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
