A hypothetical explanation of saccadic oscillations

Abnormal supranuclear eye movements in the child: a practical guide to examination and interpretation

Abnormal eye movements in the infant or voting child can be congenital or acquired. They may be a result of abnormal early visual development or a sign of underlying neurologic or neuromuscular disease. It is important to be able to detect these abnormalities and to distinguish them from normal but immature eye movements. The spectrum of disease in children differs from that in adults. Serious, potentially fatal but treatable disorders can be acquired in infancy, and abnormal eye movements in a sometimes apparently well child should never be labeled as congenital or benign without careful investigation. Eye movement analysis can indicate the presence of an underlying condition and help the clinician to classify different neurologic diseases. It is important to carefully examine the ocular motor system in any children at risk of neurologic disease. This review provides a practical guide to the examination and interpretation of eye movements in the child and includes recent literature on eye movement disorders of childhood. We describe supranuclear abnormalities of the ocular motor system in the order in which we would normally examine it: saccades, pursuit, convergence, vestibulo-ocular reflex, and optokinetic nystagmus. Nystagmus, internuclear ophthalmoplegia, cranial nerve abnormalities, and "miswiring" phenomena (such as Duane's syndrome and synergistic divergence) are not discussed.

Properties of horizontal saccades accompanied by blinks

Using the magnetic search coil technique to record eye and lid movements, we investigated the effect of voluntary blinks on horizontal saccades in five normal human subjects. The main goal of the study was to determine whether changes in the dynamics of saccades with blinks could be accounted for by a superposition of the eye movements induced by blinks as subjects fixated a stationary target and saccadic movements made without a blink. First, subjects made voluntary blinks as they fixed on stationary targets located straight ahead or 20 degrees to the right or left. They then made saccades between two continuously visible targets 20 or 40 degrees apart, while either attempting not to blink, or voluntarily blinking, with each saccade. During fixation of a target located straight ahead, blinks induced brief downward and nasalward deflections of eye position. When subjects looked at targets located at right or left 20 degrees, similar initial movements were made by four of the subjects, but the amplitude of the adducted eye was reduced by 65% and was followed by a larger temporalward movement. Blinks caused substantial changes in the dynamic properties of saccades. For 20 degrees saccades made with blinks, peak velocity and peak acceleration were decreased by approximately 20% in all subjects compared with saccades made without blinks. Blinks caused the duration of 20 degrees saccades to increase, on average, by 36%. On the other hand, blinks had only small effects on the gain of saccades. Blinks had little influence on the relative velocities of centrifugal versus centripetal saccades, and abducting versus adducting saccades. Three of five subjects showed a significantly increased incidence of dynamic overshoot in saccades accompanied by blinks, especially for 20 degrees movements. Taken with other evidence, this finding suggests that saccadic omnipause neurons are inhibited by blinks, which have longer duration than the saccades that company them. In conclusion, the changes in dynamic properties of saccades brought about by blinks cannot be accounted for simply by a summation of gaze perturbations produced by blinks during fixation and saccadic eye movements made without blinks. Our findings, especially the appearance of dynamic overshoots, suggest that blinks affect the central programming of saccades. These effects of blinks need to be taken into account during studies of the dynamic properties of saccades.

Reversible inactivation of monkey superior colliculus. II. Maps of saccadic deficits

Neurons in the superior colliculus (SC) are organized as maps of visual and motor space. The companion paper showed that muscimol injections into intermediate layers of the SC alter the trajectory of the movement and confirmed previously reported effects on latency, amplitude, and speed of saccades. In this paper we analyze the pattern of these deficits across the visual field by systematically comparing the magnitude of each deficit throughout a grid of targets covering a large fraction of the visual field. We also translate these deficits onto the SC map of the visual/movement fields to obtain a qualitative estimate of the extent of the deficit in the SC. We found a consistent pattern of substantially increased saccadic latency to targets in the contralateral visual hemifield, accompanied by slight and inconsistent increases and decreases for saccades to the ipsilateral hemifield. The initial and peak speed of saccades was reduced after the injection. The postinjection amplitude of the saccades were either hypometric or normometric, but rarely hypermetric. Although errors in the initial direction of the postinjection saccades were small, they consistently formed a simple pattern: an initial direction with minimal errors (a null direction) separating regions with clockwise and counterclockwise rotations of the initial direction. However, the null direction did not go through the center of the inactivated zone, as would be expected if the SC alone were determining saccade direction, e.g., with a population code. One hypothesis that can explain the misalignment of the null direction with the lesion site is that another system, acting in parallel with the SC, contributes to the determination of saccadic trajectory.

