Gaze step distributions reflect fixations and saccades: a comment on

In three experimental tasks Stephen and Mirman (2010) measured gaze steps, the distance in pixels between gaze positions on successive samples from an eyetracker. They argued that the distribution of gaze steps is best fit by the lognormal distribution, and based on this analysis they concluded that interactive cognitive processes underlie eye movement control in these tasks. The present comment argues that the gaze step distribution is predictable based on the fact that the eyes alternate between a fixation state in which gaze is steady and a saccade state in which gaze position changes rapidly. By fitting a simple mixture model to Stephen and Mirman's gaze step data we reveal a fixation distribution and a saccade distribution. This mixture model captures the shape of the gaze step distribution in detail, unlike the lognormal model, and provides a better quantitative fit to the data. We conclude that the gaze step distribution does not directly suggest processing interaction, and we emphasize some important limits on the utility of fitting theoretical distributions to data.     

Automated classification and scoring of smooth pursuit eye movements in the presence of fixations and saccades

Ternary eye movement classification, which separates fixations, saccades, and smooth pursuit from the raw eye positional data, is extremely challenging. This article develops new and modifies existing eye-tracking algorithms for the purpose of conducting meaningful ternary classification. To this end, a set of qualitative and quantitative behavior scores is introduced to facilitate the assessment of classification performance and to provide means for automated threshold selection. Experimental evaluation of the proposed methods is conducted using eye movement records obtained from 11 subjects at 1000 Hz in response to a step-ramp stimulus eliciting fixations, saccades, and smooth pursuits. Results indicate that a simple hybrid method that incorporates velocity and dispersion thresholding allows producing robust classification performance. It is concluded that behavior scores are able to aid automated threshold selection for the algorithms capable of successful classification.     

Object files across eye movements: Previous fixations affect the latencies of corrective saccades

One of the factors contributing to a seamless visual experience is object correspondence—that is, the integration of pre- and postsaccadic visual object information into one representation. Previous research had suggested that before the execution of a saccade, a target object is loaded into visual working memory and subsequently is used to locate the target object after the saccade. Until now, studies on object correspondence have not taken previous fixations into account. In the present study, we investigated the influence of previously fixated information on object correspondence. To this end, we adapted a gaze correction paradigm in which a saccade was executed toward either a previously fixated or a novel target. During the saccade, the stimuli were displaced such that the participant’s gaze landed between the target stimulus and a distractor. Participants then executed a corrective saccade to the target. The results indicated that these corrective saccades had lower latencies toward previously fixated than toward nonfixated targets, indicating object-specific facilitation. In two follow-up experiments, we showed that presaccadic spatial and object (surface feature) information can contribute separately to the execution of a corrective saccade, as well as in conjunction. Whereas the execution of a corrective saccade to a previously fixated target object at a previously fixated location is slowed down (i.e., inhibition of return), corrective saccades toward either a previously fixated target object or a previously fixated location are facilitated. We concluded that corrective saccades are executed on the basis of object files rather than of unintegrated feature information.     

Eye fixations and cognitive processes

This paper presents a theoretical account of the sequence and duration of eye fixation during a number of simple cognitive tasks, such as mental rotation, sentence verification, and quantitative comparison. In each case, the eye fixation behavior is linked to a processing model for the task by assuming that the eye fixates the referent of the symbol being operated on.     

Latency and accuracy characteristics of saccades and corrective saccades in children and adults

Accuracy and latency characteristics of the first saccade to a target together with the frequency and latency of corrective saccades were studied in children (mean age = 8.5) and adults. The independent variables manipulated were fixation-light offset to target-light onset warning interval (0 and 300 msec) and the presence and location of nontarget stimuli. Although saccade accuracy was significantly affected by nontarget lights, children could respond as accurately as adults and, in replication of previous findings, as quickly when a 300 msec warning interval was given. No speed-accuracy trade off was found for either group as a function of the warning signal condition. Children were as likely to make corrective saccades as adults, but did so with a significantly longer latency. Corrective saccade latencies were greater when a change in direction was required but this effect did not interact with age.     

Presaccadic attention allocation and express saccades

Express saccades are visually-guided saccades that are characterized by an extremely short latency of about 100 ms. The present experiments tested the hypothesis that a disengagement of visual attention is necessary for the generation of express saccades. All subjects produced large numbers of express saccades in the gap paradigm, in which the fixation stimulus is removed 200 ms before target onset (Exp. 1), but not in the overlap paradigm, in which the fixation stimulus remains on during the entire trial (Exp. 2). By means of peripheral cues (Exps. 3–5) and central cues (Exps. 6–7), visual attention was directed at the target location for the saccade before the actual appearance of the saccade target. In all experiments, the location cues facilitated rather than abolished express saccades. The generation of express saccades was facilitated even when the currently fixated visual stimulus was not removed before target onset (fixation-overlap; Exps. 5–7). The results are explained by the hypothesis that a disengagement of a separate fixation system is necessary for the generation of express saccades, a hypothesis that is in line with current neurobiological findings.     

