Afferent signals from the extraocular muscles of the pigeon modify the electromyogram of these muscles during the vestibulo-ocular reflex.

There is no general agreement on whether afferent signals from the extraocular muscles play any part in oculomotor control. However, we have previously shown that they modify the responses of cells in the oculomotor control system during the vestibulo-ocular reflex (VOR). If, as we suspect, these signals have an important role in the control of the VOR from moment-to-moment, we should be able to demonstrate similar, functionally significant, modifications at the output of the reflex. We have recorded the electromyographic activity of several extraocular muscles of the right eye during the VOR and while imposing movements on the left eye. We describe how the activity of the muscles, reflected in the electromyogram, is modified in specific ways depending on the parameters of the imposed eye movements. The effects of the extraocular afferent signals on the eye-muscle responses to vestibular drive during the slow phase of the VOR appear to be corrective. Thus the present results provide strong evidence that afferent signals from the extraocular muscles are concerned in the control of the reflex from moment-to-moment, and suggest that the wider question of their role in oculomotor control merits further consideration.     

无眼动条件下中文阅读的研究

设计了计算机控制的中文阅读材料的不同显示方式,并对各种阅读过程中的徙动波形进行记录和分析。实验结果表明,无眼动的阅读速度较正常有眼动高,分别为853字/分和640字/分。通过固定窗口显示方式和知动窗口显示方式条件下中文阅读的比较,排除了强迫阅读的因素,说明有无眼动是产生阅读速度差异的主要原因;根据以上实验结果,可得出结论,在有眼动的情况下,除了saccade抑制影响阅读和识别外,主要的因素在于高级中枢对位置信号的处理,眼动的驱动控制及运动过程影响了高级中枢对阅读内容和识别内容的解码速率。无眼动阅读的速度主要受到高级中枢解码速率和记忆的影响,而不是周边视觉系统的限制。实验结果还表明,在无眼动的情况下,周边信息对阅读不仅不起帮助作用,反而起到干扰作用。     

小脑控制不同类型眼动的神经编码机制：从小脑皮层到小脑核团

近年来许多研究发现,小脑作为运动控制的主要脑区,除参与运动控制外也与孤独症、精神分裂症、奖励相关的认知功能和社会行为有关,因此小脑相关研究越来越受到重视。研究小脑参与运动学习和运动控制的神经机制是神经科学中最重要的课题之一。眼睛运动的肌肉协调和生物运动特征比其他类型的运动更简单,这使眼动成为研究小脑在运动控制中作用的理想模型。作为收集外界信息的主要方式之一,视觉对日常生活至关重要。为确保清晰视觉,3种主要类型的眼动（眼跳、平滑追随眼动（SPEM）和注视）需受小脑的精确控制,以确保静止或移动的物体保持在视小凹的中心。异常眼动可导致视力障碍,并可作为诊断各种疾病的临床指标。因此,眼动控制研究具有重要的医学和生物学意义。虽然对小脑皮层和顶核在调节眼动中的作用有基本了解,但眼动动力学编码的确切神经机制,尤其是小脑顶核控制追随眼动和注视的神经机制仍不清楚。本综述总结了目前小脑在运动和认知等方面的主要研究问题与小脑相关研究的潜在应用价值,以及近年来有关小脑控制眼动的相关文献,并深入探讨了利用单细胞记录和线性回归模型分析小脑皮层和顶核同一神经元同时参与控制不同类型的眼动,而不同类型眼动的不同动力学参数编码原则不同。此外,基于检测微眼跳的研究结果,我们讨论了小脑顶核参与控制视觉注视的可能神经机制。最后,讨论了最近技术进步给小脑研究带来的新机遇,为今后与小脑相关的研究和脑控义肢的优化控制（例如通过单独改善运动参数优化义肢控制）提供了新思路。     

