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PubMed1997

Tonic vergence and vergence adaptation.

In the absence of an adequate visual stimulus, the eyes are typically converged by approximately 0.25 to 0.75 meter angles (MA). This vergence response (VR) was believed to reflect the level of tonic innervation to the extraocular muscles, and accordingly has been termed tonic vergence (TV). However, this estimation fails to consider the magnitude of the anatomical position of rest. The true typical value of TV is approximately 23 degrees. This paper will consider various aspects of this parameter, including both clinical and laboratory methods of measurement, and the relationship between TV and the distance heterophoria. In addition, the role of vergence (or prism) adaptation, i.e., the apparent change in TV after periods of sustained fixation, is discussed. This shift appears to result from the relatively prolonged decay of the slow fusional vergence response (VR), with no evidence for a change in the level of tonic innervation. On occasion, the decay of slow fusional vergence may take hours or even days to reach completion. This extended rate of decay will have a significant impact upon the clinical measurement of a number of binocular parameters, most notably the assessment of heterophoria under truly dissociated conditions (i.e., in the absence of any fusional VR). Furthermore, both the magnitude and rate of decay of vergence adaptation appear to vary with age, as well as the presence of oculomotor imbalance. It is concluded that the output of the slow fusional vergence mechanism, as reflected by the degree of vergence adaptation, makes a major contribution to the aggregate, sustained VR in most visually normal patients.

Adaptation, Ocular↗
PubMed2005

Neuroanatomical correlates of voluntary inhibition of accommodation/vergence under monocular open-loop viewing conditions.

The purpose of this work is to identify human neural circuits involved in inhibition of accommodation/vergence by contrasting the cortical functions subservient to negative voluntary accommodation/vergence (NVA) with those evoked by active fixation in darkness (FIX). Five subjects with normal corrected acuity were studied using positron emission tomography and the HO bolus technique. The dominant right eye viewed a laser speckle pattern (633 nm) whose direction and velocity of motion were determined by the refractive state of the eye. The speckle pattern was presented at a distance of 1.8 m (0.55 D). The non-dominant eye was patched. Subjects performed two tasks counterbalanced for order effects: (i) attempted fixation on the remembered target in darkness with the dominant eye open and 'fixating'; and (ii) voluntary reduction of the laser speckle flow during each alternate 20-s epoch when a convex +2.0 D lens was placed in front of the right eye causing the speckle pattern to move downwards at 3 degrees /s. Comparison of the condition of NVA with the condition of FIX indicated widespread occipital activation. Decreases in absolute regional cerebral blood flow occurred in the superior parietal cortex (BA 5), frontal cortex (BA 8 and 10) and within the postcentral/precentral gyrus (BA 1/2/3/4) bilaterally where deactivation clusters eclipsed the presumed neck and shoulder areas. Negative accommodation/vergence appears to be driven by a reduction of parasympathetic tone, and has the effect of shutting down brain regions known to be involved in regulating visual search as well as a centrally controlled eye-head-neck-shoulder motor programme responsible for posturing gaze.

Accommodation, Ocular↗
PubMed1987

Flicker adaptation in the peripheral retina.

With strict fixation, a flickering light spot smaller than 3 deg presented to the peripheral retina will rapidly appear to lose contrast and stop flickering within 35 s, before fading away completely. The time required for this adaptation to occur decreases with: decreasing depth of modulation (97-9%); decreasing stimulus diameter (2 deg-7 min arc); increasing retinal eccentricity (20-50 deg); and increasing flicker frequency (1-7 Hz). Interestingly, the effect does not depend upon the regularity of the flickering stimulus, and it occurs twice as fast for stimuli presented to the temporal retina as for stimuli presented to the nasal retina. When changes in retinal eccentricity are compensated for by taking into account the cortical magnification factor, the time needed for perceived flicker to disappear remains constant at all eccentricities. With dichoptic stimulation interocular transfer is about 35%, suggesting a cortical contribution to flicker adaptation. The results indicate that the visual system adapts rather easily to peripheral flickering stimuli. Similarities as well as differences to motion adaptation are discussed.

Adaptation, Ocular↗
PubMed2005

Saccade-vergence interactions in macaques. I. Test of the omnipause Multiply Model.

Horizontal vergence eye movements are movements in opposite directions used to change fixation between far and near targets. The occurrence of a saccade during vergence causes vergence velocity to be transiently enhanced. The goal of this study was to test in the monkey the previously described Multiply Model (Zee et al. 1992) that holds that, in humans, the speeding of vergence during a saccade may be the result of the disinhibition of a subgroup of vergence-related neurons by the saccadic omnipause neurons (OPNs). In agreement with the Multiply Model: 1) the onset of the enhancement was closely related to saccadic onset, and thus linked to the onset of the OPN pause; 2) the magnitude of the vergence velocity enhancement was strongly dependent on saccade-vergence timing. Contrary to the Multiply Model: 1) the peak of the vergence velocity enhancement was dependent on saccadic peak velocity; 2) the dependency on saccadic peak velocity was not the indirect result of a dependency on saccadic duration and therefore on the duration of the OPN pause; 3) the decline of the vergence enhancement, identified by the time of the peak of the enhancement velocity, occurred too early to be linked to the end of the OPN pause; 4) vergence enhancement had a saccadic-like peak-velocity/size main sequence. Overall, the evidence is incompatible with the OPN Multiply hypothesis of vergence enhancement. Alternative models are described in an accompanying paper.

