Eye position signals in the abducens and oculomotor nuclei of monkeys during ocular convergence

Many neurons in oculomotor pathways encode signals related to eye position. For example, motoneurons in the third, fourth, and sixth cranial nuclei discharge at highly regular rates during fixation intervals. During fixations of far targets, their tonic discharge is linearly related to conjugate eye position. Previous studies provided evidence that premotor cells in brainstem pathways also encoded conjugate eye position. McConville et al. (this volume), however, measured eye movements during binocular fixations when the eyes were converged and concluded that the position signal encoded by premotor position-vestibular-pause (PVP) cells in the vestibular nuclei is related to monocular (right or left) eye position rather than to conjugate eye position. This surprising relationship would not have been noticed in earlier studies that measured the movements of only one eye (using a single eye coil) or that measured only the conjugate movements of the two eyes (using bitemporal EOG electrodes). How general a feature of oculomotor signal processing is this finding? In this paper, we re-examine the eye position signal in abducens and oculomotor neurons when the movements of the two eyes are conjugate and when they are disjunctive and therefore disassociated. The data suggest that abducens neurons (AMNs and AINs) and oculomotor neurons (putative medial rectus motoneurons), unlike PVP cells, are not monocular but encode mixtures of right and left eye position signals.

Vergence eye movements under natural viewing conditions

PURPOSE

To determine whether there are any fundamental differences in vergence dynamics under different viewing conditions, both in instrument space and free space.

METHODS

Symmetric vergence responses were measured for a variety of conditions under the traditional instrument space as well as the more natural free space viewing environment. Vergence eye movements were recorded objectively in three subjects using the infrared reflection technique. Within each environment, four conditions were tested: aperiodic self-initiated voluntary gaze shifts between two simultaneously viewed targets; periodic (0.33 Hz) voluntary gaze shifts between the same two targets; gaze shifts in total darkness to the near target following initial far target fixation in the light; and gaze shifts in total darkness to the two remembered target positions. In addition, an experiment was performed in instrument space using randomized step changes of target disparity, in which the responses served as the standard for comparison. For all conditions, target disparities ranged from 0.5 degrees to 10 degrees. The peak velocity of each vergence response was calculated and plotted versus its amplitude.

RESULTS

It was found that the data for all conditions tested fell within the standard "main sequence" cluster, indicating similarity in dynamics and thus similarity in the motoneuronal controller signal. Also, the data from investigators who claimed differences in dynamics were also typically found to fall within the normal cluster.

CONCLUSIONS

This indicates that the vergence motoneuronal controller signal produced the same dynamics for a particular response amplitude, independent of both viewing environment and test condition.

[Treatment of fusional disorders in patients with brain damage]

BACKGROUND

We trained fusional convergence systematically using 3 orthoptic devices (fusion trainer, prisms, cheiroscope) in brain damaged patients with a severe deficit of convergent fusion.

PATIENTS AND METHODS

12 patients were selected, 6 with vascular cerebral lesions and 6 with traumatic brain damage. The chronicity (time since lesion) was 15.8 months (range: 2-108). Near and far visual acuity, accommodation, convergent fusional range, stereopsis (Titmus and TNO test), maximal reading duration and subjective complaints (e.g. eye strain, headache) were evaluated. To separate improvements during treatment from spontaneous recovery of function or measurement artefacts we performed repeated measurements before treatment (baseline period) during a time span of 4-6 weeks. Follow-up measurements could be performed in 9 of 9 patients 10 months after cessation of treatment.

RESULTS

No improvements were seen during the baseline period. 11 of 12 patients showed a significant recovery of fusional range during training which remained stable during the 10-month follow-up period. 10 of 11 patients showed significant improvements of local stereopsis (Titmus test) and 4 of 5 patients of global stereopsis (TNO test). All patients showed a significant increase of near visual acuity of about 10% (decimal visual acuity), improved reading ability and a reduction of subjective symptoms resulting from fusional deficiency, e.g. eye strain, headache.

CONCLUSIONS

These results suggest a considerable potential for recovery of oculomotor and visual functions in brain damaged patients after training.

[Post-traumatic anomalous innervation of the lateral rectus muscle by trigeminal fibers]

BACKGROUND

Congenital or acquired anomalous synkinetic movements are common in Neuro-Ophthalmology. Examples are the Stilling-Türk-Duane syndrome, the Marcus Gunn phenomenon or secondary oculomotor synkinesis in oculomotor nerve palsy.

