---------- Forwarded message ----------
From: MCLAREN, Donald <[log in to unmask]>
Date: Thu, Sep 15, 2011 at 12:47 PM
Subject: Re: [SPM] PPI with event-related designs: delta vs. boxcar
To: Bob Spunt <[log in to unmask]>
Bob,
Yes. If the neural activity is above the mean, then the PPI term will
always be positive.
You're absolutely right that in PPI, it looks like the timing has more
of an issue. I am not at all surprised by this; however, I'm not sure
if its as easy to quantify its impact as it is with the canonical
approach.
The impact will depend on how variable the neural activity is over the
sampling range. As the range gets longer, the variability is likely to
increase.
If you look at the correlations, the correlation between block and
event versions is ~.7-.8 in the second half of the run and ~.35 at the
beginning of the run. This is mainly driven by 1 trial that has a
significant change in the estimated neural activity over the course of
6 seconds.
On an unrelated note, I wonder if you could use the correlation as a
metric of the stability of the neural activity when someone performs a
task and whether that is related to performance?
Now getting back to what is being modeled. When you model something a
single event, with duration=0, then you only look at the neural
activity in the first time bin to determine the interaction. If one
assumes that the neural activity change is slow or stable across the
first few time bins, then it will make very little difference. If your
task is longer, say 4-6 seconds as in your case and you model it as an
event, you are asking what is the interaction between the neural
activity at the beginning of the task and region X, not the neural
activity during the task. As you noticed, you can get very different
timeseries, which would lead to different maps.
It might not be bad to do the analysis both ways and point out how
different they are when you mismodel the the data for PPI and show
that it is not robust to change. Maybe write a paper on the issue.
Also, I'd like to point my gPPI toolbox can use VOI.mat files as well.
It'll create the design, estimate it, and create contrasts. You should
be able to feed your VOI.mat right into my script.
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 Thu, Sep 15, 2011 at 2:44 PM, Bob Spunt <[log in to unmask]> wrote:
> Hi Donald,
>
> Thanks for the response. The time bin was identical in both cases. I'm
> attaching the PPI structures from which the plots were created - I
> just plotted PPI.ppi from each. Just in case you're motivated to poke
> around, I've also attached the code I used to build them. I didn't use
> your gPPI code - the VOIs are created using SPM8's new VOI batch
> utility and the PPIs using the default SPM8 code (which I've cut and
> pasted into this file as a subfunction for easy manipulation).
>
> Regarding your comment and positive and negatives, my PSY regressor
> here is just one condition, as I was building these PPIs for the sake
> of doing your gPPI. If the estimated neural activity is above the mean
> for the VOI for every trial, then is it correct that the PPI regressor
> will only have positive effects because PSY is not de-meaned and thus
> only has positive and zero values?
>
> Sincerely,
> Bob
> -------------------------------------------------------------------------------
> Bob Spunt
> Postdoctoral Fellow
> Social Cognitive Neuroscience Lab - www.scn.ucla.edu
> Department of Psychology
> University of California, Los Angeles
>
>
>
> On Thu, Sep 15, 2011 at 10:10 AM, MCLAREN, Donald
> <[log in to unmask]> wrote:
>> Also, your time bin will have an effect as well.
>>
>> 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
>> =====================
>> This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
>> HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
>> intended only for the use of the individual or entity named above. If the
>> reader of the e-mail is not the intended recipient or the employee or agent
>> responsible for delivering it to the intended recipient, you are hereby
>> notified that you are in possession of confidential and privileged
>> information. Any unauthorized use, disclosure, copying or the taking of any
>> action in reliance on the contents of this information is strictly
>> prohibited and may be unlawful. If you have received this e-mail
>> unintentionally, please immediately notify the sender via telephone at (773)
>> 406-2464 or email.
>>
>>
>>
>>
>> On Thu, Sep 15, 2011 at 10:38 AM, MCLAREN, Donald
>> <[log in to unmask]> wrote:
>>> I've never looked closely at this effect before. I'll have to take a
>>> closer look at the code and see what might be going on.
>>>
>>> How did you produce these two plots? I am very surprised that they are
>>> almost all positive in one case, then switch to having some negatives
>>> in the second case. Both should have positives and negatives.
>>>
>>>
>>> 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
>>> =====================
>>> This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
>>> HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
>>> intended only for the use of the individual or entity named above. If the
>>> reader of the e-mail is not the intended recipient or the employee or agent
>>> responsible for delivering it to the intended recipient, you are hereby
>>> notified that you are in possession of confidential and privileged
>>> information. Any unauthorized use, disclosure, copying or the taking of any
>>> action in reliance on the contents of this information is strictly
>>> prohibited and may be unlawful. If you have received this e-mail
>>> unintentionally, please immediately notify the sender via telephone at (773)
>>> 406-2464 or email.
>>>
>>>
>>>
>>>
>>> On Thu, Sep 15, 2011 at 3:32 AM, Bob Spunt <[log in to unmask]> wrote:
>>>> Dear SPM experts,
>>>>
>>>> When conducting PPI with event-related designs, I've found that the
>>>> PPI regressors are dramatically affected by how trials are modeled,
>>>> i.e. either with a delta (duration = 0) or boxcar (duration = stimulus
>>>> offset minus onset) function. For example, the attached plots show PPI
>>>> regressors from identical seeds in identical designs, where the only
>>>> parameter varying is the use of a delta vs. boxcar function to model
>>>> the stimulation (trials varied in length from about 3-5 seconds). The
>>>> correlation between these regressors is only about .50. This seems
>>>> surprisingly low.
>>>>
>>>> The reason I bring this up because, when one is merely concerned with
>>>> modeling the psychological response, the difference between modeling
>>>> trials as delta vs. boxcar functions is often small (although
>>>> certainly not insignificant; Grinband et al., 2008, NeuroImage).
>>>> However, this difference seems to be amplified when doing PPI with
>>>> event-related designs. Is this a reasonable conclusion?
>>>>
>>>> Thanks in advance for any comments,
>>>> Bob
>>>>
>>>> -------------------------------------------------------------------------------
>>>> Bob Spunt
>>>> Postdoctoral Fellow
>>>> Social Cognitive Neuroscience Lab - www.scn.ucla.edu
>>>> Department of Psychology
>>>> University of California, Los Angeles
>>>>
>>>
>>
>
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