Ch 11 - Cerebral Asymmetry

Anatomical Asymmetries in the Brain

Laterality = the 2 hemispheres have separate functions

• L hemisphere : specialized for language (contains Wernicke’s & Broca’s) + controls R body

• R hemisphere : specialized for nonverbal info (logic, decisions, thinking) + controls L body

→ both hemispheres are involved in almost all behaviours

• lateralization is relative, NOT absolute (always evolving & adapting)

• one hemisphere just tends to take the lead in diff areas (ex. some ppl have lang area on R side instead of L)

→ actions & thoughts seem unified, despite contributions from both hemispheres

Cerebral Asymmetry

ex. Planum temporale is larger in the L hemisphere than R in majority of ppl

• Planum temporale = main area involved in lang perception, processing, & planning

  • on the superior temporal gyrus

• wernicke’s is around this area

• possibly bc of of role in lang

ex. Primary auditory cortex is larger in R hemisphere

• possibly bc of music

• takes over the R hemisphere area where the plant temporale is smaller

→ MRI studies of brains show consistent anatomical differences b/w hemispheres (looked at over 17000 brains)

Surface Asymmetry Map

→ theres also surface/topographical asymmetry

• diff areas of the brain specializing in different things allows better performance as diff things at the same time instead of the whole brain doing everything at a subpar level

Neuronal Asymmetry

→ neurons in diff parts of the lobe have diff patterns of dendritic branching

• THUS, neurons are ALSO specialized b/w areas & b/w hemispheres

study : research comparing dendrites in pyramidal cells b/w Broca’s area, frontal operculum, & facial area of the motor cortex showed:

• difference b/w areas & b/w hemispheres

• the Brian of L-handed subjects looked diff than R-handed subjects

note: small study (only 6 subjects) → doesn’t rly show power or signify for an FMRI study with such a small n

Genetic Asymmetry

→ research has been looking at how gene expression patterns diff b/w the 2 hemispheres of the brain

findings:

• some genes are expressed differently b/w the 2 hemispheres

  • observed as early as 5weeks post conception

• likely that epigenetic modification also differ b/w the 2 hemispheres

  • possibly why ppl have heterochromia (2 diff coloured eyes) - mod only occurred in one hemisphere → affecting that eye

• its possible that differences in gene expression lead to anatomical & neuronal asymmetries

ex. possibly gave rise to anatomical asymmetry like the motor area being larger on one side

Asymmetries in Neurological patients

→ asymmetry was initially studied in ppl w neurological diseases (ex. epilepsy)

note: epilepsy starts in one hemisphere and spreads (some involves both sides)

• possible medical care included : implantation of electrodes, electrical brain stimulation & brain surgery (ex cutting corpus callosum)

  • observing the results of these investigations revealed lots of info abt brain function

clinical relevance : understanding differences b/w L & R hemisphere brain fxn can be important for understanding deficits

Asymmetries in Neurological patients : Brain Stimulation

→ an early treatment for epilepsy was removing the cortex where the seizures originated

goal : improve quality of life

• prep : map out speech & motor control areas before surgery to avoid damage to those areas

→ planning studies like the one mentioned & brain stimulation found:

• some fxns (motor control & sensation) are symmetrical

  • thus removing one side of the cortex doesn’t rly affect it

• some fxns are asymmetrical (language)

• R hemisphere has increased perceptual fxns (combing info to create full perceptual picture)

• stimulating L frontal or temporal lobes : facilitates speech production

• stimulation can block fxns when the subject is using that brain region to perform a task

Identifying Speech & movement areas

• placed electrodes (in awake pt) to stimulate/ record activity → used to identify areas of the brain and their fxn

Patients w Lateralized Lesions : Double Association

Lateralized lesions = lesions on one side of the brain

• lesions can be caused by stroke, surgery or other brain injuries

Double Association = an experimental technique used to show the association of a particular fxn to a particular brain region

process: compares 2 pts with lesions in diff regions of the brain & looks at 2 diff deficits in functions (ex. reading and writing)

ex. a double dissociation b/w reading and writing means that reading & writing must be done in diff areas

shown by: when 1 pt has impaired reading but not writing, while the other pt is impaired in writing but not reading

• impairment in reading (not writing) would be assoc w the brain region damaged in the 1st pt

• impairment in writing (not reading) would be assoc w the brain region damaged in the 2nd pt

Commissurotomy Patients

Epilepsy = from over excitation of neurons, which can begin in once hemisphere and spread to the other via the corpus callosum

Commissurotomy = cuts the corpus callosum & anterior commissure to prevent to spread of seizure activity across hemispheres

