Lecture 12 - Cognitive neuroscience: methods Flashcards
Methods we cover
Neuroimaging:
fMRI
PET
Electroencephalography (EEG)
Brain stimulation:
Transcranial Magnetic Stimulation (TMS)
How MRI works
MRI employ powerful magnets which produce a strong magnetic field
This forces protons which exists in every cell in our body to align with that magnetic field
The machine then emits a radiofrequency pulse which pushes makes the protons out of their usual position
The electromagnetic energy taken for these protons to return to normal is picked up and measured by the machine
mri diagram
What are we measuring with MRI?
MRI detects different tissue types depending on how quickly they release energy and return to their normal position when the radiofrequency pulse is turned off
In structural MRI we use this to measure different tissue types, the contrast of the image helps differentiate
But in functional MRI we can measure the relative oxygenation of blood in the brain
BOLD =
Blood Oxygenation Level Dependent
- When we use neurons in our brain to think or process information we use up a supply of oxygen in the blood vessels surrounding that part of the brain
Our body works to replace the oxygen that we’ve used so send more oxygenated blood to the site
We actually get an oversupply of oxygen to the active brain area
= the haemodynamic response
Oxygenated blood has different ….
magnetic properties to deoxygenated blood
Delayed response
The response picked up by MRI is ~6 seconds after the stimulus
Rather than measuring individual neuronal activity we measure activity in a region of the brain
Therefore it better correlates to local field potentials (the synaptic potential pooled across groups of neurons)
The importance of contrast
We can use oxygenation in different parts of the brain to see what’s active when people are using a certain cognitive process or mental state
Example: comparing brain activity when people compare faces to houses
Structurally they are similar, but we can ask the question of whether there is specific processing in the brain for social stimuli such as faces
Results in fMRI showed the fusiform face area activated for faces, but not houses
Task design
We create tasks that tap into measures of cognition we are interested in
We can do this as a block, event, related or mixed design
Block designs pool together the haemodynamic response into blocks of 20-30 seconds of sustained activity
Event-related designs create distinct peaks so we can measure the direct response to different stimuli
Summary of MRI
MRI is non-invasive
Offers good spatial resolution
Poor temporal resolution (BOLD response ~6 seconds after stimuli)
Correlational measure of cognition
PET machine diagram
How PET works
We inject people with tracers (specially designed radio-active ligand
This travels to a tissue/organ/area of the body and binds there
We can then produce images of this area using PET
For brain imaging radioligands are often made to bind to specific receptors in the brain
E.g.
Glucose metabolism – 18F-fluorodeoxyglucose (most commonly used)
Serotonin receptor activity- 11C-WAY100635 and 18F-altanserin
Dopamine receptor activity - 11C-raclopride, 11C-methylspiperone
Research Example: dopamine receptors in drug addicts
Dopamine receptor binding radioligand - [11C]raclopride
Used in PET scans in those with cocaine and meth addictions
Show less dopamine receptors in the striatum in drug abusers
Similar results are found in alcohol and nicotine addiction
Electroencephalogram (EEG)
Electrodes are pasted on your scalp
They detect tiny electrical charges from the local field potential
A measure of brain activity
What is EEG measuring?
The activity measured comes from the activity of thousand of neurons beneath each electrode
Voltage fluctuations show on an EEG reading
EEG - sleep cycles
Event related potentials (ERPs)
A small voltage generated in the brain in response to specific events (Blackwood and Muir, 1990)
Summed activity of postsynaptic potentials when a large number of neurons fire in synchrony
Early ERP: peak within the first 100 ms of stimuli as first response
Later cognitive ERP: information processing
Segments of EEG, time-locked to a stimulus
Summary EEG
Average of thousand of neurons underneath an electrode
Can cover the whole scalp
Portable methods exist (but are less accurate)
Spatial resolution is not as good as MRI/PET, temporal resolution is good
Mostly measures signals from the cortex
Struggles with measure deeper brain structures (as the signal doesn’t reach that far)
TMS - Transcranial magnetic stimulation
Magnetic pulses over the scalp (1.5T, as strong as an MRI scanner in a localised point, but short)
Induces small electric currents - alter the firing patterns of neurons
Interfere with cognitive processes such as:
perception
motor control
higher-level cognitive processes.
TMS - How it works
Electrical burst is short-lasting, it rises to maximum and reverses back towards zero in 0.2 milliseconds
stimulate superficial areas of the cortex, but not areas further from the scalp, or deeper brain structures.
Which coil to use
The maximum electric field induced is in the ring-shaped area underneath the coil
Single coil: used in early studies, less focal and activates a large region
Figure of eight coils intensify the electrical field at the point of overlap – good to target specific regions
Double cone good for bilateral or wide-area stimulation as covers both sides of the head
H coil less focal but covers large networks and deeper brain region
TMS - How do we know what intensity to use?
Calibrating motor evoked potential (MEP)
The intensity threshold is determined for each participant separately
Experimenters place the coil over the motor cortex
They then increase the threshold until they find the intensity that move a muscle from their hand
This intensity is then set and used for that participant
How do we know what frequency to use with TMS?
Depending on the outcome you want to achieve – a single short pulse can be delivered, or repetitive pulses rTMS.
As a general protocol:
Frequency of <1 Hz decreases activity in an area
Frequency of >1Hz increases activity in an area (rTMS)
Research Example: TMS for treatment of depression
Daily left prefrontal TMS for several weeks improves symptoms of depression
This uses a repetitive pulse over the DLPFC:
120% MEP
10Hz delivered in 4 second pulses
37.5 minutes
Figure-eight coil
TMS - Summary
TMS uses magnetic pulses to invoke small electric currents in the brain
Coils are selected based on the research question – figure of eight allows more precision to specific areas
Intensity of TMS is determined on an individual basis, thresholding for motor evoked potential in each person (what makes a finger twitch)
As a general principle low frequency (<1Hz) reduces activity in a target area, whilst high frequency (>1Hz) increases activity in a target area
Disadvantages to TMS
Context dependency: it will depend on how excited the cortex is at the time the stimulus is applied
Very small risk of seizures, one in 30,000 treatments (0.0003%)
tms VS tDCS
Transcranial direct current stimulation
An alternative brain stimulation to TMS
Constant low direct current delivered via electrodes on the head
Uses electrical energy directly, rather than magnetic
More portable and usually battery powered – does not need a large cart like TMS
Current generated are lower in tDCS, compared to TMS which the magnetic pulse passes though the scalp and skull with no problem
Real-life example: tDCS trialed by the NHS
Comparing methods
