Nature of variability in saccades

Modeling kinematic variability reveals displacement and velocity based dual control of saccadic eye movements

Noise is a ubiquitous component of motor systems that leads to behavioral variability of all types of movements. Nonetheless, systems-based models investigating human movements are generally deterministic and explain only the central tendencies like mean trajectories. In this paper, a novel approach to modeling kinematic variability of movements is presented and tested on the oculomotor system. This approach reconciles the two prominent philosophies of saccade control: displacement-based control versus velocity-based control. This was achieved by quantifying the variability in saccadic eye movements and developing a stochastic model of its control. The proposed stochastic dual model generated significantly better fits of inter-trial variances of the saccade trajectories compared to existing models. These results suggest that the saccadic system can flexibly use the information of both desired displacement and velocity for its control. This study presents a potential framework for investigating computational principles of motor control in the presence of noise utilizing stochastic modeling of kinematic variability.

What is a blink? Classifying and characterizing blinks in eye openness signals

Blinks, the closing and opening of the eyelids, are used in a wide array of fields where human function and behavior are studied. In data from video-based eye trackers, blink rate and duration are often estimated from the pupil-size signal. However, blinks and their parameters can be estimated only indirectly from this signal, since it does not explicitly contain information about the eyelid position. We ask whether blinks detected from an eye openness signal that estimates the distance between the eyelids (EO blinks) are comparable to blinks detected with a traditional algorithm using the pupil-size signal (PS blinks) and how robust blink detection is when data quality is low. In terms of rate, there was an almost-perfect overlap between EO and PS blink (F1 score: 0.98) when the head was in the center of the eye tracker's tracking range where data quality was high and a high overlap (F1 score 0.94) when the head was at the edge of the tracking range where data quality was worse. When there was a difference in blink rate between EO and PS blinks, it was mainly due to data loss in the pupil-size signal. Blink durations were about 60 ms longer in EO blinks compared to PS blinks. Moreover, the dynamics of EO blinks was similar to results from previous literature. We conclude that the eye openness signal together with our proposed blink detection algorithm provides an advantageous method to detect and describe blinks in greater detail.

Rethinking simultaneous suppression in visual cortex via compressive spatiotemporal population receptive fields

When multiple visual stimuli are presented simultaneously in the receptive field, the neural response is suppressed compared to presenting the same stimuli sequentially. The prevailing hypothesis suggests that this suppression is due to competition among multiple stimuli for limited resources within receptive fields, governed by task demands. However, it is unknown how stimulus-driven computations may give rise to simultaneous suppression. Using fMRI, we find simultaneous suppression in single voxels, which varies with both stimulus size and timing, and progressively increases up the visual hierarchy. Using population receptive field (pRF) models, we find that compressive spatiotemporal summation rather than compressive spatial summation predicts simultaneous suppression, and that increased simultaneous suppression is linked to larger pRF sizes and stronger compressive nonlinearities. These results necessitate a rethinking of simultaneous suppression as the outcome of stimulus-driven compressive spatiotemporal computations within pRFs, and open new opportunities to study visual processing capacity across space and time.

Reduced maximal range of ocular movements and its response to acute levodopa challenge in Parkinson's disease

Introduction

Although restriction of vertical ocular range of motion is known to be the hallmark of progressive supranuclear palsy (PSP), the maximal amplitude of ocular movement has not been quantitatively assessed despite of accumulating evidences of oculomotor dysfunction in Parkinson's disease (PD). Here, we evaluated the maximal oculomotor range and its response to levodopa in PD, and compare findings to atypical parkinsonism.

Methods

We recruited 159 healthy controls (HC) as well as 154 PD, 30 PSP, and 16 multiple system atrophy (MSA) patients. Oculomotor range was assessed using a kinetic perimeter-adapted device for the vertical and horizontal axes (four positions). Parameters were reassessed after levodopa challenge and compared among PD, PSP, and MSA patients.

Results

Maximum oculomotor range in PD patients was reduced as compared to HC. Levodopa improved oculomotor range in all directions; corrective effects of upward range positively correlated with improvements in Unified Parkinson's Disease Rating Scale III and bradykinesia sub-scores among PD patients. Although oculomotor range was markedly restricted among PSP and MSA patients, the beneficial effects of levodopa was less pronounced. Reduced oculomotor range of motion was more significant among PSP as compared to PD or MSA patients; MSA patients did not significantly differ from PD patients. The range of upward gaze was optimally sensitive for differentiating among PD, PSP, and MSA patients.

Conclusion

Maximum oculomotor range was reduced among PD patients significantly improved by levodopa treatment. Variations in, as well as the positively effects of levodopa on, the range of upward gaze assist diagnostic differentiation among PD, PSP, and MSA patients.

Adaptive coding across visual features during free-viewing and fixation conditions

Theoretical studies have long proposed that adaptation allows the brain to effectively use the limited response range of sensory neurons to encode widely varying natural inputs. However, despite this influential view, experimental studies have exclusively focused on how the neural code adapts to a range of stimuli lying along a single feature axis, such as orientation or contrast. Here, we performed electrical recordings in macaque visual cortex (area V4) to reveal significant adaptive changes in the neural code of single cells and populations across multiple feature axes. Both during free viewing and passive fixation, populations of cells improved their ability to encode image features after rapid exposure to stimuli lying on orthogonal feature axes even in the absence of initial tuning to these stimuli. These results reveal a remarkable adaptive capacity of visual cortical populations to improve network computations relevant for natural viewing despite the modularity of the functional cortical architecture.

