Object recognition during foveating eye movements

The effect of impaired velocity signals on goal-directed eye and hand movements

Information about position and velocity is essential to predict where moving targets will be in the future, and to accurately move towards them. But how are the two signals combined over time to complete goal-directed movements? We show that when velocity information is impaired due to using second-order motion stimuli, saccades directed towards moving targets land at positions where targets were ~ 100 ms before saccade initiation, but hand movements are accurate. Importantly, the longer latencies of hand movements allow for additional time to process the sensory information available. When increasing the period of time one sees the moving target before making the saccade, saccades become accurate. In line with that, hand movements with short latencies show higher curvature, indicating corrections based on an update of incoming sensory information. These results suggest that movements are controlled by an independent and evolving combination of sensory information about the target's position and velocity.

Visual rehabilitation indicating neuroplasticity in an esotropic adult patient with diplopia after sudden visual acuity loss in the non-amblyopic eye: A case report

BACKGROUND

The aim of this case report is to present the successful management of both diplopia and amblyopia in a specific clinical situation, demonstrating neuroplasticity of the visual system in an adult patient. Causes of diplopia include eye pathologies in monocular diplopia and ischemic ocular motor nerve palsies, sudden life-threatening and chronic conditions in central nervous system in binocular diplopia. Strabismic amblyopia and nonarteritic anterior ischemic optic neuropathy are quite often ophthalmic conditions, first one is caused by suppression during developmental period and the latter one by ischemia of the optic nerve in adults. Coexistence of aforementioned conditions may cause unusual clinical situation in which ability of nervous system to functional reorganization could be demonstrated.

CASE PRESENTATION

In our adult patient, diplopia was incited by the loss of suppression of the strabismic amblyopic eye, which was the consequence of a sudden decrease of the visual acuity in the previously better eye in the course of nonarteritic anterior ischemic optic neuropathy. This led to impairment in daily activities.

RESULTS

Visual training rehabilitation improved distance and near visual acuity in the amblyopic eye over three months, and prescribing two pairs of glasses with prisms enabled the patient to return to daily activities.

CONCLUSION

The discussed patient lost the suppression of the strabismic amblyopic eye. Management of amblyopia is usually undertaken in children, however considering neuroplasticity we successfully attempted to improve visual functioning of our patient, despite lower intensity of neuroplasticity functions in an adult brain.

Serial dependence for oculomotor control depends on early sensory signals

To create an accurate percept of the world, the visual system relies on past experience and prior assumptions.¹ For example, although the retinal projection of an object moving in depth changes drastically, we still perceive the object at a constant size and velocity.^2,3 Consequently, if we see the same object with a constant retinal size at two different depth levels, the perceived size differs (illustrated by the Ponzo illusion). Past experience also directly influences perceptual judgments, an effect known as serial dependence.^4,5 Such sequential effects have also been reported for oculomotor behavior, even on the trial-by-trial level.^6-10 An integration of past experiences seems like a smart and sophisticated mechanism to reduce uncertainty and improve behavior in a world full of statistical regularities. By leveraging the Ponzo illusion to dissociate perceived size and speed from retinal signals, we show that serial-dependence effects for oculomotor control are mediated by retinal error signals. These sequential effects likely take place in early sensory processing because they transfer to different visual stimuli. In contrast to recently reported history effects for perceptual decisions,¹¹ sequential effects for oculomotor control deviate from perceptual mechanisms by not integrating spatial context and by ignoring size and velocity constancy. Although this dissociation might appear suboptimal, we argue that this effect reveals the different goals of the oculomotor and perceptual systems. The oculomotor system tries to reduce retinal error signals to bring and keep the target close to the fovea, whereas the visual system interprets retinal input to achieve an accurate representation of the world.¹².

