Low-level predictive inference in reading: the influence of transitional probabilities on eye movements

We report the results of an investigation into the ability of transitional probability (word-to-word contingency statistics) to account for reading behaviour. Using a corpus of eye movements recorded during the reading of newspaper text, we demonstrate both the forward [P(n/n-1)] and backward [P(n/n+1)] transitional probability measures to be predictive of first fixation and gaze durations: the higher the transitional probability, the shorter the fixation time. Initial fixation position was also affected by the forward measure; we observed a small rightward shift for words that were highly predictable from the preceding word. Although transitional probability is sensitive to word class, with function words being generally more predictable from their context than content words, the measures accounted equally well for the data for both classes.     

Semantic guidance of eye movements in real-world scenes

The perception of objects in our visual world is influenced by not only their low-level visual features such as shape and color, but also their high-level features such as meaning and semantic relations among them. While it has been shown that low-level features in real-world scenes guide eye movements during scene inspection and search, the influence of semantic similarity among scene objects on eye movements in such situations has not been investigated. Here we study guidance of eye movements by semantic similarity among objects during real-world scene inspection and search. By selecting scenes from the LabelMe object-annotated image database and applying latent semantic analysis (LSA) to the object labels, we generated semantic saliency maps of real-world scenes based on the semantic similarity of scene objects to the currently fixated object or the search target. An ROC analysis of these maps as predictors of subjects’ gaze transitions between objects during scene inspection revealed a preference for transitions to objects that were semantically similar to the currently inspected one. Furthermore, during the course of a scene search, subjects’ eye movements were progressively guided toward objects that were semantically similar to the search target. These findings demonstrate substantial semantic guidance of eye movements in real-world scenes and show its importance for understanding real-world attentional control.     

On Pulfrich-illusion eye movements and accommodation vergence during visual pursuit

When the Pulfrich illusion is perceived with stationary fixation, and visual pursuit of the pendulum is then initiated, rapid vergence changes occur which correspond to the illusory elliptical path. During steady-state visual tracking of the illusion, however, the eyes move along a planar path without systematic changes in vergence. These latter pursuit movements with monocular filter involve large fixation disparities relative to unobstructed vision (0.5 degree to 1 degree divergence); hence, it is proposed that the planar tracking path probably results from strong dominance of the oculomotor system by stimuli from the unobstructed eye. During visual tracking with monocular filter and a target moving along a nonillusory elliptical path in depth, appropriate changes in vergence occur, but comparable vergence changes also arise when the target is fully hidden from one eye. This response apparently represents a superposition of accommodation vergence upon smooth pursuit movements; similar responses also occur during monocular tracking of a target moving around a circular path in depth.     

The central fixation bias in scene viewing: selecting an optimal viewing position independently of motor biases and image feature distributions

Observers show a marked tendency to fixate the center of the screen when viewing scenes on computer monitors. This is often assumed to arise because image features tend to be biased toward the center of natural images and fixations are correlated with image features. A common alternative explanation is that experiments typically use a central pre-trial fixation marker, and observers tend to make small amplitude saccades. In the present study, the central bias was explored by dividing images post hoc according to biases in their image feature distributions. Central biases could not be explained by motor biases for making small saccades and were found irrespective of the distribution of image features. When the scene appeared, the initial response was to orient to the center of the screen. Following this, fixation distributions did not vary with image feature distributions when freely viewing scenes. When searching the scenes, fixation distributions shifted slightly toward the distribution of features in the image, primarily during the first few fixations after the initial orienting response. The endurance of the central fixation bias irrespective of the distribution of image features, or the observer's task, implies one of three possible explanations: First, the center of the screen may be an optimal location for early information processing of the scene. Second, it may simply be that the center of the screen is a convenient location from which to start oculomotor exploration of the scene. Third, it may be that the central bias reflects a tendency to re-center the eye in its orbit.     

The attenuation of perceived motion smear during combined eye and head movements

The extent of perceived motion smear was compared for targets that underwent similar velocities of retinal image motion during the vestibulo-ocular reflex (VOR) in the dark, the visually enhanced VOR (VVOR), VOR suppression, and fixation. Compared to the extent of perceived motion smear during fixation, observers reported significantly less smear when the target moved either in the same direction or against the direction of the head movement during the VVOR and VOR. We also confirmed a previous finding that perceived smear is attenuated asymmetrically during VOR suppression, with attenuation occurring primarily for targets that move against the direction of the observer's head motion. The results support the hypothesis that the visual system employs extra-retinal signals that accompany eye and head movements to reduce the perception of motion smear for targets that move physically in the opposite direction of eye and/or head movements.     