Variables Contributing to the Coordination of Rapid Eye/Head Gaze Shifts

In this article results of several published studies are synthesized in order to address the neural system for the determination of eye and head movement amplitudes of horizontal eye/head gaze shifts with arbitrary initial head and eye positions. Target position, initial head position, and initial eye position span the space of physical parameters for a planned eye/head gaze saccade. The principal result is that a functional mechanism for determining the amplitudes of the component eye and head movements must use the entire space of variables. Moreover, it is shown that amplitudes cannot be determined additively by summing contributions from single variables. Many earlier models calculate amplitudes as a function of one or two variables and/or restrict consideration to best-fit linear formulae. Our analysis systematically eliminates such models as candidates for a system that can generate appropriate movements for all possible initial conditions. The results of this study are stated in terms of properties of the response system. Certain axiom sets for the intrinsic organization of the response system obey these properties. We briefly provide one example of such an axiomatic model. The results presented in this article help to characterize the actual neural system for the control of rapid eye/head gaze shifts by showing that, in order to account for behavioral data, certain physical quantities must be represented in and used by the neural system. Our theoretical analysis generates predictions and identifies gaps in the data. We suggest needed experiments.     

Cursive writing with smooth pursuit eye movements

The eyes never cease to move: ballistic saccades quickly turn the gaze toward peripheral targets, whereas smooth pursuit maintains moving targets on the fovea where visual acuity is best. Despite the oculomotor system being endowed with exquisite motor abilities, any attempt to generate smooth eye movements against a static background results in saccadic eye movements [1, 2]. Although exceptions to this rule have been reported [3-5], volitional control over smooth eye movements is at best rudimentary. Here, I introduce a novel, temporally modulated visual display, which, although static, sustains smooth eye movements in arbitrary directions. After brief training, participants gain volitional control over smooth pursuit eye movements and can generate digits, letters, words, or drawings at will. For persons deprived of limb movement, this offers a fast, creative, and personal means of linguistic and emotional expression.     

A method for the chronic electromagnetic recording of eye and head movements in monkeys

A search-coil method for two-dimensional chronic registration of eye and head movements is described. The method is based on the analysis of the electromotive force induced by a magnetic field in a search coil. The output parameters of the original dual system were measured using both the standard search coil and that implanted into monkey's eye. The precision of eye movement recording was evaluated in real time. Standard deviation of spontaneous noise level for both channels was equal to 0.16 degrees (deg). The same parameters representing eye movement error during gaze fixation in the horizontal (in the range of -20/+20 deg) and vertical (in the range of -13/+13 deg) directions were equal to 0.27-0.38 and 0.23-0.31 deg, respectively. The obtained errors were comparable with the angular size of the peripheral target stimuli (0.20 deg), which had to be traced by an animal with saccadic movements.     

Does the brain use sliding variables for the control of movements?

Delays in the transmission of sensory and motor information prevent errors from being instantaneously available to the central nervous system (CNS) and can reduce the stability of a closed-loop control strategy. On the other hand, the use of a pure feedforward control (inverse dynamics) requires a perfect knowledge of the dynamic behavior of the body and of manipulated objects. Sensory feedback is essential both to accommodate unexpected errors and events and to compensate for uncertainties about the dynamics of the body. Experimental observations concerning the control of posture, gaze and limbs have shown that the CNS certainly uses a combination of closed-loop and open-loop control. Feedforward components of movement, such as eye saccades, occur intermittently and present a stereotyped kinematic profile. In visuo-manual tracking tasks, hand movements exhibit velocity peaks that occur intermittently. When a delay or a slow dynamics are inserted in the visuo-manual control loop, intermittent step-and-hold movements appear clearly in the hand trajectory. In this study, we investigated strategies used by human subjects involved in the control of a particular dynamic system. We found strong evidence for substantial nonlinearities in the commands produced. The presence of step-and-hold movements seemed to be the major source of nonlinearities in the control loop. Furthermore, the stereotyped ballistic-like kinematics of these rapid and corrective movements suggests that they were produced in an open-loop way by the CNS. We analyzed the generation of ballistic movements in the light of sliding control theory assuming that they occurred when a sliding variable exceeded a constant threshold. In this framework, a sliding variable is defined as a composite variable (a combination of the instantaneous tracking error and its temporal derivatives) that fulfills a specific stability criterion. Based on this hypothesis and on the assumption of a constant reaction time, the tracking error and its derivatives should be correlated at a particular time lag before movement onset. A peak of correlation was found for a physiologically plausible reaction time, corresponding to a stable composite variable. The direction and amplitude of the ongoing stereotyped movements seemed also be adjusted in order to minimize this variable. These findings suggest that, during visually guided movements, human subjects attempt to minimize such a composite variable and not the instantaneous error. This minimization seems to be obtained by the execution of stereotyped corrective movements. Received: 18 February 1997 / Accepted in revised form: 29 July 1997     

Detection and correction of erroneous eye movements by the brain error detector while tracking moving object

The models for research of function of the brain error detector of eye movements are described. The quantitative estimation of temporary parameters of erroneous eye saccades detection and correction of the healthy people is given. The data on distinctions in duration of the latency, speed and other parameters of erroneous and correctional saccades, are used for discussion on types of sensory signals used by the brain detector for revealing and correction of erroneous eye movements.     

