Dear Funny
I would use al least 5-10 seconds of fixation cross. This would insure that the response to the rating procedure (with is about 10-15 sec long and very powerful in almost all brain areas) do not overlap with your video responses.
Possibly, since you contrast video responses between each other this should not be a big problem, however the rating responses can contribute to variation/noise of your video data.
I am not an expert in statistics, but I personally faced this kind of problem previously, and I think you should consider this reservation. Good idea is to make few pilot experiments and to see empirically, what is the signal.
Good luck,
Vladimir
--- On Thu, 6/23/11, Fanny Eugène <[log in to unmask]> wrote:
> From: Fanny Eugène <[log in to unmask]>
> Subject: Re: [SPM] Advice on timing for event-related design
> To: [log in to unmask]
> Date: Thursday, June 23, 2011, 7:46 PM
> Hi,
>
> Sorry I wasn't very clear. The fixation will be in addition
> to the 10 second trial. Each trial includes a thermal
> stimulation (6 sec) and a rating scale (3 sec).
>
> thanks,
>
> Fanny
>
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