Spontaneous eye movements in goldfish: oculomotor integrator performance, plasticity, and dependence on visual feedback

To quantify performance of the goldfish oculomotor neural integrator and determine its dependence on visual feedback, we measured the relationship between eye drift-velocity and position during spontaneous gaze fixations in the light and in the dark. In the light, drift-velocities were typically less than 1 deg/s, similar to those observed in humans. During brief periods in darkness, drift-velocities were only slightly larger, but showed greater variance. One hour in darkness degraded fixation-holding performance. These findings suggest that while visual feedback is not essential for online fixation stability, it may be used to tune the mechanism of persistent neural activity in the oculomotor integrator.     

Visually evoked eye movements in the mouse (Mus musculus)

Visually evoked eye movements were observed and measured in one inbred strain of the mouse (C57BL/6J) and in one inter-strain hybrid. This was done by observing pupillary movements while a mouse was restrained inside of a test apparatus that presented the mouse with slowly moving stripes and allowed the experimenter to view the mouse's pupil using i.r. illumination. Slow eye movements (smooth pursuits) occurred only in the direction of drum rotation. Rapid eye movements (saccades) occurred only in the opposite direction. The eye position often appeared to follow a fixed location on the drum while the direction of drum rotation was being reversed. Eye movements did not occur when the stripes were rotated in the dark. Mutants having retinal degeneration (C57BL/6Jrd le/rd le) had oscillatory eye movements independent of visual stimulus luminance or motion. Hybrid animals performed more frequent following movements than inbred animals. It is concluded thatM. musculus has visually-evoked eye movements, including the optokinetic nystagmus, and that the test described is suited to screen mice for subtle hereditary defects in vision.     

Dynamics in longitudinal eye movements and corneal shape

The magnitude and character of longitudinal movements of the eye were studied in the context of high speed videokeratoscopy. It was of interest to determine whether these dynamic changes in the eye movements can affect the corneal shape and its estimation. A high speed videokeratoscope with a sampling frequency of 50 Hz was used for measuring the corneal apex movements as well as for measuring variations in the best-fit sphere radius and central radius of curvature. The magnitude of the measured longitudinal apex movements could reach over 200 microm showing a slow trend. The estimated local changes of the apex movements that could be associated with the cardiopulmonary system were about 40-50 microm. The temporal variations in the equivalent estimated central radius of curvature ranged between 10 and 15 microm. Spectral analysis of the longitudinal eye movements revealed strong signatures of the pulse and respiration signals as well as the assumed blink control signal. No clear association between the longitudinal apex movements and the corneal curvature was found. However, very slow significant changes in the corneal shape were observed. The central radius of curvature of the cornea revealed slow changes of up to 120 microm. Understanding the nature of such changes will be of benefit in ophthalmic applications requiring highly accurate measurements of corneal shape, such as contact lens design and refractive surgery.     

Isometric force development in human horizontal eye muscles and pulleys during saccadic eye movements

Purpose: The connective tissue elements forming the check ligaments and portals of the human eye muscles have recently been ascribed with a pulley function. Active positioning of the pulleys over orbital layer contraction during eye movements has been suggested. Other studies have instead demonstrated fibrous tissue connections between all parts of the muscle and the pulleys. We aimed to compare the isometric force developed at the muscle tendon and at the pulleys of the horizontal eye muscles, and to investigate which eye muscle structures might exert force on the pulleys. Methods: Isometric force development was recorded from the lateral and medial rectus muscles in six patients operated for strabismus under topical anaesthesia. Two strain gauge probes were used, each attached with 5–0 silk sutures either to the muscle tendon or to the pulley. The eye muscles were activated by horizontal saccadic eye movements in steps from 30 degrees in the off‐direction to 30 degrees in the on‐direction of the muscles. Results: The forces developed at the tendon and pulley were almost identical with respect to amplitude and other parameters. No differences were found in forces developed at the pulleys of the medial and lateral rectus muscles. Conclusions: The results support the presence of fibrous tissue connections between all eye muscle fibres and pulley structures, rather than orbital fibre control of the pulley.     