PATIENT AND METHOD

A 29-year-old woman with N.V-, N.VI-, N.VII- and N.VIII-lesion after blunt cranio-cerebral trauma developed a left sided trigemino-abducens synkinesis. The abduction of the left eye by gaze effort was limited to 30 degrees before the midline. With tight jaw closure the eye reached the midline. Eye muscle surgery was performed (recess-resect procedure) because of homonymous diplopia. This is the third published case of a posttraumatic trigemino-abducens synkinesis.

RESULTS

Postoperatively the left globe was able to abduct 20 degrees over the midline using maximum coinnervation, with a left exotropia of 5 degrees. Without coinnervation, i.e., without jaw closure an residual esotropia of 26 degrees at distance and 14 degrees at near remained. The utilization of the synkinesis to compensate the esodeviation and to avoid diplopia was hardly possible even after a combined operation for esotropia. It is difficult to determine the amount and type of surgery since one has to take into account that the coinnervation produces an abduction of the eye depending on the intensity of jaw closure.

DISCUSSION

The trigemino-abducens synkinesis may be explained by aberrant regeneration of the peripheral never at the temporal petrous bone. Here, aberrant regrowing fibres of the mandibular nerve destined for the masseter muscle may reach the lateral rectus muscle via adjacent distal sheaths of the degenerated proprioceptive fibres of the lateral rectus muscle. Other mechanisms (ephaptic transmission or central reorganisation between the N.V- and N.VI-nuclei in the brain stem) are very unlikely.

Tonic accommodation, tonic vergence and surround propinquity

Previous studies have typically assessed tonic accommodation (TA) and tonic vergence (TV) by placing subjects in total darkness. However, a recent investigation demonstrated that even under such apparently stimulus-free conditions, subjects' awareness of the proximity of the adjacent surroundings and overall room topography (i.e. surround propinquity) may still influence the accommodative response. Accordingly, the aim of the present study was to measure TA and TV while subjects (n = 34) viewed an illuminated distant target (viewing distance = 6 m) to minimize this propinquity effect. TA was measured using an objective, open-field, infrared optometer under two conditions: first, while viewing the distant target monocularly through a 0.5 mm pinhole, and second in total darkness. Additionally, TV was assessed by determining the heterophoria subjectively while subjects viewed the distant target biocularly through 0.5 mm pinholes. The mean level of TA using the pinhole technique (mean = 0.28 D) was significantly lower than that observed in total darkness (mean = 0.60 D). Furthermore, a significant positive correlation (r = 0.86) was found when comparing the difference between the two TA measurements with the level obtained in total darkness. Those subjects having apparently high levels of TA in darkness exhibited the largest propinquity effect. Thus measurements of accommodation recorded in total darkness are likely to have overestimated both the magnitude and normal range of TA. The mean level of TV was 0.58 metre angle. No significant correlation was observed between TV and either of the two TA measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Dark focus of accommodation and vergence posture

We evaluated the dark focus of accommodation and vergence posture in 20 boys and girls, 10 with and 10 without accommodative esotropia, before and after they wore glasses for at least 1 month. Refractive error was measured by Nidek Autorefractometer AR1600. We used two definitions of the dark focus, DFcus (Non-Cyclo R) and DFcus (Cyclo R). DFcus (Non-Cyclo R) = Dark R - Non-Cyclo R. DFcus (Cyclo R) = Dark R - Cyclo R. (Dark R: refractive state in the dark; Non-Cyclo R: non-cycloplegic refractive error; Cyclo R: cycloplegic refractive error.) Vergence posture was measured by prism cover test during distance fixation while uncorrected. DFcus (Cyclo R) was significantly greater in esotropic subjects than in non-esotropic subjects, although DFcus (Non-Cyclo R) did not differ significantly. A significant hyperopic shift in Dark R was observed after wearing glasses. DFcus (Cyclo R) was significantly decreased after wearing glasses, suggesting that wearing glasses is an important variable of the dark focus. Unlike the dark focus, the vergence posture did not change in either group after wearing glasses. The dissociation of the dark focus from vergence posture seems to be inconsistent with previous findings. It is postulated that change in dark vergence after wearing glasses is responsible for the results.

