The vestibulo-ocular reflex during human saccadic eye movements

Eye and head movements were recorded in normal humans during rapid refixations with the head still (saccades) or moving (gaze saccades) to determine if the vestibulo-ocular reflex was operating at such times. Subjects made self-electrooculogram for large saccades and with the eyecoil/magnetic field method for smaller movements. The putative function of the vestibulo-ocular reflex during a gaze saccade is to adjust the movement of the eye for the movement of the head by adding the saccadic command and the vestibular signal. This action, referred to here as linear summation, would maintain gaze-saccade accuracy by making gaze velocity (eye in space) independent of head velocity. It would also preserve the duration of the eye saccades of about 200 deg. When a subject increased his head velocity voluntarily, for example, from 420 to 805 deg/s, mean gaze velocity rose from 540 to 820 deg/s and duration dropped from 380 to 250 ms. Linear summation did not occur. By means of a yoke clenched in the teeth, the subject's head could be momentarily and unexpectedly slowed by collision of the yoke with a lead weight during a 180 deg gaze saccade. The perturbation decreased head velocity by about 150-200 deg/s, decreased gaze velocity by about the same amount and did not change eye velocity (in the head); another indication that the vestibulo-ocular reflex was not working. Nevertheless, gaze-saccade duration was automatically increased so that the over-all accuracy of the movement was not changed. Subjects made saccades between targets at +/- 20 deg without attempted head movements. Simultaneously the experimenter struck the yoke, clenched in the subject's teeth, with a rubber hammer. The hammer blow caused a transient head velocity of about 70 deg/s. Gaze velocity transiently rose or fell, depending on the direction of the blow, by similar amounts and a quantitative analysis suggested that the vestibulo-ocular reflex was essentially absent. Again, duration was automatically altered so that saccade accuracy was not changed. Subjects looked back and forth between targets 20, 40 and 60 deg apart as their head turned through the straight ahead position, actively or passively, at velocities up to 600 deg/s (active) or 300 deg/s (passive).(ABSTRACT TRUNCATED AT 400 WORDS)

Saccadic undershoot is not inevitable: saccades can be accurate

Saccades normally take the eye 90% of the way to a target, followed by a 10% corrective saccade. An exception to this rule occurs with the range effect. When targets appear in a set of positions, saccades overshoot the near positions and undershoot the far. This phenomenon, previously reported, was confirmed with more accurate methods. The range effect increases if a visual discrimination task is added. It is established rapidly in only a few trials. Latencies of corrective saccades from overshoots and undershoots were the same. Centripetal saccades were more accurate than centrifugal. Thus, undershooting is not inevitable.

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.

The systems approach to the oculomotor system

Recent progress in understanding the oculomotor system is briefly reviewed. This progress is largely due to technological advances such as the ability to record from neurons in behaving animals. Furthermore, parts of the oculomotor system are now well-enough understood that the techniques of exact science, such as quantitation and mathematical description, are becoming useful. This, in turn, leads to the use of the language of systems analysis, and the vestibulo-ocular reflex is examined as an example of such a description. Systems analysis not only organizes current knowledge but leads to predictions by way of hypotheses known as models. A model of time integration by neurons is given as an example. It is put forward to illustrate that our biggest problem at the moment is an inability to test such models at the neuronal network level.

Motion of the eye immediately after a saccade

Dynamic overshoot is a small saccade that follows a main saccade, in the opposite direction, with no delay. To re-examine prior reports of dynamic overshoot, the properties of dynamic over-shoot were studied in six normal subjects. The post-saccadic drift of eye movements was studied as well. Horizontal eye movements were recorded with the magnetic-field/search-coil method. System noise level was 0.05 deg. Dynamic overshoot occurred with a frequency of about 13% and was more frequent for saccades 10 deg or less. Its mean size was 0.15 deg and its peak velocity showed it to be saccadic in nature. Binocular recordings for three subjects showed that when dynamic overshoot occurred it was almost always in the abducting eye which also had the least post-saccadic drift. The abducting eye seldom had dynamic overshoot and consistently had a more pronounced post-saccadic drift, almost always in the onward direction. We suggest that, at the end of a saccade, the eye normally is brought to rest by a braking pulse and dynamic overshoot occurs when the braking pulse is accidentally too large. It would appear to serve no useful purpose. Why dynamic overshoot is monocular and coincides with the eye having less post-saccadic drift is unclear.

