I am sorry that I still have problem with this rule: the sum of them should
be zero.
In another email, I saw:
>I have a basic question, let say I have tow conditions: activation and
>rest. (100 scans).
>Vector onsets for the activation condition is: 10 30 50 70 90.
>Duration: 10 Vector onsets for the rest condition is: 20 40 60 80.
>Duration: 10
>
>Is it the same to specify only the activation condition (contrast
>[1]) than specifing both conditions (contrast [1 - 1])?
>No, it's not the same:
>- [1 -1] tests for positive differences between Activation and Rest;
>- [1] tests for positive differences between Activation and "Anything
that is not declared", i.e., your scans 0 to 9 that are not declared in your
vectors.
In the second case, the sum is also not zero. Should it be invalid?
Robin
_____
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
Behalf Of y8003012003
Sent: Wednesday, September 28, 2005 8:33 PM
To: [log in to unmask]
Subject: Re: [SPM] are the two designs same?
it is invalid because the sum of them was not 0.
but I also want to know if the following are right:
[-1 -1 -1 -1 4]
and what it mean.
Thank you very much
-----原始邮件-----
发件人:"Robin CHARK"
发送时间:2005-09-28 19:53:52
收件人:[log in to unmask]
抄送:(无)
主题:Re: are the two designs same?
One follow-up question. If I want to test the significancy of the first 4
conditions only, can I define it as [1 1 1 1 0]? Thanks. Robin > hi > >>
Dear all: >> >> If I had a five conditions fMRI experiment, and made the
following two >> t-contrasts using spm2 : >> >> [1 1 1 1 -1] and [1 0 0 0
-1; >> >> 0 1 0 0 -1; >> >> 0 0 1 0 -1; >> >> 0 0 0 1 -1] >> >> Are they all
valid? And are they the same? If not, what are they >> meaning separately?
>> >> Tank you very much! >> > well, they are not all valid and they are not
the same .. > 1. When you enter a contrast, the sum of coef must be 0 and
thus [1 1 1 > 1 -1] is invalid (you can try but SPM contrast manager will
refuse to do > that) .. if you want to oppose one condition against the
others the > correct contrast would be [1 1 1 1 -4] > 2. in the second case
you test if any of these contrasts (or > combinations) is significant .. but
those ones are correct as they all > sum up to 0 > > best > cyril
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