Eye proprioception and visual localization in humans: influence of ocular dominance and visual context

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.

Ocular dominance in cataract surgery: research status and progress

Ocular dominance (OD), a commonly used concept in clinical practice, plays an important role in optometry and refractive surgery. With the development of refractive cataract surgery, the refractive function of the intraocular lens determines the achievement of the postoperative full range of vision based on the retinal defocus blur suppression and binocular monovision principle. Therefore, OD plays an important role in cataract surgery. OD is related to the visual formation of the cerebral cortex, and its plasticity suggests that visual experience can influence the visual system. Cataract surgery changes the visual experience and transforms the dominant eye, which confirms the plasticity of the visual system. Based on the concept and mechanism of OD, this review summarizes the application of OD in cataract surgery.

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.

Oculomotor performance in patients with neck pain: Does it matter which angle of neck torsion is used in smooth pursuit eye movement test and is the agreement between angles dependent on target movement amplitude and velocity?

BACKGROUND

Neck torsion manoeuvre is thought to affect eye movement control via afferent sensory drive in neck pain disorders patients. Literature reports inconsistencies regarding the angle of neck torsion most commonly used across the studies.

OBJECTIVES

The goal of this study was to determine the level of agreement in oculomotor performance between two most commonly used neck torsion angles during smooth pursuit neck torsion test (SPNT).

DESIGN

A cross-sectional design was used in thirty-two neck pain patients and thirty-two healthy individuals.

METHOD

Gain and SPNT_diff were measured during SPNT test at 30° and 45° of neck torsion angle, at 30°, 40° and 50° of target movement amplitudes and three different target movement velocities (20°s^-1, 30°s^-1 and 40°s^-1) using eye tracking device. Bland-Altman plots and correlation analysis were used to study the agreement between the two angles.

RESULTS

Small to medium correlations and wide bias confidence intervals suggest medium level of agreement in gain or SPNT_diff between the two neck torsion angles for chronic neck pain patients, but higher in healthy individuals. Higher agreement in gain was observed at lager target movement amplitudes and at slower target movement velocities, however this trend was not observed for SPNT_diff.

CONCLUSION

Level of agreement between the two angles in SPNT test depends on the amplitude and velocity of the moving target. In cases when subjects within the same study are not able to perform 45° of neck torsion, 50° amplitude and 20°s^-1 velocity of target movement are more suitable to reach higher agreement between the angles.

Is Altered Oculomotor Control during Smooth Pursuit Neck Torsion Test Related to Subjective Visual Complaints in Patients with Neck Pain Disorders?

Subjective visual complaints are commonly reported in patients with neck pain, but their relation to objectively measured oculomotor functions during smooth pursuit neck torsion tests (SPNTs) has not yet been investigated. The aim of the study was to analyse classification accuracy of visual symptom intensity and frequency based on SPNT results. Forty-three patients with neck pain were referred by orthopaedic outpatient clinics where they were required to fill out 16-item proformas of visual complaints. Infrared video-oculography was used to measure smooth pursuit eye movements during neutral and neck torsion positions. Parameters of gain and SPNT difference (SPNT_diff) were taken into the Naïve Bayes model as classifiers, while intensity and frequency of visual symptoms were taken as predicted class. Intensity and, to a lesser degree, frequency of visual symptoms previously associated with neck pain or focal vision disorders (computer vision syndrome) showed better classification accuracy using gain at neck torsion position, indicating cervical driven visual disturbances. Moreover, SPNT_diff presented with slightly lower classification accuracy as compared to gain at neck torsion position. Our study confirmed the relationship between cervical driven oculomotor deficits and some visual complaints (concentrating to read, words moving on page, blurred vision, difficulty judging distance, sore eyes, heavy eyes, red eyes, and eyes strain).

The influence of neck torsion and sequence of cycles on intra-trial reliability of smooth pursuit eye movement test in patients with neck pain disorders

The sensory mismatch commonly observed in patients with neck pain disorders could alter intra-trial reliability in simple implicit smooth pursuit eye movement tasks. This could be more pronounced when neck is in torsioned position (SPNT). The aim of this study was to explore the effects of neck torsion, target movement velocity and amplitude on intra-trial reliability of smooth pursuit eye movements in patients with neck pain disorders and healthy individuals. SPNT test was evaluated in 32 chronic neck pain patients and 32 healthy controls. Ten cycles were performed using video-oculography at three different velocities (20° s^-1, 30° s^-1 and 40° s^-1) and at three different amplitudes (30°, 40° and 50°) of target movement. Intra-trial reliability and differences between average gain and SPNT difference from the second to fifth cycle and from the sixth to ninth cycle were assessed using ICC_3.1 and factorial analysis of variance, respectively. Intra-trial reliability for gain and SPNT difference at all target movement amplitudes and velocities proved to be good to excellent in both observed groups. Patients with neck pain disorders presented with a trend of inferior gain performance between the sixth and ninth cycle at 30° s^-1 of target movement as compared to healthy individuals which was only evident when neck was in torsioned position. Although intra-trial reliability of smooth pursuit neck torsion test is good to excellent, the effects of learning are not as pronounced in patients with neck pain disorders.

