Saccadic adaptation to moving targets

Saccades are so called ballistic movements which are executed without online visual feedback. After each saccade the saccadic motor plan is modified in response to post-saccadic feedback with the mechanism of saccadic adaptation. The post-saccadic feedback is provided by the retinal position of the target after the saccade. If the target moves after the saccade, gaze may follow the moving target. In that case, the eyes are controlled by the pursuit system, a system that controls smooth eye movements. Although these two systems have in the past been considered as mostly independent, recent lines of research point towards many interactions between them. We were interested in the question if saccade amplitude adaptation is induced when the target moves smoothly after the saccade. Prior studies of saccadic adaptation have considered intra-saccadic target steps as learning signals. In the present study, the intra-saccadic target step of the McLaughlin paradigm of saccadic adaptation was replaced by target movement, and a post-saccadic pursuit of the target. We found that saccadic adaptation occurred in this situation, a further indication of an interaction of the saccadic system and the pursuit system with the aim of optimized eye movements.     

Which retinal and extra-retinal information is crucial for circular vection?

In contradistinction to conventional wisdom, we propose that retinal image slip of a visual scene (optokinetic pattern, OP) does not constitute the only crucial input for visually induced percepts of self-motion (vection). Instead, the hypothesis is investigated that there are three input factors: 1) OP retinal image slip, 2) motion of the ocular orbital shadows across the retinae, and 3) smooth pursuit eye movements (efference copy). To test this hypothesis, we visually induced percepts of sinusoidal rotatory self-motion (circular vection, CV) in the absence of vestibular stimulation. Subjects were presented with three concurrent stimuli: a large visual OP, a fixation point to be pursued with the eyes (both projected in superposition on a semi-circular screen), and a dark window frame placed close to the eyes to create artificial visual field boundaries that simulate ocular orbital rim boundary shadows, but which could be moved across the retinae independent from eye movements. In different combinations these stimuli were independently moved or kept stationary. When moved together (horizontally and sinusoidally around the subject's head), they did so in precise temporal synchrony at 0.05 Hz. The results show that the occurrence of CV requires retinal slip of the OP and/or relative motion between the orbital boundary shadows and the OP. On the other hand, CV does not develop when the two retinal slip signals equal each other (no relative motion) and concur with pursuit eye movements (as it is the case, e.g., when we follow with the eyes the motion of a target on a stationary visual scene). The findings were formalized in terms of a simulation model. In the model two signals coding relative motion between OP and head are fused and fed into the mechanism for CV, a visuo-oculomotor one, derived from OP retinal slip and eye movement efference copy, and a purely visual signal of relative motion between the orbital rims (head) and the OP. The latter signal is also used, together with a version of the oculomotor efference copy, for a mechanism that suppresses CV at a later stage of processing in conditions in which the retinal slip signals are self-generated by smooth pursuit eye movements.     

Learned timing of motor behavior in the smooth eye movement region of the frontal eye fields

Proper timing is a critical aspect of motor learning. We report a relationship between a representation of time and an expression of learned timing in neurons in the smooth eye movement region of the frontal eye fields (FEF(SEM)). During prelearning pursuit of target motion at a constant velocity, each FEF(SEM) neuron is most active at a distinct time relative to the onset of pursuit tracking. In response to an instructive change in target direction, a neuron expresses the most learning when the instruction occurs near the time of its maximal participation in prelearning pursuit. Different neurons are most active, and undergo the most learning, at distinct times during pursuit. We suggest that the representation of time in the FEF(SEM) drives learning that is temporally linked to an instructive change in target motion, and that this may be a general function of motor areas of the cortex.     

Contrast dependence of smooth pursuit eye movements following a saccade to superimposed targets

Dorsal stream areas provide motion information used by the oculomotor system to generate pursuit eye movements. Neurons in these areas saturate at low levels of luminance contrast. We therefore hypothesized that during the early phase of pursuit, eye velocity would exhibit an oculomotor gain function that saturates at low luminance contrast. To test this, we recorded eye movements in two macaques trained to saccade to an aperture in which a pattern of dots moved left or right. Shortly after the end of the saccade, the eyes followed the direction of motion with an oculomotor gain that increased with contrast before saturating. The addition of a second pattern of dots, moving in the opposite direction and superimposed on the first, resulted in a rightward shift of the contrast-dependent oculomotor gain function. The magnitude of this shift increased with the contrast of the second pattern of dots. Motion was nulled when the two patterns were equal in contrast. Next, we varied contrast over time. Contrast differences that disappeared before saccade onset biased post-saccadic eye movements at short latency. Changes in contrast occurring during or after saccade termination did not influence eye movements for approximately 150 ms. Earlier studies found that eye movements can be explained by a vector average computation when both targets are equal in contrast. We suggest that this averaging computation may reflect a special case of divisive normalization, yielding saturating contrast response functions that shift to the right with opposed motion, averaging motions when targets are equated in contrast.     

