Hello SPM experts.
Regarding the situation described below, i.e. the problem of modelling one
event.
What if this situation had occurred, but your original intention was to take
this contrast forward to a between group comparison?
So, what if you had wanted to examine the BOLD signal associated with
correct-incorrect, and then compare this contrast between groups 1 and 2 -
but you didn't observe any incorrect events at the first level.
Would you dare jump straight to the between group comparison and compare the
signal for CORRECT trials only between the 2 groups?
For context, this is a fixed effects analysis - it was only a pilot study.
How comfortable would you feel with this?
I like to hear your views.
With many thanks
Rachel
--------------------------------------------------------
Dr Rachel L. C. Mitchell.
Lecturer in Cognitive Psychology, University of Reading.
Senior Honorary Research Fellow, Institute of Psychiatry.
Research Psychologist, Berkshire Healthcare NHS Trust.
Correspondence Address:
School of Psychology and Clinical Language Sciences,
Whiteknights Road
University of Reading
Reading
Berkshire
RG6 6AL
Tel: +44 (0)118 378 8523
Direct Dial: +44 (0)118 378 7530
Fax: +44 (0)118 378 6715
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Date: Mon, 12 Mar 2007 14:19:29 +0100
Reply-To: Volkmar Glauche <[log in to unmask]>
Sender: "SPM (Statistical Parametric Mapping)" <[log in to unmask]>
From: Volkmar Glauche <[log in to unmask]>
Subject: Re: Modeling one event
In-Reply-To: <[log in to unmask]>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Dear Ninad,
a late follow-up on your question (I've just come across the same
problem) - but without a solution yet:
1) If an event does not occur at all, then SPM will not create any column
for it in your design matrix. There is no way around it - this column
would be all-zero anyway.
2) If you have a parametric modulation, which does not modulate anything
(e.g. a constant parameter for all occurences of your event), then only
the main effect regressor will be built, since your parametric modulation
would be an all-zero column.
Currently, there is no easy way to automatically specify contrasts when
conditions or modulations are missing. A way out of this would be to
build contrasts based on column names/condition names instead of column
numbers, but this will require a major redesign of SPMs contrast manager
batch.
Volkmar
On Wed, 3 Jan 2007, Ninad Gujar wrote:
> Hi,
>
> Happy New Year to everyone !
>
> I am still trying to solve the same problem from last year.
>
> Our study consists of five sessions, each with 136 volumes. Now, each
> session had 4 events.
>
> Whenever there are no events to model e.g. no misses I tried embedding in
> the script the parameter to accept the value 'none' and SPM does accept
the
> value as 'none' . Is this the right way to do it ?
>
> Now, some of the session events have only one value to model. i.e. only
> onemiss
> SPM does not create the column for the regressor and only include one
column
> for the event.
>
> I checked in the SPM structure and all the values including the onset and
> the corresponding regressor seem to exist.
> Is there a problem if my event has only one value ?
>
> Some of our code: (for one event it looks like this)
>
> SPM.Sess(ses).U(count).P(1).name = 'ArousalScore';
> SPM.Sess(ses).U(count).P(1).h = 1; % order of polynomial
> expansion
> SPM.Sess(ses).U(count).P(1).i = [1 2];
> SPM.Sess (ses).U(count).P(1).P = [6.65];
> SPM.Sess(ses).U(count).ons = [274];
> SPM.Sess(ses).U(count).dur = 0;
>
> I need to create the column for this since while creating the contrasts at
a
> much broader level e.g. modeling just the misses I combine all the misses
> from all sessions.
>
> Any help would be most appreciated !
>
> Thanks,
>
> Ninad Gujar
>
>
--
Volkmar Glauche
-
Department of Neurology [log in to unmask]
Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
Breisacher Str. 64 Fax 49(0)761-270-5416
79106 Freiburg http://fbi.uniklinik-freiburg.de/
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