Animals↗
PubMed2005

Correlates of motor planning and postsaccadic fixation in the macaque monkey lateral geniculate nucleus.

There is significant controversy regarding the ability of the primate visual system to construct stable percepts from a never-ending stream of brief fixations and rapid saccadic eye movements. In this study, we examined the timing and occurrence of perisaccadic modulation of LGN single-unit activity in awake-behaving macaque monkeys while they made spontaneous saccades in the dark and made visually guided saccades to discrete stimuli located outside the receptive field. Our hypothesis was that the activity of LGN cells is modulated by efference copies of motor plans to produce saccadic eye movements and that this modulation depends neither on the presence of feedforward visual information nor on a corollary discharge of signals directing saccadic eye movements. On average, 25% of LGN cells demonstrated significant perisaccadic modulation. This modulation consisted of a moderate suppression of activity that began more than 100 ms prior to the initiation of a saccadic eye movement and continued beyond the termination of the saccadic eye movement. This suppression was followed by a large enhancement of activity after the eyes arrived at the next fixation. Although members of all three LGN relay cell classes (magnocellular, parvocellular, and koniocellular) demonstrated significant saccade-related suppression and enhancement of activity, more cells demonstrated postsaccadic enhancement (25%) than perisaccadic suppression (17%). In no case did the timing of the modulation coincide directly with saccade duration. The degree of modulation observed did not vary with LGN cell class, LGN receptive field center location, center sign (ON-center or OFF-center), or saccade latency or velocity. The time course of modulation did, however, vary with saccade size such that suppression was longer for longer saccades. The fact that activity from a percentage of LGN cells from all cell classes was modulated in relationship to saccadic eye movements in the absence of direct visual stimulation suggests that this modulation is a general phenomenon not tied to specific types of visual stimuli. Similarly, because the onset of the modulation preceded eye movements by more than 100 ms, it is likely that this modulation reflects higher order motor-planning rather than a corollary of mechanisms in direct control of eye movements themselves. Finally, the fact that the largest modulation is a postsaccadic enhancement of activity may suggest that perisaccadic modulations are designed more for the facilitation of visual information processing once the eyes land at a new location than for filtering unwanted visual stimuli.

Action Potentials↗
PubMed1998

Saccade-vergence trajectories under free- and instrument-space environments.

PURPOSE: The purpose of this study was to examine in detail the binocular fixation top-view trajectories of saccade-vergence responses to asymmetrical targets, and to compare latency difference between saccade and vergence, under the free- and instrument-space viewing environments. METHODS: Binocular eye movements were recorded using the infrared reflection technique in five visually-normal subjects. Responses were obtained for various asymmetrical target positions under both free- and instrument-space environments. RESULTS: Four types of top-view trajectories that represented normal variations in saccade and vergence control were found: straight, overshoot, undershoot, and saccade-vergence. Also, it was found that under the instrument-space environment, there was a predominance of saccade-vergence trajectories and a scarcity of overshoot trajectories, whereas under the free-space environment, there was a predominance of overshoot trajectories, and a scarcity of saccade-vergence trajectories. Further, under the instrument-space environment, latency was significantly longer for saccade than vergence (35.9 +/- 15.7 msec; t = 5.1, degrees of freedom (df) = 4, P < 0.01), whereas under the free-space environment, there was no latency difference (-10.5 +/- 14.8 msec; = -1.6, df = 4, P > 0.05). CONCLUSIONS: The differences in response profiles under the two viewing environments could be accounted for by differences in timing of saccade and vergence onset. Moreover, in contrast to some recent investigations that suggest higher center control of individual trajectories, which was dependent on the naturalistic scene, these trajectories could be accounted for by known neural and oculomotor mechanisms, with the higher centers using a priori information about spatial location of the target, to assist in the synchrony of saccade and vergence onset under the free-space environment.

Adult↗
PubMed2005

Influence of gap and overlap paradigms on saccade latencies and vergence eye movements in seven-year-old children.