Unreferenced spatial localization under monocular and dichoptic viewing conditions

Knowledge of eye position in the brain is critical for localization of objects in space. To investigate the accuracy and precision of eye position feedback in an unreferenced environment, subjects with normal ocular alignment attempted to localize briefly presented targets during monocular and dichoptic viewing. In the task, subjects' used a computer mouse to position a response disk at the remembered location of the target. Under dichoptic viewing (with red (right eye)-green (left eye) glasses), target and response disks were presented to the same or alternate eyes, leading to four conditions [green target-green response cue (LL), green-red (LR), red-green (RL), and red-red (RR)]. Time interval between target and response disks was varied and localization errors were the difference between the estimated and real positions of the target disk. Overall, the precision of spatial localization (variance across trials) became progressively worse with time. Under dichoptic viewing, localization errors were significantly greater for alternate-eye trials as compared to same-eye trials and were correlated to the average phoria of each subject. Our data suggests that during binocular dissociation, spatial localization may be achieved by combining a reliable versional efference copy signal with a proprioceptive signal that is unreliable perhaps because it is from the wrong eye or is too noisy.

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

A conceptual model of the visual control of posture

In order to isolate the visual contribution to the control of postural balance, experiments in which subjects are exposed to large-field visual motion (optokinetic) stimuli are reviewed. In these situations, at motion onset, the visual stimulus signals subject self-motion but inertial (vestibulo-proprioceptive) cues do not. Visually evoked postural responses (VEPR) thus induced can be quickly suppressed by cognitive status or simple repetition of the stimulus, if the inertial self-motion cues available to the subject are reliable. In the conceptual model presented here, the process of assessing the reliability, and degree of matching, of visual and inertial signals is carried out by a General comparator; in turn able to access the Gain control mechanism of the visuo-postural system. Complexity and congruency in the visual stimulus itself are assessed by a Visual comparator, e.g., the presence of motion parallax in the visual stimulus can reverse the sway response direction. VEPR can also be re-oriented according to the position of the eyes in the head and the head on the trunk. This indicates that ocular and cervical proprioceptors must also access the gain control mechanism so that visual stimuli can recruit and silence different postural muscles appropriately. The overall gain of the visuo-postural system is also influenced by less easily defined idiosyncratic factors, such as visual dependence and psychological traits; interestingly both these factors have been found to be associated with poor long term outcome in vestibular disorders. The experimental results and model presented illustrate that the visuo-postural system is a wonderful example of interaction between physics (e.g., stimuli geometry, body dynamics), neuroscience and the border zone between neurology and psycho-somatic medicine.

Visual vertigo: Vertigo of oculomotor origin

Since Róbert Bárány proposed his hypothesis on vestibulo-ocular reflex (VOR), dizziness associated with vertigo has been interpreted as being vestibular in origin. However, there have been many contradictory findings showing modulations of VOR, which have caused confusion as to VOR's role and accuracy. Further, there seems to be an influence of VOR when the anatomical inner ear structures are congenitally absent. Many people report vertiginous symptoms when they are exposed to visually challenging situations. These people with visually induced vertigo are usually found to have only mildly abnormal labyrinthine findings. Accurate visual information via binocular vision in animals, including humans, is important for the survival. Understanding how visual information is used in balance can help us to apply a different approach to the mechanism of vertigo. This article will review how accurate binocular viewing is possible for precise images through a complex oculomotor system and the proprioceptive senses of the external ocular muscles (EOMs). The proprioceptive senses from EOMs appear to affect motor efferents of the body. Oculomotor activities during viewing are important not just for learning but also for executing whole body motor responses. An error in the oculomotor afferents will cause a reaction to the error signal. This can be troubling for proper balancing during movement. Especially, common oculomotor causes (including fatigue of EOMs which is common in today's lifestyle) can contribute to many vertiginous conditions.

Angle of gaze and optic flow direction modulate body sway

Optic flow is a crucial signal in maintaining postural stability. We sought to investigate whether the activity of postural muscles and body sway was modulated by eye position during the view of radial optic flow stimuli. We manipulated the spatial distribution of dot speed and the fixation point position to simulate specific heading directions combined with different gaze positions. The experiments were performed using stabilometry and surface electromyography (EMG) on 24 right-handed young, healthy volunteers. Center of pressure (COP) signals were analyzed considering antero-posterior and medio-lateral oscillation, COP speed, COP area, and the prevalent direction of oscillation of body sway. We found a significant main effect of body side in all COP parameters, with the right body side showing greater oscillations. The different combinations of optic flow and eye position evoked a non-uniform direction of oscillations in females. The EMG analysis showed a significant main effect for muscle and body side. The results showed that the eye position modulated body sway without changing the activity of principal leg postural muscles, suggesting that the extraretinal input regarding the eye position is a crucial signal that needs to be integrated with perceptual optic flow processing in order to control body sway.

