[FieldTrip] Mismatch between channel label and position in hdr.grad (Neuromag 306 system)

Jarang Hahm jarang.hahm at gmail.com
Mon Jun 25 04:57:51 CEST 2012

Dear FieldTrip user.

Hi, I'm processing MEG data of Neuromag 306 system after removing some bad
channels using fieldtrip-20120620 toolbox.

To repair the bad channels with its neighbours, I firstly found the
neighbor of all channel based on distance using 'ft_prepare_neighbours'.

However, the outcome of neighbour was far more than default distance, 4cm,
from a target channel.

So, I checked thoroughly the code 'ft_prepare_neighbours', finding it used
information of channel label and position from data’s hdr.grad to find
neighbour of the target.

The problem is channel label and position in hdr.grad are arranged

As far as I found, both information come from different source: the channel
label was from code 'fiff_read_meas_info', while the channel position was
from ‘channelposition’ in 'ft_datatype_sens'.

And  the channel label is sorted into the sensor location, e. g. MEG 0113,
MEG 0112, MEG 0111, MEG 0122, etc, however, the channel position is
arranged along the sensor type (magnetometer and gradiometer) according to
its ending number (1, 2, or 3), e.g. MEG 0111, MEG 0121, MEG 0131, ....,
MEG 0112, MEG 0122, MEG 0132, ...MEG 0113, MEG 0123, MEG 0133, etc.

Thus, the label and position of channel in hdr.grad were mismatched so
that we got somewhat weired neighbour after using 'ft_prepare_neighbours'.

It should be matched correctly to find the proper neighbour and ensure the
result of repair as well.

In fact, the code 'channelposition' in 'ft_datatype_sens' yield the channel
label as well as the channel position. It might help to solve this problem.

Is there any other way to solve this problem?


Jarang Hahm
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