Hi Jack. Yes, EVs 1,2,3 are probably all estimable, but it may well be
that contrasts between them are not (once noise is taken into account).
(Identification of this issue will be a simple addition to FEAT we're just
working on now.)
In general you should make "distinguishing judgments" such as you are
doing by using contrasts, as you suggest towards the bottom. This gives
the most unambiguous test of what you want to know, and it does sound like
with the EV1 and 2 regressors being so similar that the data cannot
provide a significant answer to this. In general it is dangerous to run a
design where you want to separate out different events that are so close
to each other in time!
Cheers.
On Sat, 30 Oct 2004, Jack Grinband wrote:
> Steve,
>
> >Is this heading in the right direction or are your issues something
> >different?
>
> This is exactly the issue that I'm struggling with.
>
> Let's say you have 4 regressors: EV1=cue1, EV2=working memory, EV3=cue2,
> and EV4=response. EV1, EV2, and EV3 are temporally continuous and the
> convolution makes them collinear. Brain Area 1 (BA1) is significantly
> correlated to EV1, BA2 is correlated to EV2, and BA3 is correlated to EV3.
> BA1, BA2, and BA3 are non-overlapping.
>
> 1. What can we say about the estimability of EV1, EV2, and EV3? It seems
> to me this should be related to the variance of the residual between each
> EV and it's reduced data.
>
> 2. With regard to false positives: Assume that cue1 lasts a very short
> time (a few hundred milliseconds) and the delay period/working memory is
> equally short. I don't doubt that we can detect a change in amplitude
> that's correlated with EV1 and EV2, but there may be enough noise in the
> data that variance associated with EV1 is attributed to EV2 and vice
> versa. So, it's not that I'm afraid of not seeing activation but rather
> that the activation is attributed to the wrong regressor. How confident
> can I be that regressors parse the variance correctly?
>
> 3. What can we say about the function of BA1, BA2, and BA3? That is, can I
> make the statement that BA1 activity represents cue encoding, while BA2
> activity represents working memory? Or must I always do a contrast between
> EV1 and EV2, such that the [1, -1] contrast represents cue encoding while
> the [-1, 1] contrast represents working memory? If the latter, what about
> EV3 and EV4... do I need to include them in the contrast as well?
>
> thanks a lot!
>
> jack
>
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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