Visual motion shifts saccade targets

Saccades are made thousands of times a day and are the principal means of localizing objects in our environment. However, the saccade system faces the challenge of accurately localizing objects as they are constantly moving relative to the eye and head. Any delays in processing could cause errors in saccadic localization. To compensate for these delays, the saccade system might use one or more sources of information to predict future target locations, including changes in position of the object over time, or its motion. Another possibility is that motion influences the represented position of the object for saccadic targeting, without requiring an actual change in target position. We tested whether the saccade system can use motion-induced position shifts to update the represented spatial location of a saccade target, by using static drifting Gabor patches with either a soft or a hard aperture as saccade targets. In both conditions, the aperture always remained at a fixed retinal location. The soft aperture Gabor patch resulted in an illusory position shift, whereas the hard aperture stimulus maintained the motion signals but resulted in a smaller illusory position shift. Thus, motion energy and target location were equated, but a position shift was generated in only one condition. We measured saccadic localization of these targets and found that saccades were indeed shifted, but only with a soft-aperture Gabor patch. Our results suggest that motion shifts the programmed locations of saccade targets, and this remapped location guides saccadic localization.     

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.     

Perceptual localization of visual stimuli flashed during saccades

Subjects were asked to make a saccade to a visual target flashed in the dark during a prior primary saccade, and to report its apparent position by moving an adjustable light spot to that position. When targets were presented at the beginning of the primary saccade, subjects perceptually mislocated them in the direction of the saccade, whereas when targets were presented immediately before the end of the primary saccade, the flashed targets were mislocated in the opposite direction. The perceptually localized position of the target was primarily determined by its retinal position. However, at all actual and retinal positions of the target, the localized position shifted from the position that would be predicted if the location of the target was determined only by its retinal position to the prior primary saccade direction. The results were discussed in relation to extraretinal eye position signals. Subjects moved their eyes not to the actual position of the target, but to its apparent position. In some trials, there was a discrepancy between perceptual and oculomotor localization, which was interpreted as having been caused by the imprecise localization ability of the oculomotor system.     

Localization of targets across saccades: Role of landmark objects

Saccadic eye movements are required to bring different parts of the visual world into the foveal region of the retina. With each saccade, the images of the objects drastically change their retinal positions—nevertheless, the visual world appears continuous and does not seem to jump. How does the visual system achieve this continuous and stable percept of the visual world, despite the gross changes of its retinal projection that occur with each saccade? The present paper argues that an important factor of this type of space constancy is formed by the reafferent information, i.e., the visual display that is found when the eyes land. Three experiments demonstrate that objects present across the saccade can serve as landmarks for postsaccadic relocalization. The basic experimental manipulation consisted of a systematic displacement of these landmark objects during the saccade. The effectiveness of the landmarks was determined by analysing to what degree they modify the perceived shift of a small saccade target that was blanked for 200 ms during and after the saccade. A first experiment studied the spatial range where objects become effective as landmarks. The data show that landmarks close to the saccade target and horizontally aligned with the target are specifically effective. The second experiment demonstrates that postsaccadic localization is normally based on relational information about relative stimulus positions transferred across the saccade. A third experiment studied the effect of a prominent background frame on transsaccadic localization; the results suggest that background structures contribute only little to transsaccadic localization.     

Saccades reveal that allocentric coding of the moving object causes mislocalization in the flash-lag effect

The flash-lag effect is a visual misperception of a position of a flash relative to that of a moving object: Even when both are at the same position, the flash is reported to lag behind the moving object. In the present study, the flash-lag effect was investigated with eye-movement measurements: Subjects were required to saccade to either the flash or the moving object. The results showed that saccades to the flash were precise, whereas saccades to the moving object showed an offset in the direction of motion. A further experiment revealed that this offset in the saccades to the moving object was eliminated when the whole background flashed. This result indicates that saccadic offsets to the moving stimulus critically depend on the spatially distinctive flash in the vicinity of the moving object. The results are incompatible with current theoretical explanations of the flash-lag effect, such as the motion extrapolation account. We propose that allocentric coding of the position of the moving object could account for the flash-lag effect.     

