Setting up TTL shared events in Pro Lab

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The combination of eye tracking data with other biometric data streams (e.g., EEG, GSR, or ECG) is becoming a common practice in different areas of human behaviour research. Often, biometric data streams have to be recorded in a separate recording software outside Pro Lab (e.g., eye tracking data in Pro Lab and EEG data in its dedicated EEG recording software). To be able to analyse biometric data recorded during the presentation of visual stimuli, it is essential to know when stimuli were shown to the participant. Also, depending on the research methodology, it can be crucial that this stimulus information is timestamped with a high-timing accuracy. Pro Lab includes a synchronization solution -Stimulus-onset markers (TTL)- that allows to send out accurate shared events at the onset of every stimulus to any compatible biometric device that is being recorded simultaneously during the stimulus presentation. Pro Lab uses TTL technology to send the events from the parallel port of the computer to any device with a TTL input port. Each event can contain one numerical value that can be used to identify the stimulus shown (1 - 255 for external devices with input TTL ports of 8 bits).

How to send stimulus-onset markers to external devices

To be able to send shared events to external devices, the presenting computer where Pro Lab is running must have installed a parallel port card. These simple steps have to be followed to configure and send Stimulus-onset markers in Pro Lab:


  1. Go to Advanced Settings in the Design section and enable the option Send Stimulus-onset markers (TTL).
  2. Choose between the automatic and manual mode. If the automatic mode is selected, choose the preferred marker bit depth. If the manual mode is selected, choose the preferred shared event value of each stimulus (Read the next section to learn about automatic and manual mode).


  1. Connect the parallel port of your computer to the TTL input port of the device that will receive the shared events. Use an appropriate cable with the right connections between the ports. Usually, the biometric device manufacturers offer specific cables that directly connect with the parallel of the computer.
  2. Go to the Record section and check if the parallel port of the computer is recognized by Pro Lab. If the TTL Parallel port card has black background, the parallel port is correctly set up on your computer and Pro Lab is ready to send the shared events during the recording. If the TTL Parallel port card has red background, please make sure that the computer where Pro Lab is running has a proper parallel port card installed (not a converter) and it has been properly set up.
  3. Once the TTL Parallel port card is enabled in Pro Lab, the stimulus presentation and eye tracking data recording can start in Pro Lab. We recommend that you start first the recording of your external data and after the recording in Pro Lab. This will ensure that all the shared events will be registered in your external data stream.


After the recording, you can check that all data streams recorded contain the same shared events.

  1. In Pro Lab, you can check it by opening the recording and enabling the Sync Events type in the Events section. The shared events will appear as independent events named “TTL out”. They are timestamped with the estimated time when the computer sent the shared event to the external device. In most of the cases, the timestamp will correspond with the stimulus onset time event.
  2. The stimulus-onset marker events can also be found in the column “Event” and “Event value” of the data export file.

Automatic Vs Manual mode

The Stimulus-onset markers (TTL) option can be configured in two modes: Automatic and Manual.

The automatic option is the fastest and most convenient option in many experiments. When this mode is selected, Pro Lab will automatically assign to each stimulus a unique value. Pro Lab will start with a value of 1 for the first stimulus of the first timeline and will increment the value by one for the following new stimuli. There is one exception to this: when a stimulus with the same name is repeated in the same or different timeline, Pro Lab will assign the same value to all of them. An example can be seen in the figure below. The example contains seven different image stimuli, all of them preceded by the same fixation cross. Pro Lab has automatically assigned an increasing TTL marker value to all the different stimuli and the same TTL marker value -2- to the fixation cross. Pro Lab will increase by one the TTL marker value for every new stimulus until it reaches the maximum accepted value. Pro Lab will restart then the TTL marker value count and will assign a value of 1 to the next new stimulus in the timeline. The maximum number of TTL marker values will depend on the Marker bit depth selected. A marker bit depth of 8 means that the external device has a TTL input port with 8 pins and therefore the user can send up to 255 different values. Pro Lab also offers marker bit depths of 3 (up to 7 different values) and 1 (only 1 value).

The manual option is ideal for all the users that prefer to assign a specific value to the different stimuli. With this option, the Stimulus property TTL marker value can be edited and any value from 0 to 255 can be entered.

TTL setup in Tobii Pro Lab

Computer parallel port

It is becoming more common that modern computers don’t have a parallel port card installed by default. However, the parallel port and TTL technology is still one of the most reliable and accurate ways of sending shared events from a computer. Also, TTL technology is still the most common input port of many biometric devices. Parallel port cards can be found in many stores that sell computer components (e.g. and can be installed easily in most PC. Tobii Pro recommends PCI type of cards installed in a stationary computer.