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Chapter 4

Chapter 4 notes

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Ted Petit

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Chapter 4
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
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
oLeft Brain interprets sound waves that make up the speech and
interpret it as the words nice shoes
oRight 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
oComplete commissurotomies is successful treatment for epilepsy
frequency and severity of seizures were reduced
Complete commissurotomies results in two hemispheres that are intact but
Motor, visual, auditory, and somatosensory abilities of each hemisphere are

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Chapter 4
Allowed learning that RH is capable of language although also specialized
for other functions
oAlthough 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)
oSpecial 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
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
Intracarotid Amobarbital Testing (IAT)
Aka. Wada test reflects role of Juhn Wada who used pioneered its use in the late
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
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

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Chapter 4
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
Techniques that investigate laterality of visual functions must take
care to present stimuli peripherally and not to allow the fovea to
observe the stimulus
oAchieved 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)
oStimuli is presented very briefly (150 > ms)
oAllows 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
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
oWhen 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
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