Fixation classification: how to merge and select fixation candidates

Eye trackers are applied in many research fields (e.g., cognitive science, medicine, marketing research). To give meaning to the eye-tracking data, researchers have a broad choice of classification methods to extract various behaviors (e.g., saccade, blink, fixation) from the gaze signal. There is extensive literature about the different classification algorithms. Surprisingly, not much is known about the effect of fixation and saccade selection rules that are usually (implicitly) applied. We want to answer the following question: What is the impact of the selection-rule parameters (minimal saccade amplitude and minimal fixation duration) on the distribution of fixation durations? To answer this question, we used eye-tracking data with high and low quality and seven different classification algorithms. We conclude that selection rules play an important role in merging and selecting fixation candidates. For eye-tracking data with good-to-moderate precision (RMSD < 0.5∘), the classification algorithm of choice does not matter too much as long as it is sensitive enough and is followed by a rule that selects saccades with amplitudes larger than 1.0∘ and a rule that selects fixations with duration longer than 60 ms. Because of the importance of selection, researchers should always report whether they performed selection and the values of their parameters.

Time perception in film is modulated by sensory modality and arousal

Considerable research has shown that the perception of time can be distorted subjectively, but little empirical work has examined what factors affect time perception in film, a naturalistic multimodal stimulus. Here, we explore the effect of sensory modality, arousal, and valence on how participants estimate durations in film. Using behavioral ratings combined with pupillometry in a within-participants design, we analyzed responses to and duration estimates of film clips in three experimental conditions: audiovisual (containing music and sound effects), visual (without music and sound effects), and auditory (music and sound effects without a visual scene). Participants viewed clips from little-known nature documentaries, fiction, animation, and experimental films. They were asked to judge clip duration and to report subjective arousal and valence, as their pupil sizes were recorded. Data were analyzed using linear mixed-effects models. Results reveal duration estimates varied between experimental conditions. Clip durations were judged to be shorter than actual durations in all three conditions, with visual-only clips perceived as longer (i.e., less distorted in time) than auditory-only and audiovisual clips. High levels of Composite Arousal (an average of self-reported arousal and pupil size changes) were correlated with longer (more accurate) estimates of duration, particularly in the audiovisual modality. This effect may reflect stimulus complexity or greater cognitive engagement. Increased ratings of valence were correlated with longer estimates of duration. The use of naturalistic, complex stimuli such as film can enhance our understanding of the psychology of time perception.

Speed-accuracy tradeoffs influence the main sequence of saccadic eye movements

Several studies have proposed that an optimal speed-accuracy tradeoff underlies the stereotyped relationship between amplitude, duration and peak velocity of saccades (main sequence). To test this theory, we asked 8 participants to make saccades to Gaussian-blurred spots and manipulated the task’s accuracy constraints by varying target size (1, 3, and 5°). The largest targets indeed yielded more endpoint scatter (and lower gains) than the smallest targets, although this effect subsided with target eccentricity. The main sequence depended on several interacting factors: saccade latency, saccade gain and target size. Early saccades, which were faster than amplitude-matched late saccades, followed the target-size dependency one would expect from a speed-accuracy tradeoff process. They had higher peak velocities and shorter durations for larger targets than for smaller targets. For late saccades, however, the opposite was found. Deviations from the main sequence also covaried with saccade gain, in line with the idea that motor noise underlies part of the endpoint variability. Thus, our data provide partial evidence that the saccadic system weighs the detrimental effects of motor noise on saccade accuracy against movement duration and speed, but other factors also modulate the kinematics. We discuss the possible involvement of parallel saccade pathways to account for our findings.

Automatic processing of gaze movements to quantify gaze scanning behaviors in a driving simulator

Eye and head movements are used to scan the environment when driving. In particular, when approaching an intersection, large gaze scans to the left and right, comprising head and multiple eye movements, are made. We detail an algorithm called the gaze scan algorithm that automatically quantifies the magnitude, duration, and composition of such large lateral gaze scans. The algorithm works by first detecting lateral saccades, then merging these lateral saccades into gaze scans, with the start and end points of each gaze scan marked in time and eccentricity. We evaluated the algorithm by comparing gaze scans generated by the algorithm to manually marked "consensus ground truth" gaze scans taken from gaze data collected in a high-fidelity driving simulator. We found that the gaze scan algorithm successfully marked 96% of gaze scans and produced magnitudes and durations close to ground truth. Furthermore, the differences between the algorithm and ground truth were similar to the differences found between expert coders. Therefore, the algorithm may be used in lieu of manual marking of gaze data, significantly accelerating the time-consuming marking of gaze movement data in driving simulator studies. The algorithm also complements existing eye tracking and mobility research by quantifying the number, direction, magnitude, and timing of gaze scans and can be used to better understand how individuals scan their environment.

Predictive Model for Dyslexia from Fixations and Saccadic Eye Movement Events

BACKGROUND

Dyslexia is a disorder characterized by difficulty in reading such as poor speech and sound recognition. They have less capability to relate letters and form words and exhibit poor reading comprehension. Eye-tracking methodologies play a major role in analyzing human cognitive processing. Dyslexia is not a visual impairment disorder but it's a difficulty in phonological processing and word decoding. These difficulties are reflected in their eye movement patterns during reading.

OBJECTIVE

The disruptive eye movement helps us to use eye-tracking methodologies for identifying dyslexics.

METHODS

In this paper, a small set of eye movement features have been proposed that contribute more to distinguish between dyslexics and non-dyslexics by machine learning models. Features related to eye movement events such as fixations and saccades are detected using statistical measures, dispersion threshold identification (I-DT) and velocity threshold identification (I-VT) algorithms. These features were further analyzed using various machine learning algorithms such as Particle Swarm Optimization (PSO) based SVM Hybrid Kernel (Hybrid SVM - PSO), Support Vector Machine (SVM), Random Forest classifier (RF), Logistic Regression (LR) and K-Nearest Neighbor (KNN) for classification of dyslexics and non-dyslexics.

