The Tobii Eye tracking system produces remarkably "clean" and accurate information about the gaze position of the eyes.  However, you will notice that the gaze position often appears quite unsteady.


While some of this "noise" may be attributable to instrument "noise", in most cases this represents the fundamental instability of the oculomotor system.  Even when attempting to fixate steadily, the eyes are constantly in motion.  These are known as the involuntary eye movements and include drifts, microsaccades and tremor.


This noise can make it more difficult to interpret the underlying pattern of eye movements.  Therefore, in order to analyse the eye movements, the data is first modelled to create a series of saccades and fixations.


To view the modelled data, select Model in the main toolbar.  The modelled data will be shown in orange next to the actual data.  In most cases, the modelled data will provide a good fit with the actual data as shown below.


The modelled data is used to extract the summary statistics.


When the Model data option is selected, the X-Y display will show the modelled scan path with each fixation shown as a circle, the diameter of which represents the duration of the fixation, a shown below.



In Play mode, the modelled data will be shown, removing most of the noise from the recording.