Coordinated orienting movements can be accurately performed without direct sensory control. Ocular saccades, for instance, have been shown to be reprogrammed after target disappearance when an intervening eye movement is electrically triggered before the saccade onset. Saccadic eye movements can also be executed toward memorized targets, even when the subject has been passively moved in darkness. Two hypotheses have been proposed to account for this goalinvariance property: either (a) the goal is reconstructed and memorized in the stable frame of reference linked to the environment ("allocentric coordinates") or (i) the goal is selected and memorized in the sensors-related maps ("egocentric coordinates") and is continuously updated by efferent copies of the motor commands. In this paper, we shall describe a formal neural network based on this second hypothesis. The results of the simulation show that target position can be memorized and accurately updated in a topologically ordered map, using a velocity-signal feedback. Moreover, this network has been submitted to a simple learning procedure by using the intermittent visual recurring afferent signal as the teaching signal. A similar mechanism could be involved in control of limb movement.Among the various types of orienting movements, visually triggered oculomotor saccades have been extensively studied in the past 2 decades. The stereotyped dynamic characteristics of saccades initially led to the consideration of saccades as an example of ballistic movements elicited from retinal stimulation by a "built-in temporal-pattern generator" (1). In 1975, Robinson (2) argued that visual-target position first has to be recomputed in a craniotopic frame of reference by adding an eye-position efferent copy. His model predicted that when the eyes were displaced by an internal command after the target had disappeared and before the saccade started, the saccade would still end where the target had been. This prediction was confirmed later by Mays and Sparks (3,4) in monkey and by Viviani and Velay (5) in humans. Tweed and Vilis (6) extended the Robinson model to the three-dimension eye-rotation space by using the formalism of quaternionic algebra to code the desired and instantaneous eye position. Moreover, Guitton et al. (7) proposed that target position and gaze position are reconstructed in space coordinates for the programming of eye/head coordinated movements.However, despite intensive investigations in the various neural structures involved in saccade generation, the neural coding of desired eye position in the orbit (and of target in craniotopic coordinates) has never been found. Becker and Jurgens (8) proposed a modification of the Robinson model, in which a dynamic motor-error signal is computed as the difference between the retinotopic target position and the instantaneous eye displacement, provided by an integrator that can be reset. Waitzman et al. (9) have recorded singlecell activities in the intermediate layers of monkey superior colliculus that are c...

Off-vertical axis rotation in darkness induces a perception of body motion which lasts as long as rotation continues. Perceived body motion is the combination of two simultaneous displacements. The most easily perceived is a translation without rotation along a conical path, at the frequency of the actual rotation. Meanwhile, the subjects feel as if they were always facing towards the same direction. The summit of the cone is generally below the head, from the waist to below the feet, and subjects have a sense of progression in the direction opposite to actual spinning. Some subjects feel, on the contrary, the summit of the cone above their heads, and the progression in the direction of spinning. Subjects also perceived another body motion, although it was faint for some of them. It consists of a rotation at low velocity in the same direction as progression along the cone. The axis of the cone is perceived as slowly rotating along a larger cone. These motion perceptions increase with tilt angle and rotation velocity. They probably result from the analysis by the Central Nervous System of the acceleration acting on the otoliths. The perceived trajectory would be reconstructed from estimates of gravity, and kinematic variables such as head translational acceleration and velocity, and head rotational velocity. The same variables would account for OVAR-induced nystagmus. Motion sickness would result from the impossibility of reconstructing a consistent body movement from most sets of values of these variables.

SUMMARY1. The aim of the present study is to describe the behaviour of identified secondorder vestibular neurones in the alert cat during eye saccades. A selection of neurones which are involved in horizontal eye movements has been made. The activity has been compared with a selected sample of abducens motoneurones recorded in the same animals.2. Alert head-fixed cats were used for this study. Eye movements were recorded by the scleral search coil technique. Abducens motoneurones were identified by antidromic stimulation from the VIth nerve with chronically implanted electrodes.They were recorded extracellularly. 3. Second-order vestibular neurones were identified by orthodromic stimulation from the vestibular organs. They were recorded intra-axonally and injected with horseradish peroxidase after recording of their physiological characteristics. Their morphology was reconstructed from frozen sections.4. All the recorded vestibular neurones showed various amounts of eye position sensitivity. The firing rate (F) -horizontal eye position (H) characteristics are compared for abducens and vestibular neurones. The population average values are F = 33 + 4 H for motoneurones and F = 51 + 2-4 H for vestibular neurones.5. All recorded vestibular neurones showed an increase of discharge rate during contralateral horizontal saccades and a strong decrease or pause during ipsilateral saccades. Firing rate -horizontal eye velocity sensitivity has been calculated.6. Results suggest a strong inhibitory input on vestibular neurones from the saccadic generator. This mechanism underlies the suppression of the vestibulo-ocular reflex during saccades. Our results suggest that in the cat, for saccades of amplitude smaller than 20 deg, there is a variable degree of suppression which is provided by a projection of excitatory bursters (EBNs) on second-order vestibular neurones through inhibitory type II neurones.7. We also conclude from this study that the eye position sensitivity of vestibular second-order neurones is in fact a motor signal indicating a motor error, i.e. the amount of head or eye movement which remains to be done in order to align gaze on target with the eyes centred in the orbit.

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Off-vertical rotation (OVAR) in darkness induced continuous horizontal nystagmus in humans at small tilts of the rotation axis (5 to 30 degrees). The horizontal slow eye velocity had two components: a mean velocity in the direction opposite to head rotation and a sinusoidal modulation around the mean. Mean velocity generally did not exceed 10 deg/s, and was less than or equal to the maximum velocity of optokinetic after-nystagmus (OKAN). Both the mean and modulation components of horizontal nystagmus increased with tilt angle and rotational velocity. Vertical slow eye velocity was also modulated sinusoidally, generally around zero. The amplitude of the vertical modulation increased with tilt angle, but not with rotational velocity. In addition to modulations in eye velocity, there were also modulations in horizontal and vertical eye positions. These would partially compensate for head position changes in the yaw and pitch planes during each cycle of OVAR. Modulations in vertical eye position were regular, increased with increases in tilt angle and were separated from eye velocity by 90 deg. These results are compatible with the interpretation that, during OVAR, mean slow velocity of horizontal nystagmus is produced by the velocity storage mechanism in the vestibular system. In addition, they indicate that the otolith organs induce compensatory eye position changes with regard to gravity for tilts in the pitch, yaw and probably also the roll planes. Such compensatory changes could be utilized to study the function of the otolith organs. A functional interpretation of these results is that nystagmus attempts to stabilize the image on the retina of one point of the surrounding world. Mean horizontal velocity would then be opposite to the estimate of head rotational velocity provided by the output of the velocity storage mechanism, as charged by an otolithic input during OVAR. In spite of the lack of actual translation, an estimate of head translational velocity could, in this condition, be constructed from the otolithic signal. The modulation in horizontal eye position would then be compensatory for the perceived head translation. Modulation of vertical eye velocity would compensate for actual changes in head orientation with respect to gravity.