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

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 pupil foreshortening error (PFE)—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.     

Eye tracking research and technology: Towards objective measurement of data quality

Two methods for objectively measuring eye tracking data quality are explored. The first method works by tricking the eye tracker to detect an abrupt change in the gaze position of an artificial eye that in actuality does not move. Such a device, referred to as an artificial saccade generator, is shown to be extremely useful for measuring the temporal accuracy and precision of eye tracking systems and for validating the latency to display change in gaze contingent display paradigms. The second method involves an artificial pupil that is mounted on a computer controlled moving platform. This device is designed to be able to provide the eye tracker with motion sequences that closely resemble biological eye movements. The main advantage of using artificial motion for testing eye tracking data quality is the fact that the spatiotemporal signal is fully specified in a manner independent of the eye tracker that is being evaluated and that nearly identical motion sequence can be reproduced multiple times with great precision. The results of the present study demonstrate that the equipment described has the potential to become an important tool in the comprehensive evaluation of data quality.     

A microcomputer-based software package for eye monitoring research

A software package is described that collects and reduces eye behavior (eye position and pupil size) data using an IBM-compatible personal computer. Written in the C language for speed and portability, the package includes several unique features: data can be collected simultaneously from other sources (e.g., EEG, EMG), logically defined events can be detected in real time on any data channel, and either of two types of data matrix can be produced. Data reduction algorithms and data structures are described.     

Image processing for improved eye-tracking accuracy

Video cameras provide a simple, noninvasive method for monitoring a subject’s eye movements. An important concept is that of the resolution of the system, which is the smallest eye movement that can be reliably detected. While hardware systems are available that estimate direction of gaze in real time from a video image of the pupil, such systems must limit image processing to attain real-time performance and are limited to a resolution of about 10 arc minutes. Two ways to improve resolution are discussed. The first is to improve the image processing algorithms that are used to derive an estimate. Offline analysis of the data can improve resolution by at least one order of magnitude for images of the pupil. A second avenue by which to improve resolution is to increase the optical gain of the imaging setup (i.e., the amount of image motion produced by a given eye rotation). Ophthalmoscopic imaging of retinal blood vessels provides increased optical gain and improved immunity to small head movements but requires a highly sensitive camera. The large number of images involved in a typical experiment imposes great demands on the storage, handling, and processing of data. A major bottleneck had been the real-time digitization and storage of large amounts of video imagery, but recent developments in video compression hardware have made this problem tractable at a reasonable cost. Images of both the retina and the pupil can be analyzed successfully using a basic toolbox of image-processing routines (filtering, correlation, thresholding, etc.), which are, for the most part, well suited to implementation on vectorizing supercomputers.     

A novel evaluation of two related and two independent algorithms for eye movement classification during reading

Nystr?m and Holmqvist have published a method for the classification of eye movements during reading (ONH) (Nyström & Holmqvist, 2010). When we applied this algorithm to our data, the results were not satisfactory, so we modified the algorithm (now the MNH) to better classify our data. The changes included: (1) reducing the amount of signal filtering, (2) excluding a new type of noise, (3) removing several adaptive thresholds and replacing them with fixed thresholds, (4) changing the way that the start and end of each saccade was determined, (5) employing a new algorithm for detecting PSOs, and (6) allowing a fixation period to either begin or end with noise. A new method for the evaluation of classification algorithms is presented. It was designed to provide comprehensive feedback to an algorithm developer, in a time-efficient manner, about the types and numbers of classification errors that an algorithm produces. This evaluation was conducted by three expert raters independently, across 20 randomly chosen recordings, each classified by both algorithms. The MNH made many fewer errors in determining when saccades start and end, and it also detected some fixations and saccades that the ONH did not. The MNH fails to detect very small saccades. We also evaluated two additional algorithms: the EyeLink Parser and a more current, machine-learning-based algorithm. The EyeLink Parser tended to find more saccades that ended too early than did the other methods, and we found numerous problems with the output of the machine-learning-based algorithm.     

