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Subject:

which interval to jitter in a rapid event-related design?

From:

Richard Morris <[log in to unmask]>

Reply-To:

Richard Morris <[log in to unmask]>

Date:

Thu, 9 Jul 2009 16:30:09 +1000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (57 lines)

Dear fMRI gurus,

I'm interested in designing an experiment examining reward-related  
positive and negative prediction errors in the striatum. But as a  
relative newbie I need some help with a rapid event-related design.  
The basic problem is I don't know which intervals in my task I need to  
vary or jitter and which intervals can remain fixed.

The task consists of a series of trials in which participants are  
presented with a hand of cards and must learn to predict whether it is  
a winning hand or not. Thus, they are presented with cards and they  
make a prediction (win or lose) and then they are told whether the  
cards won or lost that hand (the outcome). So there are at least two  
intervals which can be varied: an interval between trials (SOA?) and  
an interval between card presentation and outcome (ISI).

I'm primarily interested in activity during the outcome. My hypothesis  
is that when a win is unexpected, the outcome should evoke more  
activity (in the striatum) relative to when the win is expected  
(correctly predicted). Conversely, when a loss is unexpected, the  
outcome should evoke less activity than an expected loss.

As I understand it, if the interval between trials or the cards and  
the outcome is too short (e.g., < 14 seconds), then the BOLD signal  
from the outcome must be deconvolved from any BOLD signal which still  
persists from earlier stimulus presentations (e.g., the cards  
presentation). To assist with this, experimenters typically jitter the  
SOA and this is the point I need some guidance on.

Which interval do I need to jitter? Is it the interval between cards  
and outcome (the ISI) or the interval between trials (the ITI)?

Furthermore, I'm not sure that it will be easy to deconvolve the BOLD  
signal during the outcome from any BOLD that is still persisting from  
the immediately preceding card presentation. I think the BOLD signal  
to the card presentation will be opposite to the BOLD evoked by the  
outcome, and so any differences between expected and unexpected  
outcomes will be masked or obscured. In particular, the cards which  
precede an unexpected win are likely to evoke a small BOLD signal  
(while the outcome will evoke a large BOLD signal) and conversely, the  
cards which precede an expected win are likely to evoke a large BOLD  
signal (while the outcome evokes a small BOLD signal). Thus, the  
hypothesized difference in BOLD evoked by expected and unexpected wins  
(expected < unexpected) will be reduced by the persisting (and  
opposite) differences produced by the card presentation (expected >  
unexpected).

I believe the same problem exists for unexpected and expected losses  
for similar reasons, but I'll spare you the details right now.

Thanks so much for reading this far and I really appreciate any help  
or guidance you can provide.

All the best,

Rich.

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