Methods in cognitive neuroscience Flashcards
What experimental methods can be used with animals?
Single cell recording
Lesion methods
What is single cell recording?
Used with animals
- In vivo (living animals) orr in vitro (in a cell slice, kept alive with artificial CSF - given them glucose and oxygen etc.)
What is a lesion method?
- Damage (‘ablate’) brain area or impair its function and then observe the effect on task performance
Can determine whether an area ‘is involved’ in a task - However, not very specific as it could damage areas surrounding the specific area you ablate so the effect isn’t always reliable
What lesion methods are there?
- Aspiration - remove tissue (not so specific)
- Neurochemical - specific damage (more specific)
- Chemical cooling (reversible)
- Optogenetics (increase/reduce activity using light signals)
What are neuropsychology methods?
- More popular currently
- Look at behavioural, cognitive or emotional effects of damage occurring ‘naturally’ to the brain
- e.g. strokes, tumours, head trauma, neurodegenerative disease & neurological disorders – but PLASTICITY!
What is a double dissociation?
Where you have two people, one has different damage to the other, one has a problem with task x but not y, and the other has problems with task y but not x and so you can link the task problems with the damage
Why are single dissociations difficult to interpret?
- Lack of impairment on task X could reflect task difficulty (tasks X + Y both depend on area A, but task Y is more difficult - therefore more sensitive)
- There are several different explanations of impaired performance on a single task or in a single domain
- -> Task B could simply be much harder than Task A, so poorer performance in general could look like a specific problem with familiarity memory. But this could be a difficulty effect – and patients with temporal lobe lesions could simply be worse at tasks that are more difficult whereas their performance on easy tasks is preserved.
What do double dissociations indicate?
They are more informative and indicate that functions X and Y are independent and involve different underlying neural substrates
What would be done before brain surgery?
You would stimulate it with electrodes to test the lateralisation and find where the language centres are so that you don’t damage them (e.g. with epilepsy, look at where the language centres are so that you don’t damage them when cutting out the areas where the seizures are being caused)
What are the strengths of neurosurgery methods?
- Measure functions directly in human brain and can draw causal inferences – that the brain region stimulated is definitely involved in that process
- Patient can be awake and can report subjective sensations or experiences (e.g., vivid memories)
- No pain involved as no pain receptors in the brain
What are the limitations of neurosurgery methods?
- Since a lot of human cortex is ‘association’ – may not be clear what process is associated with brain area. So we may have to draw on prior knowledge
- All subjects in these experiments are undergoing procedures due to neurological disease (e.g., epilepsy or treatment-resistant depression). Could be damage or brain may have ‘changed’
- Therefore, not clear how generalizable the results are to the ‘typical’ brain – re-organisation of functions…
What does PET stand for?
Positron emission tomography
What do PET scans do?
- Radioactive substance, goes into brain, then see where it is coming from and can see what part of the brain is active
- PET measures changes in local blood flow correlated with mental activity, using radiotracers such as H215O
- Have to be exposed to radiation so cannot be used very often or it may increase risk of cancer
- Not used for cognitive neuroscience much anymore, but used to study binding in the brain (e.g. drug addicts – are the reward parts/ dopamine systems damaged? – and people wit Alzheimer’s)
What are the strengths of PET scans?
- Provides knowledge about which areas of the brain are associated with certain functions
- Reasonable information about localisation (though not great)
- Can be paired with specific radioligands to study receptor binding in brain (e.g. density of D2 dopamine receptors in addicts) or even complex molecules such as beta (β) amyloid
- Can be used to compare two different groups and the activity of their brains
What are the limitations of PET scans?
- Invasive (needle injection of radioactive tracer)
- Repeat scanning not permitted (within ~ 1 yr)
- Limited to ‘block design’ functional paradigms
unable to properly visualise individual trials
even less ecologically valid than fMRI
poor temporal resolution (minutes to seconds) - Expensive & limited to research centres with capacity to produce radiotracers
What does MRI stand for?
Magnetic Resonance Imaging
What do MRI scans do?
- Commonly used in hospitals (e.g. look for brain tumour or brain bleed after car accident)
- Only takes 4-5 minutes
- Different types of structural MRI scan
o T1 (anatomical): fast to acquire, excellent structural detail (e.g. white and grey matter and CSF)
o T2 (pathological): slower to acquire, therefore usually lower resolution than T1. Excellent for detecting swelling in the brain (tissue oedema)
What does VBM stand for?
Voxel-based morphometry
What do VBM scans do?
- Method which can be used with structural MRI data
- Morphometry methods can be used to investigate shape and volume of entire brain and individual brain structures (e.g. amygdala)
- Used to rely on manual segmentation of brain structures – but open to bias & time-intensive
- In the last 2 decades, fully automated methods such as VBM have been introduced – a whole -brain approach, where independent comparisons are made at every voxel in brain
What is a voxel?
- A 3D cube (e.g., 3 × 3 × 3 mm) that is the basic unit of structural or functional MRI analysis
- Depending on resolution of scanning sequence, could be 10,000-100,000 voxels in average human brain
What is normalisation?
Fitting brain into standard space & orientation
What is VBM group analysis?
- Next step is to build a template that combines all of the subjects’ brains (using DARTEL), then compare the brains of group A with group B
- Do group A have more grey matter volume?
- E.g. OCD patients have ↑ cerebellar (a) and ↓ insula (b) grey matter volume than controls
- Can only say there is an association between OCD and these findings
What are the strengths of VBM?
Unbiased approach which tests whole brain & can distinguish between grey versus white matter
What are the limitations of VBM?
