Changes In Eye Position Research Articles

Tonic eye movements induced by bilateral and unilateral galvanic vestibular stimulation (GVS) in guinea pigs

Galvanic vestibular stimulation (GVS) stimulates primary vestibular afferents innervating the semicircular canals (SCCs) and otoliths found in the inner ear of humans and other mammals, including guinea pigs. To determine which pathways contribute to eye movements generated by this artificial vestibular stimulation in guinea pigs, low current intensities of GVS were passed either bilaterally between the tensor-tympani muscles of the two ears (up to 30 μA) or unilaterally between one tensor-tympani electrode and an indifferent on the back of the neck (up to 60 μA). Both forms of GVS were found to selectively generate tonic eye movements without nystagmus, characteristic of the otolith-ocular reflex; the axis of eye rotation did not align with any semicircular canal plane, but was oriented close to the expected axis of eye rotation that would occur in response to the net stimulation of otolith afferents. The induced eye rotation was predominantly vertical with a smaller horizontal deviation and very little torsion. Consistent with the results of previous human studies, the tonic eye movements were found to exhibit bilateral gain enhancement, whereby bilateral GVS generated twice the amplitude of eye rotation as unilateral anodal or cathodal stimulation alone. Eye movement responses to unilateral GVS were symmetrical in amplitude during equivalent intensities of anodal and cathodal stimulation, consistent with the known responses of more regularly and intermediately discharging primary vestibular afferents to GVS. These results together suggest that more regularly discharging otolith-ocular projections may mediate the tonic changes in eye position induced during maintained, low-intensity GVS in guinea pigs.

A sub-Tenon’s capsule injection of lidocaine induces extraocular muscle akinesia and mydriasis in dogs

The effect of local anaesthetic on the extraocular muscles was investigated in dogs by injecting lidocaine into the space between Tenon’s capsule and the sclera. A cross-over design was used with both eyes from five Beagle dogs randomly injected, under general anaesthesia, with 1mL of 2% lidocaine (1mL-lidocaine group), 2mL of 2% lidocaine (2mL-lidocaine group) or 2mL of normal saline (control group). Each eye was assigned to all treatments with a minimum 14day interval between injections. Changes in eye position, pupil diameter, and intraocular pressure (IOP) were evaluated during the procedure.All eyes in the 2mL-lidocaine group exhibited akinesia and mydriasis (pupil diameter >10mm) with an onset time of 6.5±4.9 and 4.2±4.3min, respectively. In the 1mL-lidocaine group, akinesia was induced in nine eyes and mydriasis occurred in seven eyes at 10.7±5.8 and 5.4±2.4min after the injection, respectively. No changes in eye position or pupil diameter were observed in the control group. Akinesia was maintained for 44.3±26.7min in the 1mL-lidocaine group and for 88.5±17.2min in the 2mL-lidocaine group. Duration of mydriasis was 51.7±28.9min in the 1mL-lidocaine group and 82.9±15.6min in the 2mL-lidocaine group. Marked chemosis and sub-conjunctival haemorrhage occurred in 16/30 and 15/30 eyes, respectively. No significant change in IOP was observed between the mean pre- and post-injection values in all groups. These results suggest that a sub-Tenon’s injection of 2mL of 2% lidocaine provided effective extraocular muscle akinesia and mydriasis in dogs.

