Lecture 7: Eye Tracking I

Question 1

How can we interpret gaze / what gaze are indications for?

Answer 1

• Gaze signals attention and interest
• Eye movement depends on activity (for example reading)
• Gaze can reveal mental states (nervousness, sleepiness, alertness, relaxation, …)
• Gaze is non-verbal communication

Question 2

Describe / Sketcht the anatomie of an eye

Answer 2

Question 3

What do we need to see and what is the time period for the stable projection of a human brain?

Answer 3

To see we need a stable projection of light on our photo receptors for perception and cognition by our brain, so that we finally recognize what we are seeing.

Time period for the stable projection is in the range of 200 – 300 milliseconds.

Question 4

Which many antagonistic pairs of muscles has an eye?

Answer 4

4:
for horizontal, vertical, and rotational movements

Additionally one for the size of the pupil

Question 5

What is age-related macular degeneration (AMD) and what is the size of the fovea?

Answer 5

AMD:
Similar to the skin, the macula can get ‘wrinkly by age’. This is called age-related macular degeneration (AMD).

The size of the fovea is about 1° to 1.5°

Question 6

Why is there the need for a individual callibration before eye-tracking?

Answer 6

As the pupillary axis is not identical with the line of sight a calibration to the individual is necessary (for best accuracy).

Question 7

The retina has two types of photosensors, which ones?

Answer 7

rods and cones.

The rods only detect brightness (night vision).
The cones exist in three different types for three different color ranges (red, green, blue).

Question 8

Briefly define “Eye Tracking”, “Eye Detection” and “Facial Expression Recognition”

Answer 8

Eye Tracking is the estimation of the gaze direction

Tracking the position of the eyes is called eye detection

Tracking the state of the eye (closed, open, angry looking) is part of facial expression recognition

Question 9

List three types of eye-tracking

Answer 9

Electro-occulography
Measurement of the eye’s electrical field
Advantage: Works with closed eyes
Disadvantage: obtrusive

Scleral coils
Coil integrated into contact lens; measurement of orientation of coil in magnetic field
Advantage: High space and time resolution
Disadvantage: extremely obtrusive as the coil needs wires

Video-based
Image processing of video stream
Advantage: unobtrusive
Disadvantage: depends on light conditions

Question 10

Describe how Video-based Eye Tracking work

Answer 10

Many eye trackers use an infrared LED to create a corneal reflection.

There is a white and a black pupil method. The white pupil method requires an infrared light source in the camera’s optical axis (red eyes on flash picture effect).

Question 11

What is a fast methods to calibrate eye trackers?

Answer 11

smooth pursuits

Question 12

What is a typical calibration procedure for eye-trackers?

Answer 12

Typical calibration procedures present a number of calibration points, for example 9, and ask the user to fixate them.

The eye tracker estimates the vector from the corneal reflection to center of the pupil.

Question 13

What means “accuracy” in terms of eye-tracking?

Answer 13

The average difference between the target center and the center of the gaze points

Question 14

What means “precision” in terms of eye-tracking?

Answer 14

There are different definitions:
* Root means square of the distances of successive gaze points
* Standard deviation of gaze points
* Average distance of gaze points to center of gaze points

Question 15

What are Remote Eye Trackers?
What is a disadvantage of built-in webcams?

Answer 15

Remote eye trackers are fixed at the display. The calibration is done with calibration points on the screen. The eye tracker reports screen coordinates.

The disadvantage of built-in webcams is a low accuracy.

Question 16

List some Quality Criterions for Eye Trackers.

Answer 16

• Accuracy and precision (typically 0.5°)
• Sample rate (typically 60 Hz)
• Latency
• Stability and robustness (e.g. under different light conditions)
• Comfort

Question 17

When can we say it is a Fixation, Saccade, or Smooth Pursuit?

Answer 17

Saccades:
faster than 30 °/s
involuntary and voluntary

Fixations:
periods of relative non-movement between saccades

Smooth pursuits:
slower than 30 °/s
requires a moving object that is tracked by the eye – always voluntary

Question 18

What do smooth pursuit detection require?

Answer 18

The movement of the targets. Typical detection calculates the correlation between eye and target movements.

Question 19

What is a essential part of eye-movement data analysis?

Answer 19

Separating and labeling fixations and saccades in eye-tracking protocols.

Question 20

What is a method to visualization Eye Movements?

Answer 20

Scan Paths or Heatmaps

Question 21

What can be the meaning, if we detect longer reading times or long regressions through eye-tracking?

Answer 21

Longer reading times may indicate confusion, or difficulty to comprehend.

Long regressions may indicate difficulty in comprehending or incorporating a unit into the previous context

Question 22

Name a reading pattern

Answer 22

F-Pattern:

• Users won’t read your text thoroughly
• The first two paragraphs must state the most
  important information
• Start subheads, paragraphs, and bullet points
  with information-carrying words