The composition of central programs subserving horizontal eye movements in man

A hypothesis is presented which describes, in biomechanical terms, the central programs underlying horizontal eye movements in man. It is suggested that eye movements are produced by means of programmed shifts of the so-called invariant muscle characteristics (static force vs angle of gaze). These shifts lead to a change of the equilibrium point resulting from the interaction of agnnist and antagonist muscles and, as a consequence, to movement and the attainment of a new position of gaze. A reciprocal or a coactivation command to agonist and antagonist muscles occurs when their characteristics shift with respect to the coordinate in the same or opposite directions, respectively. It is proposed that during pursuit and saccadic eye movements a supperposition of the both central commands occurs. During a saccade, the reciprocal command develops evenly up to a certain level. The initial and final levels of the reciprocal command dictate the respective position of gaze and therefore the size of the saccade. The coactivation command develops to a maximum level and is slowly switched off when the new position of gaze has been achieved. The magnitude of the coactivation command seems to be not connected with an absolute position of gaze. It provides probably a stability of the movement and, in particular, prevents overshoot and oscillation during the saccade. The same timing of these commands occurs during pursuit movements, but the magnitude of the coactivation command and the rates of the development of the both commands are less in this case and correlate with the velocity of the movement. This hypothesis enables the tension changes in the muscle during saccadic and pursuit movements to be simulated in qualitative accordance with unique experimental data obtained by Collins et al. (1975). The functional significance of superposition of these motor commands and similarity in the efferent organization of eye and limb movements are discussed.     

Motion Control in Saccade and Smooth Pursuit for Bionic Eye Based on Three-dimensional Coordinates

Saccade and smooth pursuit are two important functions of human eye.In order to enable bionic eye to imitate the two functions,a control method that implements saccade and smooth pursuit based on the three-dimensional coordinates of target is proposed.An optimal observation position is defined for bionic eye based on three-dimensional coordinates.A kind of motion planning method with high accuracy is developed.The motion parameters of stepper motor consisting of angle acceleration and turning time are computed according to the position deviation,the target's angular velocity and the stepper motor's current angular velocity in motion planning.The motors are controlled with the motion parameters moving to given position with desired angular velocity in schedule time.The experimental results show that the bionic eye can move to optimal observation positions in 0.6 s from initial location and the accuracy of 3D coordinates is improved.In addition,the bionic eye can track a target within the error of less than 20 pixels based on three-dimensional coordinates.It is verified that saccade and smooth pursuit of bionic eye based on three-dimensional coordinates are feasible.     

New methods for removing saccades in analysis of smooth pursuit eye movement

New computation methods for removing saccades in analysis of smooth pursuit eye movement characteristics were developed. They have removed saccades more completely than previous methods, and were very effective especially for noisy data recorded by the EOG method. The fully developed method was applicable to eye movement data in tracking of pseudo-random target movement as well as deterministic target movement. Furthermore, the methods were also useful for extracting the number and magnitudes of saccades more precisely.     

The representation of time for motor learning

We have identified factors that control precise motor timing by studying learning in smooth pursuit eye movements. Monkeys tracked a target that moved horizontally for a fixed time interval before changing direction through the addition of a vertical component of motion. After repeated presentations of the same target trajectory, infrequent probe trials of purely horizontal target motion evoked a vertical eye movement around the time when the change in target direction would have occurred. The pursuit system timed the vertical eye movement by keeping track of the duration of horizontal target motion and by measuring the distance the target traveled before changing direction, but not by learning the position in space where the target changed direction. We conclude that high temporal precision in motor output relies on multiple signals whose contributions to timing vary according to task requirements.     

