Biomedical subjects

J L Demer

Publications and source records attributed to J L Demer.

At least 37 records · Page 2Linked to original sources

PubMed2000

Effect of adaptation to telescopic spectacles on the initial human horizontal vestibuloocular reflex.

Gain of the vestibuloocular reflex (VOR) not only varies with target distance and rotational axis, but can be chronically modified in response to prolonged wearing of head-mounted magnifiers. This study examined the effect of adaptation to telescopic spectacles on the variation of the VOR with changes in target distance and yaw rotational axis for head velocity transients having peak accelerations of 2,800 and 1,000 degrees /s(2). Eye and head movements were recorded with search coils in 10 subjects who underwent whole body rotations around vertical axes that were 10 cm anterior to the eyes, centered between the eyes, between the otoliths, or 20 cm posterior to the eyes. Immediately before each rotation, subjects viewed a target 15 or 500 cm distant. Lighting was extinguished immediately before and was restored after completion of each rotation. After initial rotations, subjects wore 1.9x magnification binocular telescopic spectacles during their daily activities for at least 6 h. Test spectacles were removed and measurement rotations were repeated. Of the eight subjects tolerant of adaptation to the telescopes, six demonstrated VOR gain enhancement after adaptation, while gain in two subjects was not increased. For all subjects, the earliest VOR began 7-10 ms after onset of head rotation regardless of axis eccentricity or target distance. Regardless of adaptation, VOR gain for the proximate target exceeded that for the distant target beginning at 20 ms after onset of head rotation. Adaptation increased VOR gain as measured 90-100 ms after head rotation onset by an average of 0.12 +/- 0.02 (SE) for the higher head acceleration and 0.19 +/- 0.02 for the lower head acceleration. After adaptation, four subjects exhibited significant increases in the canal VOR gain only, whereas two subjects exhibited significant increases in both angular and linear VOR gains. The latencies of linear and early angular target distance effects on VOR gain were unaffected by adaptation. The earliest significant change in angular VOR gain in response to adaptation occurred 50 and 68 ms after onset of the 2,800 and 1,000 degrees /s(2) peak head accelerations, respectively. The latency of the adaptive increase in linear VOR gain was approximately 50 ms for the peak head acceleration of 2,800 degrees /s(2), and 100 ms for the peak head acceleration of 1,000 degrees /s(2). Thus VOR gain changes and latency were consistent with modification in the angular VOR in most subjects, and additionally in the linear VOR in a minority of subjects.

Adaptation, Ocular↗

PubMed2000

Evidence for active control of rectus extraocular muscle pulleys.

PURPOSE: Connective tissue structures constrain paths of the rectus extraocular muscles (EOMs), acting as pulleys and serving as functional EOM origins. This study was conducted to investigate the relationship of orbital and global EOM layers to pulleys and kinematic implications of this anatomy. METHODS: High-resolution magnetic resonance imaging (MRI) was used to define the anterior paths of rectus EOMs, as influenced by gaze direction in living subjects. Pulley tissues were examined at cadaveric dissections and surgical exposures. Human and monkey orbits were step and serially sectioned for histologic staining to distinguish EOM fiber layers in relationship to pulleys. RESULTS: MRI consistently demonstrated gaze-related shifts in the anteroposterior locations of human EOM path inflections, as well as shifts in components of the pulleys themselves. Histologic studies of human and monkey orbits confirmed gross examinations and surgical exposures to indicate that the orbital layer of each rectus EOM inserts on its corresponding pulley, rather than on the globe. Only the global layer of the EOM inserts on the sclera. This dual insertion was visualized in vivo by MRI in human horizontal rectus EOMs. CONCLUSIONS: The authors propose the active-pulley hypothesis: By dual insertions the global layer of each rectus EOM rotates the globe while the orbital layer inserts on its pulley to position it linearly and thus influence the EOM's rotational axis. Pulley locations may also be altered in convergence. This overall arrangement is parsimoniously suited to account for numerous aspects of ocular dynamics and kinematics, including Listing's law.

Actins↗

PubMed2000

Three-dimensional location of human rectus pulleys by path inflections in secondary gaze positions.

