Neuromag event values

Lauri Parkkonen lauri at NEURO.HUT.FI
Wed Apr 18 23:43:38 CEST 2007


To clear the confusion with the Neuromag trigger channels, here is the
"evolution" of trigger handling in the Neuromag MEG systems:

1. The 122-channel system has only analog-like trigger channels. There
is a separate channel for each trigger line or bit, and the names of
those channels are STI 001 ... STI 006. The signals are TTL-level
pulses, i.e., about +5 volts for the active state and about zero otherwise.

2. With the first 306-channel systems, the triggers are already handled
as digital signals but for backwards compatibility with some analysis
software modules, the system emulates the analog-like channels STI 001
.. STI 006, again using TTL-level pulses. The compound digital trigger
channel STI 014 first existed as an "undocumented feature", and in the
early versions of the acquisition software it's calibration coefficient
was erroneously not set to unity, which resulted in strange values if
the calibration coefficient was applied. This is most likely what Myles
is seeing in his datafile. There are workarounds: 1) divide the trigger
values with the calibration coefficient within FT to get back to the
normal binary representation, or 2) run a small utility on the data file
which fixes the calibration info (I can send that utility). Note that
this problem exists only with the rather early versions of the front-end
acquisition software.

3. The new 306-channel systems (installed in or after Aug 2005) have a
redesigned trigger system. The primary trigger channel is STI101 (note
that there is no space in the name anymore -- this applies to all
channel names in the new system) which holds 16 trigger bits or lines.
Optionally, one may use the two trigger interface units as separate,
which gives another 16 trigger lines on STI102. There are also trigger
channels STI201 (for triggers from the internal function generator) and
STI301 (for housekeeping functions), both of which should normally be
ignored in FT.

I hope this clarifies this trivial but in practice somewhat complicated

Best regards,

alotof eve wrote:
> Hi, My data is also from Vectorview, 306 sensors.
> The trigger in my data is on 'STI101'. You might check this by commond:
>   unique({event.type})
> My unique event.type is: 'STI101' 'STI301' 'trial'
> Then I use:
> [event(find(strcmp('bckpanel trigger', {event.type}))).value]
> The value I got are exactly the same as the triggers I set.
> But I did this before timelock analysis.
> */Myles Reilly <Myles.Reilly at HSC.UTAH.EDU>/* wrote:
>     It's a Vectorview, 306 sensors. The first one actually.
>     Myles
>     -----Original Message-----
>     From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On
>     Behalf Of Daniel Kislyuk
>     Sent: Tuesday, April 17, 2007 4:12 PM
>     Subject: Re: [FIELDTRIP] Neuromag event values
>     What Neuromag system do you have? With the newer models it's enough to
>     look at the channel STI 014 only, where the events are very clearly
>     coded in an easily extractible way.
>     -daniel
>     On 4/18/07, Myles Reilly wrote:
>     > Hello,
>     >
>     > I have been trying timelockanalysis on continuous Neuromag data.
>     > It does do the average but the results don't match averages
>     calculated
>     by
>     > Neuromag applications. In Fieldtrip I can see a single event
>     > named 'trigger' that has 2 values, [3.0953e-004]
>     > and [6.1907e-004]. There are actually two triggers with the values 1
>     and 2.
>     >
>     > Interestingly 3.0953 * 2 = 6.196 so I wonder if there is a
>     calibration
>     > factor at work here.
>     >
>     > I suspect both triggers are being averaged together.
>     >
>     > Thank you for any direction you can give me on this,
>     >
>     > Myles
>     >
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