Kenneth Holmqvist

An adaptive algorithm for fixation, saccade, and glissade detection in eyetracking data.

Event detection is used to classify recorded gaze points into periods of fixation, saccade, smooth pursuit, blink, and noise. Although there is an overall consensus that current algorithms for event detection have serious flaws and that a de facto standard for event detection does not exist, surprisingly little work has been done to remedy this problem. We suggest a new velocity-based algorithm that takes several of the previously known limitations into account. Most important, the new algorithm identifies so-called glissades, a wobbling movement at the end of many saccades, as a separate class of eye movements. Part of the solution involves designing an adaptive velocity threshold that makes the event detection less sensitive to variations in noise level and the algorithm settings-free for the user. We demonstrate the performance of the new algorithm on eye movements recorded during reading and scene perception and compare it with two of the most commonly used algorithms today. Results show that, unlike the currently used algorithms, fixations, saccades, and glissades are robustly identified by the new algorithm. Using this algorithm, we found that glissades occur in about half of the saccades, during both reading and scene perception, and that they have an average duration close to 24 msec. Due to the high prevalence and long durations of glissades, we argue that researchers must actively choose whether to assign the glissades to saccades or fixations; the choice affects dependent variables such as fixation and saccade duration significantly. Current algorithms do not offer this choice, and their assignments of each glissade are largely arbitrary.

Pictures and spoken descriptions elicit similar eye movements during mental imagery, both in light and in complete darkness

This study provides evidence that eye movements reflect the positions of objects while participants listen to a spoken description, retell a previously heard spoken description, and describe a previously seen picture. This effect is equally strong in retelling from memory, irrespective of whether the original elicitation was spoken or visual. In addition, this effect occurs both while watching a blank white board and while sitting in complete darkness. This study includes 4 experiments. The first 2 experiments measured eye movements of participants looking at a blank white board. Experiment 1 monitors eye movements of participants on 2 occasions: first, when participants listened to a prerecorded spoken scene description; second, when participants were later retelling it from memory. Experiment 2 first monitored eye movements of participants as they studied a complex picture visually, and then later as they described it from memory. The second pair of experiments (Experiments 3 and 4) replicated Experiments 1 and 2 with the only difference being that they were executed in complete darkness. This method of analysis differentiated between eye movements that are categorically correct relative to the positions of the whole eye gaze pattern (global correspondence) and eye movements that are only locally correct (local correspondence). The discussion relates the findings to the current debate on mental imagery.

Entry points and reading paths on the newspaper spread: Comparing semiotic analysis with eye-tracking measurements.

The aim of this article is to compare general assumptions about newspaper reading with eye-tracking data from readers’ actual interaction with a newspaper. First, we extract assumptions about the way people read newspapers from socio-semiotic research. Second, we apply these assumptions by analysing a newspaper spread; this is done without any previous knowledge of actual reading behaviour. Finally, we use eye-tracking to empirically examine so-called entry points and reading paths. Eye movement data on reading newspaper spreads are analysed in three different ways: the time sequence in which different areas attract attention is calculated in order to determine reading priorities; the amount of time spent on different areas is calculated in order to determine which areas have been read most; the depth of attention is calculated in order to determine how carefully those areas have been read. General assumptions extracted from the socio-semiotic framework are compared to the results of the actual behaviour of subjects reading the newspaper spread. The results show that the empirical data confirm some of the extracted assumptions. The reading paths of the five subjects participating in the eye-tracking tests suggest that there are three main categories of readers: editorial readers, overview readers and focused readers.

Eye tracker data quality: what it is and how to measure it

Data quality is essential to the validity of research results and to the quality of gaze interaction. We argue that the lack of standard measures for eye data quality makes several aspects of manufacturing and using eye trackers, as well as researching eye movements and vision, more difficult than necessary. Uncertainty regarding the comparability of research results is a considerable impediment to progress in the field. In this paper, we illustrate why data quality matters and review previous work on how eye data quality has been measured and reported. The goal is to achieve a common understanding of what data quality is and how it can be defined, measured, evaluated, and reported.

Reading or scanning? A study of newspaper and net paper reading

Net paper readers have been shown to read deeper into articles than newspaper readers. It has also been claimed that newspaper readers rather scan than read newspapers. Do these findings mean that net paper readers read proportionally more than newspaper readers? This paper presents results showing that in fact net paper readers scan more and read less than newspaper readers. We furthermore investigate whether this result can be expained by the difference in layout, navigation structure and purpose of reading between the two media.

A vector-based, multidimensional scanpath similarity measure

A great need exists in many fields of eye-tracking research for a robust and general method for scanpath comparisons. Current measures either quantize scanpaths in space (string editing measures like the Levenshtein distance) or in time (measures based on attention maps). This paper proposes a new pairwise scanpath similarity measure. Unlike previous measures that either use AOI sequences or forgo temporal order, the new measure defines scanpaths as a series of geometric vectors and compares temporally aligned scanpaths across several dimensions: shape, fixation position, length, direction, and fixation duration. This approach offers more multifaceted insights to how similar two scanpaths are. Eight fictitious scanpath pairs are tested to elucidate the strengths of the new measure, both in itself and compared to two of the currently most popular measures - the Levenshtein distance and attention map correlation.