The influence of long-term visual near-work on accommodation and vergence: a field study

A study of the influence of sustained, visual near-work upon a set of oculomotor functions (ZCSV i.e. Zone of Clear Single Vision) was carried out on a randomly drawn group (N = 43) of young, female, healthy and experienced accountants (mean age 24.6 years) who were daily engaged in full-time interactive VDT work. PRA (Positive Relative Accommodation), NRA (Negative Relative Accommodation), PRC (Positive Relative Convergence) and NRC (Negative Relative Convergence) was measured twice: 1) in the morning before the subjects started work and 2) at the end of the working day. Results showed significant reductions in the four ZCSV components over the working day. Vergence reductions accounted for over 60% of the ZCSV changes.

Ophthalmic findings in dyslexic schoolchildren

The ophthalmic findings of 55 dyslexic 12 to 13-year-old Finnish schoolchildren and 50 age, sex, and social class-matched control children were evaluated. On a neuropsychological basis the children could be divided into six subgroups: general deficiency, general language, visuomotor, naming, mixed, and normal. The two groups did not differ significantly from each other in visual acuity, cycloplegic refraction, the amount of phorias and tropias, stereo acuity, fusion, or accommodation. Convergence near point > or = 8 cm was, however, statistically more frequent in the dyslexic group. This finding was also significant in the general deficiency subgroup compared with the other subgroups. The most conspicuous common denominator in those with dyslexia was revealed to be the convergence insufficiency type of exodeviation, occurring in 38% of the general deficiency dyslexic subgroup and in 36% of the visuomotor dyslexic subgroup. This finding suggests a low accommodative convergence/accommodation ratio in these children.

Disparity vergence eye movements exhibit preprogrammed motor control

To determine the control strategy used to mediate disparity vergence eye movements, an optical technique was employed which eliminated the visual feedback normally associated with a vergence movement. The resulting "open-loop" vergence responses from four subjects exhibited multiple step-like movements. This finding augments previous demonstrations of multiple-step tracking strategies in vergence eye movements and supports the theory that the initial portion of the vergence response is under preprogrammed control. Additional evidence for preprogramming is given by the similarity of the initial dynamics between open- and closed-loop responses as evidenced by similar ratios of peak velocity to amplitude (i.e. "main sequence" plots).

Dark vergence in relation to fixation disparity at different luminance and blur levels

Fixation disparity was measured psychophysically when subjects viewed a central fusion stimulus at 50 cm distance. Reducing luminance and increasing target blur resulted in an exophoric shift of fixation disparity. Fixation disparity was significantly more exophoric in subjects with distant dark vergence, i.e. the resting position of vergence. This was true for the bright and sharp target, as well as for targets impaired by blurring or dimming. These results are interpreted as a bias of fixation disparity towards the individual resting position of vergence not only for degraded stimuli but also for strong central fusion stimuli.

Instability of ocular torsion during fixation: cyclovergence is more stable than cycloversion

We investigated spontaneous variation of binocular torsion. Variation was expressed as SD of torsional eye positions measured over periods up to 32 sec. Subjects viewed a single dot target for periods of 32 sec. In half of the trials a large random-dot background pattern was superimposed on the dot. The movements of both eyes were measured with scleral induction coils. Spontaneous torsional movements were largely conjugate: cyclovergence was much more stable than cycloversion. This difference was not due to roll head movements. Stability of cyclovergence was improved by the background pattern. Although overall stability (SD of position) of cycloversion was unaffected by a background, the background induced or enhanced a small-amplitude torsional nystagmus in 3 out of 4 subjects. We hypothesize that the difference in stability of cycloversion vs cyclovergence reflects the greater importance of torsional retinal correspondence, compared to absolute torsional position. In two subjects we found evidence for the existence of cyclophoria, manifested by systematic shifts in cyclovergence caused by the appearance and disappearance of the background.

