Proprioceptive and retinal afference modify postsaccadic ocular drift.

Drift of the eyes after saccades produces motion of images on the retina (retinal slip) that degrades visual acuity. In this study, we examined the contributions of proprioceptive and retinal afference to the suppression of postsaccadic drift induced by a unilateral ocular muscle paresis. Eye movements were recorded in three rhesus monkeys with a unilateral weakness of one vertical extraocular muscle before and after proprioceptive deafferentation of the paretic eye. Postsaccadic drift was examined in four visual states: monocular viewing with the normal eye (4-wk period); binocular viewing (2-wk period); binocular viewing with a disparity-reducing prism (2-wk period); and monocular viewing with the paretic eye (2-wk period). The muscle paresis produced vertical postsaccadic drift in the paretic eye, and this drift was suppressed in the binocular viewing condition even when the animals could not fuse. When the animals viewed binocularly with a disparity-reducing prism, the drift in the paretic eye was suppressed in two monkeys (with superior oblique pareses) but generally was enhanced in one animal (with a tenotomy of the inferior rectus). When drift movements were enhanced, they reduced the retinal disparity that was present at the end of the saccade. In the paretic-eye-viewing condition, postsaccadic drift was suppressed in the paretic eye and was induced in the normal eye. After deafferentation in the normal-eye-viewing state, there was a change in the vertical postsaccadic drift of the paretic eye. This change in drift was idiosyncratic and variably affected the amplitude and velocity of the postsaccadic drift movements of the paretic eye. Deafferentation of the paretic eye did not affect the postsaccadic drift of the normal eye nor did it impair visually mediated adaptation of postsaccadic drift. The results demonstrate several new findings concerning the roles of visual and proprioceptive afference in the control of postsaccadic drift: disconjugate adaptation of postsaccadic drift does not require binocular fusion; slow, postsaccadic drift movements that reduce retinal disparity but concurrently increase retinal slip can be induced in the binocular viewing state; postsaccadic drift is modified by proprioception from the extraocular muscles, but these modifications do not serve to minimize retinal slip or to correct errors in saccade amplitude; and visually mediated adaptation of postsaccadic drift does not require proprioceptive afference from the paretic eye.

Relationships among visual acuity demands, convergence, and nystagmus in patients with manifest/latent nystagmus.

BACKGROUND: We investigated the role convergence plays in nystagmus dampening, in particular, relationships among visual acuity demands, convergence, and nystagmus. Previously we showed that subjects with idiopathic infantile nystagmus exhibit a range of responses to acuity targets, one of which is nystagmus blockage syndrome. We report herein eye movement responses to acuity targets of patients with manifest/latent nystagmus. METHODS: Fourteen patients, 11 with latent or manifest latent nystagmus and 3 with combined manifest latent with infantile nystagmus, were asked to indicate the direction of the gap in Landolt C optotypes while their eye movements were recorded. RESULTS: The tested patients exhibited various responses to acuity demands: (1) dampening of nystagmus with convergence (i.e., nystagmus blockage syndrome) (5/14 patients), (2) changes in vergence without nystagmus dampening (2 patients), (3) decrease of nystagmus without convergence (2 patients), and (4) little change in nystagmus or vergence (5 patients). In nystagmus blockage syndrome the amount of convergence increased with acuity demands in two of five patients and the convergence duration in four of five patients; nystagmus dampening increased with acuity demands in one of five patients and the blockage duration in four of five patients. CONCLUSIONS: Many, but not all, patients with manifest/latent nystagmus, similar to those with infantile nystagmus, used convergence to dampen their nystagmus. The convergence response tended to increase with acuity demands, but the amount of dampening was idiosyncratic and not predictably related to the measured convergence across patients.

Congenital periodic alternating nystagmus. Diagnosis and Management.

