Neuroscience is the study of how the structure and dynamics of the brain
control all aspects of human bodily function from basics such as
respiration, heartbeat, and digestion to the higher cognitive functions of
sensory processing, control of locomotion, reasoning, memory, language,
and the sense of self-awareness called consciousness.
This module will have a good deal to say about these higher cognitive
functions, and about language in particular; because the brain implements
these cognitive functions, the fundamentals of brain structure and
dynamics should be understood.
This lecture outlines these fundamentals.
1. History of neuroscience
Though it seems self-evident to us, the realization that the brain is the physical
organ that underlies cognition is fairly recent.
The ancient Egyptians thought that the heart was the seat of human intelligence
and routinely removed the brain in the course of mummification, presumably on
the grounds that the owner wouldn't need it in the next life.
By the 6th century BC some Greek philosophers had come to the view that the
brain was the place where the mind was located, though no less a thinker than
Aristotle still took the Egyptian view and regarded the brain as a cooling
mechanism for the blood.
In Roman times the philosopher Galen (c.129-200 AD) based his view that the
brain controls bodily function on dissection of animal brains, though like his
predecessors he continued to believe that the brain was the 'seat' of a nonphysical substance or entity which endowed living creatures with their cognitive
functions.
1.
History of neuroscience
During the Middle Ages (c.500 - c.1500 AD) the emphasis was on the nonphysical basis of cognition, the soul, and little or no progress was made in study
of the brain.
With the renewed interest in empirical science during the Renaissance,
however, interest in the brain revived. The anatomist Andreas Vesalius (1514-64
AD) emphasized the importance of dissection as a means of understanding the
human body as a physical mechanism; he published a detailed account of the
brain's anatomy, and correctly proposed the nerves as a means by which the
brain controlled bodily functions.
History of neuroscience
The polymath philosopher, mathematician, and scientist René Descartes (15961650 AD) developed a view of the relationship between the mind and the body
that has been highly influential ever since and continues to vex philosophers of
mind as the 'mind-body problem':
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that the physical body (brain included) is a machine, and that the mind / soul, a
non-physical substance, inhabits the machine and gives it its cognitive
functions.
Descartes proposed the pineal gland, an actual brain structure, as the place
where physical and non-physical meet.
History of neuroscience
The first truly scientific advances, in the modern sense of 'science', were made
in the late 19th century.
1.
For example, Golgi (1843-1926) invented a way of staining brain tissue to
reveal that it was made from a dense interconnection of microscopic cells or
neurons
Ramon y Cajal (1852-1934) made detailed studies of the neural structuring of
the brain and proposed the neuron as the basis for all brain function.
Hermann von Helmholtz (1821-94) demonstrated that neurons interact with one
another electrically. Since then neuroscience has grown rapidly to become a
worldwide research discipline.
2. The nervous system
The human nervous system comprises the
brain, spinal cord, and the network of nerves
that branch to all parts of the body.
The brain controls the body by means of
electrochemical signals which travel to and
from the brain via the spinal cord and along
the nerves.
2. The nervous system
The brain receives input signals from the
sensory organs, sends signals to the
muscular system to effect bodily movement,
and monitors the operation of internal
organs like the heart and lungs by two-way
signals.
The brain and the spinal cord are generally
referred to as the central nervous system,
and the nerve network as the peripheral
nervous system.
The remainder of this lecture focuses on the
central nervous system and on the brain in
particular.
3. Brain anatomy
Everyone has seen pictures of a human
brain. Here's an example.
To quote the website from which this picture
was taken, 'The human brain is a 3-pound
(1.4-kilogram) mass of jelly-like fats and
tissues', which sounds fairly unpromising as
the organ which makes us us,
but the website goes on to say 'yet it's the
most complex of all known living structures'.
The present part of the lecture examines
this complexity in terms of the brain's main
components, the next part looks at its
microstructure.
3. Brain anatomy
The picture shows three of the brain's main
components: the highly-convoluted cortex,
the cerebellum on the lower left, and, below
the cerebellum, the stump of the brain stem
where it has been severed from the spinal
cord.
Much of the brain's structure is, however,
hidden by the cortex; the following crosssectional diagram shows the hidden
components.
3. Brain anatomy
The various brain components are associated with different functions:
Cortex: This is a sheet of tissue whose thickness varies from 2 to 6 mm and
which constitutes the outer layer of the brain; it is crumpled to allow it to fit
inside skull. Seen from the top, the cortex is divided into two halves joined by a
large bundle of nerves called the corpus callosum:
3. Brain anatomy
The cortex is associated with the so-called
'higher' cognitive functions: sensory perception,
voluntary movement, reasoning, and language.
The cerebellum, meaning 'little brain', is like a little
version of the cortex. It is associated primarily
with control of bodily movement.
The limbic system is associated with the emotions
and with memory.
The basal ganglia coordinate movement.
The brain stem deals with fundamental bodily
functions over which we have no conscious
control, such as breathing and heart rate.
3. Brain anatomy
These various components overlap to some
degree in terms of their functionality.
This is because the human brain is a product of
gradual development by evolution; a designer
would have given it a more efficient structure.
4. Neurons
The human body is made up entirely of different kinds of cells.
The cells of the nervous system are called nerve cells or neurons, and these
transmit signals to one another and to other types of cell via an
electrochemical process.
The brain alone consists of about 100 billion neurons.