PURPOSE: Connective tissue pulleys serve as the functional mechanical origins of the extraocular muscles (EOMs). Anterior to these pulleys, EOM paths shift with gaze to follow the scleral insertions, whereas posterior EOM paths are stable in the orbit. Inflections in EOM paths produced by gaze shifts can be used to define the functional location of pulleys in three dimensions (3-D). METHODS: Contiguous magnetic resonance images in planes perpendicular to the orbital axis spanned the anteroposterior extents of 22 orbits of 11 normal adults with the eyes in central gaze, elevation, depression, abduction, and adduction. Mean EOM cross-sectional area centroids represented in a normalized, oculocentric coordinate system were plotted over the length of each EOM to determine paths. Path inflections were identified to define pulley locations in 3-D. RESULTS: All rectus EOM paths exhibited in secondary gaze positions distinct inflections 3 to 9 mm posterior to globe center, which were consistent across subjects. The globe center and the lateral rectus pulley translated systematically in the orbit with lateral gaze, whereas other pulleys remained stable relative to the orbit. CONCLUSIONS: Distinct inflections in rectus EOM paths in secondary gaze positions confirm the existence of pulleys and define their locations in 3-D. The globe and lateral rectus pulley translate systematically with gaze position. The EOM pulleys may simplify neural control of eye movements by implementing a commutative ocular motor plant in which commands for 3-D eye velocity are effectively independent of eye position.

Adult↗

PubMed2000

Asymmetry of ocular motor and perceptual vestibular processing in humans with unilateral vestibular deafferentation.

To investigate the effect of asymmetrical vestibular input on the perceived straight-ahead direction, we compared 7 subjects (age 59 +/- 8 yrs, mean +/- SD) who had chronic (>10 mos) unilateral vestibular deafferentation with 10 age matched controls (age 61+/-6 younger controls (age 28 +/- 7 yrs). Despite the age difference, the two control groups performed similarly and were therefore pooled. Eye and head movements were recorded using search coils as subjects underwent 30 s trials of sinusoidal, whole body oscillation (0.4-2 Hz, peak velocities 0-120 degrees /s) in darkness while attempting to maintain gaze on a remembered target 5 m distant. As a control, most stimulus oscillations were randomly superimposed on an imperceptible, constant velocity of +/-0.5 degrees /s that produced a whole-body offset of 15 degrees by the end of the trial. Following oscillation, subjects remained motionless in darkness and were asked to orient both gaze and a manipulandum to the remembered target location. In control subjects, mean final gaze and manipulandum positions were within 15 degrees of the target for all testing conditions. There was no dependence of final gaze and manipulandum positions on the frequency or velocity of the preceding whole-body oscillations (p > 0.05). In four of seven unilaterally deafferented subjects there was an ipsilesional bias of final eye position of > or =10 degrees. These subjects moved both eye and manipulandum to the ipsilesional side, with the error increasing at higher stimulus velocities. For the 120 degrees /s peak head velocity, mean ipsilesional gaze bias ranged from 10-37 degrees and mean manipulandum bias ranged from 26-108 degrees. Although the errors depended on velocity p < 0.01), errors were independent of frequency (p > 0.1). In the remaining three subjects with vestibular deafferentation, final gaze and manipulandum positions [were not statistically different from controls.] Early gain (eye velocity / head velocity) of the VOR averaged 0.82 +/- 0.01 for the first 10 s of all trials and was similar in all groups (p > 0.1). Gain during the final 10 s gain averaged 0.78 +/- 0.01 for control subjects, but was significantly lower at 0.70 +/- 0.01 for unilaterally deafferented subjects, whose eye positions reached the limit of the ocular motor range. We conclude that many humans with chronic unilateral vestibular deafferentation have a large ipsilesional dynamic bias of eye position and the perceived straight ahead direction reflecting persistent asymmetry of vestibular processing.

Adult↗

PubMed1999

Horizontal vestibulo-ocular reflex and head stability in response to torso perturbations during visual search.

Eye, head, and torso movements were recorded using magnetic search coils while six normal human subjects made unconstrained eye and head movements as they searched for targets in a panoramic visual environment. Torso movements were imposed by pseudorandom rotations of a servomotor-driver chair in which subjects were seated; body motion was partially transmitted to the head as a perturbation. Horizontal vestibulo-ocular reflex (VOR) gain (eye velocity divided by head velocity) and head gain (head velocity divided by torso velocity) were determined. Measurements were performed with unaided vision and while subjects wore x4 binocular telescopic spectacles. Since the head was free to move during the experiment, much of the perturbation delivered to the torso was compensated by head rotation on the neck. During the 50 ms immediately following chair rotation, the head corrected 98% of the torso motion. For the interval 50-80 ms after the perturbation 81-85% of the perturbation was corrected by head movement. The degree of head compensation did not significantly depend on magnification or type of visual target. The density distribution for VOR gain was calculated over the entire course of each trial and was found to be sharply centered between 0.9 and 1.0 for trials with unmagnified vision. The gain density distribution with x4 telescopes was broader and centered around 1.5, reflecting visual enhancement. Gain of the VOR was also determined during four discrete epochs covering the period from 50 ms before to 130 ms after the onset of each imposed torso rotation. The first, second, and fourth epochs were 50 ms each, while the third epoch was 30 ms. The torso began to rotate in the second epoch (0-50 ms), and the onset of head rotation was in the third epoch (50-80 ms). Gains of the VOR determined during the first three epochs were in response to self-generated head rotation and were not significantly different from each other, averaging 1.0+/-0.4 (n=1604, mean+/-SD) with unaided vision and increased significantly (P<0.05) to 1.4+/-0.6 (n=2464) with telescopic spectacles. Gain of the VOR during the fourth (80-130 ms) epoch was in response to the imposed perturbation; this averaged 0.9+/-0.3 (n=1380) with unaided vision and increased significantly to 1.1+/-0.4 (n=2185) with telescopic spectacles. The wearing of telescopic spectacles thus induced an enhancement of VOR gain, which was dependent on the context of the associated head movement. The greater enhancement of VOR gain during self-generated head movement suggests that the large enhancement may be at least partially mediated by the motor program itself. However, the smaller, but still significant gain enhancement with telescopic spectacles observed during unpredictable, externally imposed head motion had a latency too short to be mediated by visual pursuit. We propose that the smaller gain enhancement during passive rotation is due to a small, context-dependent, parametric increase in the gain of canal or proprioceptive mediated eye movements.

