Eye tracking

Introduction

Seeing is the most fundamental sense we have. In process of evolution, it has very important place, because this very sense was necessary to survive among other animals. The evolution of human seeing leads to nowadays, when we have one of the most advanced organs for seeing, our eyes. Of course, there are animals with "better eyes" which can see also polarized, infrared or ultraviolet light, but these abilities were improved in order to theirs needs in process of evolution.

There are a lot of processes hidden behind our ability to see which are topic of a lot of researches even now, when the medicine is very developed. The processes by which we can see the world and the connections, through the neural net, that we use to make sense of it. The visual system allows us to assimilate information from the environment to help guide our actions.

Eye tracking is the process of measuring either the point of gaze ("where we are looking") or the motion of an eye relative to the head. There are a number of methods for measuring eye movements. An instrument that does eye tracking is called an eye tracker.

How we can see

The eye is often described as like a camera, but it is the quite uncamera-like features of perception which are most interesting. How is information from the eyes coded into neural terms, into the language of the brain, and reconstituted into experience of surrounding objects? The task of eye and brain is quite different from either a photographic or a television camera converting objects merely into images. There is a temptation, which must be avoided, to say that the eyes produce pictures in the brain. A picture in the brain suggests the need of some kind of internal eye to see it - but this would need a further eye to see its picture... and so on in an endless regress of eyes and pictures. This is absurd. What the eyes do is to feed the brain with information coded into neural activity - chains of electrical impulses - which by their code and the patterns of brain activity, represent objects. We may take an analogy from written language: the letters and words on this page have certain meanings, to those who know the language. They affect the reader's brain appropriately, but they are not pictures. When we look at something, the pattern of neural activity represents the object and to the brain is the object. No internal picture is involved.

Gestalt writers did tend to say that there are pictures inside the brain. They thought of perception in terms of modifications of electrical fields of the brain, these fields copying the form of perceived objects. This doctrine, known as isomorphism, has had unfortunate effects on thinking about perception. Ever since, there has been a tendency to postulate properties to these hypothetical brain fields such that visual distortions, and other phenomena, are 'explained'. But it is all too easy to postulate things having just the right properties. There is no independent evidence for such brain fields, and no independent way of discovering their properties. If there is no evidence for them, and no way of discovering their properties, then they are highly suspect. Useful explanations relate observables.

The Gestalt psychologists did however point to several important phenomena. They also saw very clearly that there is a problem in how the mosaic of retinal stimulation gives rise to perception of objects. They particularly stressed the tendency for the perceptual system to group things into simple units. This is seen in an array of dots.

Reading

The process of reading is a process of retrieving and comprehending a form of stored information or ideas. By analogy, in computer science, reading is acquiring of data from some sort of computer storage. By knowing how people reads a text, we could change the format of text, font, size to make it more convenient to read and understand.

The eyes reading a text doesn't move smoothly over the text, but they make short-timed "stops" to focus eye lenses. This also the way we look at everything around us. As is explained above, eyes aren't like a camera, there is also process of coding, processing and decoding the image and that takes some time. And also, our width of view is wide enough to read more letters or even words at the same time. It will be wasting of our brain if it will have to handle so many images.

Advice for the appropriate choice of reading rate includes reading flexibly, slowing down when the concepts are closer together or when the material is unfamiliar, and speeding up when the material is familiar and the material is not concept rich. Speed reading courses and books often encourage the reader to continually speed up; comprehension tests lead the reader to believe their comprehension is constantly improving. However, competence in reading involves the understanding that skimming is dangerous as a default habit.

Speed reading is characterized by an analysis of trade-offs between measures of speed and comprehension, recognizing that different types of reading call for different speed and comprehension rates, and that those rates may be improved with practice. On the picture is an example of fixations and saccades over text. This is the typical pattern of eye movements during reading. The eyes never move smoothly over still text.

Technologies and techniques

The most widely used current designs are video-based eye trackers. A camera focuses on one or both eyes and records their movement as the viewer looks at some kind of stimulus. Most modern eye-trackers use contrast to locate the center of the pupil and use infrared and near-infrared non-collumnated light to create a corneal reflection (CR). The vector between these two features can be used to compute gaze intersection with a surface after a simple calibration for an individual.

Two general types of eye tracking techniques are used: Bright Pupil and Dark Pupil. Their difference is based on the location of the illumination source with respect to the optics. If the illumination is coaxial with the optical path, then the eye acts as a retroreflector as the light reflects off the retina creating a bright pupil effect similar to red eye. If the illumination source is offset from the optical path, then the pupil appears dark.

Bright Pupil tracking creates greater iris/pupil contrast allowing for more robust eye tracking with all iris pigmentation and greatly reduces interference caused by eyelashes and other obscuring features. It also allows for tracking in lighting conditions ranging from total darkness to very bright. But bright pupil techniques are not effective for tracking outdoors as extraneous IR sources interfere with monitoring.

Eye tracking setups vary greatly; some are head-mounted, some require the head to be stable (for example, with a chin rest), and some function remotely and automatically track the head during motion. Most use a sampling rate of at least 30 Hz. Although 50/60 Hz is most common, today many video-based eye trackers run at 240, 350 or even 1000/1250 Hz, which is needed in order to capture the detail of the very rapid eye movements during reading, or during studies of neurology.

Eye movements are typically divided into fixations and saccades, when the eye gaze pauses in a certain position, and when it moves to another position, respectively. The resulting series of fixations and saccades is called a scanpath. Most information from the eye is made available during a fixation, but not during a saccade. The central one or two degrees of the visual angle (the fovea) provide the bulk of visual information; the input from larger eccentricities (the periphery) is less informative. Hence, the locations of fixations along a scanpath show what information loci on the stimulus were processed during an eye tracking session. On average, fixations last for around 200 ms during the reading of linguistic text, and 350 ms during the viewing of a scene. Preparing a saccade towards a new goal takes around 200 milliseconds.

Scanpaths are useful for analyzing cognitive intent, interest, and salience. Other biological factors (some as simple as gender) may affect the scanpath as well. Eye tracking in HCI typically investigates the scanpath for usability purposes, or as a method of input in gaze-contingent displays, also known as gaze-based interfaces.

Applications

• Cognitive Studies
• Medical Research
• Human Factors
• Computer Usability
• Translation Process Research
• Vehicle Simulators
• In-vehicle Research
• Training Simulators
• Virtual Reality
• Adult Research
• Infant Research
• Adolescent Research
• Geriatric Research
• Primate Research
• Sports Training
• fMRI / MEG / EEG
• Commercial eye tracking (web usability, advertising, marketing, automotive, etc)
• Finding good clues

Resources

• http://en.wikipedia.org/wiki/Eye_tracking
• http://en.wikipedia.org/wiki/Visual_perception
• http://en.wikipedia.org/wiki/Reading_(activity)
• http://en.wikipedia.org/wiki/Speed_reading
• http://en.wikipedia.org/wiki/Eye
• http://www.grand-illusions.com