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

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.

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.

Effects of smooth pursuit and second-order stimuli on visual motion prediction

The purpose of this study was to determine whether smooth pursuit eye movements affect visual motion prediction using a time‐to‐contact task where observers anticipate the exact instant that a partially occluded target would coincide with a stationary object. Moreover, we attempted to clarify the influence of second‐order motion on visual motion prediction during smooth pursuit. One target object moved to another stationary object (6 deg apart) at constant velocity of 3, 4, and 5 deg/s, and then the two objects disappeared 500 ms after the onset of target motion. The observers estimated the moment the moving object would overlap the stationary object and pressed a button. For the pursuit condition, both a Gaussian window and a random dots texture moved in the same direction at the same speed for the first‐order motion, whereas a Gaussian window moved over a static background composed of random dots texture for the second‐order motion. The results showed that the constant error of the time‐to‐contact shifted to a later response for the pursuit condition compared to the fixation condition, regardless of the object velocity. In addition, during smooth pursuit, the constant error for the second‐order motion shifted to an earlier response compared to the first‐order motion when the object velocity was 3 deg/s, whereas no significant difference was found at 4 and 5 deg/s. Therefore, our results suggest that visual motion prediction using a time‐to‐contact task is affected by both eye movements and motion configuration such as second‐order motion.

Passive eye movements induced by electromagnetic force (EMF) in rats

Accurate information on eye position in the orbit is available from visual feedback, efference copy of the oculomotor commands and proprioceptive signals from the extraocular muscles (EOM). Whereas visual feedback and oculomotor commands have been extensively studied, central processing of EOM proprioceptive signals remains to be elucidated. A challenge to the field is to develop an approach to induce passive eye movements without physically contacting the eyes. A novel method was developed to generate passive eye movements in rats. A small rare-earth magnet disk (0.7 mm diameter, 0.5 mm thickness) was attached to the surface of a rat's eyeball. A metal rod (5 mm diameter) wrapped with an electromagnetic (EM) coil was placed near the magnet (8-15 mm). By passing currents to the EM coil, electromagnetic force (EMF) was generated and acted upon the magnet and induced passive eye movements. The EMF induced well-defined passive eye movements, whose directions were dependent on current polarity and amplitudes and peak velocities were dependent on current intensity and duration. Peak velocities of the EMF-induced eye movements were linearly related to amplitudes, exhibiting main sequence relationships similar to that of saccades in awake rats and eye movements induced by electrical microstimulation of the abducens nucleus in anesthetized rats. Histological examination showed that repetitive EMF stimulations did not appear to result in damages in the EOM fibers. These results validated the EMF approach as a novel tool to investigate EOM proprioceptive signals and their roles in visual localization and gaze control.

A noninvasive electromagnetic perturbation approach to probe extraocular proprioception

BACKGROUND

Extraocular proprioception has been shown to participate in spatial perception and binocular alignment. Yet the physiological approaches used to study this sensory signal are limited because proprioceptive signaling takes place at the same time as visuomotor signaling. It is critical to dissociate this sensory signal from other visuomotor events that accompany eye movements.

METHODS

We present a novel noninvasive and quantifiable method for probing extraocular proprioception independent of other visuomotor processing by attaching a rare-earth magnet to a real-time model eye and placing an electromagnet <20 mm from the eye. An electromagnet can increase or decrease angular displacements and velocities of the model eye.

RESULTS

Electromagnetic activation rapidly affected (<2 ms) the rotation kinematics of the eye, which were correlated linearly with both the current supply and the distance of the electromagnet relative to the eye.

CONCLUSIONS

This method circumvented the constraints of conventional physiological manipulation of extraocular proprioception, such as manually or mechanically tugging on the eye ball. It can be applied to produce the discrepancy between the intended and the executed eye movements, so that proprioceptive reafference signals are dissociated from corollary motor discharges and other visuomotor events.

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.

Motion perception during involuntary eye vibration

Retinal motion caused by reflexive or voluntary eye movements is rarely misinterpreted as object motion, as if the visual system discounted the contribution of these eye movements to retinal motion. Yet, involuntary eye movements caused by mechanical eye vibration is often interpreted as object motion unless the vibration has high frequency, in which case only image blur may be noticed. In these latter conditions, however, a light flickering above the fusion limit is vividly perceived to undergo oscillatory motion over its static surround. We determined the conditions of this phenomenon, showing that the perceived frequency of illusory oscillation equals the difference between flicker frequency and the frequency of vibration of the eyes. This outcome is explained as a result of the low-pass temporal frequency characteristic of vision, which further predicts that the same effect should occur if the flickering light is vibrated and observed with static eyes. This prediction was corroborated empirically. We also determined the minimal amplitude of oscillation required to perceive motion as a function of postural stability and the presence of static references, finding an amplitude threshold of approximately 1 arcmin with postural stability in dim-light conditions, which increases to approximately 2 arcmin with postural instability in the dark.

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.