Brain Study Techniques
Introduction
There are two methods of studying a nervous system:
In vitro (literally, in glass) means studying an organ outside
of the living body and in an artificial environment. In vitro of the brain,
then, implies removing the nervous system from a living organism and keeping
it alive in a medium such as a petri dish.
Because this method severely harms the organism it studies, it is
inappropriate for higher life forms like human beings. Hence, in vitro
will not be covered in the rest of this tutorial.
The preferred method is in vivo (literally in the living),
which involves studying the nervous system without removing it from its
living organism.
Invasive Study
There are number of methods of studying the brain by opening its surface
and attaching recording devices.
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In Animals: single cell recording of action potential from single
cells or multiple cells in response to stimuli or cognitive behavior
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For example, the diagram below shows the action potential frequency
of a cell associated with a specific receptive field in a monkey's field
of vision. The frequency increases as a light stimulus is brought closer
to the receptive field.
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In Humans: subdural
surface electrodes are placed in a grid directly on the surface of the
brain beneath the dura matter
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This is often done in preparation for surgery. For example, in order
to determine the focus of seizures for removal in epileptic patients
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Used for frontal and temporal lobes, affecting motor, memory, and language
functions
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Lesion: Technique whereby very small regions of the brain are removed
(microlesioned). Used by experimenters to test the pattern of spared
and impaired abilities
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Permanent:
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In gray matter: surgical removal, neurotoxic lesion
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In white matter: cutting connections between areas
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Temporary:
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Compare a patient's abilities before and after the lesion
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Cooling a region (decreasing temperature by 20¡ F) to lower metabolism
Quasi-invasive Study
This involves the neuropsychological study of the brain, which
integrates the psychological observation on behavior of the mind with neurological
observations on the brain and nervous system. In particular, quasi-invasive
methods study human lesion patients:
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Permanent lesions
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Caused by nature: e.g., strokes or trauma
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Caused by neurosurgeons for medical treatment. E.g., removal of tumors
or epileptic foci
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These are typically larger lesions than in controlled animal studies
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Lesions are more random, often due to strokes caused by tied vasculature
of the brain
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Positive aspects:
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Best at determining if the brain region (affected by the lesion) is
critical to cognitive functions
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Negative aspects:
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Study in animals have a homology problem inherent with them:
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Lesions may not correspond to the same regions in humans
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Higher cognitive functions in humans (such as language) may not have
corresponding functions in animals
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The cause of problems in humans may be due to a difference in their
brains:
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There may be developmental reorganization. E.g., chronic epilepsy
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There may be recovery-related reorganization, E.g., acute deficits/chronic
defects)
Non-invasive Study
This involves the measuring of the structure and function in intact,
'normal' human brains. A general overview:
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Very sensitive detectors are placed on the surface of the brain, either
on the scalp or completely removed from the head
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The detectors pickup changes in electrical activity (metabolic activity)
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Sophisticated computer algorithms are used to interpret the signals
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The signals are then converted into images
The tutorial on Brain Imaging covers in detail non-invasive techniques