Initial component control in disparity vergence: a model-based study

The dual-mode theory for the control of disparity-vergence eye movements states that two components control the response to a step change in disparity. The initial component uses a motor preprogram to drive the eyes to an approximate final position. This initial component is followed by activation of a late component operating under visual feedback control that reduces residual disparity to within fusional limits. A quantitative model based on a pulse-step controller, similar to that postulated for saccadic eye movements, has been developed to represent the initial component. This model, an adaptation of one developed by Zee et al. [1], provides accurate simulations of isolated initial component movements and is compatible with the known underlying neurophysiology and existing neurophysiological data. The model has been employed to investigate the difference in dynamics between convergent and divergent movements. Results indicate that the pulse-control component active in convergence is reduced or absent from the control signals of divergence movements. This suggests somewhat different control structures of convergence versus divergence, and is consistent with other directional asymmetries seen in horizontal vergence.

Transforming sensory perceptions into motor commands: evidence from programming of eye movements

The visual stimulus for a saccadic eye movement is encoded in place-coded maps in cerebral cortex and the dorsal superior colliculus. In contrast, the motor command for the saccade is encoded by the temporal discharge properties of ocular motoneurons and premotor burst neurons in the brain-stem reticular formation. Thus, there is need for a spatial-temporal transformation of neural signals, and recent findings suggest that the superior colliculus might contribute to this process. The ventral, output layers of the superior colliculus encode the metric of the desired saccade in polar coordinates. However, premotor neurons in the pontine and mesencephalic reticular formation are organized to generate horizontal and vertical saccades, respectively. Studies of oblique saccades in patients with slow vertical components--due to Niemann-Pick type C disease--support the interpretation that the saccadic command from the reticular formation is encoded in Cartesian coordinates. Currently, saccades are thought to be generated under local, brain-stem feedback control in which current eye displacement is continuously subtracted from desired eye displacement to compute motor error--the remaining movement required for the eye to acquire the target. If the superior colliculus is positioned in the feedback loop, then there is a need for transformation of premotor signals back into a place-coded version of motor error. Recent studies suggest that, during the saccade, this might be achieved by a wave of activity spreading rostrally, which traverses the collicular map in a direction corresponding to progressively smaller movements and finally activates a group of neurons concerned with fixation. These new hypotheses are ripe for testing by basic and clinical studies. By confronting the issue of what signal transformations are required to program visually guided saccades, new experimental approaches have emerged. Such computational approaches offer insights into how the brain controls behavior not just by measuring stimulus and response, but by asking what "currency" is being used by interacting populations of neurons at any stage in the process.

Evidence for independent feedback control of horizontal and vertical saccades from Niemann-Pick type C disease

We measured the eye movements of three sisters with Niemann-Pick type C disease who had a selective defect of vertical saccades, which were slow and hypometric. Horizontal saccades, and horizontal and vertical pursuit and vestibular eye movements were similar to control subjects. The initial movement of oblique saccades was mainly horizontal and most of the vertical component occurred after the horizontal component ended; this resulted in strongly curved trajectories. After completion of the horizontal component of an oblique saccade, the eyes oscillated horizontally at 10-20 Hz until the vertical component ended. These findings are best explained by models that incorporate separate feedback loops for horizontal and vertical burst neurons, and in which the disease selectively affects vertical burst neurons.

Electrophysiological observations in idiopathic opsoclonus-myoclonus syndrome

To supplement existing knowledge regarding the pathophysiology of the opsoclonus-myoclonus syndrome, electrophysiological findings are reported in three patients with idiopathic opsoclonus-myoclonus. Surface electromyography (EMG) revealed < 100-ms synchronous discharges correlating with the clinical myoclonus. Short duration EMG discharges, with no back-averaged cortical correlate, normal gross electroencephalogram, and no exaggerated responses with either evoked potential testing or long latency EMG responses were observed. The clinical and electrophysiological findings we describe are consistent with a brainstem origin of the myoclonus in this syndrome, with concurrent abnormalities in cerebellar circuits, similar to those described for paraneoplastic cases. The constellation of these electrophysiological findings differentiates the myoclonus in these patients from other clinical myoclonic entities.

