Dear Maria,
>Would a change in heart rate cause a global blood flow increase in the brain
>which would then cause the oxygen extraction to decrease. Does SPM99 control
>for this, if so how?
Changes in blood pressure (e.g. second to heart rate), within normal
physiological limits, tend not to impact cerebral perfusion significantly
due to the brain's capacity for autoregulation. A faint on the other hand
exemplifies the other extreme. Other factors such as blood CO2 level
(reduced in hyperventilation, anxiety etc.) however, do impact global
cerebral perfusion.
In the context of 'normal' fMRI experiments any such change is unlikely
to influnce your results unless:
1) It survives the high pass temporal filtering applied to
your data - (This is probably the answer you are looking for).
2) The changes are time locked to your stimuli of interest. - Should
this be the case then selecting global scaling would be an option.
This effectively converts your data into units of percentage change
with respect to the mean volume intensity for every volume individually.
Good luck!
Afraim
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Dr A Salek-Haddadi
Clinical Research Fellow
Institute of Neurology
UK
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