[FieldTrip] centre of head bias

Eelke Spaak eelke.spaak at donders.ru.nl
Tue Feb 17 07:47:43 CET 2015

Hi Kaelasha,

The center-of-head bias is due to noise. Since you can assume the
noise to be uncorrelated across experimental conditions, you can
assume this bias will not be present in a contrast. To verify this for
yourself, simply plot the beamforming results for two conditions
separately; you will see a strong center-of-head bias. Subtract one
big blob from another equally big blob and they should disappear :)
So, plot the (normalized) difference, and you will likely notice less
center-of-head bias.


On 17 February 2015 at 06:23, Kaelasha Tyler <ktyler at swin.edu.au> wrote:
> Hi all,
> A question about the centre head bias.
> Does computing a contrast of conditions remove the issue of the centre head
> bias? Read below.
> The Localizing oscillatory sources tutorial talks about the possibility of a
> beamformer over estimating power in the centre of the head, and suggests
> several methods of counteracting this.
> After suggestion use of the NAI to counter this bias, the tutorial goes on
> to show this beamformer method using contrasting conditions and says that
> using this approach we can "assume that the noise bias is the same for the
> pre- and post-stimulus interval and it will thus be removed."
> The tutorial uses the following code to do this:
> sourceDiff = sourcePost_con;
> sourceDiff.avg.pow = (sourcePost_con.avg.pow - sourcePre_con.avg.pow) ./
> sourcePre_con.avg.pow;
> My question again: Does using this approach and computing the contrast
> condition remove the centre of head bias for this contrasted condition?
> Thanks!
> Kaelasha
> PhD Candidate
> Brain and Psychological Sciences Research Centre
> Swinburne University of Technology
> Melbourne
> Australia

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