How does it work?
- Can be used to measure neural activity.
- Electrode inserted into neuron.
- Measures activity of single neuron.
Problem: Can't stick electrodes into human brains.
Even if we could - would only get small number of brain's neurons and we don't know which brain regions are involved yet.
STICK ELECTRODES ON SCALP INSTEAD.
- Raw data from each electrode.
-EEG records voltage changes produced by synaptic activity by placing electrodes on scalp.
POSITIVES: High temporal resolution (fast)
- Can reveal differences in processing speed for different stimuli.
NEGATIVES: Poor spatial resolution .
- not due to amount of electrodes which fit on head.
- Waves from different spatial locations flow across the scalp and interact (add up+cancel out).
-Makes it difficult to interpret and localise signal sources in EEG.
PET (Positron Emission Tomography)
- MEASURES CEREBRAL BLOOD FLOW (CBF)
How does it work?
-Inject radioactive tracers into blood and see where they end up.
1) Inject radioactive water - used to measure CBF
2) Water stays in brain - amound there is proportional to flow in that region.
3) More activity = more flow.
Positives and Negatives of PET:
POSITIVES: Gives measurements in absolute units.
- Poor spatial resolution
-Poor temporal resolution (speed)
- Radioactivity means same subjects can't be retested too much
- Injection of radioactivity takes time to get to brain (minutes) - not good for psychology studies (eg changing perceps).
-Limitations overcome by MRI - no radioactivity.
FMRI - Fucntional Magnetic Resonance.
- Used to obtain maps of functional activation in the brain (i.e. which parts are active following presentation of a stimuli or performance of a cognitive task)
- Blood oxygen level dependent (BOLD) fMRI is often used.
B - Blood
O - Oxygen
L - Level
D - Depdendence
How does it work?
- Red blood cells contain haemoglobin
- 2 types - OXY-haemoglobin and DEOXY - haemoglobin
OXY-haemoglobin + DEOXY-haemoglobin = Total haemoglobin (or cerebral blood flow, CBV)
How does fMRI work?
- works because blood is magnetic.
De-oxyhaemoglobin - Paramagnetic (Magnetic) - DECREASES MR signal (a lot)
Oxy-haemoglobin - Diamagnetic (Not really magnetic) - INCREASES MR signal (a bit)
Signal = brightness of image.
WHY? As deoxyhemoglobin is paramagnetic it affects the signal emitted from the surrounding protons in MRI - (don't worry about this).
In simple terms how 'bright' the imagine is in the fMRI depends on how oxygenated the blood is (hence the term blood oxygen level dependent (BOLD)).
- Deoxyhaemogobin appears as brain extracts oxygen for metabolism.
- If brain becomes active Oxgen consumption SHOULD increase and there SHOULD be an increase in haemoglobin - WRONG.
-Deoxyhaemoglobin levels go DOWN because blood flow has increased so much.
-Washing away of deoxyhaemoglobin produces an increase on BOLD fMRI signal - (positive BOLD signal - mapping signal)
- Early incerase in deoxyhaemoglobin produces a decrease int he BOLD fMRI signal (DIP) -
but: - CBF overcompensates for oxygen consumption and BOLD signal increases.
- At the start of the stimulus there is an transient increase in deoxyhaemoglobin.
- Due to increased oxygen consumption before CBF has increased enough to 'over compensate'
INCREASED SPATIAL LOCALISATION:
- dip shows greater spatial localisation compared to other aspects of the haemodynamic (blood) response.
Eg - If you are shown a line of a particular orientation then particular patches of your visual brain will be activated.
The positive BOLD response only allows us to map the whole of the visual cortex in response to a particular line.
The initial dip allows us to see these little patches.