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PubMed1997

[The advantages and disadvantages of bifocal lenses].

The bifocal glasses present three main disadvantages: the jump of the image when the visual axis passes from the far vision glass to the reading segment, the prismatic effect on the near vision point that entails an apparent displacement of the fixed object as well as a degradation of the quality of its image, and the absence of intermediate vision in total presbyopia. Bifocals however keep their indications, even after the availability of the progressive glasses, by people accustomed with this kind of correction, and by others who are not supporting deformations in the lateral vision of the progressive glasses. Suggestions are made for choosing the model of bifocal which is the most appropriate in function of the different situations.

Accommodation, Ocular↗
PubMed1976

Does accommodative micropsia exist?

Subjects matched the size of a test target seen under various conditions of accommodation and convergence to the size of a reference target seen under constant conditions. Convergence, as expected, had a substantial effect on perceived size, while accommodation by itself had little or none.

Accommodation, Ocular↗
PubMed2003

Cortical correlates of vestibulo-ocular reflex modulation: a PET study.

To elucidate cortical correlates of vestibulo-ocular reflex (VOR) modulation, we observed cortical activation during fixation suppression and habituation of caloric vestibular nystagmus in 12 normal subjects, using PET. Significant positive correlation between regional cerebral blood flow (rCBF) and slow phase eye velocity of caloric nystagmus was observed in the middle and posterior insula, inferior parietal lobule, temporal pole, right fusiform gyrus, lingual gyrus, and cerebellar vermis and hemisphere. The rCBF increase in the insular region and the inferior parietal lobule was lateralized depending on the direction of the nystagmus. Caloric nystagmus was suppressed as a result of visual fixation, during which time the area around the right frontal eye field, temporal pole, inferior temporal gyrus, a broad area in the visual cortex, including fusiform and lingual gyrus, cerebellar uvula/nodulus and flocculus, exhibited positive correlation with fixation suppression of caloric nystagmus, while vestibular cortices exhibited negative correlation. The caloric nystagmus habituated with repetition of stimulation. With habituation, we observed activation in the right anterior cingulate gyrus, left superior parietal lobule and right cuneus, and deactivation in the anterior insula, cingulate gyrus, inferior parietal lobule and occipito-temporal visual cortex. The region that showed significant co-activation with fixation suppression and habituation of caloric nystagmus was the right cuneus, and significant co-deactivation was observed in the anterior insula, cingulate gyrus, inferior parietal lobule and middle temporal visual cortex. The present results support previous observations that the parieto-insular cortex and inferior parietal lobule are involved in processing of vestibular information, and, in addition, suggest that activation may depend on the direction of nystagmus. Deactivation of vestibular cortices during visual fixation supports the concept of inhibitory visual-vestibular interaction in the cortex. Significant activation of the cingulate, superior parietal and visual cortices, and cerebellar vermis accompanying reduction of caloric response with repeated stimuli suggests possible involvement of these regions in vestibular habituation. Common activation of the cuneus in visual cortex and deactivation of vestibular and visuo-spatial association cortices by both visual suppression and habituation of VOR suggests that these two mechanisms are not completely independent but may share some cortical and subcortical regions.

Adult↗
PubMed1975

Observations upon the role of the peripheral retina in the execution of eye movements.

A variety of experimental situations are described concerning optokinetic nystagmus and tracking movements of the eyes, which have shown that in certain circumstances the peripheral retina can exert a powerful influence upon the resultant response. The teleological significance of the findings is discussed in the context of the mechanisms subserving ocular-following movements.

Fixation, Ocular↗
PubMedDate not supplied

Recovery of vestibulo-ocular reflex and gaze disturbance in patients with unilateral loss of labyrinthine function.

