[Eeglablist] interpretation of weights on scalp maps

Scott Makeig smakeig at ucsd.edu
Thu Feb 27 16:09:58 PST 2003

You are correct. The plotting convention in EEGLAB is that 0 = green. 
Thus, component map projections colored red and blue may be equal in 
amplitude (but opposite in polarity). The (back)projection of a 
component to the scalp data (e.g., in uV) is the product of this map 
with the component 'activation' time course. So, pos*pos=pos, 
neg*neg=pos,pos*neg=neg,... The same applies to any linear decomposition 
of the data, properly considered.

Scott Makeig

Jed Meltzer wrote:
> I have a conceptual question about the weights in an
> ICA decomposition.  As far as I understand it, each
> point in time on each channel is multiplied by the
> inverse weight and sphere matrices to output each
> point in time in the component activations.  Some of
> these weights are negative.  Does this mean that a
> positive deflection in the component corresponds to a
> negative deflection in the channel?
> The reason I ask is because I don't know how to
> interpret the scalp maps of components.  Are red parts
> and blue parts equally important?  That is, do they
> both represent that the component maps strongly onto
> the channels in the colored region?  That would mean
> that the green regions are the ones that are not
> affected strongly by the component.  Furthermore, one
> could then infer that if a red region and a blue
> region are close together, then the underlying source
> may be modelled by a single dipole located in between
> the two colors.  
> Or am I understanding this incorrectly?  Does blue
> mean minimal projection?  
> -Jed Meltzer
> Yale University
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