Stable Visual Working Memory Representations Across Changes in Eye Position

Stable Visual Working Memory Representations Across Changes in Eye Position

Responses of Cells in the Midbrain Near-Response Area in Monkeys with Strabismus

To investigate whether neuronal activity within the supraoculomotor area (SOA-monosynaptically connected to medial rectus motoneurons and encode vergence angle) of strabismic monkeys was correlated with the angle of horizontal misalignment and therefore helps to define the state of strabismus. Single-cell neural activity was recorded from SOA neurons in two monkeys with exotropia as they performed eye movement tasks during monocular viewing. Horizontal strabismus angle varied depending on eye of fixation (dissociated horizontal deviation) and the activity of SOA cells (n = 35) varied in correlation with the angle of strabismus. Both near-response (cells that showed larger firing rates for smaller angles of exotropia) and far-response (cells that showed lower firing rates for smaller angles of exotropia) cells were identified. SOA cells showed no modulation of activity with changes in conjugate eye position as tested during smooth-pursuit, thereby verifying that the responses were related to binocular misalignment. SOA cell activity was also not correlated with change in horizontal misalignment due to A-patterns of strabismus. Comparison of SOA population activity in strabismic animals and normal monkeys (described in the literature) show that both neural thresholds and neural sensitivities are altered in the strabismic animals compared with the normal animals. SOA cell activity is important in determining the state of horizontal strabismus, possibly by altering vergence tone in extraocular muscle. The lack of correlated SOA activity with changes in misalignment due to A/V patterns suggest that circuits mediating horizontal strabismus angle and those that mediate A/V patterns are different.

Differential lateral rectus compartmental contraction during ocular counter-rolling.

The lateral rectus (LR) and medial rectus (MR) extraocular muscles (EOMs) have largely nonoverlapping superior and inferior innervation territories, suggesting functional compartmental specialization. We used magnetic resonance imaging (MRI) in humans to investigate differential compartmental activity in the rectus EOMs during head tilt, which evokes ocular counter-rolling, a torsional vestibulo-ocular reflex (VOR). MRI in quasi-coronal planes was analyzed during target-controlled central gaze in 90° right and left head tilts in 12 normal adults. Cross sections and posterior partial volumes of the transverse portions of the four rectus EOMs were compared in contiguous image planes 2 mm thick spanning the orbit from origins to globe equator, and used as indicators of contractility. Horizontal rectus EOMs had significantly greater posterior volumes and maximum cross sections in their inferior compartments (P < 10(-8)). In orbit tilt up (extorted) compared with orbit tilt down (intorted) head tilts, contractile changes in LR maximum cross section (P < 0.0001) and posterior partial volume (P < 0.05) were significantly greater in the inferior but not in the superior compartment. These changes were not explainable by horizontal or vertical eye position changes. A weaker compartmental effect was suggested for MR. The vertical rectus EOMs did not exhibit significant compartmental contractile changes during head tilt. Mechanical modeling suggests that differential LR contraction may contribute to physiological cyclovertical effects. Selective activation of the two LR, and possibly MR, compartments correlates with newly recognized segregation of intramuscular innervation into distinct compartments, and probably contributes to noncommutative torsion during the VOR.

Changes in pupil diameter entrained by cortically initiated changes in attention

The diameter of the pupil is affected by changes in ambient illumination, color, spatial structure, movement, and mental effort. It has now been found that pupil diameter can be affected by cognitive processes. That is, it can be entrained by alternations between broadly spread and narrowly focused attention that are cued exogenously (attention is "summoned" by the cue) or endogenously (attention changes under the perceiver's intentional control). Pupil diameter also is affected by post-eye-blink constrictions that occur most often when attention is narrowed, and possibly by changes evoked by the near reflex, although changes in attention state parsimoniously account for the entirety of the results. Changes in pupil diameter produce differences in spherical aberration that alternately blur (when the pupil dilates) and sharpen the retinal image (when the pupil constricts), affecting the relative sensitivity of large receptive fields that mediate broadly spread attention compared with smaller receptive fields that mediate more narrowly focused attention. Results for endogenously cued, intentional changes in attentional spread provide definitive behavioral evidence for cortical feedback to subcortical nuclei that control pupil diameter, either directly or through pupil-constricting eye blinks. Analyses of convergent and divergent changes in eye position indicate that the near reflex was activated long after the initiation of relatively gradual attentionally cued changes in pupil diameter, and further, that when it occurs, the near reflex facilitates ongoing changes in pupil diameter.