Muscle Vibration-Induced Illusions: Review of Contributing Factors, Taxonomy of Illusions and User’s Guide

Limb muscle vibration creates an illusory limb movement in the direction corresponding to lengthening of the vibrated muscle. Neck muscle vibration results in illusory motion of visual and auditory stimuli. Attributed to the activation of muscle spindles, these and related effects are of great interest as a tool in research on proprioception, for rehabilitation of sensorimotor function and for multisensory immersive virtual environments. However, these illusions are not easy to elicit in a consistent manner. We review factors that influence them, propose their classification in a scheme that links this area of research to perception theory, and provide practical suggestions to researchers. Local factors that determine the illusory effect of vibration include properties of the vibration stimulus such as its frequency, amplitude and duration, and properties of the vibrated muscle, such as contraction and fatigue. Contextual (gestalt) factors concern the relationship of the vibrated body part to the rest of the body and the environment. Tactile and visual cues play an important role, and so does movement, imagined or real. The best-known vibration illusions concern one’s own body and can be classified as ‘first-order’ due to a direct link between activity in muscle spindles and the percept. More complex illusions involve other sensory modalities and external objects, and provide important clues regarding the hidden role of proprioception, our ‘silent’ sense. Our taxonomy makes explicit this and other distinctions between different illusory effects. We include User’s Guide with tips for anyone wishing to conduct a vibration study.

The distinctive vertical heterophoria of dyslexics

In this study, we looked for the presence of vertical heterophoria (VH) in 42 dyslexic children (22 males and 20 females) aged 118.5±12.9 months who were compared with a control group of 22 nondyslexic children (eleven males and eleven females) aged 112±9.8 months. Dyslexics presented a low-level (always <1 prism diopter) VH combined with torsion. This oculomotor feature clearly separates the dyslexic group from the normal readers group. It is independent of the type of dyslexia. The essential feature of this VH is a lability that appears during specific stimulation of sensory receptors involved in postural regulation. This lability is demonstrated using a vertical Maddox test conducted under very specific conditions in which postural sensors are successively stimulated in a predetermined order. A quantitative variation in this VH may be seen during the Bielchowsky Head Tilt Test, which reveals hypertonia of the lower or upper oblique muscles. Vertical orthophoria can be achieved by placing low-power prisms asymmetrically within the direction of action of the superior or inferior oblique muscles. The selection of power and axis is not only guided by elements of the eye examination but also from observation of postural muscle tone. All these elements suggest that the VH could be of postural origin and somehow related to the vertical action of the oblique muscles. VH and torsion are not harmful per se. There is no statistical relationship between their level and the various parameters used to assess the reading skills of dyslexic children. VH and torsion could be a clinical marker of global proprioceptive dysfunction responsible for high-level multisensory disturbances secondary to poor spatial localization of visual and auditory information. This dysfunction might also explain the motor disorders concomitant to dyslexia.

Unilateral vestibular loss impairs external space representation

The vestibular system is responsible for a wide range of postural and oculomotor functions and maintains an internal, updated representation of the position and movement of the head in space. In this study, we assessed whether unilateral vestibular loss affects external space representation. Patients with Menière's disease and healthy participants were instructed to point to memorized targets in near (peripersonal) and far (extrapersonal) spaces in the absence or presence of a visual background. These individuals were also required to estimate their body pointing direction. Menière's disease patients were tested before unilateral vestibular neurotomy and during the recovery period (one week and one month after the operation), and healthy participants were tested at similar times. Unilateral vestibular loss impaired the representation of both the external space and the body pointing direction: in the dark, the configuration of perceived targets was shifted toward the lesioned side and compressed toward the contralesioned hemifield, with higher pointing error in the near space. Performance varied according to the time elapsed after neurotomy: deficits were stronger during the early stages, while gradual compensation occurred subsequently. These findings provide the first demonstration of the critical role of vestibular signals in the representation of external space and of body pointing direction in the early stages after unilateral vestibular loss.

Looking at eye dominance from a different angle: is sighting strength related to hand preference?

Sighting dominance (the behavioural preference for one eye over the other under monocular viewing conditions) has traditionally been thought of as a robust individual trait. However, Khan and Crawford (2001) have shown that, under certain viewing conditions, eye preference reverses as a function of horizontal gaze angle. Remarkably, the reversal of sighting from one eye to the other depends on which hand is used to reach out and grasp the target. Their procedure provides an ideal way to measure the strength of monocular preference for sighting, which may be related to other indicators of hemispheric specialisation for speech, language and motor function. Therefore, we hypothesised that individuals with consistent side preferences (e.g., right hand, right eye) should have more robust sighting dominance than those with crossed lateral preferences. To test this idea, we compared strength of eye dominance in individuals who are consistently right or left sided for hand and foot preference with those who are not. We also modified their procedure in order to minimise a potential image size confound, suggested by Banks et al. (2004) as an explanation of Khan and Crawford's results. We found that the sighting dominance switch occurred at similar eccentricities when we controlled for effects of hand occlusion and target size differences. We also found that sighting dominance thresholds change predictably with the hand used. However, we found no evidence for relationships between strength of hand preference as assessed by questionnaire or by pegboard performance and strength of sighting dominance. Similarly, participants with consistent hand and foot preferences did not show stronger eye preference as assessed using the Khan and Crawford procedure. These data are discussed in terms of indirect relationships between sighting dominance, hand preference and cerebral specialisation for language and motor control.

