Dear Experts,
I was hoping you could help with the following issue that I couldn't find an answer to online or on JISC.
I am analysing an fMRI task in which participants were asked to respond to stimuli. Following the analysis of behavioural data, it seems there is a certain proportion of invalid trials (depending on participant, 0-10%) when participants responded instantly (response times 0-0.2s) when the 'usual' response time is in the order of 0.7-1s. I would like to exclude the trials with ultra-short reaction times from fMRI analysis, but I don't know how best to go about it.
I have the onset times (and durations) of the trials which are invalid. So far I have tried to model 'invalid' as another condition and specifying it in the design matrix, but I have a problem with participants who do not have any invalid trials as spm needs at least one onset time per condition. For those I specified one onset time, 0 seconds, which is outside of the scanning time of interest for my task. I don't know if it makes sense to convolve this with the HRF however?
I have also thought about trying to convert the onset times to scan numbers and excluding the relevant scans, but (a) not sure how to do it; (b) often an invalid trial is right before or after a valid trial; duration of a trial is 1.75 seconds and TR 2 seconds - so I would probably inadvertently remove some valid trials too.
I am wondering if there is a better way of doing it?
Thanks in advance for your advice.
Best wishes,
Jakub
Jakub Nagrodzki
Academic Foundation Year 2 Doctor
Addenbrooke's Hospital
Cambridge University Hospitals NHS Foundation Trust
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