• a corpus callostomy is a type of this

initial behavioural results : good clinical outcome (reduced seizure activity) + no obvious behavioural changes

closer inspection : behavioural changes become apparent

→ this provided info abt the organization of brain fxns

Commissurotomy Patients : Effects

→ 2 hemispheres are split & can act independently - creates 2 separate experiences/views of the world

• cortical connections w sensory & motor systems and their bodily receptors and effectors are Unaffected

  

→ experimental application : researchers can control which hemisphere visual info goes to & then observe the result

normally, : visual & auditory info is presented to both hemispheres independently

• info in L visual field → R hemisphere

• info in R visual field → L hemisphere

note: eye movements usually enable info directly in from of the subject to be processed by both hemispheres (takes time tho)

→ brief presentation in only the L or R visual field presents the info to a single hemisphere & bc of the commissurotomy that info cannot be shared w the other hemisphere

ex. present an image of a women in L field & name “Anne” in R field

w/o split : subject combines name & the lady

w split : anne or lady w NO connection

ex. present “key” in L visual field

w/o split : L visual input is sent to R hemisphere then to L hemisphere via corpus callosum

w split : L visual input is sent to R hemisphere (isn’t sent to L hemisphere)

Split-Brain Patients

info in R visual field : presented to L hemisphere

• CAN verbalize what they saw (bc sent to L hemisphere for lang)

• CAN identify what they saw on R w R-hand ( L hemisphere → R body)

info in L visual field : presented to R hemisphere

• CANT verbalize seeing anything (bc it doesn’t specialize in lang)

• can identify what they saw on L with L-hand (R hemisphere → L body)

  • However, when asked to explain their hands selection the L hemisphere (verbal) interprets the choice of the L-hand & MAKES UP A REASON

    • sometimes they dont know that they’re making up an excuse

Split-Brain Patients : The interpreter example

see R side : L brain can assoc + verbalize why

• thus, L hemisphere sees milk → sends to R-hand to select cow

see L side : R brain can assoc, but CANT verbalize why

• thus, R hemisphere sees beach → send to L-land to select beachball

  • still makes assoc, but cant verbalize or write why bc its only sent to the R-hemisphere which ISNT for lang

Split-Brain Patients : Facial Recognition

→ not just vision is affected by split-brain

• visual recognition of faces : pts choose face seen on L visual field → sent to R hemisphere

• vocal naming of a face : pts choose face seen on R visual field → sent to L hemisphere where it CAN be verbalized → sent to R-hand to select

note: the switching b/w L-hemisphere to R body & vice versa occurs in the hind brain which is uncut in this procedure

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Debate:

• split brain pts get one consciousness in ONE of the hemispheres

or

• get 2 DIFF consciousnesses ( 1 in each hemisphere)

note: see more extremes in behaviour w pts having done a full cumissurotomy

ex. arms doing opposite movements like pulling up and down they’re pants

Carotid Sodium Amobarbital Injection : WADA Test

= injecting sodium amobarbital into the carotid artery will briefly anesthetize the ipsilateral (same side) hemisphere → clinicians can check for impairments of speech & language & other cog functions

• see behavioural changes within seconds of injection (wears off)

practical application: before brain surgery its important to confirm which hemisphere is the lang hemisphere for a pt

• for most ppl lang is localized to the L hemisphere (both L & R-handed ppl)

→ can use this test to check which hemisphere, when anesthetized, impacts lang more

results:

• 98% of R-handers have lang on L hemisphere

• 70% of L-handers have lang on L hemisphere

  • 30% of l-handers have lang NOT in the L hemisphere

Asymmetry in the Auditory System

→ auditory system is LESS differentiated (less segregation of info b/w fields) than the visual system

Dichotic-listening task = present diff stimuli to the L and R ears, then asks the subject to recall the info presented

• lang stimuli presented to the R ear are MORE memorable

  • goes to L hemisphere (more lang specialized)

• melodies presented to the L ear are MORE memorable

  • goes to R hemisphere (more hearing spec)

Kimura’s Dichotic Listening Model = tests how with monaural presentation to an ear the ipsilateral pathway in the brain allows the sound to be sent to both hemispheres, however w dichotic presentation (both ears) the ipsilateral pathway is suppressed and each hemisphere focuses only on the sound from the opposite ear

• Monaural presentation :

  • sound at L ear → sent to R-hemisphere & L-hemisphere via ipsilateral pathway (thus they can verbalize what was heard)

  • sound at R ear → sent to L-hemisphere (can verbalize) & to R-hemisphere via ipsilateral pathway

→ even w the corpus callosum cut, both hemispheres are sent the auditory info via the ipsilateral pathway

• Dichotic Presentation = present a diff sound at each ear at the same time → hemispheres focus on only the sound from the opp. ear bc the ipsilateral pathway is suppressed.

