Influence of mechanical disturbance on oculomotor behavior

Gaze and attention: Mechanisms underlying the therapeutic effect of optokinetic stimulation in spatial neglect

Left smooth pursuit eye movement training in response to large-field visual motion (optokinetic stimulation) has become a promising rehabilitation method in left spatial inattention or neglect. The mechanisms underlying the therapeutic effect, however, remain unknown. During optokinetic stimulation, there is an error in visual localisation ahead of the line of sight. This could indicate a change in the brain's estimate of one's own direction of gaze. We hypothesized that optokinetic stimulation changes the brain's estimate of gaze. Because this estimate is critical for coding the locus of attention in the visual space relative to the body and across sensory modalities, its change might underlie the change in spatial attention. Here, we report that in healthy participants optokinetic stimulation causes not only a directional bias in the proprioceptive signal from the extraocular muscles, but also a corresponding shift of the locus of attention. Both changes outlasted the period of stimulation. This result forms a step in investigating a causal link between the adaptation in the sensorimotor gaze signals and the recovery in spatial neglect.

Acute Effect of Video Feedback on Self-Regulation and Proprioceptive Control of Standing Back Tuck Somersault in the Absence of Vision

The purpose of this study was to assess the immediate effect of video feedback on the regulation and control of the standing back tuck somersault in the absence of vision. Two groups of male parkour athletes performed the standing back tuck somersault under both open and closed eyes conditions. The first group received video feedback, while the second group received verbal feedback. Concurrent analysis, including kinetic data from a force plate (Kistler Quattro-Jump) and kinematic data in two-dimensional by Kinovea freeware, was employed for motion and technical performance analysis. The results indicate that the loss of vision during the standing back tuck somersault affected only the take-off and ungrouping angle, as well as the vertical velocity and displacement. These effects were consistent regardless of the type of feedback provided (i.e., video feedback or verbal feedback). Furthermore, a significant Vision × Feedback interaction was observed at the level of technical performance. This suggests that the use of video feedback enabled the parkour athletes to maintain a high level of technical performance both with and without vision (i.e., 13.56 vs. 13.00 points, respectively, p > .05 and d = 2.233). However, the verbal feedback group technical performance declined significantly under the no-vision condition compared with the vision condition (13.14 vs. 10.25 points, respectively, with and without vision, p < .001 and d = 2.382). We concluded that when the movement is proprioceptively controlled (i.e., without vision), the video feedback enables the athletes to globally assess the technical deficiencies arising from the lack of vision and to correct them. These findings are discussed based on parkour athletes' ability to evaluate the kinematic parameters of the movement.

Proprioceptive contribution to oculomotor control in humans

Stretch receptors in the extraocular muscles (EOMs) inform the central nervous system about the rotation of one's own eyes in the orbits. Whereas fine control of the skeletal muscles hinges critically on proprioceptive feedback, the role of proprioception in oculomotor control remains unclear. Human behavioural studies provide evidence for EOM proprioception in oculomotor control, however, behavioural and electrophysiological studies in the macaque do not. Unlike macaques, humans possess numerous muscle spindles in their EOMs. To find out whether the human oculomotor nuclei respond to proprioceptive feedback we used functional magnetic resonance imaging (fMRI). With their eyes closed, participants placed their right index finger on the eyelid at the outer corner of the right eye. When prompted by a sound, they pushed the eyeball gently and briefly towards the nose. Control conditions separated out motor and tactile task components. The stretch of the right lateral rectus muscle was associated with activation of the left oculomotor nucleus and subthreshold activation of the left abducens nucleus. Because these nuclei control the horizontal movements of the left eye, we hypothesized that proprioceptive stimulation of the right EOM triggered left eye movement. To test this, we followed up with an eye-tracking experiment in complete darkness using the same behavioural task as in the fMRI study. The left eye moved actively in the direction of the passive displacement of the right eye, albeit with a smaller amplitude. Eye tracking corroborated neuroimaging findings to suggest a proprioceptive contribution to ocular alignment.

