Manipulation of stimulus onset delay in reading: evidence for parallel programming of saccades

On-line eye movement recording of 12 subjects who read short stories on a cathode ray tube enabled a test of direct control and preprogramming models of eye movements in reading. Contingent upon eye position, a mask was displayed in place of the letters in central vision after each saccade, delaying the onset of the stimulus in each eye fixation. The duration of the delay was manipulated in fixed or randomized blocks. Although the length of the delay strongly affected the duration of the fixations, there was no difference due to the conditions of delay manipulation, indicating that fixation duration is under direct control. However, not all fixations were lengthened by the period of the delay. Some ended while the mask was still present, suggesting they had been preprogrammed. But these "anticipation" eye movements could not have been completely determined before the fixation was processed because their fixation durations and saccade lengths were affected by the spatial extent of the mask, which varied randomly. Neither preprogramming nor existing serial direct control models of eye guidance can adequately account for these data. Instead, a model with direct control and parallel programming of saccades is proposed to explain the data and eye movements in reading in general.     

On the control of visual fixation durations in free viewing of complex images

The mechanisms for the substantial variation in the durations of visual fixations in scene perception are not yet well understood. During free viewing of paintings, gaze-contingent irrelevant distractors (Exp. 1) and non-gaze-related time-locked display changes (Exp. 2) were presented. We demonstrated that any visual change-its onset and offset-prolongs the ongoing fixation (i.e., delays the following saccade), strongly suggesting that fixation durations are under the direct control of the stimulus information. The strongest influence of distraction was observed for fixations preceded by saccades within the parafoveal range (<5° of visual angle). We assume that these fixations contribute to the focal in contrast to the ambient mode of attention (Pannasch & Velichkovsky, Visual Cognition, 17, 1109-1131, 2009; Velichkovsky, Memory, 10, 405-419, 2002). Recent findings about two distinct "subpopulations of fixations," one under the direct and another under the indirect control of stimulation (e.g., Henderson & Smith, Visual Cognition, 17, 1055-1082, 2009), are reconsidered in view of these results.     

Information influences the pattern of eye fixations during sentence comprehension

How does the pattern of eye fixation vary as an informative part of a word is encountered? If the processing of information lags behind the movement of the eyes, then we should expect no variation in the pattern; but if processing is immediate, then the movements of the reader's eyes should correspond to the distribution of information being inspected. An experiment is reported which examined the ways that the text ahead of the point of current fixation can be used to guide the eyes to future fixations, by monitoring fixations during a sentence comprehension task. The patterns of eye fixations upon words with uneven distributions of information (where, for example, words predictable from the sight of their first few letters but not from their last few letters are defined as containing informative beginnings) were observed, and it was found that more and longer fixations were produced when subjects looked at the informative parts of words, particularly at the informative endings of words. The results support the suggestion that eye movements are under the moment-to-moment control of cognitive mechanisms.     

Brief foveal masking during scene perception

In the present experiment, participants were exploring line drawings of scenes in the context of an object-decision task, while eye-contingent display changes manipulated the appearance of the foveal part of the image. Foveal information was replaced by an ovoid noise mask for 83 ms, after a preset delay of 15, 35, 60, or 85 ms following the onset of fixations. In control conditions, a red ellipse appeared for 83 ms, centered around the fixation position, after the same delays as in the noise-mask conditions. It was found that scene exploration was hampered especially when foveal masking occurred early during fixations, replicating earlier findings. Furthermore, fixation durations were shown to increase linearly as the mask delay decreased, which validates the fixation duration as a measure of perceptual processing speed.     

Linguistically guided anticipatory eye movements in scene viewing

The present study replicated the well-known demonstration by Altmann and Kamide (1999) that listeners make linguistically guided anticipatory eye movements, but used photographs of scenes rather than clip-art arrays as the visual stimuli. When listeners heard a verb for which a particular object in a visual scene was the likely theme, they made earlier looks to this object (e.g., looks to a cake upon hearing The boy will eat …) than when they heard a control verb (The boy will move …). New data analyses assessed whether these anticipatory effects are due to a linguistic effect on the targeting of saccades (i.e., the where parameter of eye movement control), the duration of fixations (i.e., the when parameter), or both. Participants made fewer fixations before reaching the target object when the verb was selectionally restricting (e.g., will eat). However, verb type had no effect on the duration of individual eye fixations. These results suggest an important constraint on the linkage between spoken language processing and eye movement control: Linguistic input may influence only the decision of where to move the eyes, not the decision of when to move them.     