Rhythmic movement and rhythmic auditory cues enhance anticipatory postural adjustment of gait initiation

Abstract

The purpose of this study was to determine whether the rhythmic movements or cues enhance the anticipatory postural adjustment (APA) of gait initiation. Healthy humans initiated gait in response to an auditory start cue (third cue). A first auditory cue was given 8?s before the start cue, and a second auditory cue was given 3?s before the start cue. The participants performed the rhythmic medio-lateral weight shift (ML-WS session), rhythmic anterior-posterior weight shift (AP-WS session), or rhythmic arm swing (arm swing session) in the time between the first and second cues. In the rhythmic cues session, rhythmic auditory cues with a frequency of 1?Hz were given in this time. In the stationary session, the participants maintained stationary stance in this time. The APA and initial step movement preceded by those rhythmic movements or cues were compared with those in the stationary session. The temporal characteristics of the initial step movement of the gait initiation were not changed by the rhythmic movements or cues. The medio-lateral displacement of the APA in the ML-WS and arm swing sessions was significantly greater than that in the stationary session. The anterior–posterior displacement of the APA in the rhythmic cues and arm swing sessions was significantly greater than that in the stationary session. Taken together, the rhythmic movements and cues enhance the APA of gait initiation. The present finding may be a clue or motive for the future investigation for using rhythmic movements or cues as the preparatory activity to enlarge the small APA of gait initiation in the patients with Parkinson’s disease.     

Tuning Properties of MT and MSTd and Divisive Interactions for Eye-Movement Compensation

The primate brain intelligently processes visual information from the world as the eyes move constantly. The brain must take into account visual motion induced by eye movements, so that visual information about the outside world can be recovered. Certain neurons in the dorsal part of monkey medial superior temporal area (MSTd) play an important role in integrating information about eye movements and visual motion. When a monkey tracks a moving target with its eyes, these neurons respond to visual motion as well as to smooth pursuit eye movements. Furthermore, the responses of some MSTd neurons to the motion of objects in the world are very similar during pursuit and during fixation, even though the visual information on the retina is altered by the pursuit eye movement. We call these neurons compensatory pursuit neurons. In this study we develop a computational model of MSTd compensatory pursuit neurons based on physiological data from single unit studies. Our model MSTd neurons can simulate the velocity tuning of monkey MSTd neurons. The model MSTd neurons also show the pursuit compensation property. We find that pursuit compensation can be achieved by divisive interaction between signals coding eye movements and signals coding visual motion. The model generates two implications that can be tested in future experiments: (1) compensatory pursuit neurons in MSTd should have the same direction preference for pursuit and retinal visual motion; (2) there should be non-compensatory pursuit neurons that show opposite preferred directions of pursuit and retinal visual motion.     

The reliability of eye movement quantitative evaluation. II. Smooth pursuit eye movements.

We adopted the estimate of the intraclass coefficient of reliability, R, to evaluate the reliability of smooth pursuit eye movement quantitative analysis. At a one-week interval, we recorded twice smooth pursuit eye movements from fifteen healthy subjects by means of the binocular electrooculographic technique. R was computed for the constant and the slope of the target velocity/gain relationships. R values were rated good for the slope and excellent for the constant. Finally, we computed for each parameter the maximum variability value according to two differing methods; on the basis of the within-subjects mean square values, we defined the normal range of biological test-retest variability for the two parameters.     

Manual chronostasis: tactile perception precedes physical contact

When saccading to a silent clock, observers sometimes think that the second hand has paused momentarily. This effect has been termed chronostasis and occurs because observers overestimate the time that they have seen the object of an eye movement. They seem to extrapolate its appearance back to just prior to the onset of the saccade rather than the time that it is actually fixated on the retina. Here, we describe a similar effect following an arm movement: subjects overestimate the time that their hand has been in contact with a newly touched object. The illusion's magnitude suggests backward extrapolation of tactile perception to a moment during the preceding reach. The illusion does not occur if the arm movement triggers a change in a continuously visible visual target: the time of onset of the change is estimated correctly. We hypothesize that chronostasis-like effects occur when movement produces uncertainty about the onset of a sensory event. Under these circumstances, the time at which neurons with receptive fields that shift in the temporal vicinity of a movement change their mappings may be used as a time marker for the onset of perceptual properties that are only established later.     