Acceleration↗

PubMed1999

Latency of voluntary cancellation of the human vestibulo-ocular reflex during transient yaw rotation.

Volitional suppression of the initial vestibuloocular reflex (VOR) was studied in ten normal humans, aged 29+/-8 years (mean+/-standard deviation, SD), who were rotated about a vertical axis centered between the otoliths. Rotations consisted of steps in acceleration of 2800, 1600, 1000, or 500 degrees/S2 delivered at unpredictable times in unpredictable directions in the horizontal plane. As a test of the VOR, subjects were asked to attend to an earth-fixed target located 500 cm away that was either continuously visible or extinguished immediately before rotation. The gain of the VOR (angular eye velocity/angular head velocity) was 0.78+/-0.01 (mean+/-standard error of the mean, SE) during the period 35-45 ms after the onset of head rotation and 0.952+/-0.005 during the period 125-135 ms after the onset of head rotation. Subsequent rotations were performed during viewing of a target that moved with the head (cancellation). Cancellation was studied under three conditions of target visibility: (1) with the target always visible; (2) with the target always extinguished immediately prior to head rotation; or (3) with the target unpredictably extinguished half of the time immediately before each rotation. Cancellation responses showed individual idiosyncrasies, but certain features were common to most subjects. During cancellation, the VOR response generally differed from the earth-fixed target condition in that there was usually a small decrease in slow-phase VOR gain followed by an oppositely directed saccade. During the highest acceleration (2800 degrees/s2), the latency of the earliest statistically significant gain decrease for cancellation, as compared with the earth-fixed target condition, averaged 48+/-5 ms (mean+/-SE) from the onset of head rotation, although it was significantly shorter in one subject who had an onset at 14+/-2 ms. The latency of cancellation increased as head acceleration decreased such that, for each stimulus, cancellation began when the head was displaced an average of 1.4+/-0.1 degrees (-/+SD). Because VOR cancellation generally occurred before the availability of visual feedback or under conditions when vision was never permitted, it is inferred that cancellation is triggered by a threshold eye position or an estimate of head rotation based on integration of vestibular afferents. Cancellation occurred significantly earlier with a visible target than with an extinguished target only at the lowest peak head acceleration of 500 degrees/s2. Corrective saccades with a visible target occurred later for head accelerations of 500 and 1000 degrees/s2 than for greater head accelerations. Significant effects of target illumination on the latencies of both saccades and cancellation occurred at least 80-90 ms after the onset of head rotation, consistent with the earliest available visual feedback. This longer latency of saccades for visible as compared with extinguished targets may be analogous to a release of fixation, as occurs with express saccades. The latency difference due to target visibility was not related to prediction, since it was unchanged under conditions of random target illumination.

Acceleration↗

PubMed1999

A linear canal-otolith interaction model to describe the human vestibulo-ocular reflex.

