Hi all!
I'm performing an fMRI experiment with the following characteristics:
1. Paradigm: a visual stimulation paradigm, that contains a frequent
stimulus (presented 70 times/run) and an infrequent one (presented 30
times/run). The stimulus duration is 1 s. Subject has to press a button with
his/her right index finger in response to the frequent stimulus ("go"
stimulus) and to avoid pressing it when the infrequent one ("no go"
stimulus) appears on the screen. The inter-stimulus interval varies between
3.5 s and 4.5 s.
2. Acquisition: a Single-shot Gradient-Echo EPI sequence is used to acquire
the functional images (TR=2 s, TE=50 ms, 236 volumes acquired per run,
Reconstruction matrix: 64x64, 20 4.5 mm thick slices/volume, sequential
acquisition, voxel size=3.59 mm x 3.59 mm x 4.5 mm, ascending order).
3. Haemodynamic response is modeled with a Canonical Haemodynamic response
function (time and dispersion derivatives are considered too, in cases where
I see no activation when using only the canonical hrf).
4. I'm applying the regular preprocessing steps to the images (slice-timing
correction, registration, normalization to a standard space, smoothing).
The thing is that I'm getting go-related activation, but no no go-related
activation, even if I change the way I model the Haemodynamic response (and
thus, the design matrix) or the contrasts....
Any help?
Many thanks and regards,
--
Caro
--
Caro