Characteristics of antidromically identified oculomotor internuclear neurons during vergence and versional eye movements

1. Previous studies have shown that midbrain near response cells that increase their activity during convergent eye movements project to medial rectus motoneurons, which also increase their activity during convergence. Most neurons in the abducens nucleus decrease their firing rate during convergence, and the source of this vergence signal is unknown. Oculomotor internuclear neurons (OINs) in monkeys project primarily from the medial rectus subdivisions of the oculomotor nucleus to the contralateral abducens nucleus, although there is a smaller ipsilateral projection as well. Because of these anatomic connections, it has been suggested that the OIN input may be responsible for the vergence signal seen on abducens neurons. The behavior of the OINs during eye movements and their synaptic drive are not known. Thus the goal of this study is to determine the behavior of these neurons during conjugate and disjunctive eye movements and to determine if these neurons have an excitatory or inhibitory drive on the abducens neurons. 2. Single-unit recording studies in alert rhesus monkeys were used to characterize the behavior of OINs. Eighteen OINs were identified by antidromic activation and collision testing. The recorded OINs displayed a burst-tonic pattern of activity during adducting saccades, and the majority of these cells displayed an increase in tonic activity with convergent eye movements. 3. Identified OINs were compared with a large sample of non-activated and untested horizontal burst-tonic cells in the medial rectus subdivisions of the oculomotor nucleus. The results indicate that the OINs behave similarly to medial rectus motoneurons during vergence and versional eye movements. None of the OINs displayed vertical eye position sensitivity. 4. Microstimulation of the oculomotor nucleus where both the OINs and medial rectus motoneurons were located resulted in a large adducting twitch of the ipsilateral eye and a smaller abducting twitch of the contralateral eye. The latter effect was presumed to be the result of OIN innervation of the contralateral abducens nucleus. This result suggests that the crossed OIN pathway is predominately, if not entirely, excitatory. 5. Injection of 10% lidocaine HCl into the medial rectus subdivision of the oculomotor nucleus caused a reversible inactivation of the medial rectus motoneurons and OINs. As expected, the inactivation of medial rectus motoneurons resulted in an exophoria and weakness of adduction for the eye ipsilateral to the lidocaine injection. In addition, the lidocaine injection resulted in hypometric and slowed abducting saccades in the eye contralateral to the injection site. This result also suggest that the crossed OIN pathway is excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity analysis of relative accommodation and vergence

A sensitivity analysis was performed to determine the variation in response to changes in parameter values of a previously developed nonlinear static model of accommodation and vergence. To determine normal behavior, model simulation responses were computed using previously obtained parameter values in 4 subjects under 2 conditions. In the first, relative accommodation was evaluated by maintaining the vergence stimulus constant at 2.5 meter angles (MA) and varying the accommodative stimulus from -2.5 to 2.5 diopters (D) in 0.25-D steps. In the second, relative vergence was evaluated by maintaining the accommodative stimulus constant at 2.5 D and varying the vergence stimulus from 25 prism diopters (PD) base-in to 25 PD base-out in 5-PD steps. Sensitivity of the model parameters, consisting of controller gains for accommodation (ACG) and vergence (VCG), crosslink gains for accommodation-to-vergence (AC) and vergence-to-accommodation (CA), deadspace operators for accommodation (AE +/- AD) and vergence (VE +/- VD), and the tonic levels for accommodation (ABIAS) and vergence (VBIAS) were assessed by varying them at 50% and 150% of their normal values. It was found that the accommodation and vergence systems were most sensitive to variation in crosslink gain, moderately sensitive to variation in controller gain and tonic level, and least sensitive to variation in size of the deadspace. These results may provide a quantitative basis for the occurrence of ocular dysfunctions associated with abnormal crosslink gains, such as strabismus, in clinic patients.

A method for correction of CA/C ratio based on linear model of accommodation and vergence

Measurement of the response CA/C ratio has required that the target used to stimulate the vergence system provides no blur information to the accommodative system. Although several methods have been proposed to open the accommodative feedback loop, it is difficult to ensure that a vergence target produces no stimulus for accommodation. To avoid this problem, we have derived a formula based on a linear model of the accommodative and vergence systems that allows the CA/C ratio to be estimated when the accommodative loop is not opened completely. An experiment was conducted to verify the derived formula. Two targets, Snellen letters and a small point source, were used to provide different blur-inputs to the accommodative system. CA/C ratios were estimated from the formula using measures of accommodation and vergence obtained with these two targets for eight subjects and showed a high correlation. The formula, therefore, was shown to provide a consistent estimate of the CA/C even when the accommodative loop was not opened completely.

