http://www.sciencedirect.com/science/article/pii/S0885201417301132
In
the past two decades, a wealth of evidence has accumulated that
supports a neural constructivist approach to characterizing human
development. This approach, in which learning and individual experiences
play a central role in constructing mental representations and their
corresponding neural changes, is of course one of the central themes of
Piaget's theory. This special issue, based on the annual meeting of the
Jean Piaget Society held in Toronto in June, 2015, brings together
reports from researchers who examine neural plasticity in a variety of
ways, across varied domains of development. It provides an overview of
the state-of-the-science in examining how experiences and biology
interact to shape brain development, and we hope it stimulates
discussion of the implications of this neuroscience-based research for
the broader understanding of child development.
The importance of neural plasticity has long been recognized (e.g., Hebb, 1949),
and the role of expectable environmental input (and its absence) on
brain development is well known (e.g., from sensory deprivations
studies; Hubel & Wiesel, 1962). An early demonstration of the way in
which behavioral adaptations to more idiosyncratic (i.e.,
experience-dependent) aspects of the environment co-occur with neural
adaptations appeared in the work of Greenough and colleagues (e.g., Greenough, Black, & Wallace, 1987),
who examined rats raised in “enriched,” or relatively complex
environments that included other rats and the opportunity to explore and
play (see also Rosenzweig et al., 1962).
Compared to rats raised in captivity as usual, these rats showed better
learning and memory (e.g., on maze learning) as well as effects on
brain development, including heavier and thicker cortices, more
dendrites per neuron, and more spines per dendrite.
The
implications of these and other findings subsequently became more
widely appreciated in light of several well-publicized studies with
adults. For example, Maguire et al. (2000)
examined brain regions related to spatial memory in a sample of taxi
drivers in London, England. Taxi drivers, who have to pass a rigorous
test demonstrating knowledge of London streets, were found to have
larger posterior hippocampi (and smaller anterior hippocampi) than
age-matched controls. In addition, the number of years they had been
driving a cab was positively related to the volume of their posterior
hippocampi and negatively related to the volume of their anterior
hippocampi. Although correlational, this finding suggests that engaging
regularly in navigation (and relying heavily on spatial memory) leads to
the reshaping of relevant regions of the brain.
Similar findings have been obtained for white matter and also for measures of brain function. Elbert et al., (1995),
for example, used magnetoencephalography (MEG) to measure cortical
representations of fingers in violin players and found larger
representations in sensorimotor cortex of the digits of the left
(fingering) hand (but not the thumb), as would be expected if experience
produced these changes. In addition, the number of hours spent
practicing the piano (especially as a child) has been found to be
related to myelination, with different neural regions being implicated
at different ages (Bengtsson, Nagy, Skare, Forsman, Forssberg, & Ullén, 2005).
Findings like these, which are increasingly supported by experimental
research involving human beings, suggest that we grow our brains by
using them, and that we grow our brains in particular ways by using them
in particular ways (Zelazo, 2013).
Our
understanding of cognitive development has grown enormously with the
advent of new tools and techniques for examining processes operating at
many levels of analysis (e.g., for studying complex interactions among
genes and environment, for measuring neural activity in young children,
and for modeling developmental change using sophisticated computational
techniques). This research has made it clear that there is plasticity at
all levels of analysis, and in particular it has sharpened out
understanding of the influence of specific early experiences on
subsequent cognitive and brain development. The current special issue of
Cognitive Development is designed to provide cutting edge theoretical and empirical contributions on developmental neuroplasticity.
Lisa Oakes
provides for this special issue a theoretical piece examining
neuroplasticity in development. Rather than using a canonical framework
to explore this topic, where plasticity is discussed in terms of the
ways in which experience is changed by differences in brain structures,
processes or input, this paper discusses the idea that developmental
plasticity, itself, also effectively changes input into the system. In
this way, plasticity is not only seen in the structures and processes
that result from differences in experience, but also is seen with
respect to changes in the input as those structures and processes adapt.
The paper includes examples from research findings across two domains
(face processing and the effect of pet experience on infants’ processing
of animal images) that illustrate how, even when processing the same
information, children who have adapted to different experiences will
attend to and learn from that information differently, and thus the
input to the system will be altered.
In the next article, Bryan Kolb
presents a masterful review of the effects of various kinds of stress
(including preconception, gestational, bystander gestational, and
maternal separation) on prefrontal cortex and behavioral development,
largely in animal models. Kolb argues that neocortical development
represents more than a simple unfolding of a genetic blueprint but
rather represents a complex dance of genetic and environmental events
that interact to adapt the brain to fit a particular environmental
context. As the brain develops it progresses through a series of stages
beginning prenatally and continuing through gestation, infancy and
childhood, adolescence, and well into the third decade. The developing
normal brain shows a remarkable capacity for plastic changes in response
to a wide range of pre-conceptual, prenatal, and postnatal experiences.