Motion-in-depth effects on interceptive timing errors in an immersive environment

We often need to interact with targets that move along arbitrary trajectories in the 3D scene. In these situations, information of parameters like speed, time-to-contact, or motion direction is required to solve a broad class of timing tasks (e.g., shooting, or interception). There is a large body of literature addressing how we estimate different parameters when objects move both in the fronto-parallel plane and in depth. However, we do not know to which extent the timing of interceptive actions is affected when motion-in-depth (MID) is involved. Unlike previous studies that have looked at the timing of interceptive actions using constant distances and fronto-parallel motion, we here use immersive virtual reality to look at how differences in the above-mentioned variables influence timing errors in a shooting task performed in a 3D environment. Participants had to shoot at targets that moved following different angles of approach with respect to the observer when those reached designated shooting locations. We recorded the shooting time, the temporal and spatial errors and the head's position and orientation in two conditions that differed in the interval between the shot and the interception of the target's path. Results show a consistent change in the temporal error across approaching angles: the larger the angle, the earlier the error. Interestingly, we also found different error patterns within a given angle that depended on whether participants tracked the whole target's trajectory or only its end-point. These differences had larger impact when the target moved in depth and are consistent with underestimating motion-in-depth in the periphery. We conclude that the strategy participants use to track the target's trajectory interacts with MID and affects timing performance.

A change in perspective: The interaction of saccadic and pursuit eye movements in oculomotor control and perception

Due to the close relationship between oculomotor behavior and visual processing, eye movements have been studied in many different areas of research over the last few decades. While these studies have brought interesting insights, specialization within each research area comes at the potential cost of a narrow and isolated view of the oculomotor system. In this review, we want to expand this perspective by looking at the interactions between the two most important types of voluntary eye movements: saccades and pursuit. Recent evidence indicates multiple interactions and shared signals at the behavioral and neurophysiological level for oculomotor control and for visual perception during pursuit and saccades. Oculomotor control seems to be based on shared position- and velocity-related information, which leads to multiple behavioral interactions and synergies. The distinction between position- and velocity-related information seems to be also present at the neurophysiological level. In addition, visual perception seems to be based on shared efferent signals about upcoming eye positions and velocities, which are to some degree independent of the actual oculomotor response. This review suggests an interactive perspective on the oculomotor system, based mainly on different types of sensory input, and less so on separate subsystems for saccadic or pursuit eye movements.

Retinal error signals and fluctuations in eye velocity influence oculomotor behavior in subsequent trials

The oculomotor system makes use of an integration of previous stimulus velocities (the prior) and current sensory inputs to adjust initial eye speeds. The present study extended this research by investigating the roles of different retinal or extra-retinal signals for this process. To test for this, participants viewed movement sequences that all ended with the same test trial. Earlier in the sequence, the prior was manipulated by presenting targets that either had different velocities, different starting positions, or target movements designed to elicit differential oculomotor behavior (tracked with or without additional corrective saccades). Additionally, these prior targets could vary in terms of contrast to manipulate reliability. When the velocity of prior trials differed from test trials, the reliability-weighted integration of prior information was replicated. When the prior trials differed in starting position, significant effects on subsequent oculomotor behavior were only observed for the reliable target. Although there were also differences in eye velocity across the different manipulations, they could not explain the observed reliability-weighted integration. When comparing the same physical prior trials but tracked with additional corrective saccades, the eye velocity in the test trial also differed systematically (slower for forward saccades, and faster for backward saccades). The direction of the observed effect contradicts the expectations based on perceived speed and eye velocity, but can be predicted by a combination of retinal velocity and position error signals. Together, these results suggest that general fluctuations in eye velocity as well as retinal error signals are related to oculomotor behavior in subsequent trials.

Looking away from a moving target does not disrupt the way in which the movement toward the target is guided

People usually follow a moving object with their gaze if they intend to interact with it. What would happen if they did not? We recorded eye and finger movements while participants moved a cursor toward a moving target. An unpredictable delay in updating the position of the cursor on the basis of that of the invisible finger made it essential to use visual information to guide the finger's ongoing movement. Decreasing the contrast between the cursor and the background from trial to trial made it difficult to see the cursor without looking at it. In separate experiments, either participants were free to hit the target anywhere along its trajectory or they had to move along a specified path. In the two experiments, participants tracked the cursor rather than the target with their gaze on 13% and 32% of the trials, respectively. They hit fewer targets when the contrast was low or a path was imposed. Not looking at the target did not disrupt the visual guidance that was required to deal with the delays that we imposed. Our results suggest that peripheral vision can be used to guide one item to another, irrespective of which item one is looking at.