A system for tracking gaze on handheld devices

Many of the current gaze-tracking systems require that a subject’s head be stabilized and that the interface be fixed to a table. This article describes a prototype system for tracking gaze on the screen of mobile, handheld devices. The proposed system frees the user and the interface from previous constraints, allowing natural freedom of movement within the operational envelope of the system. The method is software-based, and integrates a commercial eye-tracking device (EyeLink I) with a magnetic positional tracking device (Polhemus FASTRAK). The evaluation of the system shows that it is capable of producing valid data with adequate accuracy.     

The corneal reflection technique and the visual preference method: sources of error.

Corneal reflection techniques for eye fixation position measurement have been used in recent years to assess such variables of visual behavior as duration of looking and the area(s) of the stimulus fixated. These techniques are especially useful when measuring visual regard in infants and young children as head restraints are not required. In an earlier article the present authors demonstrated, empirically, that the common assumption that the center of the pupil represents the line of sight is untenable. The present article considers the causes of the errors present in these techniques: it can be shown that theoretical calculations of these sources of error, calculations based both on the anatomy of the eye, and on the optics involved, produce good agreement with their empirically derived magnitudes.     

眼动追踪技术与婴幼儿研究:程序、方法与数据分析

眼动技术在婴幼儿研究中成为一种流行的研究工具。如何合理地选择和使用眼动仪进行数据收集及分析,是婴幼儿眼动研究者需要考虑的重要问题。本文从眼动仪使用的流程出发,主要对婴幼儿眼动研究过程中所涉及的四个方面的问题进行了梳理和分析:(1)正确选择仪器;(2)合理校准;(3)提高数据质量;(4)有效分析和挖掘数据。同时,针对这些方面提出了相应的操作性建议。     

Performance of a computer system for recording eye fixations using limbus reflection

A computer system was developed to measure and record locus and duration of the eye axis of gaze using a limbus reflection technique. System hardware and operating procedures for on-line data collection and correction are described. Algorithms are presented that separate fixations from movements. Based on these algorithms, system performance was measured in terms of spatial accuracy of eye fixations and temporal accuracy of saccadic movements.     

Interaction between eye and body movements to perform visual tasks in upright stance

Synergistic interactions between visual and postural behaviors were observed in a previous study during a precise visual task (search for a specific target in a picture) performed upright as steady as possible. The goal of the present study was to confirm and extend these novel findings in a more ecological condition with no steadiness requirement. Twelve healthy young adults performed two visual tasks, i.e. a precise task and a control task (free-viewing). Center of pressure, lower back, neck, head and eye movements were recorded during each task. The subjective cognitive workload was assessed after each task (NASA-TLX questionnaire). Pearson correlations and cross-correlations between eyes (time-series, characteristics of fixation) and center of pressure/body movements were used to test the synergistic model. As expected, significant negative Pearson correlations between eye and head-neck movement variables were only observed in searching. They indicated that larger precise gaze shifts were correlated with lower head and neck movements. One cross-correlation coefficient (between COP on the AP axis and eyes in the up/down direction) was also significantly higher, i.e. stronger, in searching than in free-viewing. These synergistic interactions likely required greater cognitive demand as indicated by the greater NASA-TLX score in searching. Moreover, the previous Pearson correlations were no longer significant after controlling for the NASA-TLX global score (thanks to partial correlations). This study provides new evidence of the existence of a synergistic process between visual and postural behaviors during visual search tasks.     