A control systems model of the vestibulo-ocular reflex (VOR) originally derived for yaw rotation about an eccentric axis (Crane et al. 1997) was applied to data collected during ambulation and dynamic posturography. The model incorporates a linear summation of an otolith response due to head translation scaled by target distance, adding to a semi-circular canal response that depends only on angular head rotation. The results of the model were compared with human experimental data by supplying head angular velocity as determined by magnetic search coil recording as the input for the canal branch of the model and supplying linear acceleration as determined by flux gate magnetometer measurements of otolith position. The model was fit to data by determining otolith weighting that enabled the model to best fit the data. We fit to the model experimental data from normal subjects who were: standing quietly, walking, running, or making active sinusoidal head movements. We also fit data obtained during dynamic posturography tasks of: standing on a platform sliding in a horizontal plane at 0.2 Hz, standing directly on a platform tilting at 0.1 Hz, and standing on the tilting platform buffered by a 5-cm thick foam rubber cushion. Each task was done with the subject attending a target approximately 500, 100, or 50 cm distant, both in light and darkness. The model accurately predicted the observed VOR response during each test. Greater otolith weighting was required for near targets for nearly all activities, consistent with weights for the otolith component found in previous studies employing imposed rotations. The only exceptions were for vertical axis motion during standing, sliding, and tilting when the platform was buffered with foam rubber. In the horizontal axis, the model always fit near target data better with a higher otolith component. Otolith weights were similar with the target visible and in darkness. The model predicts eye movement during both passive whole-body rotation and free head movement in space implying that the VOR is controlled by a similar mechanism during both situations. Factors such as vision, proprioception, and efference copy that are available during head free motion but not during whole-body rotation are probably not important to gaze stabilization during ambulation and postural stabilizing movement. The linearity of the canal-otolith interaction was tested by re-analysis of the whole body rotation data on which the model is based (Crane et al. 1997). Normalized otolith-mediated gain enhancement was determined for each axis of rotation. This analysis uncovered minor non-linearities in the canal-otolith interaction at frequencies above 1.6 Hz and when the axis of rotation was posterior to the head.

Adult↗

PubMed1999

Posterior fixation sutures: a revised mechanical explanation for the fadenoperation based on rectus extraocular muscle pulleys.

PURPOSE: To determine the effect of the rectus extraocular muscle pulleys on the fadenoperation, an operation designed to fixate the posterior muscle belly to the underlying retroequatorial sclera. METHODS: First, duction into the field of action of the operated-on muscle was quantified retrospectively after fadenoperation. Magnetic resonance imaging was then performed prospectively after surgery to verify anatomic changes. Forced duction testing was performed prospectively during surgery before and after faden placement. Finally, computed tomography in a cadaver containing radiographic markers was performed prospectively to determine the effect of fadenoperation on the position of the medial rectus insertion relative to its pulley. RESULTS: Mean maximum adduction after medial rectus fadenoperation was 18 degrees (range, 10 to 25 degrees; 13 eyes). Fadenoperations combined with large medial rectus recessions restricted adduction more than fadenoperations combined with smaller recessions (P = .019), but even fadenoperations without recessions substantially restricted adduction. Mean maximum abduction after lateral rectus fadenoperation was 40 degrees (range, 25 to 45 degrees; four eyes). Axial magnetic resonance imaging in two eyes demonstrated a smaller loss of muscle tangency to the globe during contraction than predicted by geometric models. Forced ductions in nine patients performed immediately after faden placement demonstrated a new mechanical restriction to duction toward the operated-on muscle. Cadaveric computed tomographic scans demonstrated posterior displacement of the medial rectus pulley during adduction after fadenoperation. CONCLUSIONS: Posterior fixation sutures do not significantly decrease muscle torque during contraction. Because posterior fixation sutures posteriorly displace the pulley sleeve during duction toward the operated-on muscle, the mechanical restriction after surgery probably represents the force deforming the pulley. This mechanical restriction may account for the limitation in duction seen after fadenoperation.

Eye Movements↗

PubMed1999

Clinical features of congenital absence of the superior oblique muscle as demonstrated by orbital imaging.

INTRODUCTION: Absence of an extraocular muscle was considered rare when demonstrable only by surgical exploration or necropsy. This study presents advances in orbital imaging to correlate clinical findings with absence of the superior oblique (SO) muscle. METHODS: We performed high-resolution coronal orbital imaging by magnetic resonance imaging (222 orbits) or computerized radiographic tomography (32 orbits) in 127 patients with strabismus. We reviewed the histories and ocular motility examinations in patients who had absence of 1 or both SO muscles. Findings were compared with patients who were clinically diagnosed with SO palsy but had demonstrable SO muscles on orbital imaging. RESULTS: SO muscles were absent in 6 patients. All had histories suggesting congenital strabismus. In patients old enough for quantitative testing who had unilateral SO muscle absence, visual acuity was a least 20/25 in all and stereopsis was better than 80 arc/s in one. Three patients were orthotropic in primary position. Five patients with unilateral SO muscle absence had clinical findings variably consistent with SO palsy, whereas a sixth patient with Duane syndrome had clinically unsuspected bilateral SO muscle absence. Versions and patterns of hypertropia in patients with SO muscle absence overlapped findings of 20 patients with SO palsy but demonstrable SO muscles. CONCLUSIONS: Imaging can frequently demonstrate absence of the SO muscle in patients with SO palsy. Such patients may have good vision and stereopsis and clinical findings indistinguishable from SO palsy without absence of the SO muscle. Orbital imaging should be considered in the evaluation of congenital SO palsy to facilitate planning of effective surgical correction.

