Will,
Thanks for your response.
If I am not mistaken, the contrast vector [1 0 0 0] will produce a con
image identical to the beta image "A" only if no events have been left
unmodelled in an implicit baseline. I understand that if a condition were
left unmodeled, then the contrast vector [1 0 0 ] would represent the
difference between the beta image A and the implicit baseline. Is my
understanding correct?
Dan
>Daniel H. Mathalon wrote:
>
>> Dear SPMers,
>>
>> We have modeled 4 trial types (A,B,C,D) in an event-related design in two
>> groups of subjects. In addition to examining condition contrasts (e.g.,
>> B-A, A-B) within and between the groups, we are interested in knowing how
>> the groups differ specifically in response to condition A. Therefore, we
>> would like to perform a two group t-test on the beta image associated with
>> A.
>>
>> It is generally understood that fMRI analysis requires contrasts between
>> conditions, which would seem to suggest that we cannot validly compare
>> groups on a single condition. Yet, to the extent that the beta image
>> reflects the peak of the fitted HRF, it seems natural to ask whether this
>> peak is larger in one group versus another.
>>
>> Can anyone clarify whether we can legitimately compare a beta image (rather
>> than a CON image) between groups, and if not, why not?
>>
>
>
>This is perfectly legitimate. In fact
>the contrast vector [1 0 0 0] will
>produce a CON image that is identical to
>the beta image for trial type A. Two
>group t-tests can then be implemented on
>the resulting CON/BETA images.
>
>Best wishes,
>
>Will.
>
>
>> Thanks.
>> Daniel H. Mathalon, Ph.D., M.D.
>> Assistant Professor
>> Department of Psychiatry
>> Yale University School of Medicine
>>
>> Psychiatry Service 116A
>> VA Connecticut Healthcare System
>> 950 Campbell Avenue
>> West Haven, CT 06516
>>
>> Phone (203) 932-5711, ext. 5539
>> FAX : (203) 937-3886
>> Pager : (203) 867-7756
>> e-mail: [log in to unmask]
>>
>>
>>
>
>
>--
>William D. Penny
>Wellcome Department of Imaging Neuroscience
>University College London
>12 Queen Square
>London WC1N 3BG
>
>Tel: 020 7833 7478
>FAX: 020 7813 1420
>Email: [log in to unmask]
>URL: http://www.fil.ion.ucl.ac.uk/~wpenny/
Daniel H. Mathalon, Ph.D., M.D.
Assistant Professor
Department of Psychiatry
Yale University School of Medicine
Mail address: Psychiatry Service 116A
VA Healthcare System
950 Campbell Avenue
West Haven, CT 06516
Phone (203) 932-5711, ext. 5539
FAX : (203) 937-3886
Pager 203-867-7756
e-mail: [log in to unmask]
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