Everson R, Knight BW, Sirovich L. Separating spatially distributed response to
 stimulation from background .1. Optical imaging. BIOL CYBERN
 1997 DEC;77(6):407-417


        We consider the problem of estimating a small stimulus-induced response to
        stimulation that is masked by a fluctuating background when measurements of the
        background in the absence of stimulation are available, as is common in optical
        imaging of the cortex and in many other experimental situations. Two related
        methods based on the Karhunen-Lohe procedure are discussed. One seeks the
        function, an indicator function, that is most parallel to the response data and most
        orthogonal to the background data. The second removes the subspace spanned
        by the background from the response. Numerical investigations on simulated
        optical imaging data show that the first method is generally superior. Connections
        between the two methods and techniques for assessing the quality of the result
        are discussed.