Using a Tobii eye-tracker with PsyToolkit

Step 1: Go to the Tobii Analytics SDK website and select download.

Step 2: Unpack the Tobii Analytics zip file as follows:

Step 3: Copy some of the files to the right locations on your Linux system (the Tobii package does not do this for you!).

Step 4: Now run the PsyToolkit install for Tobii. Do the following.

The Tobii eye tracker is connected using the internet cable. There are different ways to do this. The cable goes simply from the eye-tracker to the Linux computer.

In your network manager dialog (see image below), make sure you set the connection mode to "Link-Local only", otherwise, the connection might not be established.

The Tobii Analytics packages comes with working Linux software to test your eye-tracker. It is best to see if that works first. Only if that works, you should continue.

They provide two test packages. Out of the box, I could only compile one of them, the other one had an error in it, which I could fix easily.

=⇒ image of file folder with the working package

Now you are set to test if PsyToolkit works with Tobii. Make sure you have switched your eye-tracker on.

1) Copy the Tobii example to your HOME directory:

2) Compile the script

psycc -t tobii-test.psy

3) Run the script

If you want to use the eye-tracker, you need to add tobii to your options. Once you have that option, PsyToolkit will assume that only one eye-tracker is connected and it will connect to that one.

It is assumed that every experiment you run starts with a calibration. In your block section, you can add a tobii calibrate line to the first block section.

You typically will want to make sure people look at the fixation point at the beginning of your task. In essence, people need to "hold" their gaze at the fixation point for some time. You can use the tobii hold command.

In the example below, we show one stimulus (fixpoint), which is thus stimulus number 1. Then we tell Tobii that the participant needs to hold this stimulus number 1 for 3000 ms. People get up to 10000 ms (10 secs) to look at the stimulus and fix it. For example, it might take someone 1 second to move their gaze to the fixpoint, and then when they hold it, they might not hold it, etc. For the whole process, they get more time than the hold time itself. You can set this number as high as you want, of course. But if it is too high, people just might have some issue or the fixpoint might be too small (see tobii allow to deal with this).

Thus, the command syntax can be described as such:

tobii hold stimulus_number total_time_for_this time_to_old_in_ms

Note that the computer sticks to the tobii hold command. In other words, it does not go on until people have held the stimulus.

Similar to functions to check is someone presses a button within a certain time, we can ask if someone looks at as a stimulus.

In the example below, we build on the example above. After the fixpoint, we add my_stimulus, which is stimulus number 2. We simply do a tobii look 2 3000 to tell Tobii that the computer should wait for up to 3000 ms until participant looks at the stimulus. The response time will be stored in EYE_RT.

In the above, the stimuli might be too small to accurately check if people look at them. This can be an issue given that there is "noise" in the eye location signal. You can widen the window around stimuli. For this we use the tobii allow command. It specifies the number of pixels around the stimuli.

Thus, if your stimulus is 10 pixels wide and 10 pixels high, you might find that people cannot hold their fixation on it. But with tobii allow 100 this would allow a size of 110 pixels wide and 110 pixels high.

During testing, following calibration, or during instructions for participants, it is nice to show where people look at. You can display the eye-gaze position on screen on top of your stimuli. This technique is used in the tobii_test example as well.

In your task, or in the block, you just use the command tobii display eyes. You can stop this PsyToolkit feature with tobii hide eyes.

or in block