Digital computer estimation of eye fixations

A method is presented for estimating eye fixations to the degree of accuracy desired. In the process, the quantity of data is greatly reduced so that analysis can be accomplished at the end of each experimental run. The algorithm is particularly suitable for “slow” computers because the analog-to-digital conversion rate can be low and because the data reduction process decreases the number of calculations required to obtain the analytical results.     

Eye fixations and memory for emotional events.

Subjects watched either an emotional, neutral, or unusual sequence of slides containing 1 critical slide in the middle. Experiments 1 and 2 allowed only a single eye fixation on the critical slide by presenting it for 180 ms (Experiment 1) or 150 ms (Experiment 2). Despite this constraint, memory for a central detail was better for the emotional condition. In Experiment 3, subjects were allowed 2.70 s to view the critical slide while their eye movements were monitored. When subjects who had devoted the same number of fixations were compared, memory for the central detail of the emotional slide was again better. The results suggest that enhanced memory for detail information of an emotional event does not occur solely because more attention is devoted to the emotional information.     

Correlations between psychometric schizotypy,scan path length,fixations on the eyes and face recognition

Psychometric schizotypy in the general population correlates negatively with face recognition accuracy, potentially due to deficits in inhibition, social withdrawal, or eye-movement abnormalities. We report an eye-tracking face recognition study in which participants were required to match one of two faces (target and distractor) to a cue face presented immediately before. All faces could be presented with or without paraphernalia (e.g., hats, glasses, facial hair). Results showed that paraphernalia distracted participants, and that the most distracting condition was when the cue and the distractor face had paraphernalia but the target face did not, while there was no correlation between distractibility and participants’ scores on the Schizotypal Personality Questionnaire (SPQ). Schizotypy was negatively correlated with proportion of time fixating on the eyes and positively correlated with not fixating on a feature. It was negatively correlated with scan path length and this variable correlated with face recognition accuracy. These results are interpreted as schizotypal traits being associated with a restricted scan path leading to face recognition deficits.     

Eye fixations in prediction of recognition and recall

Eye fixations were recorded at viewing of picture-label stimuli presented under either recall or recognition instructions; both retention tests were administered. Ss performed substantially better on the retention test of which they were informed, indicating differential encoding of the same stimuli in anticipation of test type. There was no correlation between recognition and recall of items, evidence that different information from the encoded stimuli was utilized in performing each test. Encoding strategies had no effect on how Ss regarded the stimuli, but viewing patterns were related to memory performance: More word fixations was associated with better verbal recall, while fewer picture fixations was associated with better recall and with better picture recognition.     

Eye-position signals in successive saccades

Accuracy of the eye-position signal (EPS) in successive-saccade conditions was examined by analyzing the subject’s performance in tracking his gaze by pointing with his unobservable hand in the dark. The size of constant error in manual pointing was found to increase with the number of the component saccades and to be inversely proportional to the size of the largest component saccade. The results are interpreted as showing that, in successive-saccade conditions, subjects are not able to use the sum of EPSs from each component saccade, but rather use only the EPS from the largest component saccade.     

Perceiving stimulus displacements across saccades

The visual world appears stable despite frequent retinal image movements caused by saccades. Many theories of visual stability assume that extraretinal eye position information is used to spatially adjust perceived locations across saccades, whereas others have proposed that visual stability depends upon coding of the relative positions of objects. McConkie and Currie (1996) proposed a refined combination of these views (called the Saccade Target Object Theory) in which the perception of stability across saccades relies on a local evaluation process centred on the saccade target object rather than on a remapping of the entire scene, with some contribution from memory for the relative positions of objects as well. Three experiments investigated the saccade target object theory, along with an alternative hypothesis that proposes that multiple objects are updated across saccades, but with variable resolution, with the saccade target object (by virtue of being the focus of attention before the saccade and residing near the fovea after the saccade) having priority in the perception of displacement. Although support was found for the saccade target object theory in Experiment 1, the results of Experiments 2 and 3 found that multiple objects are updated across saccades and that their positions are evaluated to determine perceived stability. There is an advantage for detecting displacements of the saccade target, most likely because of visual acuity or attentional focus being better near the fovea, but it is not the saccade target alone that determines the perception of stability and of displacements across saccades. Rather, multiple sources of information appear to contribute.