Hi - yes, this is effectively the same scenario as what I was
comenting on. Probably the important factor here is that significance
of correlation for a given EV is given according to the part of the
data that it UNAMBIGUOUSLY explains in a way that no other EVs do -
hence if you have two similar EVs then the component of each that is
orthogonal to the other will be small and even [1 0] and [0 1]
contrasts will have little power. Try this out in the efficiency tool
in FEAT.
Cheers.
On 6 Nov 2008, at 09:15, Nils Richter wrote:
> Hi,
> I'm sorry, but I don't think I expressed myself clearly.
> The timecourses were not averaged for the one EV models. There were
> two one EV models, that were run seperately. Upon visual comparison
> they also looked different.
> When I put the two timecourses into one model as EV1=timecourse of
> stimulus and EV2= timecourse of response the contrasts for each of
> them showed almost no activation (as opposed to sensible activation
> when running them seperately) and the timecourse comparing them
> (contrast 3: EV1=1, EV2=-1) showed weird subcortical activation.
> Now the latter doesn't really surprise me, maybe there are no
> significant differences between the timecourses at the thresholds I
> used, but I would have assumed that activation pattern for the
> contrasts running just the individual timecourses would look like
> the activation for timecourses each run in their own model.
> I have the feeling that I am missing something really obvious here
> regarding the statistics involved in using multiple contrasts.
> Cheers,
> Nils
>
>
> -------- Original-Nachricht --------
>> Datum: Thu, 6 Nov 2008 08:12:48 +0000
>> Von: Steve Smith <[log in to unmask]>
>> An: [log in to unmask]
>> Betreff: [FSL]
>
>> Hi - I think you're saying that you have two somewhat similar
>> timecourses; when you in some way averaged them into a single EV you
>> found reasonable activation, but when you separated them, they didn't
>> show separate activations (or anything under the differential
>> contrast). This isn't necessarily surprising if they are similar to
>> each other - assuming they are both accurate models, then you can get
>> back to the 'average' result with a [1 1] contrast - but it they are
>> very similar to each other, all the other contrasts will be very low
>> efficiency (see the efficieny reporting in the FEAT model GUI, and
>> also the Smith NeuroImage paper on efficiency).
>>
>> Cheers.
>>
>>
>> On 5 Nov 2008, at 11:42, Nils Richter wrote:
>>
>>> <[log in to unmask]
>>>>
>>> <[log in to unmask]>
>>> <[log in to unmask]>
>>> Subject: comparing timecourses
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>>>
>>> Hi everybody,
>>>
>>> I have a problem regarding the comparison of two different
>>> timecourses. One is the timecourse of stimulus onset and the other
>>> is the timecourse of response onset. Because the time from stimulus
>>> to response varies from trial to trial the shift between the
>>> timecourses ist not the same for each trial.
>>> I want to see if they yield different activation. Now I have run
>>> both timecourses in one EV models and on visual inspection the
>>> activation appears to be somewhat different between the two.
>>> To quantify this difference I have tried to run the two timecourses
>>> as EVs in the following two EV model:
>>>
>>> EV1: stimulus timecourse
>>> EV2: response timecourse
>>>
>>> Contrast 1: EV1=1, EV2=0
>>> Contrast 2: EV1=0, EV2=1
>>> Contrast 3: EV1=1, EV2=-1
>>>
>>> The three contrasts all show very little activation and the
>>> contrasts 1 and 2 bear no resemblance to the contrasts in the one ev
>>> models, which I can't make sense of.
>>> There appears to be some sort of interaction between the contrasts.
>>> Cheers,
>>> Nils
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>>
>>
>> ---------------------------------------------------------------------------
>> Stephen M. Smith, Professor of Biomedical Engineering
>> Associate Director, Oxford University FMRIB Centre
>>
>> FMRIB, JR 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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---------------------------------------------------------------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR 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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