Exactly -
If you want PE1 to represent the mean activity in condition A, then
you should orthogonalise EV2 wrt EV1 (this is equivalent to demeaning
the vlaues in the 3rd col of EV2).
If you do not demean EV2, PE1 will represent the Y-intercept (i.e.
the activation pattern present when the performance score was 0).
T
On 8 Jan 2008, at 16:59, Hanne Lehn wrote:
> Hi again,
>
> Thanks for the quick reply.
>
> Just to make sure I understand you correctly: If I go for option C,
> I define EV1 (condition A) and EV2 (weighting). The first two
> columns in their onset files (onset and duration) should be
> identical. The third column should be all 1's for EV1 and different
> values (0-6) for EV2. Ex:
>
> EV1:
> 0.0 28.4 1
> 64.2 28.3 1
> 128.3 28.4 1
> 194.3 28.4 1
> ...
>
> EV2:
> 0.0 28.4 3
> 64.2 28.3 1
> 128.3 28.4 5
> 194.3 28.4 2
> ....
>
> Is that correct?
>
> Kind regards
> Hanne
>
>
>
>
> Tim Behrens wrote:
>> Hi
>>
>> A) would generally work fine.
>>
>> The only reason you might not want to use (A) is if you don't
>> have many blocks per condition
>> in this case, you can use :
>> C) Have two EVs - one defines the block timings, and the other is
>> the weighting EV
>>
>> If you just have the weighting EV, you cannot tell whether an
>> effect is really correlated with weighting, or if it is just
>> condition A itself that causes the effect
>>
>> T
>>
>>
>> On 8 Jan 2008, at 15:31, Hanne Lehn wrote:
>>
>>> Hi,
>>>
>>> I have data from a block design experiment with two conditions, A
>>> and B.
>>> I'm interested in whether activation in condition A is modulated
>>> by task
>>> performance. Performance was measured throughout the experiment
>>> and has a
>>> separate value (0-6) assigned to each block. How do I set up the
>>> appropriate first-level analysis? Do I
>>>
>>> (A) create 7 EVs for condition A, one for each level of
>>> performance, and
>>> compare their effects with the appropriate contrasts?
>>>
>>> or
>>>
>>> (B) create 1 EV for condition A which includes all blocks, then
>>> add the
>>> performance variable as a weighting factor (use performance
>>> scores instead
>>> of "1"s in the third column of my custom onset file)?
>>>
>>> Hope my question is clear.
>>> Many thanks in advance!
>>>
>>> Hanne
>>>
>
> --
> Hanne Lehn
> PhD student Neuroscience
>
> MR-Centre, St. Olav's Hospital
> 7006 Trondheim, Norway
> Phone: (+47) 73 59 88 04
> Fax: (+47) 73 86 77 08
>
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