Transsaccadic processing: stability, integration, and the potential role of remapping

While our frequent saccades allow us to sample the complex visual environment in a highly efficient manner, they also raise certain challenges for interpreting and acting upon visual input. In the present, selective review, we discuss key findings from the domains of cognitive psychology, visual perception, and neuroscience concerning two such challenges: (1) maintaining the phenomenal experience of visual stability despite our rapidly shifting gaze, and (2) integrating visual information across discrete fixations. In the first two sections of the article, we focus primarily on behavioral findings. Next, we examine the possibility that a neural phenomenon known as predictive remapping may provide an explanation for aspects of transsaccadic processing. In this section of the article, we delineate and critically evaluate multiple proposals about the potential role of predictive remapping in light of both theoretical principles and empirical findings.

Eye movements reset visual perception

Human vision uses saccadic eye movements to rapidly shift the sensitive foveal portion of our retina to objects of interest. For vision to function properly amidst these ballistic eye movements, a mechanism is needed to extract discrete percepts on each fixation from the continuous stream of neural activity that spans fixations. The speed of visual parsing is crucial because human behaviors ranging from reading to driving to sports rely on rapid visual analysis. We find that a brain signal associated with moving the eyes appears to play a role in resetting visual analysis on each fixation, a process that may aid in parsing the neural signal. We quantified the degree to which the perception of tilt is influenced by the tilt of a stimulus on a preceding fixation. Two key conditions were compared, one in which a saccade moved the eyes from one stimulus to the next and a second simulated saccade condition in which the stimuli moved in the same manner but the subjects did not move their eyes. We find that there is a brief period of time at the start of each fixation during which the tilt of the previous stimulus influences perception (in a direction opposite to the tilt aftereffect)--perception is not instantaneously reset when a fixation starts. Importantly, the results show that this perceptual bias is much greater, with nearly identical visual input, when saccades are simulated. This finding suggests that, in real-saccade conditions, some signal related to the eye movement may be involved in the reset phenomenon. While proprioceptive information from the extraocular muscles is conceivably a factor, the fast speed of the effect we observe suggests that a more likely mechanism is a corollary discharge signal associated with eye movement.

Eye proprioception used for visual localization only if in conflict with the oculomotor plan

Both the corollary discharge of the oculomotor command and eye muscle proprioception provide eye position information to the brain. Two contradictory models have been suggested about how these two sources contribute to visual localization: (1) only the efference copy is used whereas proprioception is a slow recalibrator of the forward model, and (2) both signals are used together as a weighted average. We had the opportunity to test these hypotheses in a patient (R.W.) with a circumscribed lesion of the right postcentral gyrus that overlapped the human eye proprioceptive representation. R.W. was as accurate and precise as the control group (n = 19) in locating a lit LED that she viewed through the eye contralateral to the lesion. However, when the task was preceded by a brief (<1 s), gentle push to the closed eye, which perturbed eye position and stimulated eye proprioceptors in the absence of a motor command, R.W.'s accuracy significantly decreased compared with both her own baseline and the healthy control group. The data suggest that in normal conditions, eye proprioception is not used for visual localization. Eye proprioception is, however, continuously monitored to be incorporated into the eye position estimate when a mismatch with the efference copy of the motor command is detected. Our result thus supports the first model and, furthermore, identifies the limits for its operation.

Eye proprioception may provide real time eye position information

Because of the frequency of eye movements, online knowledge of eye position is crucial for the accurate spatial perception and behavioral navigation. Both the internal monitoring signal (corollary discharge) of eye movements and the eye proprioception signal are thought to contribute to the localization of the eye position in the orbit. However, the functional role of these two eye position signals in spatial cognition has been disputed for more than a century. The predominant view proposes that the online analysis of eye position is exclusively provided by the corollary discharge signal, while the eye proprioception signal only plays a role in the long-term calibration of the oculomotor system. However, increasing evidence from recent behavioral and physiological studies suggests that the eye proprioception signal may play a role in the online monitoring of eye position. The purpose of this review is to discuss the feasibility and possible function of the eye proprioceptive signal for online monitoring of eye position.

