Dynamic interactions between accommodation and convergence

Frequency-domain equations were derived for the current dual-interaction model of accommodation and convergence control, and its adaptive behavior was related to the system's parameters. Contrary to predictions based on the steady-state performance of the model [18], dynamic analysis showed that the AC/A is sensitive to the method of measurement and a procedure is established for its reliable determination. Additionally, the results provided theoretical support for the empirical finding that low AC/A and CA/C ratios are associated with high accommodative and convergence adaptation, respectively [15]. The derived equations should help future studies relate the physiological behavior of accommodative and convergence to specific model parameters.

[Functional vision disorders after head injuries]

The authors present the results of multilateral orthoptic examinations performed in 72 persons who suffered an injury to the head. They present the signs of a "subjective posttraumatic syndrome". The patients have been divided into 3 groups in dependence of the severity of the trauma. The first group (light injuries) did not show any substantial changes, the 2-d and 3-d group exhibited an increase of the physiological exophoria as well as disturbances of convergence. There was also a lowering of the relationship between the convergence accommodation and accommodation caused by a decrease of the accommodative convergence. The patients showed also an increase or reversal of the relationship divergence--convergence.

Influence of target color and vergence of light on ocular accommodation during binocular fixation

In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.

Distance heterophoria and tonic vergence

It has previously been suggested that the difference between tonic vergence (TV) and distance heterophoria may relate either to proximally induced vergence or accommodative divergence. To identify which of these components was responsible for this difference, the present study compared TV and distance heterophoria in a population of 20 subjects. However, distance heterophoria was now assessed with open-loop accommodation. No significant difference was observed between these two measures. This suggests that the previously observed variations between TV and distance heterophoria (with accommodation closed-loop) were produced by accommodative divergence. However, in a subgroup of 7 subjects, measurements of TV were more convergent (greater than 2 delta) than the distance heterophoria. In these subjects, proximally induced vergence may have accounted for the difference in response.

Proximal and cognitively-induced accommodation

The aim of this study was to investigate proximally-induced accommodation (PIA) at two levels of cognitive demand. PIA was assessed by comparing the open-loop accommodative responses to stimuli located at viewing distances of 6 m (0.17 D) and 0.33 m (3 D), respectively. The vergence and accommodation loops were opened by subjects monocularly viewing the targets through a 0.5 mm pin-hole. PIA was measured while subjects either were encouraged to relax and listened to popular music, or performed a relatively demanding mental arithmetic task (counting backwards in sevens). The results indicated that for 4 of the 12 subjects examined, increased cognitive demand produced a marked (greater than +/- 1.00 D) change in the magnitude of PIA. This finding suggests the presence of an interaction between the output of proximally- and cognitively-induced accommodation in these subjects. Such an interaction may hinder attempts to examine individual, non-optical components of accommodation.

Effects of stimulus distance on measurements of dark convergence

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ms in some individuals.

Eye-tracking infra-red optometer

An eye-tracking infra-red optometer has been developed to measure dynamic refractive power during slow horizontal eye movement. It consists of two instruments: an optometer to measure the refractive power of the human eye, and an eye tracker. When measuring refractive power, the eye tracker is used to maintain automatically the optical axis of the optometer coincident with the visual axis of the human eye. The measurement range of the optometer is approximately -10 to +10 D; refractive power is measured with an accuracy of about 0.4 D and with a precision of 0.05 D over an eye movement range of 60 degrees. The maximum trackable eye movement velocity is over 100 deg s-1, with a time delay of about 0.13 s. Furthermore it was confirmed that eye refraction measurement during vergence is possible using this eye-tracking infra-red optometer.

Binocular convergence of excitatory and inhibitory synaptic pathways onto neurons of cat visual cortex

When a cortical neuron receives synaptic input from both eyes, do the synaptic pathways that mediate the input from each eye match? In this study, inputs from the two eyes were compared by measuring the latencies of EPSPs and IPSPs evoked by electrical stimulation of the two optic nerves. For binocular neurons, these latencies invariably matched closely, indicating that the pathways from the two eyes contain the same number of synapses; monosynaptic input from lamina A of the lateral geniculate nucleus (LGN) is always matched by monosynaptic input from lamina A1. Conversely, polysynaptic input from one eye, either excitatory or inhibitory, is invariably accompanied by similar input from the other eye. In addition, the match between the two eyes in latency indicates that for each eye a synaptic potential is mediated by the same type of afferent, either X or Y. Judging from intracellular recording, 75% of the neurons studied were binocular, that is, EPSPs could be evoked from either eye. In the remaining 25%, EPSPs could be evoked from only one eye, in agreement with extracellular receptive field studies in which 30% of cortical neurons are monocular.

The effect of stimulus duration on tonic accommodation and tonic vergence

Changes in tonic accommodation and tonic vergence were studied as a function of stimulus duration and time after near fixation. Thirteen visually normal young adults each participated in experimental sessions comprising each of 12 combinations of stimulus type (accommodative or vergence) and stimulus duration (0.25, 0.5, 1, 2, 4, or 8 min). During each session, measurements were made of either tonic accommodation or tonic vergence 30 s before stimulus onset and at 0.5, 2, 4, 6, 8, 10, 12, and 14 min after stimulus offset. The stimulus (a high contrast reduced Snellen visual acuity chart) was viewed monocularly at the accommodative nearpoint and binocularly through pinholes at the fusional nearpoint on the accommodative and vergence sessions, respectively. For tonic accommodation, stimulus duration had no significant effect on either the magnitude of the aftereffect or its rate of decay. For tonic vergence however, the aftereffect was larger and had a slower rate of decay as stimulus duration increased. The results suggest that the adaptive mechanisms within the two systems have different time constants.

