Functional roles of the lateral suprasylvian cortex in ocular near response in the cat

The lateral suprasylvian (LS) area, an extrastriate visual area in the cat, has been suggested to play an important role in processing motion in 3-dimensional visual space. In addition, the LS area is related to all three components of the ocular near response, i.e. lens accommodation, pupillary constriction, and ocular convergence: microstimulation in this area evoked these intra- and extraocular movements, and neuronal discharges associated with these movements were also found. Anatomical pathways, direct and indirect, from this area to premotor nuclei in the brainstem are known to exist. The present paper reviews studies useful for assessing the functional roles played by the LS area in triggering and modulating component movements in the ocular near response.

Ocular convergence-related neuronal responses in the lateral suprasylvian area of alert cats

Neuronal spike discharges were recorded from the lateral suprasylvian (LS) area while ocular convergence was elicited in five alert cats. Ocular convergence was elicited by presenting a visual target moving in depth. Cats were rewarded for convergence eye movement. In 9 out of 426 cells sampled in the caudal postero-medial LS area, the number of spikes was positively correlated with the peak eye velocities during ocular convergence. Significant correlation was found mostly within 400 ms preceding the moment at which the maximum velocity of ocular convergence was obtained. The result favors the hypothesis that the LS area plays an important role in the integrative control of ocular convergence.

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.

Rapid- and slow-velocity vergence eye movements

Previously we have measured rapid-velocity vergence responses to targets at different distances that provided no disparity or accommodative stimulation. To evaluate the possibility that this rapid-velocity vergence occurs during saccades, the latencies of eye movement between two long dim luminous rods were compared under two conditions. Rapid-velocity vergence with an average latency of approximately 300 ms was elicited when subjects alternately viewed horizontal rods at distances of 38 and 78 cm, and with a vertical separation of 5.2 degrees. Horizontal saccades with a comparable latency were measured when the rods were equidistant and oriented vertically. The correlation between the mean latencies of vergence and saccadic movements was 0.97, suggesting that the two movements occurred together. In a second experiment vergence responses were measured when the subject looked between a bright vertical line on a screen at 76 cm and a second pair of lines (vertically displaced between 0.15 degrees and 3 degrees) with crossed disparity to simulate a target at 38 cm. Slow-velocity vergence often occurred alone when the vertical separation between targets was small; rapid-velocity vergence intruded when the separation between the targets was larger. The results can be accounted for if proximity and disparity stimulation act through a single vergence controller, the output of which produces slow- or rapid-velocity vergence depending upon whether the saccadic system is concurrently active.

[Results of the bimedial muscle belt]

The effectivity of a retroequatorial muscle belt operation, modification of Cüppers' Fadenoperation, is demonstrated by its results in 206 patients with excess esotropia at near and variable angle esotropia. It is shown that this procedure can reduce excess esotropia at near by a significant amount and lessen the extent of variable angle esotropia without causing incomitance in horizontal eye motility. A belt, placed bimedially on both recti muscles, showed to be twice as effective on the deviation at near as on the angle at distance. Proposing a similar mode of action, the atraumatic fixation with the muscle belt can facilitate reoperations in comparison with the Fadenoperation. Damage to the muscle is limited to a minimum.

[Eye diseases and visual disability in extremely low birth weight infants]

79 consecutively treated very low birth weight newborns (less than 1000 g) were systematically examined in the postpartal period and after a follow-up of 2 to 5.5 years. Retinopathy of prematurity was found in 19 eyes of 10 children (maximum stage III-IV). At a minimum age of 2 and maximum age of 6 years, when 42 of these children could be reexamined, including all children with earlier retinopathy of prematurity, 10 children suffered from minimal to moderate retinal pathology due to inactive retinopathy of prematurity (4 eyes with dragging of vessels and macula), but 9 children exhibited a minor visual disability due to strabism, higher ametropia, anisometropia, and amblyopia. This stresses the need for opthalmological follow-up in low birth weight babies with 6 months and 3-4 years of age.

Absence of pupil response to blur-driven accommodation

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.

