Thanks for all the help


Hallvard

-------------------------------------------------------------------------------
Siterer Eugene Duff <[log in to unmask]>:

> Hi Hallvard -
> It might be a concern if you were generating contrasts comparing some events
> that were relatively brief, say, 6 seconds, to events that were on average
> considerably longer, e.g.  > 15 or 20 seconds, as a lower amplitude for the
> brief events might simply reflect the fact that the hemodynamic response is
> not reaching its peak.  You also want to be careful if the two phases always
> occur in the same order - in some regions there may be some consistent
> temporal effect that is unrelated to the phases.
> You might use a second-level regressor to determine whether duration affects
> the amplitude of responses to a given phase, which may help you determine
> whether to trust your contrasts between phases when they have different
> average durations?  Stretching your time series is definitely not a good
> idea.
> Eugene
> 2009/6/5 Hallvard Røe <[log in to unmask]>
>
>> The differences are in the duration of the trials, because the time the
>> participants did use depended on strategy used during navigation
>>  and/or how well they knew the environment. I`m not comparing groups. But
>> I`m comparing different phases during spatial navigation. The total
>> navigation time varies, as well as the time of the different phases during
>> navigation. The total time of each block was maximally 40 sec. For the two
>> phases that we are most interested in comparing, it would in average be so
>> that if the participant used a long time to complete one of these phases,
>> then the other phase would also be long. However, in the other comparison we
>> did, then the relationship was oppsite, meaning that if one of the phases
>> was long then the other one would be shorter.
>>
>> Hallvard
>>
>>
>> -------------------------------------------------------------------------------
>> Siterer Eugene Duff <[log in to unmask]>:
>>
>>  Hi Hallvard -
>>> Are the differences in the total scan time or the duration of the trials?
>>>  What is your comparison of interest (e.g. are you comparing groups)?
>>> Some variation in, say, the number of trials across subjects does not
>>> usually have to be explicitly accounted for, as the variation in the
>>> reliability of pes from different sessions is taken into account in the
>>> standard statistics.
>>> Eugene
>>> 2009/6/5 Hallvard Røe Evensmoen <[log in to unmask]>
>>>
>>>  I have completed an fMRI experiment, in which it was a systematic
>>>> difference
>>>> between the amount of time used and the subjects. How to take this into
>>>> account when running the analysis?
>>>>
>>>> - Add an average time regressor at third level
>>>> - Multiply the time periods with a factor, so that the length of the
>>>> time-periods for those was fast becomes more similar to the length of the
>>>> time periods for those that used most time
>>>> .....................
>>>>
>>>> Anyone got any suggestions?
>>>>
>>>>
>>>> Best Regards
>>>>
>>>> Hallvard Røe Evensmoen
>>>>
>>>>
>>>
>>>
>>> --
>>>
>>> Eugene Duff
>>>
>>> FMRIB Centre,
>>> University of Oxford
>>> John Radcliffe Hospital, Headington OX3 9DU  Oxford  UK
>>>
>>> Ph: +44 (0) 1865 222 739  Fax: +44 (0) 1865 222 717
>>>
>>> --
>>>
>>>
>
>
> --
>
> Eugene Duff
>
> FMRIB Centre,
> University of Oxford
> John Radcliffe Hospital, Headington OX3 9DU  Oxford  UK
>
> Ph: +44 (0) 1865 222 739  Fax: +44 (0) 1865 222 717
>
> --
>