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J Droulez

Publications and source records attributed to J Droulez.

At least 19 recordsLinked to original sources
PubMed1999

A 6-dof device to measure head movements in active vision experiments: geometric modeling and metric accuracy.

This work describes a technique for measuring human head movements in 3D space. Rotations and translations of the head are tracked using a light helmet fastened to a multi-joint mechanical structure. This apparatus has been designed to be used in a series of psycho-physiological experiments in the field of active vision, where position and orientation of the head need to be measured in real time with high accuracy, high reliability and minimal interference with subject movements. A geometric model is developed to recover the position information and its parameters are identified through a calibration procedure. The expected accuracy, derived on the basis of the pure geometric model and the sensor resolution, is compared with the real accuracy, obtained by performing repetitive measurements on a calibration fixture. The outcome of the comparison confirms the validity of the proposed solution which turns out to be effective in providing measurement of head position with an overall accuracy of 0.6 mm and sampling frequency above 1 kHz.

Calibration↗
PubMed1997

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.

Adult↗
PubMed1995

Perception of three-dimensional shape from ego- and object-motion: comparison between small- and large-field stimuli.

We compare the performance in the detection of the shape of concave, planar and convex surfaces for small-field (8 deg) and large-field (90 deg) stimuli. Shape is perceived from head translations, object translations and object rotations. We find large differences between small-field and large-field stimulation. For small-field stimulation performance is best for object rotation, intermediate for self-motion and worst for object translation. For large-field stimulation performance is similar across conditions. Few errors on the sign of the curvature are found for self-motion for both field sizes, indicating that self-motion information disambiguates the curvature sign. For object rotation with small-field stimulation, the concave-convex ambiguity is strong with many apparent deformations. In contrast, large-field curvature signs are always accurately reported, suggesting that the weight of the rigidity hypothesis depends on field size.

Depth Perception↗
PubMed1995

Measurements of human force control during a constrained arm motion using a force-actuated joystick.

1. When interacting with the environment, human arm movements may be prevented in certain directions (i.e., when sliding the hand along a surface) resulting in what is called a "constrained motion." In the directions that the movement is restricted, the subject is instead free to control the forces against the constraint. 2. Control strategies for constrained motion may be characterized by two extreme models. Under the active compliance model, an essentially feedback-based approach, measurements of contact force may be used in real time to modify the motor command and precisely control the forces generated against the constraint. Under the passive compliance model the motion would be executed in a feedforward manner, using an internal model of the constraint geometry. The feedforward model relies on the compliant behavior of the passive mechanical system to maintain contact while avoiding excessive contact forces. 3. Subjects performed a task in which they were required to slide the hand along a rigid surface. This task was performed in a virtual force environment in which contact forces were simulated by a two-dimensional force-actuated joystick. Unknown to the subject, the orientation of the surface constraint was varied from trial to trial, and contact force changes induced by these perturbations were measured. 4. Subjects showed variations in contact force correlated with the direction of the orientation perturbation. "Upward" tilts resulted in higher contact forces, whereas "downward" tilts resulted in lower contact forces. This result is consistent with a feedforward-based control of a passively compliant system. 5. Subject responses did not, however, correspond exactly to the predictions of a static analysis of a passive, feedforward-controlled system. A dynamic analysis reveals a much closer resemblance between a passive, feedforward model and the observed data. Numerical simulations demonstrate that a passive, dynamic system model of the movement captures many more of the salient features observed in the measured human data. 6. We conclude that human subjects execute surface-following motions in a largely feedforward manner, using an a priori model of the surface geometry. The evidence does not suggest that active, real time use of force feedback is used to guide the movement or to control limb impedance. We do not exclude, however, the possibility that the internal model of the constraint is updated at somewhat longer latencies on the basis of proprioceptive information.

Arm↗
PubMed1994

The visual perception of three-dimensional shape from self-motion and object-motion.

To evaluate the influence of egomotion on the three-dimensional visual processing of structure-from-motion (SFM), we compared the visual discrimination between planar and spherical surfaces during subject-translation, object-translation, or rotation of the object in depth. Performance was the best for object-rotation, intermediate for subject-translation, and the poorest for object-translation--and thus increased with the quality of retinal image stabilization achieved in the different conditions. This suggests that the major role of self-motion information was to stabilize retinal images. In view of previous results, we propose that the interactions between self-motion information and SFM are reduced to functional complementarity, in the sense that self-motion can lift visual ambiguities but does not improve the sensitivity of SFM processes.

