HC6 fMRI Flashcards
What is the hemodynamic signal?
An indirect measure of neural activity. There is a complex relationship between neural activity and hemodynamics, requiring complex methods and analysis. Similar to how astronomers infer exoplanets through spectral analysis of starlight.
Why do neurons need blood circulation?
Neurons require energy (ATP) from glucose to function, which necessitates:
- Continuous supply of glucose and oxygen through blood circulation
- Blood delivery through arteries and arterioles
- Exchange of glucose and oxygen in capillaries
- Oxygen removal from hemoglobin creating deoxyhemoglobin
- Deoxyhemoglobin moving through venules and veins to exit the brain
What are the three main components of the HRF?
- Initial dip: oxygen consumption leads to decreased blood oxygenation and signal
- Primary response: Strong increase in signal due to influx of oxygenated blood
- Negative overshoot: Signal decreases below baseline levels
What happens during neural activity according to the HRF?
Slightly delayed local increase in oxygen and glucose consumption
- Decrease in ratio of oxygenated to deoxygenated hemoglobin
- Neurovascular coupling triggers increased blood supply
- Blood oxygenation increases markedly
- Peak increase occurs several seconds after initial oxygen consumption
- Blood volume and oxygenation eventually decay (negative overshoot)
- Response expands beyond the region of neural activity
What is the additivity assumption in HRF?
When multiple stimuli occur, the total hemodynamic response is the sum of the individual HRFs for each stimulus.
How does the hemodynamic response relate to electrical potential changes?
HR of a region may not directly represent overall action potential output
- Energy consumption (and HR) can increase while neuron output remains unchanged
- Differences in HR indicate different neural activity but don’t necessarily reflect changes in action potential output
- Inhibitory input can increase energy consumption while output decreases or remains stable
How does the hemodynamic response correlate with electrophysiological measures?
Correlates with both Multi-Unit Activity (MUA, action potentials) and Local Field Potentials (LFP, synaptic input)
- In typical situations, HR, MUA, and LFP all correlate
- During long stimulation, HR correlates slightly more with LFP than with MUA
What are the main factors contributing to T2* decay?
Molecular interactions (leading to T2 decay)
- Field inhomogeneity
- Tissue susceptibility
- Total dephasing results in T2* decay
What does BOLD stand for and what are its key components?
BOLD = Blood-Oxygenation-Level Dependent signal
- Blood carries hemoglobin containing iron atoms
- Iron atoms can distort the magnetic field
- Iron’s magnetic properties differ when bound (OxyHb) vs unbound (DeoxyHb) to oxygen
- Magnetic field inhomogeneities are modulated by ratio of deoxygenated/oxygenated blood
How does blood oxygenation affect magnetic field dephasing?
More DeoxyHb (oxygen-poor blood):
Creates strong field inhomogeneities
Results in fast dephasing and faster T2*
More OxyHb (oxygen-rich blood):
- Creates weak field inhomogeneities
- Results in slow dephasing and slower T2*
Why is the phrase “fMRI shows what parts of the brain ‘light up’” misleading?
Two main reasons:
- “Lights up” implies electrical changes, but fMRI doesn’t measure electrical signals directly. Instead, it shows areas that are active as a result of firing.
- “Lights up” suggests areas turn on and off, but the brain is actually active all the time. fMRI shows what parts are most active during specific tasks.
What key considerations are needed when designing an fMRI study?
Requires significant investment (time & money)
- Must formulate relevant hypotheses
- Need to design study to provide evidence for/against specific hypotheses
- Requires understanding of both methods and psychological theories
- Expensive equipment doesn’t do the thinking for you
What is the subtraction method in fMRI experiments?
Attempts to isolate behavior by subtracting conditions that differ in only one mental process
- Related to Donders’ behavioral method of mental chronometry
Examples:
partner – friend = love?
flanker test: incongruent – congruent = conflict resolution
What are the problems with the subtraction method?
Several key issues:
1. Other mental states may be included
Example: partner – friend could = familiarity? obligation? sexuality?
2.Complex interactions
Example: Conflict monitoring shows increased activation in incongruent trials
Priming shows decreased activation in congruent trials
3. Relies on pure insertion (additivity) assumption
4. Difficulty in precisely determining what is being measured
How should you approach condition selection in fMRI studies?
Need to carefully consider which brain areas correspond to isolated behaviors
- Example: Studying “where love comes from in the brain” requires carefully designed contrasts
- Must consider all possible confounding variables and mental processes
What is the pure insertion assumption in fMRI studies?
The assumption that two or more conditions can be cognitively added, implying no interactions among the cognitive components of a task. However, this assumption is usually invalid! When it fails, comparisons between conditions become confounded, making it impossible to determine which cognitive processes are reflected in the observed activation.
What is the purpose of factorial design in fMRI studies?
Allows detection of violations in additivity assumptions through interaction effects
- Principle: Creates situations where cognitive components are intermingled in one instance and separated in another
- Enables testing for interactions between different cognitive processes
What’s an example of factorial design in fMRI research?
Love study example:
Factor 1: love vs no-love (partner – friend = romantic love)
Factor 2: romantic vs maternal (own child – other child = maternal love)
This design allows researchers to examine how different types of love might interact in the brain.
What is parametric variation in fMRI studies and why is it useful?
Principle: Increases cognitive demand without changing the fundamental nature of the task
- BOLD signal increase implies strong association between an area and the manipulated parameter
Example: Rosell-Negre et al. (2017) showed how regions involved in cognitive processing track reward magnitude during task performance
What are conjunction analyses and how do they differ from disjunction analyses?
