Lecture 3: Effects of Brain Damage and Brain Stimulation as a way of Studying the Brain Flashcards
1
Q
What is the problem of causality in brain research?
A
- Brain imaging makes it possible to examine the brain substrates of psychological processes
- However neuroimaging techniques, such as fMRI and PET (and any other brain measurement) suffer from one serious limitation.
- The fact that some brain activity is associated with a task/hypothetical psychological process does not mean that the activity causes the observed behaviour or hypothesised psychological process
2
Q
What are some non-essential activations of brain research?
A
- Some brain regions may be involved in the learning of any new task, but they may not be required once the task has been learned
- Some brain areas are recruited as a ‘back-up’ in case processing requires extra resources or effort
- Some process A essential for the task may often co-occur with another process B that is not essential for the task
- For instance, reading on a computer/laptop often goes hand in hand with typing on it, so a task of reading on a computer screen may elicit activations related to typing but typing is unlikely to be causally related to reading performance
3
Q
How can we determine causality?
A
Examine whether behaviour is affected when brain activity is disrupted in a particular way
4
Q
In humans we can determine causality through the effect of:
A
- Neurosurgery: removal of brain tissue for treatment of neurological or psychiatric disorders, most often epilepsy
- Stroke: cerebrovascular accident resulting in the disruption of blood circulation in the brain and/or brain haemorhage
- Brain trauma or tumors
- Neurodegeneration: degeneration of brain tissue in dementia
- Infection of brain tissue
5
Q
In animals we an determine causality by
A
Producing lesions experimentally
6
Q
What is neuropsychology?
A
- Neuropsychology is the area of psychology that examines the effects of brain damage on abilities and behaviour.
- If damage to a particular brain region/structure is systematically associated with a certain cognitive impairment, that region/structure is NECESSARY for the cognitive process to function.
- Therefore, that brain region must be (part of) the anatomical substrate for the given cognitive process
- Neuropsychology is thought to have emerged in the 19th century when the French neurologist Paul Broca identified post-mortem that damage to an area in the inferior frontal cortex (now referred to as Broca’s area) was the likely cause of a severe language impairment in one of his patients
7
Q
What is Clinical Neuropsychology?
A
- Clinical neuropsychology is the applied clinical variant of neuropsychology.
- They are experts on the behavioural and emotional consequences of brain damage.
- Clinical neuropsychologists assessing the effect of brain damage in patients. Diagnosing neural disorders and help patients and family adjust.
- They work in hospitals, care homes and rehabilitation centres
8
Q
What is Broca’s area?
A
- Broca studied brains of patients who had impaired speech (aphasia)
- One of his patients (Mr Leborgne) was nicknamed Tan for his inability to utter anything other than “tan”
- In 1861, through post-mortem autopsy, Broca determined that Tan had a lesion caused by syphilis in the left inferior frontal lobe
- Subsequent research has confirmed that lesions to this area indeed often result in language impairments
9
Q
What is Wernicke’s Aphasia?
A
- The ability to comprehend the meaning of words is highly impairded. Reading and writing often is as well.
- They often use sentences but with the wrong words or non existent words
10
Q
What is the corpus callosum?
A
White matter tracts (numerous acons) connecting the two hemispheres
11
Q
What is a callosotomy?
A
Severing the corpus callosum to limit the spread of epileptic activity from one brain hemisphere to the other
12
Q
What is the Wada Test?
A
Reversible numbing of the left hemisphere via the sodium amytal injection
13
Q
What have split-brain studies and wada test studies shown us?
A
That linguistic competence of the right hemisphere is very limited
14
Q
What is temporal lobe amnesia - Patient H.M.?
A
- Patient H.M. -the most famous clinical case in the human memory literature (first reported by Brenda Milner). To treat severe epilepsy, at age 27, he received bilateral, medial temporal lobe resection.
- After surgery, the epilepsy was greatly improved but H.M. showed a nearly total, profound amnesia that persisted for his entire life.
- H.M. had profound anterograde amnesia - he formed almost no new episodic memories following surgery. Despite years of memory testing experiments, he had no memory of them
- H.M. had partial retrograde amnesia - he recalled his early childhood but not the years immediately before the surgery
- He could recall early childhood, suggesting that his ability to retrieve memories might have been spared
- Working memory was relatively normal - six numbers could be remembered with constant, uninterrupted rehearsal
- His procedural memory and lexical memory (which support skills such as writing and the memory for words) were close to normal
15
Q
What is hemispatial neglect?
A
- Inattention to parts of the visual field
- Affects up to 2/3 of right hemispheric stroke patients
- Can differ from very mild to almost complete
- Can strongly affect one’s independence
- Crucially different from blindness and cortical blindness
16
Q
What are the symptoms of hemispatial neglect?
A
- Only attend to things on the right
- Move in the opposite direction from you if you come from the neglected side
- Problems reading
- Ignoring objects in their environment
- Problems navigating space
- Not using particular limbs
- Lack of insight
17
Q
What is the damaged area in neglect?
A
The parietal lobe therefore seems crucially involved in regulating attending to things
18
Q
Why don’t we know what is happening in neglect?
A
- The lateralization to the left side of the visual field suggest to some to indicate that attention is inherently biased to the right and that some function bringing it leftwards is impaired
- Another possibility is that something has gone wrong in the internal representation of space in the brain (some of it is missing and therefor not used)
- Another possibility is that initiation of the motor system in certain directions is impaired
- It is possible that neglect is caused by a multitude of problems.