Extraocular muscle afferent signals modulate visual attention

PURPOSE

Extraocular muscle afferent signals contribute to oculomotor control and visual localization. Prompted by the close links between the oculomotor and attention systems, it was investigated whether these proprioceptive signals also modulated the allocation of attention in space.

METHODS

A suction sclera contact lens was used to impose an eye rotation on the nonviewing, dominant eye. With their viewing, nondominant eye, participants (n = 4) fixated centrally and detected targets presented at 5° in the left or right visual hemifield. The position of the viewing eye was monitored throughout the experiment. As a control, visual localization was tested using finger pointing without visual feedback of the hand, whereas the nonviewing eye remained deviated.

RESULTS

The sustained passive rotation of the occluded, dominant eye, while the other eye maintained central fixation, resulted in a lateralized change in the detectability of visual targets. In all participants, the advantage in speed and accuracy for detecting right versus left hemifield targets that occurred during a sustained rightward eye rotation of the dominant eye was reduced or reversed by a leftward eye rotation. The control experiment confirmed that the eye deviation procedure caused pointing errors consistent with an approximately 2° shift in perceived eye position, in the direction of rotation of the nonviewing eye.

CONCLUSIONS

With the caveat of the small number of participants, these results suggest that extraocular muscle afferent signals modulate the deployment of attention in visual space.

Vertical alignment in monkeys with unilateral IV section: effects of prolonged monocular patching and trigeminal deafferentation

We investigated monocular viewing and trigeminal (V) deafferentation on the vertical deviation (VD) in monkeys following intracranial IV section. Two monkeys wore a patch for four to six weeks, one over the paretic eye and the other over the normal eye following IV section. Two other monkeys had combined IV and V section with the paretic eye patched postlesion. In monkeys with IV section alone, the VD lessened within the first week postlesion but then increased gradually with the same eye still patched. Thus binocular viewing was unnecessary for the later VD increase. With combined IV and V section, the VD also transiently lessened postlesion. We have proposed that the decrease in VD after IV section is adaptive, driven by an error signal using ocular proprioception and efference copy. Since V section did not eliminate the early decrease in VD, we suggest some orbital afference is transmitted centrally via other cranial nerves. However, the later increase in VD suggests either that the proprioceptive effect cannot be sustained or that mechanical changes supervene to increase the VD.

Ocular dominance and balance performance in healthy adults

PURPOSE

To evaluate the effect of ocular dominance on balance performance in healthy adult subjects.

METHODS

Ocular dominance was determined in 24 healthy subjects using the hole-in-the-paper test. Balance function was evaluated by computerized dynamic platform posturography (CDPP). Sway index (SI), antero-posterior sway (APS) and lateral sway (LS) were served as outcome parameters.

RESULTS

The outcome parameters did not differ significantly between dominant and non-dominant eye fixation both in static and angular balance tests (SI-5.47+/-0.42, 6.23+/-0.52, p=0.146 and 18.4+/-1.07, 19.11+/-1.15, p=0.142, respectively; APS--2.26+/-4.68, -5.1+/-4.6, p=0.082 and -1.94+/-3.33, -3.64+/-2.6, p=0.48, respectively; LS--1.21+/-1.46, -1.12+/-1.66 p=0.94 and -1.98+/-1.16, -1.55+/-1.39, p=0.69, respectively).

CONCLUSIONS

Ocular dominance does not seem to affect postural function in the monovision and far viewing condition.

Localization in the frontal plane is not susceptible to manipulation of afferent feedback via the Jendrassik Maneuver

We have previously shown that registered vergence eye position is altered while participants perform the Jendrassik Maneuver (JM). We proposed that the altered eye position signal registration is due to the effect of the JM which changes the gain of the sensory feedback from the eye muscles, possibly via the activity of non-twitch motoneurons. We conducted two studies to further extend and clarify one of our previous findings by examining whether the JM also affects registered eye position during localization in the frontal plane. Since the non-twitch motoneurons do not receive premotor input from areas involved in the programming of saccades, we hypothesized that localization responses associated with the saccadic system should not be affected by the JM. The data confirmed our prediction. We propose that the non-twitch motoneurons are involved in parametric adjustment of the proprioceptive feedback loops of the vergence but not the version eye movements.

