fMRI Flashcards
Hemodynamic signal
Indirect measure of neural activity
- complex relationship between neural activity and hemodynamics –> can it be trusted?
- deep understanding allows discriminating (in)valid ways of using it
Relation between hemodynamics and neural activity
Neurons require energy to function (ATP), from glucose
- continuous sypply via blood circulations:
–> via arteries, exchange glucose and oxygen in capillaries, deoxyhemoglobin and blood leaes again
Hemodynamic response function
Deoxyhemoglobin
Oxygen gets removed from hemoglobin
HRF in neural activity
- delayed local increase in oxygen and glucose consumption
- ratio oxygenated en deoxygenated heoglobin decreases
- signal through neurovascular coupling mechanism, triggeren increase in blood supply
- peak increase in blood oxygenation several seconds after
- blood volume and oxygenation decay again
- expand across larger territory than region of neural activity
Short step by step HRF
Rest –> stimulus –> inc neural activity –> inc oxygen and glucse consumption –> big inc cerebral blood flow –> fMRI BOLD response inc.
Three components HRF
- intial dip: regular oxygen consumption
- primairy response: bring lots of oxygen
- negative overshoot: signal decreases
–> additivity assumption
Additivity assumption
In case of multiple stimulu, total hemodynamic response is sum of HRF’s to individual stimuli
Hemodynamic repsonse and electrical potential changes
Measure action potentials = measure output of a neuron
- HR of a region: not sure whether it represents overall action potential output of that region
- situations possible where energy consumption increases, while neuron output stays the same
- difference in HR –> twwo conditions are different in neural activity, but action potential output does not need to be
HR vs. other electrophysiological measures
- multi-unit activity MUA: number of action potentials
- local field potentials LFP: synaptic input of neurons
–> typical situation: everything correlates
–> with long stimulation: HR slightly more correlated with LFP than with MUA
T2 and T2*
Mainfactors contributing to decay of transverse magnetization
Molecular interactions (=> T2 decay):
- field inhomogeneity
- tissue succeptibility
Total dephasing = T2* decay (always faster, because there are more factors to the decaying)
BOLD
Blood oxygenation level dependent signal
- blood carriers hemoglobin
- hemoglobin contains iron atoms
- iron atoms can distort the magnetic field
- the iron has slightly different magnetic properties depending on whether it is bound or unbound to oxygen
BOLD signal
Inhomogneities are modulated by variable ratio of deoxygenated/oxygenated blood
- level of inhomogeneity of magnetic field affects speed of dephasing
- more DeoxyHb, strong field inhomogeneities (fast dephasing)
- more OxyHb, weak field inhomogeneities (slow dephasing)
Is fMRI worth it
It is very expensive and will take a long time
- formulate relevant hypotheses
- design study to provide evidence for or against some hypothesis
- expensive machines do not think of you
–> need to know the methods and the psychological theories of mental functioning
Subtraction
Isolate behavior by substracting conditions that only differ in 1 mental process
- realted to behavioral method of Donders: mental chronometry
problems substraction
What if other mental states are included?
Underlying assumption: pure insertion or additivity assumption
- conflict monitoring
- priming
Pure assumption
Two (or more) oncditions can be congnitevely added, implying no interactions among the cognitive components of a task
- In most cases, this assumption is invalid
- if assumption fails => compasrison between conditions is confounded
Conflict monitoring
SLower in incongruent trials and activaiton increase in incongruent trials
Priming
Faster in congruent trials and activation decrease in congruent trials
Factorial design for pure assumption
Additivity assumption violations visible through interaction effects in factorial design
Principle: task where cognitive components are intermingles in one moment and separated in another instance of the paradigm => allows testing for interactions
Parametric variation
Principle: increase cogntive demand without modifying intrinsic nature of task
- BOLD increase implies heavy association of area with nature of manipulated parameter
Conjuntion analyses
Cognitive conjuntion studies are designed such that two or more distinct task pairs each share a common processing difference
- task
- stimuli
- senses
Disjunction analyses
Results conjunction analyses
Conjunction: both production and perception
Disjunction: production, no perception
Disjunction: perception, no production
Recap contrasting ecperimental conditions
- substraction: effect = A - B
- Factorial: addition and interaction
- parametric
- conjunciton
Correlational studies
Between subjects design –> relate individual differences in behavior to brain differences
Forward inference
If cognitive process X is manipulated, particular brain region R is activated => activation of brain region R is related to cognitive process X
Reverse inference
Engagement of particulalr cogntive process is inferred by activation of particular brain area
- based on literature
Overlapping HRFs
HRF peak delayed with 6 sec, total duration more than 12s from onset to back to baseline
- if you show multiple simuli within those 12 s they will overlap
3 options:
1: ISI of 16 sec -> inefficient
2: ISI of 2 sec -> signal reaches asymptomic level (low sensitivity for differences)
3: Block design: trials are blocked per condition
Block design
- alternate blovks of different conditions
- within vlock: strong HRF because additivity signal
- after a block: signal goes down again before next block
- condition-associated ups and downs in BOLD signal are late
- very efficient
Drawbacks to block design
- predictabilty of conditions (tendency to prepare)
- Impossible for some experimental questions
- impossible to estimate single-trial response function
Slow event-related design
Long ISI (with or without jitter)
- ideal for estimating HRF function by the event related response
- inefficient use of time and borign
Rapid counterbalanced event-related design
- ISIS = 0 or not longer than the trial duration
- conditions alternate in pseudo-random order
<-> alternating event-related design - peak is signal difference between conditions when particular condition occurs frequently in short period of time
- reasonable sensitivity and power
- rest
- trial order similar to that of most behavioral experiments
Rest condition
Studies often include a rest condition as a baseline
- makes comparison between different studies easier
- helps to disambiguate more activation from less deactivation
Default mode network
Very predictable set of deactivations in the contrast of an active task minus a passive no-stmulus baseline:
- number of subtraction task
- one-back repetition detection of object images
- one-back repetition detection of texture patterns
–> thus, these regions are active when you are at rest
