The proprioceptive representation of eye position in monkey primary somatosensory cortex

The cerebral cortex must have access to an eye position signal, as humans can report passive changes in eye position in total darkness, and visual responses in many cortical areas are modulated by eye position. The source of this signal is unknown. Here we demonstrate a representation of eye position in monkey primary somatosensory cortex, in the representation of the trigeminal nerve, near cells with a tactile representation of the contralateral brow. The neurons have eye position signals that increase monotonically with increasing orbital eccentricity from near the center of gaze, with directionally selectivity tuned in a Gaussian manner. All directions of eye position are represented in a single hemisphere. The signal is proprioceptive, because it can be obliterated by anesthetizing the contralateral orbit. It is not related to foveal or peripheral visual stimulation, and it represents the position of the eye in the head and not the angle of gaze in space.     

Eye movements evoked by proprioceptive stimulation along the body axis in humans

Proprioceptive input arising from torsional body movements elicits small reflexive eye movements. The functional relevance of these eye movements is still unknown so far. We evaluated their slow components as a function of stimulus frequency and velocity. The horizontal eye movements of seven adult subjects were recorded using an infrared device, while horizontal rotations were applied at three segmental levels of the body [i.e., between head and shoulders (neck stimulus), shoulders and pelvis (trunk stimulus), and pelvis and feet (leg stimulus)]. The following results were obtained: (1) Sinusoidal leg stimulation evoked an eye response with the slow component in the direction of the movement of the feet, while the response to trunk and neck stimulation was oriented in the opposite direction (i.e., in that of the head). (2) In contrast, the gain behavior of all three responses was similar, with very low gain at mid- to high frequencies (tested up to 0.4 Hz) but increasing gain at low frequencies (down to 0.0125 Hz). We show that this gain behavior is mainly due to a gain nonlinearity for low angular velocities. (3) The responses were compatible with linear summation when an interaction series was tested in which the leg stimulus was combined with a vestibular stimulus. (4) There was good correspondence of the median gain curves when eye responses were compared with psychophysical responses (perceived body rotation in space; additionally recorded in the interaction series). However, correlation of gain values on a single-trial basis was poor. (5) During transient neck stimulation (smoothed position ramp), the neck response noticeably consisted of two components – an initial head-directed eye shift (phasic component) followed by a shift in the opposite direction (compensatory tonic component). Both leg and neck responses can be described by one simple, dynamic model. In the model the proprioceptive input is fed into the gaze network via two pathways which differ in their dynamics and directional sign. The model simulates either leg or neck responses by selecting an appropriate weight for the gain of one of the pathways (phasic component). The interaction results can also be simulated when a vestibular path is added. This model has similarities to one we recently proposed for human self-motion perception and postural control. A major difference, though, is that the proprioceptive input to the gaze-stabilizing network is weak (restricted to low velocities), unlike that used for perception and postural control. We hold that the former undergoes involution during ontogenesis, as subjects depend on the functionally more appropriate vestibulo-ocular reflex. Yet, the weak proprioceptive eye responses that remain may have some functional relevance. Their tonic component tends to stabilize the eyes by slowly shifting them toward the primary head position relative to the body support. This applies solely to the earth-horizontal plane in which the vestibular signal has no static sensitivity. Received: 10 October 1997 / Accepted: 22 January 1998     

Eye position modulates the electromyographic responses of neck muscles to electrical stimulation of the superior colliculus in the alert cat

Rapid gaze shifts are often accomplished with coordinated movements of the eyes and head, the relative amplitude of which depends on the starting position of the eyes. The size of gaze shifts is determined by the superior colliculus (SC) but additional processing in the lower brain stem is needed to determine the relative contributions of eye and head components. Models of eye–head coordination often assume that the strength of the command sent to the head controllers is modified by a signal indicative of the eye position. Evidence in favor of this hypothesis has been recently obtained in a study of phasic electromyographic (EMG) responses to stimulation of the SC in head-restrained monkeys (Corneil et al. in J Neurophysiol 88:2000–2018, 2002b). Bearing in mind that the patterns of eye–head coordination are not the same in all species and because the eye position sensitivity of phasic EMG responses has not been systematically investigated in cats, in the present study we used cats to address this issue. We stimulated electrically the intermediate and deep layers of the caudal SC in alert cats and recorded the EMG responses of neck muscles with horizontal and vertical pulling directions. Our data demonstrate that phasic, short latency EMG responses can be modulated by the eye position such that they increase as the eye occupies more and more eccentric positions in the pulling direction of the muscle tested. However, the influence of the eye position is rather modest, typically accounting for only 10–50% of the variance of EMG response amplitude. Responses evoked from several SC sites were not modulated by the eye position.     