4. Neurons
There are various kinds of neuron, each for a different kind of function, but for
present purposes these do not need to be distinguished, and the discussion
will relate to a generic neuron.
Needless to say, neurons are tiny and can only be seen with the aid of a
microscope; the figure below left shows actual neurons in the cortex, and the
one on the right shows a schematic of a single neuron.
4. Neurons
The cell body receives input signal pulses via its dendrites, and when the
number and intensity of such signals reaches a threshold, the cell body starts
firing signal pulses along the axon.
Youtube. Firing neurons:
http://www.youtube.com/watch?v=GIGqp6_PG6k&feature=related
4. Neurons
A neuron typically has about 10,000 dendrites, only a few of which are shown
above for clarity; the reality looks more like the graphic on the left.
Neurons are densely interconnected in the brain. Each axon makes numerous
connections with the dendrites of other neurons; the graphic on the right
gives an idea of this.
4. Neurons
The result is a vast network of neurons sending signals to one another.
Youtube. Neuron network: http://www.youtube.com/watch?v=sX87g3AHIbc
Youtube. Neurons:http://www.youtube.com/watch?v=sQKma9uMCFk
This, and only this, is the mechanism that produces human cognition.
5. Learning
The brain is plastic in the sense that the structure of neuronal interconnection
can change over time in response to its owner's experience of the real-world
environment.
This begins at birth as the baby and then child first begins to interact with the
world, and continues throughout adulthood until death.
It is how skills and information about the world are acquired and stored in
memory.
This process is otherwise known as learning.
5. Learning
Learning occurs at the places where the axon
from one neuron connects to the dendrites of
other neurons.
These connections are called synapses, and
the pattern of synaptic connectivity is the
mechanism that underlies learning.
5. Learning
The brain grows rapidly during the first few years of
life, and as it does so each neuron makes
numerous new synaptic connections with others in
its neighbourhood.
At birth a typical neuron in the cortex makes about
2500 synaptic connections, and by the age of two or
three this has grown to 5000 on average.
Thereafter, the pattern of neuronal connectivity
established at this early stage of life is modified in
response to experience as we age via a process
called synaptic pruning: synapses that rarely send
or receive input gradually die, and those that are
often used are preserved and strengthened.
6. Brain and cognition
We know that the brain generates cognition, and that the higher cognitive
functions are generated mainly by the cortex, but how this happens is still a matter
for ongoing research.
With respect to the cortex in particular, substantial progress has been made in
understanding which regions of the cortex are involved in which cognitive
functions.
In the past this was inferred from observation of the behavioural effects on people
with various sorts of brain damage due to disease or accident, and from direct
stimulation and removal of different parts of the cortex during remedial surgery
such as removal or tumours.
6. Brain and cognition
More recently, the development of brain scanning technologies such as CT, PET,
and MRI have allowed online observation of brain function in response to various
cognitive tasks to be observed without recourse to surgery.
These have greatly enhanced our understanding of the relationship between
cognitive function and cortical dynamics, that is, of which regions of the cortex are
involved in which cognitive functions, and how the pattern of cortical activation
changes as cognitive functions unfold over time.
6. Brain and cognition
An MRI scanner is shown below.
The test subject is placed in the machine so that his or her head is in the opening
in the middle of the device and is then given one or more cognitive tasks to
perform, in the course of which brain function is observed and recorded.
6. Brain and cognition
For example, the figures below shows the effect of a painful injection into the arm
on the thalamus and on the cortex.
The coloured areas show increased neuronal activation; cognitive functions other
than pain perceptual could be similarly observed.
6. Brain and cognition
For example, the figure below shows the effect of a painful injection into the arm
on the thalamus and on the cortex.
The coloured areas show increased neuronal activation; cognitive functions other
than pain perceptual could be similarly observed.
http://www.youtube.com/watch?v=8oN-sGrPgpI
• http://www.youtube.com/watch?v=XwUn64d5Ddk&feature=related
•
6. Brain and cognition
Based on such results, maps assigning at least some cognitive functions to
particular regions of the cortex can be constructed; two such maps are shown
below.
7. Brain and language
Linguists are, of course, primarily
interested in identifying the part or
parts of the brain involved in the
implementation of human language.
Attention has long focused on two
small areas of the cortex called
'Broca's area' and 'Wernicke's area'.
This is because both were long ago
found to be involved on speech
production and comprehension.
7. Brain and language
In 1861 Paul Broca had a patient who
could say only one word: 'tan'.
When this patient died Broca
examined his brain and noticed
damage in the part of the brain shown
in the diagram.
This has become known as 'Broca's
area', and many observations since
then have confirmed that this area is
indeed involved in speech production.
7. Brain and language
That Broca's and Wernicke's areas are demonstrably involved in speech
processing does not justify the claim that these are where human language is
located in the brain.
Current research supports the idea that these two areas have to do with speech
motor functions, and that language involves many parts of the brain working in
conjunction.
The 'Brain and language' links at the end of the lecture provide an introduction to
this work.
At present, the implementation of important aspects of language such as syntax
and semantics in the brain is largely mysterious.
8. The significance of neuroscience for cognitive science and linguistics
The central question in any scientific discourse about human cognitive functioning
is this: How can the brain, a physical object, generate cognitive function and, in
humans, a sense of self-awareness or consciousness?
It's a question that this module will be returning to many times in one form or
another.