Functional Magnetic Resonance Imaging (fMRI) Research Flashcards
1
Q
Comment on analysis of fMRI.
A
- We can see local activity in the scans because our signal quality is (slightly) better due to the presence of more oxygenated blood.
- These areas of enhanced activity can then be mapped onto a structural image of the brain Statistical Parametric Mapping
- For the analysis a General Linear Model is usually fitted to brain activity at each measurement point (voxel): significantly stronger activation in region X for task A compared to task B is interpreted as involvement of the region in A
2
Q
What is an activation blob?
A
Statistical effects in experiment often colour-coded activation and de-activation.
3
Q
Discuss functional Magnetic Resonance Imaging and BOLD.
A
- In a typical fMRI experiment, BOLD signal within a region is measured while participants engage in a cognitive task
- Repeated measurement of brain activity is required for the whole brain while performing experimental tasks (e.g. A and B) because the signal is very noisy
- In conclusion, differences in BOLD signal tell us something about whether a brain regions ‘is engaged’ in the task.
4
Q
What can we do with fMRI?
A
- One of the major uses of fMRI is localising cognitive functions
- The kind of question researchers ask: ‘Which areas of the brain are responsible for the function X?’ This is a typical ‘brain mapping’ approach
- For example, we know that there are distinct regions in the frontal cortex that show difference in their cytoarchitecture (i.e. the composition of cell types)
- But what do they do? Are these areas fulfilling different roles for cognition?
- We also know that these regions are interconnected, and they might form smaller networks
- fMRI is used to measure activation when people perform different cognitive tasks with the goal to understand what each of these regions might contribute
5
Q
Describe Sohn et al. (2000)
A
- When contrasting switching with not switching between task rules (btw letter task and digit task), the researchers found a region in prefrontal cortex and another region in posterior parietal cortex to be significantly activated
- This means that these regions might be ‘involved’ in task-switching
- Note how tempting it is to conclude that these regions actually ‘do’ the switching! But can we conclude that?
- One problem is that they could also just encode the colour of the cue
6
Q
Describe Brass & Von Cramon (2004)
A
- The previous study illustrated nicely that we learn from these studies depends on how well we can exclude alternative interpretations
- Brass and von Cramon wanted to know whether the prefrontal region is really involved in task-switching per se, independent of how a switch is cued
- Their clever idea was to use different cues that indicate the same rule switch!
- They found that there was indeed a specific region in prefrontal cortex that was always activated when people switched between tasks
- Importantly, they also looked at how consistently this finding has been observed
- Mapping of brain areas to functions should always be based on multiple studies
7
Q
Discuss the process of
localising Cognitive Functions
A
- Brain mapping approaches have been most successfully used in the visual system
- The advantage here is the we already have a lot of knowledge from animal studies on how visual information is represented
- We can now use fMRI to confirm that the human visual system represents information in the same way and study the transformation of visual content as it becomes available to higher-level visual areas in the cortex
- One interesting question is how complex visual objects, such as faces are represented in the brain
8
Q
Describe Kanwisher et al. (1997)
A
- Investigated how faces are represented in the brain
- They presented their participants with images of faces and contrasted BOLD signals to when participants saw objects
- They found a region located in the fusiform gyrus responding more strongly to faces than to objects
- They could show this result reliably in most of their participants, and they could replicate it with different participants
- Does that mean that this area is specialised on the processing of faces?
- To rule out that this result was simply due to using objects, they replicated the study with faces> scrambled faces
- This contrast confirmed the initial findings: the same brain area was strongly activated for faces but not for scrambled faces
- However, they decided to be really sure and ran another control condition
9
Q
Describe Kanwisher et al. (1997)’s second study
A
*This time, they contrasted faces> houses
- The result w as the same: the region in the fusiform gyrus showed again stronger activation for faces
- Maybe this region simply processes body parts in general?