Conjunction analyses:
Design tasks where multiple distinct task pairs share a common processing difference
Work across various contexts:
- Different tasks
- Different stimuli
- Different senses (visual, auditory)
What are disjunction analyses?
The opposite of conjuction analyses. It’s an experiment in which you look at what aspects are involved in region A and not region B.
Conjunction - Example
Gauvin et al. (2016) showed internal speech monitoring occurs through conflict detection between response options, resolved by domain-general executive center (e.g., ACC)
What is Forward Inference in fMRI research?
If cognitive process X is manipulated and brain region R is activated → the activation of brain region R is related to cognitive process X
What is Reverse Inference and what are its limitations?
Infers engagement of cognitive processes based on activation of specific brain areas (based on literature)
Problems:
- Multiple mental states can activate the same brain region
- Uncertainty about strength of association between brain activity and assumed behavior
- Questions about regional specificity
Often misused in media (Example: “You Love Your iPhone. Literally.” claiming phone use activates insular cortex associated with love)
What is the purpose of correlational studies in fMRI research?
They relate individual differences in behavior to differences in brain activation, helping understand brain-behavior relationships across participants.
Chemical shift
A change in the resonance frequency of the nuclei within the molecules, in function of their chemical bonds. For example, H bond to O2 has a different resonance frequency than not bond to O2. The chemical environment of a proton determines its spinning frequency.
Shimming
Making the magnetic field more homogeneous, so that other frequencies are minimalised.
Single voxel MRS/ single voxel spectroscopy (SVS)
Measures the volume of 1 voxel, the simplest form of MRS. It cost less time. It is often used to measure metabolites (GABA) with a low signal to noise ratio. It gives less artefacts because you measure only one voxel.
MR spectrum
Reflects particle resonance (ppm = parts per million) of metabolites that are associated with specific neurotransmitters or other substances in the brain tissue.
The ppm is related to a reference
The Larmor frequency of tetramethylsilane (TMS). Every metabolite has a different spinning frequency, you see this also in the spectrum. There is a chemical shift for: 1:1000000 x Larmor freq of TMS.
Criticism on hemodynamic methods
- The hemodynamic signal is only an indirect measure of neural activity.
- Complex relationsip between neural activity and hemodynamics.
- Complexity of methods and analysis.
- Analogy with physics/astronomy: the use of indirect measures and complex procedures.
What happens if you apply a RF pulse of 180 degrees
By applying a RF pulse of 180 degrees (instead of 90), you reverse the field inhomogeneities but not the spin-spin interaction. You then get only the T2 effect.
What is a slow-event related design (Option A)?
A design with an inter-stimulus interval (ISI) of 16s, allowing HRF to return to baseline. It is inefficient due to long waiting times.
What is a fast-event related design (Option B)?
A design with an ISI of 2s, leading to an asymptotic HRF signal. It has low sensitivity for condition differences.
What is a block design (Option C)?
A design where all trials in a block belong to the same condition, alternating between blocks. It generates a strong BOLD signal due to additivity.
What are the advantages of a block design?
- High BOLD signal
- High power and sensitivity
- Efficient: many trials per unit time
- No long waiting times
What are the disadvantages of a block design?
- Predictability may lead to unwanted cognitive processes
- Can be boring for participants
- Not suitable for some experimental designs (e.g., switching tasks)
- Cannot estimate single-trial response function
What is an event-related design?
A design where short-duration events are presented in randomized order. It reduces expectation effects and allows individual trial analysis.
What is a slow event-related design?
A design with a long ISI (with or without jitter), ideal for estimating HRF. It is inefficient and can be boring.
What is a rapid counterbalanced event-related design?
A design with an ISI of zero or equal to trial duration, alternating conditions in pseudo-random order. It balances conditions and provides reasonable BOLD sensitivity.
What is a rest condition in fMRI studies?
A baseline condition used to compare with task conditions, making studies more comparable and helping to distinguish activation from deactivation.
What is the default mode network (DMN)?
A network of brain regions showing lower activation during tasks than at rest, inspiring resting-state fMRI research.
What is functional connectivity?
It is the relationship between functional activity of different brain regions. There is functional localisation but no brain regions work in isolation from others. They work in systems or networks. So it is not about the anatomical or structural connectivity.
What is the simplest index of functional connectivity?
The correlation between activity in two brain regions over time.
What is a seed region?
A specific brain region of interest used to study functional connectivity.
What is seed-based functional connectivity?
Correlating the time-varying signal of a seed region with signals from all other brain voxels.
What is scrubbing in fMRI analysis?
Excluding time points with excessive motion to reduce its impact on BOLD signal changes.
What is effective connectivity?
A model of causal influence between brain regions, but not true causal inference.
What methods are used to test effective connectivity?
- Structural equation modeling (SEM)
- Dynamic causal modeling (DCM)
- Granger causality
- Psychophysiological interactions (PPI)
What is structural equation modeling (SEM)?
A method to analyze structural relationships and pathway modulations between brain regions.
What is psychophysiological interaction (PPI)?
A method to identify brain regions whose activity depends on interactions between psychological and physiological factors.
What does a PPI effect indicate?
A context-specific relationship between brain regions, seen in differences in regression slopes of regional time series.
Can functional connectivity change during an experiment?
Yes, functional connectivity can change (PPI), while anatomical connectivity remains static.
What is the difference between anatomical, functional, and effective connectivity?
- Anatomical connectivity: Physical connections between regions
- Functional connectivity: Simultaneous activation of regions
- Effective connectivity: Influence of one region on another
