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28 Cards in this Set

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What method is used to investigate blood flow & oxygen, and why?

Optical studies are used - because BOLD fMRI is related to blood oxygenation in a very complicated way.

What colours are oxyhaemoglobin and deoxyhaemoglobin? And why is this important?

Oxyhaemoglobin is blue and Deoxyheamoglobin is red. This is important because optical measurements can be used to see how they vary following activation.

What are 4 techniques for measuring blood oxygenation & oxygen consumption?

Optical imaging, Optical Imaging Spectroscopy (OIS), Laser Doppler Flowmetry (LDF) and Seminal PET Studies.

What is optical imaging?

Animal anaesthetised, top of skull removed/thinned to translucency.




Cortex illuminated by wavelength (single colour light) & high speed camera captures images of cortex.




Stimulus presented to modality associated with cortex of interest - activates cortex.




Produces changes in blood oxygenation & concentration (vol.) - makes cortex absorb light differently.




As amount of light absorbed = different, amount of light remitted from cortex = different.




This causes a change in image intensity/brightness which camera sees & allows us to map where the activity is.

What is the rodent whisker barrel cortex?

Part of somatosensory cortex which contains representations of each whisker.




Occupies large part of rat's cortex as uses its whiskers a lot due to not being able to see very well!

How does an O.I. Spectroscopy work?

Illuminate cortex with lots of colours - since oxy/deoxyhaemoglobin = blue and red, they have a different absorption spectra - absorb different colours of light differently.

What is the main thing in the brain that absorbs light and why is this important for O.I.Spectroscopy?

Haemoglobin - so use OIS to measure haemoglobin concentration & oxygenation.



Compare oxy/deoxyhaemoglobin data with known absorption spectra to estimate charges in these 2 chromophores.

What is a chromophore?

Something that absorbs light.

How do you get TOTAL haemoglobin concentration (Hbt) and what is this similar to?

By adding oxyhaemoglobin (HbO2) and deoxyhaemoglobin (Hbr).




Similar to CBF.

What does an OIS measure?

Changes in blood volume.

How can we measure blood flow?

Use LASER DOPPLER FLOWMETRY.




Laser Doppler probe pointed at active cortical region - wave length shone into cortex, either absorbed or 'bounces' back out!

What happens when the light bounces back out of the cortical region being pointed at by a laser doppler probe?

Light collected by receivers in probe & sent back to LDF processor - looks for laser light that's different colour to the one that was sent in.




Light which has changed colour has been Doppler shifted because it hit something that was moving.




Only thing moving in brain = blood cells - by seeing if light that changed colour changes colour even more, can detect increases/decreases in CBF.

We can measure, but why does CBF increase so much?

The positive BOLD fMRI signal (most common for human brain mapping studies) occurs because an overcompensation of fresh oxygenated blood rushes into an active brain region.

Why does the increase happen?

Simplest reason = Oxygen consumption doesn't actually increase after all - flow increases for some other reason!

What do Seminal PET Studies do?

They measure CBF by injecting radioactive substances.

How are Seminal PET studies carried out?

Special form of glucose which doesn't get broken down is used to measure glucose use. Radioactive oxygen is used to measure oxygen consumption - this is inhaled not injected.

What did Raichle et al find?

Flow +50%, Glucose +50% - oxygen hardly increased.




Was confusing as everyone at that time assumed oxygen would increase when brain region increased its activity.

But what about the 'dip'?

Modern optical studies & fMRI suggest transient increase in DEoxyhaemoglobin concentration after stimulation increase.




Called a 'dip' as produces decrease in BOLD fMRI.



Why is the dip important for talking about oxygen consumption?

Some say the dip = evidence for increased oxygen consumption ('eating' oxygen = more deoxyhaemoglobin).




Further ev = deoxy increase related to oxy consumption - shows greater spatial localisation with neural activity than other aspects of haemodynamic (blood) response.




However, Dip is really small & not found in all studies.

What is hypercapnia and what does it offer?

Hypercapnia = elevated CO2 in blood - offers further evidence for increased oxygen consumption.




Hypercapnia = control condition to elevate CBF but not neural activity - i.e. increases CBF but doesn't increase any other activity in other brain regions.

How much CO2 do we breathe in and out? Why is this important?

We breathe in small qualities ~0.04, but breathe it out at 4%.




If we artificially change amount we breathe in, see how much CO2 in blood. Body wants to get rid of CO2 so hypercapnia 'fools' body & makes it increase blood flow - particularly true in CBF.

What happens if we compare Hypercapnia & Neural activation?

If we compare blood oxygenation increased by certain CBF increase - see if neural activation has used up any oxygen.




Find that CBF increase produced by hypercapnia causes oxygenation to increase!




However for same CBF increase produced by neural activity, there's less of an oxygenation increase.




WHY?! Because neural activity has used up some of the oxygen, i.e. oxygenation consumption has increased.

Why was Raichle wrong about the non-increase after neural activation?

Because of poor spatial/temporal res.




If flow & oxy consumption increases spatially/temporally - might be possible to not see change in oxygen consumption.




Also, inhaled rather than injected makes it harder to detect how much ends up in the blood - may be errors in PET oxy consumption calculations.

What do most modern PETs with better spatial/temporal & statistical power find?

They usually find the oxygen consumption increases!

What do diffusion limited theories of oxygen delivery seek to answer?

Oxygen consumption increases, but flow goes up so much more - why?

Describe Buxton & Frank's theory.

To get more oxygen from blood to brain, you need a gradient. Either more oxygen in blood than brain, or less oxygen in blood than brain.




Different gradient allows oxygen to diffuse from blood to brain, but brain is very hungry for oxygen at rest, so oxygen in brain is already used up!




Thus only way to increase gradient = increase to amount of oxygen in capillaries, through increasing blood flow (since all blood vessels are perfused at rest.




BUT increased blood flow means it goes faster! Less time to get oxygen as goes past brain faster.




Therefore, this is how we end up with a disproportionately large flow increase to support lesser increase in oxygen consumption.

What are capillaries?

Blood vessels.

What does perfused mean?

Having blood flow at rest.