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Dear SPM'ers, I want to plot the power spectrum of the estimated global AR(1) + white noise model that is used in SPM. I first create the residual forming matrix by I - S*X * pinv(S*X), where X is the design matrix and S is the filtering matrix residual_forming_matrix = eye(number_of_timepoints) - SPM.xX.xKXs.X * SPM.xX.pKX; I then multiply this matrix with the non-sphericity estimate, to obtain the correlation matrix correlation_matrix = residual_forming_matrix * SPM.xVi.V; As this matrix has a Toeplitz structure, I can obtain the auto correlation function as the first row (or column) of this matrix correlation_function = correlation_matrix(:,1); Finally I calculate the spectrum by using a Fourier transform abs(fftshift(fft(correlation_function))) Can anyone tell me if this is correct? /Anders -- -- ----------------------------------------------------------------------- Anders Eklund Phd student Medical Informatics, Department of Biomedical Engineering CMIV, Center for Medical Image Science and Visualization Tel: +46 73 6003790 mail: [log in to unmask] Fax: +46 13 101902 web: http://www.wanderineconsulting.com/ -----------------------------------------------------------------------