Temporal and spatial resolution of fmri
temporal- in the range of seconds
spatial- in the range of mm (3x3x3mm)
How does fMRI work?
- Protons spin, and have a magnetic charge
- Use strong magnetic field to line up protons
- Send a radio pulse through the lined up protons, and see how they resonate - Different protons (different tissues) resonate differently (magnetic susceptibility), allowing composition of a tissue image
- fMRI: oxygenated blood resonates differently to de-oxygenated blood, allowing composition of an (indirect) image of brain activity
- BOLD FMRI measures regional levels of blood oxygen by detecting magnetic changes in red blood cells when they become de-oxygenated
Three ingredients in BOLD signal
CMRO2 = cerebral metabolic rate of oxygen
CBF = cerebral blood flow
CBV = cerebral blood volume
How does BOLD signal work
o Cognitive functions are region specific, if a task involves a certain cognitive function, the areas involved will become more active, need more oxygen and more blood.
o More active areas therefore contain more deoxygenated blood/haemoglobin (since they use the oxygen)
o Compares level of oxygenated with deoxygenated blood magnetic properties: hemoglobin (blood cell that carries oxygen) is diamagnetic (only magnetic when exposed to external magnetic field) when oxygenated and paramagnetic (normally magnetic) when de-oxygenated
o Hemoglobin molecules resonate differently in these different magnetic states
o hemodynamic response function is an indirect measure of brain activity
Issues with BOLD signal
o you want to measure the level of de-oxygenated blood, since that would be the most direct evidence of increased brain activity
o However, BOLD cannot measure the level of de-oxygenated blood per se, only its level compared to the level of oxygenated blood: more oxygenated blood means lower BOLD
o This would mean that greater brain activity is only detected by BOLD if all oxygenated blood is used up
What is the haemodynamic response function in fMRI?
o initial dip: as neurons consume oxygen and more blood is supplied, levels of deoxyhaemoglobin initially (very briefly) decrease -> reduction of BOLD signal
o overcompensation: in response to the increased consumption of oxygen, blood flow to the region increases. Level of deoxygenated haemoglobin increases due to high consumption rate - > BOLD signal increased
o undershoot: blood flow and oxygen consumption dip before returning to original levels. This may reflect a relaxation of the venous system
o % signal change: 0.5-3%
o time to peak: signal peaks 4-6 sec after stimulus begins
What is cognitive subtraction in fMRI
- In imaging, ‘active’ area refers to physiological response that is greater relative to some other condition ► Need for baseline response, well-matched to experimental task.
- In functional neuroimaging studies, cognitive subtraction refers to an aspect of experimental design involving the comparison of two conditions or brain states
- The experimental and control tasks are considered to differ only for the processing stage(s) of interest, and differences in brain activations are considered to be related to such stage(s)
- In cognitive subtraction, you subtract the brain areas activated in the control condition from those activated in the experimental condition, so that what you end up with is only those areas that become activated in the response to the task.
Problematic assumption of cognitive subtraction
- Cognitive subtraction designs rely on the assumption of “pure insertion” – the notion that a single cognitive process can be inserted into a task without affecting the remaining processes, or that there are no interactions among the cognitive components of a task.
- BUT interactions do occur!
- Control task should be as similar to the experimental task as possible.
Alternative to cognitive subtraction
Cognitive conjunctions
What are cognitive conjunctions
• Cognitive conjunction requires a set of tasks that has a particular component in common. Look for regions of activation that are shared across several different subtractions.
• Blakemor et al (1998) Central cancellation of self-produced tickle sensation- cognitive conjunction in factorial design
–> Conducted a study using FMRI to investigate cognitive mechanisms contributing to the fact that people are unable to tickle themselves. A tactile stimulus device was developed that allowed for the following conditions: participants could move to generate tickles their own hands using the device, participants could move and no tickle sensation would be delivered, the experimenter could use the device to tickle the participants, and a hypothetical control condition where the participant did not generate a movement, nor were they tickled by the experimenter. fMRI results showed more self-produced tactile stimuli result in less activation of somatosensory cortex than externally produced tactile stimuli. In addition, activity in the anterior cerebellar cortex is less for self-produced movements that generate tactile stimuli relative to movements that do not. We propose that the cerebellum is involved in predicting the specific sensory consequences of self-generated movements, providing the signal that is used to cancel the somatosensory response to self-produced tactile stimulation.
What are parametric designs in fMRI?
- Variable of interest is treated as a continuous dimension rather than a categorical distinction.
- Associations between brain activity rather than differences between two or more conditions.
- No baseline necessary
Example of parametric design fMRI study
• Zilverstand et al 2017
For the stimulus set, we systematically manipulated perceived proximity of threat by varying a depicted spider’s context, size, and posture. An independent group participated in a subsequent fMRI anxiety provocation study (phobics n = 7; controls n = 7; all female), in which we compared a whole‐brain categorical to a whole‐brain parametric analysis. Results demonstrated that the parametric analysis provided a richer characterization of the functional role of the involved brain networks. In three brain regions—the mid insula, the dorsal anterior cingulate, and the ventrolateral prefrontal cortex—activation was linearly modulated by perceived proximity specifically in the spider phobia group, indicating a quantitative representation of an exaggerated anxiety response. In other regions (e.g., the amygdala), activation was linearly modulated in both groups, suggesting a functional role in threat monitoring.
What is functional integration?
The way in which different regions communicate with each other.
o Model how activity in different regions is interdependent.
o ‘effective connectivity’ or ‘functional connectivity’ between regions when performing a task.
o Use techniques like principal component analysis
Functional integration study
Greven et al 2017
The ventral visual stream has been associated with the detection of physical features such as faces and body parts, whereas the theory-of-mind network has been associated with making inferences about mental states and underlying character, such as whether someone is friendly, selfish, or generous. To date, however, it is largely unknown how such distinct processing components integrate neural signals. Using functional magnetic resonance imaging and connectivity analyses, we investigated the contribution of functional integration to social perception. We demonstrate that when observing someone who cues the recall of stored social knowledge compared to non-social knowledge, a node in the ventral visual stream (extrastriate body area) shows greater coupling with part of the theory-of-mind network (temporal pole).