Eye muscle proprioception is represented bilaterally in the sensorimotor cortex

The cortical representation of eye position is still uncertain. In the monkey a proprioceptive representation of the extraocular muscles (EOM) of an eye were recently found within the contralateral central sulcus. In humans, we have previously shown a change in the perceived position of the right eye after a virtual lesion with rTMS over the left somatosensory area. However, it is possible that the proprioceptive representation of the EOM extends to other brain sites, which were not examined in these previous studies. The aim of this fMRI study was to sample the whole brain to identify the proprioceptive representation for the left and the right eye separately. Data were acquired while passive eye movement was used to stimulate EOM proprioceptors in the absence of a motor command. We also controlled for the tactile stimulation of the eyelid by removing from the analysis voxels activated by eyelid touch alone. For either eye, the brain area commonly activated by passive and active eye movement was located bilaterally in the somatosensory area extending into the motor and premotor cytoarchitectonic areas. We suggest this is where EOM proprioception is processed. The bilateral representation for either eye contrasts with the contralateral representation of hand proprioception. We suggest that the proprioceptive representation of the two eyes next to each other in either somatosensory cortex and extending into the premotor cortex reflects the integrative nature of the eye position sense, which combines proprioceptive information across the two eyes with the efference copy of the oculomotor command.

How the brain makes the world appear stable

Space constancy, the appearance of a stable visual world despite shifts of all visual input with each eye movement, has been explained historically with a compensatory signal (efference copy or corollary discharge) that subtracts the eye movement signal from the retinal image shift accompanying each eye movement. Quantitative measures have shown the signal to be too small and too slow to mediate space constancy unaided. Newer theories discard the compensation idea, instead calibrating vision to each saccadic target.

Is there a role for extraretinal factors in the maintenance of stability in a structured environment?

The calibration solution to the stability of the world despite eye movements depends, according to Bridgeman et al., upon a combination of three factors which presumably all need to operate to achieve the goal of stability. Although the authors admit (sect. 4.3, para. 5) that the relative contributions of retinal and extraretinal factors will depend on the particular viewing situation, Figure 5 (sect. 4.3) makes it clear in its representation that the role of perceptual factors is relatively minor compared to extraretinal ones. It is with this representation that this commentary wishes to take issue, believing that it occurs as a result of some assumptions about terminology that may be ambiguous, as well as some misconceptions about the circumstances in which there is a need for stability.

A theory of visual stability across saccadic eye movements

We identify two aspects of the problem of maintaining perceptual stability despite an observer's eye movements. The first, visual direction constancy, is the (egocentric) stability of apparent positions of objects in the visual world relative to the perceiver. The second, visual position constancy, is the (exocentric) stability of positions of objects relative to each other. We analyze the constancy of visual direction despite saccadic eye movements.

Three information sources have been proposed to enable the visual system to achieve stability: the structure of the visual field, proprioceptive inflow, and a copy of neural efference or outflow to the extraocular muscles. None of these sources by itself provides adequate information to achieve visual direction constancy; present evidence indicates that all three are used.

Our final question concerns how information processing operations result in a stable world. The three traditionally suggested means have been elimination, translation, or evaluation. All are rejected. From a review of the physiological and psychological evidence we conclude that no subtraction, compensation, or evaluation need take place. The problem for which these solutions were developed turns out to be a false one. We propose a “calibration” solution: correct spatiotopic positions are calculated anew for each fixation. Inflow, outflow, and retinal sources are used in this calculation: saccadic suppression of displacement bridges the errors between these sources and the actual extent of movement.

Landmarks facilitate visual space constancy across saccades and during fixation

It has been demonstrated that visual objects that are present after saccadic eye movements act as landmarks for the localization of stimuli across saccades, facilitating space constancy (Deubel, 2004). We here study the temporal conditions under which landmark effects occur after saccadic eye movements, and during fixation. Two small objects were presented 6 degrees in the periphery, one above the other. Observers saccaded to the space between them. One of the objects disappeared during the saccade and reappeared with a variable delay during or after the saccade. At the same time either that object or the continuously present one jumped by 1 degrees . The observer's task was to decide which object had moved. The results revealed a strong bias to assign movement to the object that was blanked, regardless of which actually moved. If both objects were blanked, the one that was blanked for a shorter time tended to be seen as stable. The effects were stronger as the onset asynchrony between the stimuli increased. Surprisingly, analogous though weaker effects occurred during visual fixation, suggesting that similar visual mechanisms relying on visual landmarks operate both across saccades and during fixation.