19
Q
What are the key approaches in neuroimaging?
A
- You may want to run imaging experiments to see where a certain task/function is localised in the brain
Or…
- You may be interested in whether a certain task/condition has a qualitatively different pattern of activation from another condition - regardless of precisely which areas happen to be activated
20
Q
What is the logic of dissociations?
A
- Neuropsychological data can be used to test theories about the architecture of psychological processes even without knowing the exact location of the damage
- Suppose one wants to investigate the psychological processes involved in the recognition and writing of letters
- One key question that can be asked is whether the recognition and writing of vowels (e.g. a, e) rely on different psychological process from the recognition and writing of consonants (e.g. t, r)
- Say, brain damage impairs processing of vowel letters but spares processing of consonant letters
- This dissociation may indicate that the two classes of letters are processed differently
- Note that determining the exact location of damage in the brain is not crucial for the above inference
21
Q
What did Roberto Cubelli (1991) find about dissociations?
A
One of them could write consonants, but left gaps where there were vowels. The other made spelling errors in mostly on vowels instead of behind.
22
Q
What is single dissociation?
A
- However, at close scrutiny the above single dissociation is not sufficient for drawing the conclusion that there is a qualitative difference between how the mind represents vowels and consonants
- For instance, it is possible that the same mental computations are used for both, but suppose that consonant letters are easier to differentiate visually from each other than vowel letters
- This could make consonants more resilient to the effects of brain damage, but would not necessarily demonstrate a qualitative difference in the way they are processed
- But - if consonants are generally more resilient to the effects of brain damage, one should not find any patients with impaired processing of consonants and relatively spared processing of vowels
23
Q
What is double dissociation?
A
- The existence of such opposite patterns is referred to as double dissociation
- A double dissociation is hard to explain as a quantitative difference, where one type of item (here vowels or consonants) is generally more resilient to the effects of damage
24
Q
What are the strengths of neuropsychology?
A
The obvious advantage of neuropsychology over electrophysiology and neuroimaging is that it enables causal inference
25
What are the disadvantages of neuropsychology?
- The main drawback is that lesions resulting from trauma or neurological degeneration are rarely anatomically selective- they tend to affect multiple brain regions/structures
- Also, brain damage is always associated with general cognitive, emotional and personality changes whose effect on cognitive performance is very considerable and difficult to separate from the effects of damage to a specific region/structure
- Could one disrupt the brain selectively, reversibly and safely?
26
What is the Neurophysiology of transcranial magnetic stimulation?
- A large current is briefly discharged into a coil of wire held on the subject’s head.
- The current generates a rapidly changing (increasing) magnetic field around the coil of wire and this field passes into the brain.
- In the cortex, the magnetic field generates electric (ionic) current through neurons’ membranes
- TMS over the primary motor cortex induces muscle contractions resulting in finger twitches
- TMS over the primary and secondary visual cortex results in the perception of flashing patterns- “phosphenes”
- The shape and size of the phosphenes are thought to depend on the exact site, strength, duration and timing of stimulation
27
What are the effects of transcranial magnetic stimulation on performance?
- A TMS pulse typically induces a brief chaotic increase in neural activity often followed by a more sustained reduction in excitability
- This results in a disorganisation of neural activity, typically resulting in impaired performance
- Thus, the effect is similar to that of neurological lesion, only mild, reversible and safe
- For the reasons above, TMS is often referred to as a **virtual lesion** technique
28
What is the spatial resolution of transcranial magnetic stimulation?
- Typically 10-20 mm; 5-10 mm at best
- Influenced by: distance from the scalp, connectivity between target region and adjacent regions
29
What is the temporal resolution of transcranial magnetic stimulation?
- Subjects presented with sets of three letters and asked to report the letters on each trial
- On each trial, TMS applied over the visual cortex at a slightly different time
- Specific times affect performance, others do not
- TMS clearly capable of telling when the targeted area was involved in processing
- The effects of a single TMS pulse on behaviour are rather brief
- So- the temporal resolution is high
30
What can we infer from the effects of transcranial magnetic stimulation?
- Functional-anatomical inference: is cortical area ***x (or the connected network)*** is essential for performing a given task
- Chronometric (temporal) inference: at what time ***t*** does stimulation affect performance relying on a hypothetical psychological process ***x***?
31
What is a suitable control condition for somatosensory and auditory effects of transcranial magnetic stimulation?
- No TMS is not enough as a control, because at least some of the effect of TMS is due to the noise and sensation it elicits
- Sham TMS- noise but no stimulation: does not control for somatosensory component (the sensation on the scalp, muscle twitches, discomfort)
- Better: control site (over area that is unlikely to be involved in task)
- Sometimes, it is difficult to ensure that control site has equivalent somatosensory and auditory effects to test site
32
What are the limitation of transcranial magnetic stimulation?
- Although it is generally very safe, it is associated with a small risk of eliciting a seizure. To minimise the risk, low levels of stimulation is used; participants are carefully screened
- The effects of TMS on behaviour/performance are much more subtle (and hence can be harder to detect) than the effects of neurological damage (in patients)
- The effects of TMS on the brain are limited to the cortex- TMS cannot ‘reach’ deeper cortical and subcortical regions/structures (e.g. hippocampus, thalamus)
