RFX in SPM5

Floris de Lange

florisdelange@gmail.com

RFX Options

Compare 2 conditions

Conditions are MI LH (press left foot) and MI RH (press right foot); 8 subjects; threshold used is p<0.001 uncorrected, k>20 (arbitrary)

Methods used:

• One-sample T-test on difference images MI LH>MI RH

• Paired-samples T-test on MI LH and MI RH

•Measurements assumed independent

•Measurements assumed dependent

• Two-samples T-test on MI LH and MI RH

• Multiple regression analysis on MI LH and MI RH

• Full factorial

• Flexible factorial

The gold standard: one-sample T-test

Paired T-test: dependence/indepence

Independent: error covariance matrix = identity matrix (check SPM.xVi.V!)

Dependent: error covariance matrix will be estimated (check SPM.xVi.V!)

Paired-samples T-test dep.: same

Paired-samples T-test indep.: same

Dependence/independence doesn’t make a difference here, because there’s only one sample to estimate covariance from

= identical

Multiple regression analysis: same

Two-samples T test indep: worse

Degrees of freedom ↑

Variance term ↑

Two-samples T test dep: better

• the correlation between the variance of the subjects in the first group and those in the second group is estimated

• this reduces the error term

Two-samples T test: dep vs indep

Dependent measures Independent measures

Two-samples T test: con images

Dependent measures Independent measures=

Two-samples T test: ResMS images

Dependent measures Independent measures<Error terms is reduced for dependent measures by modelling the dependencies

Full factorial dep. = 2-sample T dep

Full factorial indep. = 2-sample T indep

Flex factorial dep. = 2-sample T dep

Flex factorial indep = 2-sample T indep

Summary

•There are two types of models:

• Models that specify the subject factor (e.g., one-sample, paired-samples, MRA if you specify the factor yourself)

• Models that estimate the subject factor (e.g., two-samples T-test, full factorial, flexible factorial; measurements are dependent)

• If you don’t specify the subject factor, but also don’t estimate the error covariance, you are likely to shoot yourself in the foot because the errors will be assumed to be independent, and simply added, leading to much higher estimates of the error term

Is it valid to use 2-sample T test dep?

• It can be statistically beneficial to specify the model as a “between-subjects” model without modelling subject, but instead estimating the subject-induced regularities by specifying that measures may be dependent

• SPM5 manual suggests to do analyses this way

• But is it valid? Aren’t df’s inflated?

SPM5 manual