RESULTS

The accuracy achieved using the Hybrid SVM -PSO model is 95.6 %. The best set of features that gave high accuracy are average no of fixations, average fixation gaze duration, average saccadic movement duration, total number of saccadic movements, and average number of fixations.

CONCLUSION

It is observed that eye movement features detected using velocity-based algorithms performed better than those detected by dispersion-based algorithms and statistical measures.

Successful visually guided eye movements following sight restoration after congenital cataracts

Sensitive periods have previously been identified for several human visual system functions. Yet, it is unknown to what degree the development of visually guided oculomotor control depends on early visual experience-for example, whether and to what degree humans whose sight was restored after a transient period of congenital visual deprivation are able to conduct visually guided eye movements. In the present study, we developed new calibration and analysis techniques for eye tracking data contaminated with pervasive nystagmus, which is typical for this population. We investigated visually guided eye movements in sight recovery individuals with long periods of visual pattern deprivation (3-36 years) following birth due to congenital, dense, total, bilateral cataracts. As controls we assessed (1) individuals with nystagmus due to causes other than cataracts, (2) individuals with developmental cataracts after cataract removal, and (3) individuals with normal vision. Congenital cataract reversal individuals were able to perform visually guided gaze shifts, even when their blindness had lasted for decades. The typical extensive nystagmus of this group distorted eye movement trajectories, but measures of latency and accuracy were as expected from their prevailing nystagmus-that is, not worse than in the nystagmus control group. To the best of our knowledge, the present quantitative study is the first to investigate the characteristics of oculomotor control in congenital cataract reversal individuals, and it indicates a remarkable effectiveness of visually guided eye movements despite long-lasting periods of visual deprivation.

Image complexity analysis with scanpath identification using remote gaze estimation model

Analysis of gaze points has been a vital tool for understanding varied human behavioral pattern and underlying psychological processing. Gaze points are analyzed generally in terms of two events of fixations and saccades that are collectively termed as scanpath. Scanpath could potentially establish correlation between visual scenery and human cognitive tendencies. Scanpath has been analyzed for different domains that include visual perception, usability, memory, visual search or low level attributes like color, illumination and edges in an image. Visual search is one prominent area that examines scanpath of subjects while a target object is searched in a given set of images. Visual search explores behavioral tendencies of subjects with respect to image complexity. Complexity of an image is governed by spatial, frequency and color information present in the image. Scanpath based image complexity analysis determines human visual behavior that could lead to development of interactive and intelligent systems. There are several sophisticated eye tracking devices and associated algorithms for recording and classification of scanpath. However, in the present scenario when the chances of viral infections (COVID-19) from known and unknown sources are high, it is very important that the contact less methods and models be designed. In addition, even though the devices acquire and process eye movement data with fair accuracy but are intrusive and costly. The objective of current research work is to establish the complexity of the given set of images while target objects are searched and to present analysis of gaze search pattern. To achieve these objectives a remote gaze estimation and analysis model has been proposed for scanpath identification and analysis. The model is an alternate option for gaze point tracking and scanpath analysis that is non intrusive and low cost. The gaze points are tracked remotely as against sophisticated wearable eye tracking devices available in the market. The model employs easily available softwares and hardware devices. In the current work, complexity is derived on the basis of analysis of fixation and saccade gaze points. Based on the results generated by the proposed model, influence on subjects due to external stimuli is studied. The set of images chosen, act as external stimuli for the subjects during visual search. In order to statistically analyze scanpath for different subjects, certain scanpath parameters have been identified. The model maps and classifies eye movement gaze points into fixations and saccades and generates data for identified parameters. For eye detection and subsequent iris detection voila jones and circular hough transform (CHT) algorithms have been used. Identification by dispersion threshold (I-DT) is implemented for scanpath identification. The algorithms are customized for better iris and scanpath detection. Algorithms are developed for gaze screen mapping and classification of fixations and saccades. The experimentation has been carried on different subjects. Variations during visual search have been observed and analyzed. The present model requires no contact of human subject with any equipment including eye tracking devices, screen or computing devices.

Waist-to-Hip Ratio as Supernormal Stimuli: Effect of Contrapposto Pose and Viewing Angle

In women, the waist-to-hip ratio (WHR) is an indicator of attractiveness, health, youthfulness, and reproductive potential. In the current study, we hypothesized that viewing angle and body postures influence the attractiveness of these forms based on the view dependency of WHR stimuli (vdWHR). Using eye tracking, we quantified the number of fixations and dwell time on 3D images of a female avatar in two different poses (standing and contrapposto) from eight viewing angles incrementing in 45 degrees of rotation. A total of 68 heterosexual individuals (25 men and 43 women) participated in the study. Results showed that the contrapposto pose was perceived as more attractive than the standing pose and that lower vdWHR sides of the stimuli attracted more first fixation, total fixations, and dwell time. Overall, the results supported that WHR is view-dependent and vdWHRs lower than optimal WHRs are supernormal stimuli that may generate peak shifts in responding. Results are discussed in terms of the attractiveness of women's movements (gaits and dance) and augmented artistic presentations.

Differences in eye movement range based on age and gaze direction

PURPOSE

To determine the range of eye movement in normal human subjects and to investigate the effect of age and gaze direction on eye movement.

PATIENTS AND METHODS

A prospective observational study. We enrolled 261 healthy subjects, 5-91 years of age. Photographs were obtained in the cardinal gaze positions and processed using Photoshop. The processed images were analyzed using the Image J program to measure the angle of eye movement. The angle of eye movement was quantified using a modified limbus test. We measured the angle of eye movement in adduction, abduction, elevation, and depression.

