hi,

I am analysing some data where participants were exposed to a stimulus on three different intensity levels.

So far,  these levels are coded in the design matrix as three seperate EVs. it seems to me that it is problematic to test for a linear effect across ALL three levels, by using a contrast [-1 0 1] (assuming, intensity increases from EV1 to EV3). However, this is the approach suggested in the last 2 paragraphs on this fls web page: http://www.fmrib.ox.ac.uk/fsl/feat5/glm.html.

My problem with the [-1 0 1] contrast is that the result of this test is totally independent of the PE for EV2. The contrast would also get significant, if the activation under medium intensity stimulation is lower (higher) than under minimum (maximum) intensity stimulation. [-1 0 1] seems only to test, if activation increases from the minimum to the maximum level.

Or am I getting something wrong?

 

It seems to me that in order to test for a linear effect, one would need to use a different design matrix, where one  EVs codes for the presence of a stimulus and a second EV codes for the intensity (e.g., with 1,2,3 for the linear assumption.). A linear effect could than be tested with a contrast on the second EV. Would this be the proper way to do it?

 

Or are there yet others ways to test a linear contrast with only three intensity levels?

 

cheers,

guido