Dissociated unilateral convergence paralysis in a patient with thalamotectal haemorrhage

A 47 year old male was admitted in a comatose state. CT scan showed a haemorrhage in the right pulvinar thalamus descending into the right part of the lamina quadrigemina. He presented with anisocoria, prompt bilateral pupillary light reaction, and unilateral convergence paralysis contralateral to the lesion in combination with upward gaze palsy. During an observation period of two months, the convergence reaction returned to normal. MRI showed a lacunar lesion ventral to superior right colliculus. Angiography revealed an arteriovenous malformation (right posterior cerebral artery--sinus rectus) as the possible cause of the haemorrhage.

Lateral phoria at distance: contributions of accommodation

Lateral heterophoria at distance often is cited as an index of tonic vergence, yet recent research has shown that vergence in darkness is more convergent than most measures of phoria. Two experiments were conducted to investigate the possible role of accommodation in this discrepancy. Experiment I compared measures of distance phoria of 19 young adults with measures of vergence and accommodation of the same subjects. Dark vergence was found to be correlated with but significantly more convergent than distance phoria. Hierarchical regression analysis showed that individual measures of phoria were related to subjects' negative accommodation (from the dark focus to the distant target) as well as to dark vergence. Experiment II tested 13 young adults to evaluate three simple models of the influence of accommodation on distance phoria. Distance phoria was predicted best by a model that included the subjects' negative accommodation, accommodative vergence, and dark vergence. These findings support the hypothesis that distance phoria is influenced by accommodation for the fixation target. Dark vergence is a simpler index of tonic vergence.

Development of accommodation and convergence in infancy

Paraxial photorefraction was used to assess the development of accommodation and convergence in a large sample of infants under 1 year of age. The infants viewed small dolls placed at various distances (200-25 cm). The majority of infants at all ages demonstrated appropriate convergence for target distance, regardless of manifest refractive error. However, accommodation lagged behind convergence in development. Infants under 2 months tended to demonstrate either flat accommodation responses with a fixed plane of focus at around 30 cm, or accommodated appropriately for near targets, but failed to relax their accommodation sufficiently for the more distant targets. Thus, the focussing error increased with increasing target distance. Since the manifest refractive error was estimated by extrapolating the accommodation function to 0 diopters demand, these infants demonstrated spuriously myopic behavior. After 2 months, the majority of infants showing emmetropic behavior had accommodation responses that changed appropriately with target distance. However, infants with myopic or hyperopic manifest refractive errors displayed a variety of accommodative styles.

Compensatory eye movements during near fixation after fast adaptation to lenses

When the eyes are converged on a near target, the gain of the vestibulo-ocular reflex (VOR) during a subsequent head rotation is dependent upon both the amount of vergence and the degree to which it is asymmetrical. In conditions without visual feedback, we measured the gain of the VOR in the right eye of observers as they rotated their heads to the right or left while viewing a target placed 22, 32.5 or 200 cm from the centre of head rotation. We then adapted the VOR to the magnifying effects of +5 D lenses, and retested the VOR under the former convergence conditions. We found an inverse relationship such that the greater the VOR gain due to convergence, the less the effect of the rapid adaptation to lenses.

Adaptation model of accommodation and vergence

Both accommodation and vergence have been shown to exhibit adaptation after extended near viewing. Normally, when the stimulus to accommodation is removed, the accommodation system returns rapidly towards its tonic position. However, if the stimulus is removed after an extended focusing effort, the decay is much slower. A similar effect can be observed in the vergence system. After prolonged wearing of horizontal prisms, blockage of one eye results in a much slower decay of the vergence output towards its tonic value. No previous models have been shown to simulate quantitatively these effects. An interactive dual-feedback model of accommodation and vergence was developed to simulate the adaptive behaviour found experimentally. The unique feature of the model is that the output of each controller drives a dynamic adaptive component whose output governs the time constant of the controller. The model was able to simulate the rapid and slow decays following short and long viewing intervals in each of the accommodative and vergence systems. It also simulated adaptation during alternate binocular and monocular viewing under the accommodation closed-loop condition. Thus, this model can serve as the basis for detailed quantitative evaluation of adaptive behaviour in the accommodation and vergence systems.

