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ye-movement measures represent a unique way to study how people process visual information in real time as they are engaged in a specific task. Eye tracking has been used successfully in a rich collection of studies across numerous disciplines to address visual behaviors in a vast array of cognitive tasks. Examples are reading, piloting of airplanes, chess playing, viewing of art, car driving, and during newspaper layout, Weh page design, several aspects of problem-solving, and use of devices such as copy and fax machines. Patterns of eye movements may help us answer questions about visual processes involved in attention, the time course of processiTig visual information, and how the brain controls the eyes to seleci. extract, and use visual information in communication (for a review, see Rayner, 1998). Wlien applied ttt research with individuals with disabilities, eye-movement indices have particular appeal in that they offer an alternative response mode and may reduce reliance on nieniory and comprehension abilities. Additionally,
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individuals with neurological disorders, such as those with stioke and brain injury, including motor problem.s in the head, neck, trunk, and limbs, often retain eye-movement control.
Various Eye Movements There are several types of eye movement. The eye movements to which we refer here are saccadic eye movements (or saccades). These are quick rotations of the eye that allow us to fixate images with tlie fovea, which is the area of the eye with the greatest visual acuity. Given that our intere.st in most eyemovement research related to cognition and language acttially involves study of where the eyes fixate between movements, the term "eye fixation" is often used interrhangeahly with "eye movement," although the two terms are not synonymous. Here we discuss the use of eye movements to study information processing, not their u.se for augmentative commtmication and computer and environ met! tal control, although the latter has tremendous benefits for many persons with speech motor control difficulties.
The Eye-Mind Assumption Much of the research using eye tracking to study cognition and language is ha.sed on the important assumption that there is a relationship between where we fix our gaze and what we are thinking about. Still, we know that it is possible to look at one thing while thinking abotit something else, and that we are able to process information about things we .see through peripheral vision. For this reason, it is important that researchers take great care in the design of their studies to ensure the validity of the eye-mind a.ssumption. The means of doing this depend on the specific tasks to be administered.
Applications of Eye Movements Although numerous studies have addressed basic aspects of language processing using eye tracking {e.g., psycholingiiistic studies of lexical ambiguities and varying levels of syntactic complexity), few authors to date (excluding those addressing reading and reading cli.sorders) have used eye tracking to study clinical issues in the field of
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EYE M O V E M E N T S
communication sciences and disorders. This is likelv to change in the near future as the cost of eye-iracking instrumentation continues to decrease, the quality and precision of instrunieiitation continue to improve, and research entailing eye-tracking measures is increasingly visible in our research join nals and professional conferences. The present authors ha\e been involved in eye-movement research for many years. In tlie Neurolinguistics Laboratory at Ohio L'liiversity. directed by Brooke Hallowcll, ongoing work is dedicated to developing test materials and protocols geared toward assessing language comprehension in individuals with aphasia and related neurogenic language disorders. In one scries of sttidies, individuals view controlled image displays as ihev read text or listen to auditory \erbai messages, eye-inovemeni indices are used lo capture comprehension responses, which are \ali(lairt;uil lo consider tlie requireinents ol tiie system in relation to the needs of participants. For exainpk'. some systems retiuire a fixed head position to separate eye movements from head movements for high s|)atial ac< tiiary. Such sy.stems would l)e appropriale foi young, healthy adtflts, who are highly cooperative aiitl would tolerate restraints to Lestrict Iiead and chin nuneiiient and tise a hite-bar to help fix the head. These systems. howt'\ei. may not he toleiated h} atUilts with physitai or K>giiiti\e impairments, sonic oldei' adults, or \ery ac tive \()uug t liiUlicii for whom lemote evetrackiiig systems ma)' he more appiopriate.
EYE MOVEMENTS
T OSTUDY
COGNITION
A N D COMMUNICATION
interfere with eye monitoring. Responses tiiat require limited head and b(jdy movenient, sucli as buttoti presses or eye movemetits toward a display region to select an object, letter, or icon, coutd be tisecl with any system. For research that requires sinuiltaneotis collectioti of other physiologic data, attention to
I equired to speak or .sign then tlie system must allow for some limited movement but be capable of tracking head pt>sition. If a keyboarded t esponse or other activity, such as WTiting, for which the participant must look away from the display, is reqtiired, then the camera set-up must be considered to eiisttre that it does not obstruct the visual field or
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• Response mode It is also inipoi taut to coiisidei" the type of response that will be reqtiired of participants for data collet tiou. If participant.s are
requirements of external hardware must be considered. For example, head-worn systems that minimize the amotirit of metal are recommended for tise in functional magnetic resonance imaging (fMRI) protocols, and remote .systems are recommended for use in Evoked Response Potentials (FRP) protocols in which surface electrodes are attached to head-worn helmets.