RESULTS

The ranges of eye movement were 44.9 ± 7.2° in adduction, 44.2 ± 6.8° in adduction, 27.9 ± 7.6° in elevation, and 47.1 ± 8.0° in depression. The ranges of eye movement in the younger group were higher than that in the older group in adduction, abduction, and elevation (P < 0.001, P = 0.013, and P < 0.001, respectively), except in depression (P = 0.790). There were significant negative correlations between the angles of horizontal and upward gazes and age (R = -0.294 in adduction, R = -0.355 in abduction, and R = -0.506 in elevation, all P < 0.001). However, the angle of downward gaze was not significantly correlated with age (R = 0.017, P = 0.722).

CONCLUSIONS

The angle of upward gaze most rapidly decreased with age than the angle of other gaze. Unlike the age-related decline of range in horizontal and upward gazes, only downward gaze was not impaired by increasing age. Differences in eye movement range based on gaze direction and their associated aging mechanisms should be considered when assessing eye movements.

Transsacadic Information and Corollary Discharge in Local Field Potentials of Macaque V1

Approximately three times per second, human visual perception is interrupted by a saccadic eye movement. In addition to taking the eyes to a new location, several lines of evidence suggest that the saccades play multiple roles in visual perception. Indeed, it may be crucial that visual processing is informed about movements of the eyes in order to analyze visual input distinctly and efficiently on each fixation and preserve stable visual perception of the world across saccades. A variety of studies has demonstrated that activity in multiple brain areas is modulated by saccades. The hypothesis tested here is that these signals carry significant information that could be used in visual processing. To test this hypothesis, local field potentials (LFPs) were simultaneously recorded from multiple electrodes in macaque primary visual cortex (V1); support vector machines (SVMs) were used to classify the peri-saccadic LFPs. We find that LFPs in area V1 carry information that can be used to distinguish neural activity associated with fixations from saccades, precisely estimate the onset time of fixations, and reliably infer the directions of saccades. This information may be used by the brain in processes including visual stability, saccadic suppression, receptive field (RF) remapping, fixation amplification, and trans-saccadic visual perception.

Is human classification by experienced untrained observers a gold standard in fixation detection?

Manual classification is still a common method to evaluate event detection algorithms. The procedure is often as follows: Two or three human coders and the algorithm classify a significant quantity of data. In the gold standard approach, deviations from the human classifications are considered to be due to mistakes of the algorithm. However, little is known about human classification in eye tracking. To what extent do the classifications from a larger group of human coders agree? Twelve experienced but untrained human coders classified fixations in 6 min of adult and infant eye-tracking data. When using the sample-based Cohen's kappa, the classifications of the humans agreed near perfectly. However, we found substantial differences between the classifications when we examined fixation duration and number of fixations. We hypothesized that the human coders applied different (implicit) thresholds and selection rules. Indeed, when spatially close fixations were merged, most of the classification differences disappeared. On the basis of the nature of these intercoder differences, we concluded that fixation classification by experienced untrained human coders is not a gold standard. To bridge the gap between agreement measures (e.g., Cohen's kappa) and eye movement parameters (fixation duration, number of fixations), we suggest the use of the event-based F1 score and two new measures: the relative timing offset (RTO) and the relative timing deviation (RTD).

Information presentation compatibility in a simple digital control panel design: eye-tracking study

Various designs of typical digital control panels were analyzed experimentally from both the effectiveness and efficiency points of view. Subjects performed information comparison tasks aimed at keeping vehicle velocity at the same level. The experiment involved two versions of speedometers for displaying current and target velocities (clock-face and digital). The stimuli were also differentiated by the target velocity value (20, 50 and 80 km/h) and the correct response type (increase or decrease). Subjects' performance results along with the eye-tracking data were qualitatively and quantitatively analyzed for all 24 experimental conditions.

The Lifetime of Salience Extends Beyond the Initial Saccade

Several models of selection in search predict that saccades are biased toward conspicuous objects (also referred to as salient objects). Indeed, it has been demonstrated that initial saccades are biased toward the most conspicuous candidate. However, in a recent study, no such bias was found for the second saccade, and it was concluded that the attraction of conspicuous elements is limited to only short-latency initial saccades. This conclusion is based on only a single feature manipulation (orientation contrast) and conflicts with the prediction of influential salience models. Here, we investigate whether this result can be generalized beyond the domain of orientation. In displays containing three luminance annuli (Experiment 1), we find a considerable bias toward the most conspicuous candidate for the second saccade. In Experiment 1, the target could not be discriminated peripherally. When we made the target peripherally discriminable, the second saccade was no longer biased toward the more conspicuous candidate (Experiment 2). Thus, conspicuity plays a role in saccadic selection beyond the initial saccade. Whether second saccades are biased toward conspicuous objects appears to depend on the type of feature contrast underlying the conspicuity and the peripheral discriminability of target properties.

Noise-robust fixation detection in eye movement data: Identification by two-means clustering (I2MC)

Eye-tracking research in infants and older children has gained a lot of momentum over the last decades. Although eye-tracking research in these participant groups has become easier with the advance of the remote eye-tracker, this often comes at the cost of poorer data quality than in research with well-trained adults (Hessels, Andersson, Hooge, Nyström, & Kemner Infancy, 20, 601-633, 2015; Wass, Forssman, & Leppänen Infancy, 19, 427-460, 2014). Current fixation detection algorithms are not built for data from infants and young children. As a result, some researchers have even turned to hand correction of fixation detections (Saez de Urabain, Johnson, & Smith Behavior Research Methods, 47, 53-72, 2015). Here we introduce a fixation detection algorithm-identification by two-means clustering (I2MC)-built specifically for data across a wide range of noise levels and when periods of data loss may occur. We evaluated the I2MC algorithm against seven state-of-the-art event detection algorithms, and report that the I2MC algorithm's output is the most robust to high noise and data loss levels. The algorithm is automatic, works offline, and is suitable for eye-tracking data recorded with remote or tower-mounted eye-trackers using static stimuli. In addition to application of the I2MC algorithm in eye-tracking research with infants, school children, and certain patient groups, the I2MC algorithm also may be useful when the noise and data loss levels are markedly different between trials, participants, or time points (e.g., longitudinal research).

