[Eeglablist] std_envtopo (plotchans and pvaf)

[Ana Navarro Cebrian]

Hi,
I have two questions about the function std_envtopo.m

First, I was wondering wether I could use something like the 'plotchans'
(available in envtopo) to compute the contributions in just one or a few
channels of interest. The default function computes the contributions for
the grand ERPs which gives me a lot of variability that I'm not interested
in.

Also, I'm still confuse about the pvaf for the individual clusters.
For example, I'm looking for the clusters that explain the P300 variability
and there is a cluster (out of 7 clusters) that seems to me (based on the
cluster's ERP) that explains most of the variability of the P300.
When I run std_envtopo, I get a pvaf of -550.46 for that cluster. First, I
understand that this is not talking about the P300 activity alone, but the
grand average, and this implicates a lot of variance from many areas that
I'm not interested in (and this is the reason why I'm trying to use just a
few channels of interest). Therefore, because what I think is the 'P300
cluster' may have a different signal than all the other activities (that
I'm not interested in), then I get a negative value for this cluster pvaf
(-550.46). Am I getting something wrong so far?

Also, I imagine that, because this is the percent variance accounted for,
for positive numbers, 100 should be the maximum possible value?

I hope that makes sense. Thanks in advance for your help.
Ana
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