Neuroimaging

Question 1

Hubel and Wiesel

Answer 1

famous cat experiment

neurons in visual cortex (v1) respond to lines of different orientation and direction

Question 2

EEG

Answer 2

records voltage changes produced at synapses from electrodes on scalp

high temporal resolution

poor spatial resolution

• not only because surface electrodes but waves from different spatial locations flow across the scalp and interact with each other (adding up and cancelling out) making it difficult to localise the source

Question 3

PET

Answer 3

uses a circular scintillation detector (detects radioactivity)

radioactive isotopes injected

radioactive water to butanol a be used to measure flow (amount of radioactive butanol that ends up in the brain will be proportional to flow to that region)

short lived isotope decays and emitting a positron
after travelling up to a few millimetres the positron annihilates with an electron producing a pair of annihilation photons (similar to gamma rays) moving in opposite directions

these are detected when they reach a scintillator in the scanning device

creating a burst of radiation with is detected by photomultiplier tubes.

(photons which do not arrive in paris (i.e. within a few nanoseconds) are ignored

by measuring where the annihilation photons end up their origin in the brain can be plotted.

Question 4

william james principles of psychology 1890

Answer 4

first psychologist to be interested in cereal blood flow

Question 5

cerebral blood flow and metablism- marcus raichle

Answer 5

showed that following visual stimulation CBF increased in visual areas of the brain (e.g visual cortex)

afterwards PET became a way of imaging brain activity through the ;surrogate; of localised increases in cerebral blood flow

Question 6

limitations of PET

Answer 6

poor temporal and spatial

radioactivty- can’t do longitudinal

injection and the time it takes to get to the brain means that stimuli have been applied for a long time (minutes)- so not good for psychology experiments

Question 7

MRI

Answer 7

someone placed in strong magnetic field
the protons in their hydorgen atoms (particularly water) will line up and spin together (usually in random directions)

they precess (like the earth)
and spin at the larmor frequency (which is proportional to the strength of the magnetic field)

if we then apply a radio frequency pulse (energy) at the same frequency as the larmor frequency the protons acquire energy.

like opera singers

acquiring this energy briefly knock the proton spin out of alignment

when the radio frequency pulse stops the protons come back into alignment and emit a single of their own that varies depending on the surrounding tissue

from this can build picture of part of the body of interest

Question 8

fMRI

Answer 8

BOLD- blood oxygen dependent fMRI ( used to see brain during activities)

based on fact haemoglobin in RBC has different magnetic properties when got oxygen or not

Question 9

deoxyheamoglobin is

Answer 9

paramagnetic (meaning magnetic)

and decreases MRI signal (ALOT) i.e. brightness of image

Question 10

oxyheamoglobin is

Answer 10

diamagnetic (non magnetic)

increases MRI signal (a bit)
i.e. brightness of image

Question 11

unexpected physiological phenomenon

Answer 11

blood flow increases so there is actually more oxygenated blood at site of action, depletion not seen
and washes away deoxygenated blood

the washing away of deoxygenated blood is the main process affecting fMRI single

the increase in cerebral blood flow is greater than any increase in oxygen consumption so there is an increase in oxyheamoglobain and a washing away of deoxy

Question 12

the dip

Answer 12

transient increase in deoxy at the start of stimulus presentation
could be duet oxygen consumption at a time when CBF hasn’t high enough to over compensate

creating a decrease in the fMRI signal (darker)

Question 13

use of the dip

Answer 13

improves spatial localisation

if shown a line of particular orientation then particular patches of visual cortex will be activated

the positive BOLD response only allows mapping of the whole of the cortex in response to a line but the initial dip allows us to see little patches - different bits of cortex correspond to different bits of the stimulus

can easily map change

Question 14

optical techniques

Answer 14

can accurately measure haemoglobin concentration

Question 15

hemodynamics measure

Answer 15

blood changes

Question 16

blood can be measured with light

Answer 16

Oxyhemroglbin is bright red (absorbs blue light and reflects red light) making it look red

Deoxyhaemoglobin is blueish tinge (absorb red light and reflect blue light) making it look blue

Question 17

laser doppler flowmetry

Answer 17

measures cerebral blood flow
(optical imaging spectroscopy can’t do this can only measure conc of oxy and deoxy)
Pulse opsimeter – like in hospital for sats

Records oxy and deoxy in arteries

One of greatest advances in medical research

a laser light of a certain colour (wavelength) is shone into the cortex

which is either absorbed or bunches about a bit
and comes back out

light is collected by receivers in the probe and sent back to LDF processor

it looks for laser light that is different colour to the colour sent in- light that has changed colour has been doppler shifted (like pitch of ambulance as it gets further away)
because it hits something that is moving

the only thing moving in the brain is blood cells

if light has changed colour, the cerebral blood flow changes are detected.

