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Biomedical subjects

J L Demer

Publications and source records attributed to J L Demer.

At least 55 records · Page 3Linked to original sources
PubMed1997

Deficits of gaze stability in multiple axes following unilateral vestibular lesions.

Abnormalities in the vestibulo-ocular reflex (VOR) after unilateral vestibular injury may cause symptomatic gaze instability. We compared five subjects who had unilateral vestibular lesions with normal control subjects. Gaze stability and VOR gain were measured in three axes using scleral magnetic search coils, in light and darkness, testing different planes of rotation (yaw and pitch), types of stimulus (sinusoids from 0.8 to 2.4 Hz, and transient accelerations) and methods of rotation (active and passive). Eye velocity during horizontal tests reached saturation during high-velocity/acceleration ipsilesional rotation. Rapid vertical head movements triggered anomalous torsional rotation of the eyes. Gaze instability was present even during active rotation in the light, resulting in oscillopsia. These abnormal VOR responses are a consequence of saturating nonlinearities, which limit the usefulness of frequency-domain analysis of rotational test data in describing these lesions.

Adaptation, Physiological↗
PubMed1997

Ophthalmologic findings in patients with ataxia.

PURPOSE: To determine the frequency and spectrum of ophthalmologic findings in a large, heterogeneous group of patients with ataxia. METHODS: Medical records of 184 patients from a university-based ataxia clinic were retrospectively reviewed. Patients were classified as having Friedreich's ataxia, spinocerebellar degeneration, cerebellar degeneration, familial or sporadic olivo-pontocerebellar atrophy, multisystem atrophy, spastic ataxia, myoclonic ataxia, or other diagnoses such as mitochondrial myopathy. All had complete ophthalmologic examinations, and 107 underwent electro-oculography. RESULTS: Among 184 patients with ataxia, diplopia was present in 52 (28%), best-corrected visual acuity was decreased in 29 (16%), and oscillopsia was present in 10 (5%). Diplopia was usually caused by an intermittent, small-angle heterotropia. The reduction in best-corrected visual acuity varied from mild to profound and was caused by optic atrophy, retinal degeneration, or both. Optic atrophy was present most frequently in spastic ataxia (five of 22 patients) and myoclonic ataxia (two of eight patients), followed by Friedreich's ataxia (three of 26 patients). Retinal degeneration and ophthalmoplegia were most characteristically associated with familial olivopontocerebellar atrophy and mitochondrial myopathy. Laboratory testing of ocular motility showed frequent abnormalities in all groups. Fixation instability was most characteristic of Friedreich's ataxia, whereas saccadic slowing was noticeably absent from patients with purely cerebellar degenerations. CONCLUSIONS: The ataxias may be associated with visual dysfunction caused by retinal degeneration, optic atrophy, oculomotor disturbance, or a combination of these.

Adolescent↗
PubMed1997

Positron-emission tomographic study of human amblyopia with use of defined visual stimuli.

PURPOSE: The purpose of this study was to use positron emission tomography (PET) to evaluate effects of amblyopia on cerebral blood flow and glucose metabolism in humans viewing defined visual stimuli and to correlate these effects with specific behavioral and electrophysiologic measures of visual function. METHODS: One subject with normal vision and five patients with amblyopia were prospectively studied. During monocular viewing of a checkerboard reversal stimulus by each subject, we performed PET imaging of relative cerebral glucose metabolism with use of [18F]fluorodeoxyglucose, PET imaging of relative cerebral blood flow with use of H2(15)O, and visual evoked potentials. Control studies were also performed with use of binocular occlusion and during presentation of stationary and horizontally drifting checkerboards. These data were correlated with letter acuities and contrast sensitivity functions for each eye. RESULTS: Although spatial resolution was superior for glucose metabolic imaging, PET readily demonstrated activation of calcarine cortex with use of both metabolic and blood flow tracers. Even in patients with mild amblyopia, functional activation of calcarine cortex was reduced in amblyopic eyes compared with sound eyes to a degree more closely correlated with visual acuity than were visual evoked potential amplitudes to the same stimulus. When responses to drifting versus stationary stimuli were compared, a putative motion processing center was identified in the right temporoparietal region. Activity in this motion center was relatively preserved during viewing of drifting stimuli by the affected eye of an anisometropic amblyopic subject, but was attenuated during viewing of the same stimulus by the affected eye of a strabismic amblyopic subject. CONCLUSIONS: PET imaging of blood flow and metabolism can quantitatively evaluate functional deficits resulting from amblyopia in striate as well as extrastriate visual areas. Calcarine cortical function correlates closely with severity of amblyopia, but function in a putative motion processing area may vary according to the type of amblyopia present.