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TRACKING
E Y EMOVEMENTS
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Useful links for Information on Eye-Monitoring Technology, Research, Manufacturers, and Applications www.eyetracking.net—a comprehensive collection of Internet resources to link to people and labs, manufacturers, archives of discussion issues, papers, conferences, and events, designed and maintained by Dr. Michael Liu http://ibs.derby.ac.uk/enied—a database of eye-tracking equipment developed by the University of Derby, UK, Applied Vision Research Unit http://listserv.spc.eclu/archives/eyemov-l.html—archives of posts and replies to issues related to eye-tracking research hosted by Saint Peter's College, Jersey City, NJ
• Freedom of movement It is important to consider the degree of movement that is required and typical for the experimental task. If operations involve manipulation of objects, walking, talking, or signing in virtual-reality environments, or daily movements in realworld interactions, then systems that allow for full movement are necessary; still, some tracking of head movement must be considered to
retain accuracy. Systems that are unobtrusive, such as remote systems, may be preferable in some natural settings, but with less physical control, the experimenter sacrifices spatial measurement accuracy. If the experimental task can be executed with little or no movement and participants arc alert and cooperative, then it may be preferable to explore systems used with chin rests or other restraints to limit head movement.
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• Comfort/set-up time/portability
• Cost
Systems differ in the amount of time required to position and adjust the hardware. If the system requires the use of a bite bar this will add time to the set-up. Some researchers select chairs that can he adjusted to chaTige the height of the display relative to the participant or that use head rests to achieve comfortable positions. If portability is required it is a good idea to consider a system that could be installed on a cart that may be moved to differeni lab areas. Some systems operate best under special
The cost of the system is not a good indicator of whether it will be appropriate to a researcher's needs. For example, the nuxst expensive systems may offer very high temporal and spatial accuracy, hut require a fixed head and use of a bite bar, which may not be desirable for a specific application. Systems range in cost from approximately $10,000 to $100,000. Customized systems for special environments or dedicated software will add to the cost.
Systems that are unobtrusive, such as remote systems, may be preferable in some natural settings, but with less physical control, the experimenter sacrifices spatial measurement accuracy. lighting conditions and the luminance levels must be considered. Typically, incandescent light (generated hy standard light bulbs) contains some infrared components and may degrade performance accuracy.
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• Software It is highly unlikely that a system purchased from an eye-tracking manufacturer will include software needed to conduct the type of study the buyer may have in mind. Be
TRACKING
skeptical of advertisements for "turn-key" eye-tracking systems. Consider the level of su[)pori from the manufacturer, as well as the resources available to you for prograniiiiing. There are many different algorithms for determining eye position and movement, and for takulating (le[K'ndent measures. Some systems supply these and allow modifications to a standard software package. Some tnannfacturers or iiidepfntlfnt snppoi t groups provide extensive software libraries specific to a system. Some systems are (oinpatible wilh otlicr data acquisition and aiialysis programs. OlluTS are specific to particular plaliornis. If Other data streams.
If exploited through careful experimental design, patterns of eye movements may provide valuahle information ahout visual attention and the time course of visual processes as they correspond to language and cognition.
such as speech acoustics, button presses, or clcctrophysiological measuies, arc to be analyzed in relation to the eye-iiHKiitoring data then they must be synchronized and require citstomized programming and hardwaic. adding to the complexity of the system. Monitoring of eye movements provides an online lecord of visual bcliaviors in (ogiiitivf tasks, siirb as listening to and understanding spct'th, writing, sign, or nonverbal coininunitation. If exploited tliioiigli tarelul experimental design, eye movement patteiiis may piovidf \alu;ibl(' intorination al)(>ut visual attention aiul the tinu' couise of \isual priKt'sses as tliey corres])
