I think Rik Henson would be in best position to answer, but all thoughts
are welcome.
We are trying to generate maps of the temporal delays of the hrf using the
modeled temporal derivative. We are taking the ratio of the temporal
derivative value to the beta value to derive an estimate of the time shift
of the HRF relative to the assumed peak of the HRF of 6 seconds.
Several problems arise.
1) When the beta value is small or near zero, we get VERY large values for
this ratio. Should this type of analysis only be applied to voxels that
have some minimum beta to avoid this problem?
2) When the temporal derivative is very large, we get VERY small values for
this ratio. Henson's paper warns that the ratio of temporal derivative to
beta is a good estimate of the temporal shift only within a limited range
of temporal derivative values. Should we exclude voxels whose temporal
derivative values are too large?
3) We have considered using just the temporal derivative value by itself,
rather than taking the ratio. One problem we encountered is that the
direction implied by the temporal derivative value (i.e., late or early
relative to our assumed 6 second peak for the HRF) depends on whether the
beta value is positive or negative. I believe that the temporal derivative
value implies exactly opposite shifts for positive or negative betas. Does
this mean we should further limit our search to voxels with positive beta
values?
Are there alternatives to this method that provides a more stable (less
susceptible to spuriously large or small values) estimate of the temporal
delays of the hrf?
Thanks
Daniel H. Mathalon, Ph.D., M.D.
Assistant Professor
Department of Psychiatry
Yale University School of Medicine
Psychiatry Service 116A
VA Connecticut Healthcare System
950 Campbell Avenue
West Haven, CT 06516
Phone (203) 932-5711, ext. 5539
FAX : (203) 937-3886
Pager : (203) 867-7756
e-mail: [log in to unmask]
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