or postnatally show a wide
range of changes in the brain’s
serotonin, catecholamine, and opi-
ate systems.
3
Similarly, rats raised
in social isolation make more learn-
ing errors than socially raised rats.
Finally, brief maternal deprivation
in the rat pup can alter the sensitiv-
ity of the hypothalamic pituitary
adrenal (HPA) axis (see Black,
Jones, Nelson, & Greenough, 1998,
for review), thereby potentially al-
tering the animal’s ability to regu-
late and mount a behavioral re-
sponse to threat.
Similarly, pregnant Rhesus mon-
keys exposed to different stressors
at different points in time give birth
to offspring that show seeming-
ly permanent neurobehavioral
changes. For example, at the cogni-
tive level, the achievement of object
permanence (the concept that ob-
jects out of sight continue to exist)
can be delayed, and performance
on tests of explicit memory
4
can be
impaired. At the behavioral level,
these animals display long-lasting
changes in their ability to control
their emotional state (see
Schneider, in press).
The effects of prenatal exposure
to stress are not limited to the rat or
monkey. For example, Lou et al.
(1994) reported that stress during
pregnancy affected the head cir-
cumference of human newborns.
(Head circumference is a coarse
measure of brain growth.) In addi-
tion, prenatal stress was related to
less than optimal outcome in the
newborn period. As researchers
have hypothesized for the rat and
monkey, Lou et al. speculated that
MALADAPTIVE CHANGES
Published by Blackwell Publishers, Inc.
Neural Plasticity and
Human Development
Charles A. Nelson
1
Institute of Child Development and Department of Pediatrics, University of
Minnesota, Minneapolis, Minnesota
Abstract
In this article, I argue that
experience-induced changes in
the brain may be a useful way
of viewing the course of
human development. Work
from the neurosciences sup-
ports the claim that most of the
behavioral phenomena of in-
terest to psychologists (e.g.,
cognition, perception, lan-
guage, emotion) are instantiat-
ed by the process of neural
plasticity. When development
is viewed in this manner, the
fallaciousness of the long-
standing and often con-
tentious debate over nature
versus nurture becomes ap-
parent. Moreover, by utilizing
the neuroscientific tools used
to examine the effects of expe-
rience on brain and behavioral
development (e.g., functional
neuroimaging), we may im-
prove how we conceptualize
our notions of intervention,
competence, and resilience.
Keywords
brain; plasticity; behavior
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 43
the effects of maternal stress on
fetal brain development might be
mediated by stress hormones (glu-
cocorticoids) circulating in the
bloodstream. A similar mechanism
has been proposed to account for
the observation that adults who
have survived abuse as children
show reduced volume of the hip-
pocampus (a brain structure im-
portant for explicit memory) and,
correspondingly, impairments on
memory tasks. In this case, gluco-
corticoids act toxically on the hip-
pocampus, which is well endowed
with glucocorticoid receptors (see
McEwen & Sapolsky, 1995).
Overall, there is clear evidence
that early or late exposure to stress
can deleteriously affect a range of
brain systems, and thus a range of
behaviors. In this context, one may
view stress as something akin to a
psychological lesion that exerts its
effects to varying degrees depend-
ing on what system is targeted, the
age of the organism when the stress
occurred, and whether there are
protective or exacerbating factors
that can moderate the effects.
Having begun with the bad
news, let us now turn to the good
news, which is that being exposed
to the “right” experiences can have
beneficial effects on brain and be-
havior. Greenough and his col-
leagues have demonstrated that
rats raised in complex environ-
ments (e.g., those filled with lots of
toys and other rats) perform better
than rats reared in normal laborato-
ry cages on a variety of cognitive
tasks (see Greenough & Black,
1992, for review). Correspondingly,
the brains of these rats show im-
proved synaptic contacts, and a
greater number of dendritic spines.
Even more impressive changes are
observed in perceptual-motor
tasks. For example, Black and
Greenough (see Black et al., 1998)
required rats to learn complex
motor coordination tasks and
found the rats developed more
synapses within the cerebellum, a
brain structure important for per-
formance of such tasks. Kleim, Vij,
Ballard, and Greenough (1997)
demonstrated that these changes
can be long lasting.