Strabismus and eye muscle function

Studies of external eye muscle morphology and physiology are reviewed, with respect to both motor and sensory functions in concomitant strabismus. The eye muscles have a more complex fibre composition than other striated muscle, and they are among the fastest and most fatigue-resistant muscles in the body. However, it is not generally believed that concomitant strabismus is due to a primary abnormality of the eye muscles or the ocular motor system. The gross anatomy of eye muscles, including the shape and position of the eye muscle pulleys, was not changed in strabismus. The histology of the eye muscle fibres was also basically the same, but changes have been observed in the cellular and biochemical machinery of the fibres, most notably in the singly innervated orbital fibres. Functionally, this was seen as slower contractions and reduced fatigue resistance of eye muscles in animals with strabismus and defects of binocular vision. Most likely the changes represented an adaptation to modified visual demands on the ocular motor control, because of the defects of binocular vision in strabismus from an early age. Adaptation of eye muscle function to visual demands could be seen also in the adult human ocular motor system, but here the effects could be reversed with treatment in some conditions. External eye muscles in the human have sensory organs, muscle spindles and tendon organs, responding to changes in muscle force and length. It is not known how these proprioceptors are used more specifically in ocular motor control, and there is no stretch reflex in the external eye muscles. However, a clear influence on space localization and eye position can be demonstrated with vibratory stimulation of the eye muscles, presumably activating muscle spindles. Different effects were observed in normal subjects and in adult patients with strabismus, which would indicate that the proprioceptive input from one eye of strabismic patients could be suppressed by the other eye, similar to visual suppression in concomitant strabismus. Such an interaction would most likely occur in the visual cortex, and not in the ocular motor system. Further studies of proprioceptive mechanisms, during the postnatal developmental stage and in adult concomitant strabismus may shed light on the mechanisms of childhood strabismus and may, in this respect, be a more fruitful avenue for further research than eye motor studies.

Manipulation of extraocular muscle afference has no effect on higher order perceptual judgments

Observers perceive targets as farther while performing the Jendrassik Maneuver (JM) suggesting that eye position is registered as more divergent. We examined the effects of the JM perturbation in three studies of perceptual judgment that rely on accurate registration of absolute distance: size constancy, stereoscopic depth, and the magnitude of the Pulfrich illusion. The data showed no significant differences between the JM and control conditions. The lack of an effect may be due to the fact that vergence is not a perfect cue to distance. Furthermore, the relative contribution of extraocular muscle afference to registered eye position may be less significant for higher order perceptual judgments.

Monovision: a review

In presbyopia, patients can no longer obtain clear vision at distance and near. Monovision is a method of correcting presbyopia where one eye is focussed for distance vision and the other for near. Monovision is a fairly common method of correcting presbyopia with contact lenses and has received renewed interest with the increase in refractive surgery. The present paper is a review of the literature on monovision. The success rate of monovision in adapted contact lens wearers is 59-67%. The main limitations are problems with suppressing the blurred image when driving at night and the need for a third focal length, for example with computer screens at intermediate distances. Stereopsis is impaired in monovision, but most patients do not seem to notice this. These limitations highlight the need to take account of occupational factors. Monovision could cause a binocular vision anomaly to decompensate, so the pre-fitting screening should include an assessment of orthoptic function. Various methods have been used to determine which eye should be given the distance vision contact lens and the literature on tests of ocular dominance is reviewed. It is concluded that tests of blur suppression are most likely to be relevant, but that ocular dominance is not fixed but is rather a fluid, adaptive, phenomenon in most patients. Suitable patients can often be given trial lenses that allow them to experiment with monovision in real world situations and this can be a useful way of revealing the preferred eye for each distance. Of course, no patient should drive or operate machinery until successfully adapted to monovision. Surgically induced monovision is less easily reversed than contact lens-induced monovision, and is only appropriate after a successful trial of monovision with contact lenses.

Cervical range of motion and proprioception in rugby players versus non-rugby players

In this study, we examined the effects of number of years of playing rugby on neck function. Active cervical spine range of motion and proprioception were assessed in 14 non-rugby-playing but trained sportsmen (mean age 28 years, s = 7) and 46 rugby players (26 rugby forwards: mean age 26 years, s = 5; mean years played 14 years; 20 backs: mean age 24 years, s = 5; mean years played 14 years). Active cervical range of motion in flexion, extension, left and right lateral flexion, plus left and right rotation were measured using a cervical range of motion device. The ability to reposition the head in a central position with eyes closed was taken as a measure of proprioception. Results show that rugby forwards generally had the least active cervical range of motion, particularly neck extension (forwards, 43 degrees ; backs, 55 degrees ; controls, 58 degrees ), with the decrement correlating with the number of years played. In addition, repositioning was significantly worse in rugby players after neck extension than non-rugby players (6 degrees vs. 3 degrees ). The active cervical range of motion of rugby forwards is similar to that of whiplash patients, suggesting that participation in rugby can have an effect on neck range of motion that is equivalent to chronic disability. Reduced active cervical range of motion could also increase the likelihood of injury and exacerbate age-related neck problems.