PURPOSE: The purpose of the study is to investigate diagnostic criteria and treatment methods for patients with congenital periodic alternating nystagmus (PAN). METHODS: A retrospective analysis was performed of clinical findings and serial eye movement recordings of patients with congenital PAN. Eighteen patients observed from 1983 through 1996 and diagnosed with congenital PAN are included. Five of these have ocular or oculocutaneous albinism. Nine of the 18 patients were treated. Three had Kestenbaum operations before referral to the authors, one was treated with baclofen, and five had large recessions of the four horizontal recti. The studied parameters included visual acuity (VA) and abnormal head posture (AHP); temporal aspects of PAN cycle, nystagmus waveforms, frequency, amplitude, and velocity; as well as mean foveation fraction, a mean percentage of the nystagmus cycle spent at retinal slip velocities less than 10 degrees per second. RESULTS: The authors diagnosed PAN in 9% of patients with congenital nystagmus, although most had not been diagnosed with PAN before referral, despite changing nystagmus. Sixteen patients had AHP, typically shifting. The PAN cycle was of variable duration, often with asymmetric right- and left-beating components. Although horizontal jerk nystagmus with accelerating slow phase was predominant, other waveforms were encountered in the active phase of PAN. In the quiet phase (close to null zone), similar, but less intense, oscillations than those in the active phase were characteristic. Half of the patients showed a combination of waveforms in both phases. Baclofen treatment was unsuccessful. Patients who had Kestenbaum procedures remained with AHP in the original or opposite direction, without change in nystagmus or VA. Large recessions of four horizontal recti proved uncomplicated. This treatment improved, at least for several years, AHP and VA and caused favorable changes in nystagmus parameters in all patients. Mean foveation fractions increased significantly after surgery. CONCLUSIONS: Congenital PAN often is underdiagnosed. Differing waveforms may indicate PAN. Evaluation of nystagmus, especially before surgery, for at least 3 minutes, preferably with eye movement recordings, is necessary to diagnose PAN and perhaps prevent Kestenbaum procedures, which seem inappropriate. Large horizontal recti recessions seem to provide safe and promising treatment.

Initial pursuit responses using a fixation-gapramp paradigm.

Pursuit eye movements keep the image of a small moving target near the fovea with an admixture of smooth pursuit and saccades. To probe the initiation of pursuit, four humans and one monkey attended a bright laser spot that was rear-projected on a diffusely illuminated (1 cd/m(2)) tangent screen. Movement of the viewing eye was recorded with a magnetic search coil. After presenting a stationary target for a random time (1-3 sec), the computer extinguished the target in separate trial blocks for 40, 100, 200, or 500 ms. At the end of this gap period, the target reappeared on the fovea, moving centrifugally at 10°/sec in a random direction before decelerating to a stop. The predominate initial response (< 90%) to unpredictable changes in target motions had four sequential components: a latent period, a primary smooth movement, a small saccade, and then a secondary smooth movement. None of five measured variables depended systematically on the gap duration: the latency to primary smooth pursuit (range of means 144-217 ms), average eye velocity in the first 50 msec of primary smooth pursuit (0.7-3.6°/s), latency to the first saccade (207-319 ms), amplitude of the first saccade (1.6-2.7°), or average eye velocity in the first 100 ms of the secondary smooth pursuit (7.4-11-4°/s). Our data do not support the hypothesis that a separate fixation system has to be disengaged before smooth pursuit can begin, as has been suggested for the saccadic system.

Dissociated vertical deviation (DVD): The saccadic and slow eye movements.

Dissociated vertical deviation (DVD), a vertical strabismus, is often associated with infantile esotropia, latent nystagmus and excyclo-torsion. DVD usually becomes apparent at about three years of age. The authors wished to determine whether DVD is a manifestation of an abnormal motor vergence system or is part of a visual system disorder. Vertical eye movements of six subjects with DVD, on cover-uncover and alternate cover tests, were studied using the magnetic search eye coil system. Asymmetry between the eyes' saccades were quantified during the DVD upward movement and during a trial of pure vertical saccades. In three subjects the vertical deviation increased to its full amount through a smooth vertical divergence movement, and in the other three patients through a combination of smooth and asymmetrical saccade-like movements. The latencies of the DVD, the peak velocity of the deviation movements, its time constant and the asymmetries in the saccades suggest that DVD represents an abnormal vertical vergence system.

