Chapter 4: Laterality

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9 Apr 2012
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Chapter 4- Laterality
o Laterality: functional specialization of right and left hemispheres
o Left hemisphere: specialized for language and control of right hand and fingers
o Right hemisphere: believed to have role in visual identification of objects; music, emotion, spatial
abilities and control of left hand and fingers
Harder to determine with no linguistic or right hand motor skill when LH is damaged
o There is constant communication between the two sides
o Both work together to interpret ex. Words and prosody
Module 4.1 Methods
Split Brain
o Both sides connected by commissures (anterior posterior), corpus callosum, habenular
commissure and hippocampal formation
o Cats
Severed commissural systems
When left eye trained to specific stimulus right eye doesnt exhibit recognition of
stimulus and has to be trained separately
o Epilepsy: commissures transmit information back and forth increasing it
o Severed corpus callosum: both sides of the brain cannot communicate with each other
With these patients discovered than RH comprehends spoken and written words ; unable to
speak or write; is capable of arranging letters to spell 3 or 4 letter words; is specialized for
recognition of faces (used chimeric faces)
Intracarotid Amobarbital Testing
o Aka Wada test
o Used to assess the laterality of language in people who are going to be having brain surgery
Goal is to assess expressive and receptive language ability and long term and short term
memory capabilities of two hemispheres independent of each other
o Anaesthetize hemisphere with injection to the same side, keeps contralateral arm in the air and
when functioning is gone the arm drops
Then assess cognitive functioning of patient
o Have shown that most people show LH dominance for speech though some show bilateral (have
some type of interference with different language functions) or RH (in left handers)
Review Sensory Systems
Visual System
o Left Visual field: from nasal hemiretina of left eye and temporal hemiretina of right eye; both are
sent to right side of the brain
o Right Visual field: opposite of above
o There is overlap however such that center of retina (fovea) is sent to both sides; only periphery is
only on one side
o Therefore stimulus must be presented on outside and very briefly so you don't move eyes
Auditory System
o All receptors send projections bilaterally ; info from one ear is sent to both hemispheres
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o However projections to ipsilateral hemisphere is weaker, less numerous, and send info more
slowly
o When competing stimuli sent to each ear RH will attend to left ear and LH will attend to right ear
Dichotic Listening
o Task presents different stimuli (words music tones or phenomes) to each ear
o Results in asymmetry score that favour right ear
o LH language functions; better able to comprehend and report stimuli
People with RH language function have asymmetries that favour left ear
o This method reports the most people with RH language function (20% of right handers);
might have to do with type of task perception versus production
Might be due to impurity in the task
o Tasks that use music, environmental sounds or emotional tones result in asymmetry task that
favours left ear (sends to RH that is specialized for processing non-linguistic sounds like music and
prosody)
Tachistoscopic Presentations
o Rapid presentation of stimulus of to Right visual field RVF or LVF using tachistoscope
o Tachistoscope: piece of equipment that allows rapid presentation of stimulus to RVF or LVF
o Participant then asked to report what they see
o Stimuli: words or letters: asymmetry score that favours LH (language functions); some favour RH
(RH language function)
o Stimuli: facial recognition, detection of emotional expression, spatial judgement; asymmetry that
favour RH (for spatial info, discrimination of locations, identity of faces and emotional expression)
Module 4.2 Neuroanatomical, Neurochemical, and Behavioural Findings
o Brain is asymmetric
o Left front lobe extends farther
o Right occipital lobe extends farther back
Neuroanatomical Asymmetries
General Anatomical Trends
1. Rate of Maturation: LH gyri and sulci develop faster; asymmetry before birth
2. Size of Complete hemispheres: RH larger and heavier than LH
3. Ratio of gray to white matter: gray/white matter ratio higher in LH
4. Density of neural Tissue: LH more dense (due to ratio of gray to white)
Asymmetries in Specific Structures Within the Structures
o Sylvian fissure runs farther toward occipital lobe in LH but at a greater angle in RH (seen in early
hominoids too)
o Planum temporale: region of temporal lobe under the posterior of sylvian fissure
65% longer in LH; 11% longer in RH, 24% same in both
o Heschls gyrus: 1 in LH, 2 in RH
o Tables 4.4 and 4.5
Connections Between the Two Halves of the Brain
o Corpus callosum: largest approximately 200 million fibres
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