source projection

Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Thu Aug 25 09:00:55 CEST 2005

Hi Sanja,

That is something that I never considered, actually. In our 151
channel case, we have with the default 4 cm about one circle of
sensors around the sensor of interest. In your 275 channel case that
is of course more. Intuitively, I would say that you want to have a
smallest circle of sensors that is possible, to accieve the highest
spatial resolution. Increasing the spatial resolution is, besides
making the spatial topography more easy to understand, a main goal of
planar gradients.

But constructing the synthetic planar gradients using the nearest
neighbour approach also makes the signals more noise, since, at the
sensor of interest, you are mixing in the (independent) noise of the
neighbouring sensors. In our 151ch case, that is something that we
just have to accept, but in your 275ch case, you can trade in some of
the spatial resolution (lower, if more channels as neighbours)
against the noise (beter, by averaging the noise over them).

best regards,

On 24-aug-2005, at 0:50, Sanja Kovacevic wrote:

> Hi Robert,
> Have you also tried different neighbor distances when you tested
> megplanar? I believe that for the 151 channel system, the default 4
> cm distance would lead to a smaller average number of neighbors
> than for the 275 channel system. For 275 channel data, using 3 cm
> neighbor distance I've got 5.5 neighbors on average, while using 4
> cm neighbor distance, I've got 9.1 neighbors on average. I noticed
> that when I used 4 cm neighbor distance, changes in power were more
> spread out. Do you have any suggestions on how many neighbors would
> be optimal?
> Thank you,
> Sanja

Robert Oostenveld, PhD
F.C. Donders Centre for Cognitive Neuroimaging
Radboud University Nijmegen
phone: +31-24-3619695

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