Calibration of the Hirschberg test in human infants

PURPOSE

The Hirschberg ratio has been defined as the ratio of the change in angular position of the line of sight to the change in location of the first Purkinje image relative to the pupil center. This study was designed to determine empirically whether the adult value of the Hirschberg ratio (approximately 22 prism diopters/mm) is a suitable value to use for all ages. Because many structures in the eye are growing during the 1st year, it is possible that there could be some change in the Hirschberg ratio during this period.

METHODS

A photographic technique was used to calculate the Hirschberg ratio for 323 infants between the ages of 27 and 365 days and for ten adult subjects. The study also measured angle lambda, the angle between the line of sight and the pupillary axis, in these populations.

RESULTS

The Hirschberg ratio did not change with age during infancy and was similar for adults and infants both in value and in variance. Angle lambda, however, declined rapidly, from an extrapolated value of about 8.4 degrees at birth to near 5 degrees at 5 months, a change that is assumed to reflect axial growth of the eye.

CONCLUSIONS

The results suggest that the same average Hirschberg ratio can be used to estimate angle of strabismus across age. The change in angle lambda is important to consider when evaluating angle of deviation from measures using the Hirschberg test.

The effect of stimulus contrast and interocular correlation on disparity vergence

In previous reports, we developed a metric for describing the signal strength of a dynamic random-dot stereogram (DRDS) stimulus at binocular (cyclopean) levels of the human visual system, which takes both contrast and interocular correlation into account. In this study we tested the generality of that metric in relation to the control of horizontal vergence eye movements. Signal strength was assessed by measuring the extent to which a DRDS stimulus could elicit involuntary vergence responses from a subject who was attempting to fixate steadily. Results for both step and sinusoidal disparity modulation paradigms showed that vergence velocity increased when either interocular correlation (IOC) or contrast was increased. Furthermore, IOC and contrast were found to contribute to signal strength for vergence in the same proportion as was found psychophysically. In general, the results indicate that the signals that drive this passive form of vergence are derived according to the same binocular combination rules as the signals that give rise to the perception of surfaces in DRDS stimuli.

The influence of a stationary single line in darkness on the visual perception of eye level

The angle of pitch of a visual field consisting of only a single vertical, 64 degrees-long, eccentrically-located line in otherwise total darkness influences the elevation of a target set to appear at eye level (VPEL). The influence changes linearly with the magnitude of pitch over the range from -30 degrees to +20 degrees. The average slope of the VPEL-vs-pitch function is +0.53. The influence on VPEL of a pitched visual field consisting of two parallel vertical lines is slightly greater (slope = +0.56), and the influence of the pitch of a complexly-structured well-illuminated pitched room is slightly greater yet (slope = +0.63). The pitch of a frontoparallel plane containing one horizontal line has a small influence on VPEL (slope = +0.08); the influence with two horizontal lines is slightly greater (slope = +0.18). The slope of the VPEL-vs-pitch function differs among individual subjects but is linear for each of the eight subjects. A great deal of consistency is manifested by individual subjects across all of the visual fields: an individual with a steep slope with one visual field tends to have a steep slope with all visual fields. The individual's characteristic response in total darkness is strongly correlated with the response to an erect well-illuminated visual field. The significant aspect of the pitched-from-vertical line stimulus is the change in orientation of its retinal image. An additional experiment with a small pupil (pilocarpine) indicates that cues related to other retinal gradients or to accommodation play no role in the influence of the visual field on VPEL. The experiments provide support for treating the visual influence on VPEL by means of the Great Circle Model.

The development of eye alignment, convergence, and sensory binocularity in young infants

PURPOSE

To measure and compare the development of ocular alignment, sensory binocularity, and convergence in infants during the first 5 months of life.

METHODS

Healthy infants were tested between 2 and 21 weeks of age. Ocular alignment was measured by the Hirschberg test; convergence was determined by visual examination as an illuminated toy approached an infant's face; and sensory binocularity was measured by preferential looking for fusible versus rivalrous gratings. In experiment 1 we compared the proportion of infants at different ages demonstrating orthotropic ocular alignment with those showing convergence. In experiment 2, we compared the age of onset of convergence to that of sensory binocularity.