Topology for gaze analyses - Raw data segmentation

Recent years have witnessed a remarkable growth in the way mathematics, informatics, and computer science can process data. In disciplines such as machine learning, pattern recognition, computer vision, computational neurology, molecular biology, information retrieval, etc., many new methods have been developed to cope with the ever increasing amount and complexity of the data. These new methods offer interesting possibilities for processing, classifying and interpreting eye-tracking data. The present paper exemplifies the application of topological arguments to improve the evaluation of eye-tracking data. The task of classifying raw eye-tracking data into saccades and fixations, with a single, simple as well as intuitive argument, described as coherence of spacetime, is discussed, and the hierarchical ordering of the fixations into dwells is shown. The method, namely identification by topological characteristics (ITop), is parameter-free and needs no pre-processing and post-processing of the raw data. The general and robust topological argument is easy to expand into complex settings of higher visual tasks, making it possible to identify visual strategies.

No Evidence for a Saccadic Range Effect for Visually Guided and Memory-Guided Saccades in Simple Saccade-Targeting Tasks

Saccades to single targets in peripheral vision are typically characterized by an undershoot bias. Putting this bias to a test, Kapoula [1] used a paradigm in which observers were presented with two different sets of target eccentricities that partially overlapped each other. Her data were suggestive of a saccadic range effect (SRE): There was a tendency for saccades to overshoot close targets and undershoot far targets in a block, suggesting that there was a response bias towards the center of eccentricities in a given block. Our Experiment 1 was a close replication of the original study by Kapoula [1]. In addition, we tested whether the SRE is sensitive to top-down requirements associated with the task, and we also varied the target presentation duration. In Experiments 1 and 2, we expected to replicate the SRE for a visual discrimination task. The simple visual saccade-targeting task in Experiment 3, entailing minimal top-down influence, was expected to elicit a weaker SRE. Voluntary saccades to remembered target locations in Experiment 3 were expected to elicit the strongest SRE. Contrary to these predictions, we did not observe a SRE in any of the tasks. Our findings complement the results reported by Gillen et al. [2] who failed to find the effect in a saccade-targeting task with a very brief target presentation. Together, these results suggest that unlike arm movements, saccadic eye movements are not biased towards making saccades of a constant, optimal amplitude for the task.

The pupil is faster than the corneal reflection (CR): Are video based pupil-CR eye trackers suitable for studying detailed dynamics of eye movements?

Most modern video eye trackers use the p-CR (pupil minus CR) technique to deal with small relative movements between the eye tracker camera and the eye. We question whether the p-CR technique is appropriate to investigate saccade dynamics. In two experiments we investigated the dynamics of pupil, CR and gaze signals obtained from a standard SMI Hi-Speed eye tracker. We found many differences between the pupil and the CR signals. Differences concern timing of the saccade onset, saccade peak velocity and post-saccadic oscillation (PSO). We also obtained that pupil peak velocities were higher than CR peak velocities. Saccades in the eye trackers' gaze signal (that is constructed from p-CR) appear to be excessive versions of saccades in the pupil signal. We conclude that the pupil-CR technique is not suitable for studying detailed dynamics of eye movements.

Revisiting the global effect and inhibition of return

Saccades toward previously cued locations have longer latencies than saccades toward other locations, a phenomenon known as inhibition of return (IOR). Watanabe (Exp Brain Res 138:330–342. doi:10.1007/s002210100709, 2001) combined IOR with the global effect (where saccade landing points fall in between neighboring objects) to investigate whether IOR can also have a spatial component. When one of two neighboring targets was cued, there was a clear bias away from the cued location. In a condition where both targets were cued, it appeared that the global effect magnitude was similar to the condition without any cues. However, as the latencies in the double cue condition were shorter compared to the no cue condition, it is still an open question whether these results are representative for IOR. Considering the double cue condition can provide valuable insight into the interaction of the mechanisms underlying the two phenomena, here, we revisit this condition in an adapted paradigm. Our paradigm does result in longer latencies for the cued locations, and we find that the magnitude of the global effect is reduced significantly. Unexpectedly, this holds even when only including saccades with the same latencies for both conditions. Thus, the increased latencies associated with IOR cannot directly explain the reduction in global effect. The global effect reduction can likely best be seen as either a result of short-term depression of exogenous visual signals or a result of IOR established at the center of gravity of cues.

Patients With Obsessive-Compulsive Disorder Check Excessively in Response to Mild Uncertainty

Patients with obsessive-compulsive disorder (OCD) not only respond to obsessions with perseverative checking, but also engage in more general checking, irrespective of their obsessive concerns. This study investigated whether general checking is specific to OCD and exacerbated when only mild uncertainty is induced. Thirty-one patients with OCD, 26 anxiety- and 31 healthy controls performed a visual search task with eye-tracking and indicated in 50 search displays whether a target was "present" or "absent". Target-present trials were unambiguous, whereas target-absent trials induced mild uncertainty, because participants had to rely on not overlooking the target. Checking behavior was measured by assessing search time and the number of fixations, measured with an eye-tracker. Results showed that in both target-present and target-absent trials patients with OCD searched longer and made more fixations than healthy and anxiety controls. However, the difference in checking behavior between patients with OCD and the control groups was larger in target-absent trials (where mild uncertainty was induced). Anxiety and healthy controls did not differ in checking behavior. Thus, mild uncertainty appears to specifically promote checking in patients with OCD, which has implications for treatment.

