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At least 1,693 records · Page 94Linked to original sources
PubMed2003

Primary position and listing's law in acquired and congenital trochlear nerve palsy.

PURPOSE: In ocular kinematics, the primary position (PP) of the eye is defined by the position from which movements do not induce ocular rotations around the line of sight (Helmholtz). PP is mathematically linked to the orientation of Listing's plane. This study was conducted to determine whether PP is affected differently in patients with clinically diagnosed congenital (conTNP) and acquired (acqTNP) trochlear nerve palsy. METHODS: Patients with unilateral conTNP (n = 25) and acqTNP (n = 9) performed a modified Hess screen test. Three-dimensional eye positions were recorded with dual search coils. RESULTS: PP in eyes with acqTNP was significantly more temporal (mean: 21.2 degrees ) than in eyes with conTNP (6.8 degrees ) or healthy eyes (7.2 degrees ). In the pooled data of all patients, the horizontal location of PP significantly correlated with vertical noncomitance with the paretic eye in adduction (R = 0.59). Using a computer model, PP in acqTNP could be reproduced by a neural lesion of the superior oblique (SO) muscle. An additional simulated overaction of the inferior oblique (IO) muscle moved PP back to normal, as in conTNP. Lengthening the SO and shortening the IO muscles could also simulate PP in conTNP. CONCLUSIONS: The temporal displacement of PP in acqTNP is a direct consequence of the reduced force of the SO muscle. The reversal of this temporal displacement of PP, which occurs in some patients with conTNP, can be explained by a secondary overaction of the IO muscle. Alternatively, length changes in the SO and IO muscles, or other anatomic anomalies within the orbit, without a neural lesion, may also explain the difference in location of PP between conTNP and acqTNP.

Adaptation, Ocular↗
PubMed2002

Temporal constraints on lens compensation in chicks.

If the effective focal length of a growing eye is modified by spectacle lenses, the eye compensates by altering its growth, thereby keeping images in focus, a process we presume is similar to normal emmetropization. Using chicks, we have investigated how much visual exposure the eye needs to exhibit the two principal components of ocular compensation: altered rate of elongation (a scleral mechanism) and altered choroidal thickness. We have found that surprisingly small amounts of vision through spectacle lenses can elicit robust scleral and choroidal compensation if other visual feedback is limited by keeping the animals in the dark when not wearing lenses. Furthermore, we have found that the amount of vision necessary to induce these responses can be summarized as three rules: First, several brief daily episodes are more effective than a single or a few longer daily episodes, even if the total amount of vision is the same. Second, extremely brief episodes, even if very frequent, are relatively ineffective. Third, when plus and minus lenses are worn successively on the same eye, the plus lens has the dominant effect, even if the minus lens is worn five times longer than the plus lens. In addition, we have shown that the elongation rate and choroidal thickness responses are dissociable, such that brief, infrequent lens-viewing produces only an elongation response in the case of plus lens-wear and only a choroid response in the case of minus lens-wear. We thus show that the emmetropization system does not integrate defocus in a simple, linear fashion. These non-linearities, if present in children, might explain why, although education and reading show an epidemiological correlation with myopia, the total time spent reading and doing other nearwork by individual children generally does not predict the degree of myopia. It may therefore be necessary to quantify more complex temporal patterns of nearwork over the day in order to measure the impact of nearwork on eye growth.

Adaptation, Ocular↗
PubMed1999

Effect of interrupted lens wear on compensation for a minus lens in tree shrews.

BACKGROUND: When a young animal wears a monocular minus (concave) lens that shifts the focal plane away from the cornea, the vitreous chamber elongates over a period of days, shifting the retinal location to compensate for the altered focal plane. We examined the effect of removing the lens for a portion of each day on the amount of compensation in tree shrews. METHODS: Starting 24 days after natural eye opening, juvenile tree shrews wore a goggle frame that held a -5 D lens in front of one eye, with an open frame around the fellow control eye. The goggle was removed for 0, 0.5, 1, 2, or 7 h each day (N = 5, 5, 5, 5, and 3 animals per group, respectively), starting 0.5 h after the start of each 14 h light-on period. After 21 days of treatment, measures were made of the cycloplegic refractive state (streak retinoscopy) and the ocular component dimensions (A-scan ultrasound). Normal animals that experienced 14 h each day with no lens (N = 3) were also examined. RESULTS: The treated eyes of the 0 h group developed full refractive compensation for the lens (treated eye - control eye, mean +/- SEM = -5.8+/-1.1 D) and had increased vitreous chamber depth (0.13+/-0.02 mm) and axial length (0.12+/-0.02 mm) relative to the untreated control eye. The groups in which the lens was removed for 0.5 and 1 h each day showed partial compensation for the -5 D lens, both in refractive state (-4.2+/-0.4 D; -2.9+/-1.6 D) and in vitreous chamber depth (0.12+/-0.02 mm; 0.09+/-0.02 mm). The 2, 7, and 14 h (normal) groups showed no significant refractive or axial compensation. In the 0.5 and 1 h groups, A-scan ultrasound showed a thinning of the region between the front of the retina and back of the sclera. CONCLUSIONS: The eyes of tree shrews can tolerate altered monocular visual stimulation produced by a minus lens worn for 12 h of a 14-h light cycle without developing an induced myopia. However, when the lens is worn more than 12 of 14 h each day, compensation appears to increase linearly with decreased lens-off time. If the eyes of human children respond similarly to defocus from near work or other sources, it would seem that the defocus must be present almost all the time to induce myopia. If defocus contributes to human myopia through a compensation mechanism, then an increase in the amount of time that focused images are present should reduce myopic progression.

