Ch 16 Lateralization, Language & the Split Brain

Intro Like most everything in our body, the brain is bilateral Left & right hemispheres are entirely separate except for the cerebral commisures connecting them Major differences exist between the functions of the hemispheres Lateralization of function Split-brain patients: those whose hemispheres have been separated Language is the most lateralized of all cognitive abilities (Mostly left hemi) Hemis have different abilities & can function independently

Cerebral Lateralization of Function Broca’s area: Inferior prefrontal cortex of the left hemisphere Patients with aphasia (inability to produce or comprehend language) had damage to this area Apraxia (difficulty performing movements when asked to do so out of context) almost always associated with left hemi damage, even though symptoms are bilateral

Cerebral Dominance Cerebral dominance: Idea that one hemi (usually left) plays the dominant role in controlling all complex behavioral & cognitive processes So the left hemi is commonly called the dominant hemisphere & the right is the minor hemisphere

Tests of Cerebral Lateralization Sodium Amytal Test: During neurosurgery, inject sodium amytal to anesthetize one hemisphere of the brain & have patient recite a series of words When in left hemi, patient becomes mute for a few minutes When in right hemi, no effect on language Dichotic Listening Test: Audio of #s being read played through headphones, with a different set of #s going to each ear (simultaneously) When asked to repeat all the #s, most people say more #s heard in the right ear Indicating left brain hemi for language; contralateral Functional Brain Imaging: PET or fMRI scans During language tests, more activity is shown in the left hemi

Relation Between Speech Laterality & Handedness Dextrals: right handers Sinestrals: left handers Study of handedness, hemisphere damage & aphasia showed that the left hemi is dominant for language for almost all dextrals & most sinestrals Sinestrals are more variable in which hemi controls language

Split Brain Corpus callosum: brain tissue that connects the 2 hemispheres The largest cerebral commisure Contains 200 million axons A study using cats with transected (cut) corpus callosums showed that they were equally able to learn a task using one hemi as when using both When tested using the opposite hemi, it was as if they had never learned it Effectively showing the hemis acted as 2 separate brains Conclusion: function of corpus callosum is to transmit info between the hemispheres

Commissurotomy in Human Epileptics Commissurotomy: transecting the corpus callosum Done as a treatment for severe epilepsy to prevent the spread of the over-stimulated signal from one hemi to the other Tests done by delivering info to one hemi while keeping it out of the other Like with split-brain animals, split-brain humans seem to have 2 independent brains, each with its own stream of consciousness, abilities, memories & emotions Unlike the animals, human hemis are unequal in their abilities to perform certain tasks Especially left hemi is capable of speech, right is not

Hemispheres Functioning Independently Reminder: Input from one visual field or movement/feeling from one hand go to the contralateral hemisphere

Hemispheres Functioning Independently Left hemisphere can tell what it has seen, right hemisphere can show it. Studies of split-brain patients: Present a picture to the right visual field (left brain) Left hemisphere can tell you what it was Right hand can show you, left hand can’t Present a picture to the left visual field (right brain) Subject will report that they do not know what it was Left hand can show you what it was, right can’t

Doing 2 Things at Once Your brain can learn 2 different things at once When shown 2 different pictures (one in each visual field), patients can reach into 2 bags (one with each hand) and correctly grab the items they saw However, if you ask them what was in their hands, they would say 2 of what was shown on the right & be surprised when they looked at the objects in their hands and saw 2 different items

Doing 2 Things at Once Experiment is repeated, but instead of reaching into bags, the patient can see the objects in front of them When the patient is asked to pick up what was seen sometimes the helping-hand phenomenon occurs This is when the right hand goes to pick up what was seen by the left hemi & the right hemi “realizes” that is the wrong object (not what the right hemi saw) & causes the left hand to shoot out to redirect the right hand towards the correct object

Doing 2 Things at Once Because the hemis are effectively seeing twice as much at once, split brain patients can find a visual target in a group of items more quickly than healthy individuals Chimeric figures test Visual completion Scotoma (blind spot)

Split Brain Misc. For most split brain patients, the left hemi tends to control most of everyday activities However, in some cases, the right hemi has a will of its own & will create conflicts with the left hemi Split brain hemis mostly act independently, but they can interact via brain stem Individuals can vary on hemispheric independence Emotional info about a picture presented to right hemi can be transferred to left hemi which can communicate the feeling, even when it doesn’t know what the picture was More complex tasks tend to involve both hemis Elderly display less lateralization of function

