Pro Lab and Pro Studio are able to time events with good accuracy, but its performance depends on your setup configuration. If your experiment involves presenting stimuli, synching with other biometric devices, or getting responses from the subject, and you need this done with high accuracy, please read the following recommendations.
- Use recommended components
Choose appropriate hardware components according to your timing requirements. Specifically, we recommend you use a graphics card with a dedicated processor. For a detailed list of recommended components read Tobii System Recommendations for Pro Lab and Pro Studio.
- Keep computer processes to a minimum
The computer running Pro Lab or Pro Studio should have the least number of processes running in the background. This means that you should disable or uninstall any unnecessary programs that might be running in the background (Anti-virus software, Firewall software, Anti-Spyware software, Skype or other chat/communication programs). It may also help to disable or disconnect Bluetooth, WLAN/Wifi and network connectors, and to unload CD or DVD disk drives. Ideally, use the computer only to run Tobii Pro Lab tests and analysis.
- Keep drivers up to date
Make sure that your computer drivers are up to date, especially the graphics card driver and the parallel port driver. You can do so by visiting the manufacturer’s website and checking for the latest update. Please note: It is not a good practice to rely only on Windows Update service to get updates on your drivers.
- Check your graphics card settings
If available, set the vertical sync of your graphics card to be controlled by applications. Check if this option is available in your graphics card settings under any name such as vertical sync, vertical retrace, synchronize buffer swaps, etc.
- Use native screen resolution
We recommend that you use the native screen resolution and that you create your stimuli in that size. Running your study at other resolution other than native increases the time needed for clearing the buffer, drawing your stimulus and for post-processing operations.
- Multi-screen setups
We recommend using multi-screen setups only in extended mode and with the “primary monitor” set to the stimulus presentation monitor. Otherwise, there is a risk that the timing functioning and estimation will be inaccurate. Dual or duplicated-monitor setups are not recommended if you need accurate stimulus display timing.
- Find out the pixel response time of your monitor
To increase the accuracy of the stimulus-onset events in Pro Lab, you can include the pixel response time of your monitor (Pro Lab recording settings). Information about the pixel response time can be usually found in your monitor documentation (specification sheet).
- Use DVI, HDMI or Display Port connectors
Whenever possible, use digital connectors between your computer and your monitor with no connector converters. This will reduce the probabilities of bad communication between the graphics card and the monitor, i.e., correct synchronization with the VSYNC signal of the monitor. Unless necessary, avoid using VGA connectors, especially if you have precise timing requirements. Also, our timing tests suggest that Display Port connector gives more precise timing than DVI connectors.
- If precise stimulus onset and duration is important, use a monitor with a higher refresh rate
In time-sensitive experiments you can benefit from having a monitor with a higher refresh-rate. If you replace a 60 Hz monitor with a 240 Hz monitor, the stimulus duration resolution (frame duration) will be 4.17ms (1/240Hz), instead of 16.67ms (1/60Hz). For most eye tracking applications that involve global measuring of visual attention and area of interest (AOI) visit duration related metrics, a 60Hz refresh rate monitor is sufficient. A high refresh-rate monitor is of great importance in specialized experiments which involve for example, the measurement of saccade latencies, and especially in gaze-contingent paradigms with saccade dependent stimulus image manipulation.
- Use the recommended image and video format as stimuli
The recommended image stimulus format in Pro Lab is jpeg. The recommended video stimulus format in Pro Lab is H.264. We recommend that the resolution for both types of stimuli be the same as the monitor resolution.
- Stimulus duration preceding a video stimulus
When the next stimulus in a presentation sequence is a video, Pro Lab will spend some time (order of milliseconds) preparing the video before the video onset. This will not affect the video stimulus-onset estimation but it will likely affect the duration of the preceding stimulus, making it longer than the expected duration. Therefore, we recommend not showing time-sensitive stimulus before a video. A good practice is to show an extra black image stimulus for a short time before the video stimulus.
- If precise picture stimulus duration is important, think in frames!
If you really need to have precise duration of your picture stimuli, you will have better control by designing the duration of your stimuli using the following duration calculation: Use the refresh rate of your monitor to calculate the real duration of your picture (e.g., with a 60Hz monitor, possible times are multiple of 16.67ms). Take this value, subtract about 2/3 of a refresh cycle and introduce the result as the stimulus duration in Pro Lab. If you follow this procedure, it is highly likely that your stimulus will be displayed the correct number of frames that was designed for.
Eye tracking components
- If precise stimulus onset is needed, avoid using X2-30 and X3-120 without EPU
If you need to have precise stimulus onset timing, we do not recommend using the X2-30 eye tracker or the X3-120 eye tracker without EPU. Because these eye trackers’ computational work takes place in Pro Lab’s computer, the performance of the computer could be affected. These eye trackers can be used if millisecond precision stimuli onset and duration are not needed.
- If precise stimulus onset is needed, don’t use the moderator tool for live viewing in Pro Lab
The live view option of the moderator tool in Pro Lab allows the experimenter to view the live gaze of the participant overlaid on the stimulus on a second screen. While this is an important feature for some eye tracking research, e.g. local live viewing for real-time insight and preparing post interviews, it is not essential in most experiments and can affect Pro Lab stimulus display performance. By default, the live view of the moderator tool is disabled. We recommend that you do not enable the live view option in time-sensitive experiments.
- If you use a gaze-contingent paradigm, bear in mind the total system latency
You can use a Tobii Pro eye tracker together with the Tobii Pro SDK to create your custom gaze-contingent paradigms in several stimulus presentation software applications such as Psychtoolbox or Psychopy. In some gaze-contingent paradigms, it is important to take into account not only the latency introduced by the stimulus display process, but also the eye tracker latency. This is the time interval between the moment the eye tracker takes the picture of the eye (eye tracker timestamp) until the gaze information is available at the computer and you can use it to update the stimulus information. This is known as total latency and is mainly influenced by the eye tracker sampling rate and eye tracker gaze computing time. Total latency is of significant importance in gaze-contingent paradigms such as the moving-window, where the stimulus needs to be updated during saccades. To get information about the specific latency of your Tobii Pro eye tracker, go to the corresponding product description. For example, the Tobii Pro Spectrum eye tracker has a latency of less than 5ms at 600Hz.
Last but not least… Test your timing!
If you are performing experiments that require high timing accuracy and precision, we strongly recommend testing the system latency with the stimuli you will be presenting. It is important to use the same computer, monitor and monitor connector when measuring the system latency as for all the recordings of the experiment. You can find information about how to run a timing test with a TX300 or Spectrum eye tracker in Timing Guide for Stimulus Display in Pro Lab