An investigation of the mechanisms of eye movement control

Summary Feedback mechanisms exist in all the periferal sense organs including the eye, which acts as a highly efficient position control servo system. Histological studies so far have not revealed the precise circuitry of the eye movement control system but some information about it can be obtained by a study of the sources of feedback. Existing theories have considered three types of feedback originating in the oculomotor tract, in the proprioceptive fibres of the extrinsic eye muscles and from retinal image displacement. In the present experiments an optical arrangement has been used to vary or eliminate the amount of information available from retinal image motion, and the response of the eye to simple harmonic displacement of a target has been recorded. The response curves of gain (eyeball movement divided by target motion) against frequency indicate that the system is lion linear when the image falls in the retinal region which is insensitive to position. Outside this area, retinal image position is used as negative feedback but the information from the oculomotor tract must be regenerative. There is also evidence for feedback proportional to the first derivative of eyeball position and this function is ascribed to the proprioceptive signals; this form of feedback appears to saturate for large amplitude movements, thus avoiding heavy damping of the flick movements.A schematic eye movement control system having the same characteristics as the eye is proposed. The transfer function of this system indicates that it should be unstable if the sign of the retinal image feedback loop is reversed. Experiments with this form of feedback show that steady fixation is impossible and the eye performs a pendular nystagmus.     

Fixational Eye Movements in the Earliest Stage of Metazoan Evolution

All known photoreceptor cells adapt to constant light stimuli, fading the retinal image when exposed to an immobile visual scene. Counter strategies are therefore necessary to prevent blindness, and in mammals this is accomplished by fixational eye movements. Cubomedusae occupy a key position for understanding the evolution of complex visual systems and their eyes are assumedly subject to the same adaptive problems as the vertebrate eye, but lack motor control of their visual system. The morphology of the visual system of cubomedusae ensures a constant orientation of the eyes and a clear division of the visual field, but thereby also a constant retinal image when exposed to stationary visual scenes. Here we show that bell contractions used for swimming in the medusae refresh the retinal image in the upper lens eye of Tripedalia cystophora. This strongly suggests that strategies comparable to fixational eye movements have evolved at the earliest metazoan stage to compensate for the intrinsic property of the photoreceptors. Since the timing and amplitude of the rhopalial movements concur with the spatial and temporal resolution of the eye it circumvents the need for post processing in the central nervous system to remove image blur.     

Monitoring and control of action by the frontal lobes

Success requires deciding among alternatives, controlling the initiation of movements, and judging the consequences of actions. When alternatives are difficult to distinguish, habitual responses must be overcome, or consequences are uncertain, deliberation is necessary and a supervisory system exerts control over the processes that produce sensory-guided movements. We have investigated these processes by recording neural activity in the frontal lobe of macaque monkeys performing a countermanding task. Distinct neurons in the frontal eye field respond to visual stimuli or control the production of the movements. In the supplementary eye field and anterior cingulate cortex, neurons appear not to control directly movement initiation but instead signal the production of errors, the anticipation and delivery of reinforcement, and the presence of processing conflict. These signals form the core of current models of supervisory control of sensorimotor processes.     

Vision as oculomotor reward: cognitive contributions to the dynamic control of saccadic eye movements

Humans and other primates are equipped with a foveated visual system. As a consequence, we reorient our fovea to objects and targets in the visual field that are conspicuous or that we consider relevant or worth looking at. These reorientations are achieved by means of saccadic eye movements. Where we saccade to depends on various low-level factors such as a targets’ luminance but also crucially on high-level factors like the expected reward or a targets’ relevance for perception and subsequent behavior. Here, we review recent findings how the control of saccadic eye movements is influenced by higher-level cognitive processes. We first describe the pathways by which cognitive contributions can influence the neural oculomotor circuit. Second, we summarize what saccade parameters reveal about cognitive mechanisms, particularly saccade latencies, saccade kinematics and changes in saccade gain. Finally, we review findings on what renders a saccade target valuable, as reflected in oculomotor behavior. We emphasize that foveal vision of the target after the saccade can constitute an internal reward for the visual system and that this is reflected in oculomotor dynamics that serve to quickly and accurately provide detailed foveal vision of relevant targets in the visual field.     