How humans combine simultaneous proprioceptive and visual position information

To enable us to study how humans combine simultaneously present visual and proprioceptive position information, we had subjects perform a matching task. Seated at a table, they placed their left hand under the table concealing it from their gaze. They then had to match the proprioceptively perceived position of the left hand using only proprioceptive, only visual or both proprioceptive and visual information. We analysed the variance of the indicated positions in the various conditions. We compared the results with the predictions of a model in which simultaneously present visual and proprioceptive position information about the same object is integrated in the most effective way. The results are in disagreement with the model: the variance of the condition with both visual and proprioceptive information is smaller than expected from the variances of the other conditions. This means that the available information was integrated in a highly effective way. Furthermore, the results suggest that additional information was used. This information might have been visual information about body parts other than the fingertip or it might have been visual information about the environment.     

Organismic,stimulus and task determinants of phasic and tonic heart rate and skin conductance changes

The effects of three variables (perceptual style ‘leveling-sharpening’, stimulus intensity and task) on tonic and phasic heart rate and skin conductance responses were studied. 24 levelers and 24 sharpeners received a series of 25 auditory stimuli under two instructions (task): in one task the subject had to listen carefully to possible small differences between tones (intake task) and in the other task the subject had to count back silently and could neglect the tones (rejection task). One third of each group received tones of 50 dB, one third 75 dB and one third 100 dB. Results show that task has a strong effect on tonic responses in the direction predicted from Lacey's intake-rejection hypothesis. There is hardly any effect of task on phasic responses: only at the first trial heart rate tends to decelerate more in the intake than in the rejection task. Stimulus intensity does not influence tonic responses but very strongly affects phasic responses: deceleration to 50 and 75 dB, acceleration to 100 dB. GSR amplitude increases monotonically as a function of stimulus intensity. There is no difference between levelers and sharpeners with regard to tonic responses. Concerning phasic heart rate, sharpeners decelerate more than levelers and levelers have greater GSRs than sharpeners. It is suggested that tonic changes are relatively more under cognitive control, whereas phasic changes are more of a non-cognitive reflex-like nature. A relationship between the distinction arousal-activation as proposed by Pribram and McGuinness (1975) and the results of this experiment was suggested.     

Interaction between gaze and visual and proprioceptive position judgements

There is considerable evidence that targets for action are represented in a dynamic gaze-centered frame of reference, such that each gaze shift requires an internal updating of the target. Here, we investigated the effect of eye movements on the spatial representation of targets used for position judgements. Participants had their hand passively placed to a location, and then judged whether this location was left or right of a remembered visual or remembered proprioceptive target, while gaze direction was varied. Estimates of position of the remembered targets relative to the unseen position of the hand were assessed with an adaptive psychophysical procedure. These positional judgements significantly varied relative to gaze for both remembered visual and remembered proprioceptive targets. Our results suggest that relative target positions may also be represented in eye-centered coordinates. This implies similar spatial reference frames for action control and space perception when positions are coded relative to the hand.     

