1. Ch 16 Lateralization, Language & the Split Brain

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

14. 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

15. 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

16. 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)

17. 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

18. 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

19. 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 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

20. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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)

42. 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