Week 5: Lateralisation & language Flashcards
Why does contralateral control mean?
The left side of the brain controls the right side of the body and vice versa
How do the hemispheres communicate?
Through a large band of fibres known as the corpus callosum - white matter
Do we have a dominant hemisphere?
Yes e.g. right handed people are left dominant
What anatomical direction refers to the top of the brain?
DORSAL (superior)
Which anatomical direction is rostral?
FRONT (anterior)
What anatomical direction is caudal?
BACK (posterior)
Which anatomical direction refers to the bottom of the brain?
VENTRAL (inferior)
Looking at the spinal cord, what are the anatomical directions?
Dorsal refers to the back of the neck
Opposite to this (front?) is ventral
What is a frontal plane?
Brain is cut through the middle, leaving just the front regions
What is a horizontal plane?
Brain is cut horizontally, separating the frontal/parietal lobes from the temporal/occipital lobes
What is a Sagittarius plane?
Essentially cutting apart the hemispheres
What do asymmetries in the brain allow insight into?
Potential differences in function
What are some of the asymmetries seen in the brain?
Right hemisphere: enlargement of the anterior portion
Left hemisphere: Enlargement of wernickes area and parts of the thalamus - neurons in LH tend to have longer dendrites
What is white matter?
Axons or connections
Grey matter?
Where cell bodies are located
Explain regional differences in the corpus callosom
3 different parts:
Genu - at the front near the prefrontal cortex (anterior side)
Body
Splenium - at the back near the occipital lobes (posterior)
All connect different regions of the brain
What are ipsilateral connections?
Connections that stay within the same hemisphere - don’t cross the corpus callosom
What are homotopic connections?
Connections that cross over the CC to the same region of the opposite hemisphere
What are heterotopic connections?
Connections that cross over the CC to a different region on opposite hemisphere
Where do collosal connections start and finish?
The same layer of the neocortex that they start in e.g. start in layer 1, end in layer 1 (6 layers)
What is functional homotophy
Functional connectivity synchrony between brain areas in the different hemispheres
Explain empirical studies looking at synchrony across the CC
If you look at firing rates when watching light bars move across the visual field
When have two seperate light bars - one moving up, one moving down, the firing rate is not synchronised
When have just one bar, moving in same direction across both, brain regions are firing in synchrony - shows efficiency of CC
However, this condition doesn’t produce same affects if CC is severed - don’t get synchrony anymore
What are the 4 other commissures (not including CC)? and what do they make possible?
Anterior
Habenular
Posterior
Hippocampal
Means if CC is severed completely, there can still be some information crossing the hemispheres
What does the anterior commissure do?
- Plays a role in olfactory pathway
- Pain sensation
- Connects temporal regions
This information can still cross without CC
What does the posterior commissure do?
- Plays a role in pupillary light reflexes
- Right next to pineal gland
- May play a role in hormone system
Which commissure is right next to the posterior commissure?
The habenular commissure.
- Hormonal role
Hippocampal commissure?
Called the commissure of fornix
Connects the 2 halves of the hippocampus
Explain the Myers and Sperry experiment
- They wanted to restrict visual information to strictly 1 hemisphere in cats - cut CC, cut, optic chiasm and blindfolded one eye
- Control groups: cats with only either their CC or OC severed and blindfolded, learned the discrimination task at a normal rate - retaining this perfectly when blindfold was switched. When you only cut 1 of these 2 things, the animal behaves like normal
- Cats with both regions severed, learned the task when had one eye blindfolded, however when changed eyes, they had to completely relearn the task - all learning information remained on the same hemisphere
What conclusions were drawn from the Myers and Sperry experiment?
The cat forebrain has the ability to act as two seperate forebrains, each capable of independent learning and of storing its own memories
Why would we ever cut the CC in humans seeing as it is important?
Performed on patients with life threatening cases of epilepsy - remarkably effective in controlling seizures
Few obvious side effects though
Temporal lobe epilepsy - seizures start in TL and spread across whole brain
What information is still transferred if the genu of the CC is spared?
Transfer of higher order semantic info is spared
If there was damage from the splenium of the CC forward?
Tactile information is disrupted, but tactile information still gets across because the splenium is what is connecting the occipital cortex
What happens if there is damage to the posterior portion of the the CC (including Splenium)?
Disruptions to transfers of visual, tactile, and auditory information
What areas of the brain are involved in vision sensation?