Eye movement control during scene viewing: Immediate degradation and enhancement effects of spatial frequency filtering

What controls how long the eyes remain fixated during scene perception? We investigated whether fixation durations are under the immediate control of the quality of the current scene image. Subjects freely viewed photographs of scenes in preparation for a later memory test while their eye movements were recorded. Using the saccade-contingent display change method, scenes were degraded (Experiment 1) or enhanced (Experiment 2) via blurring (low-pass filtering) during predefined saccades. Results showed that fixation durations immediately after a display change were influenced by the degree of blur, with a monotonic relationship between degree of blur and fixation duration. The results also demonstrated that fixation durations can be both increased and decreased by changes in the degree of blur. The results suggest that fixation durations in scene viewing are influenced by the ease of processing of the image currently in view. The results are consistent with models of saccade generation in scenes in which moment-to-moment difficulty in visual and cognitive processing modulates fixation durations.     

Direct control of fixation times in scene viewing: Evidence from analysis of the distribution of first fixation duration

Participants' eye movements were monitored in two scene viewing experiments that manipulated the task-relevance of scene stimuli and their availability for extrafoveal processing. In both experiments, participants viewed arrays containing eight scenes drawn from two categories. The arrays of scenes were either viewed freely (Free Viewing) or in a gaze-contingent viewing mode where extrafoveal preview of the scenes was restricted (No Preview). In Experiment 1a, participants memorized the scenes from one category that was designated as relevant, and in Experiment 1b, participants chose their preferred scene from within the relevant category. We examined first fixations on scenes from the relevant category compared to the irrelevant category (Experiments 1a and 1b), and those on the chosen scene compared to other scenes not chosen within the relevant category (Experiment 1b). A survival analysis was used to estimate the first discernible influence of the task-relevance on the distribution of first-fixation durations. In the free viewing condition in Experiment 1a, the influence of task relevance occurred as early as 81 ms from the start of fixation. In contrast, the corresponding value in the no preview condition was 254 ms, demonstrating the crucial role of extrafoveal processing in enabling direct control of fixation durations in scene viewing. First fixation durations were also influenced by whether or not the scene was eventually chosen (Experiment 1b), but this effect occurred later and affected fewer fixations than the effect of scene category, indicating that the time course of scene processing is an important variable mediating direct control of fixation durations.     

On the visual span during object search in real-world scenes

The current study investigated from how large a region around their current point of gaze viewers can take in information when searching for objects in real-world scenes. Visual span size was estimated using the gaze-contingent moving window paradigm. Experiment 1 featured window radii measuring 1, 3, 4, 4.7, 5.4, and 6.1°. Experiment 2 featured six window radii measuring between 5 and 10°. Each scene occupied a 24.8 × 18.6° field of view. Inside the moving window, the scene was presented in high resolution. Outside the window, the scene image was low-pass filtered to impede the parsing of the scene into constituent objects. Visual span was defined as the window size at which object search times became indistinguishable from search times in the no-window control condition; this occurred with windows measuring 8° and larger. Notably, as long as central vision was fully available (window radii ≥ 5°), the distance traversed by the eyes through the scene to the search target was comparable to baseline performance. However, to move their eyes to the target, viewers made shorter saccades, requiring more fixations to cover the same image space, and thus more time. Moreover, a gaze-data based decomposition of search time revealed disruptions in specific subprocesses of search. In addition, nonlinear mixed models analyses demonstrated reliable individual differences in visual span size and parameters of the search time function.     

Regarding Scenes

ABSTRACT— When we view the visual world, our eyes flit from one location to another about three times each second. These frequent changes in gaze direction result from very fast saccadic eye movements. Useful visual information is acquired only during fixations, periods of relative gaze stability. Gaze control is defined as the process of directing fixation through a scene in real time in the service of ongoing perceptual, cognitive, and behavioral activity. This article discusses current approaches and new empirical findings that are allowing investigators to unravel how human gaze control operates during active real-world scene perception.     

Eye movement control during reading: Evidence for direct control

In three experiments, subjects read text as their eye movements were monitored and display changes in the text were made contingent upon the eye movements. In one experiment, a window of text moved in synchrony with the eyes. In one condition, the size of the window was constant from fixation to fixation, while in the other condition the size of the window varied from fixation to fixation. In the other experiments, a visual mask was presented at the end of each saccade which delayed the onset of the text, and the length of the delay was varied. The pattern of eye movements was influenced by both the size of the window and the delay of the onset of the text, even when the window size or text delay was varying from fixation to fixation. However, there was also evidence that saccade length was affected by the size of the window on the prior fixation and that certain decisions to move the eye are programmed either before the fixation begins or are programmed during the fixation but without regard to the text fixated on. The results thus provide support for a mixed control model of eye movements in reading, in which decisions about when and where to move the eyes are based on information from the current fixation, the prior fixations, and possibly, other sources as well.     

