The gaze-contingency paradigm is a general term for techniques allowing a computer screen display to change in function depending on where the viewer is looking. Gaze-contingent techniques are part of the eye movement field of study in psychology.
From a system analysis point of view, eye-tracking applications should be distinguished from diagnostic or interactive system. In diagnostic mode, the eye-tracker provides data about the observer’s visual search and attention processes. In interactive mode, the eye-tracker is used as an input device. From a general point of view, an interactive system responds to the observer’s actions and interacts with him. Gaze-contingency paradigm can be classified an interactive eye-tracking applications (Duchowski 2007).
Over the past century, the way the eyes move in human activities as diverse as playing sport, viewing works of art, piloting aircraft, exploring visual scenes, recognizing face or facial expressions (Shimojo & Simion 2003), and reading language and music, has revealed some of the ocular and psychological mechanisms involved in the visual system. The gaze-contingent techniques aim to overcome limitations inherent to simple eye-movement recording. Indeed, due to an imperfect coupling between overt and covert attentions (Posner, 1980), it is not possible to exactly know which visual information the viewer is processing based on the fixation locations. By controlling precisely the information projected in different parts of the visual field, the gaze-contingent techniques permit to disentangle what is fixated and what is processed.
The technical principle of the paradigm involves a computer interfaced with both an eye-movement tracking system (Eye-tracker) and a display of the visual stimulus. A fast computer, eye-tracker and display allow reliable results (Veneri 2010, Pomplun 2001). In gaze-contingent paradigms the stimulus display is continuously updated as a function of the observers' current gaze position; for instance, Shimojo & Simion 2003 applied a central hole to see the scene only through the fovea, giving to subjects the sensation of seeing through a telescope.
Therefore, the gaze-contingent technique is a powerful method to control for the visual information feeding the visual system and to isolate information use.
The gaze-contingent technique is the basis of various experimental paradigms, each of them allowing to investigate specific cognitive processes. In the moving window paradigm (e.g., Reder, 1973; McConkie & Rayner, 1975) only the part of the visual field around the gaze location (foveal information) is displayed normally, the surrounding part of the visual field (extrafoveal and peripheral information) being altered (removed for visual scenes or replaced by chains of X in reading). The moving mask paradigm (e.g. Rayner & Bertera, 1979) is a reverse technique in comparison with the moving window paradigm. It dynamically obscures central vision (or replaces letters with X in reading), permitting only extrafoveal information use. In the boundary paradigm (e.g., Rayner, 1975; Balota, Pollatsek & Rayner, 1985; Miellet & Sparrow, 2004), an extrafoveal prime (a homophone in reading for example) is replaced by the target stimulus when the eyes cross an invisible boundary around the target area. The parafoveal magnification paradigm (Miellet, O'Donnell & Sereno, 2009) compensates for how visual acuity drops off as a function of retinal eccentricity. On each fixation and in real time, parafoveal text is magnified to equalize its perceptual impact with that of concurrent foveal text.
In the language domain this method has been successfully used in natural reading. The study of eye movements in reading allowed researchers to map out the perceptual span (moving window paradigm: e.g., Reder, 1973; McConkie & Rayner, 1975), the nature of the extrafoveal information extracted during a fixation, for instance orthographic and phonological information (boundary paradigm: e.g., Rayner, 1975; Balota, Pollatsek & Rayner, 1985; Miellet & Sparrow, 2004) or the relative influence of attention versus visual acuity drop-off in the perceptual span (parafoveal magnification paradigm: Miellet, O'Donnell & Sereno, 2009).
The gaze-contingent technique has been adapted in other tasks than reading. The moving window paradigm has been used to study the effect of culture in face recognition for example (Caldara, Zhou & Miellet, 2010). The moving mask paradigm has been used in visual learning (Castelhano & Henderson, 2008) or visual search of animals in natural visual scenes (Miellet, Zhou, He, Rodger & Caldara, 2010).
The various gaze-contingent techniques has given eye-movement researchers the ability to observe the processing of visual input in much greater detail (particularly its temporal characteristics), the perceptual span, and the nature of central versus peripheral processing in reading.
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