The influence of saccade length on the saccadic suppression of displacement detection

The decrease in sensitivity to spatial displacement which accompanies a voluntary horizontal saccadic eye movement was measured as a function of the length of the saccade. Threshold for detecting the displacement increased linearly from about 0.3 degrees to 1.2 degrees as saccade length increased from 4 degrees to 12 degrees. The variability (standard deviation) of the discrimination increased linearly with saccade length as well, and hence also linearly with the displacement threshold. These results, along with our previous finding that the increase is not a consequence of the saccadically generated spatiotemporal smearing of the retinal image (Li & Matin, 1990), support the proposal that displacement detection is based on a constant internal signal/noise ratio whose denominator is a measure of the variability of the extraretinal signal regarding eye position, and that the reduction in sensitivity is a result of a transient increase of this variability in the temporal neighborhood of a saccade.     

Visual perception of direction when voluntary saccades occur. I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade

In experiments designed to clarify the mechanisms underlying the normal stability of visual direction for stationary objects when voluntary saccades occur, Ss reported on the horizontal visual direction of a brief test [lash presented when the eye was at a specific point in the saccade (the trigger point) relative to a fixation target viewed and extinguished prior to the saccade. From these reports, PSEs (points of subjective equality) were calculated for the fixation target as measured by the test [lashes. The distance of the trigger point from the previous fixation position was systematically varied in each experiment. Different experiments required saccades of different lengths and directions. With the exception of the presentation of the test [lash the saccades were carried out in complete darkness so that the possible utilization of an extraretinal signal regarding the eye movement (change in eye position, the intention to turn the eye, or a change of attention related to the eye movement) in the determination of visual direction could be observed uncomplicated by a continuing visual context. According to classical theories, an extraretinal signal proportional to the change in eye position acts to maintain direction constancy by compensating for the Shift of the retinal image resulting from the movement of the eye. In general, direction constancy was not preserved in the present experiments, and thus the data would not be predicted by classical theories. However, the PSE varied with distance of the trigger point from the fixation target. Since this displacement of PSE from the trigger point was in the correct direction for compensation, the presence of an extraretinal signal was confirmed. However, the growth of this signal appears to be time-locked to the saccade rather than locked to eye position; it is suggested that this growth takes place over a time period which is longer than the duration of the saccade itself.     

Saccadic suppression of displacement is strongest in central vision

Perception of target displacement is severely degraded if the displacement occurs during a saccadic eye movement, but the variation of this effect across the visual field is unknown. A small target was displaced from a starting point at the midline, or 10 deg to the right or left, while the eye made a saccade from the 10 deg right position to the 10 deg left position. Saccades were detected and the target displaced on line. Assessed with a signal detection measure, suppression was stronger in central vision than in more peripheral locations for all three subjects. Leftward and rightward displacements yielded equal thresholds. The results complement the findings of others to reveal a picture of perceptual events during saccades, with both deeper saccadic suppression and faster correction of spatial values (the correspondences between retinal position and perceived egocentric direction), favouring more accurate spatial processing in central vision than in the periphery.     

A predictive coding model of gaze shifts and the underlying neurophysiology

A comprehensive model of gaze control must account for a number of empirical observations at both the behavioural and neurophysiological levels. The computational model presented in this article can simulate the coordinated movements of the eye, head, and body required to perform horizontal gaze shifts. In doing so it reproduces the predictable relationships between the movements performed by these different degrees of freedom (DOFs) in the primate. The model also accounts for the saccadic undershoot that accompanies large gaze shifts in the biological visual system. It can also account for our perception of a stable external world despite frequent gaze shifts and the ability to perform accurate memory-guided and double-step saccades. The proposed model also simulates peri-saccadic compression: the mis-localization of a briefly presented visual stimulus towards the location that is the target for a saccade. At the neurophysiological level, the proposed model is consistent with the existence of cortical neurons tuned to the retinal, head-centred, body-centred, and world-centred locations of visual stimuli and cortical neurons that have gain-modulated responses to visual stimuli. Finally, the model also successfully accounts for peri-saccadic receptive field (RF) remapping which results in reduced responses to stimuli in the current RF location and an increased sensitivity to stimuli appearing at the location that will be occupied by the RF after the saccade. The proposed model thus offers a unified explanation for this seemingly diverse range of phenomena. Furthermore, as the proposed model is an implementation of the predictive coding theory, it offers a single computational explanation for these phenomena and relates gaze shifts to a wider framework for understanding cortical function.     