Complex predictive eye pursuit in monkey: a model system for cerebellar studies of skilled movement

Smooth pursuit eye movements provide a good model system for cerebellar studies of complex motor control in monkeys. First, the pursuit system exhibits predictive control along complex trajectories and this control improves with training. Second, the flocculus/paraflocculus region of the cerebellum appears to generate this control. Lesions impair pursuit and neural activity patterns are closely related to eye motion during complex pursuit. Importantly, neural responses lead eye motion during predictive pursuit and lag eye motion during non-predictable target motions that require visual control. The idea that flocculus/paraflocculus predictive control is non-visual is also supported by a lack of correlation between neural activity and retinal image motion during pursuit. Third, biologically accurate neural network models of the flocculus/paraflocculus allow the exploration and testing of pursuit mechanisms. Our current model can generate predictive control without visual input in a manner that is compatible with the extensive experimental data available for this cerebellar system. Similar types of non-visual cerebellar control are likely to facilitate the wide range of other skilled movements that are observed.     

Whole-field integration, not detailed analysis, is used by the crab optokinetic system to separate rotation and translation in optic flow

For optimal visual control of compensatory eye movements during locomotion it is necessary to distinguish the rotational and translational components of the optic flow field. Optokinetic eye movements can reduce the rotational component only, making the information contained in the translational flow readily available to the animal. We investigated optokinetic eye rotation in the marble rock crab, Pachygrapsus marmoratus, during translational movement, either by displacing the animal or its visual surroundings. Any eye movement in response to such stimuli is taken as an indication that the system is unable to separate the translational and the rotational components in the optic flow in a mathematically perfect way. When the crabs are translated within a pseudo-natural environment, eye movements are negligible, especially during sideways translation. When, however, crabs were placed in a gangway between two elongated rectangular sidewalls carrying dotted patterns which were translated back and forth, marked eye movements were elicited, depending on the translational velocity. To resolve this discrepancy, we tested several hypotheses about mechanisms using detailed analysis of the optic flow or whole-field integration. We found that the latter are sufficient to explain the efficient separation of translation and rotation of crabs in quasi-natural situations. Accepted: 6 May 1997     

Optimal control of antagonistic muscle stiffness during voluntary movements

This paper presents a study on the control of antagonist muscle stiffness during single-joint arm movements by optimal control theory with a minimal effort criterion. A hierarchical model is developed based on the physiology of the neuromuscular control system and the equilibrium point hypothesis. For point-to-point movements, the model provides predictions on (1) movement trajectory, (2) equilibrium trajectory, (3) muscle control inputs, and (4) antagonist muscle stiffness, as well as other variables. We compared these model predictions to the behavior observed in normal human subjects. The optimal movements capture the major invariant characteristics of voluntary movements, such as a sigmoidal movement trajectory with a bell-shaped velocity profile, an N-shaped equilibrium trajectory, a triphasic burst pattern of muscle control inputs, and a dynamically modulated joint stiffness. The joint stiffness is found to increase in the middle of the movement as a consequence of the triphasic muscle activities. We have also investigated the effects of changes in model parameters on movement control. We found that the movement kinematics and muscle control inputs are strongly influenced by the upper bound of the descending excitation signal that activates motoneuron pools in the spinal cord. Furthermore, a class of movements with scaled velocity profiles can be achieved by tuning the amplitude and duration of this excitation signal. These model predictions agree with a wide body of experimental data obtained from normal human subjects. The results suggest that the control of fast arm movements involves explicit planning for both the equilibrium trajectory and joint stiffness, and that the minimal effort criterion best characterizes the objective of movement planning and control.     