What gaze direction can tell us about cognitive processes in invertebrates

Most visually guided animals shift their gaze using body movements, eye movements, or both to gather information selectively from their environments. Psychological studies of eye movements have advanced our understanding of perceptual and cognitive processes that mediate visual attention in humans and other vertebrates. However, much less is known about how these processes operate in other organisms, particularly invertebrates. We here make the case that studies of invertebrate cognition can benefit by adding precise measures of gaze direction. To accomplish this, we briefly review the human visual attention literature and outline four research themes and several experimental paradigms that could be extended to invertebrates. We briefly review selected studies where the measurement of gaze direction in invertebrates has provided new insights, and we suggest future areas of exploration.

Perceptual decisions about object shape bias visuomotor coordination during rapid interception movements

Visual processing for perception and for action is thought to be mediated by two specialized neural pathways. Using a visuomotor decision-making task, we show that participants differentially utilized online perceptual decision-making in reaching and interception and that eye movements necessary for perception influenced motor decision strategies. These results provide evidence that task complexity modulates how pathways processing perception versus action information interact during the visual control of movement.

Dynamic combination of position and motion information when tracking moving targets

To accurately foveate a moving target, the oculomotor system needs to estimate the position of the target at the saccade end, based on information about its position and ongoing movement, while accounting for neuronal delays and execution time of the saccade. We investigated human interceptive saccades and pursuit responses to moving targets defined by high and low luminance contrast or by chromatic contrast only (isoluminance). We used step-ramps with perpendicular directions between vertical target steps of 10 deg/s and horizontal ramps of 2.5 to 20 deg/s to separate errors with respect to the position step of the target in the vertical dimension, and errors related to target motion in the horizontal dimension. Interceptive saccades to targets of high and low luminance contrast landed close to the actual target positions, suggesting relatively accurate estimates of the amount of target displacement. Interceptive saccades to isoluminant targets were less accurate. They landed at positions the target had on average 100 ms before saccade onset. One account of this finding is that the integration of target motion is compromised for isoluminant targets moving in the periphery. In this case, the oculomotor system can use an accurate, but delayed position component, but cannot account for target movement. This deficit was also present for the postsaccadic pursuit speed. For the two luminance conditions, pursuit direction and speed were adjusted depending on the saccadic landing position. The rapid postsaccadic pursuit adjustments suggest shared position- and motion-related signals of target and eye for saccade and pursuit control.

The predictability of a target’s motion influences gaze, head, and hand movements when trying to intercept it

Does the predictability of a target’s movement and of the interception location influence how the target is intercepted? In a first experiment, we manipulated the predictability of the interception location. A target moved along a haphazardly curved path, and subjects attempted to tap on it when it entered a hitting zone. The hitting zone was either a large ring surrounding the target’s starting position (ring condition) or a small disk that became visible before the target appeared (disk condition). The interception location gradually became apparent in the ring condition, whereas it was immediately apparent in the disk condition. In the ring condition, subjects pursued the target with their gaze. Their heads and hands gradually moved in the direction of the future tap position. In the disk condition, subjects immediately directed their gaze toward the hitting zone by moving both their eyes and heads. They also moved their hands to the future tap position sooner than in the ring condition. In a second and third experiment, we made the target’s movement more predictable. Although this made the targets easier to pursue, subjects now shifted their gaze to the hitting zone soon after the target appeared in the ring condition. In the disk condition, they still usually shifted their gaze to the hitting zone at the beginning of the trial. Together, the experiments show that predictability of the interception location is more important than predictability of target movement in determining how we move to intercept targets.

NEW & NOTEWORTHY We show that if people are required to intercept a target at a known location, they direct their gaze to the interception point as soon as they can rather than pursuing the target with their eyes for as long as possible. The predictability of the interception location rather than the predictability of the path to that location largely determines how the eyes, head, and hand move.

