Precise timing in relation to the presentation of visual stimuli is a requirement in many types of research methodologies. Some examples are, experiments measuring eye movement reaction times, participant response times, or electrophysiological responses with high timing resolution such as EEG. In these types of experiments, it is critical to set or determine the stimulus-onset time (i.e., the point in time when a stimulus first becomes visible on the screen), as well as the stimulus duration to an accuracy down to the millisecond.
To ensure millisecond accuracy and precision, it is essential to understand the different hardware and software factors involved in the stimulus presentation, and how they affect the stimulus-onset time and display duration. Different computer and monitor factors impact the latency from the time that the CPU sends the image stimulus information to the graphics card until the image is finally displayed on a screen. Depending on the stimulus control process, these factors will to varying degrees affect the timing accuracy of the stimulus onset on the screen and its estimation by the stimulus presentation software. Computer factors can be controlled and optimized to have little impact on the stimulus display latency (e.g. by having a graphics card with a dedicated processor, or using the correct timing functions and priorities in Windows). On the other hand, the choice of monitors and their specifications have a strong impact on timing and are often forgoten when designing eye tracking studies. In the next section, we will present and discuss two important monitor properties for your stimulus presentation timing - screen refresh rate and pixel response time.
When a CRT or LCD monitor is updated with new information, pixels are not updated all at once, but rather the pixels are updated sequentially, following a specific pattern. The update, known as refresh cycle, always starts from the leftmost pixel of the first horizontal line, scanning line by line from left to right until the rightmost pixel of the last line has been updated. The number of times in a second that a display draws the image on the display following the process described is known as the refresh rate (e.g. 60Hz in a standard LCD monitor).
Pixel response time is a characteristic of LCD displays only. Pixels in a LCD display require some time to update and reach their new luminance target level when a new stimulus is displayed. The pixel response time will vary for different monitor models, with a common a pixel response time between 5 to 10ms. The pixel response time is usually reported by the manufacturer in the monitor specifications.
Pro Lab controls the stimulus display process by synchronizing the display of a stimulus with the monitor vertical scan (i.e., the start of a refresh cycle). This practice prevents image tear on the display (visual artefact where information from two or more frames are presented at the same time, creating a defective image), enables an accurate estimate of stimulus onset, and an accurate synchronization of the stimulus presentation with any data stream recorded in Pro Lab. External data streams recorded outside Pro Lab will also benefit from accurate stimulus control when sending TTL stimulus-onset markers to register onset events in any external data. A direct consequence of displaying stimuli at the start of a vertical scan is that the stimulus duration will always be a multiple of the refresh time. Depending on the refresh rate of your monitor, the stimuli duration will be more or less influenced by this. For example, with a 60Hz monitor, stimuli durations will be multiples of the refresh time: 16.67ms. The user can select any stimulus duration as short as 50ms when designing the stimuli, but Pro Lab will consider all the computer and monitor factors when estimating the stimulus-onset events and durations and will timestamp the events accordingly. This ensures that the stimulus-onset and duration estimation in Pro Lab are as close in time as possible to the actual stimulus-onset and duration on the display during the experiment.
It is also important to understand that there are factors that Pro Lab cannot automatically control. These factors are usually related to constant latencies (e.g., pixel response time). Constant latencies can be easily corrected for in Tobii Pro Lab’s recording settings. However, in time-sensitive experiments with high timing accuracy needs, jitter or variable latency must be avoided as much as possible as it is difficult to correct. In these cases, the overall system latency should be measured to ensure that the setup does not have any significant jitter that can affect the results. You can add the total constant latency of your setup under Target Screen settings (Record tab). Pro Lab will add that constant time to all the estimated onset events. Note that this latency can include any constant delay introduced by your setup, not only the monitor latency.