Effects of electrical stimulation and reversible lesions of the olivocerebellar pathway on Purkinje cell activity in the flocculus of the cat

The activity of Purkinje cells (P-cells) in the flocculus of 8 lightly anesthetized cats and one alert cat was recorded for periods of up to several hours each. The resting simple spike (SS) rate in the anesthetized cats was 37 +/- 21 Hz (mean +/- standard deviation), similar to that observed in the alert cat. Complex spikes (CSs) were evoked by an electrode placed in the climbing fiber (CF) decussation or inferior olive (IO). For each P-cell, SS activity was suppressed completely at or above a cut-off frequency of evoked CSs; median cut-off rate was 5 Hz (range 1-10 Hz, 15 cells). Reversible lesions of the CF pathway were made by microinjection of 1-10 microliter of saturated lidocaine into the IO or CF decussation. This abolished spontaneous CS activity and produced two reversible effects on SS discharge: (1) an increase in mean SS rate of 98%, from 23 +/- 13 to 40 +/- 18 Hz (11 cells); and (2) a decrease of 50% in the variability of SS firing rate. Similar effects were observed in two P-cells whose CF axons were lesioned mechanically. These results show that electrical stimulation and reversible lesions in areas previously shown to alter the gain of the vestibulo-ocular reflex (VOR) also alter CF input to the flocculus, suggesting that the gain changes were caused by changes in CS rate. This study confirms and extends the observation that a reciprocal relationship exists between CS rate and SS background rate, and therefore further suggests that the changes in the gain of the VOR might be due to changes in SS background rate.

The upper limit of human smooth pursuit velocity

The maximum smooth pursuit eye velocity was determined in five human subjects. Eye movements were recorded with the d.c.-coupled EOG. On each trial the target moved in a ramp, at a constant velocity between 10 and 200 deg/sec. In four subjects eye velocity was approximately 90% of target velocity up to a target velocity of 100 deg/sec. Eye velocity then saturated with a large variability from trial to trial (standard deviation 16 deg/sec). The fifth subject had eye velocities only 60% of these values.

Bielschowsky head-tilt test--II. Quantitative mechanics of the Bielschowsky head-tilt test

In ten recent cases of superior oblique (SO) palsy the involved eye elevated by 8.2 +/- 3.8 deg (standard deviation) on 45 deg ipsilateral head tilt. A computer model predicted a rise to only 4.1 deg but could easily achieve 8.2 deg by including reactions in the other cyclovertical muscles, notably on overaction of the superior rectus (SR). In 17 older cases the elevation on tilt rose to an average of 15 deg. The model shows that these deviations must be due to SR overaction. The analysis suggests that SR overaction is the main cause of the rapid increase in eye elevation in recent cases and further increases in older cases.

Alexander's law: its behavior and origin in the human vestibulo-ocular reflex

Alexander's law refers to the phenomenon in which the spontaneous nystagmus of a patient with a vestibular lesion is more intense when the patient looks in the quick-phase than in the slow-phase direction. Alexander's law was investigated in normal subjects as well as patients with vestibular lesions. During sinusoidal rotations of normal subjects, there was no trace of this phenomenon when subjects looked 25 degrees left and right in the dark. The phenomenon of Alexander's law is therefore created centrally and is not due to any mechanical properties of the orbit. During rotation at constant velocity in the dark, normal subjects did weakly demonstrate Alexander's law because of a mild gaze-evoked nystagmus present in normal subjects in the dark. Gaze-evoked nystagmus from a cerebellar lesion involved a pronounced demonstration of Alexander's law during rotatory nystagmus. In patients with a vestibular lesion and a mild spontaneous nystagmus in the dark that obeyed Alexander's law, the nystagmus reversed upon far gaze in the slow-phase direction. We propose that in patients with a vestibular lesion, the phenomenon of Alexander's law is created by the sum of vestibular nystagmus and an abnormally large gaze-evoked nystagmus that is consequent to the vestibular lesion. This conclusion has a number of neurological implications concerning the ways in which the nervous system attempts to compensate for vestibular lesions.

A model of the mechanics of binocular alignment

A computer model (SQUINT) of the static mechanics of human eyes is developed and applied to the diagnosis and treatment of defects of binocular alignment (strabismus). Brief discussions of ocular anatomy, binocular vision, strabismus and its surgical correction, and early strabismus modeling are provided for nonspecialists in these areas. Models of muscle force, muscle path, globe translation, and binocularity are developed. To illustrate the use of the model in clinical cases, it is applied to the diagnosis and surgical treatment of a case of superior oblique palsy. The model's implementation under the UNIX operating system is described.

