Fig. 5.

The top row shows the signal (with a b-value of 3000) from a cortical grey matter voxel and the bottom row from a white matter voxel with a three-way crossing. The left most column shows data and predictions obtained when deriving the hyperparameters from the grey matter voxel, the middle column when deriving them from the white matter voxel and the right most column when deriving them jointly from both voxels. It can be seen (lower left sub-figure) that the modelling of the white matter voxel is affected somewhat negatively when using the hyperparameters estimated from the grey matter voxel. The arrows point to data points where it can be seen that the distance to the model fit is greater than for the other two columns. When using the jointly derived hyperparameters, the $\mathcal{G}\mathcal{P}$ (lower right sub-figure) manages to capture the features of the data equally well as when the white matter derived hyperparameters are used (lower middle sub-figure).