Hi Jesper,
Thanks a lot for your response, especially because you make my life
simpler!!! Just a pecision: to only model my stimuli as "events", should
I use the custom EVs with one entry per volume, specifiying only onset
times?
And thand you for the additional explanations!
Best,
Stéphane
Stéphane Jacobs - Chercheur post-doctorant / Post-doctoral researcher
ImpAct - Inserm U1028 - Equipe Pélisson
Centre de Recherche en Neurosciences de Lyon
16 avenue du Doyen Lépine
69676 Bron Cedex, France
Téléphone / Phone: (+33) (0)4-72-91-34-20
Le 25/05/12 00:10, Jesper Andersson a écrit :
> Dear Stephane,
>
> it sounds like you are making life complicated.
>
> In your case you can just model them as "events" which will by definition have a negligible duration. Just treat the three cases as separate event types (i.e. use three EVs).
>
> The only time when the duration of events matters is if you have events of the same type (e.g. visual stimuli) which have different durations. In that case (and with events as short as yours) it is easiest to model that modulating the height of the sticks (i.e. using the third column in the three column format). But, as I said, in your case you don't need to do that.
>
> Good luck Jesper
>
> On 24 May 2012, at 14:56, Stephane Jacobs wrote:
>
>> Hello list!
>>
>> I have a design where I use visual stimuli (6ms long), tactile stimuli (2ms), and combined, visuo-tactile stimuli, composed of the two previous ones (6ms and 2ms, common onset). I have questions regarding the modeling of such short events, and also regarding the best solution to model the different length events with respect to the contrasts I want to analyse.
>>
>> Regarding how to model very short events such as mine, I've read a few threads, but did not get a clear answer - to me, that is. Steve Smith has said that " the sampling of the "high resolution temporal model" generated initially by feat_model when the model is processed is 0.05 seconds, so anything shorter than this may get lost" (https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind04&L=fsl&D=0&1=fsl&9=A&I=-3&J=on&d=No+Match%3BMatch%3BMatches&z=4&P=274675). He then advised to modify and recompile feat_model.c, which I'm willing to do if that's the best solution, but I'll definitely need help with this... :-)
>> On the other hand, in another thread (that I can't find any more,sorry!), I think it was Steve again who advised to multiply actual durations by a common factor so that they would all be superior to 100ms or so, saying that this should not modify the model sensitivity, if I undestood correctly. I've done this, using 200ms and 600ms to model my tactile and visual events, respectively. However, I clearly get stronger activations for tactile than visual events, even in areas related to common components of the task (i.e. V1 related to the permanent fixation LED - my visual stimuli activate more V5). This leads to artificial activations when I contrast tactile vs. visual stimuli, whereas the opposite contrasts give empty stat maps. Could this be because 600ms is really to much longer than the actual duration?
>> By contrast, if I model all events with a 400ms duration, which is what I had decided to use for the visuotactile events anyway (the average of tactile and visual events duration, x100ms), then contrasts against rest don't change, but the contrasts between tactile and visual events are much "cleaner".
>>
>> Having said that, I was wondering whether it would be worth it to model everything with real durations, and thus maybe modify feat_model to have a better temporal resolution? Would it make sense to model all events with 100ms or 200ms durations, to use durations>100ms that would not be so much longer than the actual ones?
>>
>> The second question is already somewhat embeded in the first: should I model my different events with different durations, knowing that I would like to run contrasts between them? If so, what duration should I pick for the visuotactile events?
>>
>>
>> I hope my message isn't too confusing... Any help with those issues woujld be greatly appreciated, as always!
>>
>> Best regards,
>>
>> Stephane
>>
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