Vestibulo-ocular reflex (VOR) gain and the ability of gaze fixation were investigated in patients with unilateral loss of labyrinthine function during the time course of compensation (four days to 6 months). A marked directional difference in the VOR gain in the acute stage (0.51 to 0.73 at four days) decreased proportionately to the logarithm of time on rotations to the affected side and to the intact side by an increase and a slight decrease of the gain, respectively. Gaze disturbance, which occurred only on rotations to the affected side, recovered in parallel with the decrease of directional difference in the VOR gain; it was still found at 1 Hz at 6 months. Recovery of gaze disturbance in unilateral lesions seems to be accomplished not only by the change in the VOR gain, but also by intensified amplification of this gain to make up for a low VOR gain.

Adult↗
PubMedDate not supplied

Surgical correction of excess esotropia at near.

A retrospective study of the surgical treatment of patients with excess esotropia at near was conducted. Thirty-one patients conformed to the following criteria: equal vision in each eye, esotropia at near exceeding distant deviation by 15 or more prism diopters, and no prior surgery. Hyperopes greater than or equal to +1.50 spherical equivalent were measured in their full spectacle correction. All patients had an otherwise normal ocular examination and no overt neurological disease. At distance fixation, approximately half of the population had an esotropia greater than 10 delta and the remainder were orthophoric. All demonstrated a near deviation measuring at least 25 delta. Patient age at time of surgery ranged from 18 months to 36 years. All but two patients underwent bilateral medial rectus recessions of 4.5 mm to 7 mm, based on the near deviation. The remaining two patients underwent right medial rectus recessions of 6 mm each. Minimum postoperative follow-up was 4 months. The majority of patients were able to achieve satisfactory alignment. Many could maintain this alignment without spectacles. Two patients developed consecutive exotropia. One of these patients required bilateral lateral rectus recessions to achieve alignment.

Adolescent↗
PubMed1984

High-frequency vestibulo-ocular reflex activation through forced head rotation in man.

The dynamic characteristics of the human vestibulo-ocular system were studied in the 0.5-30 Hz frequency range by rotating the head around a vertical axis. The rotation produced by a powerful servo-controlled vibrator was transmitted to the head by a hard-cushioned helmet and a rigid bite bar. The vestibulo-ocular reflex (VOR) gain and phase were measured during fixation of a mobile or stationary visual target and in total darkness. In a second set of experiments, the subjects evaluated the amplitude of the perceived visual instability of the target in the two fixation conditions. The results confirm earlier observations describing the VOR gain and phase in the 0.5-8 Hz range: the gain starts to decrease beyond 2 Hz and the phase rises towards 90 degrees. Beyond 8 Hz, the gain curve reverses direction and increases continuously toward 3-4 at 25-30 Hz in the three tested situations. Meanwhile, the phase curves do not vary much. In the same frequency range, the perceived visual instability is significantly altered. Therefore, the gain variation at high frequency may, in part, explain the perceived visual target instability and may be suspected of altering visually controlled tracking executed in vibrating environments.

Adult↗
PubMed1999

Listing's plane rotation with convergence: role of disparity, accommodation, and depth perception.

Earlier studies have reported temporal rotation of Listing's plane with convergence of the eyes causing torsion, which is dependent on eye elevation. The amount by which the planes rotate differs from study to study. To gain insight into the functional significance of the temporal tilt of Listing's plane for vision, we examined whether the rotation of the plane depends on the visual conditions, namely on the stimuli driving vergence. In different conditions, accommodative vergence, disparity-vergence, combinations of disparity with accommodation or depth perception were used and the resulting rotation of Listing's plane was measured. Our findings show, for the first time, that the relationship between convergence and Listing's-plane temporal rotation depends on the stimuli driving vergence. When the stimulus contains only disparity cues, vergence and Listing's plane rotate immediately and consistently among subjects. Accommodative vergence, the mutual couplings between vergence and accommodation, can influence the orientation of Listing's plane, but they do so in a idiosyncratic way. The largest rotation was elicited by stereograms combining disparity-vergence with depth perception. These findings support the idea of a functional role of Listing's plane rotation for binocular vision, perhaps for depth perception.

Accommodation, Ocular↗