The influence of action video game playing on eye movement behaviour during visual search in abstract,in-game and natural scenes

Action game playing has been associated with several improvements in visual attention tasks. However, it is not clear how such changes might influence the way we overtly select information from our visual world (i.e. eye movements). We examined whether action-video-game training changed eye movement behaviour in a series of visual search tasks including conjunctive search (relatively abstracted from natural behaviour), game-related search, and more naturalistic scene search. Forty nongamers were trained in either an action first-person shooter game or a card game (control) for 10 hours. As a further control, we recorded eye movements of 20 experienced action gamers on the same tasks. The results did not show any change in duration of fixations or saccade amplitude either from before to after the training or between all nongamers (pretraining) and experienced action gamers. However, we observed a change in search strategy, reflected by a reduction in the vertical distribution of fixations for the game-related search task in the action-game-trained group. This might suggest learning the likely distribution of targets. In other words, game training only skilled participants to search game images for targets important to the game, with no indication of transfer to the more natural scene search. Taken together, these results suggest no modification in overt allocation of attention. Either the skills that can be trained with action gaming are not powerful enough to influence information selection through eye movements, or action-game-learned skills are not used when deciding where to move the eyes.     

Fixation durations in scene viewing: Modeling the effects of local image features,oculomotor parameters,and task

Scene perception requires the orchestration of image- and task-related processes with oculomotor constraints. The present study was designed to investigate how these factors influence how long the eyes remain fixated on a given location. Linear mixed models (LMMs) were used to test whether local image statistics (including luminance, luminance contrast, edge density, visual clutter, and the number of homogeneous segments), calculated for 1° circular regions around fixation locations, modulate fixation durations, and how these effects depend on task-related control. Fixation durations and locations were recorded from 72 participants, each viewing 135 scenes under three different viewing instructions (memorization, preference judgment, and search). Along with the image-related predictors, the LMMs simultaneously considered a number of oculomotor and spatiotemporal covariates, including the amplitudes of the previous and next saccades, and viewing time. As a key finding, the local image features around the current fixation predicted this fixation’s duration. For instance, greater luminance was associated with shorter fixation durations. Such immediacy effects were found for all three viewing tasks. Moreover, in the memorization and preference tasks, some evidence for successor effects emerged, such that some image characteristics of the upcoming location influenced how long the eyes stayed at the current location. In contrast, in the search task, scene processing was not distributed across fixation durations within the visual span. The LMM-based framework of analysis, applied to the control of fixation durations in scenes, suggests important constraints for models of scene perception and search, and for visual attention in general.     

Establishing the Duration of Crimes: An Individual Differences and Eye‐Tracking Investigation into Time Estimation

The time available for viewing a perpetrator at a crime scene predicts successful person recognition in subsequent identity line‐ups. This time is usually unknown and must be derived from eyewitnesses' duration estimates. This study therefore compared the estimates that different individuals provide for crimes. We then attempted to determine the accuracy of these durations by measuring observers' general time estimation ability with a set of estimator videos. Observers differed greatly in their ability to estimate time, but individual duration estimates correlated strongly for crime and estimator materials. This indicates that it might be possible to infer unknown durations of events, such as criminal incidents, from a person's ability to estimate known durations. We also measured observers' eye movements to a perpetrator during crimes. Only fixations on a perpetrator's face related to eyewitness accuracy, but these fixations did not correlate with exposure estimates for this person. The implications of these findings are discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

The limits of visual resolution in natural scene viewing

We examined the limits of visual resolution in natural scene viewing, using a gaze-contingent multiresolutional display having a gaze-centred area-of-interest and decreasing resolution with eccentricity. Twelve participants viewed high-resolution scenes in which gaze-contingent multiresolutional versions occasionally appeared for single fixations. Both detection of image degradation (five filtering levels plus a no-area-of-interest control) in the gaze-contingent multiresolutional display, and eye fixation durations, were well predicted by a model of eccentricity-dependent contrast sensitivity. The results also illuminate the time course of detecting image filtering. Detection did not occur for fixations below 100 ms, and reached asymptote for fixations above 200 ms. Detectable filtering lengthened fixation durations by 160 ms, and interference from an imminent manual response occurred by 400-450 ms, often lengthening the next fixation. We provide an estimate of the limits of visual resolution in natural scene viewing useful for theories of scene perception, and help bridge the literature on spatial vision and eye movement control.     

