1st Level Analysis Contrasts and Inferences

1. 1st Level AnalysisContrasts and Inferences Nico Bunzeck Katya Woollett

2. What do we use fMRI for? • Functionalspecialisation: • 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

3. 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

4. Design overview Cognitive subtraction Conjunctions Factorial

5. Cognitive subtraction • 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

6. 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

7. B A Region(s) involved in the cognitive/ sensorimotor process of interest Baseline/controltask identical to A except for process of interest Activation task involving process of interest Assumptions of cognitive subtraction • The experimental task and baseline/ control task must be identical in every way except for the process of interest

8. For example… Question: brain regions are responsible for biological motion? Task A Task B Regions involved in biological motion? Activation task Point light display movie Baseline / controltask Point light display static image MT / V5 STS Frontal eye fields Cerebellum Parietal cortex Violates assumption 1: task A and B identical but the process of interest Many processes in addition to presence of biological motion in A including visual motion and eye movements

9. For example… Question: areas for biological motion? Task A Task B Regions involved in biological motion? Activation task Point light display moves Baseline / controltask Random dots moves MT / V5 STS Frontal eye fields Cerebellum Parietal cortex A better baseline to answer this question.

10. B A Region(s) involved in the cognitive/ sensorimotor process of interest Baseline/controltask identical to A except for process of interest Activation task involving process of interest Assumptions of cognitive subtraction 2. There must be no implicit processing of the component of interest in the baseline/control task

11. Task A Task B Regions involved in semantic processing? Activation task read words aloud Baseline / controltask look at words For example… Question: is inferotemporal cortex involved in the semantic processing? Violates assumption 2: Note implicit processing of the component of interest in the baseline task

12. Summary • The experimental task and baseline/ control task must be identical in every way except for the process of interest • There must be no implicit processing of the component of interest in the baseline task

13. Conjunctions • 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.

14. For example… B1 Baseline task Say “Yes” A1 Activation task Read words - Form processing Lexical orthography Sublexical orthography Badge lotus Phonological retrieval A2 Activation task Name picture B2 Baseline task Say “Yes” - Form and colour processing Object structure

15. Using Conjunctions- Summary • 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

16. Factorial Design • In factorial designs there are two or more factors • The main effects of each factoridentify brain areas that respond to that particular factor of interest • The interaction between factorsidentifies 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

17. Factor A A1 A2 B1 1 2 Factor B B2 3 4 Factorial design 2x2 Sugiura et al, 2005

18. Factorial design 2x2 • MAIN EFFECTS • Difference between columns • Difference between rows • INTERACTIONS • Difference between columns depending on status of rows (or vice versa)

19. Factor A A1 A2 B1 1 2 Factor B B2 3 4 Main effects Main effect of factor A1 (1+3)-(2+4) Factor A1 BOLD signal in voxel Y Factor A2 B1 B2

20. Factor A A1 A2 B1 1 2 Factor B B2 3 4 Main effects Main effect of factor B1 (1+2)-(3+4) Factor A1 BOLD signal in voxel Y Factor A2 B1 B2

21. Factor A A1 A2 B1 1 2 Factor B B2 3 4 Factor A1 Factor A2 Interactions … Interaction between the factors (1-2)-(3-4) BOLD signal in voxel Y B1 B2

22. Factor A A1 A2 B1 1 2 Factor B B2 3 4 Crossover interaction Interaction between the factors A1 B1 and A2 B2: (1-2)-(3-4) Factor A2 BOLD signal in voxel Y Factor A1 B1 B2

23. How do I do it in SPM • Now we have clear what comparisons we want to make… to answer our question