> Does increasing the number of basis actually increase the accuracy of > image registration in SPM and AIR ? Sometimes. Nonlinear registration is (approximately) formulated as a MAP problem, which maximises: P(warp , data) = P(data | warp) * P(warp) this is equivalent to minimising: -log(P(data | warp)) - log(P(warp)) The regularisation term encodes P(warp) as a multi-variate Gaussian, whereby (in SPM2) -log(P(warp)) = the "bending energy" of the deformation If the form and magnitude of the regularisation are an accurate estimate of the prior probability distribution, then having more basis functions should improve the result. If they are not, then increasing the number of basis functions can decrease the accuracy. > I am assuming that most image registration techniques (based on basis > function expansion) estimate the lower order coefficients first and > estimate the higher order coefficients later. In SPM, they are all estimated together. Higher frequencies tend to be regularised more heavily than lower frequencies though, so they tend to be fitted last. In SPM, there is also a rough coarse-to-fine strategy, whereby the earlier iterations use more regularisation. The coarse-to-fine strategy is a bit of a mess (although it works not too badly), and should probably be re-written to use Laplace approximations to an ML-II (REML) scheme, in order to estimate the best trade-off between regularisation and likelihood terms. The non-IID nature of the residuals makes this slightly difficult though (see my very old attempts in 1D in http://www.fil.ion.ucl.ac.uk/~john/misc/nonlin_reml.m and http://www.fil.ion.ucl.ac.uk/~john/misc/nonlin_reml_smo.m ). > In this sense, variance of > the low order coefficients is larger than the variance of the higher order > coefficients. Is my logic correct? It's fairly correct. Look around line 256 of spm_normalise.m. You will see an inverse covariance matrix, which is used to generate the penalties. Lower frequencies are penalised much less than higher frequencies. All the best, -John