Eyes Optic nerve Optic chiasm Lateral Geniculate nucleus (relay station in thalamus for visual info) Primary visual cortex
Explain a split brain patient scenario
Word ‘knight’ presented on the left visual field - once in right hemisphere, info crosses over to the left hemisphere and able to recite the word (normal brain)
Partial split in back back of CC - spare genu can get semantic meaning cross over. Can work out semantically and logically that the word presented is Knights - created images based on semantic properties that cross over
Complete split - say they didn’t see anything because the right side of the brain sees the stimulus bit the speech centre didn’t see anything
Explain the spoon-apple task of lateralisation
Spoon presented in the left visual field so goes to the right hemisphere
Apple is presented in the right visual field so goes to left hemisphere
When asked what you see - will say apple as this information is on the same side as speech centres (LH)
When asked to pick the item up with your left hand that is controlled by the right hemisphere, you will pick up the spoon as this is the information that is available for this motion
Face recognition disparities: what is the left hemisphere better at?
Better at recognising the self
Face recognition disparities: what is the right hemisphere better at?
Better at recognising familiar others
What is the right hemisphere specialised for?
Visuospatial processing
- mental rotation, spatial matching, mirror image task, block design performance (better with left hand - right hem)
What happens when the block task is attempted by both hands?
Seem to compete almost as if independent systems
How do we test language lateralisation?
Sodium amytal test
Dichotic listening task
Brain imaging
What is the sodium amytal test?
Done prior to brain surgery so that the surgeon knowns where speech originates to take care not to damage these areas
- Anesthetises the ipsilateral hemisphere by injecting a small amount of sodium amytal into the carotid artery on one side fo the neck, allowing the abilities of the contralateral hemisphere to be assessed
- When injection is on the side of the dominant speech hemisphere, the patient is totally mute for 1-2 minutes
What is a dichotic listening task?
Each ear presented with different words/phrases/numbers
The side that does better on this task is dominant
What is a limitation of using a dichotic listening task to determine hemispheric dominance?
The right hemisphere is better for melodies
Left hemisphere is better for letters
What is the hemispheric anatomy of language?
Language areas are typically left lateralised and are areas surrounding the sylvian fissure
There are some right hemisphere roles in language - anything to do with visualisation (rhythm, metaphors)
Which area is thought responsible for language production?
Broca’s area
Which area is thought responsible for language comprehension?
Wernicke’s area
What is anomia?
Difficulty finding words
What is dysarthria?
Difficulty using muscles controlled in speech
What is apraxia?
Impairment of motor planning and programming or speech articulation
What is aphasia?
Deficits in language comprehension or production
What is broca’s aphasia? And what problems may arrise from it?
Cannot produce speech
- trouble with spontaneous speaking
- trouble with repeating (dysarthria)
- listening for comprehension in certain situations (even though it is a production area - shows how areas are connected)
Wernicke’s aphasia is caused by….
Damages to connections from Broca’s area - even if wernickes area is still intact
(Angular gyrus, arcuate fasciculus)
Do people use one side of the brain more than other?
No. You use both sides of the brain equally no matter how creative or maths orientated you are
Is lateralisation only present in humans?
No it is present in many species, not exclusive to humans - maybe even invertebrates
Split brain animals?
Split brain monkeys show similar hemispheric interactions to humans in visual perception tasks
What are the theories of hemispheric lateralisation?
Motor theory
Linguistic theory
Analytic-synthetic
What does the motor theory propose?
Posits that the LH is specialised for fine motor movement (claims speech is an example of this movement specialisation, moving mouth and tongue to speak)
What evidence supports the motor theory?
Lesions to LH disrupt facial movements then RH lesions, even when not related to speech
The degree of disruption of non-verbal facial movements is positively correlated with the degree of aphasia
What is a limitation to the motor theory?
Does not suggest why motor function became lateralised in the first place
What does the linguistic theory say?
The primary function of the left hemisphere is language
What evidence is there for the linguistic theory?
Deaf people who communicate via sign language lose the ability to do so if they suffer damage to the left hemisphere - even if hold the ability to make the movements required to do so
What is the analytic-synthetic theory?
Suggests 2 fundamentally different modes of thinking - analytic and synthetic and the neural circuitry for each is fundamentally different.
As a result - the 2 are presumed to have segregated during evolution with analytic (logical, sequential, analytic) mode in the left and synthetic (makes immediate, overall synthetic judgements) in the right
What is a limitation to the analytic-synthetic theory?
It is really hard to test empirically