Eye-head-hand coordination in pointing at visual targets: spatial and temporal analysis

This study investigated whether the execution of an accurate pointing response depends on a prior saccade orientation towards the target, independent of the vision of the limb. A comparison was made between the accuracy of sequential responses (in which the starting position of the hand is known and the eye centred on the target prior to the onset of the hand pointing movement) and synergetic responses (where both hand and gaze motions are simultaneously initiated on the basis of unique peripheral retinal information). The experiments were conducted in visual closed-loop (hand visible during the pointing movement) and in visual openloop conditions (vision of hand interrupted as the hand started to move). The latter condition eliminated the possibility of a direct visual evaluation of the error between hand and target during pointing. Three main observations were derived from the present work: (a) the timing of coordinated eye-head-hand pointing at visual targets can be modified, depending on the executed task, without a deterioration in the accuracy of hand pointing; (b) mechanical constraints or instructions such as preventing eye, head or trunk motion, which limit the redundancy of degrees of freedom, lead to a decrease in accuracy; (c) the synergetic movement of eye, head and hand for pointing at a visible target is not trivially the superposition of eye and head shifts added to hand pointing. Indeed, the strategy of such a coordinated action can modify the kinematics of the head in order to make the movements of both head and hand terminate at approximately the same time. The main conclusion is that eye-head coordination is carried out optimally by a parallel processing in which both gaze and hand motor responses are initiated on the basis of a poorly defined retinal signal. The accuracy in hand pointing is not conditioned by head movement per se and does not depend on the relative timing of eye, head and hand movements (synergetic vs sequential responses). However, a decrease in the accuracy of hand pointing was observed in the synergetic condition, when target fixation was not stabilised before the target was extinguished. This suggests that when the orienting saccade reaches the target before hand movement onset, visual updating of the hand motor control signal may occur. A rapid processing of this final input allows a sharper redefinition of the hand landing point.     

Comparing limb proprioception and oculomotor signals during hand-guided saccades

We previously showed that saccades tend to overshoot briefly flashed targets that were manually displaced in the dark (Ren et al. 2006). However it was not clear if the overshoot originated from a sensory error in measuring hand displacement or from a premotor error in saccade programming, because gaze and hand position started at the same central position. Here, we tested between these hypotheses by dissociating the initial eye and hand position. Five hand/target positions (center, far, near, right, left) on a frontally-placed horizontal surface were used in four paradigms: Center or Peripheral Eye-hand Association (CA or PA, both gaze and right hand started from the center or a same peripheral location) and Hand or Eye Dissociation (HD or ED, hand or gaze started from one of three non-target peripheral locations). Subjects never received any visual feedback about the final target location and the subjects’ hand displacement. In the CA paradigm, subjects showed the same overshoot that we showed previously. However, changing both initial eye and hand positions relative to the final target (PA) affected the pattern, significantly altering the directions of overshoots. Changing only the initial position of hand (HD) did not have this effect, whereas changing only initial eye position (ED) had the same effect as the PA condition (CA ≈ HD, PA ≈ ED). Furthermore, multiple regression analysis showed that the direction of the ideal saccade contributed significantly to the endpoint direction error, not the direction of the hand path. These results suggest that these errors do not primarily arise from misestimates of the hand trajectory, but rather from a process of comparing the initial eye position and the limb proprioceptive signal during saccade programming.     

Human gaze shifts in which head and eyes are not initially aligned

Most studies of rapid orienting gaze shifts generated by combined eye and head movements have focused on an experimental condition in which gaze displacements are started with the subject's eyes in the normal straight-ahead position in the orbit. Such an experimental approach does not permit a clear identification of the input signal to the head motor system, because target offset angle is the same for both the eye and head. We have studied gaze shifts in human subjects which began with the visual axis straight ahead relative to the body (i.e., gaze or line of sight aligned with body sagittal plane) and with head offset from straight ahead at various angular positions. In our experimental conditions, the amplitude of head movement during a gaze shift was nearly equal to the angular distance between the target position and the starting head position (target-re-head), even though subjects were not specifically instructed to move their heads. This observation contrasts with other published reports in the literature showing considerable varibility amongst subjects in the amplitude of head rotation within a given task and between tasks. The difference may be related to the initial conditions which required subjects to align the eye and head on specific starting targets, since others have shown that requiring head alignment enhances head displacement. The amplitude of the saccadic eye movement was not determined by either the target's position relative to the starting eye or head positions. The value that best described the eye movement amplitude was the eye position in the orbit at the end of the saccade. This was nearly equal to target-rehead until a saturation eye position in the orbit was attained.     