The latency of eye movements is influenced by the fixation task; when the fixation stimulus is switched off before the target presentation (gap paradigm) the latency becomes short and express movements occur. In contrast, when the fixation stimulus remains on when the target appears (overlap paradigm), eye movement latency is longer. Several previous studies have shown increased rates of express saccades in children; however the presence of an express type of latency for vergence and combined movements in children has never been explored. The present study examines the effects of the gap and the overlap paradigms on horizontal saccades at far (150 cm) and at close (20 cm) viewing distances, on vergence along the median plane, and on saccades combined with convergence or divergence in 15 normal seven-year-old children. The results show that the gap paradigm produced shorter latency for all eye movements than the overlap paradigm, but the difference was only significant for saccades at close viewing distances, for divergence (pure and combined), and for saccades combined with vergence. The gap paradigm produced significantly higher rates of express latencies for saccades at close viewing distances, for divergence, and for saccades combined with divergence; in contrast, the frequencies of express latencies for saccades at far viewing distances and for convergence (pure or combined) were similar in the gap and the overlap paradigms. Interestingly, the rate of anticipatory latencies (<80 ms) was particularly high for divergence in the gap paradigm. Our collective findings suggest that the initiation of saccades at close viewing distances and of divergence is more reflexive, particularly in the gap paradigm. The finding of frequent anticipatory divergence that occurs at similar rates for seven-year-old children (this study) and for adults (Coubard et al., 2004, Exp Brain Res 154:368-381) indicates that predictive initiation of divergence is dominant.

Aging↗
PubMed1982

Ocular signs of cerebellar disease.

Ocular signs of cerebellar disease have been increasingly appreciated with the advent of means for quantitative recording of eye movements. The graphs in this article illustrate ocular flutter, dysmetria, abnormal (nonsmooth) pursuit, instability of fixation, faulty vestibular suppression, impaired optokinetic response, end-position nystagmus, and rebound nystagmus. The signs may be categorized as follows: (1) proprioceptive abnormalities manifest by flutter, dysmetria, and instability of gaze and (2) defects of vision-dependent functions manifest by abnormalities of pursuit, vestibular suppression, optokinetic response, and nystagmus.

Basal Ganglia↗
PubMed1981

Biofeedback of accommodation to reduce functional myopia.

Functional myopia may be defined as the refractive condition of the eye due to spasm of the ciliary muscle. As a result of the ciliary muscle spasm, the crystalline lens becomes more convex, creating a myopic refractive condition. The normal increase and decrease in the refractive power of the crystalline lens is know as accommodation and is controlled by the autonomic nervous system innervation to the ciliary muscle. Previous studies have reported that voluntary control of accommodation is possible by biofeedback training (Cornsweet & Crane, 1973; Randle, 1970). The present research investigated the application of biofeedback control of accommodation to reduce functional myopia. A double-reversal, multiple-baseline design was used to conduct the experiment. The results revealed that the three adult male subjects achieved the preset criterion, a 1/2-diopter reduction from initial baseline to a subsequent baseline. Further analysis of the data revealed even greater changes between initial baseline and feedback periods. Although generalization to a nonexperimental environment was not trained, each subject showed a reduction in myopia and an increase in visual acuity. The results of the experiment clearly demonstrated that functional myopia is subject to voluntary control.

Accommodation, Ocular↗
PubMed1989

Preoperative prism adaptation test in normosensoric strabismus.

In 19 patients with normosensoric esotropia, the squint angles measured with the alternate cover test were compared with those after prolonged prismatic correction of the squint angle and with those after prolonged occlusion of one eye. All patients showed an increase of the squint angle after prism adaptation. The angle was generally smaller after diagnostic occlusion of one eye than after prism adaptation. We assume that the increase in the squint angle after prism adaptation is caused by an anomalous sensorial relationship between the two eyes that was not detected with the usual psychophysical tests. Surgery tailored to the squint angle after prism adaptation seems advisable in patients with normosensoric esotropia.

Adaptation, Ocular↗
PubMed1984

Saccadic anomalies: vergence induces large departures from ball-and-socket behavior.

The configuration of muscular forces, which maintains a given orientation of the eye, varies with vergence state. As a consequence, changes in vergence produce both static and dynamic violations of simple ball-and-socket behavior: during strong convergence, the entire eye is displaced temporally within its orbit at steady state by as much as 200 microns; and the axis of ocular rotation for small horizontal saccades is consistently displaced forward within the globe by an average of about 1 mm. These phenomena occur regardless of whether vergence is maintained by accommodation or by binocular disparity. Hence, systematic errors of as much as a full degree can arise in measurement of vergence movement, unless monitoring methods are used which are insensitive to translational motion. The observed displacement on the axis of rotation for saccades may be involved in subjective shrinkage of visual targets during convergence ("experimental micropsia ").

Accommodation, Ocular↗
PubMed1990

The size of the pool for bleaching adaptation in human rod vision.

We present new psychophysical estimates of the size of the rod pool for bleaching adaptation in the human retina. We estimate that at 5 deg nasal eccentricity in the human retina the size of the adaptation pool for rods is between 5 and 7.5 min arc. This estimate is compatible with the extent of the dendritic spread of rod bipolars located in this region of the primate retina and with the area occupied by roughly 50 rods in this parafoveal region of the human retina. Thus a candidate for the site of adaptation is the bipolar cell whose receptive field is comprised of approx. 50 rods. These estimates represents the lowest measurements to date of the size of the adaptation pool for rods.

Accommodation, Ocular↗