Pleural surgery in patients with cystic fibrosis. A review of anaesthetic management

Eighteen patients with advanced cystic fibrosis complicated by pneumothorax required either pleurectomy or abrasion pleurodesis. Anaesthesia with nitrous oxide and oxygen was combined with controlled ventilation to both lungs, usually through a double lumen endobronchial tube. Spontaneous ventilation was restored at the end of operation in all but a minority of cases. Ten of 25 procedures were uncomplicated and a further five were followed by non-respiratory events. Respiratory complications developed after the remaining ten procedures which were carried out on six patients, of whom only one survived for more than 6 months. All five fatal cases suffered a recurrence of pneumothorax within 6 months whereas three patients requiring re-operation 2-6 years later survived. The principles of management are discussed and it is concluded that the operative and anaesthetic hazards of thoracic surgery in this group of high-risk cases can be avoided successfully, except when re-operation is required within 6 months.

Signals in vestibular nucleus mediating vertical eye movements in the monkey

The action potentials of single neurons were recorded extracellularly throughout the rostral vestibular nuclei and subadjacent reticular formation in three alert, juvenile, rhesus monkeys. Neuronal responses were tested during a) sinusoidal pitch oscillations in darkness, b) cancellation of the vestibuloocular reflex (VOR) during similar oscillations by fixation of a target moving with the head, c) sinusoidal vertical smooth pursuit, d) vertical saccades, and e) fixation with the head stationary. Eye movements were measured using the magnetic field-search coil technique. Of the 527 neurons isolated, 318 responded to pitch oscillation and/or vertical eye movements. The latter cells could be classified into six categories. Of this group, 273 cells were recorded from for sufficient time to allow them to be fully tested and form the basis of this report. Cells were classified as follows: pure-vestibular cells with firing rates modulated only by head velocity (15%), vestibular-pause cells that were similar to the pure-vestibular cells but paused for saccades in all directions (10%), gaze-velocity cells that modulated their rates in proportion to vertical eye velocity in space (7%), position cells with rates modulated by changes in eye position in the head but that did not burst or pause during saccades (33%), position-burst cells that also carried an eye-position signal but did burst during saccades in one direction and paused in the opposite direction (15%), and position-vestibular-pause cells that carried signals proportional to eye position in the head and head velocity and paused during all saccades (20%). Most cells that carried an eye-position signal also carried an eye-velocity signal during pursuit. Position and position-burst cells could be divided into two subcategories. Position cells that also reported head velocity represented 20% of the total sample, while those without head-velocity signals made up the remaining 13%. Position-burst cells were divided into two subcategories based on their behavior during pitch oscillation in darkness. Both carried eye-velocity signals during pursuit, but only one type (8% of the total sample) also carried an eye-velocity signal during vestibular eye movements in the dark, while the other (7%) did not. Some cells in all six categories except the pure-vestibular cells responded antidromically to stimulation of the medial longitudinal fasciculus (MLF). Only the position-vestibular-pause, position-burst, and gaze-velocity cells, however, were judged to be commonly antidromically activated, suggesting that these three cell types are the major contributors to the MLF from the rostral vestibular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Different time constants for optokinetic and vestibular nystagmus with a single velocity-storage element

Models have been proposed for the combined optokinetic system and vestibulo-ocular reflex that share a single, common, velocity-storage element that generates both optokinetic after-nystagmus and prolongs post-rotatory nystagmus. It has been thought by many that such a model requires the time course of these two types of nystagmus to be similar. The demonstration that vestibular or optokinetic habituation can selectively and significantly reduce the duration of each type of nystagmus without affecting that of the other has been taken to imply that the concept of shared velocity storage is incorrect. This need not be the case because post-rotatory nystagmus is described by two time constants, not one. Changes in the model's parameters can create time courses of this nystagmus that, when fit by single exponentials, have apparent time constants quite different from that of optokinetic after-nystagmus. Consequently, a common velocity-storage mechanism is still a tenable hypothesis for optokinetic and vestibular eye movements.

A proposed neural network for the integrator of the oculomotor system

Single-unit recordings, stimulation studies, and eye movement measurements all indicate that the firing patterns of many oculomotor neurons in the brain stem encode eye-velocity commands in premotor circuits while the firing patterns of extraocular motoneurons contain both eye-velocity and eye-position components. It is necessary to propose that the eye-position component is generated from the eye-velocity signal by a leaky hold element or temporal integrator. Prior models of this integrator suffer from two important problems. Since cells appear to have a steady, background signal when eye position and velocity are zero, how does the integrator avoid integrating this background rate? Most models employ some form of lumped, positive feedback the gain of which must be kept within totally unreasonable limits for proper operation. We propose a lateral inhibitory network of homogeneous neurons as a model for the neural integrator that solves both problems. Parameter sensitivity studies and lesion simulations are presented to demonstrate robustness of the model with respect to both the choice of parameter values and the consequences of pathological changes in a portion of the neural integrator pool.

