Hello Xavier,

Forgive me for the length of my reply, but this is the definitive explanation
from the Fieldtrip list.

4/18/07

Hi,

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 issue.

Best regards,
Lauri



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 <[log in to unmask]>/* wrote:
>
>     It's a Vectorview, 306 sensors. The first one actually.
>
>     Myles
>
>     -----Original Message-----
>     From: FieldTrip discussion list [mailto:[log in to unmask]] On
>     Behalf Of Daniel Kislyuk
>     Sent: Tuesday, April 17, 2007 4:12 PM
>     To: [log in to unmask]
>     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
>     >
>