Temporal & Spatial Precision
How precise is Labvanced? How does Labvanced ensure precise presentation of all stimuli?
Labvanced was built with the focus of ensuring the exact temporal presentation of all stimuli. We pursue a double pre-loading approach: before the experiment starts, we load all external content (images,videos, audio, etc.) in the browser cache. Then, for each trial we pre-load and pre-render the next trial such that once the trial changes, all stimuli are presented instantaneously. As a result, on devices/computer with acceptable internet bandwidth and CPU/RAM, the temporal precision should be comparable to lab settings.
How can I be sure that timing measures/stimulus presentation are correct? What does Labvanced do to measure timing precision?
Labvanced runs separate controls of timing precision for each recording. More precisely, we run a test time measurement every 5 seconds during each recording. In the end, we calculate the mean and standard deviation of these values and provide these for each recording. The mean is always positive, as a device never stops before but occasionally does stop after a desired time and thus sometimes a little too late (due to CPU usage, other lags, etc). This is equivalent to the delay in presenting a stimulus on the screen. However, to our understanding, such a constant mean offset can be counteracted post-hoc analytically.
On the other hand, a high standard deviation in the control time measurement suggests that the time measurements are generally of different quality, and therefore not really comparable. A high standard deviation is a good exclusion criterion of the subject. The median offset of all time measurements is about 20ms and the median standard deviation of all time measurements is about 12ms (across all subjects and experiments). If values are much higher (2-3 times) than that, it indicates imprecise timing measurements. Furthermore, it is possible to determine the timing precision on a custom basis (e.g. for each trial, frame, or stimulus) using UNIX timestamps in the event system.
What are the options of spatial presentation modes? How precise can stimuli be presented spatially?
There are various modes of how stimuli can be presented spatially. The default mode is the zoom mode, which will zoom all content elements until either vertical or horizontal limits are reached. Stimuli can also be fixed to pixels or visual degrees. Fixing a stimulus / frame to visual degrees will require a pre-calibration before the start of the experiment by which the physical size of the participants’ computer/ device is measured. Depending of the experimenter's choice of display mode, stimuli can be presented with very high (lab comparable) spatial accuracy.