Oculomotor Performance Research Articles

Ocular vergence under natural conditions. II. Gaze shifts between real targets differing in distance and direction

Horizontal binocular eye movements of three subjects were recorded with the scleral sensor coil--revolving magnetic field technique during voluntary shifts of gaze between pairs of stationary, real, continuously visible targets. The target pairs were located either along the median plane (requiring symmetrical vergence), or on either side of the median plane (requiring asymmetrical vergence). Symmetrical vergence was primarily smooth, but it was often assisted by small, disjunctive saccades. Peak vergence speeds were very high; they increased from about 50 degrees s-1 for vergence changes of 5 degrees to between 150 and 200 degrees s-1 for vergence changes of 34 degrees. Differences between convergence and divergence were idiosyncratic. Asymmetrical vergence, requiring a vergence of 11 degrees combined with a version of 45 degrees, was largely saccadic. Unequal saccades mediated virtually all (95%) of the vergence required in the divergent direction, whereas 75% of the vergence required in the convergent direction was mediated by unequal saccades, with the remaining convergence mediated by smooth vergence, following completion of the saccades. Peak divergence speeds during these saccades were very high (180 degrees s-1 for a change of vergence of 11 degrees); much faster than the smooth, symmetrical vergence change of comparable size (14 degrees). Peak convergent saccadic speeds were about 20% lower. This difference in peak speed was caused by an initial, transient divergence, observed at the beginning of all horizontal saccades. The waveform of disjunctive saccades did not have the same shape as the waveform of conjugate saccades of similar size. The smaller saccade of the disjunctive pair was stretched out in time so as to have the same duration as its larger, companion saccade. These results permitted the conclusion that the subsystems controlling saccades and vergence are not independent. Vergence responses were relatively slow and incomplete with monocular viewing, which excluded disparity as a cue. Monocularly stimulated vergence decreased as a function of the increasing presbyopia of our three subjects. Subjects were able to generate some vergence in darkness towards previously seen and remembered targets. Such responses, however, were slow, irregular and evanescent. In conclusion, vergence shifts between targets, which provided all natural cues to distance, were fast and accurate; they appeared adequate to provide effective binocular vision under natural conditions. This result could not have been expected on the basis of previous observations, all of which had been made with severely reduced cues to depth.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of background illumination and stimulant medication on smooth pursuit eye movements of hyperactive children.

Problems with visual information processing have been reported in children with the diagnosis of attention deficit disorder with hyperactivity (ADD H), and deficits in oculomotor control have been posited as an important factor in this phenomenon. To assess aspects of oculomotor performance, smooth pursuit eye movements (PEMs) were recorded electrooculographically in 20 ADD H and 20 age-matched control children and computer-analyzed for discrete (velocity arrests) and global (root mean square error) disruptions. The effects of stimulant medication (methylphenidate), together with manipulations designed to influence behavioral (attention) and physiological (cerebellar) processes involved in PEM performance, were examined. The tracking patterns of nonmedicated ADD H children contained significantly more discrete aberrations on baseline conditions. Although no single experimental manipulation significantly improved tracking performance in ADD H children, combining all experimental conditions did normalize PEMs in these subjects. Slight improvements in PEM performance in association with medication correlated positively with dose of medication and with medication-related improvements in behavior. In light of these data, arguments are presented in support of (a) hypoarousal as a contributing factor underlying oculomotor difficulties in ADD H children, and (b) subcortical involvement in PEM dysfunction.

Cerebellar involvement in the coordination control of the oculo-manual tracking system: effects of cerebellar dentate nucleus lesion.

When the hand of the observer is used as a visual target, oculomotor performance evaluated in terms of tracking accuracy, delay and maximal ocular velocity is higher than when the subject tracks a visual target presented on a screen. The coordination control exerted by the motor system of the arm on the oculomotor system has two sources: the transfer of kinaesthetic information originating in the arm which increases the mutual coupling between the arm and the eyes and information from the arm movement efferent copy which synchronizes the motor activities of both subsystems (Gauthier et al. 1988; Gauthier and Mussa-Ivaldi 1988). We investigated the involvement of the cerebellum in coordination control during a visuo-oculo-manual tracking task. Experiments were conducted on baboons trained to track visual targets with the eyes and/or the hand. The role of the cerebellum was determined by comparing tracking performance defined in terms of delay, accuracy (position or velocity tracking errors) and maximal velocity, before and after lesioning the cerebellar dentate nucleus. Results showed that in the intact animal, ocular tracking was more saccadic when the monkey followed an external target than when it moved the target with its hand. After lesioning, eye-alone tracking of a visual target as well as eye-and-hand-tracking with the hand contralateral to the lesion was little if at all affected. Conversely, ocular tracking of the hand ipsilateral to the lesion side became more saccadic and the correlation between eye and hand movement decreased considerably while the delay between target and eyes increased. In normal animals, the delay between the eyes and the hand was close to zero, and maximal smooth pursuit velocity was around 100 degrees per second with close to unity gain; in eye-alone tracking the delay and maximal smooth pursuit velocity were 200 ms and 50 deg per second, respectively. After lesioning, delay and maximum velocity were respectively around 210 ms and 40 deg per second, that is close to the values measured in eye-alone tracking. Thus, after dentate lesioning, the oculomotor system was unable to use information from the motor system of the arm to enhance its performance. We conclude that the cerebellum is involved in the "coordination control" between the oculomotor and manual motor systems in visuo-oculo-manual tracking tasks.