Adult↗

PubMed1999

Magnetic resonance imaging after surgical transposition defines the anteroposterior location of the rectus muscle pulleys.

INTRODUCTION: Connective tissue pulleys serve as the functional origins of the rectus extraocular muscles (EOMs) and constrain the sideslip of the posterior EOM bellies after transposition surgery. Anterior to the pulleys, EOM paths appreciably displace to reach their transposed insertions. The inflection points in the EOM paths from minimal posterior displacement to maximal anterior displacement should define the anteroposterior location of the EOM pulleys after transposition. METHODS: Contiguous cross-sectional magnetic resonance images were obtained in planes perpendicular to the long axis of the orbit over its entire anteroposterior extent before and after operation in 6 patients who underwent rectus muscle transposition surgery. Four patients underwent full tendon width transposition of the vertical rectus muscles laterally for lateral rectus palsy. Two of these patients had augmentation of the transposition with sutures that fixated the temporal margins of the transposed muscles posteriorly to the sclera adjacent to the borders of the lateral rectus muscle. One patient underwent full tendon width transposition of the horizontal rectus muscles superiorly for superior rectus palsy. One patient underwent full tendon width transposition of both lateral rectus muscles inferiorly for "A" pattern esotropia. Paths of EOMs were defined relative to the area centroid of the orbit. Pulley locations were inferred from EOM paths. The postoperative change in EOM pulley location was obtained by subtracting the preoperative pulley location from the postoperative pulley location for each image plane. RESULTS: For all patients, the postoperative change in EOM belly location was relatively small posterior to the globe-optic nerve junction. The 2 patients with abducens palsy who underwent placement of posterior augmentation sutures, however, demonstrated a significantly larger displacement of the posterior vertical rectus paths compared with similar patients who did not receive augmentation sutures. For all horizontally transposed vertical rectus muscles and inferiorly transposed lateral rectus muscles, the inflection of the EOM path began 3 mm anterior to the globe-optic nerve junction. For the superiorly transposed medial rectus muscle and lateral rectus muscle, the inflection began 6 mm anterior to the globe-optic nerve junction. CONCLUSIONS: The anteroposterior locations of the EOM pulleys can be defined by analysis of EOM displacement after transposition surgery. Augmentation of transpositions by posterior suturing displaces the EOM pulleys substantially more than nonaugmented transpositions.

Abducens Nerve↗

PubMed1998

Gaze stabilization during dynamic posturography in normal and vestibulopathic humans.

Dynamic posturography by measurement of center of pressure (COP) is a widely employed technique for evaluating the vestibular system. However, the relationship of COP motion to vestibulo-ocular reflex (VOR) function and image stability on the retina has not been determined previously. To assess these relationships, we report gaze, head, and trunk stability during dynamic posturography in 11 normal volunteers, 7 subjects with unilateral vestibular lesions, and 3 subjects with bilateral vestibular lesions. Posturographic tasks consisted of standing still and standing on a platform that was sliding (0.2 Hz), tilting (0.1 Hz), or covered with a foam cushion 6 cm thick while tilting (0.1 Hz). Each perturbation was imposed in the anterior-posterior and repeated in the medial-lateral direction, in both light and darkness. Subjects viewed (or in darkness remembered) a target located 50, 100, or 500 cm distant. COP, angular eye position, and angular and linear orbit and trunk positions were measured using magnetic search coils and flux gate magnetometer sensors. With the target visible, the velocity of image motion on the retina was on average always less than 1 degree/s, well within the range consistent with high visual acuity. In darkness, gaze velocity increased for normal and vestibulopathic subjects. During tilt, vestibulopathic subjects had a significantly greater gaze velocity than controls. Gain of the angular VOR (eye velocity/head velocity) was significantly lower in darkness than in light and in vestibulopathic as compared to control subjects. Gain of the VOR was significantly correlated with gaze instability, but variation in VOR gain accounted for only 20-40% of the variance. In darkness, the velocity of the COP was significantly greater in vestibulopathic than control subjects for every condition tested. In light, this difference was small and often not significant. Although spectral analysis of the COP indicated frequencies above 1 Hz that were not observed in motion of the trunk and orbit, root mean square (RMS) velocities of the trunk and orbit in the horizontal plane were higher in darkness and in vestibulopathic subjects, mirroring COP findings. Only in vestibulopathic subjects tested in darkness was there a correlation between COP velocity and gaze velocity; COP velocity was otherwise uncorrelated with gaze. Gaze velocity was greater with near than with distant targets. Vertical VOR gain was higher with near targets. No other significant effects of target distance were found. Head movement strategy, VOR gain, and COP were all unaffected by target proximity. These data show that gaze velocity measurements during dynamic posturography in darkness are sensitive to vestibular loss. With a visible target, both COP and gaze stability of vestibulopathic subjects are difficult to distinguish from normal. During visual feedback, it is likely that image stabilization over the range of frequencies tested is achieved through better head stability and through visual tracking, allowing vestibulopathic subjects to maintain adequate visual acuity.