Oculomatic: High speed, reliable, and accurate open-source eye tracking for humans and non-human primates

BACKGROUND

Video-based noninvasive eye trackers are an extremely useful tool for many areas of research. Many open-source eye trackers are available but current open-source systems are not designed to track eye movements with the temporal resolution required to investigate the mechanisms of oculomotor behavior. Commercial systems are available but employ closed source hardware and software and are relatively expensive, limiting wide-spread use.

NEW METHOD

Here we present Oculomatic, an open-source software and modular hardware solution to eye tracking for use in humans and non-human primates.

RESULTS

Oculomatic features high temporal resolution (up to 600Hz), real-time eye tracking with high spatial accuracy (<0.5°), and low system latency (∼1.8ms, 0.32ms STD) at a relatively low-cost.

COMPARISON WITH EXISTING METHOD(S)

Oculomatic compares favorably to our existing scleral search-coil system while being fully non invasive.

CONCLUSIONS

We propose that Oculomatic can support a wide range of research into the properties and neural mechanisms of oculomotor behavior.

Introduction of the Utrecht Tasks for Attention in Toddlers Using Eye Tracking (UTATE): A Pilot Study

Attention capacities underlie everyday functioning from an early age onwards. Little is known about attentional processes at toddler age. A feasible assessment of attention capacities at toddler age is needed to allow further study of attention development. In this study, a test battery is piloted that consists of four tasks which intend to measure the attention systems orienting, alerting, and executive attention: the Utrecht Tasks of Attention in Toddlers using Eye tracking [UTATE]. The UTATE assesses looking behavior that may reflect visual attention capacities, by using eye-tracking methods. This UTATE was studied in 16 Dutch 18-month-old toddlers. Results showed that the instrument is feasible and generates good quality data. A first indication of sufficient reliability was found for most of the variables. It is concluded that the UTATE can be used in further studies. Further evaluation of the reliability and validity of the instrument in larger samples is worthwhile.

Factor Structure of Attention Capacities Measured With Eye-Tracking Tasks in 18-Month-Old Toddlers

OBJECTIVE

Attention capacities are critical for adaptive functioning and development. Reliable assessment measures are needed for the study of attention capacities in early childhood. In the current study, we investigated the factor structure of the Utrecht Tasks of Attention in Toddlers Using Eye-tracking (UTATE) test battery that assesses attention capacities in 18-month-old toddlers with eye-tracking techniques.

METHOD

The factor structure of 13 measures of attention capacities, based on four eye-tracking tasks, was investigated in a sample of 95 healthy toddlers (18 months of age) using confirmatory factor analysis.

RESULTS

Results showed that a three-factor model best fitted the data. The latent constructs reflected an orienting, alerting, and executive attention system.

CONCLUSION

This study showed support for a three-factor model of attention capacities in 18-month-old toddlers. Further study is needed to investigate whether the model can also be used with children at risk of attention problems.

GraFIX: a semiautomatic approach for parsing low- and high-quality eye-tracking data

Fixation durations (FD) have been used widely as a measurement of information processing and attention. However, issues like data quality can seriously influence the accuracy of the fixation detection methods and, thus, affect the validity of our results (Holmqvist, Nyström, & Mulvey, 2012). This is crucial when studying special populations such as infants, where common issues with testing (e.g., high degree of movement, unreliable eye detection, low spatial precision) result in highly variable data quality and render existing FD detection approaches highly time consuming (hand-coding) or imprecise (automatic detection). To address this problem, we present GraFIX, a novel semiautomatic method consisting of a two-step process in which eye-tracking data is initially parsed by using velocity-based algorithms whose input parameters are adapted by the user and then manipulated using the graphical interface, allowing accurate and rapid adjustments of the algorithms' outcome. The present algorithms (1) smooth the raw data, (2) interpolate missing data points, and (3) apply a number of criteria to automatically evaluate and remove artifactual fixations. The input parameters (e.g., velocity threshold, interpolation latency) can be easily manually adapted to fit each participant. Furthermore, the present application includes visualization tools that facilitate the manual coding of fixations. We assessed this method by performing an intercoder reliability analysis in two groups of infants presenting low- and high-quality data and compared it with previous methods. Results revealed that our two-step approach with adaptable FD detection criteria gives rise to more reliable and stable measures in low- and high-quality data.

At 120 msec you can spot the animal but you don't yet know it's a dog

Earlier studies suggested that the visual system processes information at the basic level (e.g., dog) faster than at the subordinate (e.g., Dalmatian) or superordinate (e.g., animals) levels. However, the advantage of the basic category over the superordinate category in object recognition has been challenged recently, and the hierarchical nature of visual categorization is now a matter of debate. To address this issue, we used a forced-choice saccadic task in which a target and a distractor image were displayed simultaneously on each trial and participants had to saccade as fast as possible toward the image containing animal targets based on different categorization levels. This protocol enables us to investigate the first 100-120 msec, a previously unexplored temporal window, of visual object categorization. The first result is a surprising stability of the saccade latency (median RT ∼ 155 msec) regardless of the animal target category and the dissimilarity of target and distractor image sets. Accuracy was high (around 80% correct) for categorization tasks that can be solved at the superordinate level but dropped to almost chance levels for basic level categorization. At the basic level, the highest accuracy (62%) was obtained when distractors were restricted to another dissimilar basic category. Computational simulations based on the saliency map model showed that the results could not be predicted by pure bottom-up saliency differences between images. Our results support a model of visual recognition in which the visual system can rapidly access relatively coarse visual representations that provide information at the superordinate level of an object, but where additional visual analysis is required to allow more detailed categorization at the basic level.