Question 18

optical imaging

Answer 18

similar to BLOD fMRI but with light

diff blood absorb and reflect different wavelength of light and so can be mapped to see where activity is

Question 19

the rodent whisker barrel cortex

Answer 19

part of somatosensory system (touch)
contains representations of each of the whiskers on the contralateral face
occupies a large part of the cortex

Question 20

optical imaging spectroscopy

Answer 20

can illuminate the cortex with multiple different colours

oxyhemo- redish
deoxy-blueish

so have different absorbent spectra

can estimate changes in these two chromophores (something that absorbs light)

Question 21

radioactive water

Answer 21

injected (butanol)

Question 22

radioactive oxygen

Answer 22

inhaled

Question 23

Marcus Raichle

Answer 23

measured flow, oxygen consumption and glucose concumtoion in the late 80s
after visual stimulus presented

found visual cortex blood flow increased (50%)
glucose increased (50%) but oxygen increased hardly at all !!!!!

This was confusing for people at the time because they had previously assumed that oxygen consumption would increase when a brain region increased its activity

Question 24

clever experiments for showing oxygen consumption

Answer 24

hypercapnia

allows blood flow to increase without oxygen increasing

comparing the data collected during stimulus presentation to this and see what happened to the oxygen

Question 25

hypercapnia

Answer 25

increased co2 waste product of anaerobic metabolism breathe in 0.04% breathe out 4%

Question 26

further evidence for increased oxygen consumption : hypercapnia

Answer 26

artificially changing amount of co2 in blood- by breathing in as body wants to get rid of co2 hypercapnia 'fools' the body and makes it increase blood flow co2 increases CBF apart from the brain areas that detect co2, all areas stay the same if we become hypercapnic so gives us a control condition to elevate blood flow but not neural activity can compare blood oxygenation increased

Question 27

if there is evidence for oxygen consumption why did classic PET studies show that oxygen consumption did not increase following neural activation

Answer 27

PEt has poor spatial and temporal resolution so if flow and oxygen differed spatially and temporally it might be possible not to see a change in o2 consumption also calculation regarding how much of n inhaled substnace ends up in the blood are much more difficult than if it were injected therefore there may be errors in the PET o2 consumption calculations PET studies today (with better spatial and temporal res) actually do find increases in o2 con

Question 28

Richard Buxton and LAwrence Frank why blood flow goes up so much more than o2 consumption

Answer 28

to get more oxygen from the blood to the brain you need a steep gradient for o2 to diffuse across however because brain is very hungry for oxygen even at rest all oxygen in brain is already used up! meaning oxygen tension (the amount) is very low or zero in the brain so to increase the gradient you cant do it by lowering oxygen in the brain (it's already zero) therefore have to increase it in the capillaries and this is done by increase blood flow (speeD) but faster blood flow leaves less time to get the oxygen as it passes through this is how there is a disproportionately large flow increase to support lesser increase in oxygen consumption

Question 29

Richard Hoge 1999

Answer 29

compared the blood oxygenation increases produce by a certain CBF increase (neural activation versus hypercapnia) saw if neural activation has used up any oxygen and it did

Question 30

chromophore

Answer 30

something that absorbs light

Question 31

optical imaging spectroscopy cannot measure so what do we use

Answer 31

blood flow (velocity) so use laser doppler flowmetry

Question 32

what does optical imaging spectroscopy measure

Answer 32

changes in concentration of oxyhemoglobin and deoxyheamoglobin

Question 33

What can laser doppler flowmetry essentially measure

Answer 33

the changes in colour of the light, show the increases or decreases in cerebral blood flow

Question 34

So why do we get a positive BOLD fMRI signal ?

Answer 34

because an overcompensation of fresh oxygenated blood rushes into an active brain region and washes away the deoxygenated blood (deoxyhemoglobin)

Question 35

oxygen consumption doesn't actually go up...

Answer 35

blood flow just increases

Question 36

so why is it hard to ascertain whether oxygen gets used up

Answer 36

because before we have a chance to see what oxygen is used up, it gets washed away!