Negative feedback control model of proximal convergence and accommodation

A comprehensive model has been developed to illustrate the interactions between the observer and the surrounding environment in the control of oculomotor responses to distance or 3-D space. Accommodation and vergence respond to both spatiotopic (body reference) proximal percepts and retinotopic (eye referenced) physical stimuli of blur and disparity. Both spatiotopic and retinotopic stimuli are derived respectively from perceptual and physical correlates of negative feedback for eye position. The spatiotopic and retinotopic stimulus errors are combined in the feedforward path and drive a common oculomotor controller which has a phasic-tonic organization. Spatiotopic and retinotopic stimuli are shown to be effective over complementary operating ranges. Perceptual spatiotopic errors of gaze provide optimal stimuli for near responses to large depth intervals whereas physical-retinotopic cues of blur and disparity provide quantitative information about small binocular fixation errors. Small dynamic variations of target distance are sensed both spatiotopically and retinotopically. Coarse and fine spatiotopic errors of gaze are processed differently. Large spatiotopic errors are sampled intermittently at the beginning of the near response, whereas small retinotopic position errors and spatiotopic velocity errors are sampled continuously throughout the near response. Former reports of empirically observed higher velocity of vergence responses to very large depth intervals is explained in terms of stimulus sampling modes rather than in terms of separate oculomotor control mechanisms. The model demonstrates a complementary function of top-down spatiotopic cues, which are used to initiate the near response, and bottom-up retinotopic cues, which are used to refine and complete the near response. Cross-couplings by vergence-accommodation and accommodative-vergence serve to coordinate the components of the near response when feedback from sensed response of one motor system (i.e. vergence) is more accurate than that of the other motor system (i.e. accommodation). The model presented here is concerned primarily with the near response mediated by accommodation and disjunctive eye movements and not by the independent vergence mediated by non-conjugate or yoked saccades of unequal amplitude.

Vergence errors: some hitherto unreported aspects of fixation disparity

Measurement of the monocular components of fixation disparity indicated a higher prevalence of asymmetric contributions to the total deviation than previously reported. Furthermore, the exact proportion varied from moment to moment. Two of six subjects showed significant changes in fixation disparity over a period of 1 week. For all six subjects the changes in fixation deviation of one eye were virtually independent of those changes occurring in the other eye. In other words, these monocular variances were uncorrelated. Settings of the monocular components of binocular fixation disparity were accomplished at an accuracy close to that achieved by using a monocular vernier technique. The remaining differences appeared to be due to occasional instabilities during binocular viewing. The usual method of clinical measurement in which only one element is moved is not always equivalent to that determined by summing the two monocular components. The principal process measured by subjective fixation disparity appears to be either oculomotor or localized directional shifts of a monocular nature.

Role of monkey midbrain near-response neurons in phoria adaptation

1. One striking characteristic of the way in which accommodation and convergence of the eyes are organized is that although the two responses are usually tightly coupled, a brief period of binocular viewing through prisms that require extra convergence brings about a persistent, adaptive alteration in the relationship between the two responses: the vergence during monocular viewing of a target at a given distance is biased in a convergent direction. We sought to discover the role of the near-response neurons we have previously described in the monkey midbrain in such phoria adaptation. 2. Phoria adaptation was produced in two monkeys by having them view binocularly stereoscopic targets under conditions that mimicked prism viewing, i.e., the mirrors of the stereoscope were set so as to require more convergence than that associated with a real target at the same distance as the images seen in the stereoscope. The activity of 57 near-response neurons located dorsally and dorsolaterally to the oculomotor nucleus was recorded before and after adaptation while the monkeys monocularly viewed targets at a range of distances. 3. Comparison of a neuron's response in normal binocular viewing with the response when the accommodation and vergence stimuli were in conflict allowed us to distinguish 24 neurons that behaved as though they were related exclusively to the vergence response. 5 neurons that behaved as though they were exclusively related to the accommodation response, and 12 neurons whose firing was not so simply related to either response. We were unable to classify the remaining 16 near-response cells by this method. 4. In accommodation-related neurons, the relationship between firing rate and accommodation did not alter, or only altered slightly, when the animal's phoria was adapted. 5. The relationship between firing rate and vergence was unaltered by phoria adaptation in only a small proportion of vergence-related neurons, showing that most neurons do not carry the entire signal responsible for phoria adaptation. On the other hand, in the majority of vergence-related neurons the relationship between firing rate and accommodation was altered by phoria adaptation, showing that most neurons do carry part of the phoria adaptation signal. 6. The implication is that the increase in vergence observed after adaptation is mediated at more than one site. A proportion of the phoria adaptation signal is present at the level of the midbrain vergence-related neurons, with the remainder of the signal being added later, presumably at the motoneurons.

