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.     

A perceptual-economy account for the inverted-optimal viewing position effect

In reading, fixation durations are longer when the eyes fall near the center of words than when fixation occurs toward the words' ends-the inverted-optimal viewing position (I-OVP) effect. This study assessed whether the I-OVP effect was based on the fixation position in the word or the fixation position in the visual stimulus. In Experiments 1-3, words were presented at variable locations within longer strings of symbols. On trials with short fixation durations, there were effects of fixation position in the string. When long fixations were made, there were effects of fixation position in the word. In Experiment 4, an I-OVP effect was found for meaningless number strings, and its strength depended on the task's processing demands. The findings show that (a) the I-OVP effect is unrelated to orthographic informativeness and (b) the eyes are not constrained to spend more time at the center of visual stimuli. These results support a perceptual-economy account: Fixations are held longer when the eyes are estimated to be at locations in words/stimuli in which greater amounts of information are anticipated. Implications for eye movements in reading are discussed.     

Eye movements during scene viewing: Evidence for mixed control of fixation durations

Recent behavioral and computational research on eye movement control during scene viewing has focused on where the eyes move. However, fixations also differ in their durations, and when the eyes move may be another important indicator of perceptual and cognitive activity. Here we used a scene onset delay paradigm to investigate the degree to which individual fixation durations are under direct moment-to-moment control of the viewer’s current visual scene. During saccades just prior to critical fixations, the scene was removed from view so that when the eyes landed, no scene was present. Following a manipulated delay period, the scene was restored to view. We found that one population of fixations was under the direct control of the current scene, increasing in duration as delay increased. A second population of fixations was relatively constant across delay. The pattern of data did not change whether delay duration was random or blocked, suggesting that the effects were not under the strategic control of the viewer. The results support a mixed control model in which the durations of some fixations proceed regardless of scene presence, whereas others are under the direct moment-to-moment control of ongoing scene analysis.     

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.     

Influence of a psychological perspective on scene viewing and memory for scenes

In the study, 33 participants viewed photographs from either a potential homebuyer's or a burglar's perspective, or in preparation for a memory test, while their eye movements were recorded. A free recall and a picture recognition task were performed after viewing. The results showed that perspective had rapid effects, in that the second fixation after the scene onset was more likely to land on perspective-relevant than on perspective-irrelevant areas within the scene. Perspective-relevant areas also attracted longer total fixation time, more visits, and longer first-pass dwell times than did perspective-irrelevant areas. As for the effects of visual saliency, the first fixation was more likely to land on a salient than on a nonsalient area; salient areas also attracted more visits and longer total fixation time than did nonsalient areas. Recall and recognition performance reflected the eye fixation results: Both were overall higher for perspective-relevant than for perspective-irrelevant scene objects. The relatively low error rates in the recognition task suggest that participants had gained an accurate memory for scene objects. The findings suggest that the role of bottom-up versus top-down factors varies as a function of viewing task and the time-course of scene processing.     

Influence of a psychological perspective on scene viewing and memory for scenes

In the study, 33 participants viewed photographs from either a potential homebuyer's or a burglar's perspective, or in preparation for a memory test, while their eye movements were recorded. A free recall and a picture recognition task were performed after viewing. The results showed that perspective had rapid effects, in that the second fixation after the scene onset was more likely to land on perspective-relevant than on perspective-irrelevant areas within the scene. Perspective-relevant areas also attracted longer total fixation time, more visits, and longer first-pass dwell times than did perspective-irrelevant areas. As for the effects of visual saliency, the first fixation was more likely to land on a salient than on a nonsalient area; salient areas also attracted more visits and longer total fixation time than did nonsalient areas. Recall and recognition performance reflected the eye fixation results: Both were overall higher for perspective-relevant than for perspective-irrelevant scene objects. The relatively low error rates in the recognition task suggest that participants had gained an accurate memory for scene objects. The findings suggest that the role of bottom-up versus top-down factors varies as a function of viewing task and the time-course of scene processing.     

