I'm having a little trouble decoding the images due to issues with the labeling on the scales but one possibility is that the sign of an activation (or a factor score if using PCA terminology) is arbitrary. The only thing that has a meaningful relationship to the original data is the product of the loading times the activation (factor score). Are the loadings at the voxels of interest positive or negative?
Cheers!
Joe
-----Original Message-----
From: SPM (Statistical Parametric Mapping) on behalf of Todd Penney
Sent: Fri 11/23/2007 12:55 PM
To: [log in to unmask]
Subject: [SPM] independent component analysis of fMRI data
Hello all,
I have a question regarding some ICA results I received on a series
of fMRI images from an epileptic patient who underwent a continuous
EEG-fMRI scan. The patient was scanned for interictal epileptiform
activity that occurred on her EEG.
I performed spatial ICA and found one component that I am convinced
is related to the interictal epileptic spikes. The timing of the BOLD
response corresponds to that of the epileptic spikes and the
activations are occurring in what looks like the temporal lobe (the
patient was diagnosed with right temporal lobe epilepsy). However,
the activations have a positive z-score but the BOLD response is
showing a DECREASE at each of the spike locations in the spatial brain
maps (shown by the large signal discharges in the attached images).
This is puzzling to me as I am expecting a BOLD signal increase and
positive spatial weights. I have attached two .jpg files showing the
brain maps of the component. Is it possible that perhaps the BOLD
signals are INCREASING because of the sign ambiguity when doing ICA
and that the spatial weights should be DEACTIVATIONS with the same
z-scores (which means all I have to do is multiply the time course and
spatial weights by negative 1)? If not, how is it that I am getting
positive spatial weights with a declining BOLD response at each spike?
These results are output from the GIFT ICA program.
Thank you.
Todd Penney
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