I have multiple brain maps in nifti format . So I initialized those in a cell array and I wish to convert it to numeric matrix for svm . Can anyone tell me how to do that ?

I assigned the training set , grouptrain and test set and upon running the SVMStruct=svmtrain(trainingset, grouptrain) , am getting the following error as ":" Error using svmtrain (line 241) TRAINING must be a numeric matrix. " Please help.