Studies of corneas in vivo and in vitro

We present our step-by-step procedure for the preparation of rabbit and human corneal endothelial cells in culture. We also compare biochemical differences between rabbit and human corneal stromal and endothelial cells in vivo and in vitro.     

早中期原发性开角型青光眼固视行为的改变

目的 应用微视野计MP-1研究早中期原发性开角型青光眼(primary open angle glaucoma,POAG)固视行为的改变.方法 前瞻性的筛选39只早中期POAG眼及22只正常眼进行微视野10°黄斑程序的检查.在MP-1检查黄斑光敏感度(mean sensitivity,MS)的过程中自动记录受试者的固视稳定性及固视位置的变化.MP-1结果的参数包括中央10°、6°、2°以及中央6°的鼻上、颞上、颞下和鼻下四象限的平均MS.比较POAG眼及正常眼的各MP-1参数及大视行为,并分析POAG眼MP-1各参数与固视稳定性之间的关系.结果 POAG与正常眼相比固视稳定性的下降有统计学意义(P=0.016),与固视稳定性相关的MP-1参数包括中央10°(r=0.416,P=0.008),6°(r=0.401,P=0.011),2°(r=0.376,P=0.018),中央6°的颞下象限(r=0.399,P=0.012)和鼻下象限(r=0.387,P=0.015)的平均MS.POAG眼的固视位置均为显著中心固视.结论 早中期POAG与正常对照组相比固视稳定性明显下降.固视行为的改变可能是POAG视神经病变的一个早期征兆.     

Micropursuit and the control of attention and eye movements in dynamic environments

It is more challenging to plan eye movements during perceptual tasks performed in dynamic displays than in static displays. Decisions about the timing of saccades become more critical, and decisions must also involve smooth eye movements, as well as saccades. The present study examined eye movements when judging which of two moving discs would arrive first, or collide, at a common meeting point. Perceptual discrimination after training was precise (Weber fractions < 6%). Strategies reflected a combined contribution of saccades and smooth eye movements. The preferred strategy was to look near the meeting point when strategies were freely chosen. When strategies were assigned, looking near the meeting point produced better performance than switching between the discs. Smooth eye movements were engaged in two ways: (a) low-velocity smooth eye movements correlated with the motion of each disc (micropursuit) were found while the line of sight remained between the discs; and (b) spontaneous smooth pursuit of the pair of discs occurred after the perceptual report, when the discs moved as a pair along a common path. The results show clear preferences and advantages for those eye movement strategies during dynamic perceptual tasks that require minimal management or effort. In addition, smooth eye movements, whose involvement during perceptual tasks within dynamic displays may have previously escaped notice, provide useful indictors of the strategies used to select information and distribute attention during the performance of dynamic perceptual tasks.     

The binocular coordination of eye movements during reading in children and adults

Recent evidence indicates that each eye does not always fixate the same letter during reading and there has been some suggestion that processing difficulty may influence binocular coordination. We recorded binocular eye movements from children and adults reading sentences containing a word frequency manipulation. We found disparities of significant magnitude between the two eyes for all participants, with greater disparity magnitudes in children than adults. All participants made fewer crossed than uncrossed fixations. However, children made a higher proportion of crossed fixations than adults. We found no influence of word frequency on children's fixations and on binocular coordination in adults.     

Speed and accuracy of saccades, vergence and combined eye movements in subjects with strabismus before and after eye surgery

The purpose of the study was to examine spatio-temporal characteristics of horizontal eye movements in the natural space (saccade, vergence and combined movements) in young subjects with early onset convergent or divergent strabismus. Nine young subjects (8-20 years old) were tested: three with divergent strabismus, six with convergent strabismus. A standard paradigm was used to elicit pure horizontal saccades at far and at close viewing distance, pure vergence along the median plane (convergence and divergence) and saccades combined with vergence movements. Horizontal eye movements from both eyes were recorded by a photoelectric device. Eye movements were recorded before surgery, and, for the majority of the subjects, two times after surgery. Before surgery the accuracy of convergence and divergence movements in their pure or combined form was poor with respect to normal values. The mean velocity of convergence was also abnormally slow. Strabismus surgery improved significantly the accuracy of these types of eye movements. The speed of pure convergence and of divergence combined movements increased significantly after surgery. We concluded that poor vergence eye movement’s performance, particularly those found for convergence in strabismic subjects could be due to impairment in the central structures related to sensory disparity inputs. Adaptive mechanisms promoted by the realignment of the eyes could be at the origin of the improvement in the vergence performances observed in our subjects after strabismus eye surgery.     