Dysfunction of pontine omnipause neurons causes impaired fixation: macrosaccadic oscillations with a unilateral pontine lesion

Macrosaccadic oscillations of eyes (MSO) are regarded as a form of saccadic dysmetria secondary to cerebellar dysfunction. They are usually conjugate, horizontal, and symmetric in both directions of gaze. Using magnetic search coils, we studied a patient with MSO that developed five years following head injury and involved synchronously horizontal, vertical, and torsional planes. The MSO were characterized by directional pre-ponderance and were associated with ipsilateral pontine lesion. We propose a disturbance of fixation mechanisms due to unilateral disinhibition of saccadic burst neurons in three planes. This could arise from either primary or secondary dysfunction of omnipause neurons due to impaired input from the contralateral superior colliculus. The delayed onset is suggestive of denervation supersensitivity as the underlying pathophysiology.

Neurological sequelae of the dancing eye syndrome

Fifty-four patients with dancing eye syndrome (DES), presenting over a 25-year period, were reviewed retrospectively. One third of them were on active follow up at the time of the study. Malignancy was uncommon, diagnosed in only 4 patients, neuroblastoma in 3 cases and acute lymphoblastic leukaemia (ALL) in 1. An intercurrent illness preceded onset of DES in 51% of the children and was of equivocal significance. There was no clear temporal relationship to immunization. The acute phase of the illness was severe and caused total immobilization in 88% of patients. A favourable initial response to treatment with prednisolone or corticotrophin gel was observed in all patients. Although corticotrophin seemed to produce a more rapid response, overall improvement was similar with both treatments. Long-term neurological morbidity was a major problem with 91% of children suffering either persisting symptoms or repeated relapses. A persisting disability was found in 88% and was considered severe in 61% of patients. No features prognostic of neurological severity or outcome were identified.

CONCLUSION

Demonstrable malignancy is uncommon in the dancing eye syndrome. The neurological legacy of DES is often evident well into adult life.

The movement disorder of adult opsoclonus

We present three cases of the adult opsoclonus-myoclonus syndrome in patients with systemic carcinoma. In addition to opsoclonus and myoclonus, other clinical components of the syndrome can include ataxia, tremor, gait and stance dysfunction, altered mental status, and head and face dyskinesias. The most common etiologies are idiopathic, paraneoplastic, and infectious encephalitis. Radiographic and pathological studies suggest brain-stem dysfunction with associated cerebellar and/or cerebellar pathway dysfunction. In many cases, there is evidence for the involvement of immunologic and/or inflammatory processes in the pathogenesis of this syndrome. The timely recognition of this syndrome is important because of its implications for the underlying etiology and prognosis. The appearance of this syndrome should prompt the search for an occult malignancy.

A pathophysiological approach to saccadic eye movements in neurological and psychiatric disease

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Transient opsoclonus in organophosphate poisoning

Opsoclonus transiently occurred in the acute stage of combined parathion and methyl-parathion poisoning. Despite persistent cholinesterase inhibition this symptom subsided spontaneously. This observation provides strong evidence for cholinergic pathways being reversibly involved in isolated opsoclonus.

Some characteristics of saccadic eye movements in children of primary school age

The characteristics of saccadic eye movements have been extensively studied in adults; researches have also been devoted to the saccades of preschool age children. On the contrary, for primary school-age children no data exist; we investigate the eye movements (recorded utilizing an infrared technique) of six children 7 to 11 years old. The main results indicate that the values of some parameters (for example the saccadic latency and duration) are in the same range as the values of the correspondent parameters in adults, while the values of other parameters (in particular peak velocity and mean velocity/peak velocity ratio) are distinctly different from the ones measured in adult subjects.

Opsoclonus in a patient with Guillain-Barré syndrome

A 54 year-old woman showed the unusual association of a Guillain-Barré syndrome and opsoclonus, a rare eye movement characterized by involuntary bursts of jerking, ataxic and multidirectional saccades, without an intersaccadic interval. Since opsoclonus is a phenomenon described exclusively in CNS diseases, the case reported supports the hypothesis of CNS involvement in some cases of Guillain-Barré syndrome. The latter is generally considered an immune-mediated disease and an immune pathogenesis is also supposed in opsoclonus associated with systemic malignancies; besides, in the patient reported, the self-limiting nature of the disturbance and the lack of MRI lesions suggest that opsoclonus may result from an immune-related phenomenon causing a functional and transitory dysfunction of the CNS.

