Introduction to Eye-Movement Research

1. An Introduction toEye-Movement Research Marc Pomplun Department of Computer Science University of Massachusetts at Boston E-mail: marc@cs.umb.edu Homepage: http://www.cs.umb.edu/~marc/

2. Overview: Types of Eye Movement Why Eye-Movement Research? How to Measure Eye Movements Examples of Eye-Movement Studies and Paradigms An Introduction toEye-Movement Research

3. Eye Muscles Eye Movements

4. Types of Eye Movement Fixations: • The eye is almost motionless, for example, while reading a single, short word. • The information from the scene is almost entirely acquired during fixation. • Duration varies from 100-1000 ms, typically between 200-600 ms. • Typical fixation frequency is about 3 Hz. • Fixations are interspersed with saccades.

5. Types of Eye Movement Saccades: • Quick “jumps” that connect fixations • Duration is typically between 30 and 120 ms • Very fast (up to 700 degrees/second) • Saccades are ballistic, i.e., the target of a saccade cannot be changed during the movement. • Vision is suppressed during saccades to allow stable perception of surroundings. • Saccades are used to move the fovea to the next object/region of interest.

6. Types of Eye Movement Smooth Pursuit Eye Movements: • Smooth movement of the eyes for visually tracking a moving object • Cannot be performed in static scenes (fixation/saccade behavior instead)

7. Types of Eye Movement Torsional Eye Movements: • Rotation of the eye around the viewing axis • Stabilization of visual scene by compensating body rotation (up to about 15 degrees)

8. Types of Eye Movement Vergence Eye Movements: • Slow, smooth movements changing the vergence angle (the angle between the two viewing axes) • Used for changing gaze from a near to a far object or vice versa • Can take up to one second • Execution is often interrupted if no thorough inspection of the object is required.

9. Types of Eye Movement Tremor: • Fast, low-amplitude (seconds of arc) eye-movement “jitter” • Improves the perception of high spatial frequencies • Prevents the fading of static images during fixations

10. Why Eye-Movement Research? Throughout my talks, the only types of eye movement that we will consider are fixations and saccades. Why? These eye movements indicate a person’s gaze trajectory while performing a certain task. Moreover, they yield information about a person’s visual attention.

11. Why Eye-Movement Research? About eye movements and visual attention: • Usually, saccades follow shifts of attention to provide high acuity at the attended position. • It is possible to look at an object without paying attention to it (staring). • It is possible to shift attention without eye movement (covert shifts of attention). • It is impossible to perform a saccade while not shifting attention. • During specific, natural tasks it is reasonable to assume that saccades follow shifts of attention.

12. Why Eye-Movement Research? The investigation of visual attention, in turn, is at the core of cognitive science. • Studying visual attention yields insight into general attentional mechanisms. • It can provide information on a person’s stream of conscious and unconscious processing while solving a task. • Attention is closely linked to the concept of consciousness. • Attentional mechanisms could improve artificial vision systems.

13. How to Measure Eye Movements Mirror on Eyeball • Used in first eye tracking experiments (Yarbus in 1960’s) • Suction cup attaches mirror to eyeball • Light beam is directed at mirror and reflected onto photo sensitive paper • Good spatial resolution but no temporal information • Unpleasant for the subject

14. How to Measure Eye Movements Electrooculogram (EOG) • Skin electrodes around the eyes measure potential differences • Wide range -- poor accuracy • Better for relative than absolute eye movements • Mainly used in neurological diagnosis

15. How to Measure Eye Movements Eye Coils • Subject wears contact lens with wire coil • Homogeneous magnetic field around subject allows measurement of gaze angle • Very high temporal and spatial precision • Special coil also allows measurement of torsional eye movements • Very uncomfortable

16. How to Measure Eye Movements Limbus Tracker • Photo diodes track the boundary between sclera and iris. • High temporal resolution • Poor spatial precision; only for horizontal eye movements • Inexpensive

17. How to Measure Eye Movements Purkinje Eye Tracker • Laser is aimed at the eye. • Laser light is reflected by cornea and lens • Pattern of reflected light is received by an array of light-sensitive elements. • Very precise • Also measures pupil accomodation • No head movements

18. How to Measure Eye Movements Video-Based Systems • Infrared camera directed at eye • Image processing hardware determines pupil position and size (and possibly corneal reflection) • Good spatial precision (0.5 degrees) for head-mounted systems • Good temporal resolution (up to 500 Hz) possible

19. How to Measure Eye Movements EyeLink II System: • Binocular • Head-movement compensation (head camera looking at IR markers at monitor) • Temporal resolution 500 Hz • Spatial precision about 0.5 to 1 degree • Gaze-position data available in real-time

20. How to Measure Eye Movements EyeLink II System Configuration

21. How to Measure Eye Movements Measuring vergence eye movements in anaglyphs with EyeLink II (poor subject!)

22. Eye-Movement Studies Eye movements while watching a girl’s face (early study by Yarbus, 1967)

23. Eye-Movement Studies Visual scan paths on instruments/dashboards – studies for the improvement of human-computer interfaces

24. Eye-Movement Studies Gaze trajectory measurement for the optimization of web page layout

25. Eye-Movement Studies Improving advertisements with eye-movement studies

26. Eye-Movement Studies

27. Eye-Movement Studies • The technique of attentional landscapes (see previous slide) was used to visualize attention in ambiguous pictures. • Subjects viewed ambiguous pictures that allowed two different interpretations A and B. • They were asked to press and hold a particular button while perceiving interpretation A, and a different button for interpretation B. • Afterwards, the fixations recorded during the perception of interpretations A and B were separated and separately visualized in the original image.

28. Looking at Visual Attention

29. Looking at Visual Attention

30. Eye Movement Studies Eye movements while watching (green), starting to imagine (black), and imagining a shape (red) - “reading one’s mind”

31. Eye-Movement Studies Eye movements as indicators of cognitive processes (Yarbus): • trace 1: examine at will • trace 2: estimate wealth • trace 3: estimate ages • trace 4: guess previous activity • trace 5: remember clothing • trace 6: remember position • trace 7: time since last visit

32. Eye Movement Studies

33. Eye Movement Studies

34. Eye Movement Studies Gaze-contingent display in a visual search task

35. Eye Movement Studies To be continued…