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Module 4.1: Methods
Laterality: the functional specialization of the right and left hemispheres
Broca was first to popularize the functional specialization of each hemisphere
Left Hemisphere
Specialized for language and control of the right hand and fingers
Right Hemisphere
Hughlings-Jackson: played a role in visual information of objects
Specialized for music, emotion, and spatial abilities and control of the left hand and fingers
Understanding of functions of right hemisphere took longer poor linguistic ability
Cognitive deficits resulting from RH damage tend to be subtle in comparison to the
significant linguistic impairments that follow LH damage
Tends to make it difficult to obtain information regarding its function
Neurologically normal brain function relies on constant communication between RH and LH
Both hemispheres have some degree of competency for most functions
Eg. When someone say the words “nice shoes”
o Left Brain interprets sound waves that make up the speech and interpret it as
the words “nice shoes”
o Right brain interpret sound waves that compose the emotional tone of the
speech and allows interpretation the comment as either a compliment or an
insult
Failure to communicate between RH and LF becomes especially essential in ambiguous
situations when interpretation of stimuli is important to come to a correct conclusion
Split Brain
When the corpus callosum is severed, the information that is received from the right visual
cortex can no longer be integrated with that of the right visual cortex
Results in a brain that is split into two separate halves that can no longer communicate
with each other
Information can no longer influence the other hemisphere
Hemisphere can transmit pathology
One form of epilepsy uses the commissural systems to involve both hemispheres in the
production of severe seizure activity
o Complete commissurotomies is successful treatment for epilepsy frequency
and severity of seizures were reduced
Complete commissurotomies results in two hemispheres that are intact but separate
Motor, visual, auditory, and somatosensory abilities of each hemisphere are intact
Allowed learning that RH is capable of language although also specialized for other
functions
o Although RH is largely unable to either speak or write, it is capable of arranging
letters to spell 3- or 4- letter words (capable of comprehending spoken and
written words)
o Special role in recognition of faces Levy used 2 different halves of faces that
were combined to make one face and participant had to pick out which whole
face they had observed
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Two hemispheres “saw” different faces
Most of the time, the participants picked the face that had appeared in
the left visual field which sends information to the RH
Intracarotid Amobarbital Testing (IAT)
Aka. Wada test reflects role of Juhn Wada who used pioneered its use in the late 1940s
Commonly used to assess the laterality of language in individuals who will be under-going
brain surgery
Sodium amobarbital (short-term anesthetic) is injected into the either right or left internal
carotid artery which anaesthetizes the hemisphere ipsilateral to the injection
Typically the individual has his or her contralateral arm in the air once the hemisphere
is anaesthetized, it no longer maintains motor control over the contralateral limb and
the arm drops
Tends to last for a period of minutes, during which cognitive functions of the
unanaesthetized hemisphere can be examined
Goal: to assess the language and memory capabilities of the two hemispheres independently
of each other
Many variations of IAT
Most attempt to measure expressive and receptive language ability, and short-term and
long-term memory
Revealed that most individuals exhibit LH dominance for speech, although some individuals
exhibit either bilateral or RH dominance for language
Language functions of brain may vary with hand dominance - RH language occurs more
frequently in left-handers
Bilateral representation of speech does not mean that language function is dispersed
equally across the two hemispheres exhibits some type of interference with different
language functions
Very invasive procedure
Visual System
The visual system sends information from receptors located in the retina of both eyes to both
hemispheres but information is segregated with respect to where the item was viewed in space
Visual information presented in the left visual field is transmitted to the primary visual
cortex of the RH from the nasal hemiretina of the left eye and the temporal hemiretina
of the right eye
Visual information presented in the right visual field is transmitted to the primary visual
cortex of the LH from the temporal hemiretina of the left eye and the nasal hemiretine
of the right eye
Information is perceived in the visual cortex of one hemisphere is communicated to the
other hemisphere via the corpus callosum
Divisions of information between the hemiretinas occurs only in the periphery of the retina
fovea (center of the retina that is used for fine detail and color vision) automatically transmits
information to both the right and left visual cortex
Techniques that investigate laterality of visual functions must take care to present
stimuli peripherally and not to allow the fovea to observe the stimulus
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o Achieved by having the participant stare at a point at the center of the screen
while keeping the stimulus away from the center of the visual field (>1 degree
from the center)
o Stimuli is presented very briefly (150 > ms)
o Allows eyes to have no chance to make movements from center of the screen,
preventing the fovea from viewing the stimulus and sending the information to
both hemisphere
Auditory System
Is not as segregated as the visual system all receptors in the cochlea send projections
bilaterally; auditory information received in one ear is sent to both hemispheres
Projections to the ipsilateral hemisphere are weaker and less numerous and send information
more slowly than projections to the contralateral hemisphere do
Although both hemisphere can attend to stimuli presented to either ear, situations in
which information must compete result in the domination of contralateral projections
o When competing stimuli are presented to the two ears simultaneously, the RH
will preferentially attend to the input from the left ear, and vice versa
Information is sent to the other via the commissural systems
Dichotic Listening
Takes advantage of the suppression of ipsilateral projections that takes place when stimuli
compete with each other
Dichotic listening tasks present different stimuli to each ear
Stimuli: words, music, emotional tones, phonemes
Participant are asked to either report what they heard or to listen for a target and to
indicate whether the target was present or not
o Number of correct responses is tallied for each ear, and an asymmetry score is
computed
Dichotic listening tasks that use words or phonemes as stimuli tend to result in an asymmetry
score that favours the right ear
Reflects the transmission of the stimulus from the right ear over the stronger, larger,
and faster contralateral pathway to the LH
Most people exhibit LH language functions LH is better able to comprehend and report
word or phoneme stimuli
Some people with RH language functions exhibit asymmetries that favour the left ear
Dichotic listening studies tend to report that there are more people who have RH language
functions than either IAT or split-brain studies do
Overrepresentation of RH language is an artifact of the testing condition
Dichotic tasks are not pure measures of laterality and that overrepresentation of RH
language reflects the impurities in the task when individuals are directed to attend to
a specific ear, they can modify their asymmetry scores
Dichotic listening tasks that use music, environmental sounds, or emotional tones as stimuli
tend to result in an asymmetry score that favours the left ear
Suggests that the RH is specialized for the processing of non-linguistic sounds, such as
music and prosody
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Document Summary

Laterality: the functional specialization of the right and left hemispheres. Broca was first to popularize the functional specialization of each hemisphere. Specialized for language and control of the right hand and fingers. Hughlings-jackson: played a role in visual information of objects. Specialized for music, emotion, and spatial abilities and control of the left hand and fingers. Understanding of functions of right hemisphere took longer poor linguistic ability. Cognitive deficits resulting from rh damage tend to be subtle in comparison to the significant linguistic impairments that follow lh damage. Tends to make it difficult to obtain information regarding its function. Neurologically normal brain function relies on constant communication between rh and lh. Both hemispheres have some degree of competency for most functions. Failure to communicate between rh and lf becomes especially essential in ambiguous situations when interpretation of stimuli is important to come to a correct conclusion.

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