Neuronal responses in macaque area PEc to saccades and eye position

Neurons in area PEc in the superior parietal cortex encode signals from different modalities, such as visual, extraretinal and somatosensory, probably combining them to encode spatial parameter of extrapersonal space to prepare body movements. This study reports the characterization of the functional properties of PEc non-visual neurons that showed saccade-related activity. We analyzed the pre- and post-saccadic firing activity in 189 neurons recorded in five hemispheres of three behaving monkeys. Spiking activity of PEc single neurons was recorded while the monkeys performed visually-guided saccades in a reaction time task. We found that 84% of neurons recorded from area PEc showed pre-saccadic activity with directional tuning. In 26% of neurons, we found inhibition of activity in the pre-saccadic period. The onset of this "pause" always started before the saccade and, in 51% of neurons, it was invariant among different gaze directions. The post-saccadic activity in these cells was either a phasic response with directional tuning (77%) and/or an eye position tuning (75%). The analysis of the preferred direction did not show hemispheric preference, however, for the majority of neurons, the angular difference in the preferred direction, in the pre- and post-saccadic period, was more than 60 degrees . By confirming, therefore, that PEc neurons carry information about eye position, these novel findings open new horizons on PEc function that, to date, is not well documented. The pre-saccadic activity may reflect an involvement in saccade control, whereas post-saccadic activity may indicate a role in informing on the new eye position. These novel results about saccade and eye position processing may imply a role of area PEc in gaze direction mechanisms and, possibly, in remapping visual space after eye movements.     

Neck muscle responses to stimulation of monkey superior colliculus. I. Topography and manipulation of stimulation parameters

The role of the primate superior colliculus (SC) in orienting head movements was studied by recording electromyographic (EMG) activity from multiple neck muscles following electrical stimulation of the SC. Combining SC stimulation with neck EMG recordings provides an objective and sensitive measure of the SC drive onto neck muscle motoneurons, particularly in relation to evoked gaze shifts. In this paper, we address how neck EMG responses to SC stimulation in head-restrained monkeys depend on the rostrocaudal, mediolateral, and dorsoventral location of the stimulating electrode within the SC and vary with manipulations of the eye position prior to stimulation onset and changes in stimulation current and duration. Stimulation predominantly evoked EMG responses on the muscles obliquus capitis inferior, rectus capitis posterior major, and splenius capitis. These responses became larger in magnitude and shorter in onset latency for progressively more caudal stimulation locations, consistent with turning the head. However, evoked responses persisted even for more rostral stimulation locations usually not associated with head movements. Manipulating initial eye position revealed that the magnitude of evoked responses became stronger as the eyes attained positions contralateral to the side of stimulation, consistent with a summation between a generic command evoked by SC stimulation and the influence of eye position on tonic neck EMG. Manipulating stimulation current and duration revealed that the relationship between gaze shifts and evoked EMG responses is not obligatory: short-duration (<20 ms) or low-current stimulation evoked neck EMG responses in the absence of gaze shifts. However, long-duration stimulation (>150 ms) occasionally revealed a transient neck EMG response aligned on the onset of sequential gaze shifts. We conclude that the SC drive to neck muscle motoneurons is far more widespread than traditionally supposed and is relayed through intervening elements which may or may not be activated in association with gaze shifts.     

Neck proprioceptive inputs to primate vestibular nucleus neurons

The contribution of neck proprioceptive signals to signal processing in the vestibular nucleus was studied by recording responses of secondary horizontal canal-related neurons to neck rotation in the squirrel monkey. Responses evoked by passive neck rotation while the head was held stationary in space were compared with responses evoked by passive whole body rotation and by forced rotation of the head on the trunk. Most neurons (76%; 45/59) were sensitive to neck rotation. The nature and strength of neck proprioceptive inputs varied and usually combined linearly with vestibular inputs. In most cases (94%), the direction of the neck proprioceptive input was "antagonistic" or "reciprocal" with respect to vestibular sensitivity and, consequently, reduced the vestibular response during head-on-trunk rotation. Different types of vestibular neurons received different types of proprioceptive input. Neurons whose firing behavior was related to eye position (position-vestibular-pause neurons and position-vestibular neurons) were often sensitive to the position of the head with respect to the trunk. The sensitivity to head position was usually in the same direction as the neuron's eye position sensitivity. Non-eye-movement related neurons and eye-head-velocity neurons exhibited the strongest sensitivity to passive neck rotation and had signals that were best related to neck velocity. The results suggest that neck proprioceptive inputs play an important role in shaping the output of the primate vestibular nucleus and its contribution to posture, gaze and perception.     