Abnormalities of horizontal gaze. Clinical, oculographic and magnetic resonance imaging findings. II. Gaze palsy and internuclear ophthalmoplegia

The site of lesions responsible for horizontal gaze palsy and various types of internuclear ophthalmoplegia (INO) was established by identifying the common areas where the abnormal MRI signals from patients with a given ocular-motor disorder overlapped. Patients with unilateral gaze palsy had lesions in the paramedian area of the pons, including the abducens nucleus, the lateral part of the nucleus reticularis pontis caudalis and the nucleus reticularis pontis oralis. Patients with abducens nucleus lesions showed additional clinical signs of lateral rectus weakness. Lesions responsible for bilateral gaze palsy involved the pontine tegmental raphe. Since this region contains the saccadic omnipause neurons, this finding suggests that damage to omnipause cells produces slowing of saccades rather than opsoclonus, as previously proposed. All INOs, regardless of the presence of impaired abduction or convergence, had similar MRI appearances. Frequently the lesions in patients with INO, were not confined to the medial longitudinal fasciculus (MLF) but also involved neighbouring structures at the pontine and mid-brain levels. There was a statistically significant association between the clinical severity of the INO and the presence of abnormal abduction or convergence. The findings suggest that the lesions outside the MLF, which may affect abducens, gaze or convergence pathways, are responsible for the presence of features additional to INO, depending on the magnitude of functional disruption they produce.

Catecholamine depletion produces irrepressible saccadic eye movements in normal humans

To determine the effect of catecholamine depletion on ocular motor pathways in humans, we studied the eye movements of 3 normal subjects who received the drug metyrosine (alpha-methylparatyrosine). This drug temporarily depleted dopamine and norepinephrine, as measured by a reduction in the metabolite, 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG). Saccadic, pursuit, and vestibulo-ocular eye movements were recorded using infrared oculography with subjects both on placebo and on metyrosine. The most consistent effect observed with metyrosine was an increase in the amplitude and frequency of saccadic intrusions during fixation and pursuit. Two of the 3 subjects also had shortened time constants for the vestibulo-ocular reflex, attributable in part to the sedative effect of catecholamine depletion. The increase in saccadic intrusions implies that catecholamines modulate the activity of a subpopulation of suppressor motor neurons in the human brainstem.

Pontine lesion in opsoclonus-myoclonus syndrome shown by MRI

Two patients with opsoclonus-myoclonus syndrome are reported whose magnetic resonance imaging (MRI) showed brain stem lesions. Both patients developed the opsoclonus-myoclonus syndrome after an upper respiratory illness. One case had visual hallucinations during the course of illness and MRI revealed a focal lesion in the pons involving the junction of basis and tegmentum. MRI of the second case showed a focal lesion at the upper pontine tegmentum.

Smooth pursuit eye movements in schizophrenics--what constitutes quantitative assessment?

Although a voluminous literature exists on the eye movements of schizophrenic and affective disorder patients, many of the assessments made of smooth pursuit have been qualitative in nature. Most of them have not differentiated between abnormal functioning of the smooth pursuit system and intrusion of inappropriate saccades during a smooth tracking task. Specific identification of the pursuit or saccadic defect is necessary if the origins of the abnormalities are to be understood and related to psychopathology. Analytical techniques, such as the ln(S/N) ratio, although numerical in nature, are still unable to discriminate among pursuit and saccadic defects, as shown by our analysis of simulated tracking. Thus, to understand the effects of psychiatric disorders on the ocular motor system, specific defects must be identified and quantified.

