Frame Tracker

Imagine the most straightforward of visual stimulation experiments: A full-screen flicker stimuli of 100-ms white followed by 100-ms black, repeated over and over. You expect the brightness of the entire screen to follow this pattern:

Even though to a casual observer, it may look as if that is what you get, the reality is more complex:

Here, the blue and orange curves shows the discretized brightness of the top and bottom of the screen as a function of time, respectively. The first thing that stands out is that the recorded brightness at the bottom of the screen lags the top of the screen by about 15 ms. A closer inspection reveals also that the “white” duration is consistently shorter than the “black” duration. Far worse, occasionally, the timing becomes even more irregular:

In fact, accumulated over 4000 black or white frames, the actual frame durations were distributed like this:

None of the frames were displayed for the programmed 100 ms, and a substantial number of frames (6.7% of the total) were displayed for 20 ms more or less than programmed.