Corrective saccades influence velocity judgments and interception

In daily life we often interact with moving objects in tasks that involve analyzing visual motion, like catching a ball. To do so successfully we track objects with our gaze, using a combination of smooth pursuit and saccades. Previous work has shown that the occurrence and direction of corrective saccades leads to changes in the perceived velocity of moving objects. Here we investigate whether such changes lead to equivalent biases in interception. Participants had to track moving targets with their gaze, and in separate sessions either judge the targets' velocities or intercept them by tapping on them. We separated trials in which target movements were tracked with pure pursuit from trials in which identical target movements were tracked with a combination of pursuit and corrective saccades. Our results show that interception errors are shifted in accordance with the observed influence of corrective saccades on velocity judgments. Furthermore, while the time at which corrective saccades occurred did not affect velocity judgments, it did influence their effect in the interception task. Corrective saccades around 100 ms before the tap had a stronger effect on the endpoint error than earlier saccades. This might explain why participants made earlier corrective saccades in the interception task.

Enhanced brain responses to color during smooth-pursuit eye movements

Eye movements alter visual perceptions in a number of ways. During smooth-pursuit eye movements, previous studies reported decreased detection threshold for colored stimuli and for high-spatial-frequency luminance stimuli, suggesting a boost in the parvocellular system. The present study investigated the underlying neural mechanism using EEG in human participants. Participants followed a moving target with smooth-pursuit eye movements while steady-state visually evoked potentials (SSVEPs) were elicited by equiluminant red-green flickering gratings in the background. SSVEP responses to colored gratings were 18.9% higher during smooth pursuit than during fixation. There was no enhancement of SSVEPs by smooth pursuit when the flickering grating was defined by luminance instead of color. This result provides physiological evidence that the chromatic response in the visual system is boosted by the execution of smooth-pursuit eye movements in humans. Because the response improvement is thought to be the result of an improved response in the parvocellular system, SSVEPs to equiluminant stimuli could provide a direct test of parvocellular signaling, especially in populations where collecting an explicit behavioral response from the participant is not feasible.NEW & NOTEWORTHY We constantly move our eyes when we explore the world. Eye movements alter visual perception in various ways. The smooth-pursuit eye movements have been shown to boost color sensitivity. We recorded steady-state visually evoked potentials to equiluminant chromatic flickering stimuli and observed increased steady-state visually evoked potentials when participants smoothly pursued a moving target compared with when they maintained fixation. This work provides direct neurophysiological evidence for the parvocellular boost by smooth-pursuit eye movements in humans.

Perceptual Pragmatism and the Naturalized Ontology of Color

This paper considers whether there can be any such thing as a naturalized metaphysics of color-any distillation of the commitments of perceptual science with regard to color ontology. I first make some observations about the kinds of philosophical commitments that sometimes bubble to the surface in the psychology and neuroscience of color. Unsurprisingly, because of the range of opinions expressed, an ontology of color cannot simply be read off from scientists' definitions and theoretical statements. I next consider two alternative routes. First, conceptual pluralism inspired by Mark Wilson's analysis of scientific representation. I argue that these findings leave the prospects for a naturalized color ontology rather dim. Second, I outline a naturalized epistemology of perception. I ask how the correctness and informativeness of perceptual states is understood by contemporary perceptual science. I argue that the detectionist ideal of correspondence should be replaced by the pragmatic ideal of usefulness. I argue that this result has significant implications for the metaphysics of color.

The Interaction Between Vision and Eye Movements

The existence of a central fovea, the small retinal region with high analytical performance, is arguably the most prominent design feature of the primate visual system. This centralization comes along with the corresponding capability to move the eyes to reposition the fovea continuously. Past research on visual perception was mainly concerned with foveal vision while the observers kept their eyes stationary. Research on the role of eye movements in visual perception emphasized their negative aspects, for example, the active suppression of vision before and during the execution of saccades. But is the only benefit of our precise eye movement system to provide high acuity of the small foveal region, at the cost of retinal blur during their execution? In this review, I will compare human visual perception with and without saccadic and smooth pursuit eye movements to emphasize different aspects and functions of eye movements. I will show that the interaction between eye movements and visual perception is optimized for the active sampling of information across the visual field and for the calibration of different parts of the visual field. The movements of our eyes and visual information uptake are intricately intertwined. The two processes interact to enable an optimal perception of the world, one that we cannot fully grasp by doing experiments where observers are fixating a small spot on a display.

