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Dear All! I read the 1997 Friston PPI paper and couldn't find a solution to this special problem in the helpline: How can I test with psychophysiological interaction whether brain area A differentiates between high and low levels of X (e.g., memory load or bananas) only when activity in brain area B is *low*, that is, whether there is an *inhibitory* influence of area B on high vs low level encoding of X in area A? The problem is, I don't want area A to ever differentiate low>high X significantly, but rather high>low sensitivity to fade when area B activity is high. Should I do that by selecting area B as VOI and a) assigning [1] to high X and [-1] to low X as contrast weights when setting up PPI.ppi and then testing the [-1] contrast in the subsequent spm model or rather by b) assigning [-1] to high X and [1] to low reward as contrast weights when setting up PPI.ppi and then testing the [1] contrast in the subsequent spm model? Any advice appreciated Best wishes Markus -- Dipl.-Psych. Markus Staudinger Social Cognitive Affective Neuroscience Group Department of Psychiatry Division of Medical Psychology University of Bonn Sigmund-Freud-Strasse 25 D- 53105 Bonn Germany Tel: +49 (0)228 287 19704 Fax: +49 (0)228 287 19125 E-mail: [log in to unmask]