- In a final control condition, they used pictures of other body parts, i.e. hands, as a control
*When contrasting faces > hands, the results again replicated
- Now it really looks like this brain region is specialised on faces
- This finding has been replicated many times, and it is one of the strongest in fMRI research
- It indeed led researchers to name the region ‘fusiform face area (FFA)’- because they were so sure that this is the brain’s ‘module’ for face processing
Other modules that have been found:
- Parahippocampal Place Area (PPA): houses and places
- Extrastriate Body Part Area (EBA)
- Some regions specialised for letters/tools/animals
- Study is a great example for a well controlled experiment, i.e. looking at the clear profile of activation, there is no doubt that what takes place in the FFA is face processing
10
Q
Highlight criticisms of fMRI Research
A
- One theoretical argument is that it would be simply impossible if the brain had ‘modules’ for everything- it would run out of space! We just know too many different objects!
- Is it possible that Kanwisher and colleagues have not considered something very important? Could it be that it only looks like this region is specialised on faces?
11
Q
Describe Gauhier et al. (1999)
A
- Gauthier et al (1999) asked participants to distinguish btw ‘Greebles’- strange and totally novel (and faceless) objects, which no participant had ever seen before
- During the experiment, participants learned the family structures and became experts for Greebles
- First, when participants did not know much Greebles, the FFA responded strongly to faces, but not to Greebles, as Kanwisher et al would have predicted.
- However after learning to distinguish really well btw individual Greebles and Greeble families, the FFA also responded to Greebles!
- This means, activation in FFA might reflect expertise rather than being a module for face processing per se.
- We all just happen to be experts for faces!
12
Q
Describe Malach et al. (2002)
A
- Argued that the visual system might not be organised by specific object categories, but by where in our visual field objects are usually encountered
- Organisation in ventral visual cortex might follow cortical topography i.e. eccentricity mapping
- Argued that coding is driven by resolution needs- FFA is good for everything that usually requires ‘high resolution’
- We usually need a high resolution to recognise faces, simply because we need to really see the details
- The ‘module’ for places/houses (the Parahippocampal place area, PPA) is in reality just very suited for processing the periphery, not houses per se- but that’s where places/houses usually are in our visual field
- Researchers have found evidence for all 3 coding schemes- and suggested that all might be true to some extent
- The fMRI signal might therefore reflect a mixture of all three coding schemes
- This is a great example of how difficult it actually is to interpret fMRI results- even if, in the beginning, we might think that there is clearly just one possible explanation
13
Q
Discuss the BOLD fMRI technique regarding understanding cognitive functions.
A
- The previous experiments highlight a big issue: when we interpret fMRI results, we are confronted with the problem of reverse inference
- As psychologists, we (usually) do not want to understand where cognitive functions are located, but we want to understand the mental architecture underlying these functions
- We want to use neuro-imaging as another tool to gather evidence for the engagement of mental/cognitive processes in a particular task
- However- is this valid?
14
Q
Discuss the validity of the BOLD fMRI technique regarding understanding cognitive functions.
A
- We usually apply the following logic:
1. In this study, when task A, then brain region Z is active
2. In other studies, when cognitive process X, brain region Z is active
3. Thus, in this study activity in Z engagement of cognitive process X
One problem is that (2) is not exclusive: brain region Z may be active for many tasks. - Some researchers have concluded from looking at results from many studies that more anterior regions (towards front of brain) represent more abstract info and more posterior regions (towards the back) represent more specific content
- Others concluded that most regions in the frontal cortex can actually be found to be activated in a lot of tasks
15
Q
Describe Duncan (2001, 2010, 2013)
A
- Argued that the frontal cortex shows relative, but not absolute specialisation
- This means, prefrontal regions might just be recruited ‘more strongly’ if the task at hand becomes more difficult
- This, according to Duncan, is true for other regions as well
- The problem now is that, if we find activation in a region which is part of his ‘multiple-demand’ network, we still don’t really know what the region is doing
