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Lecture 6

PSYCH 1XX3 Lecture Notes - Lecture 6: Substantia Nigra, Radioactive Tracer, Magnetic Resonance Imaging


Department
Psychology
Course Code
PSYCH 1XX3
Professor
Joe Kim
Lecture
6

Page:
of 13
Lecture 6: Neuroscience II
The Structure of the Brain
In humans, the nervous system axis or “neuraxis” curves
Dorsal always refers to the back of the axis and ventral means to the front of the axis or
“to the belly”
Because of the curve in the neuraxis, at the level of the head, dorsal is up, but at the
level of the spinal cord, dorsal is to the back
Rostral means towards the top of the axis and caudal means towards the bottom of the
axis
Locations in the brain that are more central or towards the midline of the brain are
medial and regions towards the outside of the brain are lateral
These terms can be combined to locate a very specific brain region
Ex. The medulla is a region in the hindbrain, which can be further divided into several
subregions
o One subregion is called the rostral ventral medial medulla
o This means that it is towards the top, in front of the neuraxis and towards the
midline of the brain
Studying the Brain
Lesion studies
o In 1848, Phineas Gage was a foreman of a railway construction crew, in charge of
using explosives to remove large sections of rock from the path of the railroad
o He was athletic, intelligent and full of life, by all means respected by his crew
o One day, Gage was a victim of an accident, resulting in the blasting of a 3 foot
iron rod completely though his left cheekbone and through the top of his skull
o Remarkably, he survived and except for some loss of vision and facial
disfigurement, he recovered completely
o However, his friends barely recognized him as they Gage they once knew
o He now become prone to selfish behaviour and bursts of profanity
o He became erratic and unreliable and had trouble forming and following through
on plans
o Gage’s case provided support for the view that the brain has specialized
structures for complex behaviours
o An advantage of case studies such as Gage’s is that it gives scientists a direct
measure of a brain’s structure and function
o A disadvantage is that it is hard to selectively target particular regions and draw
conclusions
o This problem can be overcome by studying specific brain lesions induced in
animal models
In such scenarios, a researcher destroys, removes or inactivates a defined
brain region and observes the result on behaviour
The accuracy of this emerging understanding of structure and function
can depend on the precision of the lesion
An alternative approach to lesioning is to electrically stimulate an area of the brain and
observe the result on behaviour to build an anatomical map related to function
o This technique was used extensively by neurologist Penfield as he performed
brain surgery to treat patients with severe epileptic seizures
o Penfield revolutionized techniques in brain surgery as he perfected his
“Montreal Procedure” to treat patients experiencing severe seizures
o In doing so, he had to be sure that critical areas of the brain were left intact
o Because the brain itself does not have pain receptors, a patient undergoing
surgery could be under local anaesthetic and fully conscious, working with
Penfield to probe the exposed brain to locate and removed scarred tissue that
caused the seizures
o Penfield used a thin wire carrying a small electric charge to stimulate the cortex
o This stimulation leads individual neurons to fire and thus Penfield could
accurately map perceptual processes and behaviours to specific brain regions
o Ex. If an area of the visual cortex was stimulated, a patient reported seeing
flashes of light and if an area of the motor cortex was stimulated, a patient
would experience a muscle twitch
Single cell recording
o Electrodes can also be used to record ongoing electrical activity in the brain
through single cell recording techniques
o A small electrode is inserted into the nervous tissue of a live animal model with
its tip held just outside the cell body of an individual neuron
o From this electrode, neural activity is recorded while the animal performs a task
or a stimulus is presented
o The pattern of firing reveals a particular neuron’s functional role
o In a typical experiment, cats were presented with specific visual stimuli while
recording from single cells in the visual cortex
o In this way, individual cell types were identified that responded to specific
categories of visual stimuli
Structural neuroimaging
o To study large-scale structure and function of brain regions, neuroscientists use
structural and functioning neuroimaging techniques
o The first structural neuroimaging technique developed was computed
tomography (CT)
During a CT scan, a series of X-ray slices of the brain are taken and pieced
together to produce a relatively quick and inexpensive picture of the
brain
These scans are often helpful to diagnose brain injuries
A major limitation with the CT scan by today’s standards is its relatively
low resolution
It is very difficult to examine fine brain anatomy with a CT scan and as
such it is not often used in neuroscience research
o For a more detailed structural image of the brain, neuroscientists use MRI
(magnetic resonance imaging)
In an MRI machine, powerful magnetic fields are generated, which align
the hydrogen atoms found throughout the brain
While these atoms are aligned, an MRI can be used to localize tissue very
precisely throughout the brain
Functional neuroimaging
o Cognitive neuroscientists can use a functional imaging technique such as the
positron emission tomography (PET scan) to learn how brain function relates to
cognitive tasks such as language and memory
o In a PET scan, a radioactive tracer of glucose or oxygen is injected into the
bloodstream
The radioactive molecules make their way to the brain and are used in
metabolic processes, which are detected by the PET scan
The logic is that more active brain areas will use more metabolic
resources and so an image of the brain’s relative pattern of activity can
be constructed
A disadvantage of the PET scan is that it requires a radioactive tracer to
be injected, a relatively invasive procedure
o Functional magnetic resonance image (fMRI) is often preferred because it can
produce a relatively clear image of the brain’s activity without the need for a
radioactive tracer
fMRI works by measured the blood oxygen dependent signal and uses
many of the same principles as the MRI
It is able to measure the relative use of oxygen throughout the brain and
operates under the same basic assumption as the PET scan (more active
areas of the brain require more metabolic resources)
A limitation of fMRI is that it provides a very rough image of brain
activation
Oxygen use by the brain often spikes a few seconds later than the spikes
of activity in the brain, which can be a very long time in terms of brain
function
As such, fMRI is not the best method to use if a researcher is interested in
precise timing of brain activation and function
o A final neuroimaging method is the electroencephalogram or EEG
The electrical activity of the brain can be recorded through the scalp by
wearing a cap of very sensitive electrodes
The EEG provides only a very rough image of the brain’s overall activity,
from populations of neurons
However, with a few modifications, the EEG can become more
informative
In an event related potential (ERP) experiment, a specific stimulus is
presented to the subject repeatedly, while the EEG is recording