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      Mapping and correcting the influence of gaze position on pupil size measurements

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      Behavior Research Methods

      Springer Nature

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          Abstract

          <p class="first" id="P1">Pupil size is correlated with a wide variety of important cognitive variables and is increasingly being used by cognitive scientists. Pupil data can be recorded inexpensively and non-invasively by many commonly used video-based eye-tracking cameras. Despite the relative ease of data collection and increasing prevalence of pupil data in the cognitive literature, researchers often underestimate the methodological challenges associated with controlling for confounds that can result in misinterpretation of their data. One serious confound that is often not properly controlled is <i>pupil foreshortening error (PFE)</i>—the foreshortening of the pupil image as the eye rotates away from the camera. Here we systematically map PFE using an artificial eye model and then apply a geometric model correction. Three artificial eyes with different fixed pupil sizes were used to systematically measure changes in pupil size as a function of gaze position with a desktop EyeLink 1000 tracker. A grid-based map of pupil measurements was recorded with each artificial eye across three experimental layouts of the eye-tracking camera and display. Large, systematic deviations in pupil size were observed across all nine maps. The measured PFE was corrected by a geometric model that expressed the foreshortening of the pupil area as a function of the cosine of the angle between the eye-to-camera axis and the eye-to-stimulus axis. The model reduced the root mean squared error of pupil measurements by 82.5 % when the model parameters were pre-set to the physical layout dimensions, and by 97.5 % when they were optimized to fit the empirical error surface. </p>

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          Most cited references 22

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          Task-evoked pupillary responses, processing load, and the structure of processing resources.

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            Pupil Size in Relation to Mental Activity during Simple Problem-Solving.

             J Polt,  Jon Hess (1964)
            Changes in pupil size during the solving of simple multiplication problems can be used as a direct measure of mental activity. The pupil response not only indicates mental activity in itself but shows that mental activity is closely correlated with problem difficulty, and that the size of the pupil increases with the difficulty of the problem. These findings relate to recent Russian research on the pupillary reflex in connection with orienting and brain stimulation.
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              Rational regulation of learning dynamics by pupil–linked arousal systems

              The ability to make inferences about the current state of a dynamic process requires ongoing assessments of the stability and reliability of data generated by that process. We found that these assessments, as defined by a normative model, were reflected in non–luminance–mediated changes in pupil diameter of human subjects performing a predictive–inference task. Brief changes in pupil diameter reflected assessed instabilities in a process that generated noisy data. Baseline pupil diameter reflected the reliability with which recent data indicated the current state of the data–generating process and individual differences in expectations about the rate of instabilities. Together these pupil metrics predicted the influence of new data on subsequent inferences. Moreover, a task– and luminance–independent manipulation of pupil diameter predictably altered the influence of new data. Thus, pupil–linked arousal systems can help regulate the influence of incoming data on existing beliefs in a dynamic environment.
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                Author and article information

                Journal
                Behavior Research Methods
                Behav Res
                Springer Nature
                1554-3528
                June 2016
                May 2015
                : 48
                : 2
                : 510-527
                Article
                10.3758/s13428-015-0588-x
                4637269
                25953668
                © 2016

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