Hi Richard

In general the idea is that you need to make sure that responses to
events you are interested in are not correlated with responses to
other events, and the easiest way to do this is vary the interval
between them.

Because you are interested in activity during the outcome, you will
want to make sure that the outcome is not systematically related to
the viewing of the hand or the prediction.  So, I think you will need
to vary the delay between prediction and outcome in order to isolate
outcome-related activity.

As far as the time between trials (SOA): If you are only interested in
comparing across trial types, you probably don't need to vary your
SOA.  However, if you are interested in the main effect of your
trials, you probably should.  If you haven't come across it already,
Rik Henson's page on design efficiency offers a good discussion of
these SOA issues:

http://imaging.mrc-cbu.cam.ac.uk/imaging/DesignEfficiency

Hope this helps,
Jonathan


On Thu, Jul 9, 2009 at 7:30 AM, Richard Morris<[log in to unmask]> wrote:
> 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.
>