Adult↗

PubMed1998

Isolated inferior rectus palsy caused by a metastasis to the oculomotor nucleus.

PURPOSE: To report a case of isolated inferior rectus palsy secondary to a metastasis to the oculomotor nucleus. METHODS: Case report. A 41-year-old woman with a history of breast cancer presented with acute onset of left hypotropia and exotropia. RESULTS: Forced generation testing confirmed weakness of the right inferior rectus muscle that was not reversed by intravenous edrophonium infusion. Magnetic resonance imaging disclosed numerous metastatic lesions to the cerebral hemispheres and brainstem. One lesion in the right midbrain was adjacent to the cerebral aqueduct in the right oculomotor nucleus. CONCLUSION: Metastasis to the oculomotor nucleus is a rare cause of isolated inferior rectus palsy; however, this entity should be considered in the differential diagnosis of an isolated inferior rectus palsy because of the life-threatening consequences of a brainstem lesion.

Adult↗

PubMed1998

Vision and vestibular adaptation.

This article summarizes six recent degree-of-freedom studies of visual-vestibular interaction during natural activities and relates the findings to canal-otolith interactions evaluated during eccentric axis rotations. Magnetic search coils were used to measure angular eye and head movements of young and elderly subjects. A flux gate magnetometer was used to measure three-dimensional head translation. Three activities were studied: standing quietly, walking in place, and running in place. Each activity was evaluated with three viewing conditions: a visible target viewed normally, a remembered target in darkness, and a visible target viewed with x2 binocular telescopic spectacles. Canal-otolith interaction was assessed with passive, whole-body, transient, and steady-state rotations in pitch and yaw at multiple frequencies about axes that were either oculocentric or eccentric to the eyes. For each rotational axis, subjects regarded visible and remembered targets located at various distances. Horizontal and vertical angular vestibulo-ocular reflexes were demonstrable in all subjects during standing, walking, and running. When only angular gains were considered, gains in both darkness and during normal vision were less than 1.0 and were generally lower in elderly than in young subjects. Magnified vision with x2 telescopic spectacles produced only small gain increases as compared with normal vision. During walking and running all subjects exhibited significant mediolateral and dorsoventral head translations that were antiphase locked to yaw and pitch head movements, respectively. These head translations and rotations have mutually compensating effects on gaze in a target plane for typical viewing distances and allow angular vestibulo-ocular reflex gains of less than 1.0 to be optimal for gaze stabilization during natural activities. During passive, whole-body eccentric pitch and yaw head rotations, vestibulo-ocular reflex gain was modulated as appropriate to stabilize gaze on targets at the distances used. This modulation was evident within the first 80 msec of onset of head movement, too early to be caused by immediate visual tracking. Modeling suggests a linear interaction between canal signals and otolith signals scaled by the inverse of target distance. Vestibulo-ocular reflex performance appears to be adapted to stabilize gaze during translational and rotational perturbations that occur during natural activities, as is appropriate for relevant target distances. Although immediate visual tracking contributes little to gaze stabilization during natural activities, visual requirements determine the performance of vestibulo-ocular reflexes arising from both canals and otoliths.

Adaptation, Physiological↗

PubMed1998

Heterotopic muscle pulleys or oblique muscle dysfunction?

