Hi Donald & thanks a lot for your answer,
We'll go for a gPPI pipeline then, however we are unsure about the connectivity quantification. What we are looking for when refering to "strength" is actually a way to quantify the connectivity that allows further stats, like comparing connectivity across
groups or conditions, or correlating connectivity with e.g. behavioural variables.
If we got you right, it seems that the parameter estimate scaling the Psycho-Physiological Interaction term associated with a given cluster can indeed be used to quantify the "coupling" between the seed region and a given cluster. This parameter represents
a difference between conditions or between condition X and a null event, or it may also represents an interaction contrast (i.e. this parameter actually reflects whatever contrast was used to define the psychological term).
Since we do obtain (with sPPI at least, we have not yet run gPPI) negative AND positive results, we do not understand your comment about whether absolute values are positive or negative is hard to determine? If the psychological term included conditions A &
B, with a contrast defined as {A > B}, then positive PPI outcome suggests a greater connectivity for A than for B (i.e. postive Beta for the interaction), right? Could you please clarify this, and the further comment about considering the physiological regressor
to determine the sign of these absolute values?
We also have two follow-up questions:
- do you recommend to include null events when building the psychological regressor for the PPI?
- to further understand the quantification issue, we have read many studies referring to PPI, and often the results suggest to us that we clearly miss something. For example, Manelis et al. (2013,
Hippocampus), provide in their Figure 5 (p. 61) data suggesting that some quantification of the connectivity can be obtained separately for each condition of a contrast (see the Y-axis).
The contrast used to build the psychological term is "{Presentation 2 - Presentation 1}", and the figure seems to provide separate connectivity estimates for each presentation....how is that possible, given that only one parameter estimates result from a typical
PPI model derived from a simple {A - B} contrast ?
Sorry for such a long email, we really find it hard to get clear answers elsewhere....
Thanks a lot in advance,
Pierre-Yves & Quentin
De : MCLAREN, Donald [[log in to unmask]]
Envoyé : samedi 31 mars 2018 00:04
À : JONIN Pierre yves
Cc : SPM
Objet : Re: [SPM] PPI analysis and connectivity quantification
Hi Pierre-Yves & Quentin,
Generalized PPI. This is the recommended approach anytime you have more than 2 conditions including null events as one of the 2 conditions.
It's the amplitude of the connectivity as its a scalar of the regressor. Whether this is "strength" or not depends on what you mean by strength. Additionally, its important to remember that the estimates represent a difference between 2 conditions or the condition
and baseline if using gPPI. Whether the absolute values are positive or negative is harder to determine from the contrasts, but might be possible by considering the physiological regressor in the contrasts.
Future research is aimed at conducting more simulations of how the gPPI can extract the absolute connectivity rather than just relative connectivity at the moment.