Saccadic Interception of a Moving Visual Target after a Spatiotemporal Perturbation

Animals can make saccadic eye movements to intercept a moving object at the right place and time. Such interceptive saccades indicate that, despite variable sensorimotor delays, the brain is able to estimate the current spatiotemporal (hic et nunc) coordinates of a target at saccade end. The present work further tests the robustness of this estimate in the monkey when a change in eye position and a delay are experimentally added before the onset of the saccade and in the absence of visual feedback. These perturbations are induced by brief microstimulation in the deep superior colliculus (dSC). When the microstimulation moves the eyes in the direction opposite to the target motion, a correction saccade brings gaze back on the target path or very near. When it moves the eye in the same direction, the performance is more variable and depends on the stimulated sites. Saccades fall ahead of the target with an error that increases when the stimulation is applied more caudally in the dSC. The numerous cases of compensation indicate that the brain is able to maintain an accurate and robust estimate of the location of the moving target. The inaccuracies observed when stimulating the dSC that encodes the visual field traversed by the target indicate that dSC microstimulation can interfere with signals encoding the target motion path. The results are discussed within the framework of the dual-drive and the remapping hypotheses.

Tracking the eye non-invasively: simultaneous comparison of the scleral search coil and optical tracking techniques in the macaque monkey

From human perception to primate neurophysiology, monitoring eye position is critical to the study of vision, attention, oculomotor control, and behavior. Two principal techniques for the precise measurement of eye position—the long-standing sclera-embedded search coil and more recent optical tracking techniques—are in use in various laboratories, but no published study compares the performance of the two methods simultaneously in the same primates. Here we compare two popular systems—a sclera-embedded search coil from C-N-C Engineering and the EyeLink 1000 optical system from SR Research—by recording simultaneously from the same eye in the macaque monkey while the animal performed a simple oculomotor task. We found broad agreement between the two systems, particularly in positional accuracy during fixation, measurement of saccade amplitude, detection of fixational saccades, and sensitivity to subtle changes in eye position from trial to trial. Nonetheless, certain discrepancies persist, particularly elevated saccade peak velocities, post-saccadic ringing, influence of luminance change on reported position, and greater sample-to-sample variation in the optical system. Our study shows that optical performance now rivals that of the search coil, rendering optical systems appropriate for many if not most applications. This finding is consequential, especially for animal subjects, because the optical systems do not require invasive surgery for implantation and repair of search coils around the eye. Our data also allow laboratories using the optical system in human subjects to assess the strengths and limitations of the technique for their own applications.

Postural And Eye-Positional Effects On Human Biting Force: An Experimental Study

Muscle groups affected on biting force are called temporal muscle as a major and masseter muscle as a minor. According to the human posture stability, forces of these muscles vary with the force directions. In this case, experimental investigation is strictly important such that biting force under different postural and eyepositional situations is changed. In this study, seven-male and seven-female within the age-range of 17-24 are considered corresponding to having with restorated molar tooth and without that type of tooth. With the help of specially designed biting fork, different postureand eye-positions are investigated for experimental biting force analysis. Changes in eye-positions are not indicated significant difference for all postural positions. On one hand, it is obtained that biting force of no-filling tooth in men becomes maximum if facial muscles give full effort to biting. On the other hand, effect of facial muscles for women is not clearly noticed depending on the postural differences.

Periodic Unilateral Eyelid Retraction in a Pediatric Patient

A healthy 11-year-old girl presented with right upper eyelid retraction since birth. An evaluation including thyroid function studies and neuroimaging was negative, and the patient was scheduled for a right levator recession to address the eyelid malposition. Intraoperatively, after the induction of inhalational general anesthesia, the patient displayed cyclic right upper eyelid retraction. Occurring in intervals of exactly 48 seconds, these cycles involved a rapid elevation of the right eyelid from a position of half-closure to a retracted position just above the superior limbus. There was no change in pupil size or eye position during these cyclic spasms, and the contralateral eyelid was unaffected. The patient underwent an uncomplicated levator recession, which improved the upper eyelid retraction. Postoperative testing, including external motility video and infrared pupillometry, demonstrated no cyclic variation in eyelid position, eye position, or pupil size in the waking state. This is a unique case of unilateral eyelid retraction with periodic spasms under conditions of anesthesia without a preexisting oculomotor paresis; it represents an unusual variation on congenital eyelid retraction and classically described cyclic oculomotor palsy.

