sensor numbers on topoplot

[Jan Mathijs Schoffelen]

Sameer,

I used to have a latin teacher who always told us: reading is half the
understanding (but then in latin of course). Since you are probably not so
much interested in my latin teacher, I'll point to you where you can find
the answer yourself (of course by reading). Many problems often solve
themselves when looking into the code, because there is no oomopa loompahs
in your computer that randomly assign numbers to electrode positions each
time you call topoplotER. However, I know of some obscure operating systems
which actually have oompa loompahs inside them, but that's beside the scope
of this e-mail.
So, let's have a look at topoplot.m which is, as Ingrid already pointed out
several times, the low level function which generates topoplot-related
figures. If you search the code for 'electrodes' you hit some relevant code
from line 597 onwards.

for i=1:size(labels,1)
  text(y(i), x(i), int2str(i) etc.....
end

Apparently, the number i converted to a string and plotted on the location
(x and y represent the coordinates as specified by lay.pos(:,1), and
lay.pos(:,2)). The number i runs from 1 to the number of sensors in your
layout. But so far I guess you already found out.

If I repeat your little procedure my bottom midline occipital sensor is
number 240, and not 289. So I cannot reproduce your finding. Did you already
look into the text file NM306all.lay, and check the 240th line?

Yours,

Jan-Mathijs


-----Original Message-----
From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On Behalf
Of Sameer Walawalkar
Sent: Thursday, September 06, 2007 11:53 PM
To: FIELDTRIP at NIC.SURFNET.NL
Subject: [FIELDTRIP] sensor numbers on topoplot

Hello,

While using topoplot, if one uses

cfg.electrodes = 'numbers';
you get a plot with the sensor number written over each of the sensor
positions. What do these numbers correspond to?

If I look at NM306 layout diagram and locate sensor 2121 (on the midline
at the very bottom of the occipital lobe), this corresponds to a sensor
number 289 on the topoplot.

Yet if I do
cfg = [];
cfg.layout           = 'NM306all.lay';
lay = prepare_layout(cfg)
reading layout from file NM306all.lay

lay =

        pos: [308x2 double]
      width: [308x1 double]
     height: [308x1 double]
      label: {308x1 cell}

>> lay.label(289)

ans =

     '2533'

>> find(strcmp(lay.label,'2121'))

ans =

    240

Can someone help me understand this?
This is necessary to understand how to assign cfg.highlights.


Thanks,
sameer

----------------------------------
The aim of this list is to facilitate the discussion between users of the
FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG
and EEG analysis. See also
http://listserv.surfnet.nl/archives/fieldtrip.html and
http://www.ru.nl/fcdonders/fieldtrip.

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.