Perception of angular head position during attempted alignment with eccentric visual objects

The perception of angular head position with respect to a visual object was investigated using three different methods: Pointer indication (P); in the dark, subjects' (Ss') heads were horizontally turned to various positions (range +/- 54 degrees); Ss then rotated a pointer carrying a light emitting diode (LED) so as to align it with head position. Active head pointing (A); again in darkness, the pointer was rotated to various positions; Ss then turned their heads so as to align them with the pointer. Reading from visible scale (V); Ss viewed a degrees scale on a circular screen; Ss' heads were turned as in P, and Ss reported head position in terms of this scale. The results obtained with all three methods indicate that head position is overestimated with respect to the visual object (LED, scale mark): object position exceeded head position by 6, 18, and 7% when measured with the P-, A-, and V-methods, respectively (median values). The observed misalignment between head and object suggests that subjective primary eye position is shifted in the direction of head rotation by a cross-talk of head position. Whether a functional advantage or merely a tolerated side-effect of other constraints, this behavior confers the impression of looking "straight ahead" in the literal sense when gaze is shifted by coordinated eye-head movements with a 10% eye and a 90% head share in total lateral displacement. In an attempt to probe a hypothesized internal representation of head position implied in head-to-object alignment, Ss were also to estimate head position in space using only neck proprioceptive information. In complete darkness, responses were often non-linear functions of head position with overestimation of large eccentricities. When a head-centered LED was added (which conveyed no position information), responses became more linear, suggesting that the mere presence of visual afferents may improve the perceptual interpretation of proprioceptive information.     

Eye and neck proprioceptive messages contribute to the spatial coding of retinal input in visually oriented activities

Summary The egocentric localization of objects in extrapersonal space requires that the retinal and extraretinal signals specifying the gaze direction be simultaneously processed. The question as to whether the extraretinal signal is of central or peripheral origin is still a matter of controversy, however. Three experiments were carried out to investigate the following hypotheses: 1) that the proprioceptive feedback originating in eye and neck muscles might provide the CNS with some indication about the gaze direction; and 2) that the retinal and proprioceptive extraretinal inputs might be jointly processed depending on whether they are of monocular or binocular origin. Application of low amplitude mechanical vibrations to either the extraocular or neck muscles (or both) of a subject looking monocularly at a small luminous target in darkness resulted in an illusory movement of the target, the direction of which depended on which muscle was stimulated. A slow upward target displacement occurred on vibrating the eye inferior rectus or the neck sterno-cleido-mastoidus muscles, whereas a downward shift was induced when the dorsal neck muscles (trapezius and splenius) were vibrated. The extent of the perceptual effects reported by subjects was measured in an open-loop pointing task in which they were asked to point at the perceived position of the target. These results extend to visually-oriented behavior the role of extraocular and neck proprioceptive inputs previously described in the case of postural regulation, since they clearly show that these messages contribute to specifying the gaze direction. This suggests that the extraretinal signal might include a proprioceptive component. The proposition that a directional body reference frame may be based on the common processing of various proprioceptive feedbacks is discussed.     

Human eye-head gaze shifts preserve their accuracy and spatiotemporal trajectory profiles despite long-duration torque perturbations that assist or oppose head motion