Does orbital proprioception contribute to gaze stability during translation?

Translational motion induces retinal image slip which varies with object distance. The brain must know binocular eye position in real time in order to scale eye movements so as to minimize retinal slip. Two potential sources of eye position information are orbital proprioception and an internal representation of eye position derived from central ocular motor signals. To examine the role of orbital proprioceptive information, the position of the left eye was perturbed by microstimulation of the left abducens nerve during translational motion to the right or left along the interaural axis in two rhesus macaques. Microstimulation rotated the eye laterally, activating eye muscle proprioceptors, while keeping central motor commands undisturbed. We found that microstimulation-induced eye position changes did not affect the translational VOR in the abductive (lateral rectus) direction, but it did influence the responses in the adductive (medial rectus) direction. Our findings demonstrate that proprioceptive inputs appear to be involved in the TVOR responses at least during ipsilateral head movements and proprioceptive influences on the TVOR may involve vergence-related signals to the oculomotor nucleus. However, internal representation of eye position, derived from central ocular motor signals, likely plays the dominant role in providing eye position information for scaling eye movements during translational motion, particularly in the abducent direction.

Static ocular counterroll reflex in skew deviation

The static ocular counterroll (OCR) reflex generates partially compensatory torsional eye movements during head roll. It is mediated by the utricle in the inner ear. Skew deviation is a vertical strabismus thought to be caused by imbalance in the utriculo-ocular pathway. We hypothesized that if skew deviation is indeed caused by damage to this reflex pathway, patients with skew deviation would show abnormal OCR. Eighteen patients with skew deviation caused by brainstem or cerebellar lesions and 18 normal participants viewed a target at 1 m. Ocular responses to static passive head roll-tilts of approximately 20° were recorded using search coils. Static OCR gain was calculated as the change in torsional eye position divided by the change in head position during sustained head roll. Perception of the subjective visual vertical (SVV) was also measured. Group mean OCR gain was reduced by 45% in patients. At an individual level, OCR gains were asymmetric between eyes and between torsional directions in 90% of patients. In addition, the hypotropic eye incyclotorting gain was lower than the hypertropic eye excyclotorting gain during head roll toward the hypotropic eye in 94% of patients. No consistent pattern of gain asymmetry was found during head roll toward the hypertropic eye. The SVV was tilted toward the hypotropic eye. Static OCR gain is significantly reduced in skew deviation. Interocular and directional gain asymmetries are also prevalent. The asymmetries provide further evidence that disruption of the utriculo-ocular pathway is a mechanism for skew deviation.

Do Common Systems Control Eye Movements and Motion Extrapolation?

People are able to judge the current position of occluded moving objects. This operation is known as motion extrapolation. It has previously been suggested that motion extrapolation is independent of the oculomotor system. Here we revisited this question by measuring eye position while participants completed two types of motion extrapolation task. In one task, a moving visual target travelled rightwards, disappeared, then reappeared further along its trajectory. Participants discriminated correct reappearance times from incorrect (too early or too late) with a two-alternative forced-choice button press. In the second task, the target travelled rightwards behind a visible, rectangular occluder, and participants pressed a button at the time when they judged it should reappear. In both tasks, performance was significantly different under fixation as compared to free eye movement conditions. When eye movements were permitted, eye movements during occlusion were related to participants' judgements. Finally, even when participants were required to fixate, small changes in eye position around fixation (<2°) were influenced by occluded target motion. These results all indicate that overlapping systems control eye movements and judgements on motion extrapolation tasks. This has implications for understanding the mechanism underlying motion extrapolation.