Adult↗
PubMed1997

Human gaze stabilization during natural activities: translation, rotation, magnification, and target distance effects.

Stability of images on the retina was determined in 14 normal humans in response to rotational and translational perturbations during self-generated pitch and yaw, standing, walking, and running on a treadmill. The effects on image stability of target distance, vision, and spectacle magnification were examined. During locomotion the horizontal and vertical velocity of images on the retina was <4 degrees /s for a visible target located beyond 4 m. Image velocity significantly increased to >4 degrees /s during self-generated motion. For all conditions of standing and locomotion, angular vestibulo-ocular reflex (AVOR) gain was less than unity and varied significantly by activity, by target distance, and among subjects. There was no significant correlation(P > 0.05) between AVOR gain and image stability during standing and walking despite significant variation among subjects. This lack of correlation is likely due to translation of the orbit. The degree of orbital translation and rotation varied significantly with activity and viewing condition in a manner suggesting an active role in gaze stabilization. Orbital translation was consistently antiphase with rotation at predominant frequencies <4 Hz. When orbital translation was neglected in computing gaze, computed image velocities increased. The compensatory effect of orbital translation allows gaze stabilization despite subunity AVOR gain during natural activities. Orbital translation decreased during close target viewing, whereas orbital rotation decreased while wearing telescopic spectacles. As the earth fixed target was moved closer, image velocity on the retina significantly increased (P < 0.05) for all activities except standing. Latency of the AVOR increased slightly with decreasing target distance but remained <10 ms for even the closest target. This latency was similar in darkness or light, indicating that the visual pursuit tracking is probably not important in gaze stabilization. Trials with a distant target were repeated while subjects wore telescopic spectacles that magnified vision by 1.9 or 4 times. Gain of the AVOR was enhanced by magnified vision during all activities, but always to a value less than spectacle magnification. Gain enhancement was greatest during self-generated sinusoidal motion at 0.8 Hz and was less during standing, walking, and running. Image slip velocity on the retina increased with increasing magnification. During natural activities, slip velocity with telescopes increased most during running and least during standing. Latency of the visually enhanced AVOR significantly increased with magnification (P < 0.05), probably reflecting a contribution of the visual pursuit system. The oculomotor estimate of target distance was inferred by measuring binocular convergence, as well as from monocular parallax during head translation. In darkness, target distance estimates obtained by both techniques were less accurate than in light, consistently overestimating for near and underestimating for far targets.

Adult↗
PubMed1997

Location and stability of rectus muscle pulleys. Muscle paths as a function of gaze.

PURPOSE: The paths of the rectus extraocular muscles (EOMs) are constrained by pulleys, connective tissue sleeves mechanically coupled to the orbital walls. This study sought to investigate, using high-resolution magnetic resonance imaging (MRI), the location and stability of EOM pulleys in normal subjects and those with strabismus. METHODS: Multiple contiguous coronal MRI scans spanning the anterior-to-posterior extent of the orbit during primary gaze, upgaze, downgaze, adduction, and abduction were analyzed digitally to determine the paths of the rectus EOMs. Pulley locations were inferred from EOM paths. RESULTS: Data for 10 orbits of six normal subjects established the normal paths of the rectus extraocular muscles in primary gaze. Muscle paths in primary position were highly uniform across normal subjects. In secondary gaze positions, rectus muscle paths at the level of the pulleys exhibited small but consistent shifts, relative to the orbit, opposite the direction of gaze, consistent with the expected mechanical effects of the intermuscular connective tissue suspensions of the pulleys. Twelve orbits of seven subjects with strabismus showed, as a group, no significant difference from normal in rectus muscle paths in primary gaze and no significant difference from normal in changes of muscle paths in secondary gaze. Two subjects with incomitant strabismus wer found to have grossly abnormal rectus muscle paths in primary gaze, suggesting heterotopic pulleys. Computer simulations of these heterotopic pulley locations accounted for the observed patterns of incomitant strabismus in both. CONCLUSIONS: High-resolution MRI can determine the location and sideslip of rectus EOM pulleys. Pulley position is highly uniform across normal subjects, consistent with the notion that musculo-orbital tissue connections determine the pulling direction of the rectus EOMs. In normal subjects and subjects with strabismus, pulleys exhibit small shifts with eccentric gaze that are consistent with secondary intermuscular, but not musculo-global, mechanical couplings. Heterotopic pulley position is a potential cause of incomitant strabismus.

