Yes - that's all you need - you may want to mask with the magnitude of
the response (do an f-test across the two EVs) to make the phase map
look more interpretable.
Cheers.
On 22 May 2009, at 18:52, Michael Scheel wrote:
> Hi Mark, thanks for the answer. Could you check if what I did is
> correct:
>
> Define 2 EVs and run feat
> EV1 - Sinus: Sinusoid with Period 32 and Phase 0.
> EV2 - Cosinus: Sinusoid with Period 32 and Phase 24.
>
> in the feat directory, look for pe1 and pe2, then do
> fslmaths pe2.nii.gz -div pe1.nii.gz phase_angle.nii.gz
> Correct?
>
> How would I then proceed?
>
> Thanks, Michael
>
>
> On 21-May-09, at 2:08 PM, Mark Jenkinson wrote:
>
>> Hi,
>>
>> Pretty much - you just need two regressors - one is the sin(a*t)
>> and the
>> other is cos(a*t). The relative values of the coefficients
>> (parameter
>> estimates)
>> for these then encodes the phase. That is:
>> tan(phase angle) = parameter estimate for cos() / parameter
>> estimate for sin()
>> which comes from: sin(a*t + phi) = sin(a*t)*cos(phi) +
>> cost(a*t)*sin(phi)
>>
>> Note that the temporal derivative of sin(a*t) is a*cos(a*t), but I
>> wouldn't
>> use that as the factor in front is quite crucial and you need to know
>> exactly how the derivative has been scaled. Instead I would just
>> turn
>> the
>> temporal derivative off and explicitly make a cosine regressor with
>> the
>> correct amplitude.
>>
>> All the best,
>> Mark
>>
>>
>> On 21 May 2009, at 19:31, Michael Scheel wrote:
>>
>>> Dear fsl experts,
>>>
>>> i have data from a retinotopic mapping experiment - and I want to
>>> analyse it using the so called 'travelling wave method', i.e. a
>>> rotating checkerboard wedge, that jumps 45 degr. every 4 sec. After
>>> 32 sec it has fully rotated and starts again. Therefore the stimulus
>>> I'd choose is a sinusoid wave with a period of 32 sec. My question
>>> is how to get a retinotopic map using fsl, i.e. how do I extract the
>>> phase information - is this the same as the temporal derivative.
>>>
>>> Thanks, Michael
>>>
>
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Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
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