>My experience is that anything below a one second duration stim time
>will need to be scaled up in amplitude by the impulse's ratio to a
That's interesting. I've used event durations of a 100ms without
modulating the amplitude very successfully in the past. I wonder if it
depends on how much variability you have in event durations? In other
words, if all the events are essentially quick (100ms) then the amplitude
of the evoked HRF only varies within a relatively small range. If these
events are mixed with blocks of say 30s, however, then the difference in
amplitude would be enormous and the fast events may very well jut look like
noise in the design matrix. In this case, though, you probably wouldn't
want to rescale the amplitude of the events because the difference in
amplitudes reflects the important difference in the expected strength of
BOLD signal, with much longer events (ie blocks) leading to significantly
larger signal (and hence greater sensitivity in the design).
I hadn't specifically considered this point previously, though, so I wonder
what others think?
Joseph T. Devlin, Ph. D.
FMRIB Centre, Dept. of Clinical Neurology
University of Oxford
John Radcliffe Hospital
Headley Way, Headington
Oxford OX3 9DU
Phone: 01865 222 738
Email: [log in to unmask]