Humans routinely use coordinated eye-head gaze saccades to rapidly and accurately redirect the line of sight (Land MF. Vis Neurosci 26: 51-62, 2009). With a fixed body, the gaze control system combines visual, vestibular, and neck proprioceptive sensory information and coordinates two moving platforms, the eyes and head. Classic engineering tools have investigated the structure of motor systems by testing their ability to compensate for perturbations. When a reaching movement of the hand is subjected to an unexpected force field of random direction and strength, the trajectory is deviated and its final position is inaccurate. Here, we found that the gaze control system behaves differently. We perturbed horizontal gaze shifts with long-duration torques applied to the head that unpredictably either assisted or opposed head motion and very significantly altered the intended head trajectory. We found, as others have with brief head perturbations, that gaze accuracy was preserved. Unexpectedly, we found also that the eye compensated well--with saccadic and rollback movements--for long-duration head perturbations such that resulting gaze trajectories remained close to that when the head was not perturbed. However, the ocular compensation was best when torques assisted, compared with opposed, head motion. If the vestibuloocular reflex (VOR) is suppressed during gaze shifts, as currently thought, what caused invariant gaze trajectories and accuracy, early eye-direction reversals, and asymmetric compensations? We propose three mechanisms: a gaze feedback loop that generates a gaze-position error signal; a vestibular-to-oculomotor signal that dissociates self-generated from passively imposed head motion; and a saturation element that limits orbital eye excursion.     

Visual-ocular motor activity in the macaque pregeniculate complex

The anatomical connections of the pregeniculate complex (PrGC) with components of the visual-ocular motor system suggested its contribution to ocular motor behavior. Subsequent studies reported saccade-related activity in the primate PrGC. To determine its contribution, we characterized pregeniculate units (n = 128) in alert macaques during ocular motor tasks and visual stimulation. We found that 36/109 saccade-related units exhibited postsaccadic bursts or pauses in tonic discharge for saccades of any amplitude or direction. In contrast to previous results, 46/109 responses preceded or coincided with the saccade, while 47/109 responses were directionally tuned. Pregeniculate units were modulated not only in association with saccades (109/128) but also with smooth eye movements and visual motion (20/128) or eye position (23/128). Multiple ocular motor signals were recorded from 19% of the units, indicating signal convergence on individual neurons. Visual responses were demonstrated in 51% of PrGC units: visual field illumination modulated the resting discharge of 33 units; the responses of 37 saccade-related units and all 23 position-dependent units were modulated by visual stimulation. Early saccadic activity in the PrGC suggests that it contributes more to gaze than postsaccadic modulation of visual or ocular motor activity. The patterns of saccadic responses and the modulation of PrGC activity in association with a variety of visual-ocular motor behaviors suggest its potential role as a relay between the parietal cortex and elements of the brain stem ocular motor pathways, such as the superior colliculus and pretectal nucleus of the optic tract.     

Gaze anchoring to a pointing target is present during the entire pointing movement and is driven by a non-visual signal

A well-coordinated pattern of eye and hand movements can be observed during goal-directed arm movements. Typically, a saccadic eye movement precedes the arm movement, and its occurrence is temporally correlated with the start of the arm movement. Furthermore, the coupling of gaze and aiming movements is also observable after pointing initiation. It has recently been observed that saccades cannot be directed to new target stimuli, away from a pointing target stimulus. Saccades directed to targets presented during the final phase of a pointing movement were delayed until after pointing movement offset ("gaze anchoring"). The present study investigated whether ocular gaze is anchored to a pointing target during the entire pointing movement. In experiment 1, new targets were presented at various times during the duration of a pointing movement, triggered by the kinematics arm moment itself (movement onset, peak acceleration/velocity/deceleration, and offset). Subjects had to make a saccade to the new target as fast as possible while maintaining the pointing movement to the initial target. Saccadic latencies were increased by an amount of time that approximately equaled the remaining pointing time after saccadic target presentation, with the majority of saccades executed after pointing movement offset. The nature of the signal driving gaze stabilization during pointing was investigated in experiment 2. In previous experiments where ocular gaze was anchored to a pointing target, subjects could always see their moving arm, thus it was unknown whether a visual image of the moving arm, an afferent (proprioceptive) signal or an efferent (motor control related) signal produced gaze anchoring. In experiment 2 subjects had to point with or without vision of the moving arm to test whether a visual signal is used to anchor gaze to a pointing target. Results indicate that gaze anchoring was also observed without vision of the moving arm. The findings support the existence of a mechanism enforcing ocular gaze anchoring during the entire duration of a pointing movement. Moreover, such a mechanism uses an internally generated, or proprioceptive, nonvisual signal. Possible neural substrates underlying these processes are discussed, as well as the role of selective attention.     

