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 . Hence Venkatesh and colleagues employed a simple regularization procedure to ensure that the matrices were invertible .
After 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 and sometimes a regularization procedure is required .