Hi Govinda Poudel,
> . If I orthogonalize EV1 with EV2 and run the model, the parameter
> estimates for EV2 will reflect percentage signal change per unit
> reaction time. My question is:
>
For this to make sense then I think you will want EV2 as variable-
height impulse function with height equal to UN-demeaned response
duration. Orthogonalisation then becomes unnecessary - EV1 will
represent the intersect and EV2 the gradient of the response to
reaction time. However to relate this to percent BOLD signal change
then you also need to do further post--processing to take the peak-to-
peak height of the EV into account and to compare the estimated
response to the size of the baseline BOLD signal.
The Featquery tool:
http://www.fmrib.ox.ac.uk/fsl/feat5/featquery.html
can help you do this using a mask or voxel coordinates.
Cheers, Mark.
----
Dr Mark Woolrich
EPSRC Advanced Research Fellow University Research Lecturer
Oxford University Centre for Functional MRI of the Brain (FMRIB),
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
Tel: (+44)1865-222782 Homepage: http://www.fmrib.ox.ac.uk/~woolrich
On 3 Feb 2009, at 09:53, Govinda Poudel wrote:
> Hi all,
>
> I want to look at relationship between BOLD response and duration of
> behavioural response (measured during a visuomotor task). In my
> understanding, I could model this by two regressors; EV1 as fixed-
> height impulse functions and EV2 as variable-height impulse function
> with height equals to demeaned response duration. If I orthogonalize
> EV1 with EV2 and run the model, the parameter estimates for EV2 will
> reflect percentage signal change per unit reaction time. My
> question is:
>
> 1) Is this the correct way of obtaining percentage BOLD signal Vs
> response duration relationship?
>
> 2) I want to create a plot with % signal change in Y axis and
> response duration in X-axis. I think this will require different EVs
> for different response duration. Is there any other way of doing this?
>
> Regards,
> Govinda Poudel
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