1st Level Analysis Contrasts 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 interest Many 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 / V5 STS Frontal eye fields Cerebellum Parietal cortex

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

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 involved in 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 task Read words B1 Baseline task Say “Yes” A2 Activation task Name picture B2 Baseline task Say “Yes” - Badgelotus 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 2x2 Factor B Factor A B1 A1 B2 A 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) Factor B Factor A B1 A1 B2 A BOLD signal in voxel Y Factor A1 Factor A2 B1B2

Main effects Main effect of factor B1 (1+2)-(3+4) BOLD signal in voxel Y Factor A1 Factor A2 B1B2 Factor B Factor A B1 A1 B2 A

Interactions … Interaction between the factors (1-2)-(3-4) BOLD signal in voxel Y B1B2 Factor A1 Factor A2 Factor B Factor A B1 A1 B2 A

Crossover interaction Interaction between the factors A1 B1 and A2 B2: (1-2)-(3-4) Factor A1 Factor A2 B1B2 Factor B Factor A B1 A1 B2 A 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