[log in to unmask]" type="cite"> Dear Donald,

Many thanks for your response. As far as sensory events go, I indeed have 8 Evs of interest (tactile left, tactile right, visual left,  visual right, tactile left/visual right, visual left/tactile right, with 2 EVs for each type of double stimuli depending on whether the subject detected both stims or only one of them).

Regarding the number of events, for each single stimulus condition (tactile or visual, left or right), I have in average 55 hits per subject, distributed in 8 runs (8 trials per run and per condition, with a few misses). As for each double stimulus condition, I have between 90 and 100 hits per subject, again distributed in 8 runs (16 trials per run and per condition). So I'm guessing this should be sufficient to get stable activations? The only conditions for which I'm aware I have too few events are the double stimulation trials where only one has been reported by the subject (10-20 per subject across the 8 runs), but should this prevent me of getting stable estimates for the other conditions?

Regarding your second point, I don't think this might be an explanation, as motor preparation wouldn't lead to robust activation of M1.

What puzzles me is the fact that I don't get any activation in motor areas when I compare a sensory event, say tactile left, versus rest, whereas I get systematic and robust activation in M1 when I compare the same events vs. its contralateral homologue (tactile right). Or, that I don't get any motor activity when I contrast hits on double stimuli versus rest, whereas I get strong activations in M1 when I compare the same events (with a double foot press) versus hits on single stimuli (with a single foot press).

Thanks again for your assistance!

Best,

Stéphane

Stéphane Jacobs - Chercheur post-doctorant / Post-doctoral researcher

ImpAct - Inserm U1028 - Equipe Pélisson
Centre de Recherche en Neurosciences de Lyon
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Le 07.06.12 19:51, MCLAREN, Donald a écrit :

[log in to unmask]" type="cite">I think you want to model at least 7 event types (tactile left, tactile right, visual right, visual left, motor left, motor right, motor both) and then contrast the events you are interested in comparing. If you are interested in correct and incorrect responses, then this would also need to be coded.

I suspect part of the issue that you might be having is that you don't have enough trials to get stable estimates for each event type that you want to compare. 

One other thought is that subjects might be preparing to make a movement, which would lead to motor cortices activating differently for different conditions.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Thu, Jun 7, 2012 at 6:33 AM, Stephane Jacobs <[log in to unmask]> wrote:

Dear list,

I have a fast event-related design experiment where subjects receive a visual or tactile stimulation on either side, and subsequently have to wait for the fixation dot to turn from green to red to give their response (on what side were they stimulated) by pressing a foot pedal. The intervall between the stimulus onset and the "go" signal is jittered between 1500 and 2500 ms.

In a first analysis that I run as a "sanity check", I included one regressor per type of sensory stimulation, and one per type of motor response (left, right and double), contrasted them vs. rest (20 sec periods included in the implicit baseline). I get sensible activations for each type of sensory stimulation, without activations related to the (correlated) motor response, which are well captured by the corresponding regressors and contrasts.

In a second analysis that is supposed to give me the contrasts I'm really interested in (i.e. differences across sensory stimulations depending on the behavioral responses), I included only the regressors corresponding to the different types of sensory events, not those modeling the motor responses, and contrasted my conditiosn of interest against each other rather than against rest. Here, I again get sensible activations with respect to the sensory stimuli, but also strong activations in primary and pre-motor cortices related to the motor response usually associated with the sensory stimulus (e.g. left M1 for right tactile stimuli) when I contrasts one condition vs. another that does not involve the same motor response (e.g. tactile left vs. tactile right). A critical contrast compares trials with double stimuli (e.g. tactile left/visual right) with a correct answer (double response) to those where the subject missed either one of the two stimuli (left or right response).

So I ran the same analysis again, only this time including an additional EV (not used in the contrasts) modeling all the motor responses (left, right, double), hoping that this would capture the activity related to motor responses and "clean" my contrasts of interest. Unfortunately, I get results very similar to those of my second analysis.

Can anyone think of a better approach to the problem? Or do I need to change my design (either make the jitter longer, or make so that repsonses are comparable across conditions (e.g. same hand, different fingers))?

Many thanks to anyone who can help, as always!

Best,

Stéphane