Muscular proprioception contributes to the control of interceptive actions

The authors proposed a model of the control of interceptive action over a ground plane (Chardenon, Montagne, Laurent, & Bootsma, 2004). This model is based on the cancellation of the rate of change of the angle between the current position of the target and the direction of displacement (i.e., the bearing angle). While several sources of visual information specify this angle, the contribution of proprioceptive information has not been directly tested. In this study, the authors used a virtual reality setup to study the role of proprioception when intercepting a moving target. In a series of experiments, the authors manipulated proprioceptive information by using the tendon vibration paradigm. The results revealed that proprioception is crucial not only to locate a moving target with respect to the body but also, and more importantly, to produce online displacement velocity changes to intercept a moving target. These findings emphasize the importance of proprioception in the control of interceptive action and illustrate the relevance of our model to account for the regulations produced by the participants.

Proprioception in extraocular muscles

Human extraocular muscles are richly endowed with sensory receptors. The precise role of afferent signals derived from these proprioceptors in ocular motor control and spatial localization has been the subject of considerable debate for more than a century. Laboratory-based and clinical studies have increasingly suggested that proprioceptive signals from extraocular muscles influence visuomotor behavior.

Proprioceptive role for palisade endings in extraocular muscles: Evidence from the Jendrassik Maneuver

A proprioceptive hypothesis for the control of eye movements has been recently proposed based on neuroanatomical tracing studies. It has been suggested that the non-twitch motoneurons could be involved in modulating the gain of sensory feedback from the eye muscles analogous to the gamma (gamma) motoneurons which control the gain of proprioceptive feedback in skeletal muscles. We conducted behavioral and psychophysical experiments to test the above hypothesis using the Jendrassik Maneuver (JM) to alter the activity of gamma motoneurons. It was hypothesized that the JM would alter the proprioceptive feedback from the eye muscles which would result in misregistration of eye position and mislocalization of targets. In the first experiment, vergence eye movements and pointing responses were examined. Data showed that the JM affected the localization responses but not the actual eye position. Perceptual judgments were tested in the second experiment, and the results showed that targets were perceived as farther when the afferent feedback was altered by the JM. Overall, the results from the two experiments showed that eye position was perceived as more divergent with the JM, but the actual eye movements were not affected. We tested this further in Experiment 3 by examining the effect of JM on the amplitude and velocity of saccadic eye movements. As expected, there were no significant differences in saccadic parameters between the control and experimental conditions. Overall, the present study provides novel insight into the mechanism which may be involved in the use of sensory feedback from the eye muscles. Data from the first two experiments support the hypothesis that the JM alters the registered eye position, as evidenced by the localization errors. We propose that the altered eye position signal is due to the effect of the JM which changes the gain of the sensory feedback from the eye muscles, possibly via the activity of non-twitch motoneurons.

Eye movements cannot explain vibration-induced visual motion and motion aftereffect

Eye movements are thought to account for a number of visual motion illusions involving stationary objects presented against a featureless background or apparent motion of the whole visual field. We tested two different versions of the eye movement account: (a) the retinal slip explanation and (b) the nystagmus-suppression explanation, in particular their ability to account for visual motion experienced during vibration of the neck muscles, and for the visual motion aftereffect following vibration. We vibrated the neck (ventral sternocleidomastoid muscles, bilaterally, or right dorsal muscles) and measured eye movements in conjunction with perceived illusory displacement of an LED presented in complete darkness (N=10). To test the retinal-slip explanation, we compared the direction of slow eye movements to the direction of illusory motion of the visual target. To test the suppression explanation, we estimated the direction of suppressed slow-phase eye movements and compared it to the direction of illusory motion. Two main findings show that neither actual nor suppressed eye movements cause the illusory motion and motion aftereffect. Firstly, eye movements do not reverse direction when the illusory motion reverses after vibration stops. Secondly, there are large individual differences with regards to the direction of eye movements in observers who all experience a similar visual illusion. We conclude that, rather than eye movements, a more global spatial constancy mechanism that takes into account head movement is responsible for the illusion. The results also argue against the notion of a single central signal that determines both perceptual experience and oculomotor behaviour.

Strabismus and sensory-motor function of eye muscles

Paul Bach-y-Rita and coworkers at the Smith-Kettlewell Institute of Visual Science of San Francisco were among the first to record activity in the muscle fibers of the eye muscles in animals. With their newly developed methods, they could describe fast and slow muscle fibers types and present possible patterns of recruitment of the fibers in different eye movements. These studies have been critical for continued animal research on eye muscle fibers and motor units in different species and in animals of different ages. Bach-y-Rita and coworkers also recorded from receptors in the muscles and demonstrated stretch reflexes different from those of skeletal muscles. Further research in animals revealed that it was difficult to delineate the functional role of the muscle receptors in oculomotor control. However, recent studies on sensory functions of human extra ocular muscles have suggested that proprioception participates in space localization, and the functions may differ in normal and strabismic subjects. The eye muscle studies initiated by Bach-y-Rita have enabled analysis of the sensory-motor components of strabismus or squint in greater detail than before.

