S7) 'Higher' cortical function Flashcards
What is the cerebral cortex?
A thin superficial layer of grey matter on the cerebral hemispheres.
Arranged as 6 layers containing cell bodies and dendrites (i.e. cortex is grey mater)
Information is processed in the complex synaptic network found between inputs and outputs.
Describe the fine structure of the cortex in terms of inputs and outputs
- Most inputs (sensory) are from the thalamus and other cortical areas (the cortex can stimulate itself- there are recurrent feedback loops where the cortex provides its own input e.g. when 1 thought that arises from the cortex leads to another thought).
- Most outputs (motor) are from the axons- pyramidal cells and project to widespread areas
Most outputs from the cortex are the axons of pyramidal neurons. Identify 3 examples of what the outputs can be.
– Outputs can be projection fibres going down to brainstem and cord (e.g. upper motor neurones)
– Outputs can be commissural fibres going between hemispheres (e.g. corpus callosum)
– Outputs can be association fibres connecting nearby regions of cortex in the same hemisphere (e.g. arcuate fasciculus)
How is information processed between inputs and outputs?
Information is processed in the complex synaptic network found between inputs and outputs.
Interneurones connect inputs and outputs in a complex way, giving rise to behaviour, emotion, memory etc.
What did phrenologists believe?
Phrenologists believed that personality traits and tendencies to crime etc. could be correlated with the size of bony prominences on the skull. This has now been discounted but was an early inspiration for cortical localisation
What we have now found is that various regions of the cortex have diff functional properties e.g. occipital lobe for vision, temporal lobe for memory
Identify 6 functions of the frontal lobe
- Motor - primary role, it contains the primary motor cortex - contains the motor homunculus - has cell bodies of UMN that activate the various muscle groups
- Expression of speech (usually left hemisphere of frontal lobe) - contains Broca’s area
- Behavioural regulation / judgement
- Cognition - mathematical ability/ frontal lobe tests
- Eye movements - frontal eye field - regulates eye movement
- Continence (medial) - paracentral lobules are vital for regulation of micturition. - damage here = urinary incontinence e.g. in Alzheimer’s disease.
The primary motor cortex - if there is damage here, what can happen?
Frontal lobe damage can result in contralateral weakness
If have damage to Broca’s area found in the left frontal lobe, what can it result in?
Damage to left frontal lobe can result in expressive dysphasia
Where is behaviour regulation/ judgement found specifically in the frontal lobe? What occurs if there is damage here?
– Prefrontal cortex
– Damage to frontal lobes can lead to (usually) impulsive, disinhibited behaviours e.g. sexual inappropriateness, aggression
Where is cognition found specifically in the frontal lobe? What occurs if there is damage here?
– Prefrontal cortex
– Frontal lobe damage (particularly the right) can cause difficulty with tasks such as complex problem solving, including calculation
The frontal lobe is important for eye movements. It contains the frontal eye fields. What occurs if there is damage here?
Damage can cause problems with conjugate gaze and other eye movement disturbances (however, diplopia without other cortical features would suggest brainstem/cranial nerve problem)
The frontal lobe contains cortical areas responsible for maintenance of continence. Identify one of these areas and what can be seen if there is damage?
– Paracentral lobules
– Damage can cause urinary incontinence
How might one test the higher cognitive ability of the frontal lobe?
Ask a patient to count down in 7s from 100
Identify 5 functions of the parietal lobe
- Sensory - contains the primary sensory cortex
- Comprehension of speech (usually left hemisphere) - Wernicke’s area
- Body image (usually right hemisphere) - lesions here can lead to body dysmorphic disorders
- Awareness of external environment (attention)
- Calculation and writing
- The visual pathways projecting through white matter - superior optic radiations pass through the parietal lobe on their way from the thalamus to occipital lobes
What occurs if there is damage to the primary sensory cortex at the parietal lobe?
Damage might result in contralateral anaesthesia affecting all modalities (modalities converge at the cortex)
Which part of the parietal lobe is comprehension of speech found and what can occur if there is damage here?
– The parietal lobe contains part of Wernicke’s area
– Damage to left parietal lobe can cause a receptive dysphasia
Body image and awareness of external environment is a function of the parietal lobe. What can occur if there is damage to the right parietal lobe?
Damage to right parietal lobe can lead to neglect.
Calculation and writing is a function of the parietal lobe. It works with the frontal lobe to perform these tasks. What can occur if there is damage to the left parietal lobe?
Damage to left parietal lobe can affect calculation ability (but maybe also frontal lobe)
What is hemispatial neglect?