  • sound at L ear → sent only to R-hemis

  • sound at R ear → sent only to L-hemis

Asymmetry in the Somatosensory System

→ somatosensory input (touch, feel) is almost completely CROSSED :

• L hand → R-hemis : recognizes patterns & shapes (ex. braille)

• R hand → L-hemis : recognizes letters easier

research:

• left-hand advantage to reading braille - bc of the R-hemispheres specialization in recognizing & processing patterns

• right-hand advantage for recognizing letters - bc sent to L-hemisphere where lang is localized

• left-hand advantage for recognizing other shapes - bc sent to R-hemisphere

THUS, recognizing braille seems to be MORE reliant on R-hemis’s pattern processing than L-hemis’s lang → thus L-hand is better for braille

Asymmetry in the Motor System

→ motor system has less clear asymmetry, however… damage to the L-hemisphere motor regions results in apraxia

• Apraxia = loss of skilled fine movements

  • thus, those w poor fine motor skills (apraxia) may have damage to the L-hemis motor regions

note: diff sensory inputs being sent to the 2 hemispheres may influence how we interpret the roles of the L vs R hemispheres motor regions

→ results In questioning whether the differences observed are a result of the motor systems being diff on the two sides, OR if its bc the sides were reacting to diff sensory info.

→ to understand hemispheric differences in motor control we use: direct observations & interference effects

understanding hemispheric differences in motor control : direct observations

findings:

• the 2 hemispheres play a complementary role in controlling movements based on native hand preference

• mouth movements start on the R side of the face : controlled by L-hemisphere

• stronger display of emotions on the L side of the face : bc of R-hemisphere specialization in emotions

understanding hemispheric differences in motor control : Interference tasks

findings:

• different behaviours disrupt specific hands when playing the piano

note: had them play diff songs w each hand to see which was impacted

→ R-hand is disrupted by speaking : bc L-hemis is focusing on lang instead

→ L-hand is disrupted by humming : bc R-hemis is trying to identify sound

• as we learn motor skills they become automatized, thus require less conscious attention → LESS impacted by interference effects

ex. a long-time pianist can easily keep playing while holding a conversation bc playing has become automatized

What do Laterality Studies tell us?

• many are indirect measures of brain function

• results of these are subject to interpretation

  • many researcher encourage skepticism when look at the field of laterality research

ex. some laterality effects, like dichotic listening, can be negated by telling the subject to focus on verbal info presented to the L ear

Role of Neuroimaging in Studying Asymmetry

→ brain-imaging studies support the asymmetries found in behavioural studies of neurological pts & intact brains

Types of neuroimgaing used:

• PET

• FMRI (the best)

• changes in blood flow velocity in the brain

→ these tasks are less invasive than traditional tasks (Wada test), BUT brain imaging is expensive

findings:

• PET & FMRI studies show a L-hemis lateralization for language

  • also supported by MORE blood flow in L-hemis during speaking tasks

Specialization Theories of Lateralization

Specialized theories = suggest diff hemispheres have UNIQUE functions

says:

• that the L-hemis is specialized for fine motor control, and that this is why speech is localized to the L-hemis bc fine motor is necessary for speech

• that the L-hemis is good at rapid temporal processing which is integral to language

• that the L-hemis is organized w many focal centres, while the R-hemis is more diffusely organized

→ all models are based on inference w little research to support them

Interaction Theories of Lateralization

Interaction theories = suggest that one hemisphere may be better at a task BUT they both cooperate in performing that task

→ both hemispheres are involved in a task, but one hemisphere is better

say:

• the 2 hemispheres both work on diff parts of processing at the same time

• both hemispheres may have the ability to perform the task, BUT one inhibits or suppresses the activity of the other

• 1 hemisphere receive info preferentially/ pays attention to the info preferentially and therefore performs a specific type of analysis of that info

Preferred Cognitive Mode

Cognitive Mode = individual differences will cause ppl to use one type of thought process instead of another

ex. some ppl prefer logic and math, while others prefer looking at the big picture

→ its possible that factors other than lateralization of brain fxns influence cognitive mode (using one type of thought process over another)

Cognitive Set = biases in how individuals approach problems

• this can influence the results of tests of lateralization

Asymmetry in NONhuman animals

→ asymmetry is present throughout the animal kingdom

• frogs & salamanders : vocalization is left lateralized

• many birds : L-hemis specialization for song production (not all birds)

• bees : lateralization in learning odors associated with reward & in remembering associations

• chimpanzees : asymmetries in the homologues of Broca’s area & planum temporale (mimic those observed in humans)

• primates : R-hemis is specialized for rapid unimanual (1-sided) movements & the L-hemis is specialized for whole-body (2-sided) movements