Shift in saccadic direction induced in humans by proprioceptive manipulation: a comparison between memory-guided and visually guided saccades

It is nowadays generally recognized that saccades to remembered targets are planned in a craniotopic frame of reference by combining retinal input with eye position signal. The origin of the eye position signal is still a matter of controversy, however. Does it arise from an efferent copy or is it supplied by the sensory receptors with which the extraocular muscles are endowed? When applied to skeletal muscles, vibration elicits spindle responses simulating a stretching of the vibrated muscle. When vibration is applied to the inferior rectus muscle (IR), it induces the illusion that a stationary fixating point is moving upward. Here we attempted to change the initial eye position signal supplied to the oculomotor system before a memory- or visuo-guided saccade to a 10° left target by applying mechanical vibration to the IR muscle. We wanted to determine whether modifying extraocular proprioceptive cues during the programming phase of a saccade might affect the latter's trajectory. In the memory-guided condition, it was observed that the saccades ended lower down when vibration was applied than in the control condition. Conversely, the visuo-guided saccades were not affected by the vibration. The above results mean first that extraocular proprioceptive cues are used as an initial eye position signal when a memory guided saccade has to be planned. Secondly, they suggest that extraocular proprioception may not be used to produce a visuo-guided saccade, or that this type of saccade is computed solely on the basis of retinal cues.     

Registered eye position: short- and long-term effects of botulinum toxin injected into eye muscle

Botulinum toxin is sometimes injected into human eye muscles as an alternative to surgery in the correction of strabismus. Within minutes of botulinum injections into ungulate eye muscles, proprioceptive discharge from muscle spindles decreases dramatically. It is only over 7–48 h, however, that surgically treated strabismus patients usually show an altered proprioceptive signal about eye position, presumably from the palisade endings attached to the global multiply innervated fibers. How quickly will botulinum toxin alter proprioceptive registration of eye position in humans? First, to examine the short-term effects, we measured open-loop pointing responses (which requires knowledge of eye position) in six strabismus patients preinjection and then over a 45 min postinjection period, and in six normal controls over the same time period. Second, to examine the long-term effects, 13 strabismus patients were tested preinjection and then daily over the next 3 weeks, and three normal controls over the same time period. We compared their open-loop pointing responses with the injected eye fixating the target to the photographically determined position of the occluded other eye (a measure of where the patient would point if eye position were determined by efference, not proprioception). There were three groups of patients: esotropes with no previous injection, exotropes with no previous injection, and exotropes with previous injection. First, all patients showed significant correction of their tropias. Second, over the short-term, there was no difference in pointing responses found between the patients and the controls (t(18) = –1.427, P = 0.1706). Third, over the long-term, however, the difference between the pointing responses and eye position information was compared among the four groups across four posttests and a significant difference found (F _3,12 = 58.988, P < 0.00001). Only in patients with no previous injections was there altered proprioceptive feedback about eye position. Also, injections into the medial rectus induced a significantly greater proprioceptive response than those injected into the lateral rectus. In humans, botulinum toxin alters proprioception from eye muscles only over the long-term. We suggest that the toxin temporarily affects proprioceptive feedback from palisade endings.     

Active and passive forearm flexor tension patterns in the continuous perceptual-motor task paradigm: The significance of motivation

The psychophysiology of skeletal muscle tension patterns during goal-directed behavior was investigated by using the continuous perceptual-motor task paradigm. Subjects were recruited to form serious-minded and playful extreme groups according to their responses to the Telic Dominance Scale (10 subjects in each group). They all performed a continuous perceptual-motor task 5 times at a ‘slow’ speed followed by a final ‘fast’ task. All performance periods lasted for 150 s. Measures of passive (tonic) and active (phasic) forearm flexor EMG activity were derived. It was concluded that serious-mindedness was significant to a tonic build-up of tension in the passive forearm flexor over the course of task performance. In contrast, the playful state was associated with high phasic response amplitudes in the active forearm flexor contingent upon changing the position of the joy-stick. Training moderated the range of state-specific EMG activation. State measures confirmed that the subjects maintained their state dominance during task performance. Error-scores and scores on felt level of arousal did not discriminate between the groups.