- Hemispatial neglect is a neuropsychological condition occurring due to damage to one hemisphere of the brain
- There is a deficit in attention to/awareness of one side of the visual field and the patient is unable to process and perceive stimuli on one side of the body or environment
Although not cortical function, remember that the superior optic radiation projects through the parietal
lobe! Knowing this, if damage occurs here, what can it result in?
Damage here can cause a contralateral inferior homonymous quadrantanopia
Identify 4 functions of the temporal lobe
- Hearing - contains the primary auditory cortex - superior surface of temporal lobe, near to Wernicke’s area
- Olfaction - contains the primary olfactory cortex - infero-medial surface of temporal lobe
- Memory - deep to temporal lobe = hippocampus (lays down new memories)
- Emotion
- Visual pathways projecting through white matter - contains 3rd visual fibres going from Lateral geniculate nucleus to the occipital lobe .: temporal lobe lesion can lead to a visual field defect.
What can occur if there is damage/ lesion to the primary auditory cortex?
Damage can lead to a number of complex effects on hearing which are not the remit of the unit.
Auditory hallucinations may be a feature of temporal lobe lesions
What can occur if there is damage to the primary olfactory cortex?
Damage can lead to a number of complex effects on smell which are not the remit of the
unit.
Olfactory hallucinations may be a feature of temporal lobe lesions
Which part of the temporal lobe is vital for consolidating declarative memories and what can occur if there is damage here?
– The hippocampus is a crucial structure for consolidating declarative memories
– Damage may lead to amnesia (but remember that there are two hippocampi, one in each temporal lobe)
– Also, some pathologies such as temporal lobe epilepsy can trigger memories, leading to a feeling of deja vu
Which part of the temporal lobe is responsible for emotion? What are the effects of lesions found here?
– Temporal lobes contain a number of limbic system structures such as the hippocampus and amygdala
– Effects of lesions are complex, but may be related to pathogenesis of some psychiatric disorders
Although not a cortical function, remember that the inferior optic radiation projects through the temporal lobe. If damage occurs here, what can it cause?
Damage here can cause a contralateral superior homonymous quadrantanopia
A caveat about interpreting clinical data concerning brain lobe function
What can give us clues about ‘global’ cortical function?
In terms of cerebral dominance, state which form of processing is over-represented in the different cerebral hemispheres (in 95% of people)
- Left: sequential processing
- Right: ‘whole picture’ processing
Knowledge of cerebral dominance allows us to predict the effects of lobe lesions (e.g. a dysphasia is likely to have arisen from left hemisphere damage)
Sequential processing is dominant in the left hemisphere.
Which two functions does this include?
- Language
- Mathematics/logic
Whole picture processing is dominant in the right hemisphere.
Which four functions does this include?
- Body image
- Visuospatial awareness (related to the disorder: neglect. where if you have a right parietal lobe lesion → fail to acknowledge existence of left ½ of the world -because the right parietal lobe tends to both halves of the space whereas the left parietal lobe only tends to the right ½ of the space. Damage to the right parietal lobe → loss of attention to the left half of the space)
- Emotion
- Musical ability
In 95% people, which hemisphere is dominant?
How do the 2 hemispheres differ in terms of which half they control?
Left hemisphere is dominant in 95% of people (think about how it controls the right hand in most people)
The right hemisphere ‘attends’ to both halves of space but the left hemisphere only ‘attends’ to the right half of space
What is the corpus callosum?
A huge bundle of white matter connecting the two hemispheres.
An early surgical treatment for severe epilepsy was to cut the corpus callosum to prevent seizure activity from propagating through the whole brain. This had some interesting neuropsychological consequences for those patients…
What could happen if there was damage to the corpus callosum?
Strangely, we don’t get major obvious deficits - it is possible that other pathways can take over from it.
Destruction of the corpus callosum can cause some interesting deficits such as alien hand syndrome and subtle effects on language processing
Language pathways are primarily found in the left hemisphere.
In light of this, where is Broca’s area and what does it do?
Broca’s area - inferolateral frontal lobe– site of speech production
(sits near to the mouth/ pharynx area of primary motor cortex- close to the facial regions of the homunculus to economise the amount of wiring needed to connect the two together)
If there is damage to the Broca’s area, what can it cause?
Damage can cause staccato speech, where the patient still understands what is being said to them (Broca’s / expressive dysphasia)
Language pathways are primarily found in the left hemisphere.
In light of this, where is Wernicke’s area and what does it do?