[Length-tension measurement of oblique eye muscles in strabismus operations for differentiating trochlear paralysis and strabismus sursoadductorius]

In a group of patients with a motility typical of a superior oblique palsy (a hypertropia increasing in adduction, in down-gaze and on head-tilt, a V-pattern and an excyclotropia), we recorded length-tension diagrams of oblique eye muscles during strabismus surgery. In 14 cases a length-tension recording was made during surgery in general anaesthesia, before and after intravenous administration of succinylcholine, that produces a fierce contraction of eye muscles. Among 14 patients that had eye motilities compatible with a superior oblique muscle palsy, 7 indeed had a non-contracting superior oblique muscle, but others had oblique muscles that contracted vividly. We also made length-tension diagrams of oblique eye muscles during strabismus surgery with local, tetracain eye-drop anaesthesia. Here, the recording was made three times, while the patient looked ahead, into the field of action of the muscle and out of the field of action of the muscle. Some patients indeed had a non-contracting superior oblique muscle and a stiff inferior oblique muscle, but others had superior oblique muscles that contracted vividly, despite an eye motility typical of a superior oblique palsy, with a positive Bielschowsky head-tilt test. This finding confirms the assumption of Kaufmann, Kolling and others that these cases have a non-paretic motility disorder. Viirre et al. found in normal monkeys that disruption of fusion by one week of occlusion of one eye allowed abberrations of conjugate horizontal and vertical eye movement like upshoot-in-adduction to become manifest.(ABSTRACT TRUNCATED AT 250 WORDS)

[Comparison of five methods for subjective assessment of ocular cyclodeviation]

METHODS

1. simple Maddox rod (Franceschetti) 2. set of scaled large diameter Maddox rods 3. synoptophore 4. synoptometer (Cüppers) 5. dark red glass in front of the Harms tangent scale (Kolling). Whereas the first two methods allow to measure in primary position only, the other three also permit secondary and tertiary position measurements. Thirty normal subjects underwent comparative measurements by means of the above techniques in monocular and binocular condition. The median values indicate that all these methods are reliable but that certain specific differences influence the results. For this reason, pre- and postoperative measurements of ocular torsion should always be performed by the same method. The most appropriate techniques have shown to be: the set of scaled Maddox rods for measurements in primary position only, the dark red glass in front of the Harms tangent screen for measuring also in secondary and tertiary positions.

Symptoms related to abnormal tonic state: experimental results and computer simulations

There have been numerous attempts to relate tonic accommodation and tonic vergence and their adaptive capabilities to a variety of parameters in vision science and clinical optometry. In this paper, investigations attempting to correlate the tonic state with nearwork symptoms will be reviewed. Results suggest that tonic vergence is more closely linked to near symptoms than is tonic accommodation. The results will be discussed in terms of a steady-state model of accommodation and vergence.

Vertical vergence adaptation for normal and hyperphoric patients

This paper discusses the use of vertical fixation disparity (VFD) measurements to determine a patient's ability to adapt to vertical prism. Vertical vergence adaptation for orthophoric patients, hyperphoric patients, and patients with vertical deviations associated with superior oblique paresis is reported. The effect of orthoptic training on the ability to adapt to vertical prism is also reported.