Perception, action, and Roelofs effect: a mere illusion of dissociation

A prominent and influential hypothesis of vision suggests the existence of two separate visual systems within the brain, one creating our perception of the world and another guiding our actions within it. The induced Roelofs effect has been described as providing strong evidence for this perception/action dissociation: When a small visual target is surrounded by a large frame positioned so that the frame's center is offset from the observer's midline, the perceived location of the target is shifted in the direction opposite the frame's offset. In spite of this perceptual mislocalization, however, the observer can accurately guide movements to the target location. Thus, perception is prone to the illusion while actions seem immune. Here we demonstrate that the Roelofs illusion is caused by a frame-induced transient distortion of the observer's apparent midline. We further demonstrate that actions guided to targets within this same distorted egocentric reference frame are fully expected to be accurate, since the errors of target localization will exactly cancel the errors of motor guidance. These findings provide a mechanistic explanation for the various perceptual and motor effects of the induced Roelofs illusion without requiring the existence of separate neural systems for perception and action. Given this, the behavioral dissociation that accompanies the Roelofs effect cannot be considered evidence of a dissociation of perception and action. This indicates a general need to re-evaluate the broad class of evidence purported to support this hypothesized dissociation.

Paul Dassonville and Jagdeep Bala challenge a prominent hypothesis that proposes the existence of two separate visual systems within the brain, one creating perception and the other guiding action

Convergence defects in patients with temporomandibular disorders

The aim of this study is to show the presence of a correlation between ocular convergence defects (OCD) and temporomandibular disorders (TMD) among a group of adult subjects. The group studied was made up of 48 subjects (12 males and 36 females). The average age was 35 with a range of 19-45 years of age. The subjects presented with TMD and muscular pain and/or dysfunction. Forty-eight subjects with TMD for the case study were matched by gender and age to 48 control subjects seeking routine dental care (control group). All the subjects were examined by the same orthoptist who classified the ocular convergence degree using two tests. The first test evaluated the distances (in centimeters) of the convergence near point (3-4 cm: normal; 5-7 cm: sufficient; > 7 cm: insufficient). The second test assessed the fusional convergence using a Berens prism test (> 25 diopters: normal; between 18-25: sufficient; < 18 diopters: insufficient). In the TMD group, 36 subjects (75%) showed a compromise of convergence: 13 (36%) were classified in the 5-7 degree range and 23 (48%) in the > 7 cm degree range. The Berens test showed ten subjects (28%) in the group < 18D and 26 (72%) in the group 18-25D. The control-group presented ten (21%) subjects with compromise of convergence: three classified in the group < 18D and seven in the group 18-25D. The TMD subjects presented a higher statistical percentage (p < 0.0001) of ocular convergence defects. The TMD patients also reported a strong association referred to specific signs and symptoms, i.e., limited maximal opening or myofascial pain. There were some subjective reports also of headaches and torcicollis (neck stiffness) which appeared significantly more frequently in subjects with a compromise of convergence. The study showed a much higher prevalence of ocular convergence defects in patients with head, neck, and shoulder pain.

Vision research: losing sight of eye dominance

Most people prefer to use their right eye for viewing. New evidence reveals that this dominance is much more plastic than that for one hand or foot: it changes from one eye to the other depending on angle of gaze. Remarkably, sighting dominance depends on the hand being directed towards the visual target.

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.

Gaze-shift dynamics in subjects with and without symptoms of convergence insufficiency: influence of monocular preference and the effect of training

We studied gaze-shift dynamics during several gaze-shift tasks and during reading, in five subjects with convergence insufficiency (C.I., a diminished ability to converge), and in ten subjects without C.I. Furthermore, we studied the effect of vergence training in order to verify previous claims that orthoptic exercises can improve vergence performance. We recorded binocular eye movements with the scleral coil technique. Subjects switched fixation between nearby and distant light emitting diodes (LEDs) arranged in isovergence arrays (distances 35 and 130 cm) in a dimly lit room. In both the C.I. and non-C.I. group, two classes of subjects occurred: vergence responders and saccadic responders. During pure vergence tasks, saccadic responders made saccades with no or little vergence; vergence responders made vergence movements with no or small saccadic components. In saccadic responders, fixation of nearby targets was monocular. Subjects with a preferred eye, according to our determination, used the preferred eye. The five C.I. subjects showed idiosyncratic responses with insufficient vergence during most trials. They all had a tendency to alternate fixation between the left and right eye. Vergence-version tasks always elicited larger vergence components than pure vergence tasks. During a reading task, vergence angles were more accurate than during gaze-shifts between LEDs. After the pre-training sessions, nine subjects (one of which had C.I.) practised a pure vergence task three times a day for at least 2 weeks. Vergence amplitudes of four of these subjects were larger after training. We conclude that vergence training can change oculomotor performance. Although C.I. is often associated with abnormal vergence dynamics, there are no typical C.I. vergence dynamics. Unstable monocular preferences may play a role in the aetiology of C.I.