Manipulating intent: evidence for a causal role of the superior colliculus in target selection

The superior colliculus (SC) is well known for its role in the motor control of saccades. Recent work has shown that it also plays a role in the selection of saccades, but a causal role in the process of target selection has not been demonstrated. We applied subthreshold microstimulation to the SC while monkeys performed a task requiring them to select a stimulus as the target for a pursuit or saccade movement. Stimulation increased the proportion of selections toward the stimulus that appeared contralateral to the site of stimulation and also decreased their latencies. For pursuit, this stimulation-induced contralateral response bias was with respect to the initial target location and not the direction of eye movement, demonstrating a causal effect on target choice distinct from any effect on motor preparation. These results show that the SC helps decide the object of the next movement, beyond its traditional responsibility of saccade production.     

Anticipatory postural adjustments to arm movement reveal complex control of paraspinal muscles in the thorax

Anatomical and empirical data suggest that deep and superficial muscles may have different functions for thoracic spine control. This study investigated thoracic paraspinal muscle activity during anticipatory postural adjustments associated with arm movement. Electromyographic (EMG) recordings were made from the right deep (multifidus/rotatores) and superficial (longissimus) muscles at T5, T8, and T11 levels using fine-wire electrodes. Ten healthy participants performed fast unilateral and bilateral flexion and extension arm movements in response to a light. EMG amplitude was measured during 25 ms epochs for 150 ms before and 400 ms after deltoid EMG onset. During arm flexion movements, multifidus and longissimus had two bursts of activity, one burst prior to deltoid and a late burst. With arm extension both muscles were active in a single burst after deltoid onset. There was differential activity with respect to direction of trunk rotation induced by arm movement. Right longissimus was most active with left arm movements and right multifidus was most active with right arm movements. All levels of the thorax responded similarly. We suggest that although thoracic multifidus and longissimus function similarly to control sagittal plane perturbations, these muscles are differentially active with rotational forces on the trunk.     

Influences of illusionary position perception on anticipatory postural control associated with arm flexion

We examined the effect of illusionary perception on anticipatory postural control associated with arm flexion with subjects in a standing position, using vibration stimulation of the Achilles’ tendon. Arm flexion was performed five times under each of the following conditions: (1) quiet standing, (2) vibration of the Achilles’ tendon at 100 Hz frequency and 1.5 mm amplitude with the trunk fixed by a stopper during quiet standing, and (3) a perceived standing position during vibration. The reproduced positions were located forward by about 20% of the foot length compared with the quiet standing position; these positions showed no significant differences among the five trials. In the first trial of arm flexion during vibration, the biceps femoris began activating approximately 40 ms before the anterior deltoid. The same time difference between activation of the two muscles was observed in the reproduced condition. As the vibration trials were repeated, this activation timing approached the value in the quiet standing condition. In both the biceps femoris and erector spinae, the mean amplitude of electromyogram for the first 50 ms after the start of activation did not differ significantly among the three conditions.     

The effect of the visual stimulation of the leading and nonleading eye on the value of the saccadic latency period and on the peak latency of rapid presaccadic potentials]

Visual targets were presented monocularly to the leading and nonleading eyes. The complex of rapid positive and negative potentials was studied using the reverse summation from the onset of saccades. The latencies of saccades and peak latencies of the averaged presaccadic potentials were measured. The dependence of the saccade latencies and peak latencies of the complex of potentials on stimulation of the leading or nonleading eye and saccade direction was not simple and was largely determined by the individual profile of asymmetry. It is suggested that during stimulation of the leading eye the processes of attention fixation and switching as well as of the space visual processing are faster than during stimulation of the nonleading eye. Thus, the leading role of the right eye is reflected not only in fixation processes but also in movement anticipation.     

Optimal response of eye and hand motor systems in pointing at a visual target

In a task requiring an optimal hand pointing (with regards to both time and accuracy) at a peripheral target, there is first a saccade of the eye within 250 ms, followed 100 ms later by the hand movement. However the latency of the hand movement is poorly correlated with that of the eye movement. When the peripheral target is cut off at the onset of the saccade, there is no correlation between the error of the gaze position and the error of the hand pointing. This suggests an early parallel processing of the two motor outputs. The duration of hand movement does not change significantly when subjects either see or not see their hand (closed or open loop). In the open loop situation, the undershoot of the hand pointing increases with target eccentricity, whatever the subjects are allowed or not to do a saccade toward the target. It suggests that the encoding of eye position by itself is a poor index for an accurately guided movement of the hand.     

