What does orthogonalisation of EVs mean, and when do I use it?
Orthogonalisation is a process of modifying an EV so that it does not share any common signal with the other EVs present. Technically, the vectors are altered so that they have zero dot product (i.e. are orthogonal). When applying orthogonalisation in FEAT it alters the current EV to be orthogonal to the specified EVs. This means that any signal which this EV shared with the other EVs is, after orthogonalisation, attributed solely to these other EVs. Orthogonalisation should be used to prevent additional EVs changing the parameter estimates (betas) associated with previous EVs. For example, when adding a confound like a motion parameter it is desirable to orthogonalise the confounds with respect to the EVs of interest. This prevents the power in the activation signals being shared between the EVs of interest and the additional confound. For more information on orthogonality see the FEAT Manual or the FSL Course Slides. • When do you add temporal derivatives and what are they for? Tempo
