Week 4, Lecture 4- Methods of Cognitive Neuroscience 2 Flashcards
Explain MRI & fMRI, Describe lesion studies, TMS, & tDCS
What are MRI’s (structural)?
Collection of static images with slices in the sagittal, coronal, horizontal (axial) planes.
How do MRI’s (structural) work
- Through contrast
The Physics: - Most soft tissue of the body is water-based
- Water molecules (H2O) contain single protons in hydrogen atoms
- Protons have weak magnetic fields
(My explanation- A magnet pulls protons towards it then a radio frequency interrupts this pull causing them to turn away from the magnet, the speed at which these protons turn back toward the magnet after the radio frequency turns off tells us which tissues are which) - On imaging: Water appears dark and fat appears bright.
Give an example of when structural MRI’s have been used.
- When researching the size of the hippocampus in London taxi drivers.
- Researching Alzheimers disease
What are Functional MRI’s (fMRI)
- They are a brain scan that uses a measure that uses comparison to look at ‘active’ brain regions.
- They use cognitive subtraction
What is cognitive subtraction?
- Brain activity during a baseline task is subtracted from brain activity during an experimental task
- The brain has a constant supply of blood and oxygen (if it didn’t, it would die); To infer from fMRI we need to compare relative differences in brain activity (Blood supply) between two or more conditions. This is done using BOLD.
Explain Blood Oxygen Level Dependent (BOLD) Signal.
It is an indirect measure of neural activity
* Neural activity consumes oxygen:
- Oxyhemoglobin is converted to deoxyhemoglobin.
- If oxygen consumption increases, more blood needs to be pumped into the region
* BOLD signal
-Deoxyhemoglobin distorts local magnetic field
- Higher BOLD signal = greater proportion of oxygenated to deoxygenated blood (MY EXPLANATION- A stronger BOLD signal means there’s more oxygen-rich blood (oxyhemoglobin), which usually means higher brain activity.
Explain the hemodynamic response function.
It is a model that describes how blood flow moves in the brain.
There is an initial dip of BOLD signal intensity, then there is an overcompensation of BOLD signal intensity, interpreted as brain region activity leading to an undershoot. This all is processed in a time of around 14secs, the hemodynamic response function peaks in 6-8 seconds (poor temporal resolution). This poor temporal resolution is stated because measurement is not straight away (We see he BOLD signal only at 6-8secs so we can interpret brain activity a little late.)
**See slides 18 +19 for diagrams
What have fMRIs shown us about brain function specialisation.
The part of the fusiform gyrus (Fusiform Face Area (FFA)) that has shown us to potentially be the part of the brain that reacts to facial recognition.
Explain the fMRI analysis pipeline.
- Correct for head movement (any movement the participant made is cancelled out so the brain can be accurately measured)
- ‘Normalization’ = individual brain scans are mapped onto a standard reference brain (saying that you put a persons brain scan onto an average brain ‘template’ so we know which areas are which)
- ‘Smoothing’ = increase signal to noise ratio (averages within/across nearby voxels) (reducing external noises like talking or cars outside so the true brain activity is easier to see) (voxel represents a tiny volume of the brain where the scanner measures the blood flow)
- Then… start statistical analyses
What are the strengths of MRI’s/fMRIs?
- Non-invasive
- Good spatial resolution (how clear)
- Lots of data!
What are the weaknesses of MRI/fMRI?
- Poor temporal resolution (Takes a while to measure potentially leading to inaccuracy)
- Some brain regions are harder to
image - Scanner is loud!
Not for everyone: - Not for metal implants or people who are claustrophobic
- Need to stay still – special populations, e.g., small children
- Relatively expensive & resource heavy
- Lots of data processing
What are the limitations (caveats) of fMRI?
- More activation =/= more processing
- Shows which regions are involved but not necessarily
those which are crucial or necessary
What are Functional near-infrared spectroscopy
(fNIRS).
- Affordable
- Also great for portability & movement
- Uses near-infrared light to measure BOLD signal
- Measures how much light scatters
- Can only measure activity close to scalp
(As the light moves through the brain, it interacts with blood, and the amount of light that gets scattered or reflected back to a detector depends on how much blood is in the area.)
Explain Positron Emission Tomography (PET).
