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Eye fixation or visual fixation is the maintaining of the visual gaze on a single location. Humans (and other animals with a fovea) typically alternate saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appear to have developed for hunting prey. There are three categories of fixational eye movements: microsaccades, ocular drifts, and ocular microtremor. Fixational eye movement has been found in a number of species, including humans, other primates, cats, rabbits, turtles, salamanders, owls. Although their existence has been known since the 1950s, the role and importance of fixational eye movement is still debated.
Reading involves fixating on a successive locations across the page or screen. Visual fixation is never perfectly steady: fixational eye movement occurs involuntarily. The term "fixation" can also be used to refer to the point in time and space of focus rather than to the act of fixating; a fixation in this sense is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs (e.g., Martin 1974).
In the current consensus, fixational eye movement contributes to maintaining visibility, by continuously stimulating neurons in the early visual areas of the brain, which mostly respond to transient stimuli. In the absence of retinal jitter (a laboratory condition called retinal stabilization), stabilized images as a visual percept rapidly fade out and completely disappear (provided the stabilization is good enough) (Pritchard, Heron & Hebb, 1960; Coppola & Purves, 1996).
Fixation is also used in experiments in vision science or neuroscience. Human subjects are often told to fixate on an object on a monitor before any experiment takes place. This serves to direct the person's attention to the point where visual information will be presented. Experiments in neurophysiology from different laboratories have shown that fixational eye movement, particularly microsaccades, strongly modulate the activity of neurons in several visual areas of the macaque brain. This topic is currently under active investigation.
Fixational eye movement might also participate to the neural code in the early visual system, although this hypothesis is still a very recent line of research.
In process tracing
- Main article: Process tracing
As eye fixations are “the” way that humans gather information and every decision requires the acquisition of information it becomes obvious that eye tracking is a way of investigating decision processes with a lot of potential. Yet the method is still underutilized (Schulte-Mecklenbeck, Kühberger, & Ranyard, 2011). Advantages of eye tracking are that it is possible to trace a lot of information used during a decision task, it can sometimes be applied as a substitute for working memory and because eye movement are not so well controllable and difficult to censor, they can be recorded nonreactively. However, there are also problems one has to deal with when working with the eye tracking method. Most of times an eye tracker is not part of a standard lab and it is still expensive. Another, more functional problem is the interpretation of eye fixations: The data reveals where participants are looking, but not what they actually are thinking. Yet, this is exactly what we want to find out more about. In cognitive psychology studies, the mostly concerned movements are saccades. During these movements no information can be acquired, because the vision is suppressed. This makes it very interesting especially for judgment and decision making tasks.
Objects of regard
Eye fixation data could be used to test predictions: For example that magic tricks work because of misdirecting the eye gaze and failing to actually see what happens even if it happens right in front of ones eyes. One can also compare eye fixations between two groups: between experts and novices or a healthy and clinical population. By looking at the distribution of fixations the investigation of display properties becomes possible. For example varying the size of products’ displays drives fixations’ amount, which is predictive for sales. This design does not enable causal hypothesis testing, however with the suitable design, the distribution of fixations could answer causal questions as well. Studies show that fixation frequencies are an indicator of relative importance: the more fixations on an alternative the greater the importance of this alternative. It has been shown that the frequency of fixations on an attribute can also be transformed into a rank order of the attributes The fixation order tells us something about the evaluation of objects. Particularly the first and last fixations can point out important features of the decision making process. Russo (1994) defined sequences of fixations as different stages. The fixations before any refixations are defined as an initial phase of orientation or screening. The last fixations should reflect the elimination of alternatives as a last checking phase prior to the announcement of ones choice.