Simultaneous recordings of human microsaccades and drifts with a contemporary video eye tracker and the search coil technique

Human eyes move continuously, even during visual fixation. These “fixational eye movements” (FEMs) include microsaccades, intersaccadic drift and oculomotor tremor. Research in human FEMs has grown considerably in the last decade, facilitated by the manufacture of noninvasive, high-resolution/speed video-oculography eye trackers. Due to the small magnitude of FEMs, obtaining reliable data can be challenging, however, and depends critically on the sensitivity and precision of the eye tracking system. Yet, no study has conducted an in-depth comparison of human FEM recordings obtained with the search coil (considered the gold standard for measuring microsaccades and drift) and with contemporary, state-of-the art video trackers. Here we measured human microsaccades and drift simultaneously with the search coil and a popular state-of-the-art video tracker. We found that 95% of microsaccades detected with the search coil were also detected with the video tracker, and 95% of microsaccades detected with video tracking were also detected with the search coil, indicating substantial agreement between the two systems. Peak/mean velocities and main sequence slopes of microsaccades detected with video tracking were significantly higher than those of the same microsaccades detected with the search coil, however. Ocular drift was significantly correlated between the two systems, but drift speeds were higher with video tracking than with the search coil. Overall, our combined results suggest that contemporary video tracking now approaches the search coil for measuring FEMs.

Loneliness and hypervigilance to social cues in females: an eye-tracking study

The goal of the present study was to examine whether lonely individuals differ from nonlonely individuals in their overt visual attention to social cues. Previous studies showed that loneliness was related to biased post-attentive processing of social cues (e.g., negative interpretation bias), but research on whether lonely and nonlonely individuals also show differences in an earlier information processing stage (gazing behavior) is very limited. A sample of 25 lonely and 25 nonlonely students took part in an eye-tracking study consisting of four tasks. We measured gazing (duration, number of fixations and first fixation) at the eyes, nose and mouth region of faces expressing emotions (Task 1), at emotion quadrants (anger, fear, happiness and neutral expression) (Task 2), at quadrants with positive and negative social and nonsocial images (Task 3), and at the facial area of actors in video clips with positive and negative content (Task 4). In general, participants tended to gaze most often and longest at areas that conveyed most social information, such as the eye region of the face (T1), and social images (T3). Participants gazed most often and longest at happy faces (T2) in still images, and more often and longer at the facial area in negative than in positive video clips (T4). No differences occurred between lonely and nonlonely participants in their gazing times and frequencies, nor at first fixations at social cues in the four different tasks. Based on this study, we found no evidence that overt visual attention to social cues differs between lonely and nonlonely individuals. This implies that biases in social information processing of lonely individuals may be limited to other phases of social information processing. Alternatively, biased overt attention to social cues may only occur under specific conditions, for specific stimuli or for specific lonely individuals.

UnAdulterated - children and adults' visual attention to healthy and unhealthy food

OBJECTIVE

Visually attending to unhealthy food creates a desire to consume the food. To resist the temptation people have to employ self-regulation strategies, such as visual avoidance. Past research has shown that self-regulatory skills develop throughout childhood and adolescence, suggesting adults' superior self-regulation skills compared to children.

METHODS

This study employed a novel method to investigate self-regulatory skills. Children and adults' initial (bottom-up) and maintained (top-down) visual attention to simultaneously presented healthy and unhealthy food were examined in an eye-tracking paradigm.

RESULTS

Results showed that both children and adults initially attended most to the unhealthy food. Subsequently, adults self-regulated their visual attention away from the unhealthy food. Despite the children's high self-reported attempts to eat healthily and importance of eating healthily, children did not self-regulate visual attention away from unhealthy food. Children remained influenced by the attention-driven desire to consume the unhealthy food whereas adults visually attended more strongly to the healthy food thereby avoiding the desire to consume the unhealthy option.

CONCLUSIONS

The findings emphasize the necessity of improving children's self-regulatory skills to support their desire to remain healthy and to protect children from the influences of the obesogenic environment.

The precorneal tear film as a fluid shell: the effect of blinking and saccades on tear film distribution and dynamics

We conducted a series of experiments to elucidate the behavior of the human precorneal tear film (PCTF) during blinking and horizontal and vertical saccades. Methodology included video-interferometry with subsequent image cross-correlation (tear film lipid layer [TFLL]) and video-microscopy (mucoaqueous subphase [MAS]). We observed that the TFLL interference pattern deteriorates rapidly with successive blinks and degrades slowly with repeated horizontal saccades during blink suppression when dark arcs of thinning appear in the fluorescein-stained PCTF. Furthermore, after full downgaze and a return to the primary position, a transient horizontal bright band appears, deep to the spreading TFLL. It may be followed by local disturbances in the interference pattern. Two horizontal dark bands form in the stained PCTF after the return saccade. PCTF disruption may occur below the lower band during blink suppression. We concluded that shearing during horizontal saccades is insufficient to disturb the tear film structure greatly. The MAS and TFLL move together as a fluid shell. The dark arcs/bands are caused by meniscus-induced thinning, imprinted onto the PCTF at the lid margin. Their stability during blink suppression suggests that the MAS has gel-like properties. The horizontal bright bands are probably due to transient corneal indentation in downgaze. In downgaze, the disturbance of the TFLL and MAS below the dark bands is possibly due to shearing across the MAS in the return phase. This could cause desiccating stress in everyday activities, such as working at a computer.

