[VoxBo] 1.8.2 - convolution with an HRF

[Aaron Heller]

Hi Group:

I have been running GLMs with VoxBo 1.8.2 and have noticed an anomaly
with some of our analyses. The problem is apparent when examining the
fits of the GLM to the data. Strangely, the shape of the fitted
parameter is different from the shape of HRF that was used to generate the
covariate. It seems that the fitted response should appear to be a
scaled version of the HRF, and that the shape should not be
fundamentally different. To illustrate the issue, I've attached files
showing the time course data (timedata.ref), the fitted parameter
estimates (paramater.ref), and the empirically defined (subject specific)
IRF that was used to generate the parameter (IRF.ref). I have attached those files.

I first thought that it could be due to the fact that the IRF  I used had
only 8 time points. However, an analysis with a subject specific IRF
of 10 time points, elicits the same results.

Anyone have any ideas why this is occurring? Thanks very much.

-aaron
-------------- next part --------------
;VB98
;REF1
;
; time series pulled from pixel surfer
; DateCreated: Mon_Jun_26_12:12:52_2006
; SourceMatrix:	/data4/cross_modal_PA/spm2_analyses_reslice/JH_020106/voxbo_analyses/study_norm_IRF/study_norm_IRF
; SelectedPoint:	3	24	16

     -0.46717892
     -0.19205732
       1.1424424
       1.2776095
    -0.014703086
     -0.65494948
     -0.53545861
     -0.55570449
-------------- next part --------------
;VB98
;REF1
;
; parameter pulled from pixel surfer
; DateCreated: Mon_Jun_26_12:13:18_2006
; SourceMatrix:	/data4/cross_modal_PA/spm2_analyses_reslice/JH_020106/voxbo_analyses/study_norm_IRF/study_norm_IRF

     -0.44734511
     -0.38864345
      0.37625595
       1.1365941
      0.57186214
     -0.30702006
     -0.51299208
     -0.42871153
-------------- next part --------------
;VB98
;REF1
;
; time series pulled from pixel surfer
; DateCreated: Fri_Jun__2_11:23:55_2006
; SourceMatrix:	/data4/cross_modal_PA/spm2_analyses_reslice/JH_020106/voxbo_analyses/IRF/IRF
; SelectedPoint:	21	26	28

      0.00000
     0.155571
     0.822625
     0.910408
     0.316657
   -0.0427020
   -0.0120947
    0.0245645