INTRODUCTION: The description of connective tissue sleeves that function as pulleys for the rectus extraocular muscles (EOMs) suggests that abnormalities of EOM pulley position might provide a mechanical basis for some forms of incomitant strabismus. Pulleys determine the paths and thus the pulling directions of EOMs. METHODS: High-resolution magnetic resonance images spanning the orbits were obtained in primary position, upgaze, and downgaze for each subject. Paths of the EOMs were measured with reference to the orbital center and permitted inference of pulley locations. RESULTS: Data from 18 orbits of orthotropic subjects defined means and SDs of normal EOM pulley coordinates. Eight patients, aged 17 to 60 years, had heterotopic EOM pulleys, defined as displaced at least 2 SDs from normal. We found one to eight heterotopic pulleys (considering both orbits) in each of four patients who had been diagnosed with marked superior oblique (SO) overaction and mild to marked inferior oblique (IO) underaction. Each patient had superior mislocation of at least one lateral rectus pulley by 1.8 to 4.9 mm. Three patients diagnosed with mild to moderate IO overaction and mild to moderate SO underaction in only one orbit had one to three heterotopic EOM pulleys. Each of those patients had at least one lateral rectus pulley inferiorly dislocated by 1.9 to 4.9 mm. The final patient, who was diagnosed with mild IO underaction and normal SO function bilaterally, had bilateral superior mislocation of the medial rectus pulleys by greater than 2 mm. Computer simulations using the Orbit program (Eidactics, San Francisco) incorporating individually measured pulley positions reproduced the clinical patterns of incomitant strabismus in all cases without postulating abnormalities of oblique muscle innervation or contractility. CONCLUSION: Heterotopic EOM pulleys can cause patterns of incomitant strabismus that have been attributed to oblique muscle dysfunction. Even isolated mislocations of less than 2 mm, coupled with smaller mislocations of the other pulleys, can produce the clinical appearance of bilateral oblique dysfunction. Pulley heterotopy should be considered in the differential diagnosis of incomitant strabismus and oblique dysfunction.

Adolescent↗

PubMed1998

Human horizontal vestibulo-ocular reflex initiation: effects of acceleration, target distance, and unilateral deafferentation.

The vestibulo-ocular reflex (VOR) generates compensatory eye movements in response to angular and linear acceleration sensed by semicircular canals and otoliths respectively. Gaze stabilization demands that responses to linear acceleration be adjusted for viewing distance. This study in humans determined the transient dynamics of VOR initiation during angular and linear acceleration, modification of the VOR by viewing distance, and the effect of unilateral deafferentation. Combinations of unpredictable transient angular and linear head rotation were created by whole body yaw rotation about eccentric axes: 10 cm anterior to eyes, centered between eyes, centered between otoliths, and 20 cm posterior to eyes. Subjects viewed a target 500, 30, or 15 cm away that was extinguished immediately before rotation. There were four stimulus intensities up to a maximum peak acceleration of 2,800 degrees/s2. The normal initial VOR response began 7-10 ms after onset of head rotation. Response gain (eye velocity/head velocity) for near as compared with distant targets was increased as early as 1-11 ms after onset of eye movement; this initial effect was independent of linear acceleration. An otolith mediated effect modified VOR gain depending on both linear acceleration and target distance beginning 25-90 ms after onset of head rotation. For rotational axes anterior to the otoliths, VOR gain for the nearest target was initially higher but later became less than that for the far target. There was no gain correction for the physical separation between the eyes and otoliths. With lower acceleration, there was a nonlinear reduction in the early gain increase with close targets although later otolith-mediated effects were not affected. In subjects with unilateral vestibular deafferentation, the initial VOR was quantitatively normal for rotation toward the intact side. When rotating toward the deafferented side, VOR gain remained less than half of normal for at least the initial 55 ms when head acceleration was highest and was not modulated by target distance. After this initial high acceleration period, gain increased to a degree depending on target distance and axis eccentricity. This behavior suggests that the commissural VOR pathways are not modulated by target distance. These results suggest that the VOR is initially driven by short latency ipsilateral target distance dependent and bilateral target-distance independent canal pathways. After 25 ms, otolith inputs contribute to the target distance dependent pathway. The otolith input later grows to eventually dominate the target distance mediated effect. When otolith input is unavailable the target distance mediated canal component persists. Modulation of canal mediated responses by target distance is a nonlinear effect, most evident for high head accelerations.

Acceleration↗

PubMed1998

Displacement of the medial rectus pulley in superior oblique palsy.

PURPOSE: The rectus extraocular muscles pass through fibromuscular connective tissue pulleys that stabilize muscle paths and control the direction of muscle pull. The authors investigated whether abnormal forces associated with superior oblique palsy can cause displacement of pulleys and muscle paths. METHODS: Coronal magnetic resonance imaging (MRI) showing significantly reduced superior oblique cross-sectional areas and lack of contractile changes with vertical gaze confirms that seven subjects had superior oblique palsies. Binocular misalignment was quantified using the Hess test. In those seven subjects with palsies and in 18 normal orbits, coronal MRI scans corrected to standardized head position were analyzed digitally to determine muscle paths in primary gaze. Horizontal and vertical coordinates of the pulleys, known histologically to lie just posterior to the equator in primary gaze, were inferred from these muscle paths. RESULTS: Normal pulley coordinates were highly uniform. Compared with both normal orbits and fellow orbits, orbits with superior oblique palsies showed a statistically significant 1.1 mm superior displacement of the medial rectus pulley. No other pulley was displaced significantly from normal. Computer simulation using a biomechanical model of ocular statics showed that, in each case, the pulley position shifts alone were insufficient to reproduce the clinical pattern of strabismus. CONCLUSIONS: The excyclotorsion of the globe that accompanies superior oblique palsy does not systematically displace the pulleys of all the rectus muscles. The only significant rectus muscle path change is for the medial rectus muscle, and it may arise as a mechanical consequence of the atrophy of the adjacent superior oblique muscle belly. Biomechanical modeling suggests that this displacement of the medial rectus pulley alone does not account for the pattern of strabismus observed in superior oblique palsy.