Plasticity in the oculomotor system

The oculomotor system, responsible for the control of eye movement, is capable of a number of kinds of plasticity. The demonstration of such plasticity has been aided by several factors, including the present delineation of the system's major subsystems and their purposes; the relatively simple relationships between the system and the mechanical apparatus on which it acts, the eyeball and its muscles; and the development of techniques for studying central nervous system (CNS) activity in awake, behaving animals. Plasticity has been demonstrated by using noninvasive techniques to disturb the normal relationship between vision and eye movements, thereby creating a need for adjustment by the oculomotor system. Current studies are concerned with the location and identity of the responsible modifiable synapses. The cerebellum and the olivocerebellar pathways are definitely involved in several forms of oculomotor plasticity. However, it remains to be determined whether the responsible changes reside within these structures or are located elsewhere in the CNS.

The use of matrices in analyzing the three-dimensional behavior of the vestibulo-ocular reflex

The vestibulo-ocular reflex rotates the eye about the axis of a head rotation at the same speed but in the opposite direction to make the visual axes in space independent of head motion. This reflex works in all three degrees of freedom: roll, pitch, and yaw. The rotations may be described by vectors and the reflex by a transformation in the form of a matrix. The reflex consists of three parts: sensory, central, and motor. The transduction of head rotation into three neural signals, which may also be described by a vector, is described by a canal matrix. The neural, motor-command vector is transformed to an eye rotation by a muscle matrix. Since these two matrices are known, one can solve for the central matrix which gives the strength of the connections between all the vestibular neurons and all the eye-muscle motoneurons. The role of the metric tensor in these transformations is described. This method of analysis is used in three applications. A lesion may be simulated by altering the elements in any or all of the three component matrices. By matrix multiplication, the resulting abnormal behavior of the reflex can be described quantitatively in all degrees of freedom. The method is also used to directly compare the differences in brain-stem connections between humans and rabbits that accommodate the altered actions of the muscles of the two species. Finally the method allows a quantitative assessment of the changes that take place in the brain-stem connections when plastic changes are induced by artificially dissociating head movements from apparent motion of the visual environment.

The behavior of the vestibulo-ocular reflex at high velocities of head rotation

The behavior of the vestibulo-ocular reflex (VOR) during both passive and active, high-velocity head movements was recorded in three normal subjects. We found that the VOR is compensatory for head velocities up to at least 350 deg . s-1, during both active and passive head rotation provided there is an attempt to visualize a real (or imagined) stationary object in space. Slow phase velocities, however, could reach values as high as 500 deg . s-1. Furthermore, during passive rotation at high velocities, the quick phase trajectory is modified--indicating an interaction between the slow phase and quick phase eye velocity commands.

Campylobacter enteritis associated with the consumption of free school milk

A large outbreak of campylobacter enteritis associated with the consumption of free school milk is described. The outbreak had an abrupt onset, and lasted for about 3 weeks; it involved mainly school children in the 2-4 and 5-7 year old age groups. During this period it was established from epidemiological and microbiological data that some 2500 children were infected. The source of the epidemic was almost certainly contaminated milk, although bacteriological proof could not be obtained. Biotyping of isolates was of considerable epidemiological value and showed the involvement of two distinct strains, one of which was dominant. Epidemiological evidence of limited person to person transmission of the infection was obtained; febrile convulsions as a prodromal sign of the illness was recognized for the first time. Strains of Campylobacter jejuni, and samples of patients' serum collected during this outbreak have enabled subsequent studies to be initiated on the serotyping of the responsible organism, on the serological response of patients infected with the organism, and on experimental infection of the bovine udder which demonstrated its potential as a source of C. jejuni in raw milk. A careful search of the literature suggests that this is the largest documented outbreak of campylobacter enteritis.

Serological studies in two outbreaks of Campylobacter jejuni infection

Two outbreaks of campylobacter enteritis in residential student populations are described. In the first outbreak, 34 of 240 students in one hall of residence became ill over the course of 4 or 5 days. A single serotype of C. jejuni was isolated from sample of faeces. Sera collected demonstrated that the students had little pre-existing antibody and antibody responses were demonstrated in those who were infected. About 30% of students with serological evidence for infection with C. jejuni did not have gastro-intestinal symptoms. In the second outbreak, 77 of 300 agricultural students developed symptoms over a period of 3 weeks. Unpasteurized milk was the source of infection. Two serotypes of C. jejuni were isolated from faeces. Antibody responses were detected to both these serotypes and to another serotype, suggesting the presence of at least 3 serotypes in the milk. Bactericidal antibody consistent with recent infection was found in all students with symptoms and in 63% of students in the absence of symptoms, indicating the possible development of immunity. The high prevalence of antibody in these students was correlated with the habitual consumption of unpasteurized milk.