Adult↗
PubMed1997

Innervation of extraocular pulley smooth muscle in monkeys and humans.

PURPOSE: Soft pulleys stabilize paths and determine pulling directions of the extraocular muscles (EOMs). This study was conducted to characterize innervation of smooth muscles (SMs) supporting these pulleys. METHODS: Cadaveric human and monkey orbits were step and serially sectioned for histochemical and immunohistochemical staining. Before perfusion, the superior cervical ganglia of one monkey had been injected with the anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L). Immunoperoxidase staining to human SM alpha-actin confirmed pulley SM. Monoclonal and polyclonal antibodies were used to demonstrate PHA-L, tyrosine hydroxylase, dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, neuronal nitric oxide synthase (NOS), and synaptophysin. The NADPH-diaphorase reaction was also used as a marker for NOS and the acetylcholinesterase (AChE) reaction for acetylcholine. RESULTS: Pulleys, consisting of collagen and elastin sleeves supported by connective tissue containing SM, were observed around rectus muscles of humans and monkeys. The human and monkey SM was richly innervated. Axons terminating in motor end plates within SM bundles were immunoreactive to PHA-L, tyrosine hydroxylase, and dopamine beta-hydroxylase, but not phenylethanolamine-N-methyltransferase, indicating innervation of pulley SM from the superior cervical ganglion by projections using norepinephrine. Smaller axons and motor end plates were also demonstrated in SM, using NADPH-diaphorase and NOS immunoreactivity, indicating nitroxidergic innervation, and using AchE, indicating cholinergic parasympathetic innervation. The pterygopalatine and, to a lesser extent, the ciliary ganglia, but not the Edinger-Westphal nucleus, contained cells immunoreactive to NOS, suggesting that nitroxidergic innervation to pulley SM is mainly from the pterygopalatine ganglion. CONCLUSIONS: The SM suspensions of human and monkey EOM pulleys are similar and receive rich innervation involving multiple neurotransmitters. These complex projections suggest excitatory and inhibitory control of EOM pulley SM, and support their dynamic role in ocular motility.

Acetylcholinesterase↗
PubMed1996

The human vertical vestibulo-ocular reflex during combined linear and angular acceleration with near-target fixation.

