[FieldTrip] centre of head bias

Schoffelen, J.M. (Jan Mathijs) jan.schoffelen at donders.ru.nl
Tue Feb 17 07:48:58 CET 2015

Hi Kaelasha,

What’s the question behind this question? In principle the normalization step described should largely alleviate the depth bias. Whether or not the remaining estimated activity near the centre of the head is to be trusted, is another story.


On Feb 17, 2015, at 6:23 AM, Kaelasha Tyler <ktyler at swin.edu.au<mailto: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?


PhD Candidate
Brain and Psychological Sciences Research Centre
Swinburne University of Technology
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