Motor and perceptual responses to horizontal and vertical eye vibration in humans

Previous studies have shown that low amplitude/high frequency mechanical vibration applied to the human eye muscles results in the illusory movement of a luminous spot fixated in total darkness. The aim of the present study was to investigate whether a vibration-induced motor response also occurs in eye muscles, and to check whether the visual illusions actually result from the proprioceptors being activated by the vibration, or whether they are simply due to the retinal slip induced by the reflex eye movement. The effects of the vibratory stimuli on the inferior rectus (IR) and lateral rectus (LR) muscles were evaluated by recording subjects' eye position changes. When applied to the IR muscle, vibration effectively elicited an upward visual illusion accompanied by a small downward ocular rotation, whereas when applied to the LR muscle, it also induced horizontal visual illusion, which was less frequent and weaker than the vertical one, but no ocular rotation. We concluded that visual illusions of this kind cannot be attributable to the retinal motion of the image of the fixated point. The difference between the vertical and horizontal vibratory motor responses is discussed as regards the particular role that oculo-muscular proprioception may play in the vertical muscles.

Effects of eye muscle proprioceptive activation on eye position in normal and exotropic subjects

BACKGROUND

Activation of muscle spindles by vibration of eye muscles is known to induce illusory movements of fixated targets, but the effects on eye position have not been studied, either in normal subjects or in patients with exotropia.

METHODS

Eye position was recorded from the covered, non-dominant eye with an infrared system in 11 subjects with normal eyes and binocular vision and in 10 patients with exotropia and abnormal binocular function. Activation of eye muscle spindles was done by vibration at 70 Hz of the inferior and lateral rectus muscles of the dominant eye, fixating a light-emitting diode in subdued light.

RESULTS

Vibratory activation of proprioceptors in the inferior rectus muscle induced an eye movement mainly directed upward in both normal and exotropic subjects. The magnitude of the movement was on average 2.7 deg in normals and 2.4 deg in exotropes. Lateral rectus vibration induced a movement that was mainly temporally directed (abduction) of an average 2.1 deg in normal subjects, but a nasally directed (adduction) movement of 4.2 deg in exotropic subjects. In normal subjects the eye movement is of the same direction as the earlier reported visual illusory movements induced by the same type of proprioceptive activation, but in exotropic subjects the movements is in the opposite direction.

CONCLUSIONS

Proprioceptive activation of eye muscles affects eye position, and the results also indicate that signals from eye muscles are processed differently in normals and strabismics, probably depending on the level of binocular function.

Shift in saccadic direction induced in humans by proprioceptive manipulation: a comparison between memory-guided and visually guided saccades

It is nowadays generally recognized that saccades to remembered targets are planned in a craniotopic frame of reference by combining retinal input with eye position signal. The origin of the eye position signal is still a matter of controversy, however. Does it arise from an efferent copy or is it supplied by the sensory receptors with which the extraocular muscles are endowed? When applied to skeletal muscles, vibration elicits spindle responses simulating a stretching of the vibrated muscle. When vibration is applied to the inferior rectus muscle (IR), it induces the illusion that a stationary fixating point is moving upward. Here we attempted to change the initial eye position signal supplied to the oculomotor system before a memory- or visuo-guided saccade to a 10 degrees left target by applying mechanical vibration to the IR muscle. We wanted to determine whether modifying extraocular proprioceptive cues during the programming phase of a saccade might affect the latter's trajectory. In the memory-guided condition, it was observed that the saccades ended lower down when vibration was applied than in the control condition. Conversely, the visuo-guided saccades were not affected by the vibration. The above results mean first that extraocular proprioceptive cues are used as an initial eye position signal when a memory guided saccade has to be planned. Secondly, they suggest that extraocular proprioception may not be used to produce a visuo-guided saccade, or that this type of saccade is computed solely on the basis of retinal cues.

The use of egocentric and exocentric location cues in saccadic programming

Theoretically, the location of a visual target can be encoded with respect to the locations of other stimuli in the visual image (exocentric cues), or with respect to the observer (egocentric cues). Egocentric localization in the oculomotor system has been shown to rely on an internal representation of eye position that inaccurately encodes the time-course of saccadic eye movements, resulting in the mislocalization of visual targets presented near the time of a saccade. In the present investigation, subjects were instructed to localize perisaccadic stimuli in the presence or absence of a visual stimulus that could provide exocentric location information. Saccadic localization was more accurate in the presence of the exocentric cue, suggesting that localization is based on a combination of exocentric and egocentric cues. These findings indicate the need to reassess previously reported neurophysiological studies of spatial accuracy and current models of oculomotor control, which have focused almost exclusively on the egocentric localization abilities of the brain.

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.