- Radioactive tracer injected
- Tracer accumulates in different tissues depending on metabolic/biochemical activity
- Scanner detects radiation in different areas emitted by tracer
What are lesions?
Neurosurgery
* Strokes
* Traumatic head injuries
* Tumors
* Viral infections
* Neurodegenerative disorders
What are the challenges of looking at lesions for research on brains?
- Some forms of brain damage (e.g., tumours, swelling) make it hard to localise the lesion
- Danger in concluding that a function is localised in a region, or the purpose of a region is to support that function
- Does the region perform other functions?, Do other regions support this function?
Explain single dissociation vs double dissociation.
Single dissociation
* A lesion patient can’t do task A but can do task B
* Could infer lesion area is involved in task A
* BUT…could be task demand artifact (people think the task is about something so they behave in ways that influence the results) or task resource artifact (A resource artifact happens when two different tasks you’re studying end up using the same mental resources or brain area)
* Double dissociation
* Two lesion patients with different single
dissociations with complementary profiles
* Patient #1 can do task B but not task A; Patient #2 can do task A but not task B
* More robust evidence for different neural processes underlying tasks A and B
Why do imaging and lesions not always give us a clear idea of where brain functions occur?
As the area can be active on fMRI but the lesion has no impact on performance: This could be due to a:
* Strategy adopted by participants
* Non-specific general cognitive resource, such as attention (When you’re paying a lot of attention, some brain areas (like the prefrontal cortex, which is involved in focusing and planning) will “light up” on an fMRI scan. But that doesn’t mean the brain area is directly helping with the task you’re testing (like math or reading). It’s just supporting the overall process by keeping you focused)
* Inhibition rather than excitation (A rain region might look like its being used but the reason it looks ‘active’ is because it is slowing down ‘inhibiting’)
* Lesion studies not powerful enough (too many uncertainties in the studies so it is inaccurate at times)
Explain Transcranial Magnetic Stimulation (TMS).
Coil carries an electric current; rapid change in current produces a magnetic field
* Magnetic field induces a current in the nearby neurons (causing them to “fire”, i.e., generate action potentials)
* This disrupts the cognitive function they may be supporting at that point in time
* Creates a simulated (“virtual”) lesion or interference
What are the TMS control conditions.
- Same region in critical vs. noncritical time period, or a different task (activating the region only sometimes to see how it effects the completion of the task, or seeing if you keep the TMS in the same place and do a different task to see if it makes a difference)
- Critical vs noncritical brain region (putting the TMS on an area that doesn’t effect performance)
- Sham (placebo)
What are the advantages of TMS?
Effects are temporary, so no brain reorganisation (unlike natural lesions)
* Focal (1cm2 or so) (localised to one area)
* Lesion can be moved within-
subjects
* As the effects are brief, you can investigate the (coarse) time course of a cognitive function
- Has been known to aid in depression
What are the challenges of TMS?
Can only stimulate cortical regions up to about 1 inch
* Can’t stimulate some regions (e.g. subcortical, brainstem)
* Minor discomfort, twitches
* Considerations
Not used on people with epilepsy
Number and rate of pulses is regulated by ethical guidelines
What is Transcranial Direct Current
Stimulation (tDCS)?
Very weak electric current applied between two stimulating electrodes
* One anodal (positive, one cathodal (negative)
* Typically, one electrode is experimental over region of interest
* And one is control over region of no interest
- Cathodal (negative) source typically decreases performance
- Anodal (positive) source typically increases performance
Explain the states that neurotransmitters can be in.
REMEMBER: Dendrites receive signals primarily via chemical signals (neurotransmitters) from the synaptic cleft.
Neurotransmitters can have excitatory effects
* Makes postsynaptic neuron more positive; action potential is
more likely (e.g., glutamate)
* Neurotransmitters can have inhibitory effects
* Makes postsynaptic neuron more negative; action potential is less likely (e.g., GABA)
* Neurotransmitters can also have a more modulatory role
* Tend to be localised to certain regions with diffuse projections throughout; regulate many neurons at once, alter how they communicate (e.g., dopamine, acetylcholine) (put simply by chat- “Neurotransmitters like dopamine and acetylcholine help control how brain cells communicate, starting in one area but affecting many parts of the brain.”)