RESULTS

Experiment 1: Most infants were orthotropic during the first month, and almost all of the others showed small amounts of exotropia. None of the infants showed accurate convergence until 6 weeks of age. By 4 months of age virtually all were orthotropic and had good convergence. Experiment 2: The onset of sensory binocular fusion occurred at 12.8 +/- 3.3 weeks. Full convergence did not occur until 13.7 +/- 3.2 weeks, although the first signs of convergence occurred slightly earlier. For individual infants there was a high correlation between the age of onset of sensory binocularity and convergence, and both onsets occurred earlier in girls than in boys.

CONCLUSIONS

Ocular alignment did not require the development of binocularity mechanisms, and the development of binocularity mechanisms did not await the onset of good ocular alignment. The relatively sudden onset of binocularity, both sensory (preference for fusion and stereopsis) and motor (convergence) at about 3 months of age and the high correlation between these measures indicate a common causal mechanism that probably involves refinements in striate cortex circuitry.

[Endocrine orbitopathy. Interventions on the external eye muscles]

Eye muscle surgery in Graves' disease is conducted to normalize eye motility disorders-diplopia, abnormal head posture, eyelid malposition. A review of the literature to determine the results of eye muscle surgery on the fibrotic eye muscles indicated the following: Correction of the motility disorder can be done precisely by recessing only one fibrotic eye muscle with a strabismus angle of up to 15; this leads to reproducible and dependable results, with a dose-effect coefficient independent of the initial strabismus angle. Indications vary as to how much to do as well as which side, dependent on whether the deviation is horizontal or vertical and whether the head posture is abnormal. Improvement of the fusional visual field is possible in nearly all cases. Hypercorrections occur more often when the muscle is not directly fixed at the sclera but adjusted on the following day. The time factor is important both before and after the operation: before the operation, the motility situation should have been stable for at least 6 months; postoperatively, a correction should be expected, which is not yet sufficient in the first few days; this should not lead to premature revision. Surgery on the vertical rectus muscles influences eyelid position: upper-lid retraction is improved and depends on the previous upper-lid motility; in contrast, an increase in lower-lid retraction is not dependent on recession of the inferior rectus. The results reported permit a precise series of steps to be drawn up as regards indication, the proportion of correction of each side, and eye muscle operations in Graves' disease.

[Effect of bulbus length of reduction of squint angle after suture fixation]

The principle of the posterior fixation suture popularized by Cüppers in 1971 is to decrease the length of the lever arm. The latter depends on the radius of the globe. For geometric reasons the axial length should have an influence on the surgical effect. There are contradictory results on this problem. Therefore, we conducted a study including 112 patients mainly with non-accommodative convergence excess (average age: 7.0 years, average axial length: 21.7 mm). We measured the axial length of both eyes before performing a retroequatorial myopexy. The amount of symmetrical surgery done in both eyes averaged between 12.0 and 14.0 mm, measuring 5.5 mm behind the limbus, i.e., from the insertion of the medial rectus. Fifty of the 112 patients received additional recession or resection of the medial rectus in one or both eyes. A statistically highly significant negative correlation was found between the axial length and the response to strabismus surgery (surgical effect = reduction of near and distance deviation). The correlation coefficient ranged between -0.29 (for all 112 patients) and -0.69 (for the uniform group of 24 patients with 13 mm surgery) with P < 0.0001. The same correlation was evident between the point of the posterior fixation suture behind the equator (angle of "retroequatoriality" in degrees) and the response to strabismus surgery. It is well known fact that larger deviation show a greater response to posterior fixation sutures.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of accommodative and binocular function in dyslexia

The visual correlates of dyslexia are the subject of controversy, and much evidence suggests that they may include some aspects of binocular and accommodative function. These factors were investigated in 43 control and 39 dyslexic children, who were matched for age, sex and performance intelligence quotient. The dyslexic group exhibited significantly lower positive and negative vergence reserves, and vergence instability when the eyes were dissociated at near. Their amplitudes of accommodation also were significantly reduced. However, other measures including dissociated and associated heterophoria and accommodative lag and facility were similar in both groups. The stability of motor ocular dominance, as assessed with a modified Dunlop test, was similar in both groups. The results of a simulated reading visual search task suggested that the vergence and accommodative dysfunction were not a major cause of the dyslexia. Further analyses, using reading-age matched groups, suggested that these ocular motor correlates were not attributable to the better reading performance in the control group. The most likely remaining explanation is that they are, in most cases, non-causal correlates of the dyslexia.