Differences between Left & Right Hemis Many functions have no difference between the hemis When there are differences, they tend to be a slight bias in favor of one hemi, NOT a clear cut, absolute difference Functions do not reside exclusively in one hemi or the other Language is the most lateralized cognitive ability, but even it is not totally absent from the right hemi Right hemi language skills like that of a preschooler

Cerebral Lateralization of Function Superiority of left hemi in controlling ipsilateral movement Most movement controlled contralaterally, but some ipsilateral & left is better at it Superiority of right hemi in spatial ability Feeling an object in hand & deciding which 2-D image shows what it would look like unfolded Specialization of right hemi for emotion Better at identifying facial expressions of emotion Superior musical ability of right hemi Dichotic listening test with musical tunes, better able to identify with left ear

Cerebral Lateralization of Function Hemispheric differences in memory Both hemis involved in memory, but differ in which is best at certain tests Left hemi specialized for episodic memory Left hemi for memory of verbal info Right hemi for nonverbal info The hemis approach cognitive tasks in different ways Left hemisphere acts as the interpreter; continuously assessing patterns of events and trying to make sense of them Left hemi dominant for language, but right is better at perceiving intonation of speech & identifying the speaker Example of how these functional lateralizations are not absolute

Anatomical Asymmetries of the Brain Frontal operculum In frontal lobe In left hemi it is the location of Broca’s area Planum temporale In temporal lobe; called Wernicke’s area Involved in comprehension of language Larger in left hemi, but only in 65% of brains Heschl’s gyrus In temporal lobe; primary auditory cortex Larger in right hemi, often 2 gyri in right & only 1 in left Difficult to define the exact border/size of these structures

Evolution of Cerebral Lateralization Analytic-Synthetic Theory: Left hemi operates in an analytical, logical, computerlike way; analyzing stimulus info input sequentially, collecting extracting relevant info & attaching a verbal label Right hemi synthesizes; concerned with overall stimulus configuration and organizes & processes info in terms of wholes Mostly pop psychology; difficult to test empirically

Evolution of Cerebral Lateralization Motor Theory: Left hemi is specialized for speech because it is a type of fine motor movements Doesn’t explain why motor function would have become lateralized Linguistic Theory: Primary role of left hemi is language Deaf people with left hemi damage have difficulty using sign language, but not pantomime

Evolution of Cerebral Lateralization All classes of vertebrates have a right side preference for feeding Once hands evolved (monkeys & apes), there was a right hand preference for feeding and other complex behaviors Left hemi bias for communication in non-human species Ex: birdsong, dogs & monkeys for calls of conspecifics

Advantages of Cerebral Lateralization Advantageous for areas of the brain that perform similar functions to be located in the same hemi May be more efficient for neurons performing a particular function to be concentrated in one hemi 2 different kinds of cognitive processes can be more easily performed simultaneously if they are lateralized to diff hemis

Cortical Localization of Language Language localization refers to the location with the hemis of the circuits that participate in language- related activities Wernicke-Gerschwind Model The predominant theory of language localization

Cortical Localization of Language Broca’s area controls speech production Wernicke’s area controls language comprehension Broca’s aphasia: Lesions of Broca’s area hypothesized to produce aphasia with symptoms that are primarily expressive Normal comprehension of written/spoken language Speech that retains meaningfulness despite being slow, labored, disjointed & poorly articulated) Wernicke’s aphasia: Lesions of Wernicke’s area produce aphasia with symptoms that are primarily receptive Poor comprehension of both written/spoken language Speech that is meaningless but still retains superficial structure, rhythm & normal intonation Word salad

Cortical Localization of Language Conduction aphasia: Caused by damage to the pathway connecting Broca’s & Wernicke’s area (arcuate fasciculus) Mostly intact comprehension & speech, with difficulty repeating words they just heard Angular gyrus controls comprehending language-related visual input Area of left temporal & parietal cortex just posterior to Wernicke’s area Damage to this area can cause alexia (inability to read) & agraphia (inability to write) No difficulty speaking or understanding speech

Wernicke-Geschwind Model 7 components (all in left hemi): Primary visual cortex Angular gyrus Primary auditory cortex Wernicke’s area Arcuate fasciculus Broca’s area Primary motor cortex