Wernicke’s area - superior temporal lobe/ at the parieto-temporal junction – interpretation of language/ comprehension of speech - sits near the primary auditory cortex in the temporal lobe
If there is damage to Wernicke’s area, what can it cause?
Damage can cause fluent, nonsensical speech where the patient does not appear to understand what is being said to them (Broca’s /receptive dysphasia)
What can a large middle cerebral artery infarct result in terms of the Broca’s and Wernicke’s area?
Large middle cerebral artery infarcts can cause global aphasia where both areas are destroyed leading to virtually no verbal language function
How are Broca’s area and Wernicke’s area connected?
Broca’s area and Wernicke’s area are connected to each other via the arcuate fasciculus
Describe the arcuate fasciculus.
White matter which connects Broca’s and Wernicke’s area.
Most unidirectional from Wernicke’s to Broca’s area.
but can be bidirectional.
Damage to the arcuate fasciculus can lead to ….
Damage to this white matter pathway can cause the inability to repeat heard words
Language pathways overall summary
Describe the pathway for repeating a heard word
Cochlea → auditory cortex → Wernicke’s area → arcuate fasciculus → Broca’s area → primary motor cortex
Describe the pathway for speaking a written word
Eyes → visual cortex → Wernicke’s area → arcuate fasciculus → Broca’s area → primary motor cortex
Describe the pathway for speaking a thought
All over the brain → Wernicke’s → arcuate fasciculus → Broca’s area → motor cortex
What is Wernicke’s aphasia (aka receptive aphasia)?
Wernicke’s aphasia is form of aphasia wherein the ability to grasp the meaning of spoken words and sentences is impaired, while the ease of producing connected speech is not very affected
How does a person with Wernicke’s aphasia present?
- speaks very fluently
- no hesitations
- but what they say makes no sense
- the person does not understand what others say
- have an intact Broca’s area .: able to produce speech and speak fluently
- but non intact Wernicke’s area where they are not able to understand what is being said hence the answers were nonsensical.
What is Broca’s aphasia (aka expressive aphasia)?
- Broca’s aphasia is a form of aphasia wherein a patient has trouble speaking fluently but their comprehension can be relatively preserved
- Patients have difficulty producing grammatical sentences and their speech is limited mainly to short utterances of less than four words
How does a person with Broca’s aphasia present?
- Hesitation, paucity of words
- not able to say the words - struggling to get answers → problems with Broca’s area (speech production problem)
- No problem with understanding the questions being asked .: Wernicke’s area intact
What types of memory are there?
- Declarative – explicit, factual information
- Non declarative – implicit, motor skills and emotions
Both types of memory are stored in different areas .: a lesion in one, would mean sparing of the other.
Where in the brain is memory stored?
Memories are thought to be stored in a relatively ‘distributed’ fashion throughout large areas of the brain
- Declarative – cerebral cortex (in a distributive fashion)
- Non declarative – tends to be stored in subcortical structures: cerebellum + basal ganglia
How are memories stored?
Short term memory (seconds to minutes) becomes long term memory (up to a lifetime) when it is consolidated.
Consolidation depends on emotional context (e.g. traumatic events - less recall), rehearsal (e.g. revising using flashcards/ notes) and association
Consolidation is needed to convert short term memories into long term memories.
What are the factors influencing consolidation?
o Emotional context (if an event has strong emotional content, then it tends to be remembered better)
o Rehearsal (you are all familiar with this idea)
o Association (if you can associate a piece of knowledge with something you already know it tends to be more easily remembered)
How is short term memory vs long term memory stored?
Short term memory:
Stored for seconds to minutes as a ‘reverberation’ or ‘echo’ in cortical circuit
Long term memory:
Stored for very long periods in the cerebral cortex, cerebellum etc. (up to a lifetime) following consolidation
Which part of the brain is responsible for declarative memory consolidation?
The hippocampus is crucial for consolidating declarative memories
The hippocampus receieves multiple inputs. What are they?
Visual system
Auditory system
Somatosensory system
Limbic system
.: It has multimodal inputs from many brain systems (making it good at associating stimuli)
The hippocampus also has a role as an ‘oscillator’, facilitating consolidation of memories in the cortex via its output pathways. What is the pathway?
primarily the fornix → mammillary bodies → thalamus → cortex)
Neuroplasticity: the molecular and cellular mechanisms of memory ..
What is the key molecular mechanism of memory consolidation?
Long term potentiation (LTP) is the key molecular mechanism of memory consolidation
→ Causes changes in glutamate receptors in synapses leading to synaptic strengthening
→ New physical connections can also form between neurones to further strengthen connections (axonal sprouting)
Learn this table.