Visual remapping in infantile nystagmus.

The possibility that patients with idiopathic infantile nystagmus achieve spatial constancy by visual remapping was investigated by comparing subjective localization of flashed test targets to their absolute position in space and to their absolute position on the retina. Nystagmats first viewed a screen-stationary reference target that was followed by a test flash. A computer used eye movement feedback to precisely control the test flash position on the retina. All six nystagmats detected test flashes throughout their nystagmus cycle. For three nystagmats test flashes (total N = 48) were delivered to the same retinal locus that were, at different times, to the right and left of the reference target. More than two-thirds of such crossover stimuli were correctly located in space: when only those stimuli at least 0.5 deg from the reference were considered, two of three subjects correctly located all stimuli. Taken together these results argue that our subjects could see throughout the nystagmus cycle and shifted their visual map in synchrony with their nystagmus as an explicit means of avoiding oscillopsia.

Absence of visual sampling in infantile nystagmus.

The possibility that patients with infantile nystagmus achieve spatial constancy by sampling the visual scene only during certain range of velocities or certain phases of their nystagmus cycle was investigated by asking patients to detect a flashed test target that was presented repeatedly during all phases of the nystagmus cycle. After observing a 543 nm fixation spot projected on a diffusely illuminated tangent screen 1 m in front of the eye, patients were asked to detect a 2 msec test flash of the spot, now locked to the retina, that occurred 200 msec after the fixation spot was extinguished. The test target appeared randomly at 3,6,9, or 12 o'clock at, in separate trials, 0.8 deg or 10.0 deg from central vision. To avoid forcing patients from guessing a direction cued by the disappearance of the fixation spot, sometimes the flash did not occur at all. To avoid dark adaptation in our patients, the diffuse background illumination was adjusted to the brightest level allowing easy detection of the test flash. In every condition, all patients reported the fixation spot stationary. The probability of detection was the same across all velocity ranges. Remarkably, patients were just as likely to detect the test flash when the eye was nearly stationary even when it was moving more than 100 deg/sec. In one patient, the background illumination was raised so that he began missing the test flash more often. Here too, the probability of detection, now reduced to about 50%, was the same across velocity ranges.(ABSTRACT TRUNCATED AT 250 WORDS)

Hysteresis and slow drift in abducens unit activity.

Two trained monkeys made saccadic eye movements to a small visual target. The activity of 39 isolated abducens units, presumed to be motoneurons or abducens internuclear neurons, was recorded in relation to these eye movements. After a calibration trial, a test trial repeatedly elicited 20 degrees horizontal saccades to primary position from either the left or right. On average, the steady-state firing rate at primary position depended on the direction of the saccade. For saccades where the neuron showed a burst in activity during the saccade (on-saccades) the steady-state firing rates were usually higher than for those saccades that showed a pause in activity during the saccade (off-saccades). For the population of units this hysteresis measured 5.4 spikes/s, which may be compared with an average primary-position rate of 97 spikes/s. The average hysteresis for individual units ranged from -2.1 to 18.5 spikes/s. The steady-state firing rate after equal saccades in the same direction and ending at the same position (primary) varied slowly over time. Across all units the variability (standard deviation) ranged from 0.5 to 11.8 spikes/s with a mean of 4.7 spikes/s. Furthermore, for any one unit the variations following on-saccades generally correlated with the variations following the off-saccades. Hysteresis, doubted by many, does exist. Fortunately, it is small enough, 5.5% of typical primary-position rate, that it can be neglected for many purposes. Nevertheless, it poses the interesting theoretical question of how the oculomotor system compensates for hysteresis. The simplest explanation of slow variations in background rate is cocontractive noise: a slow fluctuation in all abducens neurons so that these variations do not result in fluctuations of eye position.