Changes in the Interpupillary Distance Following General Anesthesia in Children With Intermittent Exotropia: A Predictor of Surgical Outcomes

To determine whether changes in ocular alignment following general anesthesia, as measured by interpupillary distance, can be used as a predictor for surgical outcomes in children with intermittent exotropia. The authors obtained digital photographs of 40 children with exotropia and 20 children with epiblepharon (control group) before and immediately after induction of general anesthesia in the primary supine position. Differences in the anatomic interpupillary distance (aIPD) for each patient were measured and compared with the preoperative angles of deviation. They were also correlated with surgical outcomes on the first day and 1 year after surgery. All 60 patients demonstrated an increase in aIPD following general anesthesia. The mean change in aIPD in patients with exotropia was similar to that in patients with epiblepharon (3.78% ± 2.53% and 3.15% ± 1.05%, respectively). In patients with exotropia, there was a significant positive linear correlation between the preoperative distance deviation (P) and the change in eye position (A): A = 0.362 P - 4.488, r(2) = 0.476 (P < .001). Five (71%) of seven patients whose changes were outside the 80% confidence interval for expected values had poor surgical outcomes on the first postoperative day and four had unsatisfactory results 1 year after surgery. Changes in aIPD following general anesthesia were strongly correlated with preoperative angle of deviation. Therefore, they may be a useful predictor of surgical outcomes in children with exotropia.

Orbital Volume and Eye Position Changes After Balanced Orbital Decompression

To evaluate the changes in eye position and orbital content expansion following medial and lateral orbital wall decompression for thyroid-associated orbitopathy. The authors used a computer software program (syngo InSpace4D, syngo 3D, syngo fusion, and syngo Volume Evaluation; Siemens AG, Forchheim, Germany) to measure the orbital expansion gained by medial and lateral orbital wall decompression and the change in eye position between the digitized preoperative and postoperative orbital CT scans. Twenty patients (16 women and 4 men) with a mean age of 45 years (range, 18-64 years) were enrolled in this study. Of the 20 patients who underwent balanced decompression for thyroid-associated orbitopathy, 18 patients had bilateral medial and lateral orbital wall decompression, and 2 patients had unilateral medial and lateral decompression. The average orbital volume expansion was 3.21 mL (13.51%) of the preoperative orbital volume (2.1 mL [8.98%] gained by medial wall decompression and 1.03 mL [4.53%] by lateral wall decompression). Postoperatively, proptosis decreased by 2.53 mm on average (p < 0.0001). The eyes became closer to each other postoperatively in the horizontal plane by 2.6 mm on average (p < 0.0001). No change in the vertical eye position was detected postoperatively. A significant nasal shift in the eye position was noticed following balanced orbital decompression. Computerized assessment of preoperative and postoperative digitized orbital CT scans helps evaluate the orbital changes in response to different orbital decompression techniques and improve the surgical outcomes in thyroid-associated orbitopathy.

Face Inversion Disproportionately Disrupts Detection of Vertical versus Horizontal, Not Long-Range versus Short-Range, Spatial Relations

Presenting a face inverted disrupts sensitivity to the spacing between the features. Recent evidence suggests that inversion disproportionately affects sensitivity to vertical over horizontal changes in eye position. One explanation is that inversion disrupts the processing of long-range (e.g., eye to mouth) more than short-range (e.g., interocular) spatial relations. Here we investigated whether the size of the shift or the direction (i.e., eyes up versus eyes down; eyes in versus eyes out) affects the pattern of the inversion effect. Our results replicated the finding of poor detection of vertical changes in inverted faces (even when these changes were very large). In the vertical condition the inversion effects were just as large for the “eyes down” (short-range, as the eye-mouth distance was small) as the “eyes up” (long-range), while horizontally “eyes in” (short-range, as the eyes were very close) and “eyes out” (long-range) both produced small inversion effects. These results argue against a long-range versus short-range spatial relations explanation of the horizontal versus vertical difference in the inversion effect. Rather, it appears the axis of change rather than the spatial distances between features is behind the difference in the size of the inversion effect.