Vergence adaptation in esotropia

Vergence adaptation and anomalous fusional movements have been used by strabismics to overcome prism. There has been controversy in the literature regarding the nature of the vergence responses by esotropes to prism, i.e., fusional, avoidance of fusion, and/or anomalous fusion. This paper reviews the various types of movements made by esotropes to prism and attempts to explain the mechanism. In addition, the relation of these vergence movements to prismatic, orthoptic, and surgical correction is discussed.

Clinical implications of vergence adaptation

Placement of a prism in front of an eye results in a change in the tonic position of the eyes, a shift in the forced fixation disparity curve, and a shift in fusional amplitudes. These changes remain in effect as long as motor fusion is maintained. Elimination of fusion by occlusion or by removal of the prism results in a slow movement of the eyes back to the preprism position. This phenomenon, known as prism adaptation or slow fusional vergence, has important clinical implications in maintaining binocular vision with anisometropic prescriptions, age-related physiological changes in the positions of the eyes, blinking, high phorias, etc. Vergence adaptation is useful in explaining previous discrepancies between alternate and unilateral cover test, pre- and postorthoptic ACA ratios, stimulus and response ACA ratios, changes in phorias after orthoptics, and the observation of patients "eating up prism." Vergence adaptation anomalies have been implicated in causing asthenopia. Adaptation has been shown to change after orthoptic therapy. This paper reviews the clinical findings associated with vergence adaptation.

A dynamic model of cross-coupling between accommodation and convergence: simulations of step and frequency responses

The near triad consists of an increase in accommodation, vergence, and pupillary constriction. All three motor systems exhibit phasic and tonic responses. The tonic response adapts readily to phasic efforts of accommodation and vergence. Cross-coupling between accommodation and vergence provides a means of dynamically adjusting the tonic set points of the two motor systems to a common near or far working distance. Accommodative vergence cross-links play a dominant role in coordinating proximal changes in accommodation and convergence. The magnitude of cross-link interactions can be modified by imbalanced strength of tonic adaptation by accommodation and vergence. Reducing adaptation of tonic accommodation increases the AC/A ratio and decreases the CA/C ratio. Reducing adaptation of tonic vergence has the opposite effect. A model is able to predict these and other interactions simply by reducing the decay time constant of one of the two motor systems. For example, reducing the time constant for tonic accommodation results in an increased AC/A ratio and decreased CA/C ratio. Reducing the time constant for tonic vergence has the opposite effect. The model predicts transient step responses by accommodative vergence when the AC/A ratio is low and transient step responses of vergence accommodation when the CA/C ratio is low. It also predicts a reciprocal relationship between the AC/A and CA/C ratios. When one cross-link ratio is high the other cross-link ratio is low. Simulated frequency responses predict the low frequency roll off of low AC/A and low CA/C ratios. The step and frequency responses of cross-link ratios are shown to be the same for proximal (perceived distance) and retinal (blur and disparity) stimuli. The model suggests that physiological variations of tonic decay time constants may play an important role in determining clinically abnormal values of AC/A and CA/C ratios.

The effect of orthoptic treatment upon the vergence adaptation mechanism

This paper is a review of the research work that has been carried out over the past few years investigating the ability of the oculomotor system to adapt to prism-induced heterophoria. Our results show that subjects with normal binocular vision can adapt to horizontal and vertical prism-induced heterophorias whether fixating at distance or near. Further studies have shown that subjects with symptomatic abnormal binocular vision have an abnormal adaptation mechanism. Finally, we have found that when orthoptic treatment results in relief from the symptoms, there is an associated improvement in the subjects' ability to adapt to prism-induced heterophoria.

[Congenital nystagmus and intermittent exotropia. Suppression of nystagmus by fusional convergence]

We report on a patient with a combination of congenital nystagmus and intermittent exotropia. When the exotropia became manifest, a gross nystagmus appeared. When, however, the exotropia was overcome by fusinal convergence, the nystagmus was largely suppressed. The angle of squint was surgically reduced by such an amount that the exodeviation was just compensated, and the fusional convergence required to overcome the remaining angle sufficed to suppress the nystagmus.