The FrameTracker

Why you need this

In visual neuroscience experiments, timing is critical. You need to know precisely what was on the screen when a response occurred, and you need to know the precise latency between changes on the screen and any response. If you control your screen from a computer, you typically don't get timing precision better than tens of milliseconds, which is very often simply not good enough.

The FrameTracker records, with sub-millisecond precision, when exactly images appear on a computer screen. This means that even for recording techniques with very high temporal resolution, such as electrophysiology, you can fully reconstruct the timing of responses.

Don't trust your computer screen blindly

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.

How the FrameTracker solves this problem

The FrameTracker has two light sensors that you affix to the top and bottom corners of your screen. When the pixels underneath the sensors change from dark to light or vice versa, the FrameTracker outputs a TTL step on the corresponding BNC connector. In fact, the graphs above where made directly from FrameTracker output. You can record these outputs with any data acquisition system (may we suggest the picoDAQ?) and have a perfect record of your stimulation sequence that is synchronized with your other experimental results.

Tutorial

Please take a look at our tutorial for practical advice on using the FrameTracker.

Other uses

The FrameTracker has many other uses beyond recording the timing of images shown on your monitor. For instance, it can be used to check whether your monitor flickers (at a rate too fast for you to notice, but perhaps not invisible to your experimental subjects or their neurons).