Within and betweengroup analysis

Once a measure or parameter of experimental response has been estimated in each fMRI time series, the resulting parameter maps can be registered in standard space. There are many possible computational algorithms for spatial registration. The most commonly used at present is an affine transformation, which applies a global and linear rescaling in three dimensions to each individual image. The most commonly adopted standard space is that represented in a stereotactic atlas of the brain originally written by Talairach and Tournoux to assist neurosurgeons in locating subcortical structures. (12> In both systems, each voxel is assigned a set of {x,y,z} coordinates which define its position. In Talairach-Tournoux space, the coordinates are defined relative to the cerebral midline and a line is drawn between the anterior and posterior commissures (intercommissural or AC-PC line). After registration, parameter maps are usually smoothed by applying a two- or three-dimensional Gaussian filter to accommodate variability in sulcogyral anatomy between subjects and error in spatial registration.

It is then possible to test a wide variety of hypotheses about the response parameters measured over several subjects at each voxel in standard space. For example, one can test the null hypothesis that there is zero mean or median power of experimental response within a group, or the null hypothesis that there is zero difference in the power of response between two groups. It is also possible to test for correlations between the power of functional response and some behavioural or symptom measure within a group. All these statistical tests can be conducted by permutation as well as by theory, and at cluster level as well as voxel level (see above).

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