1st Level Analysis Contrasts and Inferences Nico Bunzeck Katya Woollett.
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Transcript of 1st Level Analysis Contrasts and Inferences Nico Bunzeck Katya Woollett.
1st Level AnalysisContrasts and Inferences
Nico Bunzeck
Katya Woollett
What do we use fMRI for?
Functional specialisation:– Identification of regionally specific effects
that can be attributed to changing stimuli or task conditions
Functional integration:– Identification of interactions among
specialised cortical areas and how these interactions depend upon context
Planning the experiment
we should have a clear…
• Hypothesis / Question • Design that help me to answer my question• How I am going to build my SPM model• How I am going to analyse my data• What contrasts and inferences are made is
dependent on choice of experimental design
Design overview
Cognitive subtraction
Conjunctions
Factorial
Definition: the difference between two tasks can be formulated as a separable cognitive or sensorimotor component
Then, regionally specific differences in haemodynamic response, evoked by the two tasks, identify the corresponding functionally specialised area
Many subtraction designs rely on the assumption of pure insertion
Cognitive subtraction
Pure insertion
Pure insertion: A new cognitive (A) component can be purely inserted without affecting the expression of the previous ones (e.g., B).
If this were not the case the difference between tasks that did, and did not, include component B would depend on the presence of component A.
Pure insertion discounts both functional and psychological interactions and therefore represents a very restricted precondition for cognitive subtraction
1. The experimental task and baseline/ control task must be identical in every way except for the process of interest
Assumptions of cognitive subtraction
Activation task involving
process of interest
Baseline/control task
identical to A except for process
of interest
Region(s) involved in the
cognitive/ sensorimotor
process of interest
A B
For example…
Violates assumption 1: task A and B identical but the process of interestMany processes in addition to presence of biological motion in A including visual motion and eye movements
Question: brain regions are responsible for biological motion?
Activation task
Point light display movie
Baseline / control task
Point light display static image
Regions involved in biological motion?
Task A Task B
MT / V5STS Frontal eye fields Cerebellum Parietal cortex
For example…
A better baseline to answer this question.
Question: areas for biological motion?
Activation taskPoint light
display moves
Baseline / control task
Random dots moves
Regions involved in biological motion?
Task A Task B
MT / V5STS Frontal eye fields Cerebellum Parietal cortex
2. There must be no implicit processing of the component of interest in the baseline/control task
Assumptions of cognitive subtraction
Activation task involving
process of interest
Baseline/control task
identical to A except for process of
interest
Region(s) involved in the
cognitive/ sensorimotor
process of interest
AB
For example…
Activation task
read words aloud
Baseline / control task
look at words
Regions involved in semantic
processing?
Task A Task B
Violates assumption 2: Note implicit processing of the component of interest in the baseline task
Question: is inferotemporal cortex involvedin the semantic processing?
1. The experimental task and baseline/ control task must be identical in every way except for the process of interest
2. There must be no implicit processing of the component of interest in the baseline task
Summary
• Cognitive conjunctions combine a series of subtractions with the aim of isolating a process that is common to two (or more) task pairs
• The assumption of pure insertion can be avoided by extracting the presence of a main effect in the absence of an interaction
• Conjunctions have the advantage of testing the effect independently of the task context, thereby controlling for influences of the effect on the context.
Conjunctions
For example…
-A1
Activation taskRead words
B1
Baseline task
Say “Yes”
A2
Activation task
Name picture
B2
Baseline task
Say “Yes”
-
Badge
Phonological
retrieval
Form processing
Lexical orthography
Sublexical orthography
Form and colour processing
Object structure
• The only restriction is that differences between the task pairs both contain the component of interest
• The analysis results in any commonality in activation differences between the task pairs
• The resulting region should be uniquely associated with the process of interest, not any interactions specific to each subtraction
Using Conjunctions- Summary
Factorial Design
In factorial designs there are two or more factors
The main effects of each factor identify brain areas that respond to that particular factor of interest
The interaction between factors identifies brain areas where the effect of one factor varies depending on the presence or absence of the other factor
This allows to measure the effect of one factor on the expression of the other factor
Factorial design 2x2F
acto
r B
Factor A
B1
A1
B2
A2
1 2
3 4
Sugiura et al, 2005
Factorial design 2x2
MAIN EFFECTS
- Difference between columns
- Difference between rows
INTERACTIONS
- Difference between columns depending on status of rows (or vice versa)
Main effects
Main effect of factor A1
(1+3)-(2+4)
Fac
tor
B
Factor A
B1
A1
B2
A2
1 2
3 4
BOLD signal in voxel Y
Factor A1
Factor A2
B1 B2
Main effects
Main effect of factor B1
(1+2)-(3+4)
BOLD signal in voxel Y
Factor A1
Factor A2
B1 B2
Fac
tor
B
Factor A
B1
A1
B2
A2
1 2
3 4
Interactions …
Interaction between the factors (1-2)-(3-4)
BOLD signal in voxel Y
B1 B2
Factor A1
Factor A2
Fac
tor
B
Factor A
B1
A1
B2
A2
1 2
3 4
Crossover interaction
Interaction between the factors A1 B1 and A2 B2:
(1-2)-(3-4)
Factor A1
Factor A2
B1 B2
Fac
tor
B
Factor A
B1
A1
B2
A2
1 2
3 4
BOLD signal in voxel Y
How do I do it in SPM
Now we have clear what comparisons we want to make… to answer our question