Visual fields and eye movements in a harbor seal (Phoca vitulina)

The boundaries of the visual fields of a harbor seal were measured using static perimetry. In the seal lying on a plane surface (fixation point "0 degrees" straight ahead at eye-level), the visual field with fixed eyes extended over 208 degrees horizontally and reached from -12 degrees to +69 degrees vertically. The binocular visual field amounted to 67 degrees. Eye movements of 12 degrees (+/-2) degrees to both sides and 64 degrees upwards could be induced. In the seal performing eye movements, a visual field of 210 degrees in the horizontal plane and 121 degrees to the dorsal side was determined. From the measured eye movements, a visual field of 232 degrees in the horizontal plane appears possible.     

Changes in the functional visual field during search with and without eye movements

The size of the functional visual field (FVF) is dynamic, changing with the context and attentive demand that each fixation brings as we move our eyes and head to explore the visual scene. Using performance measures of the FVF we show that during search conditions with eye movements, the FVF is small compared to the size of the FVF measured during search without eye movements. In all cases the size of the FVF is constrained by the density of distracting items. During search without eye movements the FVF expands with time; subjects have idiosyncratic spatial biases suggesting covert shifts of attention. For search within the constraints imposed by item density, the rate of item inspection is the same across all search conditions. Array set size effects are not apparent once stimulus density is taken into account, a result that is consistent with a spatial constraint for the FVF based on the cortical separation hypothesis.     

The Simon effect of spatial words in eye movements: Comparison of vertical and horizontal effects and of eye and finger responses

Spatial stimulus location information impacts on saccades: Pro-saccades (saccades towards a stimulus location) are faster than anti-saccades (saccades away from the stimulus). This is true even when the spatial location is irrelevant for the choice of the correct response (Simon effect). The results are usually ascribed to spatial sensorimotor coupling. However, with finger responses Simon effects can be observed with irrelevant spatial word meaning, too. Here we tested whether a Simon effect of spatial word meaning in saccades could be observed for words with vertical (“above” or “below”) and horizontal (“left” or “right”) meanings. We asked our participants to make saccades towards one of the two saccade targets depending on the color of the centrally presented spatial word, while ignoring their spatial meaning (Experiment 1 and 2a). Results are compared to a condition in which finger responses instead of saccades were required (Experiment 2b). In addition to response latency we compared the time course of vertical and horizontal effects. We found the Simon effects due to irrelevant spatial meaning of the words in both saccades and finger responses. The time course investigations revealed different patterns for vertical and horizontal effects in saccades, indicating that distinct processes may be involved in the two types of Simon effects.     

Unconscious cueing effects in saccadic eye movements - Facilitation and inhibition in temporal and nasal hemifield

The current study investigated whether subliminal spatial cues can affect the oculomotor system. In addition, we performed the experiment under monocular viewing conditions. By limiting participants to monocular viewing conditions, we can examine behavioral temporal-nasal hemifield asymmetries. These behavioral asymmetries may arise from an anatomical asymmetry in the retinotectal pathway. The results show that even though our spatial cues were not consciously perceived they did affect the oculomotor system: relative to the neutral condition, saccade latencies to the validly cued location were shorter and saccade latencies to the invalidly cued location were longer. Although we did not observe an overall inhibition of return effect, there was a reliable effect of hemifield on IOR for those observers who showed an overall IOR effect. More specifically, consistent with the notion that processing via the retinotectal pathway is stronger in the temporal hemifield than in the nasal hemifield we found an IOR effect for cues presented in the temporal hemifield but not for cues presented in the nasal hemifield. We conclude that unconsciously processed spatial cues can affect the oculomotor system. In addition, the observed behavioral temporal-nasal hemifield asymmetry is consistent with retinotectal mediation.