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Laterality – the functional specialization of the right and left
Early studies of the brail demonstrated that the left (LH0 is specialized
for language and the control of the right and fingers.
Although the functions of the right hemisphere (RH) were not readily
understood. Hughlings – Jackson suggested that it played a role in
visual identification of objects. The RH is specialized for music,
emotion and spatial abilities as well as control of the left hand fingers.
The poor linguistic ability of the RH , tends to make it difficult to
obtain information regarding its function .
Neurologically normal brain function relies on constant communication
between RH and LH.
RH interrupts the sound waves that composed the emotional tone of
speech (Prosody )
MODULE ; 4.1
When the corpus callosum was severed , the information that is
received from the right visual cortex can no longer is integrated with
that of the right visual cortex .
Severing the corpus callosum results in a brain that is split into two
separate halves that can no longer communicate with each other .
In the 1960s , they began to perform complete commissutomies (cutting
the corpus callosum and all of the other forebrain commissures on
individuals with intractable epilepsy . Vogal and Bogen observed that
complete commissurotomy was a successful treatment and that the
frequency and severity of seizures were reduced.
Motor , visual , auditory and somatosensory abilities of each
hemisphere are intact .
RH was capable of comprehending spoken and written words
RH played a special role in the reorganization of faces .
Intracarotd Ambarbital testing
Intracarotid amorbarbital testing (IAT) is also known as the Walda test
, reflecting the role of the neurosurgeon Juhn wada , who pioneered its
use in the late 1940s .
IAT is commonly used to asses the laterality of language in individuals
who will be under surgery .
Sodium amobarbitual is injected to the right or left internal carotid
Patient’s hands in the air, when anathesied the contralateral hand
The neuropsychologist then asses the cognitive functions of the
The goal of IAT is to asses language and memory capabilities of the two
hemispheres independently .
Anesthesia tends to last for a period of minutes , during which the
cognitive functions of the unanaesthesized hemisphere can be
Studies using IAT have revealed that most individuals exhibit LH
dominance for speech , although some individuals exhibit either
bilateral or RH dominance for language .
Bilateral representation does not mean that language is dispersed
evenly , rather some type of interference with different language
IAT is very invasive
The visual system sends information from receptors located in the
retina of both eyes to both hemispheres.
Visual information that is presented in the left side of space (left visual
field ) is transmitted to the primary visual cortex of the RH from the
nasal hemiretina of the left eye ( the half of the retina that is closest to
your nose ) and the temporal hemirtina of the right eye (the half of the
retina that is closest to the temples .
Important to remember that the division of information between the
hemiretinas occurs in the periphery of the retina .
The foeva (center of the retina is used for fine detail and color vision )
automatically transmits information to both the right and left visual
To allow the fovea to observe the stimulus the participant stares at a
point at the center of the screen while keeping the stimulus away from
the center of the visual field.
The auditory system is not as segregated as the visual system , as all
receptors in the cochlea send projections bilaterally .
Projections in ipsilateral hemisphere are weaker and send information
more slower than projections to the contralateral hemisphere.
The RH will preferentially attend to input from the right ear.
Dichotic listening takes advantage of the suppression of the ipsilateral
projections that takes place when stimuli compete with each other .
Presenting stimuli to each ear .
Dichotic listening tasks that use words or phenomenon as stimuli tend
to result in as asymmetry score that favors the right ear.
Most people language functions and for them the LH is better able to
comprehend ad report words or phonemes stimuli . However some
people with RH language functions exhibit asymmetries that favor the
left ear .
Dichotic listening reports more people with RH language functions
than the IAT or spilt brain studies.
Some critics say is am artifact of testing conditions.
Dichotic listening requires the perception of sounds
It is suggested that most dichotic tasks are not pure measures of
laterality and that over representation of RH language reflects the
impurities I the task.
Dichotic listening that use music or environmental sounds , or
emotional tones as stimuli tend to result in an asymmetry score that
favors the left ear.
These results suggest that the RH is specialized for the processing of
non linguistic sounds , such as music and prosody .
Take advantage of the segregation in the visual system .
Involves the rapid presentation of stimuli to either the RVF or the LVF
using a tachistoscope ( a specialized piece of equipment that allows the
rapid presentation of stimuli to eitheeer the RVF or LVF) or computer
Participants are asked to report what they observed.
Takes that use words or letters as stimuli tend to result in an
asymmetry score that favors the LH . More correct responses
presented in the RVF.
Most people exhibit LH language functions
However , some people exhibit asymmetries that favor the LVF ,
presumably reflecting RH language function.