The need for an eclectic, rather than systems, approach to the study of the primate oculomotor system

The failure of the Systems Approach to add to our understanding of the primate oculomotor system is traced out from the introduction of this approach 30 years ago to recent developments which open up the possibility of studying, separately, stimulus and psychological inputs to the smooth pursuit subsystem. Prior failure to have a valid technique for separating complex central factors from underlying reflex mechanisms compromises current understanding of oculomotor performance and its putative neurological substrate.

Eye movements of human albinos.

Albino mammals are known to suffer from misrouted optic projections and there is a growing body of evidence suggesting that human albinos have similar aberrant anatomical pathways. The present study examined the possible consequences of such aberrant pathways on the oculomotor performance of five adult human albinos. Optokinetic nystagmus to drifting grating patterns and pursuit eye movements were measured. The subjects' congenital nystagmus was also measured under different conditions of gaze position and ambient room illumination. Two of the subjects showed clear instances of an inversion in the optokinetic response and there were probable inversions observed for a third subject. The magnitude of the optokinetic nystagmus was appropriate for the rate of pattern drift, but inverted in direction. In all cases smooth pursuit was severely impaired, but reversals of the appropriate direction of pursuit eye movements were not observed. Changes in the congenital nystagmus under conditions of light and darkness were found for four of the five subjects and varied greatly between subjects. The results suggest that human albinos share many of the oculomotor deficits found in other albino species.

Compensation of oculomotor deficits in monkeys with neonatal cerebellar ablations.

The oculomotor performance of 11 monkeys, who had various degrees of cerebellar ablation shortly after birth, is described in this study. Detailed numerical results were obtained on three of these adult macaques after extensive training of specific pursuit eye movements and fixation. The vestibular nuclei were kept intact. In the presence of the intracerebellar nuclei, quite extensive neonatal ablations of the cerebellar cortex leave the adult monkey without any discernible oculomotor deficits. If ablation also includes the nuclei on one side, compensation is never complete even several years later: while vestibulo-ocular and saccadic responses seem normal, there are deficits in pursuit and gaze holding performance. The residual deficits vary with the extent of the ablation and are comparable with the pattern exhibited by acute hemicerebellar ablations. Monkeys who had their cerebellum including the nuclei essentially completely removed just after birth, could pursue, albeit with limited velocity, and hold gaze, albeit in a limited zone. Their vestibulo-ocular responses seemed unaffected.

Intra-individual variation in oculomotor performance in man.

Peak velocity of saccade, maximum velocity of smooth pursuit, and peak velocity of slow phase of vestibular and optovestibular nystagmus were measured three times daily on three separate days in 6 healthy subjects in order to estimate the intra-individual variation of the results of oculomotor tests. Analysis of variance revealed that the oculomotor performance of the individual subjects varied systematically, depending on whether the tests were performed in the morning, at noon, or late in the afternoon. In particular, the velocity of saccade displayed lowest values in the afternoon. The present results indicate that oculomotor performance may depend on the individual biological rhythm of fatigue; however, it is also possible that eye motor behaviour is coupled to an independent rhythm of motor activity. Circadian rhythmicity of oculomotor performance should be considered in follow-up studies and in examination of patients with reduced vigilance.

Oculomotor disorders in cortical lesions in man: The role of unilateral neglect

The saccades of 22 patients with lesions of the left (6) and right (16) hemisphere were analysed. Seven patients with a right hemispheric lesion presented a left unilateral neglect (UN) syndrome. In the first test session the saccades were performed in response to a predicted sequence of targets; in the second session the luminous stimuli were unpredicted and randomized. In patients with UN we observed the absence of saccadic response to 25% of the targets, a lengthening of the reaction time and a staircase pattern in the left hemifield. In the predicted stimulus sequence the oculomotor performance was not significantly better. Hemianopia, a defect of arousal and an altered visuospatial orientation are the most important elements that account for the alterations of oculomotor strategy in the UN syndrome.

Saccadic eye movements to peripherally discriminated visual targets.

Two experiments required subjects to identify a peripheral target embedded among nontarget stimuli and fixate it as quickly as possible with a single saccadic eye movement. Experiment 1 varied both the target distance and its angular position between trials; the mean oculomotor latency, the proportion of erroneous movements, and the proportion of (correct) movements followed by a corrective saccade all increased as a function of target distance. Experiment 2 held target distance constant (12.7 degrees) and used verbal instructions to manipulate the speed and accuracy of the subject's oculomotor performance between conditions. The speed/accuracy trade-off was similar for all subjects. The reduced uncertainty about target distance in Experiment 2 made each subject's oculomotor performance more efficient. Error trials not only included apparent perceptual errors (initial movements to nontarget stimuli) but also motor errors - that is, instances when the initial erroneous movement was followed, with an extremely short latency, by a large saccade to the target. The characteristics of these motor errors suggest that the saccade is not planned in terms of its amplitude and direction in retinal coordinates.

