Before revision:  However, the number of dimensions of fMRI signals are too large (hundreds of thousands of voxels in one volume) to be efficiently learnt by the MVPA. Researchers thus need to reduce the features to tens or hundreds of dimensions through feature selection/extraction that requires expert knowledgeand may lead to a selection bias .
After revision:  Conventionally, the SVM-MVPA requires careful feature selection/extraction that requires expert knowledgeand may lead to a selection bias .