Effect of a disparity pattern on the perception of direction: non-retinal information masks retinal information

The direction of an object can theoretically be determined from the binocular disparity information alone. However, there is no certain empirical evidence for this. This study examines whether the binocular disparity information alters the perceived direction. Observers make an effort to rotate their eyes beyond their movable limit for a while before observing the display. This is done to alter the reliability of the eye position signal from proprioception and efference copy. The results show that the perceived direction changes according to the amount of disparity information in the stimulus.

Neuronal mechanisms of visual stability

Human vision is stable and continuous in spite of the incessant interruptions produced by saccadic eye movements. These rapid eye movements serve vision by directing the high resolution fovea rapidly from one part of the visual scene to another. They should detract from vision because they generate two major problems: displacement of the retinal image with each saccade and blurring of the image during the saccade. This review considers the substantial advances in understanding the neuronal mechanisms underlying this visual stability derived primarily from neuronal recording and inactivation studies in the monkey, an excellent model for systems in the human brain. For the first problem, saccadic displacement, two neuronal candidates are salient. First are the neurons in frontal and parietal cortex with shifting receptive fields that provide anticipatory activity with each saccade and are driven by a corollary discharge. These could provide the mechanism for a retinotopic hypothesis of visual stability and possibly for a transsaccadic memory hypothesis, The second neuronal mechanism is provided by neurons whose visual response is modulated by eye position (gain field neurons) or are largely independent of eye position (real position neurons), and these neurons could provide the basis for a spatiotopic hypothesis. For the second problem, saccadic suppression, visual masking and corollary discharge are well established mechanisms, and possible neuronal correlates have been identified for each.

Efference copy and its limitations

Efference copy, an internal brain signal informing the visual system of commands to move the eye, was the dominant explanation for visual space constancy for over a century. The explanation is not viable, however; the signal is to small, to slow, and too unreliable to support the perception of perfect constancy. Newer theories recognize that detailed image information does not survive refixation in any case. Efference copy is a viable explanation of static position perception and sensorimotor interaction, but the rich, stable visual world is an illusion.

The functions of the proprioceptors of the eye muscles

This article sets out to present a fairly comprehensive review of our knowledge about the functions of the receptors that have been found in the extraocular muscles--the six muscles that move each eye of vertebrates in its orbit--of all the animals in which they have been sought, including Man. Since their discovery at the beginning of the 20th century these receptors have, at various times, been credited with important roles in the control of eye movement and the construction of extrapersonal space and have also been denied any function whatsoever. Experiments intended to study the actions of eye muscle receptors and, even more so, opinions (and indeed polemic) derived from these observations have been influenced by the changing fashions and beliefs about the more general question of how limb position and movement is detected by the brain and which signals contribute to those aspects of this that are perceived (kinaesthesis). But the conclusions drawn from studies on the eye have also influenced beliefs about the mechanisms of kinaesthesis and, arguably, this influence has been even larger than that in the converse direction. Experimental evidence accumulated over rather more than a century is set out and discussed. It supports the view that, at the beginning of the 21st century, there are excellent grounds for believing that the receptors in the extraocular muscles are indeed proprioceptors, that is to say that the signals that they send into the brain are used to provide information about the position and movement of the eye in the orbit. It seems that this information is important in the control of eye movements of at least some types, and in the determination by the brain of the direction of gaze and the relationship of the organism to its environment. In addition, signals from these receptors in the eye muscles are seen to be necessary for the development of normal mechanisms of visual analysis in the mammalian visual cortex and for both the development and maintenance of normal visuomotor behaviour. Man is among those vertebrates to whose brains eye muscle proprioceptive signals provide information apparently used in normal sensorimotor functions; these include various aspects of perception, and of the control of eye movement. It is possible that abnormalities of the eye muscle proprioceptors and their signals may play a part in the genesis of some types of human squint (strabismus); conversely studies of patients with squint in the course of their surgical or pharmacological treatment have yielded much interesting evidence about the central actions of the proprioceptive signals from the extraocular muscles. The results of experiments on the eye have played a large part in the historical controversy, now in at least its third century, about the origin of signals that inform the brain about movement of parts of the body. Some of these results, and more of the interpretations of them, now need to be critically re-examined. The re-examination in the light of recent experiments that is presented here does not support many of the conclusions confidently drawn in the past and leads to both new insights and fresh questions about the roles of information from motor signals flowing out of the brain and that from signals from the peripheral receptors flowing into it. There remain many lacunae in our knowledge and filling some of these will, it is contended, be essential to advance our understanding further. It is argued that such understanding of eye muscle proprioception is a necessary part of the understanding of the physiology and pathophysiology of eye movement control and that it is also essential to an account of how organisms, including Man, build and maintain knowledge of their relationship to the external visual world. The eye would seem to provide a uniquely favourable system in which to study the way in which information derived within the brain about motor actions may interact with signals flowing in from peripheral receptors. The review is constructed in relatively independent sections that deal with particular topics. It ends with a fairly brief piece in which the author sets out some personal views about what has been achieved recently and what most immediately needs to be done. It also suggests some lines of study that appear to the author to be important for the future.