Frequency response analysis of human saccadic eye movements: estimation of stochastic muscle forces

A frequency response method is used to estimate parameters of a fourth-order model of the oculomotor system and the active state tensions during a saccadic eye movement. The lateral and medial rectus muscle of each eye is modeled as a parallel combination of an active state tension generator with a viscosity and elastic element, connected to a series elastic element. The eyeball is modeled as a sphere connected to a viscosity and elastic element. Each of these elements is assumed to be ideal and linear. The active state tension for each muscle is modeled by a low-pass filtered pulse-step waveform. Initial estimates of the oculomotor mechanical components are based on physiological evidence. Initial estimates of the active state tension are based on an extrapolation of the eye movement trajectory. Horizontal saccadic eye movements were recorded from infrared signals reflected from the anterior surface of the cornea and then digitized. Parameter estimates were calculated for the model by using a conjugate gradient search program which minimizes the integral of the absolute value of the squared error between the model and the data. The predictions of the model are shown to be in good agreement with the data. Final estimates of motoneuronal activity demonstrate that the agonist muscle is maximally stimulated during the early portion of a saccadic eye movement regardless of the amplitude of the saccade; only the duration of the maximal stimulation affects the size of the saccade. The antagonist muscle is completely inhibited during the period of maximum agonist muscle stimulation. Furthermore, it is demonstrated that saccade motoneuronal activity is a stochastic phenomenon.

Anatomy of physiologically identified eye-movement-related pause neurons in the cat: pontomedullary region

Pause neurons (PNs) are inhibitory neurons close to the midline at the pontomedullary junction that fire tonically and then cease firing just prior to quick eye movements of visual or vestibular origin. Previous physiological evidence has shown that these neurons have a role of central importance in the generation of rapid eye movements in any direction and all major models of ocular motor control incorporate PNs as major elements. In this study in cats, we injected horseradish peroxidase intracellularly into somata or axons of physiologically identified PNs. After appropriate tissue preparation, cell body and axonal reconstructions were performed, with the aid of a camera lucida-equipped microscope. Fifty-three PNs were stained and reconstructed. These consisted of 17 cell bodies and dendrites and 36 axons. Seven of these included both cell bodies and axons. PN somas lay close to the midline in the nucleus raphe pontis and centralis superior, had extensive dendritic arborizations tending to arise from either pole of the elongated soma, and had axons which typically crossed the midline and bifurcated into long branches which extended rostrally and caudally, inferior to the medial longitudinal fasciculus. There were major terminal arborizations and boutons in areas just rostral and caudal to the abducens nucleus in areas where two types of premotor neurons, excitatory and inhibitory burst neurons, are concentrated. Many axosomatic contacts were noted. Other terminal arborizations and boutons were found close to the midline in a region rostral to abducens nucleus containing other neurons known to burst prior to quick eye movements, and in the nucleus reticularis gigantocellularis. Rostral stem axons could be traced to the level of the trochlear nucleus and inferior to the medial longitudinal fasciculus. The caudal stem axons could be traced parallel to the midline and inferior to the medial longitudinal fasciculus and as far caudally as the hypoglossal nucleus.

On plotting amplitude-transition functions for voluntary eye saccades

The first of two oppositely directed saccades may be small if the time interval between the two saccades is short. We have examined the merits of plotting the amplitude of the first saccade against saccadic interval. This plot provides a useful supplement to other approaches in the literature, as it is insensitive to variation due to extraneous factors or to task. Reasons for this are discussed.

Skewness of saccadic velocity profiles: a unifying parameter for normal and slow saccades

It has become customary to make use of so-called main sequence plots to characterize the dynamic properties of saccades. However, such a description does not account for the fact that the ratio between the accelerating and the decelerating fraction of the eye movement is not constant for all saccades. In this paper we introduce a new parameter, skewness, that characterizes this much neglected aspect of the saccade velocity profile. Human saccade data in this report demonstrate a clear relation between saccade duration (D) and skewness (S). When saccadic eye movements become extremely slow, due to fatigue or diazepam, the main sequence relation breaks down, while the S-D relation still holds. Despite large differences in amplitude, saccades of a fixed duration appear to have the same shape of velocity profile. A unifying equation relating the saccade parameters amplitude, maximum velocity and skewness, which is valid for both normal and slow saccades, is proposed.

Caloric-induced nystagmus with isoelectric electroencephalogram

Caloric vestibular testing induced nystagmus in a patient with an isoelectric electroencephalogram after cardiopulmonary arrest. This has been demonstrated previously in patients in a chronic persistent vegetative state with intact brainstem reflexes, but never in a patient with an isoelectric electroencephalogram. Animal studies indicate that the quick phase of nystagmus and horizontal saccades are generated in the paramedian pontine reticular formation. The present case supports the conclusion that caloric-induced nystagmus originates in the brainstem in rudimentary form.

