Dear SPMers,

I am a new user of SPM(2) and I have two (probably very 
naive) unrelated questions. I did not have much success 
when I looked for answers in previous mailings - I 
apologise in advance if they have already been discussed.

1) Together with a colleague who is using AFNI, I am 
trying to model data from an event-related experiment in 
which trials of interest (stimulus-response compatible 
trials and incompatible ones) are intermixed randomly. We 
are hoping to model interactions between the 
characteristics of trials of interest and the context set 
up by the trials that precede them (e.g., an incompatible 
trial preceded by a compatible one, etc.).

 From our understanding of the literature, previous 
imaging papers (e.g., Jon Cohen's studies, etc.) have 
focused on context as it is set up by the trial 
immediately preceding the trial of interest (n-1). 
However, this does not fully take into account the 
cumulative effects of potential repetitions or changes 
preceding the n-1 trial. For example, we have up to three 
series of trials of an identical type preceding the trial 
of interest. How should we model these cumulative effects?

Has anybody dealt with this problem before, or do you know 
of where I could read about it? We thought of adding an 
additional non-linear regressor (e.g., an exponential 
function, but which one?) to modulate the HRF. How does 
one do this in SPM2?

2) Our data were acquired using a spiral sequence, rather 
than EPI, and when I load images using the EPI template in 
SPM I get some bad distortions - what can I do about this? 
There were some previous messages on this topic on the 
list, but ... I did not understand the solution.

Many thanks for any suggestion,

Gaia

~~~~~~~~

Gaia Scerif, PhD
School of Psychology
University of Nottingham
Nottingham NG7 2RD
United Kingdom

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