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Accommodation (eye focusing) means the eye's ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power. When this accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera. The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible.

Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic Lateral Sclerosis (ALS) is a chronic, progressive, degenerative disorder affecting the motor neuron cells and the motor tracts in the brain and spinal cord. Also called Lou Gehrig's Disease. The disorder causes muscle weakness and atrophy; symptoms commonly appear in middle to late adulthood, with death in two to five years. The cause is unknown, and there is no known cure. In the last phase, the person may only be able to control his or her eyes.

Assistive software

Software that enables a person with special needs to access the computer with more efficiency, effectiveness or satisfaction.

Blink rate

A blink is defined as a reflexive closing and opening of the eye. Blink rate represents the frequency of blinks recorded over a specific time. For EPOG, a blink is the loss of data between fields. Blink rate is used as an indicator of, for example, fatigue, mental workload, and exposure to adverse conditions (e.g. sun, wind, and airborne particles).


Calibration is defined as the method by which gaze tracking systems map eye (and sometimes head) positional data captured by the hardware of the system (often video images of the eye) to the actual gaze vector from eye to host personal computer of the end user. This calibration is then used to calculate and translate gaze vectors from the eye of the end user to cursor, pointer or gaze fixation positions on the host computer screen.


A camera that tracks the movement of the head or a feature on the face to control the computer, e.g. via an on-screen pointer.

Cerebral Palsy (CP)

Cerebral Palsy (CP) is the term used to describe a group of chronic conditions affecting body movement and muscle coordination. It is usually caused by brain damage in the developing fetus, during birth, or from illness just after birth. No two cases of CP are the same, and the term is used to describe a variety of conditions depending on which part of the brain was damaged. For example, some people with CP may have learning disabilities, speech problems, hearing impairment or epilepsy.

"Cerebral" refers to the brain and "palsy" to muscle weakness/poor control.


Cone is a receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for color vision.


Convergence means the ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.


Cornea is the transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye. Contact lenses are fitted over the cornea. The diameter of the cornea of an adult is typically 11-12 mm.

Dwell time

Gaze dwell time is the total duration that a fixation remains within a specific area. Gaze dwell time can be used as an indirect indicator of mental workload by noting its effect on a number of different observed variables.

Electro-Oculography (EOG)

Relies on measurement of skin’s potential differences, using electrodes placed around the eye. A widely used method in the 1960’s. Measures eye movements relative to head position. Not generally suitable for Point of Regard (POR) measurement (unless head is also tracked).

Eye tracking

Eye tracking means figuring out the movement of eye as a person is looking at something. Video-based eye tracking devices observe a person's pupil to determine the direction of their gaze.

Eye-tracking can be used for input, by directly controlling a pointer on the screen, or even for communicating the gaze direction to a co-worker looking at the same document (much like a telepointer). Eye-tracking can also be used to study how user interfaces are being used and may give insight into screen layout issues and how users are performing their tasks.

Note: Eye Tracking can also refer to the ability of the eyes to smoothly and effortlessly follow a moving target (in vision research)


Eye-writing (or eye-typing) means the process by which text is produced using eye control.


Fixation occurs when we look (focus) at things, the ability to direct and maintain steady visual attention on a target. The eyes remain relatively still during fixations. Fixations typically last from 200 to 600 ms, depending on the task. Fixations happen between saccades.

Fixational eye movements

Fixational eye movements fix the eye to keep the image of the target object close to the fovea, and prevent the image fading. The fovea is the part of the retina responsible for accurate image resolution. Fixations last for about 100 ms and are associated with slow and fast miniature eye movements.

Fixation frequency

Fixation frequency can be expressed as the dwell time of a fixation divided by the number of fixations. Normal eye movement during scanning is characterised by a series of stops and very rapid shifts between stopping points. A fixation is defined as a dwell of 150ms or more.

Number of fixations - This defines the number of fixations in a specific area and aims to determine the number of times the eye fixates in a specific area.

Percentage of fixations - This defines the number of fixations in a given area divided by the total number of fixations and aims to determine the percentage of time the eye fixates in specific areas.


Fovea refers to the center of the retina that can produce the sharpest eyesight and contains the most cones.

Gaze awareness

Gaze awareness is the ability to tell what someone is looking at by watching the direction of their eyes.

Note: Eye aware device recognizes that eyes are present; Gaze aware device may also detect the direction of gaze at some level.


An infra-red device that tracks head movement to control the computer, e.g. via an on-screen pointer.


Iris is the colored part of the eye located between the lens and cornea; it regulates the entrance of light.

Locked-in syndrome

Locked-in syndrome is condition in which a patient is aware and awake, but cannot move or communicate due to complete paralysis of the body. It is resulting from interruption of motor pathways in the ventral pons, usually by infarction. This disconnection of the motor cells in the lower brain stem and spinal cord from controlling signals issued by the brain leaves the patient completely paralyzed and mute, but able to receive and understand sensory stimuli; communication may be possible by code using blinking, or movements of the jaw or eyes, which can be spared.

Optokinetic eye movements

Optokinetic and vestibular eye movements are a mix of smooth image stabilising movements to either a continuously moving visual field or stimulation of the semicircular canals and rapid saccadic eye movements.

Point of Regard (POR)

The "point of regard" (Young & Sheena 1975) means the position of eye in space, the direction of gaze, where/what a person is looking at.

Pupil diameter

Pupil diameter is the measure through the centre of the adjustable opening in the iris of the eye, terminated at both ends by its circumference. It aims to gain an objective assessment of differences in mental workload. An average range of pupil diameters is 2-8mm (Boff and Lincoln 1988); differences in pupil diameter can be associated with differences in mental workload (Beatty 1982) and fatigue (e.g. Zwahlen et al. 1984).


Pursuit eye movements are slow, smooth, and conjugate movements used to stabilise the retinal image as a moving target is tracked. This is usually an involuntary action related to compensatory eye movement.


Saccades occur when we move our eyes between fixations. See Saccadic eye movements.

Saccadic eye movements

Saccadic eye movements are voluntary, conjugate movements resulting in changes of fixation observed during visual search. This brings the retinal image being viewed onto the fovea.


Usability in design is about producing products and systems that perform their function effectively, efficiently, safely and comfortably.

ISO 9241-11 Usability: The effectiveness, efficiency, and satisfaction with which specified users achieve specified goals in particular environments.

User-centred design

User-centred design takes, as its starting point, the needs of the user, rather than something that is "bolted on" later. The design is based on the needs and goals of users, with users involved in the design process, design with usability as a primary focus.

Vergence eye movements

Vergence eye movements are the slow movements of the two eyes in opposite directions to facilitate fusion of the retinal images of the two eyes viewed at different distances.

Vestibular eye movements

Optokinetic and vestibular eye movements are a mix of smooth image stabilising movements to either a continuously moving visual field or stimulation of the semicircular canals and rapid saccadic eye movements.


AAC - Augmentative and Alternative Communication

ALS - [[[#Amyotrophic Lateral Sclerosis|Amyotrophic Lateral Sclerosis]]

API - Application Program Interface

CP - Cerebral Palsy

EOG - Electro-Oculography

EPOG - Eye-Point-Of-Gaze

OKN - Optokinetic Nystagmus

POG - Photo-Oculography

POR - Point of Regard

VOG - Video-Oculography

VOR - Vestibulo-Ocular Reflex


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