H16; the developing brain
In predetermined development, genes dictate the structure of the brain, which enables the
particular functions of the brain, which determines the kinds of experiences we have. This is a
traditional view of how genes affect cognition.
In contrast, Gottlieb also outlines a probabilistic developmental perspective in which brain structure,
and even the expression of genes, can be influenced by experience as well as vice versa.
Plasticity refers to experience-dependent changes in neural functioning.
The newly formed neurons must then migrate outwards toward the region where they will be
employed in the mature brain. This occurs in two ways. Passively, older cells tend to be pushed to
the surface of the brain. Structures such as the hippocampus are formed this way. There is also an
active mechanism by which newer cells are guided to particular destinations, pushing past the older
cells. identified radial glial cells that act like climbing ropes, ensuring that newly formed neurons are
guided to their final destination. The convoluted surface of the brain, the neocortex, is formed in this
The vast majority of neurons are formed prior to birth, so the expansion in brain volume during
postnatal development is due to factors such as the growth of synapses, dendrites, and axon
bundles; the proliferation of glial cells; and the myelination of nerve fibres.
synaptic density in various regions of human cortex. This is a measure of the degree to which
neurons are connected to each other and is unrelated to number of neurons per se or how active the
During development a process of fine-tuning the brain to the needs of the environment renders
some connections redundant.
The protomap theory argues that the regional layout of the cortex is established at the prenatal
stages of development. The early proliferating zone is assumed to specify the ultimate layout of
different cortical regions. This may be achieved through the radial glial fibers that carry new neurons
away from the proliferation zone and through regional differences in various molecular signals
(called transcription factors) that affect the neurons’ structure, migration, and survival. Different
doses of these signals determine the dimensions of the various lobes of the brain, such that, for
example, a dose above a certain threshold may instruct a new neuron to develop features
characteristic of a frontal lobe neuron (e.g. in terms of its connectivity) but below that dose it may
resemble a parietal neuron. This suggests a simple mechanism for creating individual differences in
brain structure and also for evolutionary development (e.g. a shifting dose could enable an
evolutionary jump in frontal lobe enlargement).
The protocortex theory argues that different regions of the cortex are initially equivalent but
become specialized as a result of projections from the thalamus (O’Leary, 1989). This is assumed to
be influenced by postnatal sensory experience. The protocortex theory predicts that regions of
cortex can initially be interchanged and that the transplanted visual cortex would now respond
to touch or sound because it would be innervated by somatosensory or auditory projections
from the thalamus.
A critical period has two defining features: first, learning can only take place within a limited time
window; and, second, the learning is hard to reverse in the face of later experience.
Against sensitive period, child had been locked away for long time. Could not speak. When placed in
foster her grammar and vocabulary improved.
both age-of-acquisition and level of proficiency determine the neural substrates of second
language processing in adults.
For syntactic judgments, the age-of-acquisition was critical This suggests a sensitive period for
grammar in terms of neural efficiency. For semantic judgments the pattern of activity was related
to proficiency level in the second language.
What general properties of the nervous system give rise to sensitive periods in development? one
possibility is that there is a strict maturational timetable in which a set of neurons are readied for
learning (e.g. by synaptogenesis) and are then later “fossilized” (e.g. reducing plasticity, removing
weaker connections) according to a strict timetable. A second possibility is that a set of neurons are
readied for learning but that the process is self-terminating to some extent, i.e. the sensitive period
will “wait” for suitable exposure. In human infants born with dense cataracts over both eyes, there
is a rapid increase in visual acuity when the cataracts are surgically removed, even as late as nine
months after birth. This suggests that the development of visual acuity will, to some extent, “wait”
for an appropriate environment. However, this is only partly true, as 9-year-old children who had
cataracts removed in the first 6 months of life had some difficulties in visual processing of faces.
For some, innate is synonymous with the idea that behaviour is a product of natural selection. For
others the word there is still a role for experience to play, perhaps within a sensitive period of
mechanisms of gene–environment interplay. Their review highlights four types of mechanism:
1.Environmental influences can alter the effects of genes. Although the sequenceof DNA is normally
fixed in a given individual and across all cells in his orher body, the timing and the degree of
functioning of genes in the DNA canbe affected by the environment (so-called epigenetic
events). For example, increased maternal nurturing by a rat affects expression of a stress-reducing
gene in its offspring that persists throughout their lifetime.
2.Heritability varies according to environmental circumstances. As noted previously, the amount
of variation in a population that is due to genetic factors is dependent on the environmental
context. In an “equal opportunities” environment heritability tends to be maximized, but in
populations with a large environmental risk (e.g. to certain pathogens) or high social control (e.g.
onacceptable behavior) heritability will be minimized.
3.Gene–environment correlations (rGE) are genetic influences on people’s exposure to different
environments. For example, people will seek out different environments (e.g. drug taking and
novelty seeking) depending on their genotype. Also, the environment that a parent creates for raising
his or her children will dependon the parent’s own dispositions (intellect, personality, mental
illnesses),which are partly genetic in origin.
4.Gene X–environment interactions (G x E) occur when susceptibility to a trait depends on a
particular combination of a gene and environment. The effects of the gene and environment
together exceed what would be expected from the sum of the parts.