Porterfield and Wells on the motions of our eyes

William Porterfield (ca 1969-1771) and William Charles Wells (1757-1817) conducted experimental investigations on eye movements related to accommodation, binocular vision, and vertigo. Porterfield gave a correct interpretation of Scheiner's experiment and invented an optometer to measure the near and far points of distinct vision. He also demonstrated the involvement of the crystalline lens in accommodation by examining vision in an aphakic person. Wells devised an alternative means of measuring the limits of vision and noted his own deterioration of sight with age; he studied the effects of belladonna on pupil size and accommodation. Their analyses of binocular visual direction contrasted Porterfield's view that perceived location was innately determined with Well's argument that visual direction was innate whereas visual distance was learned. Both Porterfield and Wells investigated the involvement of eye movements in binocular vision and in postrotary visual motion. Porterfield maintained that the eyes did not move following body rotation, whereas Wells, using an afterimage as stabilised retinal image, described the characteristics of postrotary nystagmus and their dependence on head orientation. Despite the neglect of Well's work, he should be considered as laying the foundations for the study of vestibular-visual interaction, even though the function of the vestibular system was not known at that time.     

Investigating eye movement acquisition and analysis technologies as a causal factor in differential prevalence of crossed and uncrossed fixation disparity during reading and dot scanning

Previous studies examining binocular coordination during reading have reported conflicting results in terms of the nature of disparity (e.g. Kliegl, Nuthmann, & Engbert (Journal of Experimental Psychology General 135:12-35, 2006); Liversedge, White, Findlay, & Rayner (Vision Research 46:2363-2374, 2006). One potential cause of this inconsistency is differences in acquisition devices and associated analysis technologies. We tested this by directly comparing binocular eye movement recordings made using SR Research EyeLink 1000 and the Fourward Technologies Inc. DPI binocular eye-tracking systems. Participants read sentences or scanned horizontal rows of dot strings; for each participant, half the data were recorded with the EyeLink, and the other half with the DPIs. The viewing conditions in both testing laboratories were set to be very similar. Monocular calibrations were used. The majority of fixations recorded using either system were aligned, although data from the EyeLink system showed greater disparity magnitudes. Critically, for unaligned fixations, the data from both systems showed a majority of uncrossed fixations. These results suggest that variability in previous reports of binocular fixation alignment is attributable to the specific viewing conditions associated with a particular experiment (variables such as luminance and viewing distance), rather than acquisition and analysis software and hardware.     

Attending to a misoriented word causes the eyeball to rotate in the head

Torsional eye movements are triggered by head tilt and a rotating visual field. We examined whether attention to a misoriented form could also induce torsion. Thirty-six observers viewed an adapting field containing a bright vertical Une, and then they viewed a display that was composed of two misoriented words (one rotated clockwise, the other counterclockwise, by 15°, 30°, or 45°). The subjects were instructed to attend to one of the words. The subjects’ adjustments of a reference line to match the tilt of the afterimage showed that attention to a misoriented word produces torsional eye movement (verified with direct measurements on 4 additional subjects). This eye movement reduces the retinal misorientation of the word by about 1°. The results of this study reinforce the linkage between selective attention and eye movements and may provide a useful tool for dissecting different forms of “mental rotation” and other adjustments in internal reference frames. Apparent-motion displays confirming that the eye rotated in the head may be downloaded from www.psychonomic.org/archive.     

面孔-人格知觉中眼部线索及其效应

人们常常会基于他人面孔特征对其人格做出相应推论。在影响面孔-人格知觉的五官特征中, 眼部特征最具复杂性。总体上, 眼部线索可分为可变和不可变两大类。可变线索包括注视方向、眼睑开放程度与眼部区域的表情等; 不可变线索包括眼睛大小、巩膜着色程度、虹膜颜色以及巩膜暴露指数等。从以上线索效应的潜在作用机制来看, 可以将眼部线索分为三大类, 即受病理因素影响的眼部线索、与特定群体相关的眼部线索和受主观意识或生理因素调控的眼部线索。此外, 也进一步指出了未来研究还需加以考虑的其他眼部特征效应, 以及待拓展深入的研究视角。     

老年人与青年人阅读空格文本的注视位置效应

采用EyeLink2000眼动仪,以14名老年人和14名青年人为被试,探讨他们阅读空格汉语文本的注视位置效应.阅读材料分为正常无空格文本和词间空格文本两种.结果发现:(1)在阅读词间空格文本和正常无空格文本时,老年人与青年人都表现出单次注视的首次注视是位于词的中心,多次注视的首次注视是位于词的开头;(2)老年人阅读正常无空格文本时对词尾的注视概率显著低于词间空格文本,而青年人则没有差异.结果表明老年人与青年人在阅读空格文本时存在注视位置效应.     

