Neuroscience -PSY108

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Electrophysiology:

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.

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EEG (Electroencephalogram)

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.

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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.

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Positives and Negatives of PET:

POSITIVES: Gives measurements in absolute units.

NEGATIVES:

- 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.

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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)

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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)).

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Haemodynamic Response:

- 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.

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The 'DIP'

- 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.

 

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