Eye movements, scanpaths, and dyslexia.

The oculomotor performance of 25 dyslexic and 19 normal children was evaluated to determine whether or not dyslexia involves deficits in oculomotor function or visual perception. When the children were required to follow a meaningless target or to solve pictorial tasks, the two age-matched groups could not be differentiated. When they were required to read selections commensurate with their measured reading level, the dyslexic children were markedly different on a number of parameters. It appears that the dyslexic's difficulty lies beyond visual perception, perhaps in the language area itself. The dyslexic's characteristic deficit seems to involve the integration of visual input into the language-acquisition function.

The effect of luminance on human smooth pursuit of perifoveal and foveal targets

Two subjects tracked targets located in the perifovea (6° below the line of sight) while oculomotor performance was recorded with an infrared contact lens optical lever. A point target was either 1.85 log units above absolute foveal threshold (photopic) or 0.5 log units above absolute scotopic threshold. Transfer characteristics of smooth pursuit of sinusoidal motion (amplitude = 2.18°. frequency = 0.13–5.00 Hz) were measured. It was found that (1) eccentric smooth pursuit is easily accomplished with no practice. (2) smooth pursuit characteristics were largely uninfluenced by target luminance, (3) the phase of smooth pursuit showed greater lag with the scotopic target than with photopic target, and (4) predictive tracking occurs in the perifoveal retina. Similar results were obtained in the fovea where overall smooth pursuit gain was higher but, once again, large variations in luminance did not have systematic or large effects on smooth pursuit gain.

Hand-eye coordination: The role of “motor memory”

Ten Ss attempted to track a visual target, which was moved either by hand (test run) or by a tape recording of the same hand movements (control run). On some test runs, movement of the target lagged behind that of the hand by a fixed amount (.18, .36, .72, or 1.44 sec). The number of saccades occurring on a given test run was divided by the number on the matched control run, and the test/control ratio was studied as a function of the hand-to-target lag time. With lag times of .36 sec or less, the test/control ratios were less than 1. indicating that manual control of the target had a significant effect on oculomotor performance. This effect disappeared at a lag close to .72 sec. The results are discussed in terms of a hypothetical “motor memory.”

Akinesia, hypokinesia, and bradykinesia in the oculomotor system of patients with Parkinson's disease

Detailed characteristics of voluntary saccadic eye movement were measured in 14 Parkinsonian patients and 12 age-matched controls by d-c electrooculography (5% attenuation at 450 Hz). Subjects performed rapid, alternating gaze shifts between two fixed visual targets separated horizontally by 25° of arc. On average, the patients took about twice as long as the controls ( p < .001) to complete a cycle of the alternating task. Since the dynamic characteristics of the patients' saccadic movements were normal for the amplitude achieved, the fault was not due to a reduced ability to make fast, coordinated, muscle movements. The slowed performance was found to be due to two main factors: an increased target fixation time manifest as delayed initiation of voluntary movement (akinesia); and increased transit time between targets (bradykinesia) due to the normal delays of about 200 msec between corrective saccades which were necessitated by an abnormal ( p < .001) tendency to undershoot the target (hypokinesia). The marked akinesia found in this investigation of voluntary eye movements contrasts strongly with the normal latency of saccadic response to sudden target movement found in the same patients during a previous investigation. The present findings support the previously drawn conclusion that peripheral neuromuscular components of the oculomotor system are unimpeded by the disease. Comparison with observations of other authors suggests that the impairment of oculomotor performance described here may be simlar to that found in skeletal muscle systems during Parkinson's disease.

Quantitative criteria of oculomotor performance and fatigue.

Quantitative criteria of oculomotor performance and fatigue.

X. - Diurnal variation in oculomotor performance

Summary 1. In 18 subjects kept awake for more than 24 hours, on one or more occasions, there was a diurnal variation in oculomotor performance, as judged by the blinking rate, binocular fixation, and lateral oscillations of the eyes. 2. Poorest performance occurred in the early hours of the morning, coinciding with the development of drowsiness. 3. Without intervening sleep, there was a spontaneous recovery of oculomotor performance, along with an increase in alertness, later in the morning or afternoon. 4. An intermediate degree of oculomotor impairment manifested itself in poorer oscillations in post-fixation, as compared to pre-fixation, trials, and in the appearance of diplopia toward the end of the five-minute fixation period. 5. In the execution of lateral sweeps, the templeward moving eye completed its excursion faster than did the nasalward moving eye. This disparity also held in drowsiness, when the sweep was slowed in both eyes. 6. Amphetamine had no effect on oculomotor performance of subjects when the subjects were wide-awake,but ledto improvement of impaired performance observed during drowsiness. 7. Small amounts of alcohol often imitated the impairment of oculomotor performance that occurred in drowsiness.