The purpose of this study was to examine the effect of fixation target distance on the human vestibuloocular reflex (VOR) during eccentric rotation in pitch. Such rotation induces both angular and linear acceleration. Eight normal subjects viewed earth-fixed targets that were either remote or near to the eyes during whole-body rotation about an earth-horizontal axis that was either oculocentric or 15 cm posterior (eccentric) to the eyes. Eye and head movements were recorded using magnetic search coils. Using a servomotor-driven chair, passive whole-body rotations were delivered as trains of single-frequency sinusoids at frequencies from 0.8 to 2.0 Hz and as pseudorandom impulses of acceleration. In the light, the visually enhanced VOR (VVOR) was recorded while subjects were asked to fixate targets at one of several distances. In darkness, subjects were asked to remember targets that had been viewed immediately prior to the rotation. In order to eliminate slip of the retinal image of a near target when the axis of rotation of the head is posterior to the eyes, the ideal gain (compensatory eye velocity divided by head velocity) of the VVOR and VOR must exceed 1.0. Both the VOR and VVOR were found to have significantly enhanced gains during sinusoidal and pseudorandom impulses of rotation (P < 0.05). Enhancement of VVOR gain was greatest at low frequencies of head rotation and decreased with increasing frequency. However, enhanced VOR gain only slightly exceeded 1.0, and VVOR gain enhancement was significantly lower than the expected ideal values for the stimulus conditions employed (P < 0.05). During oculocentric rotations with near targets, both the VOR and VVOR tended to exhibit small phase leads that increased with rotational frequency. In contrast, during eccentric rotations with near targets, there were small phase lags that increased with frequency. Visual tracking contributes during ocular compensatory responses to sustained head rotation, although the latency of visual tracking reflexes exceeds 100 ms. In order to study initial vestibular responses prior to modification by visual tracking, we presented impulses of head acceleration in pseudorandom sequence of initial positions and directions, and evaluated the ocular response in the epoch from 25 to 80 ms after movement onset. As with sinusoidal rotations, pseudorandom eccentric head rotation in the presence of a near, earth-fixed target was associated with enhancement of VVOR and VOR gains in the interval from 25 to 80 ms from movement onset. Despite the inability of visual tracking to contribute to these responses, VVOR gain significantly exceeded VOR gain for pseudorandom accelerations. This gain enhancement indicates that target distance and linear motion of the head are considered by the human ocular motor system in adjustment of performance of the early VOR, prior to a contribution by visual following reflexes. Vergence was appropriate to target distance during all VVOR rotations, but varied during VOR rotations with remembered targets. For the 3-m target distance, vergence during the VOR was stable over each entire trial but slightly exceeded the ideal value. For the 0.1-m near target, instantaneous vergence during the VOR typically declined gradually in a manner not corresponding to the time course of instantaneous VOR gain change; mean vergence over entire trials ranged from 60 to 90% of ideal, corresponding to target distances for which ideal gain would be much higher than actually observed. These findings suggest a dissociation between vergence and VOR gain during eccentric rotation with near targets in the frequency range from 0.8 to 2.0 Hz.

Acceleration↗
PubMed1996

Anticipatory smooth eye movements of high velocity triggered by large target steps: normal performance and effect of cerebellar degeneration.

We studied frequencies and dynamic characteristics of anticipatory smooth eye movements (ASEM) in humans who were tracking step target movements of 20-70 deg amplitude. During presentation of periodic steps of constant amplitude healthy subjects showed frequent high velocity ASEM reaching maximal peak velocities of 5-40 deg/sec. There was no effect of ASEM on the frequency of anticipatory saccades. Randomization of target step amplitude or onset reduced the frequency of ASEM but did not completely abolish fast ASEM. In patients with cerebellar degeneration who exhibited impaired smooth pursuit, fast ASEM were absent and the number of slow ASEM was minimal. In conclusion, large sequential target steps can elicit much higher ASEM velocities than typically described in the literature. Similar to slow ASEM triggered by small steps, these fast ASEM do not require specific training and are not canceled by unpredictable step target motion. However, fast ASEM depend on the intact function of the cerebellum which gives further evidence of their generation by the smooth pursuit oculomotor subsystem.

Adolescent↗
PubMed1996

Structure-function correlations in the human medial rectus extraocular muscle pulleys.

PURPOSE: Fibroelastic pulleys function like the trochlea to fix the position and pulling direction of the recti extraocular muscles within the orbit. This study characterized the fine structure of the human medial rectus muscle pulley. METHODS: Human medial rectus muscle pulley tissue was dissected at autopsy, immersed in aldehyde fixative solution, and processed for and examined with light and electron microscopy. RESULTS: Pulley structure were located within posterior Tenon's fascia, closely surrounding the medial rectus muscle. Pulleys were comprised of a dense collagen matrix with alternating bands of collagen fibers precisely arranged at right angles to one another. This three-dimensional organization most likely confers high tensile strength to the pulley. Elastin fibrils were interspersed in the collagen matrix. Fibroblasts and mast cells were scattered throughout the relatively acellular and avascular collagen latticework. Connective tissue and smooth muscle bundles suspended the pulley from the periorbita. Smooth muscle was distributed in small, discrete bundles attached deeply into the dense pulley tissue. CONCLUSIONS: Fine structural observations confirm the existence and substantial structure of a pulley system in association with the medial rectus extraocular muscle. The presence of pulleys must be considered in models of the oculomotor plant. The cytoarchitecture and placement of pulleys suggest that they are internally rigid structures and are consistent with the idea that they determine functional origins for the extraocular muscles. However, the nature of the connective tissue-smooth muscle struts suspending the pulley system to the orbit supports the notion that the pulley position, and thus the vector force of the eye muscles, may be adjustable.

