If I follow your design, you can either win 3 points or loss 3 points.
Do you want to know which regions for greater for wins and those
regions that are greater for losses OR do you want to know which
regions are greater for +3 compared to baseline and those regions that
are greater for -3 compared to baseline?
The latter can not be assessed with with a parametric modulator. It
would require separate event types for the +3 and -3 trials. PMs can
be used to find the difference in the two trial types though.
PMs must have a mean of 0 (this is done by SPM, so you can enter any
values that you want). Thus, with only 2 trial types, the PM for +3
will be the inverse of -3. However, if you use PM for +3 trials, then
the positive contrast gives you areas where +3 > -3 and the negative
contrast gives you areas where -3> +3.
Hope this helps.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (773)
406-2464 or email.
On Fri, Apr 13, 2012 at 6:30 AM, Liam Mason <[log in to unmask]> wrote:
> Hi there,
>
> I have a simple design looking at anticipatory activity for low versus high
> probability of winning points. What I want to know is whether people
> optimistically attend to the value of the win, rather than the loss,
> regardless of the probability. So in any given trial where there are 3
> points at stake, does the region I'm interested in code for +3? Secondly,
> are there other regions that code -3?
>
> One way would be to add 2 parametric regressors for each trial, one with the
> positive value (+3) and the other with a negative value (-3). The problem
> with this is that the parameter estimability is very poor, because the two
> parametric regressors are perfectly (negatively) correlated.
>
> Does anyone have any suggestions?
>
> Liam
>
> ---
>
|