The effect of surgery on the AC/A ratio

The accommodative convergence/accommodation (AC/A) ratio is used clinically as a diagnostic tool in the assessment of convergence excess and near esotropia and the differentiation of true and simulated divergence excess. An abnormally high AC/A ratio has been implicated in the aetiology of some forms of strabismus but the evidence for alteration in the ratio following surgery is inconclusive. In a prospective study of 38 patients the effect of routine squint surgery was investigated in concomitant strabismus. The AC/A ratio was found to decrease significantly after surgery in both esotropes and exotropes. Furthermore a trend was demonstrated which suggested an increased risk of overcorrection in esotropes with a high (> or = 7:1) pre-operative AC/A ratio.

Progressive increase in the angle of deviation in congenital esotropia

The majority of patients with congenital esotropia in this consecutive series of 41 patients showed an increase in the angle of deviation when followed over an average of 3 months. The ages at the initial measurement and surgery, the cycloplegic refraction, and the clinical response to patching could not be used to identify those patients with a progressive increase in the quantity of the deviation. It was found, however, that if the preliminary surgical plan was adjusted (as it was in the majority of the patients in this series) to reflect the latest measurements obtained the day before surgery, a relatively high percentage of patients (90%) could be found to have achieved satisfactory alignment by the 6-week postoperative visit.

Evidence for nonretinal feedback in combined version-vergence eye movements

Recently, a quantitative model for the general of rapid eye movements in direction and depth was proposed. In this scheme, the saccadic and the vergence system share a common initiation system and are controlled by local feedback loops based on efference copy signals. We have used a remembered-target double-step paradigm to test the idea that both subsystems are guided by extraretinal signals. The subject was instructed to move the binocular point of fixation to the remembered positions indicated by a double-step movement of the target, in direction and depth. Since both binocular refixations were made in complete darkness, correct execution of this task requires information about both the stored visual coordinates of the final target and the coordinates of the first movement. Binocular eye movements from five subjects were compared with predictions from two feed-forward models and a feedback model. Analysis of the pooled direction data showed that the feedback model performed best and fitted well. Qualitatively the same result was obtained in the vergence component, but in this case the goodness of fit was considerably less. These results, confirmed in each individual subject, show that the saccadic and vergence subsystem can use nonretinal information about a prior movement in direction and depth. Further analysis showed that the gain of the direction response of the second movement was, on average, roughly correct. By contrast, the vergence component of these responses was only about 60% of the required amplitude. Since the fit procedure gave the same weighting factors to the second target and to the first movement, we propose that the low vergence gain reflects mechanisms operating after the calculation of the motor error signal, possibly at the execution stage. Finally, we discuss the possibility of a central control stage keeping track of the ongoing movement sequence, based on a comparison of desired and current eye position signals.

An investigation of prism adaptation latency

PURPOSE

The latency of adaptation of the human vergence system to a change in convergence or divergence forced upon it by a prism was investigated in this pilot study.

METHODS

Adaptation was stimulated by a 5-s period of binocular vision through a prism of 0 delta, -8 delta, or 8 delta. Immediately thereafter and for a further period of 45 s, lateral heterophoria was monitored subjectively by an automated version of Duane's screen and parallax test. Adaptation was calculated from the phoria 40 s after the end of binocular vision. Tests were performed at 0.4 m. In the first test session, there was screening during the binocular period to test for flaws in the screening method. In the second session, there was no screening during the binocular period and phoria measurement was started without changing the power of the prism. In the third session, the prism was restored to 0 delta after the period of binocular vision. Immediately after the last 5-s test, tests were repeated with 1 s of binocular vision. The subject had excellent visual acuity, stereoacuity, and stereolatency.

RESULTS

The test functioned correctly and showed good repeatability. The greatest adaptation to 8 delta was 59%. This was obtained with only 1 s of binocular vision. There was adaptation to -8 delta with 5 s of binocular vision but it was obscured by adaptation to 0 delta or 8 delta which persisted from previous tests.

DISCUSSION

The reason why the subject's latency was not found more precisely is explained. Additional evidence is presented in support of the finding that prism adaptation can take place within 1 s (perhaps less) of binocular vision, and the persistence and dominance of adaptation to base-out prism at near is pointed out. Guidelines are proposed for the experimental measurement of prism adaptation latency.