The influence of calibration method and eye physiology on eyetracking data quality

Recording eye movement data with high quality is often a prerequisite for producing valid and replicable results and for drawing well-founded conclusions about the oculomotor system. Today, many aspects of data quality are often informally discussed among researchers but are very seldom measured, quantified, and reported. Here we systematically investigated how the calibration method, aspects of participants’ eye physiologies, the influences of recording time and gaze direction, and the experience of operators affect the quality of data recorded with a common tower-mounted, video-based eyetracker. We quantified accuracy, precision, and the amount of valid data, and found an increase in data quality when the participant indicated that he or she was looking at a calibration target, as compared to leaving this decision to the operator or the eyetracker software. Moreover, our results provide statistical evidence of how factors such as glasses, contact lenses, eye color, eyelashes, and mascara influence data quality. This method and the results provide eye movement researchers with an understanding of what is required to record high-quality data, as well as providing manufacturers with the knowledge to build better eyetrackers.

It depends on how you look at it: Scanpath comparison in multiple dimensions with MultiMatch, a vector-based approach

Eye movement sequences—or scanpaths—vary depending on the stimulus characteristics and the task (Foulsham & Underwood Journal of Vision, 8(2), 6:1–17, 2008; Land, Mennie, & Rusted, Perception, 28, 1311–1328, 1999). Common methods for comparing scanpaths, however, are limited in their ability to capture both the spatial and temporal properties of which a scanpath consists. Here, we validated a new method for scanpath comparison based on geometric vectors, which compares scanpaths over multiple dimensions while retaining positional and sequential information (Jarodzka, Holmqvist, & Nyström, Symposium on Eye-Tracking Research and Applications (pp. 211–218), 2010). “MultiMatch” was tested in two experiments and pitted against ScanMatch (Cristino, Mathôt, Theeuwes, & Gilchrist, Behavior Research Methods, 42, 692–700, 2010), the most comprehensive adaptation of the popular Levenshtein method. In Experiment 1, we used synthetic data, demonstrating the greater sensitivity of MultiMatch to variations in spatial position. In Experiment 2, real eye movement recordings were taken from participants viewing sequences of dots, designed to elicit scanpath pairs with commonalities known to be problematic for algorithms (e.g., when one scanpath is shifted in locus or when fixations fall on either side of an AOI boundary). The results illustrate the advantages of a multidimensional approach, revealing how two scanpaths differ. For instance, if one scanpath is the reverse copy of another, the difference is in the direction but not the positions of fixations; or if a scanpath is scaled down, the difference is in the length of the saccadic vectors but not in the overall shape. As well as having enormous potential for any task in which consistency in eye movements is important (e.g., learning), MultiMatch is particularly relevant for “eye movements to nothing” in mental imagery and embodiment-of-cognition research, where satisfactory scanpath comparison algorithms are lacking.

Combined eyetracking and keystroke-logging methods for studying cognitive processes in text production

Writers typically spend a certain proportion of time looking back over the text that they have written. This is likely to serve a number of different functions, which are currently poorly understood. In this article, we present two systems, ScriptLog+TimeLine and EyeWrite, that adopt different and complementary approaches to exploring this activity by collecting and analyzing combined eye movement and keystroke data from writers composing extended texts. ScriptLog+TimeLine is a system that is based on an existing keystroke-logging program and uses heuristic, pattern-matching methods to identify reading episodes within eye movement data. EyeWrite is an integrated editor and analysis system that permits identification of the words that the writer fixates and their location within the developing text. We demonstrate how the methods instantiated within these systems can be used to make sense of the large amount of data generated by eyetracking and keystroke logging in order to inform understanding of the cognitive processes that underlie written text production.

Right Visual Field Advantage in Parafoveal Processing: Evidence from Eye-Fixation-Related Potentials.

Readers acquire information outside the current eye fixation. Previous research indicates that having only the fixated word available slows reading, but when the next word is visible, reading is almost as fast as when the whole line is seen. Parafoveal-on-foveal effects are interpreted to reflect that the characteristics of a parafoveal word can influence fixation on a current word. Prior studies also show that words presented to the right visual field (RVF) are processed faster and more accurately than words in the left visual field (LVF). This asymmetry results either from an attentional bias, reading direction, or the cerebral asymmetry of language processing. We used eye-fixation-related potentials (EFRP), a technique that combines eye-tracking and electroencephalography, to investigate visual field differences in parafoveal-on-foveal effects. After a central fixation, a prime word appeared in the middle of the screen together with a parafoveal target that was presented either to the LVF or to the RVF. Both hemifield presentations included three semantic conditions: the words were either semantically associated, non-associated, or the target was a non-word. The participants began reading from the prime and then made a saccade towards the target, subsequently they judged the semantic association. Between 200 and 280ms from the fixation onset, an occipital P2 EFRP-component differentiated between parafoveal word and non-word stimuli when the parafoveal word appeared in the RVF. The results suggest that the extraction of parafoveal information is affected by attention, which is oriented as a function of reading direction.