Wernicke-Geschwind Model During a conversation, auditory info is received by primary auditory cortex & sent to Wernicke’s area, where they are comprehended To respond, Wernicke’s area generates the neural representation of the thought underlying the reply & transmits it to Broca’s area (via left arcuate fasciculus) where it activates the appropriate program of articulation that fires the neurons of primary motor cortex & then muscles of articulation

Wernicke-Geschwind Model When reading aloud, signal received by primary visual cortex is transmitted to left angular gyrus, which translates visual form of word into its auditory code & transmits it to Wernicke’s area for comprehension Wernicke’s area then triggers responses in arcuate fasciculus, Broca’s area & motor cortex to elicit speech sounds

Problems with the Wernicke-Geschwind Model Many aspects of this theory are oversimplifications Removal of Broca’s area but no surrounding area has no lasting effects on speech Speech problems may be due to swelling of surrounding area No permanent speech difficulties with lesions to arcuate fasciculus No permanent alexia or agraphia with lesions of angular gyrus Much of Wernicke’s area can be removed with no long term language deficits

Problems with the Wernicke-Geschwind Model More recent data has shown: No aphasic patients have damage restricted to just Broca’s or Wernicke’s area Aphasic patients almost always have significant damage to subcortical white matter Large anterior lesions are more likely to produce expressive symptoms; large posterior lesions more likely to produce receptive symptoms Global aphasia (severe disruption of all language related abilities) is usually related to massive lesions of anterior cortex, posterior cortex & underlying white matter Aphasic patients sometimes have brain damage not near the Wernicke-Geschwind areas

Further Study of the Wernicke-Geschwind Model More detailed info about areas related to language function by testing with electrical stimulation (as opposed to damage/lesions) Sites at which stimulation blocked/disrupted speech are scattered throughout frontal, temporal & parietal cortex (not just restricted to Wernicke-Geshwind areas) No specific areas that caused specific speech disturbances (ex: pronunciation, naming objects) Right hemi stimulation almost never disrupted speech Major differences among individuals & their neural organization of language abilities

Current State of Wernicke-Geschwind Model Original model supported in 2 ways Broca’s & Wernicke’s areas play important roles in language Tendency for aphasias associated with anterior damage to involve expressive deficits & posterior damage to involve receptive deficits

Current State of Wernicke-Geschwind Model Original model not supported Aphasia typically associated with widespread damage, not just to Wernicke-Geschwind areas Aphasia can result even when damage does not involve any Wernicke-Geschwind areas Broca’s & Wernicke’s aphasias rarely exist in pure forms; aphasia almost always involves both expressive & receptive symptoms Major differences in locations of cortical language areas in different individuals Generally, the theory has been abandoned by researchers

Cognitive Neuroscience of Language Currently used in language research Defined by 3 premises: Premise 1: Each complex language related process (speech, comprehension, reading) is the combo of several constituent cognitive processes, which may be organized separately in different parts of the brain. Premise 2: Areas of the brain involved in language are not dedicated solely to that purpose Ex: language areas can also function in memory Premise 3: Brain areas for language are small, widely distributed & specialized

Functional Brain Imagine & Localization of Language fMRI study showed that areas of the brain involved in silent reading were patchy, variable among patients & not limited to classic Wernicke- Geschwind areas Far more activity in left hemi PET study of activity in temporal lobe while naming objects in categories Involved brain areas outside classic Wernicke- Geschwind areas Area activated depended on category of the objects

Cognitive Neuroscience of Dyslexia Dyslexia: pathological difficulty in reading, not resulting from general visual, motor, or intellectual deficits 2 types: Developmental: Becomes apparent as child learns to read 5-11% of English speaking kids 2-3x higher in boys than girls Acquired: Caused by brain damage in people who could already read Rare

Developmental Dyslexia Has a major genetic component (50% heritability) No single kind of brain pathology has been found in all cases Disorder has various forms, likely with different neural correlates Results from disturbance of phonological processing (representation & comprehension of speech sounds)

Acquired Dyslexia 2 types: Surface: Deep: Inability to pronounce words based on their memories of the words (lexical procedure) but can still apply rules of pronunciation in reading (phonetic procedure) Difficulty pronouncing words that don’t follow common rules of pronunciation (ex: have, lose, steak) & will often incorrectly pronounce them based on rules (ex: rhyming with cave, hose, beak) Deep: Inability to apply rules of pronunciation in their reading (lost phonetic procedure) Incapable of pronouncing nonwords