Visual perception of direction when voluntary saccades occur: II. Relation of visual direction of a fixation target extinguished before a saccade to a subsequent test flash presented before the saccade

Previously reported experiments demonstrated changes in the relation of visual direction to retinal locus for stimulation during voluntary saccades as compared to this relation before saccade initiation. The quantitative features of these results led to the prediction, confirmed in the present experiments, that there are shifts in visual direction for stimulation presented before the saccade itself. In the present report, monotonically increasing shifts were mapped with stimuli presented as early as 240 msec before the saccade up to the saccade itself. Such shifts cannot be accounted for readily by “inflowing” processes, and while “outflowing” processes seem to be implicated, their quantitative characteristics would need to be considerably different from those required by classical outflow theories.     

Saccadic eye movements and localization of visual stimuli

Visual localization phenomena were studied before, during, and after a saccade. Light flashes of 5 and 9 msec duration presented before and during the eye movement were mislocated in the saccade direction, the localization error being a time function. When the 9-msec duration stimulus and saccade did not overlap in time, a stripe was reported, when they did not, the stimulus was perceived as a point. If a long-duration stimulus moved perpendicularly to the saccade direction with the same “sigmoidal” velocity, a curvilinear trace was perceived, regardless of the linear trace of the image on the retina. A stimulus with stabilized retinal image was perceived as a stationary point during the saccade. A possible theory to deal with the data was suggested by modifying the algebra of outflow-inflow theories.     

Pursuit compensation during self-motion

The pattern of motion in the retinal image during self-motion contains information about the person's movement. Pursuit eye movements perturb the pattern of retinal-image motion, complicating the problem of self-motion perception. A question of considerable current interest is the relative importance of retinal and extra-retinal signals in compensating for these effects of pursuit on the retinal image. We addressed this question by examining the effect of prior motion stimuli on self-motion judgments during pursuit. Observers viewed 300 ms random-dot displays simulating forward self-motion during pursuit to the right or to the left; at the end of each display a probe appeared and observers judged whether they would pass left or right of it. The display was preceded by a 300 ms dot pattern that was either stationary or moved in the same direction as, or opposite to, the eye movement. This prior motion stimulus had a large effect on self-motion judgments when the simulated scene was a frontoparallel wall (experiment 1), but not when it was a three-dimensional (3-D) scene (experiment 2). Corresponding simulated-pursuit conditions controlled for purely retinal motion aftereffects, implying that the effect in experiment 1 is mediated by an interaction between retinal and extra-retinal signals. In experiment 3, we examined self-motion judgments with respect to a 3-D scene with mixtures of real and simulated pursuit. When real and simulated pursuits were in opposite directions, performance was determined by the total amount of pursuit-related retinal motion, consistent with an extra-retinal 'trigger' signal that facilitates the action of a retinally based pursuit-compensation mechanism. However, results of experiment 1 without a prior motion stimulus imply that extra-retinal signals are more informative when retinal information is lacking. We conclude that the relative importance of retinal and extra-retinal signals for pursuit compensation varies with the informativeness of the retinal motion pattern, at least for short durations. Our results provide partial explanations for a number of findings in the literature on perception of self-motion and motion in the frontal plane.     

Cast shadow can modulate the judged final position of a moving target

Observers tend to localize the final position of a suddenly vanished moving target farther along in the direction of the target motion (representational momentum). We report here that such localization errors are mediated by perceived motion rather than by retinal motion. By manipulating the cast shadow of a moving target, we induced illusory motion to a target stimulus while keeping the retinal motion constant. Participants indicated the vanishing point of the target by directing a mouse cursor. The resulting magnitude of localization errors was modulated on the basis of the induced direction of the target. Such systematic localization biases were not obtained in a control condition in which the motion paths of the ball and shadow were switched. Our results suggest that cues to object motion trajectory, such as cast shadows, are used for the localization task, supporting a view that a predictive mechanism is responsible for the production of localization errors.     

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.     