Cross axis VOR induced by pursuit training in monkeys: further properties of adaptive responses

We have shown recently in alert monkeys that repeated interaction between the pursuit and vestibular systems in the orthogonal plane induces adaptive changes in the VOR. To examine further properties of adaptive cross axis VOR induced by pursuit training, sinusoidal whole body rotation was applied either in the pitch or yaw plane while presenting a target spot that moved orthogonally to the rotation plane with either 90 degrees phase-lead or 90 degrees phase-lag to the chair signal. After one hour of training at 0.5 Hz (+/- 10 degrees), considerable phase-shift was observed in orthogonal eye movement responses consistent with the training paradigms by identical chair rotation in complete darkness, with further lead at lower frequencies and lag at higher frequencies. However, gains (eye/chair) induced by phase- shift pursuit training was different during pitch and yaw rotation. Although frequency tuning was maintained during pitch in the phase-shift paradigms, it was not maintained during yaw, resulting in higher gains at lower stimulus frequencies compared to the gains during yaw. This difference may reflect otolith contribution during pitch rotation. To understand further the nature of signals that induce adaptive cross axis VOR, we examined interaction of pursuit, whole field-visual pattern and vestibular stimuli. Magnitudes of the cross axis VOR with a spot alone on one hand and with a spot and pattern moving together in the same plane on the other during chair rotation were similar, and when one of the two visual stimuli was stationary during chair rotation, our well trained monkeys did not induce the cross axis VOR. These results suggest that the cross axis VOR induced by pursuit training shares common mechanisms with the cross axis VOR induced by whole field-slip stimuli and that if conflicting information is given between the two visual stimuli, adaptive changes are inhibited. Horizontal GVPs were recorded in the cerebellar floccular lobe during pitch rotation coupled with horizontal pursuit stimuli. These GVPs did not respond to pitch in the dark before training, but responded after 60 min of pursuit training with eye velocity sensitivities similar to those before training. Adaptive change in the VOR was specific to smooth eye movements but not to saccades in our paradigms.     

Active sensing in a freely walking spider: look where to go

The Central American hunting spider Cupiennius salei, like most other spiders, has eight eyes, one pair of principal eyes and three pairs of secondary eyes. The principal eyes and one pair of the secondary eyes have almost completely overlapping visual fields, and presumably differ in function. The retinae of the principal eyes can be moved independently by two pairs of eye muscles each, whereas the secondary eyes do not have such eye muscles. The behavioural relevance of retinal movements of freely moving spiders was investigated by a novel dual-channel telemetric registration of the eye muscle activities. Walking spiders shifted the ipsilateral retina with respect to the walking direction before, during and after a turning movement. The change in the direction of vision in the ipsilateral anterior median eye was highly correlated with the walking direction, regardless of the actual light conditions. The contralateral retina remained in its resting position. This indicates that Cupiennius salei shifts it visual field in the walking direction not only during but sometimes previous to an intended turn, and therefore “peers” actively into the direction it wants to turn.     

Influence of gravity compensation on muscle activity during reach and retrieval in healthy elderly

IntroductionArm support like gravity compensation may improve arm movements during stroke rehabilitation. It is unknown how gravity compensation affects muscle activation patterns during reach and retrieval movements. Since muscle activity during reach is represented by a component varying with movement velocity and a component supposedly counteracting gravity, we hypothesized that gravity compensation decreases the amplitude of muscle activity, but does not affect the pattern. To examine this, we compared muscle activity during well defined movements with and without gravity compensation in healthy elderly.MethodsTen subjects performed reach and retrieval movements with and without gravity compensation. Muscle activity of biceps, triceps, anterior, middle and posterior parts of deltoid and upper trapezius was compared between the two conditions.ResultsThe level of muscle activity was lower with gravity compensation in all muscles, reaching significance in biceps, anterior deltoid and trapezius (p ? 0.026). The muscle activation pattern did not differ between movements with and without gravity compensation (p ? 0.662).DiscussionGravity compensation only influenced the level of muscle activity but not the muscle activation pattern in terms of timing. Future studies should examine if the influence of gravity compensation is comparable for stroke patients. This may stimulate early and intensive training during rehabilitation.