Post-saccadic oscillations in eye movement data recorded with pupil-based eye trackers reflect motion of the pupil inside the iris

Current video eye trackers use information about the pupil center to estimate orientation and movement of the eye. While dual Purkinje eye trackers suffer from lens wobble and scleral search coils may be influenced by contact lens slippage directly after saccades, it is not known whether pupil-based eye trackers produces similar artifacts in the data. We recorded eye movements from participants making repetitive, horizontal saccades and compared the movement in the data with pupil- and iris movements extracted from the eye images. Results showed that post-saccadic instabilities clearly exist in data recorded with a pupil-based eye tracker. They also exhibit a high degree of reproducibility across saccades and within participants. While the recorded eye movement data correlated well with the movement of the pupil center, the iris center showed only little post-saccadic movement. This means that the pupil moves relative to the iris during post-saccadic eye movements, and that the eye movement data reflect pupil movement rather than eyeball rotation. Besides introducing inaccuracies and additional variability in the data, the pupil movement inside the eyeball influences the decision of when a saccade should end and the subsequent fixation should begin, and consequently higher order analyses based on fixations and saccades.

Background, an important factor in visual search

The ability to detect an object depends on the contrast between the object and its background. Despite this, many models of visual search rely solely on the properties of target and distractors, and do not take the background into account. Yet, both target and distractors have their individual contrasts with the background. These contrasts generally differ, because the target and distractors are different in at least one feature. Therefore, background is likely to play an important role in visual search. In three experiments we manipulated the properties of the background (luminance, orientation and spatial frequency, respectively) while keeping the target and distractors constant. In the first experiment, in which target and distractors had a different luminance, changing the background luminance had an extensive effect on search times. When background luminance was in between that of the target and distractors, search times were always short. Interestingly, when the background was darker than both the target and the distractors, search times were much longer than when the background was lighter. Manipulating orientation and spatial frequency of the background, on the other hand, resulted in search times that were longest for small target-background differences. Thus, background plays an important role in search. This role depends on the individual contrast of both target and distractors with the background and the type of feature contrast (luminance, orientation or spatial frequency).

Parsing eye-tracking data of variable quality to provide accurate fixation duration estimates in infants and adults

Researchers studying infants' spontaneous allocation of attention have traditionally relied on hand-coding infants' direction of gaze from videos; these techniques have low temporal and spatial resolution and are labor intensive. Eye-tracking technology potentially allows for much more precise measurement of how attention is allocated at the subsecond scale, but a number of technical and methodological issues have given rise to caution about the quality and reliability of high temporal resolution data obtained from infants. We present analyses suggesting that when standard dispersal-based fixation detection algorithms are used to parse eye-tracking data obtained from infants, the results appear to be heavily influenced by interindividual variations in data quality. We discuss the causes of these artifacts, including fragmentary fixations arising from flickery or unreliable contact with the eyetracker and variable degrees of imprecision in reported position of gaze. We also present new algorithms designed to cope with these problems by including a number of new post hoc verification checks to identify and eliminate fixations that may be artifactual. We assess the results of our algorithms by testing their reliability using a variety of methods and on several data sets. We contend that, with appropriate data analysis methods, fixation duration can be a reliable and stable measure in infants. We conclude by discussing ways in which studying fixation durations during unconstrained orienting may offer insights into the relationship between attention and learning in naturalistic settings.

Random walk of motor planning in task-irrelevant dimensions

The movements that we make are variable. It is well established that at least a part of this variability is caused by noise in central motor planning. Here, we studied how the random effects of planning noise translate into changes in motor planning. Are the random effects independently added to a constant mean end point, or do they accumulate over movements? To distinguish between these possibilities, we examined repeated, discrete movements in various tasks in which the motor output could be decomposed into a task-relevant and a task-irrelevant component. We found in all tasks that the task-irrelevant component had a positive lag 1 autocorrelation, suggesting that the random effects of planning noise accumulate over movements. In contrast, the task-relevant component always had a lag 1 autocorrelation close to zero, which can be explained by effective trial-by-trial correction of motor planning on the basis of observed motor errors. Accumulation of the effects of planning noise is consistent with current insights into the stochastic nature of synaptic plasticity. It leads to motor exploration, which may subserve motor learning and performance optimization.

Low-level cues and ultra-fast face detection

Recent experimental work has demonstrated the existence of extremely rapid saccades toward faces in natural scenes that can be initiated only 100 ms after image onset (Crouzet et al., 2010). These ultra-rapid saccades constitute a major challenge to current models of processing in the visual system because they do not seem to leave enough time for even a single feed-forward pass through the ventral stream. Here we explore the possibility that the information required to trigger these very fast saccades could be extracted very early on in visual processing using relatively low-level amplitude spectrum (AS) information in the Fourier domain. Experiment 1 showed that AS normalization can significantly alter face-detection performance. However, a decrease of performance following AS normalization does not alone prove that AS-based information is used (Gaspar and Rousselet, 2009). In Experiment 2, following the Gaspar and Rousselet paper, we used a swapping procedure to clarify the role of AS information in fast object detection. Our experiment is composed of three conditions: (i) original images, (ii) category swapped, in which the face image has the AS of a vehicle, and the vehicle has the AS of a face, and (iii) identity swapped, where the face has the AS of another face image, and the vehicle has the AS of another vehicle image. The results showed very similar levels of performance in the original and identity swapped conditions, and a clear drop in the category swapped condition. This result demonstrates that, in the early temporal window offered by the saccadic choice task, the visual saccadic system does indeed rely on low-level AS information in order to rapidly detect faces. This sort of crude diagnostic information could potentially be derived very early on in the visual system, possibly as early as V1 and V2.