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Sending Triggers with Pixel Mode

This guide is an introduction to sending digital TTL triggers using VPixx Technologies’ “Pixel Mode.” While in Pixel Mode, your VPixx device continuously generates digital outputs based on the colour of the top left pixel of the display. By changing the colour of this specific pixel on a given frame in an experiment, researchers can send frame-accurate triggers to their recording system of choice. Pixel Mode can replace and enhance a photodiode-based triggering system—no programming required!

A VIEWPixx /EEG sends unique digital TTL triggers to an EEG system, synchronized to the onset of different visual stimuli

The guide includes an introduction to TTL triggers and their use, an explanation of digital outputs and how to set and interpret them, and finally, instructions for how to use Pixel Mode with your VIEWPixx /EEG. The guide ends with some useful tips, and instructions for how to use Pixel Mode with your other VPixx devices. 

Introduction to TTL triggers

A common requirement in research is to be able to relate neurological, physiological and behavioural responses to the onset of a visual event.

Suppose we are conducting an electroencephalography (EEG) experiment examining neural responses to pairs of objects varying on one or more feature dimensions. Specifically, we are interested in event-related potentials that occur in the first 250 ms following the appearance of these objects on our display.

In our EEG data, we need a way to identify, or mark, when the objects appear onscreen, so that the EEG data can be time-locked to it. The more consistent and reliable this segmentation is across trials, the more confidence we have in aligning multiple trials’ worth of data and averaging our results. This event marker is what supports trial-averaging, and the extraction of the event-related activity sequence. 

Raw EEG data from several channels. How do we know when the stimulus first appeared? Source: CC BY-SA 3.0,

If the effect we are looking for is small, better time-locking may give us the statistical power we need to actually detect it. So ideally, our marker of stimulus onset should have very little variable error across trials; in other words, it must be precise.  

It would also be helpful if the marker is sensitive to the kind of objects being displayed. For example, the marker could signal what trial type is being presented, what iteration of this trial type is being presented, and details of the feature dimensions which vary. This information becomes useful when sorting and categorizing EEG data during analysis.

How do we communicate temporally precise, informative markers from our display to the EEG? We use transistor-transistor logic (TTL) triggers. These triggers are simple, rapid digital signals that can be sent between systems via inexpensive cables. TTL triggers are recorded by our EEG system, to help characterize our EEG data.

Example of a 100 ms TTL pulse

In general, we need to be careful of timing issues introduced by the computer. The experiment software, the processing done by the graphics processing unit (GPU), and the computer operating system can add delays that create a variable discrepancy between when the computer reports that a visual event has occurred, and when the visual information actually appeared on the screen.

In some experiments, those discrepancies are small enough to be negligible, on the order of a few milliseconds. However, since time-averaging benefits from precise time-locking, we want the most precise event marker for our stimulus onset that we can get. 

 One way to improve precision is to bypass our computer altogether and create a trigger based on when the pixels on our monitor physically change. We can mount a light-sensitive device called a photodiode on one corner of our monitor.  A photodiode amplifier can send out a TTL signal when it detects a change in the luminance of that patch of screen. If we change the patch luminance on the same video frame in which the stimuli appear, we have a fast and reliable measure of when the stimulus actually showed up on the display.

A photodiode mounted over a patch of white pixels in the top right corner of a display

While economical and effective, photodiode setups take up space and require a bit of legwork to get running. They can interfere with full-screen stimuli like natural scenes, or search tasks. Perhaps most critically, changing luminance patches on your display can be a source of distraction for your participant.

TTL triggers are a way to insert markers of visual event onsets into our recording data. Ideally, we want these triggers to be precise, informative and minimally obtrusive. VPixx has designed a special triggering mode called “Pixel Mode,” which serves a similar purpose as a photodiode, but offers customizable triggers with no extra hardware, minimal onscreen distractions, and no programming required. In the next section, we will cover the basics of how Pixel Mode operates.


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