Saccade abnormalities in patients with ocular flutter

In a group of 8 patients with opsoclonus or ocular flutter, the eye movements were recorded by electro-oculography (EOG). The spontaneous eye movement pattern and the amplitude and peak velocity of the refixation saccade were analysed. The EOG recording demonstrated frequent bursts of horizontally and vertically directed saccades elicited without any intersaccadic interval. Three patients also demonstrated an increased frequency of square waves. In 6 of the 8 patients the peak velocity of the voluntary saccades was increased; in 5 patients this condition was found for saccades in both directions. Over-shooting oscillations or slightly hypermetric voluntary saccades occurred in 5 patients. It is proposed that the ocular flutter and the increased saccadic velocities found in the present series of patients is caused by a disorder of the burst cells in the brain stem saccadic control system.

Selective saccadic palsy caused by pontine lesions: clinical, physiological, and pathological correlations

Two patients suffered a selective deficit of voluntary saccades and quick phases of nystagmus after hypoxic-ischemic insults during open-heart surgery. All voluntary saccades, in both horizontal and vertical planes, were slow, and quick phases of vestibular and optokinetic nystagmus were absent. Smooth pursuit, the vestibuloocular reflex, the ability to hold steady eccentric gaze, and vergence eye movements were all preserved. Pathological studies in 1 patient confirmed neuronal necrosis and gliosis, consistent with ischemic lesions involving the median and paramedian pontine reticular formation and median basis pontis but sparing the rostral mesencephalon and rostral interstitial nucleus of the medial longitudinal fasciculus. These findings, taken with data from experimental studies, support the hypothesis that each functionally defined class of horizontal eye movements is controlled by a separate neural substrate that projects independently to the abducens nuclei. In addition, these data suggest that the rostral interstitial nucleus of the medial longitudinal fasciculus is dependent on inputs from the paramedian pontine reticular formation for the programming of normal vertical saccades.

Horizontal and vertical saccadic eye movement abnormalities in Huntington's chorea

With a newly developed infrared reflection technique, voluntary saccadic eye movements (VOLS), visually evoked saccades (VES) and unsuppressed visually evoked reflex saccades (USVERS) were measured in 11 patients with Huntington's chorea. Abnormalities, including latency increase, peak velocity decrease and undershoot dysmetria with multiple step saccades were found in all types of saccadic eye movements. Peak velocity decrease and undershoot dysmetria can be explained by dysfunction of the brainstem reticular formation in Huntington's chorea. USVERS and square wave jerks occurred abnormally frequently and showed direction-dependent differences. Both were more frequent in horizontal than in vertical direction. Frequency of USVERS and square-wave jerks tended to be correlated. These findings point to disinhibited superior colliculi as a possible common supranuclear origin of USVERS and square-wave jerks in Huntington's chorea.

Blink-induced saccadic oscillations

A patient with a neurodegenerative disease had abnormal saccades only when he blinked. These saccades were hypermetric and were followed immediately, without any intersaccadic interval, by a large, oppositely directed saccade (dynamic overshoot). To explain these findings, we hypothesize that a blink-related neural signal can modulate the activity of pause cells that normally inhibit saccadic burst neurons during fixation. In pathological circumstances, abnormal function of pause cells could lead to large-amplitude saccadic oscillations. In normal subjects, blinks could induce short bursts of low-amplitude flutter.

A model of the smooth pursuit eye movement system

Human, horizontal, smooth-pursuit eye movements were recorded by the search coil method in response to Rashbass step-ramp stimuli of 5 to 30 deg/s. Eye velocity records were analyzed by measuring features such as the time, velocity and acceleration of the point of peak acceleration, the time and velocity of the peaks and troughs of ringing and steady-state velocity. These values were averaged and mean responses reconstructed. Three normal subjects were studied and their responses averaged. All showed a peak acceleration-velocity saturation. All had ringing frequencies near 3.8 Hz and the mean steady-state gain was 0.95. It is argued that a single, linear forward path with any transfer function G(s) and a 100 ms delay (latency) cannot simultaneously simulate the initial rise of acceleration and ring at 3.8 Hz based on a Bode analysis. Also such a simple negative feedback model cannot have a steady-state gain greater than 1.0; a situation that occurs frequently experimentally. L.R. Young's model, which employs internal positive feedback to eliminate the built-in unity negative feedback, was felt necessary to resolve this problem and a modification of that model is proposed which simulates the data base. Acceleration saturation is achieved by borrowing the idea of the local feedback model for saccades so that one nonlinearity can account for the acceleration-velocity saturation: the main sequence for pursuit. Motor plasticity or motor learning, recently demonstrated for pursuit, is also incorporated and simulated. It was noticed that the offset of pursuit did not show the ringing seen in the onset so this was quantified in one subject. Offset velocity could be characterized by a single exponential with a time constant of about 90 ms. This observation suggests that fixation is not pursuit at zero velocity and that the pursuit system is turned on when needed and off during fixation.