Slow eye movements

Monkeys and humans are able to perform different types of slow eye movements. The analysis of the eye movement parameters, as well as the investigation of the neuronal activity underlying the execution of slow eye movements, offer an excellent opportunity to study higher brain functions such as motion processing, sensorimotor integration, and predictive mechanisms as well as neuronal plasticity and motor learning. As an example, since there exists a tight connection between the execution of slow eye movements and the processing of any kind of motion, these eye movements can be used as a biological, behavioural probe for the neuronal processing of motion. Global visual motion elicits optokinetic nystagmus, acting as a visual gaze stabilization system. The underlying neuronal substrate consists mainly of the cortico-pretecto-olivo-cerebellar pathway. Additionally, another gaze stabilization system depends on the vestibular input known as the vestibulo-ocular reflex. The interactions between the visual and vestibular stabilization system are essential to fulfil the plasticity of the vestibulo-ocular reflex representing a simple form of learning. Local visual motion is a necessary prerequisite for the execution of smooth pursuit eye movements which depend on the cortico-pontino-cerebellar pathway. In the wake of saccades, short-latency eye movements can be elicited by brief movements of the visual scene. Finally, eye movements directed to objects in different planes of depth consist of slow movements also. Although there is some overlap in the neuronal substrates underlying these different types of slow eye movements, there are brain areas whose activity can be associated exclusively with the execution of a special type of slow eye movement.

Extraretinal eye position signals determine perceived target location when they conflict with visual cues

To examine the role of extraretinal eye position information (EEPI) in visual perception of target location in normal room illumination, subjects participated in experiments in which EEPI was manipulated using the eye press maneuver with either monocular or binocular viewing. The viewing condition and eye press caused EEPI and retinal information about target location to conflict. Pointing responses in eye press trials were all in the direction of EEPI showing that EEPI is the dominant source of information in egocentric visual space perception. In binocular viewing, version and vergence occur in response to the eye press to maintain fusion and EEPI based on these movements also determine perceived location. An unanticipated finding was that the eye press was variable in its effectiveness in rotating the eye, which contributed to large variability in pointing errors and suggested the method would be a poor choice for future work.

Egocentric visual target position and velocity coding: role of ocular muscle proprioception

Limited knowledge is available regarding the processes by which the brain codes the velocity of visual targets with respect to the observer. Two models have been previously proposed to describe the visual target localization mechanism. Both assume that the necessary information is derived from the coding of the position of the eye in the orbit, either through a copy of the muscular activation (out flow model) or through eye muscle proprioception (in flow model). Eye velocity coding might be derived from velocity sensitive ocular muscle proprioceptors or from position coding signals through differentiation. We used techniques based on manual pointing and manual tracking of visual target, combined with passive deviation of one covered eye, to demonstrate that ocular muscle proprioception is involved in (i) eye-in-head position coding, hence in target localization function; (ii) long-term maintenance of ocular alignment (phoria); and (iii) sensing of visual target velocity with respect to the head. These observations support other data now available, describing the processes by which the brain codes position and velocity of visual targets. Such findings might interest engineers in the field of robotics who are facing the problem of providing robots with the ability to sense object position and velocity in order to create an internal model of their working environment.

