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Taak 3 Father Kribs1. What is single and double association and give examples.
Kolb & Whishaw, H11, Cerebral asymmetry
The functions of the two hemispheres are lateralized, or dissociated. To conclude that the
cortical area has a special or lateralized function, however, it is also necessary to show
that lesions in other areas of the brain do not produce a similar deficit.
Single dissociation
When dissecting complex mental tasks into their subcomponents, a researcher can
establish a "single dissociation" between functions. This is done by demonstrating that a
lesion to brain structure A disrupts function X but not function Y. Such a demonstration
allows one to infer that function X and function Y are independent of each other in some
way.
Dr. Oliver Sacks has described many famous cases of dissociation in his books. An
example of "single dissociation" is a patient who cannot name an object when he or she
can only see it, but can when he or she uses other senses like touching or smelling.
Patient D.F. was unable to place a card in a slot, but could do so when told to place it "as
if mailing a letter". From this the conclusion was drawn that judging orientation is one
ability and visual control of an action another.
Double dissociation
In the strongest experimental method for demonstrating the lateralization of function,
called double dissociation by Teuber, two areas of the neocortex are functionally
dissociated by two behavioral tests. Each test is affected by a lesion in one zone but not
in the other.
Lesions in the left hemisphere of right-handed patients consistently produce deficits in
language functions that are not produced by lesions in the right hemisphere. Thus, the
functions of the two hemispheres are dissociated.
In a series of patients withtraumatic brain injury, one might find two patients, A and B.
Patient A has difficulty performing cognitive tests for, say auditory memory but has no
problem with visual memory. Patient B has the opposite problem. By
using neuroimaging (or neuropathology post-mortem) to identify the overlap and
dissociation between lesioned areas of the brain, one can infer something about the
localization of visual and auditory function in the normal brain.
Establishing a single dissociation between two functions provides limited and potentially
misleading information, whereas a double dissociation can conclusively demonstrate that
the two functions are localized in different areas of the brain.
To make the difference between single and double dissociations easier to understand,
Parkin gives the following example: If your TV set suddenly loses the color you can
conclude that picture transmission and color information must be separate processes
(single dissociation: they cannot be independent because you cannot lose the picture and
still have the color). If on the other hand you have two TV sets, one without sound and
one without a picture you can conclude that these must be two independent functions
(double dissociation).
Paul Broca and Carl Wernicke had patients who proved the double dissociation between
generating language (speech) and understanding language. Broca's patients could no
longer speak but could understand language (non-fluent aphasia) while Wernicke's
1
, patients could no longer understand language but could produce jumbled speech (fluent
aphasia). Post-mortems revealed lesions in separate areas of the brain in each case.
2. What is TBI?
Kolb & Whishaw, H26 Neurological disorders
Traumatic brain injuries
The two most important factors in the incidence of head injury are age and sex. Head
injury can affect brain function by causing direct damage to the brain: by disrupting blood
supply; by inducing bleeding, leading to increased intracranial pressure; by causing
swelling, leading to increased intracranial pressure; by opening the brain to infection; and
by producing the scarring of brain tissue (the scarred tissue becomes a focus for later
epileptic seizures). There are two main types of brain trauma: open-head injury and
closed-head injury.
Open head injuries
Open-head injuries are TBIs in which the skull is penetrated, as in gunshot or missile
wounds, or in which fragments of bone penetrate the brain substance. In many cases, the
injury does not cause the victim to lose consiousness. Open-head injuries tend to prodcue
distinctive symptoms that may undergo rapid and spontaneous recovery. The neurological
signs may be highly specific, and many of the effects of the injuries closely resemble
those of the surgical excision of a small area of cortex.
Closed head injuries
Closed head injuries result from a blow to the head, which can subject the brain to a
variety of mechanical forces:
- Damage at the site of the blow, a bruise (contusion) called a coup, is incurred
where the brain has been compacted by the bone’s pushing inward, even when
the skull is not fractured.
- The pressure that produces the coup may push the brain against the opposite side
or end of the skull, producing an additional bruise, known as a countercoup.
- The movement of the brain may cause a twisting or shearing of nerve fibers,
producing microscopic lesions.
- Bruises and strains caused by the impact may produce bleeding (hemorrhage).
Because the blood is trapped wihtin the
skull, it acts as a growing mass
(hematoma), exerting pressure on
surrounding sructures.
- As with blows to other parts of the body,
blows to the brain produce edema, another
source of pressure of the brain tissue.
Closed head injuries resulting from traffic
accidents are particularly severe because the
head is moving when the blow is struck, thereby
increasing the velocity of the impact and
multiplying the number and severity of small
lesions throughout the brain. Computerized
tomographic scans of accident victims suffering
prolonged coma (loss of consciousness) shows
diffuse brain injury and enlarged ventricles, signs
associated with poor outcomes.
Closed head injuries are commonly accompanied
by coma. According to Lezak, the duration of
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