Adult↗
PubMed1993

The influence of long-term practice on mental rotation of 3-D objects.

We evaluated the influence of long-term practice on the performance of a mental rotation task in which subjects judged whether two 3-D objects presented in different orientations were identical. Stimuli and experimental conditions were analogous to those used by Shepard and Metzler. Sixteen subjects were selected, to test the influence of aptitude for mental imagery on this learning process. Subjects participated in 12 to 15 sessions over 6 weeks. Two catalogues of different stimuli were alternatively used during three (or six) consecutive sessions to determine the influence of complexity and familiarity of figures. For all subjects, the inverse of the velocity of mental rotation along the sessions was adequately fitted by a decreasing exponential curve. However, evidence for mental rotation did not disappear, even after 15 sessions. Asymptotic variations can be attributed to differences in stimuli as well as imaging skills of subjects. Our results lead to a new interpretation of the mental rotation process.

Adult↗
PubMed1993

Stereo-motion cooperation and the use of motion disparity in the visual perception of 3-D structure.

When an observer views a moving scene binocularly, both motion parallax and binocular disparity provide depth information. In Experiments 1A-1C, we measured sensitivity to surface curvature when these depth cues were available either individually or simultaneously. When the depth cues yielded comparable sensitivity to surface curvature, we found that curvature detection was easier with the cues present simultaneously, rather than individually. For 2 of the 6 subjects, this effect was stronger when the component of frontal translation of the surface was vertical, rather than horizontal. No such anisotropy was found for the 4 other subjects. If a moving object is observed binocularly, the patterns of optic flow are different on the left and right retinae. We have suggested elsewhere (Cornilleau-Pérès & Droulez, in press) that this motion disparity might be used as a visual cue for the perception of a 3-D structure. Our model consisted in deriving binocular disparity from the left and right distributions of vertical velocities, rather than from luminous intensities, as has been done in classical studies on stereoscopic vision. The model led to some predictions concerning the detection of surface curvature from motion disparity in the presence or absence of intensity-based disparity (classically termed binocular disparity). In a second set of experiments, we attempted to test these predictions, and we failed to validate our theoretical scheme from a physiological point of view.

Acceleration↗
PubMed1991

A neural network model of sensoritopic maps with predictive short-term memory properties.

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 goal-invariance property: either (i) the goal is reconstructed and memorized in the stable frame of reference linked to the environment ("allocentric, coordinates") or (ii) 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.

Computer Simulation↗
PubMed1990

Visual perception of surface curvature. The spin variation and its physiological implications.

In the context of the models of structure from motion visual processing, we propose that the optic-flow field is a source of information for the perception of the curvature of a smooth surface in motion. In particular, it is shown how the spin variation (SV), a second spatial derivative of the retinal velocity field, is mathematically related to the curvature of the surface. Under the hypothesis that the visual system relies on SV to analyse the structure of a moving surface, a neural scheme for SV detection is proposed and psychophysical predictions are developed. Results obtained on artificial images show that the SV scheme presents a rather weak sensitivity to noise in conditions of low image velocity.

Computer Simulation↗
PubMed1989

Target velocity based prediction in saccadic vector programming.

Two experiments have been designed to test whether the saccadic system takes target motion into consideration in computing saccade amplitude. In one experiment, while the subject fixated straight ahead, either a horizontal ramp-step-ramp or a horizontal step-ramp target moved from left to right. After the step, the subject had to make a saccade and follow the target. In the second set of experiments, the target, after an initial step, moved extrafoveally from up to down at fixed velocity; a tone, signaling the subject to make a saccade to the target and follow it, was delivered either after a variable delay (previewed condition) or simultaneously with the initial target step (non-previewed condition). In both experiments, eye position at saccade end was statistically different from target position 100 msec before saccade onset only when the target slow motion was presented before the step (i.e. in horizontal ramp-step-ramp and in previewed H-step V-ramp paradigms), suggesting that target motion could be used by the saccadic system to extrapolate the future target position, only if the subject is given enough time to observe the target ramp motion before the step.