Reorganization of the brain
based on selective experience is not
limited to the rat. For example,
Mühlnickel, Elbert, Taub, and Flor
(1998) have reported that adults
suffering from tinnitus (ringing in
the ears) show a dramatic reorgani-
zation of the region of the cortex
that deals with hearing. This same
group (Elbert, Pantev, Wienbruch,
Rockstroh, & Taub, 1995) has also
shown that in musicians who play
string instruments, the region of
the somatosensory cortex (the re-
gion of the brain that subserves the
sense of touch) that represents the
fingers of the left hand (the hand
requiring greater fine-motor learn-
ing) is larger than the area that rep-
resents the right hand (which is
used to bow), and larger than the
left-hand area in nonmusicians.
Finally, Ramachandran, Rogers-
Ramachandran, and Stewart (1992)
have observed similar findings in
patients who have experienced
limb amputation. The region of the
somatosensory cortex that sits adja-
cent to the region previously repre-
senting the missing limb en-
croaches on this area. This, in turn,
may account for why patients ex-
perience sensation in the missing
limb (e.g., the forearm) when this
new area (e.g., that representing
the cheek) is stimulated (Rama-
chandran et al., 1992). Collectively,
this work suggests that the adult
human cortex can be reorganized
based on experiences that occur rel-
atively late in life.
What of plasticity during child-
hood? Tallal and Merzenich
(Merzenich et al., 1996; Tallal et al.,
1996) have speculated that in some
children, difficulty in parsing ongo-
ing speech into sound segments
leads to difficulty discriminating
speech sounds. These authors have
reported improvements in both
speech discrimination and lan-
guage comprehension when such
children are given intensive train-
ing in speech processing. Although
the brains of these children were
not examined, presumably changes
at the level of the auditory-thala-
mo-cortical pathway were modi-
fied by this experience.
These are but a few examples of
how the brain is modified by expe-
rience; many others, in various do-
mains of functioning, and at vari-
ous points in development, could
be provided. This is not to say,
however, that experience-induced
changes are possible in all domains
of behavior at all points in time. For
example, there is evidence from
studies of deprivation that children
not exposed to normal caretaking
environments during their first few
years of life may suffer long-lasting
changes in their socioemotional
functioning (although some indi-
viduals show sparing or recovery
of function that is quite remark-
able). Similarly, we have known for
many years that being able to see
with only one eye in the first few
years of life yields intractable
deficits in binocular depth percep-
tion, and that prolonged linguistic
deprivation yields similarly in-
tractable long-term deficits in lan-
guage, speech perception, or both.
Even these cases, however, speak to
the importance of experience, as
without normative experiences
normal development goes awry.
The foregoing observations
suggest that the “innate”-versus-
IMPLICATIONS OF WORK
ON NEURAL PLASTICITY
FOR BEHAVIORAL
DEVELOPMENT
ADAPTIVE CHANGES
Copyright © 1999 American Psychological Society
44 VOLUME 8, NUMBER 2, APRIL 1999
”learned” debate is fallacious. An
example from the literature on
face perception illustrates this
point (some aspects of language
development may be another ex-
ample). Some investigators have
argued that face recognition is in-
nate, by which they (presumably)
mean that it develops without
benefit of experience. However,
we know that infants come into
contact with faces as soon as they
are born. Thus, it seems just as rea-
sonable to argue that experience
drives the development of the
neural tissue (perhaps selected by
evolutionary pressures, given the
importance of face recognition in
survival) that takes on this func-
tion and that this tissue becomes
specialized rather quickly. (The
brain structures responsible for
face recognition in the adult are in
the right temporal cortex and in-
clude the fusiform gyrus.) This
process would allow the ability to
recognize faces to appear early in
development, but this is not the
same thing as saying this ability is
innate qua innate. Conversely, to
argue that face recognition is
“learned” does not do justice to
the fact that such learning by de-
fault necessitates changes in the
brain, which in turn alter which
genes are expressed (i.e., activat-
ed). Even if one argues that an
ability is “genetic” (although
proving such a case would seem
insurmountable without benefit of
being able to specify the genes),
gene expression is influenced by
experience, and once experience
occurs, the brain is altered, which
in turn alters gene expression, and
so on and so forth.
What are the practical implica-
tions of the approach I am advo-
cating? Two come to mind. The
first pertains to intervention. By
understanding precisely how the
brain is modified by experience,
we can better identify the experi-
ences needed to bring children
back on a normal developmental
trajectory, or prevent them from
moving off this trajectory. In addi-
tion, we can target our interven-
tions more judiciously, rather than
targeting the whole child. Finally,
using the tools of the neuroscien-
tist, we may be able to examine
the brain before and after an in-
tervention, and in so doing better
determine where in the nervous
system change occurred. For ex-
ample, if damage to the auditory-
thalamo-cortical pathway appears
to be responsible for some lan-
guage-learning disorders, might
noninvasive procedures such as
event-related potentials (electrical
activity generated by the brain in
response to discretely presented
events) or functional magnetic res-
onance imaging be used to (a)
confirm or disconfirm this hy-
pothesis and (b) evaluate the ef-
fectiveness (or lack thereof) of a
given intervention and its effects
on brain structure and function
(see Nelson & Bloom, 1997)?