- Grey matter volume is a useful measure but it is a composite of different properties of cortex
- Reductions in volume could be due to cortical thinning, lower surface area or less folding
- Highly dependent on normalisation process
- These criticisms have led to the development of surface-based morphometry (SBM) methods which distinguish between different cortical properties
What does surface-based morphometry look at?
- White matter (innermost)
- Grey matter (outer grey)
- The darker grey is the pial surface
What has surface-based morphometry been used to examine?
- Has been used to examine neuroanatomical basis of personality traits
E.g.: Found that neuroticism was positively correlated with prefrontal cortical thickness, but negatively correlated with cortical folding (aka local gyrification) in overlapping areas
What are the strengths of structural MRI scans?
- Can be used to measure the volume, shape and properties of different brain areas
- Can study brain development, recovery after brain damage, or experience-dependent plasticity
- Not dependent on task design and highly stable
What are the limitations of structural MRI scans?
- The relationship between volume and function is unclear – e.g., is greater GMV a good or bad thing?
- SBM only assesses cortical structure (VBM looks at entire brain)
- Less suitable than fMRI for studying networks in the brain – tells you about individual regions
What does fMRI stand for?
Functional magnetic resonance imaging
What are fMRI scans?
- Blood Oxygen Level Dependent (BOLD) signal
- Increase in blood flow to parts of the brain that the person uses to perform the task
- 45 mins
What is the BOLD response?
Blood oxygen level dependent signal
- Replenish what has been lost. Takes longer for our brain to respond
What does group mapping do?
- Blocks of static stimuli, blocks of moving stimuli and see what parts of the brain are active
- May be slightly different locations but the block should be around about the same.
- Contrast indicates that MT is more concerned with visual processing vs. visual stimuli in general
- Note: this contrast relies on subtraction logic – condition of interest matched as fully as possible with control condition
What must we be aware of with fMRI scans?
- Be aware that brain activity measured using fMRI cannot be separated from the task at hand
- Therefore, it is crucial that fMRI task is well- designed and the contrasts are meaningful
- In between-group studies (e.g. people with schizophrenia vs. controls), it is key that the task chosen actually elicits activity in the brain region hypothesised to differ in patient group
- E.g., if you think that the disorder affects the motor system (e.g. Parkinson’s), you wouldn’t use an auditory fMRI task to compare groups
What are some functional connectivity methods?
- Track relationships between activity in connected regions by measuring time series
- Can be exploratory (correlations between timeseries or PPI) or
- Can test strength or direction of known anatomical connection
What are the strengths of fMRI scans?
- Non-invasive technique & no radioactive chemicals involved so repeat scans possible
- Excellent spatial resolution (especially now, with 7 Tesla scanners becoming available)
- Provides measure of quantitative changes in specific brain areas
- A global technique. Can visualise neural networks in the brain, not just single brain areas & investigate functional connectivity
What are the limitations of fMRI scans?
- Poor time (temporal) resolution – can only study neural changes over seconds, so cannot discriminate different processes that happen close together in time (eg. language processing)
- Relies on subtraction knowledge
- Scanner is noisy & space is very limited inside
- Participants have to stay very still during scan
o Limited in ecological validity - Expensive form of research (£400-800 per hour)
o Therefore, numbers of participants are often limited – smaller sample sizes, limits populations
o Focusing on the imaging studies may take away funding from other studies
What does DTI stand for?
Diffusion Tensor Imaging
What is DTI?
A method of studying the white-matter connections in the brain – based on measuring movement of water molecules
What does TMS stand for?
Transcranial Magnetic Stimulation
What doe TMS scans do?
Mapping different parts of the brain but can also be used in a therapeutic sense (e.g. PTSD – prefrontal cortex isn’t working properly or is over active so you could use TMS over a block of sessions to cause a change in plasticity or synapses)
What are the strengths of TMS?
- An experimental (not correlational) method – can infer causality because effects of activating/inactivating brain region are immediate
- Temporarily impairs/enhances brain activity
- Informative both in terms of timing and location of cognitive processes
What are the limitations of TMS?
- Stimulation effects are very brief
- Unsuitable for tasks that involve lengthy processing- - - Restricted area of cortex that can be stimulated – only regions on the outside of the brain
- Relatively poor localisation / specificity
- Can induce headaches or skin rashes
What are EEG scans?
Oscillatory waveforms & event related potentials
- ERP (event-related potential) experiments
Continuous recording of electrical activity from the cortex
Have to use the same stimulus over and over because there is so much ‘noise’ but this could mean participants habituate to this
What does MEG stand for?
Magnetoencephalography
What do MEG scans do?
- MEG measures the magnetic fields naturally present outside the head due to electrical activity in the brain
- Sensors make no contact with scalp
- A good compromise between time and spatial resolution
- Very expensive and rely on rare elements which may be hard to find
What are the strengths of EEG and MEG scans?
- Non-invasive techniques – suitable for many populations, including babies and children
- Can differentiate between different neurological conditions or behavioural states (e.g. sleep stages)
- Good for investigating the time course of cognitive processes in the brain
o good for studying timing at a millisecond (ms) level and sequences of cognitive processes that contribute to a particular task (e.g. early vs. late visual processing)
What are the limitations of EEG and MEG scans?
- Can be time-consuming
- MEG is expensive, EEG isn’t
- Environment must be very quiet & shielded
- Poor on localisation of sources of activity:
o must locate a 3D source from 2D surface information
o tends to record more ‘global’ activity of the brain - However, fMRI can help determine the likely source of brain activity when combined with EEG/ERP/MEG methods
