Adrian -
> There has been some discussion about fractions of image onsets in the past
> but I wanted to bring up that point again to gain more insight.
>
> Is it true that using a TR of 3.0s and entering fMRI_T as 30, an IMA value
> of 1.5 will cover durations of e.g. 4.46s to 4.54s? That is, SPM99 would
> then sample with 100ms bins?
Yes, but see below.
> Using "E-Prime" for stimulus presentation you get a log that claims to be
> 1ms exact. However, with a repetition rate of 60Hz for the stimulus
> projector, a range of 16 to 20ms seems sufficient. Does it really make sense
> in this situation to enter fMRI_T as 150 to achieve sampling at 20ms? Are
> fractional onset values in this range of real use for the subsequent data
> evaluation in SPM99?
This kind of temporal resolution is unnecessary though - remember
that the BOLD signal we measure derives from a temporally smooth
haemodynamic response - with little power above 0.5Hz - so sampling
at rates above this will have negligible effect.
Rik
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DR R HENSON
Institute of Cognitive Neuroscience &
Wellcome Department of Cognitive Neurology
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