Adaptation, Ocular↗
PubMed2002

[The vision of welders in France].

PURPOSE: A study was conducted to measure the impact of welding on the vision of welders. METHODOLOGY: This study was conducted in France by the occupational medicine staff of large companies on 1.131 people, namely 850 welders and 281 control subjects. This investigation included two examinations at the beginning and the end of a year. The investigative procedure examined the different welding processes, the percentage of working time spent on welding activity, the length of exposure in years, as well as the medical variables: the optical correction type and history of ocular traumatology. The Visiotest or the Ergovision were used for the visual examination, equipment in common use by occupational medicine departments. RESULTS: The welders were comparatively young (59.53% of them were less than 45 years old). Moreover, for 69.75% of the welders, more than 75% of their activity was devoted to welding. All currently used welding processes were represented, including the modern PLASMA-TIG welding process. No excessive blood alcohol levels were observed in all subjects, but welders did smoke slightly more than the control subjects (40% vs 33%). Self-medication was rather less frequent among the welders, except as regards the use of eye drops, where the proportions were clearly inverted. Optical correction for hyperopia was similar between the two groups; however, as regards myopia, the welders were corrected less often. Lastly, contact lens use was exceptional among the welders. Nearsightedness varied logically with age, but also, inexplicably, with the welding processes. Vision recovery time after exposure to glare was much longer among the welders, except for the PLASMA-TIG processes. No difference was observed in the other parameters of the study. No change in the visual functions studied was noted between the two examinations. DISCUSSION: The examination techniques used showed no impairment of the studied visual functions, probably because companies use protective and preventive eye care methods. Moreover this study is the first to examine the type of welding used by workers and particularly the modern PLASMA-TIG process. The vision recovery time after exposure to glare seems better for the PLASMA-TIG process may be the result of the lower luminous intensity of this process. CONCLUSION: This study was conducted for preventive purposes to contribute to better monitoring of safety and comfort for welding workers and has shown no alteration of the visual function among welders in general.

Adaptation, Ocular↗
PubMed1989

Adaptive changes of vertical vestibulo-ocular reflex induced by tenectomy of vertical recti muscles and neuronal behavior related to vertical eye movement in the region of the interstitial nucleus of Cajal in alert cats.

Experiments were performed in alert cats to examine adaptive changes of the vertical vestibulo-ocular reflex (VOR) induced by tenectomy of the vertical recti muscles of one eye and behavior of neurons in the region of the interstitial nucleus of Cajal (INC) during this adaptation. Continuous pitch rotations (0.11 Hz) of a constant amplitude (+/- 10 degrees) with only the operated eye uncovered produced a gradual and significant increase in VOR gain that started within 15 min, reaching a plateau within 1 h. When only the normal eye was allowed to view again, VOR gain quickly decreased to the control values within 10 min. Such reversible gain change was not observed when visual input was not allowed to the operated eye, indicating that these changes are adaptive ones to reduce visual-vestibular conflict. A total of 18 neurons were recorded from the INC region during the VOR adaptation. These include 9 eye-movement-related neurons (8 vertical burst-tonic neurons and 1 vertical tonic neuron) and 9 other neurons that did not show close correlation with eye movement but were activated by pitch rotations (tentatively called pitch cells). During adaptive gain increase of the vertical VOR, all the vertical burst-tonic and tonic neurons showed a significant gain increase, whereas a significant change in response gain was observed in only 1 of the 9 pitch cells, indicating that vertical burst-tonic neurons in the INC region are involved in the adaptive gain increase in the vertical VOR.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2002

Do you really need your oblique muscles? Adaptations and exaptations.

BACKGROUND: Primitive adaptations in lateral-eyed animals have programmed the oblique muscles to counterrotate the eyes during pitch and roll. In humans, these torsional movements are rudimentary. PURPOSE: To determine whether the human oblique muscles are vestigial. METHODS: Review of primitive oblique muscle adaptations and exaptations in human binocular vision. RESULTS: Primitive adaptations in human oblique muscle function produce rudimentary torsional eye movements that can be measured as cycloversion and cyclovergence under experimental conditions. The human torsional regulatory system suppresses these primitive adaptations and exaptively modulates cyclovergence to facilitate stereoscopic perception in the pitch plane. It also recruits the oblique muscles to generate cycloversional saccades that preset torsional eye position immediately preceding volitional head tilt, permitting instantaneous nonstereoscopic tilt perception in the roll plane. CONCLUSIONS: The evolution of frontal binocular vision has exapted the human oblique muscles for stereoscopic detection of slant in the pitch plane and nonstereoscopic detection of tilt in the roll plane. These exaptations do not erase more primitive adaptations, which can resurface when congenital strabismus and neurologic disease produce evolutionary reversion from exaptation to adaptation.

Adaptation, Ocular↗