The effect of the first glimpse at a scene on eye movements during search

Previewing scenes briefly makes finding target objects more efficient when viewing is through a gaze-contingent window (windowed viewing). In contrast, showing a preview of a randomly arranged search display does not benefit search efficiency when viewing during search is of the full display. Here, we tested whether a scene preview is beneficial when the scene is fully visible during search. Scene previews, when presented, were 250 ms in duration. During search, the scene was either fully visible or windowed. A preview always provided an advantage, in terms of decreasing the time to initially fixate and respond to targets and in terms of the total number of fixations. In windowed visibility, a preview reduced the distance of fixations from the target position until at least the fourth fixation. In full visibility, previewing reduced the distance of the second fixation but not of later fixations. The gist information derived from the initial glimpse of a scene allowed for placement of the first one or two fixations at information-rich locations, but when nonfoveal information was available, subsequent eye movements were only guided by online information.     

Describing scenes hardly seen

Knowledge about scene categories, the so-called gist, can be extracted very rapidly, while recognition and naming of individual scene objects is a more effortful process. We investigate this phenomenon by presenting action scenes involving two actors for durations varying between 100 and 300 ms. Incoherence was created by mirroring individual scene actors. Upon masked presentation participants had to report content, actors and objects and to indicate whether the scene was meaningful or not. Scene coherence was judged correctly at all presentation durations. Actors were correctly identified in about one-third of the cases even with presentation durations of 100 ms, and identification rate increased up to 80% with longer durations. Identification depended on scene coherence, on the position of agents in the scene, and on the position of actors relative to the fixation cross. These interdependencies of scene and object perception indicate that the visual system seems to be very sensitive to meaningful interactions of living entities. A series of fixations is not necessary to identify actors of a scene.     

Characterising the visual buffer: real-world evidence for overwriting early in each fixation

What happens to the pictorial content of fixations when we move our eyes? Previous studies demonstrate that observers are very poor at detecting changes in natural scenes that occur across saccades, blinks, and artificial interruptions ('change blindness'). They suggest that the visual 'snapshots' of what is on the retina during a fixation are not retained and fused over successive fixations. I find similar results when volunteers are performing the complex real-life task of making a cup of tea. Volunteers can access the snapshot of the current fixation but not those of previous fixations. I suggest that volunteers are reporting the content of a low-level visual store that holds a veridical snapshot of the current fixation, rather than the retina itself. The snapshots are not 'wiped' by the saccade and remain in the buffer until they are overwritten by a new snapshot. The overwrite occurs in an all-or-none manner and can be at any time within the first 400 ms of each new fixation, with 50% of overwrites being within the first 100 ms.     

Not fixating at the line of text comes at a cost

Previous research on eye guidance in reading has investigated systematic tendencies with respect to horizontal fixation locations on letters within words and the relationship between fixation location in a word and the duration of the fixation. The present study investigates where readers place their eyes vertically on the line of text and how vertical fixation location is related to fixation duration. Analyses were based on a large corpus of eye movement recordings from single-sentence reading. The vertical preferred viewing location was found to be within the vertical extent of the font, but fixations beyond the vertical boundaries of the text also frequently occurred. Analyzing fixation duration as a function of vertical fixation location revealed a vertical optimal viewing position (vOVP) effect: Fixations were shortest when placed optimally on the line of text, and fixation duration gradually increased for fixations that fell above or below the line of text. The vOVP effect can be explained by the limits of visual resolution along the vertical meridian. It is concluded that vertical and horizontal landing positions in single-sentence reading are associated with differences in fixation durations in opposite ways.     

Aberrant first fixations when looking at inverted faces in various poses: The result of the centre‐of‐gravity effect?

Face recognition is essential in everyday human life, and all faces are encountered in different poses. However, when a face is inverted, difficulties arise for recognition and eye movements may (Barton, Radcliffe, Cherkasova, Edleman, & Intriligator, 2006) or may not be disrupted (Williams & Henderson, 2007). The present study explored the effects of orientation and pose on recognition and eye movements during a standard old/new recognition task in order to resolve whether inversion disrupts eye movements. Eye‐tracking data looked at the first fixations, the number of fixations, and the duration of fixations over a face. A standard inversion effect was observed, but the three‐quarter view advantage was not observed. Eye‐movement data revealed that the eyes were the most sampled feature (in terms of first fixation, number of fixations, and duration of fixation) for all upright faces, however, other features were sampled first for inverted faces. These results are consistent with Barton et al.'s (2006) but not Williams and Henderson's (2007) findings: possible explanations for this are discussed with the caveat that the same images were used from learning to test.