Effects of brain damage and stimulation as a window into the mind Flashcards
the problem of causality
brain imaging makes it possible to examine the brain substrates of psychological processes
Can only correlate
non-essential activations
- 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 (see Multiple Demand system in the next lecture on Brain Anatomy)
- 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
determining causality
- The only definitive solution is to examine whether behaviour/performance is affected when the brain activity is disrupted in a particular area. - brain damage - know that damage has caused reduction in function
- In animals, lesions can be produced experimentally
determining causality in humans
- In humans, one can examine the effect of:
- Neurosurgery:
- Stroke:
- Brain trauma or tumours
- Neurodegeneration:
neuropsychology
- 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
major limitation of neuroimaging techniques
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
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 haemorrhage
neurodegeneration
degeneration of brain tissue in dementia
Infection of brain tissue
when did neuropsychology emerge?
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
Broca’s area and speech
- 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
split brain research
- Sperry and Gazzinga
- Studies patients who underwent callosotomy: cutting through (severing) of the Corpus Callosum to limit the spread of epileptic activity from one brain hemisphere to another
- Picture to LVF - say nothing is there but draw saddle with left hand
reversible numbing of the left hemisphere via the sodium amytal injection (WADA test)
- Chemicals out one hem to sleep
- Language usually on left
- Right hem cannot verbalise and left hem cannot detect it
- The localisation of language is heavily biased towards the left hemisphere
- Split-brain studies and Wada test studies have shown that the linguistic competence of the right hemisphere is very limited
hemispatial neglect
Damage to right parietal lobe
can see everything
stop attending to left side visual field
difference between seeing and attending to something
temporal lobe amnesia
- Brenda Milner (1918)
- Patient H.M. -the most famous clinical case in the human memory literature (first reported by Brenda Milner).
what did HM have to treat his epilepsy?
bilateral, medial temporal lobe resection.
what happened to HM after surgery?
the epilepsy was greatly improved but H.M. showed a nearly total, profound amnesia that persisted for his entire life
what did HM have?
- 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
neuroimaging key approaches
- One may want to run imagining experiments to see where a certain task/function is localised in the brain
Or… - One may be interested in whether a certain task/condition in a qualitatively different pattern of activation from another condition - regardless of precisely which areas happen to be activated
- Some argue this approach is more useful for psychologists because it can tell one whether the two experimental conditions rely on the same processes or on qualitatively different processes
- More interested in how
neuropsychology key approaches
- Some research is concerned primarily with localisation
- This approach is often referred to as classical neuropsychology tradition
- Other research is primarily concerned with determining the cognitive architecture by identifying behavioural performance that relies on qualitatively similar or on qualitatively different processes, regardless of the exact location of damage
- This approach is often referred to as cognitive neuropsychology and it relies on the logic of dissociations
what can neuropsychological data be used for? - the logic of dissociations
to test theories about the architecture of psychological processes even without knowing the exact location of the damage
example of using neuropsychological data and dissociations
- Suppose one wants to investigate the psychological processes involved in the recognition and writing of graphemes - 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
examples of dissociations
- Indeed, the Italian neuropsychologist Roberto Cubelli (Cubelli, 1991) found this patter in two patients
- On of them could write consonants, but left gaps where there were vowels
- The other made spelling errors in mostly on vowels, e.g. diatro instead of dietro (“behind”).
single dissociation
- However, at close scrutiny this 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 (read and written)
- 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
double dissociation
- Yet, such evidence has been provided by Kay and Hanley (1994)
- They identified a patient, who made more spelling errors in consonants than vowels
- 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
what is the gold standard of experimental psychology?
(be it in behavioural or imaging studies) is to analyse the data at the level of groups of subjects rather than individuals
group level analysis
reduces the contribution of irrelevant factors (idiosyncrasies associated with individual stimuli or individual participants) and emphasises the effect of the experimental manipulation
may not always be possible in neuropsychology
- Suppose one finds individual patients with very specific effects of brain damage and other similar cases are very rare
- In such circumstances, it seems appropriate to report single-case studies, in which the pattern of performance in one or a small number of individual patients are reported
- … at least until other similar cases are identified.
modern structural brain imaging
enables one to identify the degree of overlap between the regions of damage in different patients and compute the area of maximal overlap
strengths of neuropsychology
The obvious advantage of neuropsychology over electrophysiology and neuroimaging is that it enables causal inference
limitations 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
magnetic brain stimulation: early days
- From what we know now, it is likely that they caused direct stimulation of the retina, thus producing the visual sensations!
- The magnetic field strength from these systems was too low for brain stimulation to occur.
the neurophysiology of TMS
- 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
effects of TMS over the primary motor and visual cortices
- 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” - disruption visual field
- The shape and size of the phosphenes are thought to depend on the exact site, strength, duration and timing of stimulation
- Can only stimulate cortex
effects of TMS on performance
- A TMS pulse typically induces a brief increase in excitability, often followed by a more sustained reduction in excitability
- The induced activation causes 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
the neurophysiology of TMS: macroscopic response
- Evoked neuronal activity (EEG)
- Changes in blood flow and met (PET, fMRI, NIRS, SPECT)
- Muscle twitches (EMG)
- Changes in behav
TMS spatial resolution
- Typically 10-20 mm; 5-10 mm at best
- Influenced by: distance from the scalp, connectivity between target region and adjacent regions
TMS: temporal resolution
- 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
- Stimulating specific brief moments impairs performance
- TMS clearly capable of telling when the targeted area was involved in processing
- So- the temporal resolution is high
- Need to know where and when to stimulate
can TMS improve performance?
- Yes
- The target process is facilitated when a competing process is disrupted
- Walsh, Ellison, Battelli and Cowey (1998): stimulating motion processing area V5 improves colour discrimination
what can we infer from the effects of TMS?
- Functional-anatomical inference: is cortical area x essential for performing a given task?
- Chronometric (temporal) inference: at what time t does stimulation affect performance relying on a hypothetical psychological process x?
- Process interaction inference: if disrupting process x increases effectiveness of process y, it shows the two processes normally compete
advantages of TMS
- It shares the key advantage of neuropsychology: the capacity to determine causation
- It has relatively good spatial resolution, it is typically much more anatomically selective than neurological damage
- It has excellent temporal resolution (due to the brevity of the TMS pulse), hence it is capable of chronometric inference - safe
- Because it is reversible, it allows different conditions to be compared within the same group of subjects- always preferable to comparing conditions over groups
control conditions for TMS
- Need to control for somatosensory and auditory effects of TMS
- 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
limitations of TMS
- 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)
- 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)
- 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
