Cognitive neuroscience
Chapter 2. Introducing the brain
The neuron.
All neurons have basically the same structure. They
consist of three components: a cell body (soma),
dendrites and an axon. The cell body contains the
nucleus and other organelles. The nucleus contains the
genetic code, and this is involved in protein synthesis.
From the cell body, a number of dendrites enable
communication with other neurons. Dendrites receive information from other neurons in close proximity. The
axon, by contrast, sends information to other neurons. The terminal of an axon flattens out into the synapse,
where the neurotransmitters enable communication between a presynaptic and postsynaptic neuron.
Neurotransmitters bind to receptors on the dendrites or cell body of the postsynaptic neuron and create a
synaptic potential. The synaptic potential is conducted passively through de dendrites and soma of the
postsynaptic neuron. These passive currents form the basis of EEG.
Each neuron is surrounded by a cell membrane that acts as a barrier to the passage to e.g. charged sodium (Na+)
and potassium (K+) ions. The balance between these ions on the inside and outside of the membrane is such that
there is normally a resting potential of -70 mV across the membrane. Voltage-gated ion channels are of particular
importance in the generation of an action potential and are only found in axons. An action potential in one part
of the axon opens adjacent voltage-sensitive Na+ channels, and so the action potential moves progressively down
the length of the axon. The conduction of the action potential along the axon may be speeded up if the axon is
myelinated. Myelin blocks the normal Na+/K+ transfer and so the action potential jumps, via passive conduction,
down the length of the axon at the points at which the myelin is absent (nodes of Ranvier).
,Protein receptors in the membrane of the
postsynaptic neurons bind to
neurotransmitters. Many of the
receptors are transmitter-gated ion
channels. This sets up a localized flow of
charged Na+, K+ or Cl-, which creates the
synaptic potential. Some
neurotransmitters have an inhibitory
effect on the postsynaptic neuron. This
can be achieved by making the inside of
the neuron more negative than normal
and hence harder to depolarize. Other
neurotransmitters have excitatory effects
on the postsynaptic neuron. Note that it
is not the chemicals that make neurons
excitatory and inhibitory. Rather it is the
effect that they have on ion channels in
the membrane.
Organization of the brain.
Neurons are organized within the brain to form white matter and gray matter. Gray matter consists of neuronal
cell bodies. White matter consists of axons and glia. The brain consists of the cerebral cortex, white matter fibers
and another collection of gray matter structures (the subcortex), which includes the basal ganglia, the limbic
system and the diencephalon. White matter tracts may project different cortical regions within the same
hemisphere (association tracts), or project between different cortical regions in different hemispheres
(commisures) or may project between cortical and subcortical structures (projection tracts). The brain also
contains four ventricles filled with cerebrospinal fluid (CSF). The CSF carries waste metabolites, transfers some
messenger signals and provides a protective cushion for the brain.
The cerebral cortex.
Gyri are raised surfaces of the cortex and sulci are dips or folds, this permits efficient
packaging of the brain. Most of the cortex contains six main cortical layers, termed
the neocortex. Other cortical regions are the mesocortex and the allocortex. The
lateral surface of the cortex of each hemisphere is divided into four lobes: the frontal,
parietal, temporal and occipital lobes. The dividing line between the lobes is
sometimes prominent, but in other cases, the boundary cannot readily be observed.
Finally, the insula lies beneath the temporal lobe.
The subcortex.
The basal ganglia are large rounded masses that lie in each hemisphere. They
surround and overhang the thalamus in the centre of the brain. There are involved
in regulating motor activity, and the programming and termination of action.
Disorders of the basal ganglia are for example Huntington’s disease and Parkinson’s.
the main structures comprising the basal ganglia are: the caudate nucleus, the
putamen and the globus pallidus.
The limbic system is important for relating the
organism to its environment based on current
needs and the present situation, and based on
previous experience. It is involved in the detection
of emotional responses. E.g. the amygdala has
been implicated in the detection of fearful stimuli
and the hippocampus is important for learning and
memory.
,The two main structures that make up the diencephalon are the thalamus and the hypothalamus. The thalamus
is the main sensory relay for all senses (except smell) between the sense organs and the cortex. It also contains
projections to almost all parts of the cortex and
basal ganglia. At the posterior end of the thalamus
lie the lateral geniculate nucleus and the medial
geniculate nucleus. These are the main sensory
relays to the primary visual and primary auditory
cortices. The hypothalamus lies beneath the
thalamus and regulates e.g. body temperature,
regulation of endocrine functions etc.
The midbrain and hindbrain.
The midbrain consists of a number of structures. The superior colliculi and
the inferior colliculi are gray matter nuclei. The superior colliculi integrate
information from several senses, whereas the inferior colliculi are specialized
for auditory processing. They provide a fast route that enables rapid orienting
to sensory stimuli before the stimulus is consciously heard.The cerebellum
consists of highly convoluted folds of gray matter. It is organized into two
interconnected lobes. The cerebellum is important for dexterity and smooth
execution of movement. The pons is a key link between the cerebellum and
the cerebrum. It receives information from visual areas to control eye and
body movements. The medulla oblongata protrudes from the pons and
merges with the spinal cord. It regulates vital functions such as breathing,
swallowing, heart rate and the wake-sleep cycle.