Adolescent↗

PubMed1998

Nonvascular contractile cells in sclera and choroid of humans and monkeys.

PURPOSE: To investigate by histochemistry and immunohistochemistry the distribution and innervation of nonvascular contractile cells in the sclera and choroid of humans and monkeys. METHODS: Globes were obtained from 2 macaque monkeys and 19 human cadavers that ranged in age from fetal life to 94 years. Immunohistochemistry was performed using monoclonal antibody against human smooth muscle (SM) alpha-actin and tyrosine hydroxylase (TH). The nicotinamide-adenine dinucleotide phosphate (NADPH)- diaphorase reaction was used as a marker for nitric oxide synthase. RESULTS: The scleras of all but fetal, newborn, and infant globes exhibited myofibroblasts, amelanotic, fibroblastlike cells having SM alpha-actin immunoreactivity. In the choroid of all but fetal eyes, SM cells were present in the suprachoroidal layer, forming a reticulum of flattened laminae, and in the choriocapillaris where ovoid-to-spindle-shaped SM cells were arrayed in parallel layers immediately adjacent to Bruch's membrane. Contractile cells in the sclera and choroid were most concentrated subfoveally and were sparse anteriorly. Nerve terminals positive for NADPH- diaphorase were colocalized with SM alpha-actin-positive cells in the sclera and choroid, whereas TH-positive nerve terminals colocalized with SM cells in the choroid. Clusters of ganglion cells were present on the posterior surface of globes near SM cells. CONCLUSIONS: The posterior choroid and sclera of humans and monkeys contain nonvascular contractile cells. The presence of nerve terminals and adjacent ganglion cells suggests neural control of these contractile cells. The absence of such contractile cells in fetal, newborn, and infant eyes is an argument against a major role of these cells in promoting ocular enlargement. These contractile cells may instead participate in regulation of refractive state by maintenance of ocular size in the face of intraocular pressure or in intermediate-term regulation of choroidal thickness.

Actins↗

PubMed1997

The human horizontal vestibulo-ocular reflex during combined linear and angular acceleration.

We employed binocular magnetic search coils to study the vestibulo-ocular reflex (VOR) and visually enhanced vestibulo-ocular reflex (VVOR) of 15 human subjects undergoing passive, whole-body rotations about a vertical (yaw) axis delivered as a series of pseudorandom transients and sinusoidal oscillations at frequencies from 0.8 to 2.0 Hz. Rotations were about a series of five axes ranging from 20 cm posterior to the eyes to 10 cm anterior to the eyes. Subjects were asked to regard visible or remembered targets 10 cm, 25 cm, and 600 cm distant from the right eye. During sinusoidal rotations, the gain and phase of the VOR and VVOR were found to be highly dependent on target distance and eccentricity of the rotational axis. For axes midway between or anterior to the eyes, sinusoidal gain decreased progressively with increasing target proximity, while, for axes posterior to the otolith organs, gain increased progressively with target proximity. These effects were large and highly significant. When targets were remote, rotational axis eccentricity nevertheless had a small but significant effect on sinusoidal gain. For sinusoidal rotational axes midway between or anterior to the eyes, a phase lead was present that increased with rotational frequency, while for axes posterior to the otolith organs phase lag increased with rotational frequency. Transient trials were analyzed during the first 25 ms and from 25 to 80 ms after the onset of the head rotation. During the initial 25 ms of transient head rotations, VOR and VVOR gains were not significantly influenced by rotational eccentricity or target distance. Later in the transient responses, 25-80 ms from movement onset, both target distance and eccentricity significantly influenced gain in a manner similar to the behavior during sinusoidal rotation. Vergence angle generally remained near the theoretically ideal value during illuminated test conditions (VVOR), while in darkness vergence often varied modestly from the ideal value. Regression analysis of instantaneous VOR gain as a function of vergence demonstrated only a weak correlation, indicating that instantaneous gain is not likely to be directly dependent on vergence. A model was proposed in which linear acceleration as sensed by the otoliths is scaled by target distance and summed with angular acceleration as sensed by the semicircular canals to control eye movements. The model was fit to the sinusoidal VOR data collected in darkness and was found to describe the major trends observed in the data. The results of the model suggest that a linear interaction exists between the canal and otolithic inputs to the VOR.

Adolescent↗