Left hemisphere involved with speech disorders caused by brain damage Lateralization and Split Brain Marc Dax 1836 Paul Broca 1860s Unaware of Dax s work Left hem And aphasia Broca s Aphasia Labored and poorly articulated speech Agrammatism telegraphic speech o Haulting Anomia naming deficit Agraphia writing impairment Region of brain damage left o Inferior frontal cortex third frontal gyrus anterior to face region of motor cortex Wernicke s Aphasia Apraxia Cerebral Lateralization Left superior temporal gyrus Poor comprehension of spoken and written language Fluent and spontaneous speech but incoherent Paraphasia sound and word substitutions senseless Alexia reading impairment Liepmann early 1900s Associated with LH damage Difficulty performing movements with either side of body when asked to do so Differences between left and right cerebral hemispheres Cerebral commissures connect two halves of brain via projection fibers Largest commissure o Corpus callosum o Transfers learned info from one hemisphere to other o When cut each hem functions independently Tests of Cerebral Lateralization Determining which hemisphere is dominant 1 Sodium amytal test Wada test o Anesthetize one hemisphere check for language function 2 Dichotic listening test a Heard different sounds in each ear at same time LH right ear b Hear more sounds in dominant hemisphere c d Right ear advantage LH for verbal stimuli Left ear advantage RH for music 3 Functional brain imaging a fMRI or PET used to see which half is more active when doing language test Speech Lateralization or Handedness LH dominant speech Both right and left handed people Greater variability among left handed people Right handed aphasics 60 LH 2 RH damage Left handed aphasics 30 LH 24 RH damage LH dominant for speech in o 92 right handed people o 69 left handed or ambidextrous o 30 left handed or ambidextrous with early LH damage Sex Differences in Brain Lateralization Some evidence male brain is more lateralized McGlone s studies of unilateral LH stroke victims o Men 3X more likely to suffer aphasia Suggests stronger lateralization Some fMRI studies suggest females use both hemispheres in language Female brain may be less lateralized Split Brain Commisurotomy Procedure Cutting off corpus callosum Monkeys 1930s o Little behavior effects o Stopped spread of epileptic seizure activity First Human 1940s o Relief for intractable epilepsy o Minor motor disturbances o Conclusion CC serves no function Myers and Sperry Experiment Split Brain Cats 1953 o Transected corpus callosum and optic chiasm so visual info could not cross contralateral hemisphere o Blindfold one eye then move it Chose stimulus followed by reward 2 hemispheres communicating with one another Learning curve they relearn it when blindfold was moved o Findings Each hemisphere learns independently Split brain cats with one eye patch Learn task and controls No memory when patch transferred to other eye Intact cats learning transfers between two hemispheres Similar with split brain monkeys Commissurotonomy in Humans with Epilepsy Limits convulsive activity Sperry and Gazzaniga o Test split brain Split brain humans differ from split brain animals human brains have different abilities Most human LH capable of speech RH are not Cross Cuing Doing 2 Things at Once Facial feedback from other hemisphere o Represents communication between hemispheres via nonneural route o i e RH might make face frown when LH gives incorrect spoken answer Each hemisphere can learn independently and simultaneously of split brain Helping hand phenomenon o 2 different visual stimuli hand that knows may correct the other Dual foci of attention split brain hemispheres can search for the target item in an array faster than intact controls can Chimeric figures task only symmetrical version of right half of faces recognized o Competition between hemispheres The Z Lens Zaidel Contact lens used to restrict visual input to one hemisphere Can assess one hemisphere s understanding of spoken instructions Lens is opaque on one side so that input only reaches one hemisphere Differences between L and R Hemispheres No substantial difference Key point Lateralization of function is statistical rather than absolute Differences have been observed Anatomical brain asymmetries observed in Frontal Operculum Broca s Area Near face area of primary motor cortex Language production Planum Temporale Wernicke s Area Temporal lobe posterior lateral fissure Language comprehension Primary Auditory Cortex Heschl s Gyrus Although asymmetries are seen in language not all are larger in the left Left planum temporale larger in only 65 of human brains Heschl s Gyrus larger on right o 2 in right one in left Frontal Operculum visible surface suggest right larger but left greater volume Theories of Cerebral Symmetry 1 Analytic Synthetic Theory a Suggests 2 fundamentally different models of thinking i Analytic mode LH 1 Pieces of the whole a Operates in logical sequential analytic fashion b Analyzing and providing verbal labels ii Synthetic mode RH 1 The whole a Primarily the synthesizer b Organizes and processes info in terms of gestalt c Makes immediate overall synthetic judgment 2 Motor Theory a LH specialized for fine motor movement i e speech b evidence i i lesions of LH disrupt facial movements more than RH even when not related to speech ii degree of disruption of nonverbal facial movements is positively correlated 3 Linguistic Theory with degree of aphasia a Primary function of LH is language b Based on studies of deaf people who communicate using ASL c Signing ability is lost if people suffer damage to LH even when able to make movements won t make sense d Or is this showing ASL is a language and is language highly analytical When Did Cerebral Lateralization Evolve Lateralization of function may have been present at beginning of vertebrate evolution Right handedness may have evolved from preference of right side of body for feeding LH controlling movements o i e birds dogs monkeys Survival Advantages of Cerebral Lateralization increase neural efficiency to concentrate functioning in one hemisphere o better to have one highly skilled hand than 2 moderately skilled hands 2 cognitive processes may be more readily performed simultaneously if both are lateralized to same hemisphere Once some lateralization of function advantageous to other abilities to lateralize o Language lateralized to LH because of fine motor control