On Sat, 1 May 2004 09:15:21 +0100, Stephen Smith <[log in to unmask]> wrote:
>> Ah. So f-test results for the ps-plots, then. Great. Let me make sure I
>> understand -- I can model using 5 gamma basis functions and do an
>> f-test across all five, which gives me one map that is the best fit
>> linear combination of all basis functions. (How do I know the weighting
>> of each?)
>Yep, that's right. You can get a feel for the relative weightings by
>looking to see how strongly each individual one fits in the partial model
>fits on the right side of the web page. You can get the exact relative
>values by extracting the relevant voxel values from the relevant pe?.hdr
I'm still a bit confused by the meaning of the F-test.
1. Does the F-test give you the best single linear combination of basis functions across all voxels
or is it the best linear combination of basis functions for each single voxel? In other words, for
the F-test, is the HRF constrained to be identical across all voxels?
2. Is the parameter estimate for each individual basis function for the single contrasts the same as
the parameter estimates for the F-test? For example, if basis function 1, 2, and 3 account for
10%, 20%. and 50% of a voxel's variance ( plus 20% residual) for each of the individual contrasts, is
this ratio the same for the F-test?
3. How can I get the three basis functions used by flobs? What I would like to do is take the
parameter estimate for each function, determine the fraction of the total variance accounted for by
each function (pe1/(pe1+pe2+pe3+residual)), then determine the HRF
(HRF=fraction1*function1+fraction2*function2+fraction3*function3). This should give me
something different from the peristimulus plots, i.e the peristimulus plots are the hemodynamic
response for a given stimulus but not the hemodynamic response function that I want to convolve
with my stimulus. Is this all valid reasoning?