Lecture 8 - fMRI - Methods Flashcards
Does fMRI measure neural activity?
fMRI does not measure neural activity directly. It measure blood oxygen levels in the brain. Neural activity requires energy from ATP. ATP is a product of glucose metabolism, which requires oxygen. Therefore, increased neural activity increases blood oxygen levels to the activated area of the brain. Therefore, fMRI measures neural activity indirectly.
What signal is measured in fMRI scans?
The signal measured during fMRI scans is the Blood Oxygen-Level Dependent (BOLD) signal.
What does BOLD signal stand for?
Blood Oxygen-Level Dependent signal. This is the signal measured during fMRI scans.
What quality of oxygenated blood makes it useful as a measurement in fMRI scanning?
Oxygenated blood (or more specifically oxyhaemaglobin) is diamagnetic, which means that it generates a stronger signal in response to the magnetic field generated in an MRI machine.
Why can we not compare the BOLD signals between brain regions?
Different parts of the brain have different Hemodynamic Response Functions (HRFs) and therefore, even though one area of the brain has a stronger BOLD signal than another region, it does not intake that there is more neural activity in that region, as the increased BOLD signal may be due to the HRF of that region.
One can only ever compared BOLD signals of the same region under different conditions.
What is the HRF in regards to fMRI and what does it represent?
The HRF stands for the Hemodynamic Response Function and describes the signal obtained from the change in blood oxygen levels due to neural activity.
In terms of neural activity, what is reflected by the BOLD signal?
The complete picture of what the BOLD signal reflects in regards to neural activity is still not understood, however, it is proposed that the BOLD signal represents the activity and energy consumption of neurons receiving information from the dendrites and dendrites firing, as well as action potentials generated by neurons in response to input from dendrites.
What are EINs and what relevance do they have for measuring BOLD signals in fMRI?
The BOLD signal represents oxygen levels in the blood. Blood oxygen levels increase when neural activity increases. The oxygen is used for and in response to activity of local cortical Excitation-Inhibition Networks, which are groups of neurons working together to excite or inhibit each other in response to a stimulus. This back and forth of excitation and or inhibition requires a lot of energy and in turn a lot of oxygen and in turn generates a BOLD signal reflective of this activity.
How does the low temporal resolution of MRI scanning impact the way BOLD signals are interpreted in fMRI scanning?
Because it takes time to read a BOLD signal we can only read/collect a certain frequency of signals/timepoints. This means that we need to have an understanding of the Hemodynamic Response Function to be able to compare our BOLD signals and estimate whether they are reflective of the HRF and in turn neural activity. Therefore, timing of when to start measuring the BOLD signal after stimulus onset is important.
How are BOLD signals analysed to generated fMRI scans? Hint: Statistical Parametric Mapping
A statistical model (Statistical Parametric Mapping) is used to analyse BOLD signals during an fMRI scan. The BOLD signals obtained during the scan are mapped onto the model and if they fit then we can assume there is neural activity in that region. The characteristic brain scans fMRI is known for are reflecting whether the BOLD signals obtained fit the HRFs for those brain regions, i.e whether there is neural activity in those brain regions, which are then overlaid onto MRI structural scans and assigned colours to represent activity and/or inhibition.
Why can you not get a reliable and useful result from a single fMRI scan?
The signals obtained from single fMRI trials are too noisy to interpret in any meaningful way. Results that provide any useful insight are generally made up of averaging BOLD signals across many trials for individual participants as well as across many participants. Only then can a useful statistical map representing neural activity be generated.
What is a voxel?
A voxel is the smallest point that we can receive a signal for in MRI/fMRI scanning. It is the equivalent to a pixel for photos. A voxel is representative of the accumulation of signals (i.e. neural activity) from an area of the brain.
What is the difference between fMRI and BOLD fMRI?
fMRI is Functional Magnetic Resonance Imaging. This is could be PET or BOLD fMRI. BOLD fMRI uses blood oxygen levels to measure brain activity.
What type of analysis is used when analysing the BOLD signal in fMRI scanning.
Statistical Parametric Mapping is used to analyse BOLD signal data from BOLD fMRI scans. A General Linear Model (GLM) is used to determine whether the increase in BOLD singals recorded fit the model of the HRF. The signals are compared between experimental conditions for many trials. This information is then compared between the participants in the sample.
What is the HRF? And how is it used in BOLD fMRI?
HRF stands for Hemodynamic Response Function and is representative of the characteristic BOLD signals obtained when there is neural activity that requires an increased supply of oxgen. The HRF is used to analyse BOLD signals to determine whether there is activity in a specific brain region.