[A new method of surgery of nystagmus blockage syndrome]

Structural surgery is based on a model in which eye movements follow the ball-joint principle, i.e., there is absolute freedom of movement in all possible directions. Nystagmus blockage or convergence excess is regarded as a surplus of the medial rectus muscle. The relation between the weight of the globe and the muscle mass is taken into account. The medial rectus muscle is weakened by splitting it into 3 parts. The central part remains in its physiologic anatomic position in order to preserve normal convergence and binocularity, while the upper and lower portions prevent divergence. Additional prior weakening is accomplished by performing a tenonectomy.

Topology of the near response triad

The near response complex comprises three elements: accommodation, convergence, and pupillary constriction. The synkinesis between vergence and accommodation is well understood functionally, if not neuroanatomically. The latencies of near response components were measured in four healthy, experienced subjects to determine how the pupillary component is mediated. Addition of disparity stimulus to blur, yielding a near stimulus, reduces the latency of vergence eye movements and of accommodation by an amount that is significantly greater than the corresponding reduction in pupil latency. None of the existing hypotheses: vergence-pupil, accommodation-pupil or symmetric dual interaction, can account for this difference. Therefore, we present a new hypothesis: asymmetric dual interaction between accommodation and vergence.

On the neurophysiological organization of binocular vision

The considerable mixing in the visual cortex, of signals from left and right eyes, provides an abundant population of binocularly activated neurons. Based on this and on the fact that cortical cells respond best to different ranges of retinal disparities, it has been proposed that these neurons form the physiological substrate of stereoscopic depth discrimination. We outline reasons here for addressing first the more fundamental issue of the rules of convergence in the visual cortex, for input from the two eyes. We show that most of this convergence may be described by a linear summation process. However, there is a nonlinear mechanism that maintains binocular interaction regardless of large differences in stimulus strength between the eyes. This finding suggests that a cell which appears to be dominated by one eye, when monocular tests are conducted, may respond equally under binocular conditions. In this case, binocular processing for all cortical cells could be uniform and independent of the ocular dominance values determined monocularly. With respect to a neural mechanism for the processing of information concerning different depths in space, we propose an alternative to the conventional notion. First, we identify fundamental problems with the current view. Second, we describe a procedure which allows us to distinguish between the conventional view and our alternative proposal. Standard receptive field mapping techniques are not adequate for determining phase-disparity relationships of the type we require. Therefore, we have employed a reverse correlation procedure which enables efficient and detailed mapping of receptive field structure. Third, we describe preliminary data concerning the physiological mechanism of stereoscopic depth discrimination.

New directions for oculomotor research

This paper reviews major trends in the study of the oculomotor system since Westheimer published his doctoral research on this topic 35 years ago. Westheimer introduced the use of linear system analysis for the study of eye movements, an approach used a great deal by others ever since. Westheimer himself abandoned this approach within a decade, in part, because this kind of analysis becomes ambiguous when predictive properties of oculomotor system performance become prominent. We discuss the implications of ignoring the prominence of predictive eye movements and describe recent evidence for their prevelence and power. This leads us to propose that a new approach to the study of oculomotor performance is required. We also discuss the recent trend to apply the "connectionist" (or "neural network") approach in studies of the oculomotor system, and point out that the "symbolic", rather than the "adaptive", nature of predictive eye movements makes successful extension of these models to oculomotor performance unlikely. Our new approach emphasizes the use of natural stimulation in subjects free from bodily restraints. Accurate measurement of eye, head and torso movements under such conditions has become possible recently and data obtained in this manner has led to the discovery of a number of unexpected characteristics of oculomotor system performance. These developments have encouraged us to abandon the modular view of the oculomotor system, popular since Dodge launched the modern era of oculomotor research in 1903, which postulates five, or more, largely independent "subsystems". We suggest that only two subsystems (a fast saccadic and a somewhat slower smooth) are used to fixate and track a central representation of objects located in three-dimensional space. We show that this two-subsystem approach is consistent with current knowledge of oculomotor system neuroanatomy and neurophysiology.

Binocular vergence influences the assignment of stereo correspondences

A modified version of the double-nail illusion was used to measure the influence of binocular vergence on stereoscopic matching. The method is more sensitive than previous procedures and allows quantitative results for various stimulus markers. The results show that matches in the fixation plane are strongly preferred over matches with high disparities.

[Analysis of vergence functions by step stimulus]

Step responses of fusional vergence i.e. convergence and divergence were elicited by a computer-controlled phase-different haploscopic device. Eye movements of convergence and divergence were recorded by a limbus tracker. Elicited responses were analyzed using the repetitive linearization method of nonlinear curve fitting to exponential approximation. The results showed that the time constant of the exponential curve was identical between convergence and divergence. The destination of curves, i.e., the bias term of the exponential equation, always coincided with the stimulus position. These results were different from the behavior of accommodative step response which has been reported previously.