Biomechanical Phenomena↗
PubMed1995

Evaluation of vestibular and visual oculomotor function.

The visual system interacts synergistically with the vestibular system. A normally functioning vestibulo-ocular reflex is necessary but not sufficient for optimum visual acuity during head motion. Studies of dynamic visual acuity, the acuity achieved during relative motion of visual targets or of the observer, indicate that motion of images on the retina markedly compromises vision. The vestibulo-ocular reflex normally provides a substantial measure of stabilization of the retina during head movements, but purely vestibular compensatory eye movements are not sufficiently precise for optimal vision under all circumstances. Other mechanisms, including visual tracking, motor preprogramming, prediction, and mental set, interact synergistically to optimize the gain (eye velocity divided by head velocity) of compensatory head movements. All of these mechanisms are limited in their capacity to produce effective visual-vestibular interaction at higher rotational frequencies and velocities. It is under these conditions that vestibular deficits give rise to symptoms of oscillopsia. Patients having vestibular lesions exploit mechanisms of visual-vestibular interaction to compensate by substitution for deficient vestibular function. Thus, for accurate topographic clinical diagnosis of vestibular lesions, testing conditions should isolate purely vestibular responses. This may be done by testing reflex eye movements during passively generated rotations in darkness, or perhaps by testing during other types of motion under conditions of extreme frequency and velocity sufficient to attenuate the effects of visual-vestibular interaction. This article reviews clinical tests of vestibular function in relation to synergistic interactions with vision.

Aging↗
PubMed1995

Magnetic resonance imaging of the functional anatomy of the superior oblique muscle.

PURPOSE: To study the size and contractile changes of the normal superior oblique (SO) muscle using high-resolution magnetic resonance imaging (MRI), and to evaluate the abnormalities in these characteristics produced by SO palsy. METHODS: Multiple coronal MRI image planes were obtained using a surface coil to span the antero-posterior extent of each orbit and were repeated in multiple directions of gaze. Digital image analysis was used to measure muscle cross-sectional area for evaluation of size and contractility. RESULTS: Data for 16 orbits of 11 subjects without SO palsies established norms for morphology and contractility. On SO contraction in down gaze, maximum cross sectional area was significantly (P < 0.001) greater than on relaxation in up gaze, and the point of maximum cross-section shifted posteriorly with contraction. In seven subjects with clinically unilateral chronic SO palsies, the affected muscle was significantly smaller than normal (P < 0.001) and lacked contractile changes; clinically normal fellow SO muscles exhibited normal cross-sections and contractile changes. CONCLUSION: High-resolution MRI, coupled with quantitative morphometric analysis, can demonstrate the size and contractility of the normal SO muscle. The technique, which can be employed using widely available clinical equipment, is also sensitive enough to detect anatomic and functional changes expected in chronic SO palsies.

Adolescent↗
PubMed1995

Evidence for fibromuscular pulleys of the recti extraocular muscles.

PURPOSE: Magnetic resonance imaging (MRI) shows that the paths of recti extraocular muscle (EOM) bellies remain fixed in the orbit during large ocular rotations and across large surgical transpositions of their insertions. These findings imply that recti EOMs pass through pulleys coupled to the orbit and anterior to the muscle bellies, because the insertions must move with the globe. The present study was conducted to locate anatomically and to characterize histologically the pulley tissues. METHODS: High-resolution MRI images were collected from volunteers, using multiple gaze directions to infer the locations of, and occasionally to visualize, recti EOM pulleys. Fresh cadaver orbits were exenterated and dissected to evaluate mechanical and structural properties of the orbital connective tissues. Lipid was cleared from whole specimens to reveal tissue relationships. Other specimens were selectively step- and serial-sectioned for histochemical and immunohistochemical staining. RESULTS: Magnetic resonance imaging demonstrated dense connective tissue structures within posterior Tenon's fascia near the equator of the globe adjacent to the recti EOMs. Histochemistry showed these structures to be pulleys--fibroelastic EOM sleeves consisting of dense bands of collagen and elastin, suspended from the orbit and adjacent EOM sleeves by bands of similar composition. A monoclonal antibody to human smooth muscle alpha-actin demonstrated substantial smooth muscle in the pulley suspensions and in posterior Tenon's fascia. Tenon's fascia itself was seen to be suspended at its periphery from the orbital walls like a drumhead. CONCLUSIONS: The human orbit contains specialized musculofibroelastic tissues in and just posterior to Tenon's fascia that serve as compliant pulleys and determine the pulling directions of recti EOMs. In this sense, the pulleys are the functional origins of the recti EOMs and are determinants of ocular motility.