This review examines the many ways in which early experiences alter
brain development, including environmental events such as sensory
stimuli, early stress, psychoactive drugs, parent-child relationships,
peer relationships, intestinal flora, and diet. This sensitivity of the
brain to early experiences has important implications for understanding
neurodevelopmental disorders as well as the effect of behavioral and
medical interventions in children and adolescents.
In the article that follows, Daphne Maurer
describes lessons from her research on children treated for cataracts.
She and her colleagues have taken advantage of a natural experiment:
children born with cataracts that blocked all patterned input until the
cataracts were removed and the child fitted with compensatory contact
lenses. Their longitudinal studies indicate that early visual input sets
up the neural architecture for later refinement. When it is absent,
there are sleeper effects: damage to visual capabilities that develop
long after birth. Later deprivation has no such effects, a result
indicating that there are critical periods during which visual
development is damaged. Yet even in adulthood some recovery is possible.
The issue of timing of experiences is also addressed by Janet Werker
and colleagues, who present a series of experiments that tested
infants’ detection of content congruence (the matching of speech sounds
to their corresponding mouth movements) in non-native audiovisual speech
and whether sensitivity to congruence declines predictably with the
trajectory of perceptual attunement previously established in auditory
perception studies. They further explore whether the addition of
congruent visual information alters subsequent auditory discrimination
of these same speech contrasts, possibly constituting a shift in the
timing of the sensitive period for auditory speech perception. They
found that only infants younger than 11 months old detected content
incongruence even in an unfamiliar language and that familiarization to
audiovisual speech changed six-month-olds’ later auditory
discrimination, but did not affect the discrimination of nine- and
11-month-olds, indicating that prior to the closing of the sensitive
period for non-native speech discrimination, auditory discrimination of
speech sounds may be changed with the addition of visual information.
Next, Stephen Lomber
provides a review article examining the overarching question: What is
the function of auditory cortex when it is deprived of normal acoustic
input? Lomber reviews the literature suggesting that auditory cortex of
the deaf may be recruited in the service of vision and that the visual
abilities of the deaf may in fact be enhanced. Further, using the
elegant paradigm of reversible cooling deactivation of auditory cortex
in the congenitally deaf cat, he considers whether these functions are
distributed uniformly across deaf auditory cortex, or if specific
functions are differentially localized to distinct portions of the
affected cortices, and concludes that the neural bases for enhanced
visual functions in the deaf can be localized to distinct regions of
deaf auditory cortex. Finally, he demonstrates that crossmodal
compensatory effects are specific and appear to enhance those functions
that the deprived and replacement modalities hold in common.
In the next article, Marla Sokolowski
and colleagues again take a cross-species approach and examine
gene-environment interplay in behavior primarily in the fruit fly,
Drosophila melanogaster. Her main focus is the foraging gene, a cGMP
dependent protein kinase which influences naturally occurring behavioral
variation including the rover/sitter polymorphism. This gene plays a
role in food-related behaviours, learning and memory and social
behaviors in many organisms including social insects and mammals.
Sokolowski and colleagues have used foraging and its interactors as a
model to study pleiotropy, how a gene accomplishes its multiple and
varied functions. They address questions about the ways in which DNA
sequence variation interacts with epigenetic modifications to affect
behaviour, and how early behavior gets under the skin to affect later
life fitness and behaviour.
The next article, by Charles Nelson
and colleagues, explores neuroplasticity through the lens of one of the
most fundamental developmental processes, that of face perception. In
this paper, the authors explore the developmental consequences of having
limited racial experience versus exposure to a diverse racial
environment with regard to infants’ looking preferences to different
race faces. A leading account has been that face processing undergoes a
process of perceptual narrowing such that the perceptual window through
which faces are perceived is “broadly tuned” early in life and narrows
with experience. This account would predict that if infants are exposed
to multiple face categories during infancy, they would develop a more
broadly-tuned perceptual system and be able to recognize a broader array
of faces. In contrast, their data shows that racial exposure to
caregivers has little influence on face preference among infants and
that face preferences tend to disappear over time, contributing to our
understanding of the settings in which environmental input impacts
cognitive specialization.
In the final review article, Eric Nelson
provides an overview of neuronal maturation from the perspective of
neuroplasticity and adaptation and further provides examples of these
developmental learning principles that stem from the neurobiology of
social development. He discusses several examples of acquisitions that
might constitute times of heightened plasticity, both ones occurring
early in postnatal life such as the filial imprinting observed in avian
species as well as mammals, face processing, and language acquisition,
as well as ones occurring later in social development such as peer
integration and romantic behavior. Overall, the review emphasizes the
numerous ways in which brain development is influenced by
environmentally specific social experiences.
While this
collection represents only a sample of the contemporary research on the
topic, the selections were carefully chosen to provide a meaningful
survey of the range of questions being addressed.
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