Triggers, events, codes

What is a trigger ?

A trigger is a temporal marker, a short pulse coded in bits that is recorded at the same time you acquire your MEG data and which will be saved in the same data file. For instance, you display a visual stimulus to the subject but the MEG acquisition system does not know that (your PC stim does though). You basically need to send a trigger from the PC stim to the acquisition sytem to temporally mark that something has happened. Triggers do not simply mark an event in time, they also carry information about that event. Hence, if you want to distinguish in your raw MEG data file at which time the subject saw stimulus X and at which time he saw stimulus Y, you would just need to code your event X by the numeral 1 and your event Y by the numeral 2 (that's just an example of course).

Triggering using our MEG system

Triggers are basically a simple voltage change (or "TTL") sent by the PC stim via the parallel port to acquisition system (via the BNC lines you see on the shelf above the PC stim). Thus, a physical line can be ON (state 1, a pulse is sent) or OFF (state 0, no pulse, no voltage change). The voltage of the pulse is typically 5V but its duration can be variable (and defined as you wish for instance in matlab). The edges from 0V to 5V or 5V to 0V are what can be typically detected by most analysis softwares; most often it is the onset that is being used (from 0 to 5V).

The acquisition system has 8 physical lines which can be on or off at the same time. Hence, you have 8 bits to play with to code you events, or a total of [(2^8)-1 = 255] possible triggers that you could use.

The lines are named STI00n and are attributed the (2^n-1) bit. Hence:

STI001 <=> 2^0 = 1

STI002 <=> 2^1 = 2

STI003 <=> 2^2 = 4

STI004 <=> 2^3 = 8

STI005 <=> 2^4 = 16

STI006 <=> 2^5 = 32

STI007 <=> 2^6 = 64

STI008 <=> 2^7 = 128

The other STI of the system are used to map the response keys:

STI007 <=> 2^6 = 64

STI008 <=> 2^7 = 128

STI009 <=> 2^8 = 256

STI010 <=> 2^9 = 512

STI011 <=> 2^10 = 1024

STI012 <=> 2^11 = 2048

STI013 <=> 2^12 = 4096

STI014 <=> 2^13 = 8192

STI015 <=> 2^14 = 16384

STI016 <=> 2^15 = 32768

Fortunately, participants can have 10 response choices in your paradigms! Unfortunately, as you noticed, STI007 and STI008 are lines of overlap between the trigger lines and the responses lines. Keep this in mind when coding your triggers.

Finally, STI101 emulates an analogue line which sums the voltages across all sixteen STI lines. Hence, if lines STI001 and line STI014 are ON or triggered at the same time, STI101 will read as 16385.

Flavors of trigger definition

Two populations of researchers coexist at Neurospin MEG:

Those who use one bit to code one item following this logic:

if STI001 = 1, then a stimulus is ON

if STI001 = 0, then there is no stimulus

if STI001 = 1 & STI002 = 1, stim 1 is ON

if STI001 = 0 & STI002, stim 2 is OFF

etc

Those who use a decimal based event coding approach:

stimulus 1 = 2^0 = 1

stimulus 2 = 2^1 = 2

stimulus 3 = 2^0 + 2^1 = 3

etc

Either work fine: the first one is elegant but the second option, while messier, is more flexible with respect to the number of triggers you can generate. As a tip (but not yet tested!), you could code additional information in the duration of your trigger. For instance, the louder a sound or the brighter a stimulus is in your set, the longer your trigger duration.

Details on sending and receiving triggers from the PC stim are provided under the stimulus delivery section as a function of the stimulus delivery software you wish to use.

Additional information to go on: Responses buttons mapping Acquisition setting