Actins↗
PubMed1994

Comparison of standardized echography with magnetic resonance imaging to measure extraocular muscle size.

Using both standardized echography and high-resolution, surface coil magnetic resonance imaging, we measured the transverse diameter of recti extraocular muscles to evaluate comparative accuracy. Maximum transverse diameter of each rectus muscle in primary gaze was measured in 39 orbits of 20 subjects by using standardized A-scan echography. In the same orbits, we obtained multiple coronal magnetic resonance imaging planes by using a surface coil to span the anteroposterior extent of each orbit. Digital image analysis with the National Institutes of Health Image program was used to measure transverse muscle diameter. Average size for each rectus muscle was similar for the echographic and magnetic resonance imaging techniques. However, linear regressions of individual ultrasound against magnetic resonance imaging measurements showed coefficients of determination (R2) moderate (0.29) for inferior recti muscles and low (< or = 0.11) for other recti muscles. For most recti muscles, significant variability in echographic estimates of muscle width results from factors other than true muscle size. Results indicate that ultrasound measurements of muscle size in individual patients should be interpreted cautiously.

Adolescent↗
PubMed1994

Dynamic visual acuity: a test for oscillopsia and vestibulo-ocular reflex function.

A method has been developed of clinically quantifying dynamic visual acuity (DVA), the acuity during imposed head motion in the pitch axis. In this method, visual acuity is measured using a computer-controlled projection system during vertical, sinusoidal relative motion either of the optotypes (the letters to be read) or of a servodriven swinging chair in which subjects are seated. In normal persons, the vestibulo-ocular reflex (VOR) functions adequately during imposed head movements to limit retinal image instability, making DVA independent of head velocity. During head movements the wearing of telescopic spectacles, which magnify the visual effects, can overwhelm even normal visual-vestibular interactions, producing retinal image instability leading to creation of artificial experimental oscillopsia in normal subjects. With the head stationary, optotype motion at frequencies and velocities beyond the capabilities of visual tracking also results in retinal image motion. Using these methods, DVA for both optotype and head motion was found to be degraded in a predictable fashion when the velocity of the image on the retina exceeded 2 degrees per second. In two patients with total peripheral vestibular loss, DVA was markedly impaired during head motion. As a result of the VOR deficit, this impairment of DVA was observed, even without the use of telescopic spectacles, and was predictably related to the velocity of imposed head motion. Dynamic visual acuity during imposed head motion is a quantitative and clinically feasible measure of oscillopsia that reflects functionally significant abnormalities of the VOR.

Adult↗
PubMed1993

Optokinetic-vestibular interaction in patients with increased gain of the vestibulo-ocular reflex.

We studied optokinetic nystagmus (OKN), optokinetic afternystagmus (OKAN) and visual-vestibular interaction in five patients with markedly elevated vestibulo-ocular reflex (VOR) gain due to cerebellar atrophy. All had impaired smooth pursuit, decreased initial slow phase velocity of OKN, and impaired ability to suppress the VOR with real or imagined targets. OKN slow phase velocity gradually built up over 25-45 s, reaching normal values for low stimulus velocities (< or = 30 deg/s). Initial velocity of OKAN was increased, but the rate of decay of OKAN was normal. These findings can be explained by models that include separate velocity storage and variable gain elements shared by the vestibular and optokinetic systems.

Adult↗