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Eye tracking labs at the MPI

Eye movements during reading,
by courtesy from SMI

Introduction

The Max-Planck-Institute owns three Eye Trackers which record eye movements and store the data on disk or video tape for later analysis. They serve different functions:

• ET Lab 1: high accuracy eye tracking in reading experiments with bite bars and specially designed software

• ET Lab 2: high accuracy eye tracking with computer-based stimuli controlled by the NESU experiment software

• ET Lab 3: video-based eye tracking in real scenes

Eye Tracking Lab 1

This eye tracking setup is intended to present sequences of textual or graphic stimuli in a special way and to later derive the reading or perception pattern from the data. The setup finally allows the user to differentiate between fixations and saccades and to determine the sequence of fixations. Afterwards statistics can be calculated regarding how often and how long certain objects (either words or drawings) are fixated by the subjects.

The eye tracking is carried out by measuring the scattered light emitted by an infrared LED and reflected by the iris. Since this eye tracker only uses one reflection, head movements cannot be compensated. Therefore, a bite bar has to be used. Otherwise, small head movements would lead to large measurement errors.

The eye tracker was bought from the German AMTECH company. The hardware setup consists of the eye tracker and two PCs.The master PC controls the experiment and generates the stimuli and the slave PC is samples and maps the data generated by the eye tracker.

The software consists of a variety of components. First, a flexible and special experiment control program runs the experiment, which is made up of a sequence of text pages. The text pages themselves can be efficiently generated with the help of a special generator. Parts of these stimuli are so-called calibration pages which show a number of special spots on the screen. After subjects have looked to these spots, calibration factors can be calculated such that the eye tracker data can be mapped to screen coordinates. These mapped data can then be further analyzed in terms of fixation patterns. Of course, visualization tools help in analyzing the results. This analysis software was mainly developed at the University of Aachen.

Technical details on Eye Tracking Lab 1.

Eye Tracking Lab 2

This lab is similar to Lab 1. It was designed to do eye tracking when sequences of textual or graphic stimuli are presented via a computer display. However, here, the focus was the full control of experiments by the general NESU experiment software, which added another stream of data in standard experiments. Scientists using this device are mainly interested in object fixation parameters.

The eye tracking is carried out by measuring the scattered light emitted by an infrared LED and reflected by the iris. To compensate for the inherent sensitivity of this measurement principle with respect to head movements, these movements are registered in parallel. 4 markers are put on the sides of the computer display. The position and movement of the head is calculated on the bases of images of these markers from a head mounted camera. This correctly determines the eye position. We are very satisfied with this method.

The eye tracker was bought from the German SMI company. The hardware setup consists of the eye tracker and two PCs where the master PC is controls the experiment and generates the stimuli. The slave PC samples and maps generated by the eye tracker. Both PCs communicate via an Ethernet link such that it is possible to operate the eye tracker from both keyboards and do the visualization of the eye data on both displays.

The software consists of three components. The eye tracker is controlled by ready-made software. On the master PC, a TSR program is used as a transparent interface for eye tracker commands and data. The NESU software takes over control of the eye tracker simply by issuing commands to the TSR program. The following figure shows a typical moment in the preparation of an experiment. Shown is the subject with the head-mounted recording devices and the subjects' eye displayed on a monitor.

Analysis software finally does the mapping between objects and eye movement patterns such as shown in the following figure. This figure shows a typical eye movement pattern recorded with the SMI system. The software has a couple of modes for analysis which cannot be explained in this document.

Technical details Eye Tracking Lab 2.

Eye Tracking Lab 3

This eye tracking setup is intended to make eye movement recordings for real scenes. These could be scenes with multiple objects located on a table where the subjects are asked to point to these objects and describe what they are doing. The eye movement patterns can be recorded in parallel and afterwards compared with the gesture and speech patterns. The following figures show the headband assembly of this eye-tracker.

Here we are using traditional equipment which records eye movements based on the reflections of the pupil and the cornea. A video camera which is also mounted on the head-band records the scene such that the eye movement pattern is superponed to the video signal of the scene. This eye tracker was also bought from the German SMI company.

Of course, here the analysis of the eye movements is video-based.

Technical details Eye Tracking Lab 3.

Last updated: February 11, 2000 23:38

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