Visuomotor fields of the superior colliculus: a quantitative model

Electrophysiological and electrical stimulation studies in the monkey have disclosed that both the retinal surface and the metrics of saccades are topographically represented in the superior colliculus. This mapping of sensory and motor space onto the colliculus is nonhomogeneous in that the central region is over-represented in both the visual and the motor map. Single unit studies have revealed that visual receptive fields of collicular neurons are typically quite large and are characterized by a skewed (asymmetrical) sensitivity profile. Analyses by McIlwain [J. Neurophysiol. 38, 219-230 (1975)] in the cat have suggested that this skewness property reflects mainly the spatial distortion inherent in the afferent mapping. In this paper we describe a quantitative model, based on a logarithmic mapping function combined with a Gaussian connectivity function in the colliculus, which can account for the extent and the shape of collicular receptive fields. Collicular neurons in the deeper layers have movement-related bursts of activity for saccades in a limited amplitude and direction range related to their location in the collicular map. These movement fields, like visual receptive fields, may be quite extensive and typically have a skewed profile. In our model, an efferent-mapping function is defined, which relates the locus of a population of recruited cells to the metrics of the ensuing saccade. The parameters of this function, which was taken to be identical with the afferent mapping function, were estimated from Robinson's [Vision Res. 12, 1795-1808 (1972)] electrical stimulation data. Based on the assumption that the population-activity profile resembles a two-dimensional Gaussian function, the shape and the size of movement fields can then be described with just 2 or 3 free parameters. Electrophysiological data recorded from a small sample of collicular visuomotor neurons were used to illustrate the procedure, which we designed to enable application of our model to the experimental data. The best fit was obtained when the mapping function was slightly anisotropic. Suggestions on how the model could be improved and extended are offered in the Discussion.

Reconstruction of neural control signals for saccades based on an inverse method

The failure to obtain realistic saccadic velocity profiles, in earlier simulation studies using a linear oculomotor plant model and a stylized pulse-step input, has led to the development of quite complex plant models. The stylized-input assumption has later been shown to be unrealistic: it is known now that the pulse has a smooth shape in which the decay period is longer than the rising edge. To explore the applicability of linear models without imposing a priori constraints on the shape of the control signal, we have used Fourier deconvolution for reconstructing the neural-control signal. From the results obtained with this inverse method, we conclude that it is not necessary to use a complicated model in order to obtain realistic saccade velocity profiles. Furthermore, the results illustrate clearly that a second order linear plant model necessitates active braking at the input in order to explain the occurrence of short-lasting saccades.

Saccadic intrusions and oscillations

There are a number of different types of involuntary saccadic eye movements which differ from normal saccades in that they take the fovea away from its target. This article presents a classification and description of these abnormal eye movements and indicates how they may be used to increase the precision of neurologic diagnosis.

Effect of aging on the accuracy of visually guided saccadic eye movement

Changes in oculomotor behaviors with aging were studied in normal young and elderly subjects. Saccadic eye movements induced by presentation of a visual target were analyzed. Elderly subjects commonly showed an elongation of the time to locate the target, accompanied by an increase in reaction times (mean increase, 100 ms) and a decrease in saccadic velocities. The decrease in the velocity was particularly notable when a large-amplitude saccade was executed. In spite of the slowed motor responses, most elderly subjects preserved the function necessary to execute a correct saccade toward the visual target. The saccadic slowing was accompanied by an increase in saccade duration. Although a longer time was necessary for elderly subjects to locate the target, the accuracy of the initial saccades was not different from that of young subjects. One group of elderly subjects showed extremely long reaction times. These subjects, displaying no abnormal neurological symptoms, were not able to locate the visual target with initial saccades. They had to execute multistep saccades typically seen in patients with degenerative neurological diseases.