A review of the role of efference copy in sensory and oculomotor control systems

Efference copy is an internal copy of a motor innervation. In the oculomotor system it provides the only extraretinal signal about eye position that is available without delay, and it is shown to be the most important extraretinal source of information for perceptual localization and motor activity. Efference copy accompanies all voluntary eye movements and some involuntary ones, including pursuits, saccades, and the fast phases of vestibular and optokinetic nystagmus. Not all eye movements are accompanied by an efference copy; its presence is determined by a movement's function, not it dynamics. Because the gain of the efference copy mechanism is less than 1, and it does not take account of oculomotor delays and kinematics, it is supplemented by other mechanisms in achieving space constancy. It functions differently for perception and for visually guided behavior. There is only one efference copy for both eyes, reflecting Hering's law, and it is subject to adaptation.

Neuronal death of the cancellation theory?

The question of how the brain can construct a stable representation of the external world despite eye movements is a very old one. If there have been some wrong statements of problems (such as the inverted retinal image), other statements are less naive and have led to analytic solutions possibly adopted by the brain to counteract the spurious effects of eye movements. Following the MacKay (1973) objections to the analytic view of perceptual stability, Bridgeman et al. claim that the idea that signals canceling the effects of saccadic eye movements are needed is also a misconception, as is the claim that stability and position encoding are two distinct problems. It must be remembered, however, that what made the theory of “cancellation” formulated by von Holst and Mittelstaedt (1950) so appealing was the clinical observation of perceptual instability following ocular paralysis. Following the concept of corollary discharge, the theory of efference copy had the advantage of simultaneously solving three problems: the stability of the visual world during the saccade, the same visual stability across saccades, and the visual constancy problem of allowing the subject to know where an object in space is.

Motion perception during saccades

Although the retinal image is displaced by each saccade performed we do not perceive the visual environment moving concordant with the saccades. In this study experiments were designed in which additional movement of most of the visual scene was applied during saccades. The subjects perceived the intrasaccadic movement after the saccade. The perceived speed of this movement was decreased and the threshold amplitude was increased compared to perception during fixation. The intrasaccadic movement perception was based on a novel aftereffect of motion perception. The velocity of retinal slip did not affect the threshold. If the retinal slip speed during saccades was temporally reduced by an intrasaccadic movement parallel to the saccade, the threshold amplitude was identical to the threshold amplitude obtained by intrasaccadic movement opposite to the saccade increasing retinal slip speed. Horizontal intrasaccadic movements were detected at lower thresholds than vertical movements independent of saccade direction. In addition, the thresholds were not effected by the saccade amplitude suggesting that neither speed, duration, nor direction of eye movement related retinal slip affects the amount of suppression. Our results suggest that saccadic suppression is related to delayed central processing of retinal information during saccades. This processing does not involve saccade parameters such as direction and amplitude.

Ocular proprioception and efference copy in registering visual direction

We measured the roles of eye muscle proprioception ("inflow") and efference copy ("outflow") in registering eye position. During monocular fixation, pressing on the side of an occluded eye results in a passive rotation, changing the proprioception without affecting oculomotor efference. As we have shown previously, a constant press on the side of the viewing eye induces active resistance to rotation, changing efference because oculomotor innervation compensates for the eyepress; the viewing eye's fixation remains constant. Using these two types of eyepress, both perceived target deviations and pointing biases in an unstructured visual field were measured in 8 subjects under efference copy, proprioception and control (no eyepress) conditions. Eye deviation was measured photoelectrically. Physiological gains of efference copy and proprioception was about 5/8 and 1/4 respectively. There was no statistically significant difference between perceptual judgement and open-loop pointing. The sum of gains of efference copy and proprioception, about 7/8, indicates incomplete registration of eye eccentricity in an unstructured field, and quantitatively accounts for several previously unexplained results in the literature.

The role of ocular muscle proprioception in visual localization of targets

The role of ocular muscle proprioception in the localization of visual targets has been investigated in normal humans by deviating one eye to create an experimental strabismus. The passively deviated eye was covered and the other eye viewed the target. With a hand-pointing task, targets were systematically mislocalized in the direction of the deviated nonviewing eye. A 4- to 6-degree error resulted when the nonviewing eye was offset 30 degrees from straight ahead. When the eye was deviated, the perceived "straight-ahead" was also displaced, by a similar amount, in the same direction. Since the efferent motor commands to the displaced and to the nondisplaced eyes are presumably identical by the law of equal innervation, the mislocalization of visual objects must be attributed to the change in proprioceptive information issued from the nonviewing, deviated eye. Thus proprioception contributes to the localization of objects in space.