Chapter 6. The developing brain
Gottlieb makes a distinction between two key ideas in development which he termed predetermined
development versus probabilistic development. In predetermined development, genes dictate the structures of
the brain, which enables the particular functions of the brain, which determines the kinds of experiences we
have. In probabilistic development the brain structure and even the expression of genes can be influenced by
experience as well as vice versa.
Prenatal development.
The nervous system derives from the neural tube. By around 5 weeks the neural tube has organized into a set of
bulges and convolutions that will go on to form various parts of the brain. Closer to the hollow of the neural tube
are several proliferative zones in which neurons and glial cells are produced by division of neuroblasts and
glioblasts. The newly formed neurons must then migrate outwards. This occurs passively and by radial glial cells
that act like climbing ropes, ensuring that newly formed neurons are guided to their final destination. Regional
differences in various molecular signals affect the
neurons’ structure, migration and survival. Different
doses of these signals determine the dimensions of the
various lobes of the brain, such that, a dose above a
certain threshold may instruct a new neuron to develop
features characteristic of a frontal lobe neuron but below
that dose it may resemble a parietal neuron. This suggests
a simple mechanism for creating individual differences in
brain structure and for evolutionary development.
, Postnatal development.
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 growth of glial
cells; and the myelination of nerve fibers.
In structural MRI, the increase in white matter volume over the first two decades of life may reflect the time
course of myelination. The prefrontal cortex is one of the last areas to achieve adult levels of myelination, and
this, together with the late fine-tuning and elimination of synapses in this region, may contribute to the
development to mature social behavior during adolescence and the control of behavior in general. Following
birth, all of our everyday experiences result in tiny changes to the structure of our brain, in the form of altering
the pattern of synaptic connections. Plasticity refers to experience-dependent changes in neural functioning.
One can’t take gray matter density/thickness as a simple proxy of cognitive ability as it depends on the underlying
mechanisms: developmental pruning of synapses (thinner is better) or experience-dependent changes (thicker
is better).
Functional development.
Animal studies show that there is a high degree of structural and functional plasticity
in the early brain. However, it doesn’t mean that all neurons are fully interchangeable. Filial imprinting
Spontaneous patterns of activity prior to birth are already shaping neural activity and The process by which a young
parcellating them into different networks. animal comes to recognize the
Thomas and Johnson (2008) provide an overview of the general properties of the parent.
nervous system giving rise to sensitive periods in development. One possibility is that
Critical period
there is a strict maturational timetable in which a set of neurons are readied for A time window in which appropriate
learning (e.g. by synaptogenesis) and are then later “fossilized” (e.g. reducing environmental input is essential for
plasticity, removing weaker connections according to a strict timetable). A second learning to take place.
possibility is that a set of neurons are readied for learning but that the process is self-
terminating to some extend (i.e. the sensitive period will “wait” for suitable Sensitive period
exposure). For example, in filial imprinting there is evidence that a particular gene is A time window in which appropriate
switched on at the start of the sensitive period but is switched off again 10 hours after environmental input is particularly
imprinting has occurred. In human infants born with dense cataracts over both eyes, important for learning to take place.
there is a rapid increase in visual acuity when the cataracts are surgically removed,
even as late as 9 months after birth. This suggests that the development of visual
acuity will, to some extent, “wait” for an appropriate environment.
The discussion of the extent to which any form of knowledge or ability can be said to innate creates division
between empiricists (who believed that the mind is a blank slate) and nativists (who believed that at least some
forms of knowledge are innate). There are different ways to interpret the word ‘innate’. First in the context of
the existence of instinct; the idea that behavior is a product of natural selection. In this sense of the word ‘innate’,
there is a readiness for certain knowledge to be acquired, but the knowledge itself is not strictly innate. A second
way is that knowledge or bahavior can be said to be innate if it comes about in the absence of appropriate
experience.
Nature and nurture of individual differences.
Behavioral genetics is concerned with studying the inheritance of behaviors and cognitive skills. The classic
methods of behavioral genetics are twin studies and adoption studies which provide ways of disentangling nature
and nurture. Heritability is the proportion of variance in a trait, in a given population, that can be accounted for
by genetic differences among individuals. The degree to which an monozygotic correlation is less than 1.0 is
assumed to reflect unshared environment. The remaining portion of variance is attributed to shared
environment. Heritability is a statistical measure that doesn’t say anything directly about particular genes or
their function. In order to do that one needs to link relevant date from cognitive neuroscience with individual
differences in the genetic code itself. The two approaches that one could adopt for analysis of genetic differences
are called genotype-first and phenotype-first. The advantage of the genotype-first approach is that the genetic
analysis is limited to one specific gene and is relatively straightforward to conduct. It also avoids the problem of
multiple comparisons when testing multiple genes. An example of a phenotype-first approach is termed genome-
wide association study (GWAS). In this approach the presence/absence, or continuous variation in a trait, is
linked to variations at many different sites in the genetic code.