The total time fixation as well as the total time of a sequence of fixations can reveal important aspects of the decision process. The mean fixation duration is an indicator of the processing depth or effort, and thus, reveal an important aspect of the decision process too. There is evidence ( Pieters and Warlop, 1999) that fixation duration on a chosen brand is longer than on a nonchosen alternative. There are mostly pair comparison and fixation duration is indicating that there are different checking processes (Russo & Leclerec, 1994). Open question: Duration of saccade may improve the accuracy of the total duration. Sequences of fixation There are within-alternative and between attributes transitions. The definition of by-alternative processing is a fixation transition within an attribute between alternatives. A single transition within an attribute but between alternatives is defined as by-attribute processing. There is evidence that an increase in involvement (heightened by giving participants whichever brand they chose) not only led to longer fixation durations but also to more by-brand and fewer by-attribute transitions. ( Van Raaij, 1977) There are research that focus on how to distinguish between these two processes, such as Russo & Dosher (1983). They defined by-alternative processing as sequence that all three attributes had to be fixated without interruption. A by-attribute comparisons required contiguous by-attribute evaluations of all three attribute. There is evidence that by-attribute processing is the preferred process for decision making (Arieli et al., 2009), even if its not possible to decide accurately e.g. between two gambles without by-alternative processing. Eye fixations as complementary data Different kinds of verbal protocol and eye fixation as a complementary data were compared (Gog et al., 2005). Such analysis reveals more information than only a single method. Fixations as a monitor of attention Some experiments investigate the peripheral process. In such studies, it is essential to ensure that an individual gaze at a fixation point and to identify if the eye has moved. With the gaze-contingent stimulus alteration (McConkie & Rayner, 1975) it can be imposed whererver an individual is currently looking. With this method the impact of emotional words, when presented parafoveally (Calvo & Castillo, 2009) and the left hemisphere advantage for peripherally presented words (Jordan et al., 2009). Critics: - Eye fixation show where people are looking, but not what they are thinking. A clear interpretation is hardly doable as a fixation e.g. can occur to an elimination or to another consideration of other alternatives, or it can be learning or eliminating. - There are only a few JDM theories that specify a decision process at the level of fixations on individual alternatives or even smaller units such as individual attributes. (e.g., a theory that propose a more or less continuously developing decision process or another theory that addresses a model of the differentation of value over time).
History: Eye tracking for information of cognitive activity of decisions. [History of Eye tracking is already described in Wikipedia] Russo investigated 1978 in different methods of process tracing, such as eye tracking, verbal protocol or information display boards. They assume that different methods have different advantages and disadvantages. Further, they propose that a combined usage of different methods may reveal more information of process tracing background than a single method. They define eye fixation as a method with high quality but mention the difficulty of a valid interpretation and the difficulty to isolate relevant cognitive parts of behaviour. A big step forward in process tracing with the method eye tracking is provided from Russo & Leclerc (1994).
Why use eye tracking to measure decision processes? One potential of the eye tracking method is that new theories, predicting behavior at the level of eye fixation might elicit, thus entail the possibility to find out more about decision processes. At the same time it brings up the question how process based theories should look like in the future
In the eye of eye tracking data, how should process based theories look like? Why move from simple theories to the complex detail of process-based theories? – depends on paradigm What kind of jdm processes might eye fixation data illuminate? Example: Identifying heuristics (1. Approximate one attribute to zero, either because its difference was small or its importance was negligible. 2. Count majority of confirming attributes) Data might prompt the devising of experimental designs and visual displays that could provide a new class of tests Investigation of already existing heuristics. Preference drives perception: People look more at what they prefer. – Will inducing people to look at a stimulus longer or more often increases their preference for it? Will attention drive preference?
Eye fixations are the primary way that humans acquire information. To be able to use this immense amount of data, JDM theories need to be developed that require such detailed information acquisition data.
Neurobiology of eye fixation
Developmental aspects of eye fixation
- Eye contact
- Eye movements
- Eye tracking
- Fixational eye movement
- Fixation reflex
- Preferential looking
- Saccadic masking
- Troxler's fading
- Visual attention
- Visual field