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).
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:
After the recording, you can check that all data streams recorded contain the same shared events.
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.
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. www.amazon.com) and can be installed easily in most PC. Tobii Pro recommends PCI type of cards installed in a stationary computer.