Problem 5 Flashcards
Positron emission tomography
PET
Measures local variations in cerebral blood flow that are correlated with metal activity
- Radioactive substance is introduced into the blood stream
- Radiation is monitored by the PET instrument
- A postiron is emitted
- If the positron collides with an electron = 2 photons are created, which travel in opposite directions
–> PET Scanner determines where the collision took place
functional magnetic resonance imaging
fMRI
Putting a person inside a scanner that performs a series of cognitive tasks whilst BOLD images representing the brain are collected
- the images show small changes in the brightness levels of certain brain areas, that reflect brain activity
- The areas in which the brightness changes relative to the task can then be determined using statistical analyses
–> exploits the fact that local blood flow increases in active parts of the brain
What is the main difference between EEG and MEG + PET and fMRI ?
PET and fMRI do not directly measure neural events
–> they measure metabolic changes that correlate with neural activity
What is the main difference between PET and fMRI ?
With fMRI, imaging is focused on the magnetic properties of the deoxygenated form of hemoglobin (deoxyhemoglobin)
–> this form is weakly magnetic in the presence of a magnetic field
THUS: fMRI detectors measure the BOLD effect, which can be used to indirectly measure neuronal activity
Blood oxygen level-dependent effect
BOLD
Refers to the value of the ratio of oxygenated to deoxygenated hemoglobin
–> measured by fMRI detectors
fMRI results report an increase in the BOLD effect even though one might expect the proportion of deoxygenated hemoglobin to be greater in the area surrounding active brain tissue.
Why is that ?
When a brain region becomes active, the amount of blood that is being directed to that area obvi increases
–> neural tissue is unable to absorb all of the excess oxygen
BUZ: changes in blood flow/BOLD response occur rather slowly
Block design experiment
The recorded neural activity is integrated over a block of time during which the participant either
a) presented with a stimulus
b) performs a task
–> the recorded activity pattern is then compared to other blocks that have been recorded
What do PET and fMRI have in common ?
They both detect changes in metabolism/blood flow in the brain while the participant is engaged in cognitive tasks
–> can identify brain regions that are activated during these tasks this way + test hypotheses about functional anatomy
Structural imaging
Is based on the fact that different types of tissue have different physical properties
–> specialized to visualize these different anatomical properties of the brain in static maps
e.g. CT, MRI
Functional imaging
Is based on the assumption that neural activity produces local physiological changes in that brain region
–> this can be used to produce dynamic maps of the moment-to-moment acitivy of the brain when engaged in cognitive tasks
Disadvantages of CT scans
- Exposure to radiation due to x-ray
- Cannot distinguish between grey and white matter
- Cannot be adapted for functional imaging purposes
Why is MRI superior to the CT?
- No use of radiation
- Better spatial resolution, so folds of gyri can be seen
- Better discrimination between white + grey matter
- Can be adapted for use in detecting changes in blood oxygenation (fMRI)
The strength of a magnetic fiel is measured in units called … ?
Tesla
–> typical scanners have a field strength between 1.5 and 3 T
What are the advantages vs disadvantages of PET?
Advantages
Less signal distortion around air cavities
Disadvantages
1. Worse temporal + spatial resolution than fMRI
- Use of radiation
Why is fMRI superior to PET ?
- No use of radiation
2. High temporal and spatial resolution
Name Problems associated with functional imaging.
- It is susceptible to error due to the fact that this is a statistical science
–> thus analyzing + interpreting the data is rather difficult
- Interpret individual differences
- Small spatial distortions can produce spurious results
- Each persons head might be aligned slightly differently in the scanner over time
- -> might lead to false positive results
How do functional imaging researchers deal with the problem of individual differences ?
By Averaging data over many participants so one is left with regions of activity that are common to most of us
–> for this one first has to perform Stereotactic normalization + smoothing
Stereotactic normalization
Each brain is mapped unto a standard reference brain
–> done by dividing each brain into thousands of voxels
Smoothing
Spreads some of the raw activation level of a given voxel to a neighboring voxel
–> the closer the neighbor is the more activation it gets
e.g.: An inactive voxel surrounded by active ones gets switched on and vice versa
Preprocessing
Refer to the processing stages or measures before a statistical analysis can take place
e.g.: Stereotactic normalization, smoothing
What do signal and noise refer to in the context of voxels ?
Signal
–> the thing of interest which corresponds to the larger cluster of activity
Noise
–> Isolated voxel
Name the 2 main advantages of smoothing.
- Enhances the signal to noise ratio
- Facilitates detection of common regions of activity across individuals
–> as spatial extent of active regions is increased - so when averaging the activity across individuals one is more likely to find common regions
Statistical analysis
T-test to compare means
–> is the mean activity at a particular voxel in the experimental condition greater than in the baseline condition
BUT: Significance level is too great, thus one uses a correction method to determine a suitable p-value
Random field theory
Is a correction method that chooses a statistical threshold (p-value) based on spatial smoothness
Family wise error
FWE
A method for correcting for many statistical comparisons based on the number of tests being conducted
False Discovery Rate
A method for correcting for many statistical comparisons based on the number of positive results obtained
–> more conservative
Excitation vs inhibition
Refers to the nature of the mechanism by which neurons communicate
–> increase vs decrease of the activity of a brain region triggered by the activity in another region
Explain the question of necessity vs sufficiency
This question points out the fact that a region may be active during performance of a task but it might not be critical to that task
–> so active due to a particular strategy the individual adopted
THUS: functional imaging tells us which brain regions are sufficient to performing a task but not really which are crucial or necessary
What dictates how the MR signal will behave in a BOLD image ?
The proportion of deoxyhemoglobin relative to oxyhemoglobin within a particular voxel
–> high concentration of oxyhemoglobin give a higher signal (brighter image)
Hemoglobin
Is a protein molecule that carries oxygen
–> deoxyhemoglobin disrupts the T2 signal, whereas oxygenated hemoglobin elicits a stronger signal
Why do fMRI detectors measure the BOLD effect ?
Because in essence, it measures the time course of the blood flow change process (HRF)
–> amount of blood that is directed to a given area increases if it becomes active
THUS: measures the difference in no oxygen to oxygen as a region becomes active in response to a task
