Observed peak fluctuations in the timeseries "on the order of" 1 or 2% are well-within the expected size of thermal noise and BOLD effects over most of the brain. Note these values are only approximate guides, not hard limits, so your values of 1.2 should be fine. BOLD effects in the visual cortices can be much bigger (e.g. 5% or more). But if you see values of 10% fluctuations widespread over many voxels, I think it's more likely due to motion or artifacts in the data.
Best regards,
Paul
----- Original Message -----
From: "meikei" <[log in to unmask]>
To: [log in to unmask]
Sent: Sunday, February 21, 2010 10:48:12 PM GMT -08:00 US/Canada Pacific
Subject: [SPM] % signal change calculated by MarsBar
Dear all,
I have a question about % signal change of fMRI data. I read that "quantitative check on measured effect size can be used to screen abnormally large values likely caused by artifacts in the data" on a % signal change document written by Paul Mazaika. I suspect that some of my scans are problematic and I wonder if it's possible to check it with the above method. But how do I know if the values are abnormally large? I read that cognitive effects give signal changes on the order of 1% while I notice that the range of value for cognitive events seems to be within -1 to 1. Is this always true? Are the results out of this range (e.g. -1.251, 1.208 etc) erroneous?
Thanks very much for your answers!
Best wishes,
Meikei
--
Paul K. Mazaika, PhD.
Center for Interdisciplinary Brain Sciences Research
Stanford University School of Medicine
Office: (650)724-6646 Cell: (650)799-8319
CONFIDENTIALITY NOTICE: Information contained in this message and any
attachments is intended only for the addressee(s). If you believe
that you have received this message in error, please notify the
sender immediately by return electronic mail, and please delete it
without further review, disclosure, or copying.
|