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Chapter 4-laterality .docx

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University of Toronto Scarborough
Ted Petit

Chapter 4: Laterality 1 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) Chapter 4: Laterality 2 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 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 cortexChapter 4: Laterality 3 Techniques that investigate laterality of visual functions must take care to present stimuli peripherally and not to allow the fovea to observe the stimulus 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 o
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