Finger trajectory tracking
What is trajectory tracking?

In many behavioral experiments, you show a stimulus and your measures are taken at the end of the trial – the response and the response time.

In trajectory tracking experiments, the participant is asked to point at the response (preferably with the finger on a tablet, but a mouse would do the trick too). The software monitors the full finger trajectory – from the moment it starts moving until it reaches the response location. This means that you can measurable behavior throughout the trial, right from time=0.

The idea is that the finger/mouse movement in each time point during the trial reflects the cognitive state at that time, so you can measure how the cognitive process evolves. It’s a sort of “behavioral EEG”.

Why do trajectory tracking experiments require a special software package?

Typical trajectory tracking experiments are similar to non-tracking experiments in many ways: you show a stimulus and you record the response. Still, the fact that we can measure the behavior continuously throughout the trial creates some particular characteristics for trajectory tracking experiments:

  • A typical tracking experiment would impose certain restrictions on the finger movement. For example, you may require that the finger starts moving very quickly after the stimulus was presented, or you may impose some limits on the speed or direction of movement.

  • In a tracking experiment, it makes a lot of sense to change the stimulus during the trial: because we can measure behavior continuously, we can see how the participant reacts in real time to changes in stimulus.

  • The stimulus may also change as a result of the finger movement (e.g., the stimulus may appear or change when the finger reaches a certain location on screen; you may drag a shape with your finger; etc.)


Because of these characteristics, relatively many things happen during a typical trial of a trajectory tracking experiment – more than in a typical trial of a non-tracking experiment. This means that a typical program of a trajectory tracking experiment would have to do quite a lot of things. TrajTracker provides you with a set of functions and classes that do many of these things – so it may save you time and bugs when you program your experiments.

Is it hard?

It depends. Creating an experiment may be very easy in case you use one of the paradigms supported off-the-shelf by TrajTracker. If you have to write a full experiment yourself from scratch, you should expect more programming work than a typical experiment that only acquires a single response per trial.

In both cases, should you decide to examine the full trajectory data (rather than a single measure per trial), this complicates the analysis considerably.

Mouse or finger?

In our experience, finger trajectories are far more accurate than mouse trajectories. Arguably, finger movement may be more natural and may rely less on a visual feedback loop.

There are softwares available for mouse-trajectory tracking (e.g. Mouse Tracker). If you decide to use such software, you may consider using it with a touchscreen rather than with a real mouse.

TrajTracker Experiment is built on top of Expyriment, which still has limited support to Android devices. You can use TrajTracker with MS-Windows-based tablets, or with a computer with touchscreen.