Richard Dewhurst

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.

Fixation and saliency during search of natural scenes: the case of visual agnosia

Models of eye movement control in natural scenes often distinguish between stimulus-driven processes (which guide the eyes to visually salient regions) and those based on task and object knowledge (which depend on expectations or identification of objects and scene gist). In the present investigation, the eye movements of a patient with visual agnosia were recorded while she searched for objects within photographs of natural scenes and compared to those made by students and age-matched controls. Agnosia is assumed to disrupt the top-down knowledge available in this task, and so may increase the reliance on bottom-up cues. The patients deficit in object recognition was seen in poor search performance and inefficient scanning. The low-level saliency of target objects had an effect on responses in visual agnosia, and the most salient region in the scene was more likely to be fixated by the patient than by controls. An analysis of model-predicted saliency at fixation locations indicated a closer match between fixations and low-level saliency in agnosia than in controls. These findings are discussed in relation to saliency-map models and the balance between high and low-level factors in eye guidance.

2011 Special Issue: Modeling eye movements in visual agnosia with a saliency map approach: Bottom-up guidance or top-down strategy?

Two recent papers (Foulsham, Barton, Kingstone, Dewhurst, & Underwood, 2009; Mannan, Kennard, & Husain, 2009) report that neuropsychological patients with a profound object recognition problem (visual agnosic subjects) show differences from healthy observers in the way their eye movements are controlled when looking at images. The interpretation of these papers is that eye movements can be modeled as the selection of points on a saliency map, and that agnosic subjects show an increased reliance on visual saliency, i.e., brightness and contrast in low-level stimulus features. Here we review this approach and present new data from our own experiments with an agnosic patient that quantifies the relationship between saliency and fixation location. In addition, we consider whether the perceptual difficulties of individual patients might be modeled by selectively weighting the different features involved in a saliency map. Our data indicate that saliency is not always a good predictor of fixation in agnosia: even for our agnosic subject, as for normal observers, the saliency-fixation relationship varied as a function of the task. This means that top-down processes still have a significant effect on the earliest stages of scanning in the setting of visual agnosia, indicating severe limitations for the saliency map model. Top-down, active strategies-which are the hallmark of our human visual system-play a vital role in eye movement control, whether we know what we are looking at or not.

Improving the accuracy of video-based eye-tracking in real-time through post-calibration regression

Lack of accuracy in eye-tracking data can be critical. If the point of gaze is not recorded accurately, the information obtained or action executed might be different from what was intended. Depending on the system, researchers often have to be content with accuracies in the range of 1–2° while operator experience and participant characteristics also have a significant effect on accuracy.

Training Eye Movements: Can Training People Where to Look Hinder the Processing of Fixated Objects?

An experiment designed to test the effects of different forms of training in a visual-search-like task is reported. Observers were presented with a series of displays containing a central letter and a ring of peripheral characters, one of which was a digit. Odd digits (catch trials) required a space-bar response; even digits required a different response contingent on the identity of the central letter. Two forms of training provided information either about the location of the peripheral digit, or about a quick way to classify the central letter. The aim was to relate training to Findlay and Walkers (1999, Behavioral and Brain Sciences 22 661–721) model of saccadic eye-movement control by affecting the hypothesised Move and Fixate centres respectively. The results showed that training benefited search, but training of the Move centre alone generated significantly longer re-inspections of the central region (in a feedback condition). This highlights that the emphasis often placed upon broadening the range of visual search when training eye movements may be misplaced. More specifically, special attention should be given, not only to advising people how to move their eyes, but also to improving the ability to effectively process important visual stimuli when fixating.

Does attention move or spread during mental curve tracing

There are two theories that attempt to explain how attention is deployed when lines are traced. Initially, it was believed that a covert zoom lens moved along the line. Recent evidence has, however, suggested that attention spreads along the line, rather than moving along it, perhaps as part of an effortful object-parsing process. Three experiments tested the spreading and moving accounts of line tracing. Participants were presented with two intertwined lines and were required to trace one to find the correct target. On half the trials, a masked change occurred, most often near the top of the target line, that reversed the required response. If attention spreads along the line, the participants should have been able to notice the change whenever it occurred during the tracing process. However, the participants found it harder to spot the change if it occurred late in the tracing process. This suggests that resources were less frequently available to detect changes on portions of the line that had already been traced when the change occurred. The results argue against a spreading trace of attention that encompasses the whole line.

How task demands influence scanpath similarity in a sequential number-search task

ABSTRACT More and more researchers are considering the omnibus eye movement sequence—the scanpath—in their studies of visual and cognitive processing (e.g. Hayes, Petrov, & Sederberg, 2011; Madsen, Larson, Loschky, & Rebello, 2012; Ni et al., 2011; von der Malsburg & Vasishth, 2011). However, it remains unclear how recent methods for comparing scanpaths perform in experiments producing variable scanpaths, and whether these methods supplement more traditional analyses of individual oculomotor statistics. We address this problem for MultiMatch (Jarodzka et al., 2010; Dewhurst et al., 2012), evaluating its performance with a visual search‐like task in which participants must fixate a series of target numbers in a prescribed order. This task should produce predictable sequences of fixations and thus provide a testing ground for scanpath measures. Task difficulty was manipulated by making the targets more or less visible through changes in font and the presence of distractors or visual noise. These changes in task demands led to slower search and more fixations. Importantly, they also resulted in a reduction in the between‐subjects scanpath similarity, demonstrating that participants’ gaze patterns became more heterogenous in terms of saccade length and angle, and fixation position. This implies a divergent strategy or random component to eye‐movement behaviour which increases as the task becomes more difficult. Interestingly, the duration of fixations along aligned vectors showed the opposite pattern, becoming more similar between observers in 2 of the 3 difficulty manipulations. This provides important information for vision scientists who may wish to use scanpath metrics to quantify variations in gaze across a spectrum of perceptual and cognitive tasks.