Spinocerebellar degeneration and slow saccades in three generations of a kinship: clinical and electrophysiologic findings

Four members of a family with spinocerebellar degeneration and slow saccadic eye movements are described. Detailed electrophysiological studies revealed abnormalities of neurological pathways not apparent clinically. The patients had slow saccades as measured electrophysiologically, as well as absence of rapid eye movements (REM) despite REM stages of sleep. These studies suggest that although saccadic eye movement and REM are mediated through the pontine paramedian reticular formation, other characteristics of REM sleep are not necessarily mediated through the same neurons.

Convergence-initiated voluntary flutter: a normal intrinsic capability in man

The proposal that there is an inherent capability in humans to produce bursts of fluttering saccades was tested by comparing Purkinje image eye movement recordings in subjects with voluntary nystagmus and control subjects. Voluntary nystagmus is composed of recurrent saccades without an intersaccade interval and has been proposed to be an inherited event. No difference in saccade peak velocity-amplitude curves or microsaccades during visual fixation was found between the two groups. With training control subjects learned to produce runs of saccadic flutter identical to voluntary nystagmus. This learned flutter was composed of recurrent complete saccades rather than saccades interrupted in midflight. Voluntary flutter is thus not a genetic trait but a learned event that is usually undeveloped in man. These observations can be explained by the Robinson model of saccade generation and indicate that similar models must have an inherent ability to produce saccadic flutter.

Saccadic oscillations associated with the quick phases of caloric nystagmus in severe diffuse brain damage

Caloric nystagmus patterns associated with ocular dysmetria, ocular flutter or flutter dysmetria were studied in ten patients being in a vegetative state, among whom were 9 patients with head injury and 1 with complications caused by a grand mal status. Brain damage was complicated by hypoxemia and especially by brain stem herniation. It was more often observed in very young children and appears to be associated with a poor clinical course. Physostigmine seems to have an activating and provocating effect on these saccadic oscillations. In view of Zee and Robinson's hypothesis on the pathophysiology of saccadic oscillations, it is suggested that these nystagmus patterns may reflect a disturbance of brain stem midline structures (pausing neurons) or an abnormal supranuclear (cerebellum) control.

Characteristics of saccades in human infants

Infants (14-151 days) and adults were shown two-dimensional geometric forms or stimuli from a set of highly textured patterns. Their eye movements were recorded by an infrared corneal reflection eye movement recorder as they freely scanned the stimuli. For both infants and adults, linear relationships were found between the peak velocities of fast eye movements and their amplitudes (main sequences). Infants viewing texture stimuli had main sequences with slopes comparable to those of adults. Infants viewing simple geometric forms made slower saccades. They also showed more eye movement oscillations which analyses showed were probably back-to-back saccades. Both the slower saccades and saccadic oscillations were attributed to factors related to the attentional value of the stimuli.

Precise non-contacting measurement of eye movements using the corneal reflex

Using a real-time, analog, zero-crossing technique, it is possible to locate the centre of the corneal reflex of a collimated infra-red beam accurately. Performance at tracking horizontal eye movements in this manner is similar to contact lens techniques. System noise is less than 30 are sec; dynamic range is 30 arc sec to 36 deg; linearity is 2%; data are currently sampled at 1 kHz; velocity resolution, for a velocity bandwidth of 125 Hz, is 2 deg/sec. With automatic acquisition of the corneal reflex, the subject can be set up for measurement within a 1 min time period. Recordings of residual eye movements of fixation and of a wide range of saccades are comparable with previous findings. Post-saccadic oscillations can be resolved for all sizes of saccades. The amplitude-peak-velocity characteristic of saccades is illustrated.

Neuro-otological abnormalities in Friedreich's ataxia

Ten patients with an accepted diagnosis of Friedreich's ataxia have been examined neuro-otologically, and oculomotor, vestibular and auditory function assessed. Brainstem auditory evoked potentials (BAEPs) were also recorded. A high incidence of various eye movement disorders was noted. Some of these were indicative of cerebellar dysfunction. Reduced vestibular function and impaired hearing were common to most of the patients. BAEPs were also abnormal in the majority; reasons underlying these abnormalities are discussed. Neuro-otologically, the patients did not constitute an homogeneous group. The findings cast doubt upon the accuracy and validity of the currently accepted criteria for the diagnosis and classification of the spinocerebellar degenerations.