A second implication of the per-
spective advocated in this essay
pertains to our understanding of
competence and resilience. As
work by Masten and Garmezy
(Masten et al., in press) has
shown, not all children reared in
suboptimal conditions (including
those at significant risk for psy-
chopathology) move off a normal
trajectory. Similarly, not all chil-
dren who suffer frank brain dam-
age experience disastrous out-
comes. And, in both cases, many
of those who do fall off a normal
trajectory show some recovery—
even considerable recovery in
some instances. Might we view
those children who are otherwise
at risk but do not show any dele-
terious effects as an example of
neural sparing, and those who
show some deficits followed by a
return to a normal trajectory as an
example of recovery of function?
If so, can we identify how the
brains of these children incorpo-
rated experience differently than
the brains of children who show
no sparing or recovery of func-
tion? And can we evaluate these
changes using the latest tools from
the neurosciences?
It is indisputable that some as-
pects of pre- and postnatal human
development have their origin in
the expression of genetic scripts
conserved through evolution and
expressed at key points in develop-
ment. One example may be the
prenatal expression of the genes
that regulate the formation of body
parts (i.e., homeotic genes); anoth-
er may be the postnatal expression
of genes (not yet identified) that
lead to the cascade of hormonal
changes that usher in puberty.
Even in these cases, of course, ex-
perience may influence the out-
come; for example, the presence of
teratogens may corrupt the influ-
ence of the homeotic genes (e.g.,
thalidomide can cause limb defor-
mities), and culture or other expe-
riences can influence pubertal tim-
ing. These examples notwith-
standing, the changes in behavior
that occur over time and that are of
most interest to behavioral scien-
tists (e.g., changes in perception,
cognition, social-emotional behav-
ior) are likely mediated by experi-
entially induced changes in the
nervous system. Thus, it may serve
us well to rethink how the brain
develops and changes across the
life span by considering the impor-
tant role of experience in sculpting
neural systems. In so doing, we
may be able to shed some of the
contentious history that has
plagued our discipline for years
(e.g., nature vs. nurture; innate vs.
learned), and embrace new theo-
retical and empirical approaches to
human development and brain
function.
CONCLUSIONS
Published by Blackwell Publishers, Inc.
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 45
Notes
1. Address correspondence to
Charles A. Nelson, Institute of Child
Development, University of Minn-
esota, 51 East River Rd., Minneapolis,
MN 55455; e-mail: canelson@tc.umn.
edu.
2. Social imprinting is the process
by which an animal forms a relation-
ship of some kind to another animal,
such as the famous case of the duck-
lings that followed Konrad Lorenz
around.
3. Serotonin and catecholamines are
neurotransmitters that play a role in so-
cial and emotional behavior. The natu-
ral opiates produced by the body can
induce feelings of euphoria or well-
being.
4. Explicit memory is a form of
memory that can be stated explicitly
or declared, that can be brought to
mind as an image or proposition in the
absence of ongoing perceptual sup-
port, or of which one is consciously
aware.
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Acknowledgments—The writing of this
essay was made possible by a grant to
the author from the John D. and
Catherine T. MacArthur Foundation and
the J.S. McDonnell Foundation, through
their support of their research network
on Early Experience and Brain
Development. The author wishes to
thank Floyd Bloom, John Bruer, and
Richard Weinberg for their helpful com-
ments on an earlier draft of this article.
Copyright © 1999 American Psychological Society
Recommended Reading
Black, J.E., Jones, T.A., Nelson, C.A.,
& Greenough, W.T. (1998). (See
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Greenough, W.T., & Black, J.E. (1992).
(See References)
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G., & Rowntree, S. (1998). Age,
experience, and the changing
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behavioral Reviews, 22, 143–159.
Nelson, C.A. (in press). The neurobi-
ological bases of early interven-
tion. In J.P. Shonkoff & S.J.
Meisels (Eds.), Handbook of early
childhood intervention (2nd ed.).
New York: Cambridge University
Press.
Nelson, C.A., & Bloom, F.E. (1997).
(See References)
