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Dear Francis, Please forgive the delay in replying. I've been on Holdiay. ...... > There are a number of ways forward. First you could model both the > behavioural effects and the response to task preformance per se and > then the interaction between these. You could model all the > behavioural variables togther after some suitable orthogonalization > (and interactions among them). FIRSTLY, LETS SAY I PROCEED WITH THE FIRST OPTION YOU SUGGEST AND I DECIDE ON 3 BEHAVIOURAL MEASURES (E.G. ERROR, REACTION TIME, AND MOVEMENT DISTANCE). I WOULD CARRY OUT THE ANALYSIS (AS I SET OUT IN MY ORIGINAL EMAIL ABOVE -USER DEFINED OPTION ETC.) <underline>SEPARATELY</underline> FOR EACH OF THE BEHAVIOURAL VARIABLE. IS CARRING OUT SEPARARTE ANALYSIS CORRECT OR SHOULD (CAN) I CARRY OUT THE ANALYSES FOR ALL THREE VARIABLES AT THE SAME TIME ? IF I CAN, HOW DOES SPM 96 PROMPT ME FOR THE SECOND AND THIRD BEHAVIOURAL VARIABLE ? Parametric regressors such as yours are entered as 'User specified' covariates. Simply type 3 when prompted for '# of waveforms or conditions'. I would analyse all regresors together. SECONDLY, REGARDING THE INTERACTION BETWEEN THESE VARIABLES. HOW DO I ACTUALLY CARRY OUT TESTING FOR INTERACTIONS IN SPM96? You simply create a new covariate that is the product of the mean corrected covariates subtending the interaction. For example if A and B were vectors with two variables in them, the interaction AxB is simply AxB = spm_detrend(A,0).*spm_detrend(B,0); ...... > Yes. To say that activation has increased you would have to model a > task x error interaction. MY QUESTIONS HERE ARE RELATED TO THOSE (NEW) AT 2 ABOVE. HOW WOULD I MODEL SUCH A TASK X ERROR INTERACTION ? WOULD THIS INVOLVE LET'S SAY, AN ANALYSIS BASED ON A DELAYED BOX CAR DESIGN FOR THE TASK PER SE (BASED THE SCANNING DESIGN I USED -BASELINE-LEARNING PERIOD-BASELINE) AND THEN CARRYING OUT AN ANALYSIS BASED ON THE USER SPECIFIED OPTION, USING THE ERROR VALUE? I AM GUESSING HERE. Yes absolutely. In this instance the box-car would be A above and the error rate B. I hope this helps - Karl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%