Adult↗
PubMed1989

Neural correlates of horizontal vestibulo-ocular reflex cancellation during rapid eye movements in the cat.

1. The aim of the present study is to describe the behaviour of identified second-order 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.

Abducens Nerve↗
PubMed1989

Use of target velocity in saccadic programming.

Previous studies indicate that in response to a step-ramp visual target movement, the saccade amplitude approximates target displacement 100 ms before saccade onset. This study examines whether the saccadic system takes target motion into consideration when computing saccadic amplitude, if target movement is seen by the subject before he is requested to make a saccade. In the first experiment, while the subject fixated at the target (laser dot) and maintained fixation, the target jumped to the left and moved to the right at a fixed velocity. At some predetermined site, the target jumped a step to the right and continued to move in that direction. After the target step, the subject had to make a saccade and follow target motion. In the second experiment, while the subject fixated at the target and maintained fixation, the target jumped to the right and up and moved down at a fixed velocity. At some predetermined site, an auditory signal was given, and the subject had to make a saccade and follow target motion. Results in both experiments showed that the eye position was statistically different from the target displacement at 100 ms before saccade onset, indicating that the saccadic system uses target velocity in computing saccade amplitude.

Acceleration↗
PubMed1989

Visual perception of surface curvature: psychophysics of curvature detection induced by motion parallax.

The continuous approach to optic-flow processing shows that the curvature of a moving surface is related to a second spatial derivative of the velocity field, the spin variation (Droulez & Cornilleau-Pérès, 1989). With this approach as a theoretical framework, visual sensitivity to the curvature of a cylinder in motion was measured using a task of discrimination between cylindrical and planar patches. The results confirm the predictions suggested by the theory: (1) Sensitivity to curvature was always greater when the cylinder axis and the frontal translation were parallel than when they were orthogonal. The ratio of curvature detection thresholds in the two cases was between 1.3 and 2.5; the value predicted from the spin variation theory is about 2. (2) Sensitivity to curvature increased strongly with the velocity of the motion but was only weakly affected by its amplitude and the duration of viewing for the range of values used in our experiments.

Adult↗
PubMed1988

Eye movements induced by off-vertical axis rotation (OVAR) at small angles of tilt.

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.

Adult↗
PubMed1988

Motion perceptions induced by off-vertical axis rotation (OVAR) at small angles of tilt.

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.

Adult↗
PubMed1986

Some collicular efferent neurons code saccadic eye velocity.

The activity of identified tecto-reticulo-spinal neurons (TRSNs) was studied in alert head-fixed cats during orienting towards moving visual stimuli. Eye movements and dorsal neck muscle activity were recorded simultaneously. Burst parameters of TRSNs showing visuomotor properties were analysed quantitatively. It could be demonstrated that some neurons generate presaccadic bursts whose instantaneous frequency profile is closely correlated with the profile of saccadic eye velocity. This correlation could be revealed only under conditions in which cats made orienting saccades to 'catch' a target moving in the preferred direction of the neuron's visual receptive field. Latency between bursts and saccades varied depending upon the degree of attention toward the target and saccade direction.

Animals↗
PubMed1986

Adaptive changes in perceptual responses and visuomanual coordination during exposure to visual metrical distortion.

The abilities of the human visual system to perform metrical judgements (comparison of lengths, estimation of angles...) involve the existence of some geometrical structure in the visual perceptual space. The question arises whether this geometrical structure is rigidly determined or is subject to adaptive changes. We have tried to answer the question by using a paradigm in which subjects are exposed to a vertically lengthened visual world and then asked to compare simultaneously presented lengths and to evaluate angles between two lines. Their perceptual responses clearly indicate a plastic adaptation to the deformed environment, though the adaptation was never complete after several days of continuous exposure to strong (25%) lengthening. For a maximum time of exposure of 7 days the rate of adaptation was found to be roughly independent of the initial degree of optical distortion. Visuomanual coordination was also investigated in these subjects, but the responses were less conclusive in this case, because of the high inter-subject variability.

Adaptation, Ocular↗