On one hand, if the perceptual span includes all or many of the words on a line of text, then eye movement measures wouldn’t likely reveal much information about cognitive processing; however, if the reader gains only useful information from the word directly focused on, then eye movement behavior could shed light on what role the eyes play in reading disorders such as dyslexia.
This picture shows the acuity of foveal vision in reading (during one eye stop). The lower line of text simulates the acuity of vision with the relative acuity percentages
To do a test close one eye, fixate the upper line at the fixation point and try to read the words to the right and left without moving your eyes. The result should be similar to the incrementally blurred lower line of text - except that you never have the impression of a blurred text. The reason: Your visual perception is already the result of a massive computational analysis made by your brain. Your system "knows" that the upper line is not blurred, so you don't see it as blurred. But the difficulty of recognition increases with the distance from the fixation point.
Until the second half of the 19th century, researchers had at their disposal three methods of investigating eye movement. The first, unaided observation, yielded only small amounts of data that would be considered unreliable by today's scientific standards. This lack of reliability arises from the fact that eye movement occurs frequently, rapidly, and over small angles, to the extent that it is impossible for an experimenter to perceive and record the data fully and accurately without technological assistance. The other method was self-observation, now considered to be of doubtful status in a scientific context. Despite this, some knowledge appears to have been produced from introspection and naked-eye observation. For example, Ibn al Haytham, a medical man in 11th-century Egypt, is reported to have written of reading in terms of a series of quick movements and to have realised that readers use peripheral as well as central vision.
Leonardo DaVinci, (1452-1519) was the first in Europe to recognize the special optical qualities of the eye. He derived his insights partly through introspection but mainly through a process that could be described as optical modelling. Based on dissection of the human eye he made experiments with water-filled crystal balls. He wrote "The function of the human eye, ... was described by a large number of authors in a certain way. But I found it to be completely different.
His main experimental finding was that there is only a distinct and clear vision at the line of sight, the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the modern distinction between foveal vision (a more precise term for central vision) and peripheral vision.
There appear to be no records of eye movement research until the early 19th century. At first, the chief concern was to describe the eye as a physiological and mechanical moving object, the most serious attempt being Hermann von Helmholtz's major work Handbook of physiological optics (1866). The physiological approach was gradually superseded by interest in the psychological aspects of visual input, in eye movement as a functional component of visual tasks. As early as the 1840s, there was speculation on the relationship between central and peripheral vision.
The subsequent decades saw more elaborate attempts to interpret eye movement, including a claim that meaningful text requires fewer fixations to read than random strings of letters. In 1879, the French ophthalmologist Louis Émile Javal used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements for which he coined the term saccades. In 1898, Erdmann & Dodge used a hand-mirror to estimate average fixation duration and saccade length with surprising accuracy.
The first devices for tracking eye movement took two main forms: those that relied on a mechanical connection between participant and recording instrument, and those in which light or some other form of electromagnetic energy was directed at the participant's eyes and its reflection measured and recorded. In 1883, Lamare was the first to use a mechanical connection, by placing a blunt needle on the participant's upper eyelid. The needle picked up the sound produced by each saccade and transmitted it as a faint clicking to the experimenter's ear through an amplifying membrane and a rubber tube. The rationale behind this device was that saccades are easier to perceive and register aurally than visually. In 1889, Edmund B. Delabarre invented a system of recording eye movement directly onto a rotating drum by means of a stylus with a direct mechanical connection to the cornea. Other devices involving physical contact with the surface of the eyes were developed and used from the end of the 19th century until the late 1920s; these included such items as rubber balloons and eye caps.
Mechanical systems suffered three serious disadvantages: questionable accuracy due to slippage of the physical connection, the considerable discomfort caused to participants by the direct mechanical connection (and consequently great difficulty in persuading people to participate), and issues of ecological validity, since participants' experience of reading in trials was significantly different from the normal reading experience. Despite these drawbacks, mechanical devices were used in eye movement research well into the 20th century.
Attempts were soon made to overcome these problems. One solution was to use electromagnetic energy rather than a mechanical connection. In the "Dodge technique", a beam of light was directed at the cornea, focused by a system of lenses and then recorded on a moveable photographic plate. Erdmann & Dodge used this technique to claim that there is little or no perception during saccades, a finding that was later confirmed by Utall & Smith using more sophisticated equipment. The photographic plate in the Dodge technique was soon replaced with a film camera, but was still plagued by problems of accuracy, due to the difficulty of keeping all parts of the equipment perfectly aligned throughout a trial and accurately compensating for the distortion caused by the diffractive qualities of photographic lenses. In addition, it was usually necessary to restrain a participant's head by using an uncomfortable bite-bar or head-clamp.
In 1922, Schott pioneered a further advance called electro-oculography (EOG), a method of recording the electrical potential between the cornea and the retina. Electrodes may be covered with special contact paste before been placed to the skin. So, it is unnecessary now to make an incisions in patient's skin. Common misconception about EOG is that measured potential is the electromyogram of extraocular muscles. In fact, it is only the projection of eye dipole to the skin, because higher frequencies, corresponding to EMG, are filtered out. EOG delivered considerable improvements in accuracy and reliability, which explain its continued use by experimentalists for many decades.
Eye trackers bounce near infra-red light off the interior of the eyeball, and monitor the reflection on the eye to determine gaze location.
With this technique, the exact position of fixation on screen is determinable.