 |
 |
Dear Heather, If I understand your model setup correctly, you haven't explicitly modeled rest. Instead you have one EV for each of the force conditions plus one for all force conditions together. It is this latter EV that is causing rank deficiency. The reason is that this "common force" EV is a linear combination of the individual force EVs: common = 1/10*EV1 + 1/20*EV2+1/30*EV3+1/40*EV4*1/50*EV5 But the common force EV doesn't need to be included in the model to extract the information that you're interested in. With just the 5 EVs, you can compute a main effect of force minus rest using the contrast [1 1 1 1 1]. If you mask this inclusively with each of the force conditions relative to rest, you will guarantee that your effect isn't coming from a subset of conditions and instead is present in each. You can do this with avwmaths. Then you can look for parametrically changing force effects with whatever shape you are expecting. FOr a linear change, you can use [-2 -1 0 1 2] and for a second-order effect you could use [4 1 0 1 4]. You should note, however, that since you have an odd number of conditions, each of these contrasts ignores the middle value (contrast weight of zero) so you'll have to plot out effect sizes in any area of interest to guarantee that the data meet your expectations. Good luck. Joe -------------------- Joseph T. Devlin, Ph. D. FMRIB Centre, Dept. of Clinical Neurology University of Oxford John Radcliffe Hospital Headley Way, Headington Oxford OX3 9DU Phone: 01865 222 738 Email: [log in to unmask]