Perception onset time during fixations in free viewing

In this study, we investigated when visual perception begins in fixations. During picture viewing, the picture was degraded at the beginning of selected saccades and changed back to the original after varying intervals. Participants manually responded whenever they detected changes. The change-backs were undetected when they occurred <6 msec after the end of the saccade, marked by the peak of the overshoot in dual Purkinje image eyetracker data, and detection reached asymptote 32 msec after that marker. Eye velocity at the change-backtime also affected detection likelihood. Apparently, perception begins around the time at which the eyes stop rotating at the end of a saccade, giving a psychological justification for measuring fixation durations from then. This also specifies the deadline for gaze-contingent display changes to occur without detectable image motion. Investigators using the dual PurMnje image eyetracker should consider the peak of the overshoot as the fixation onset time and measure intrafixational presentation times from then.     

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.     

Expertise in pictorial perception: eye-movement patterns and visual memory in artists and laymen

In two sessions with free scanning and memory instructions, eye-movement patterns from nine artists were compared with those of nine artistically untrained participants viewing 16 pictures representing a selection of categories from ordinary scenes to abstraction: 12 pictures were made to accommodate an object-oriented viewing mode (selection of recognisable objects), and a pictorial viewing mode (selection of more structural features), and 4 were abstract. The artistically untrained participants showed preference for viewing human features and objects, while the artists spent more scanning time on structural/abstract features. A group by session interaction showed a change of viewing strategy in the artists, who viewed more objects and human features in the memory task session. A verbal test of recall memory showed no overall difference in the number of pictures remembered, but the number of correctly remembered pictorial features was significantly higher for artists than for the artistically untrained viewers, irrespective of picture type. No differences in fixation frequencies/durations were found between groups across sessions, but a significant task-dependent-group by session interaction of fixation frequency/duration showed that the artistically untrained participants demonstrated repetition effects in fewer, longer fixations with repeated viewing, while the opposite pattern obtained for the artists.     

The role of eye fixations in concentration and amplification effects during multiple object tracking

When tracking spatially extended objects in a multiple object tracking task, attention is preferentially directed to the centres of those objects (attentional concentration), and this effect becomes more pronounced as object length increases (attentional amplification). However, it is unclear whether these effects depend on differences in attentional allocation or differences in eye fixations. We addressed this question by measuring eye fixations in a dual-task paradigm that required participants to track spatially extended objects, while simultaneously detecting probes that appeared at the centres or near the endpoints of objects. Consistent with previous research, we observed concentration and amplification effects: Probes at the centres of objects were detected more readily than those near their endpoints, and this difference increased with object length. Critically, attentional concentration was observed when probes were equated for distance from fixation during free viewing, and concentration and amplification were observed without eye movements during strict fixation. We conclude that these effects reflect the prioritization of covert attention to particular spatial regions within extended objects, and we discuss the role of eye fixations during multiple object tracking.     

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.     

Spatial frequency filtering and the direct control of fixation durations during scene viewing

The present study employed a saccade-contingent change paradigm to investigate the effect of spatial frequency filtering on fixation durations during scene viewing. Subjects viewed grayscale scenes while encoding them for a later memory test. During randomly chosen saccades, the scene was replaced with an alternate version that remained throughout the critical fixation that followed. In Experiment 1, during the critical fixation, the scene could be changed to high-pass and low-pass spatial frequency filtered versions. Under both conditions, fixation durations increased, and the low-pass condition produced a greater effect than the high-pass condition. In subsequent experiments, we manipulated the familiarity of scene information during the critical fixation by flipping the filtered scenes upside down or horizontally. Under these conditions, we observed lengthening of fixation durations but no difference between the high-pass and low-pass conditions, suggesting that the filtering effect is related to the mismatch between information extracted within the critical fixation and the ongoing scene representation in memory. We also conducted control experiments that tested the effect of changes to scene orientation (Experiment 2a) and the addition of color to a grayscale scene (Experiment 2b). Fixation distribution analysis suggested two effects on the distribution fixation durations: a fast-acting effect that was sensitive to all transsaccadic changes tested and a later effect in the tail of the distribution that was likely tied to the processing of scene information. These findings are discussed in the context of theories of oculomotor control during scene viewing.     

