perhaps some of your subjects fell asleep?
Seems like a funny reply, but I seriously had that issue once, and i
initially thought of similar technical issues. But when asking the
subjects later on, the subjects with low activation confirmed that they
were very drowsy during the experiment. A much more banal explanation ;-)
And as it appeared, over time the activation per block got less and
less, which was another clear indication what happened. So check the
temporal evolution of task induced signal change for the duration of
your session to see whether this might affect you.
Good luck,
Bas
Op 09-12-11 14:26, gj schreef:
> Hi,
>
> For a non-negligible handful of our subjects, the visual cortical activations for our visual stimuli (simple contrast of complex visual stimuli against implicit baseline of crosshairs) are severely reduced and sometimes almost nonexistent unless we drastically lower our threshold. Does anyone have an idea why this may be?
>
> We have an event-related design (ISI ~7-8 sec), but analyze it as mini-epochs as the visual stimuli themselves are about 3-4sec long. Jitter is in the length of the stimuli. We temporally high-pass filter the signal at 128s. Modeled events convolved with canonical HRF. The realignment parameters and bad spikes were included in the model as nuisance regressors.
>
> The following do not seem to explain the bad data:
> - Signal-to-fluctuation noise ratios
> - Signal-to-noise ratios
> - Maximum range of movement during session
> - Removing the realignment parameters and bad spikes from the model (to see whether those were explaining away the activity, but still end up with similar reduced activations)
>
> I've also doublechecked the stimulus onsets and durations, and looked for any bad coregistrations.
>
> Any pointers greatly appreciated!
>
--
--------------------------------------------------
Dr. S.F.W. Neggers
Division of Brain Research
Rudolf Magnus Institute for Neuroscience
Utrecht University Medical Center
Visiting : Heidelberglaan 100, 3584 CX Utrecht
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Mail : Huispost B01.206, P.O. Box 85500
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: http://www.neuralnavigator.com (CEO)
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