汉-俄双语者阅读俄语文本的空格效应研究

采用EyeLink Ⅱ眼动仪,以汉-俄双语者(15名)和母语为俄语者(15名)为被试,要求他们阅读有空格、删除空格、去空格隔词加粗三种类型词切分方式的俄语句子,探讨不同词切分方式对两组被试俄语文本阅读的影响。结果表明:和有空格条件相比,去空格隔词加粗条件下的平均注视时间和阅读时间更长,注视次数更多,阅读速度更慢;而删除空格条件下的阅读成绩最低。在各呈现条件下,母语为俄语者的成绩都显著好于汉-俄双语者;删除空格后,汉-俄双语者比母语为俄语者的阅读时间更长,注视次数更多,阅读速度更慢。对于像俄语这种有词间空格的语言来说,删除词间空格不仅对母语读者的阅读产生干扰,对于第二语言读者的阅读产生的干扰会更大。汉-俄双语者的俄语水平和俄语文本的呈现方式影响他们的阅读。     

Evaluating and reporting data quality in eye movement research

In order to judge the degree of confidence one should have in the results of an experiment using eye movement records as data, it is necessary to have information about the quality of the eye movement data themselves. Suggestions are made for ways of assessing and reporting this information. The paper deals with three areas: characteristics of the eye movement signal, algorithms used in reducing the data, and accuracy of the eye position data. It is suggested that all studies involving eye movement data should report such information. Appendices include linear interpolation algorithms for mapping from the eye movement signal to stimulus space and a way of obtaining an index of accuracy for each data point.     

Systematic influence of gaze position on pupil size measurement: analysis and correction

Cognitive effort is reflected in pupil dilation, but the assessment of pupil size is potentially susceptible to changes in gaze position. This study exemplarily used sentence reading as a stand-in for paradigms that assess pupil size in tasks during which changes in gaze position are unavoidable. The influence of gaze position on pupil size was first investigated by an artificial eye model with a fixed pupil size. Despite its fixed pupil size, the systematic measurements of the artificial eye model revealed substantial gaze-position-dependent changes in the measured pupil size. We evaluated two functions and showed that they can accurately capture and correct the gaze-dependent measurement error of pupil size recorded during a sentence-reading and an effortless z-string-scanning task. Implications for previous studies are discussed, and recommendations for future studies are provided.     

小学生对不同词语类型预视加工的发展*

采用边界范式,以60名小学生为被试,探讨了小学生不同词语类型的预视效应。结果表明:(1)五年级学生表现出更高的阅读效率;(2)预视类型的主效应显著,出现明显的预视效应;(3)不同词语类型的预视效应差异不显著;(4)未出现副中央凹-中央凹效应。研究结果支持阅读眼动的序列加工模型。     

词边界信息在中文阅读眼跳目标选择中的作用：来自中文二语学习者的证据

采用EyeLink 1000眼动仪, 选取韩、美、日、泰四国留学生各20名, 在正常无空格和词间空格两种呈现方式下阅读中文语句, 以此来探讨二语学习者在阅读中文时词边界信息在眼跳目标选择中的作用。结果发现, 二语学习者在中文阅读中存在着一致的眼动模式, 即在单次注视事件中, 倾向于注视词的中间部分, 在多次注视事件中, 倾向于注视其开端部分, 然后再计划一次词内再注视。词边界信息能够有效地引导中文二语学习者的眼动行为和眼跳计划, 更多地将首次注视位置落在词的中间部分。最后, 词边界信息在眼跳目标选择中的促进作用没有受到母语文本呈现方式熟悉性的调节。我们认为, 中文二语学习者在阅读中的眼跳目标选择可能采用的是战略-战术策略。