When circles become triangular: how transsaccadic predictions shape the perception of shape

Human vision is characterized by a consistent pattern of saccadic eye movements. With each saccade, internal object representations change their retinal position and spatial resolution. This raises the question as to how peripheral perception is affected by imminent saccadic eye movements. Here, we suggest that saccades are accompanied by a prediction of their perceptual consequences (i.e., the foveation of the target object). Accordingly, peripheral perception should be biased toward previously associated foveal input. In this study, we first exposed participants to an altered visual stimulation where one object systematically changed its shape during saccades. Subsequently, participants had to judge the shape of briefly presented peripheral saccade targets. The results showed that targets were perceived as less curved for objects that previously changed from more circular in the periphery to more triangular in the fovea. Similarly, shapes were perceived as more curved for objects that previously changed from triangular to circular. Thus, peripheral perception seems to depend not solely on the current input but also on memorized experiences, enabling predictions about the perceptual consequences of saccadic eye movements.

Counterproductive effect of saccadic suppression during attention shifts

During saccadic eye movements, the processing of visual information is transiently interrupted by a mechanism known as "saccadic suppression" [1] that is thought to ensure perceptual stability [2]. If, as proposed in the premotor theory of attention [3], covert shifts of attention rely on sub-threshold recruitment of oculomotor circuits, then saccadic suppression should also occur during covert shifts. In order to test this prediction, we designed two experiments in which participants had to orient towards a cued letter, with or without saccades. We analyzed the time course of letter identification score in an "attention" task performed without saccades, using the saccadic latencies measured in the "saccade" task as a marker of covert saccadic preparation. Visual conditions were identical in all tasks. In the "attention" task, we found a drop in perceptual performance around the predicted onset time of saccades that were never performed. Importantly, this decrease in letter identification score cannot be explained by any known mechanism aligned on cue onset such as inhibition of return, masking, or microsaccades. These results show that attentional allocation triggers the same suppression mechanisms as during saccades, which is relevant during eye movements but detrimental in the context of covert orienting.

Visual perception and saccadic eye movements

We use saccades several times per second to move the fovea between points of interest and build an understanding of our visual environment. Recent behavioral experiments show evidence for the integration of pre- and postsaccadic information (even subliminally), the modulation of visual sensitivity, and the rapid reallocation of attention. The recent physiological literature has identified a characteristic modulation of neural responsiveness-perisaccadic reduction followed by a postsaccadic increase-that is found in many visual areas, but whose source is as yet unknown. This modulation seems optimal for reducing sensitivity during and boosting sensitivity between saccades, but no study has yet established a direct causal link between neural and behavioral changes.

Keep your eyes on the ball: smooth pursuit eye movements enhance prediction of visual motion

Success of motor behavior often depends on the ability to predict the path of moving objects. Here we asked whether tracking a visual object with smooth pursuit eye movements helps to predict its motion direction. We developed a paradigm, “eye soccer,” in which observers had to either track or fixate a visual target (ball) and judge whether it would have hit or missed a stationary vertical line segment (goal). Ball and goal were presented briefly for 100–500 ms and disappeared from the screen together before the perceptual judgment was prompted. In pursuit conditions, the ball moved towards the goal; in fixation conditions, the goal moved towards the stationary ball, resulting in similar retinal stimulation during pursuit and fixation. We also tested the condition in which the goal was fixated and the ball moved. Motion direction prediction was significantly better in pursuit than in fixation trials, regardless of whether ball or goal served as fixation target. In both fixation and pursuit trials, prediction performance was better when eye movements were accurate. Performance also increased with shorter ball-goal distance and longer presentation duration. A longer trajectory did not affect performance. During pursuit, an efference copy signal might provide additional motion information, leading to the advantage in motion prediction.