Control of human eye movements: II. a model for the extraocular plant mechanism

Control of eye movements is essential in accomplishing visual or perceptive tasks. The brain and central nervous system process retinal information and send nervous signals to the extraocular muscles, which exert forces that cause the eye to move. A model for the human extraocular plant, which consists of the nervous input signals, the extraocular muscles, the orbit and the globe, is proposed. The derivation is based on anatomical and physiological data as well as experiments concerned with a variety of eye movements under normal and abnormal conditions. The nervous activity controlling eye movements was estimated from electromyography and single unit studies of the extraocular nuclei. The equations describing muscle properties were discussed in a previous paper by the authors; these results were incorporated into the present model. The characteristics of the isolated globe and its visco-elastic interaction with the orbit were computed from length- tension curves and isotonic experiments. Simulations using the resulting representation accurately depicted the isotonic experiments on the isolated globe and on the total extraocular plant, the isometric forces during three different types of eye movements, and the weighted globe experiment. A future paper will show that the model accurately simulates normal eye movements of different types and amplitudes.     

Detection of error eye movements by cerebral detector errors and their correction

The models of the investigation of the function of cerebral error detector of eye movements are described. The data which quantifies the temporal parameters of error detection and correction of eye saccades healthy people is stated. The data on the differences in the duration of the latent period, speed and other indicators of error and correct saccades is used to discuss the types of sensory signals used by brain detector for error detection and correction of eye movements.     

On the predominance of anti-compensatory eye movements in vestibular nystagmus

The predominance of anti-compensatory eye movements in vestibular nystagmus recorded during sinusoidal and post-rotational tests is interpreted in terms of a mathematical model of the vestibulo-ocular system. Namely, a direct pathway between the vestibular nuclei and the saccadic mechanism is assumed. In the range of frequencies of natural head movements this pathway carries on a signal proportional to head angular velocity. Therefore, during active head movements the saccadic mechanism is forced to produce quick eye rotations in the direction of head movement and, thus, to cooperate in the task of picking up visual targets outside the visual field. During passive head movements giving rise to nystagmus the assumed pathway contributes to reduce the error in eye resetting due to the saccadic delay. Analytical considerations and simulation results seem to prove the adequacy of the proposed model.Work supported by the National Research Council (C.N.R.), Rome, Italy     

The signal-flow diagram of the oculomotor control system, — and its transferability to the more intricate skeletomotor control system

A signal-flow diagram of the oculomotor control system has been derived which is able to describe the three modes of its action 1) pursuit movements 2) voluntary saccadic movements and 3) the passive non-innervated state of extraocular muscles which exists during sleep. It has been taken into consideration that in the smooth pursuit system there is a neural integrator in order to bring back to zero the error between the position of the eye and an external constant reference point. [Evidence for integration in oculomotor pathways we have from experiments by Cohen and Komatsuzaki (1972) who used stimulation of the pontine reticular formation.] All this is achieved by a system of variable structure with three states. In skeletomotor systems likewise there are smooth compensatory movements and voluntary movements and a state without any innervation. Some neurological diseases can be interpreted as an impairment of switching at special spots of the signal-flow diagram or as a disconnection of signalpathways, respectively. From this can be concluded that the signal-flow diagram derived for the rather lucid oculomotor control system should be able to describe the basic function of skeletomotor control systems, too.     

Application of linear system analysis to the horizontal vestibulo-ocular reflex of the alert rhesus monkey using pseudorandom binary sequence and single frequency sinusoidal stimulation

Horizontal eye movements of the alert rhesus monkey resulting from both pseudorandom binary sequence (PRBS) and single frequency sinusoidal rotational stimulation were analyzed using a PDP 11/40 computer in order to generate gain, phase, and coherence estimates at discrete frequencies between 0.008 and 1.28 Hz. A computer simulation of vestibular induced eye movements was used to validate our analysis procedures and to determine the effects of digital noise. Frequency domain transfer functions derived from gain and phase estimates revealed that the responses to PRBS stimulation and to single frequency sinusoids were not appreciably different. PRBS testing was accomplished in approximately one third the time required for sinusoidal testing and yielded highly reproducible data. We conclude that PRBS stimulation is a reliable and efficient method for assessing linear system parameters of the horizontal vestibulo-ocular reflex. PRBS testing may be particularly and advantageous in studies of vestibulo-oculomotor plasticity in which rapid assessment of alterations in system dynamics is essential.