Many thanks... as a quick follow-up to the question of "Does the
adjustment decision matter?", consider the case of a PPI comparing two
conditions, A > B. If one were to adjust using either the contrasts A
> Baseline or A > B, this would bias the VOI timecourse to look more
like the timecourse of A, which presumably would spuriously increase
the correlation among the VOI and regions of the brain associated with
A. Is this reasoning correct?
On Mon, May 9, 2011 at 8:44 AM, MCLAREN, Donald
<[log in to unmask]> wrote:
> See inline responses below.
> Best Regards, Donald McLaren
> D.G. McLaren, Ph.D.
> Postdoctoral Research Fellow, GRECC, Bedford VA
> Research Fellow, Department of Neurology, Massachusetts General Hospital and
> Harvard Medical School
> Office: (773) 406-2464
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> On Sat, May 7, 2011 at 4:07 PM, Bob Spunt <[log in to unmask]> wrote:
>> This topic has been discussed on numerous occasions on the listserv,
>> but I'm still unsure about a few things. When extracting a VOI
>> timecourse (e.g., for a PPI analysis):
>> 1. Can adjusting for different contrasts produce large differences in
>> results? In other words, is the decision important? And if so, why
>> don't folks typically report this in empirical articles using PPI?
> I'm not sure anyone has looked at this closely enough.
>> 2. Is there ever a case in which it is desirable to adjust for a
>> t-test contrast (e.g., the comparison of only two conditions)? In
>> spm_regions the default is to only allow adjustment for F-contrasts,
>> but of course adjustment can be made for t-contrasts if desired.
> No. Even in the case of two conditions, you'd still want an F-test with 2
>> 3. My understanding is that the F-test omnibus (coding for all effects
>> of interest) is the favored option for adjustment. But there is some
>> ambiguity in what "all effects of interest" refers to. Should all
>> effects expected to relate to neural activity be included (so that
>> only motion regressors/constants are removed), or only effects "of
>> interest"? For example, if I've modeled response time and skipped
>> trials as covariates of no interest, should these be included in the
>> omnibus (because they are expected to relate to neural activity, not
>> to artifactual changes in signal intensity)?
> Effects of interest, in my conceptualization, means any signal related to
> neural activity. I believe that my gPPI code, uses all columns identified as
> being associated with a task. The code can be found at
>> Many thanks in advance for any tips.
>> Bob Spunt
>> Doctoral Student
>> Department of Psychology
>> University of California, Los Angeles