Motion and position shifts induced by the double-drift stimulus are unaffected by attentional load

The double-drift stimulus produces a strong shift in apparent motion direction that generates large errors of perceived position. In this study, we tested the effect of attentional load on the perceptual estimates of motion direction and position for double-drift stimuli. In each trial, four objects appeared, one in each quadrant of a large screen, and they moved upward or downward on an angled trajectory. The target object whose direction or position was to be judged was either cued with a small arrow prior to object motion (low attentional load condition) or cued after the objects stopped moving and disappeared (high attentional load condition). In Experiment 1, these objects appeared 10° from the central fixation, and participants reported the perceived direction of the target’s trajectory after the stimulus disappeared by adjusting the direction of an arrow at the center of the response screen. In Experiment 2, the four double-drift objects could appear between 6 ° and 14° from the central fixation, and participants reported the location of the target object after its disappearance by moving the position of a small circle on the response screen. The errors in direction and position judgments showed little effect of the attentional manipulation—similar errors were seen in both experiments whether or not the participant knew which double-drift object would be tested. This suggests that orienting endogenous attention (i.e., by only attending to one object in the precued trials) does not interact with the strength of the motion or position shifts for the double-drift stimulus.     

Quantifying the interactions between allo- and egocentric representations of space

Under many circumstances, humans do not judge the location of objects in space where they really are. For instance, when a background is added to a target object, the judged position of a target with respect to oneself (egocentric position) is shifted in the opposite direction as the placement of such a background with respect to the body midline. It is an ongoing debate whether such effects are due to a uni- or bi-directional interaction between allo- and egocentric spatial representations in the brain, or reflect a response strategy, known as the perceived midline shift. In this study, the effects of allocentric stimulus coordinates on perceived egocentric position were examined more precisely and in a quantitative manner. Furthermore, it was investigated whether the judged allocentric position (with respect to a background) is also influenced by the egocentric position in space of that object. Allo- and egocentric coordinates were varied independently. Also, the effect of background luminance on the observed interactions between spatial coordinates was determined. Since background luminance had an effect on the size of the interaction between allocentric stimulus coordinates and egocentric judgments, and no reverse interaction was found, it seems that interactions between ego- and allocentric reference frames is most likely only unidirectional, with the latter affecting the former. This interaction effect was described in a quantitative manner.     

Different effects of eyelid blinks and target blanking on saccadic suppression of displacement

Displacements of visual stimuli during saccadic eye movements are often not noticed. We have demonstrated that saccadic suppression of image displacement can be eliminated by blanking the stimulus for a short period during and after the saccade (Deubel, Schneider, & Bridgeman, 1996). Here we report an experiment in which target visibility was interrupted after the saccade, either by distal target blanking or by voluntary eyeblink. The data show that the effect of blinking is different from blanking; interruption of vision due to a blink did not enable subjects to detect target displacements any better than they had done in the no-blank condition. The results provide evidence for an extraretinal signal that distinguishes between endogenous and exogenous sources of temporary object disappearance after the saccade.     

Transsaccadic integration of bystander locations

The present study investigated whether and how the location of bystander objects is encoded, maintained, and integrated across an eye movement. Bystander objects are objects that remain unfixated directly before and after the saccade for which transsaccadic integration is being examined. Three experiments are reported that examine location coding of bystander objects relative to the future saccade target object, relative to the saccade source object, and relative to other bystander objects. Participants were presented with a random‐dot pattern and made a saccade from a central source to a designated saccade target. During this saccade the position of a single bystander was changed on half of the trials and participants had to detect the displacement. Postsaccadically the presence of the target, source, and other bystanders was manipulated. Results indicated that the location of bystander objects could be integrated across a saccade, and that this relied on configurational coding. Furthermore the present data provide evidence for the view that transsaccadic perception of spatial layout is not inevitably tied to the saccade target or the saccade source, that it makes use of objects and object configurations in a flexible manner that is partly governed by the task relevance of the various display items, and that it exploits the incidental configurational structure in the display's layout in order to increase its capacity limits.     

Environmental invariants in the representation of motion: Implied dynamics and representational momentum,gravity, friction,and centripetal force

Memory for the final position of a moving target is often shifted or displaced from the true final position of that target. Early studies of this memory shift focused on parallels between the momentum of the target and the momentum of the representation of the target and called this displacementrepresentational momentum, but many factors other than momentum contribute to the memory shift. A consideration of the empirical literature on representational momentum and related types of displacement suggests there are at least four different types of factors influencing the direction and magnitude of such memory shifts: stimulus characteristics (e.g., target direction, target velocity), implied dynamics and environmental invariants (e.g., implied momentum, gravity, friction, centripetal force), memory averaging of target and nontarget context (e.g., biases toward previous target locations or nontarget context), and observers’ expectations (both tacit and conscious) regarding future target motion and target/context interactions. Several theories purporting to account for representational momentum and related types of displacement are also considered.