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.     

Covert visual attention and extrafoveal information use during object identification

Three experiments are reported that examined the relationship between covert visual attention and a viewer's ability to use extrafoveal visual information during object identification. Subjects looked at arrays of four objects while their eye movements were recorded. Their task was to identify the objects in the array for an immediate probe memory test. During viewing, the number and location of objects visible during given fixations were manipulated. In Experiments 1 and 2, we found that multiple extrafoveal previews of an object did not afford any more benefit than a single extrafoveal preview, as assessed by means of time of fixation on the objects. In Experiment 3, we found evidence for a model in which extrafoveal information acquired during a fixation derives primarily from the location toward which the eyes will move next. The results are discussed in terms of their implications for the relationship between covert visual attention and extrafoveal information use, and a sequential attention model is proposed.     

汉字间空格大小对青年人和老年人阅读的影响：眼动研究

拼音文字阅读的眼动研究发现,老年人会采取一种“风险”阅读策略来弥补因视力和认知的自然老化所造成的阅读困难。本研究通过控制字间空格的大小对青年人和老年人汉语阅读过程的眼动模式进行了比较,结果发现,相比青年人,老年人读得更慢,有更长的注视时间和更多的回视,这些研究结果与拼音文字一致;但更为重要的发现是,相比青年人,老年人有更短的向前眼跳距离,缩小汉语字间空格给老年人阅读造成了更为显著的困难,但扩大字间空格两组被试表现出相似的阅读眼动模式。结果说明,汉语阅读中老年人并不使用拼音文字研究中发现的“风险”阅读策略,反而采用了更为谨慎的策略,原因可能在于,作为拼音文字阅读中的重要视觉线索,空格是影响两种语言下老年人阅读加工策略不同的重要因素。     

Recognition memory strength is predicted by pupillary responses at encoding while fixation patterns distinguish recollection from familiarity

Thirty-five healthy participants incidentally encoded a set of man-made and natural object pictures, while their pupil response and eye movements were recorded. At retrieval, studied and new stimuli were rated as novel, familiar (strong, moderate, or weak), or recollected. We found that both pupil response and fixation patterns at encoding predict later recognition memory strength. The extent of pupillary response accompanying incidental encoding was found to be predictive of subsequent memory. In addition, the number of fixations was also predictive of later recognition memory strength, suggesting that the accumulation of greater visual detail, even for single objects, is critical for the creation of a strong memory. Moreover, fixation patterns at encoding distinguished between recollection and familiarity at retrieval, with more dispersed fixations predicting familiarity and more clustered fixations predicting recollection. These data reveal close links between the autonomic control of pupil responses and eye movement patterns on the one hand and memory encoding on the other. Moreover, the data illustrate quantitative as well as qualitative differences in the incidental visual processing of stimuli, which are differentially predictive of the strength and the kind of memory experienced at recognition.     

Recognition memory strength is predicted by pupillary responses at encoding while fixation patterns distinguish recollection from familiarity

Thirty-five healthy participants incidentally encoded a set of man-made and natural object pictures, while their pupil response and eye movements were recorded. At retrieval, studied and new stimuli were rated as novel, familiar (strong, moderate, or weak), or recollected. We found that both pupil response and fixation patterns at encoding predict later recognition memory strength. The extent of pupillary response accompanying incidental encoding was found to be predictive of subsequent memory. In addition, the number of fixations was also predictive of later recognition memory strength, suggesting that the accumulation of greater visual detail, even for single objects, is critical for the creation of a strong memory. Moreover, fixation patterns at encoding distinguished between recollection and familiarity at retrieval, with more dispersed fixations predicting familiarity and more clustered fixations predicting recollection. These data reveal close links between the autonomic control of pupil responses and eye movement patterns on the one hand and memory encoding on the other. Moreover, the data illustrate quantitative as well as qualitative differences in the incidental visual processing of stimuli, which are differentially predictive of the strength and the kind of memory experienced at recognition.     

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.