trialfun_neuromag

[Lauri Parkkonen]

Hello Jyrki and Robert,

Just a little addition: the trigger lines in all 306-channel systems are
purely digital so there is no noise. The channel 'STI 014' (or 'STI101'
in the most recent model with 32bits/sample) holds the binary trigger
word, i.e., one bit per trigger line. For backwards compatibility with
some old analysis setups, there are also analog-like channels named 'STI
001', 'STI 002', ... representing just one trigger line each. These
channels are digitally derived from the binary trigger word thus they do
not contain noise either.

With the best regards,
Lauri

> Dear Jyrki,
>
> I recall that depending on the type of system, the neuromag trigger
> channels can have _analog_ noise on them. I.e. instead of containing
> clean trigger values embedded in a real zero-baseline signal, they
> contain a lot of small-amplitude values everywhere. That messes up
> the trigger detection, since the trigger detection searches for
> changes in the value in the trigger channel. Even a small change
> (=noise) will be detected as trigger. Hence you will end up with a
> different trigger value at almost each sample. Not only does that
> result in a huge amount of triggers, it also causes the trigger
> values that will be used by FieldTrip to be inaccurate (since there
> is noise on those values).
>
>
> The relevant code here is line 21 in trialfun_neuromag
>   trigindx = find(trigger & [0 diff(trigger)]);
>                    ^^^^^     ^^^^^^^^^^^^^^
>                      1             2
>
> which finds the samples in the trigger channel where the trigger
> value is different from zero (part 1) and where the trigger value
> changed w.r.t. teh previous samle (i.e. flank detection, part 2).
>
> I suggest that you read the trigger channel and look at it by hand
> (ie. plot it in matlab and zoom in) to see whether the analog noise
> problem applies to you. In that case the flang detection should be
> replaced by something more robust (e.g. the trigger value should
> change with at least a certain amount). If you know of a robust
> method for the flank detection, I would be happy to incorporate it
> into FT.
>
> best regards,
> Robert



--
-----------------------------------------------
Lauri Parkkonen
Brain Research Unit, Low Temperature Laboratory
Helsinki University of Technology
Otakaari 3 A, 02150 ESPOO, FINLAND
tel: +358-9-4512965
fax: +358-9-4512969
mailto:lauri at neuro.hut.fi
http://boojum.hut.fi