Before revision:  Computing geodesic distance requires that the functional connectivity matrices being compared are invertible (or positive-definite ) . As this requirement depends on the fMRI scanning length and the parcellation used, it is not always attainable .
After revision:  Computing geodesic distance requires that the functional connectivity matrices being compared are invertible (or positive-definite and hence invertible matrices . As this requirement depends on the fMRI scanning length and the parcellation used, it is not always attainable .