3Physiology of EyeCornea is a transparent structure that covers the iris and pupil; a part of the focusing system of an eye.Pupil is the adjustable opening at the center of the iris that allows varying amounts of light to enter the eye.Lens helps to focus light on the retina.Retina includes rods (94%), which are sensitive to light and cones (6%) that capture colors. Cones are concentrated in the centre of the retina - the fovea
4Eye Movements Eyes move all the time (even during sleep) Several different movement types, such asPursuitTremorRotationDriftBut the most interesting types areFixationSaccade
5FixationEye is a (relatively) still and “fixated” to the certain point. E.g. reading a single word.All the information from the scene is (mainly) acquired during fixation.Duration varies from ms, typically ms.Typical fixation frequency is < 3 HzInterspersed with saccades...
6Saccade “Jumps” which connect fixations Very rapid -- duration is typically only msVery fast (up to 600 o/s) and therefore the vision system is suppressed during the movementBallistic; the end point of saccade cannot be changed during the movementSaccades are used to move the fixation pointIf larger than 30 degree movement is required, head moves along with eyes
7Vision1 Vision field is divided in to three regions Fovea provides the sharpest visionParafovea previews foveal informationPeripheral vision reacts to flashing objects and sudden movementsPeripheral vision has 15-50% of the acuity of the fovea and it is also less colour-sensitive
8Vision2 0 1 5 Foveal Parafoveal Peripheral In real life the regions are asymmetricE.g., in reading so-called Perceptual span (size of the effective vision), is 3-4 letter spaces to the left of fixation and letter spaces to the right1O of visual angle is roughly equivalent to 3-4 letter spacesParafovealPeripheralFoveal[deg]
9Visual PerceptionThe perception of a complex scene involves a complicated pattern of fixations, where the eye is held (fairly) still, and saccades, where the eye moves to foveate a new part of the sceneThe main issue is “Where to look next?” Answer may be that perception is influenced withCognition, e.g. “meaningful” and parts previewed with parafoveal visionVisual cues; highly visually attractive areas attracted by peripheral vision
11Electronic methodsThe most used method is to place skin electrodes around the eyes and measure the potential differences in eyeWide range -- poor accuracyBetter for relative than absolute eye movementsMainly used in neurological diagnosis
12Mechanical methods Based on contact lenses with Very accurate mirror planes + reflecting IR-lightcoil + magnetic fieldVery accurateVery uncomfortable for users who are not used to wear lensesUsable only for lab studies
13Single point video-based methods Tracking one visible feature of the eyeball, e.g.:limbus (bondary of sclera and iris)pupilA video camera observes one of the user's eyesImage processing software analyzes the video image and traces the tracked featureBased on calibration, the system determines where the user is currently lookingHead movements not allowedBite bar or head rest is needed
14Two point video-based method1 The same idea as in the single point method except now two features of eye are tracked – typicallycorneal reflectionpupilUses IR light (invisible to human eye) toproduce corneal reflectioncause bright or dark pupil, which helps the system to recognize pupil from video imageBright pupilCorneal reflection
15Two point video-based methods2 The optics of the system can be mounted onheadfloorIf optics are floor mounted, the system is not in contact with the userGenerally head movements are not restricted and they can be separated from eye movements, but...With floor mounted optics the system has to track the user’s head in order to keep the eye in the field of view of camera, which limits the head movements. The performance can be improved withservo controlled tracking mirrors, ora camera taking a wide-angled view of the user’s head and by using artificial neural network, the system searches the eye from the image.
16Some terms Accuracy Spatial Resolution The expected difference in degrees of visual angle between true eye position and mean computed eye position during a fixation.Because of the vision system and physiology of eye the accuracy is usually 0.5-1O.Spatial ResolutionThe smallest change in eye position that can be measured.Temporal Resolution (sampling rate)Number of recorded eye positions per second.
17ASL 4000-series (1996) Main components Floor mounted opticsControl unit2 computers (control & subject)Head movements (partially) compensated with tracking mirrors and extended head movement optionsTemporal resolution 50 HzSpatial resolution 0.5OTracks only one eyePoor analysing software and no API
18SMI EyeLink (1999) Contains Temporal resolution 250 Hz Head mounted optics2 computers (control & subject)Temporal resolution 250 HzSpatial resolution <0.01OTracks both eyesReasonable analysis softwareWIN API’s for Microsoft Visual C++