The role of the cerebellum in ‘real’ and ‘imaginary’ line bisection explored with 1-Hz repetitive transcranial magnetic stimulation

The role of the cerebellum is well characterized for many motor processes and for some cognitive tasks, although its contribution to lateralized spatial judgement has never been probed directly. To address this omission, we investigated the effects of cerebellar disruption on two different line bisection tasks in eight healthy subjects. Based on previous evidence of crossed cerebellar-cortical connections we predicted a shift in the perceived midline that would occur in opposite directions depending on the cerebellar hemisphere targeted. Repetitive transcranial magnetic stimulation (rTMS), given at 1-Hz (600 pulses), was used as a non-invasive way to interfere with processing in the cerebellar cortex. Performance was assessed for both 'physical' line bisection using a newly developed Landmark variant task and for 'mental' line bisection using number pairs. The effects for number line bisection were lateralized--left but not right cerebellar rTMS increased rightward errors, whereas for physical line bisection rTMS to either hemisphere did not affect performance. Effects due to neck muscle contraction and changes in eye position were ruled out with appropriate control stimulation sites, and eye-tracking. The results confirm the role of the cerebellum in spatial judgement, and, for the first time, demonstrate direct cerebellar involvement in the generation of the midline in 'imaginal' (number) space. The difference between number line and physical line bisection effects is discussed with reference to pre-existing models of cerebellar hemispheric specialization and functional topography.

Learned Timing of Motor Behavior in the Smooth Eye Movement Region of the Frontal Eye Fields

Proper timing is a critical aspect of motor learning. We report a relationship between a representation of time and an expression of learned timing in neurons in the smooth eye movement region of the frontal eye fields (FEF(SEM)). During prelearning pursuit of target motion at a constant velocity, each FEF(SEM) neuron is most active at a distinct time relative to the onset of pursuit tracking. In response to an instructive change in target direction, a neuron expresses the most learning when the instruction occurs near the time of its maximal participation in prelearning pursuit. Different neurons are most active, and undergo the most learning, at distinct times during pursuit. We suggest that the representation of time in the FEF(SEM) drives learning that is temporally linked to an instructive change in target motion, and that this may be a general function of motor areas of the cortex.

Eccentric gaze dynamics enhance vection in depth

This study examined the role of eccentric gaze dynamics in the generation of visual illusions of self-motion (i.e., vection). In Experiment 1, observers maintained their gaze either upward, downward, leftward, or rightward with respect to the center of a radially expanding optic flow pattern, which simulated forward self-motion in depth through a 3D cloud of objects. Real-time vection strength ratings and changes in horizontal and vertical eye positions were recorded simultaneously. Vection strength was found to increase progressively over the course of each 30-s presentation of radial flow. Eye tracking revealed strong optokinetic responses, consistent with ocular following responses (OFRs). Reported increases in vection strength in all four gaze conditions were typically preceded by reductions in slow-phase eye velocity. Similar results were found in Experiment 2, where displays simulated self-motion over a ground plane, which provided superior perspective. We conclude in both cases that enhancements of vection strength over time were temporally contingent upon the changing character of OFR while viewing these displays.

Advancing age alters the influence of eye position on sound localization

Vision and audition provide spatial information about the environment to guide natural behavior. Because the eyes move in the head while the ears remain head-fixed, input conveying eye position in the head is required to maintain audiovisual congruence. Human perception of auditory space was previously shown to shift with changes in eye position, regardless of the target's frequency content and spatial cues underlying horizontal and vertical localization. In this study, we examined whether this interaction is altered by advancing age. Head-restrained young (18-44 yo), middle-aged (45-64 yo), and elderly (65-81 yo) human subjects localized noise bursts under conditions of transient and sustained ocular deflection. All three age groups demonstrated a time-dependent shift of auditory space in the direction of eye position. Moreover, this adaptation showed a clear decline with advancing age, but only for peripheral auditory space (beyond ±10° from midline). Alternatively, adaptation in the periphery may occur, but is more sluggish than in the central field and therefore not fully observed in this experiment. The age-dependent effect cannot be readily explained by senescent peripheral hearing loss, suggesting a change in central processing of auditory space in relation to the control of gaze.