Modeling the vestibulo-ocular reflex of the squirrel monkey during eccentric rotation and roll tilt

Model simulations of the squirrel monkey vestibulo-ocular reflex (VOR) are presented for two motion paradigms: constant velocity eccentric rotation and roll tilt about a naso-occipital axis. The model represents the implementation of three hypotheses: the internal model hypothesis, the gravito-inertial force (GIF) resolution hypothesis, and the compensatory VOR hypothesis. The internal model hypothesis is based on the idea that the nervous system knows the dynamics of the sensory systems and implements this knowledge as an internal dynamic model. The GIF resolution hypothesis is based on the idea that the nervous system knows that gravity minus linear acceleration equals GIF and implements this knowledge by resolving the otolith measurement of GIF into central estimates of gravity and linear acceleration, such that the central estimate of gravity minus the central estimate of acceleration equals the otolith measurement of GIF. The compensatory VOR hypothesis is based on the idea that the VOR compensates for the central estimates of angular velocity and linear velocity, which sum in a near-linear manner. During constant velocity eccentric rotation, the model correctly predicts that: (1) the peak horizontal response is greater while facing-motion than with back-to-motion; (2) the axis of eye rotation shifts toward alignment with GIF; and (3) a continuous vertical response, slow phase downward, exists prior to deceleration. The model also correctly predicts that a torsional response during the roll rotation is the only velocity response observed during roll rotations about a nasooccipital axis. The success of this model in predicting the observed experimental responses suggests that the model captures the essence of the complex sensory interactions engendered by eccentric rotation and roll tilt.This research was supported by NASA contracts NASW-3651, NAG2-445, NAS9-16523, the NASA Graduate Student Researchers Program, and the GE Forgivable Loan Fund. Preliminary findings were presented in a PhD thesis (Merfeld 1990) and at the Sixteenth Barany Society Meeting in Tokyo in 1990 (Merfeld et al. 1991).     

Vestibular perception of passive whole-body rotation about horizontal and vertical axes in humans: goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccades

This study was aimed at complementing the existing knowledge about vestibular perception of self-motion in humans. Both goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccade (VM-CS) tasks were used, respectively as concurrent and retrospective magnitude estimators for passive whole-body rotation. Rotations were applied about the earth-vertical and earth-horizontal axes to study the effect of the otolith signal in self-rotation evaluation, and both in yaw and pitch to examine the horizontal and vertical semi-circular canals. Two different magnitudes of constant angular acceleration (50°/s² and 100°/s²) were used. The main findings were (1) strong correlation between both oculomotor responses of both tasks, (2) greater accuracy with rotations about the earth-vertical than the earth: -horizontal axis, (3) greater accuracy for yaw than for pitch rotations, (4) greater accuracy for high acceleration than for low, and (5) no effect of the delay (2s or 12s) in the VMCS task. Adequacy of both tasks as subjective magnitude estimators of vestibular perception of self-motion is discussed.On leave from the Laboratoíre de Physiologie Neurosensorielle, CNRS, Paris, FrancePresent address: Laboratoire de Physiologie de la Perception et de l'Action, CNRS, Collége de France, 15, rue de l'Ecole de Médecine, F-75270 Paris Cedex 06, France     

Spatial orientation in humans: perception of angular whole-body displacements in two-dimensional trajectories

Vestibular perception of whole-body passive rotation in the horizontal plane was studied by applying two-dimensional (2D) motion to eight blindfolded healthy volunteers: pure rotations in place, corner-like trajectories and arcs of a circular trajectory were randomly applied by means of a remotely controlled robot. Angles embedded in the 2D trajectories were 45°, 90°, 135° and 180°. Stimulation of semicircular canals was the same for all trajectories but was accompanied by concurrent otolith stimulation during circular motion. Subjects participated in two successive experimental sessions. In the first session they were instructed to use a pointer to reproduce the total angular displacement after the motion (REPRODUCTION); in the second session they had to keep pointing towards a remote (15 m) memorised target during the motion (TRACKING). In REPRODUCTION subjects tended to overestimate their rotation angle by 28 ± 11% (mean ± SD). There was no systematic effect of the trajectory. Overestimation also occurred when subjects were required to rotate in darkness by 180° (by controlling a joystick). In TRACKING there was virtually no overestimation (6 ± 17%) and the movement of the pointer matched the dynamics of angular motion. We conclude that (a) the brain can separate and memorise the angular component of complex 2D motion; however, a large inter-individual variability in estimating its amplitude exists; (b) in the range of linear accelerations used in the study, no appreciable effect of otolith-canal perceptual interaction was shown; (c) angular displacements can be dynamically transformed into matched pointing movements; (d) overestimation seems to be typical of delayed judgements of angular displacement and of self-controlled rotations in place. This could be due to the characteristics of the physiological calibration of the vestibular input. Received: 30 October 1996 / Accepted: 18 June 1997     

Motor learning in the “podokinetic” system and its role in spatial orientation during locomotion

The present study characterizes a previously reported adaptive phenomenon in a somatosensory-motor system involved in directional control of locomotor trajectory through foot contact with the floor. We call this the “podokinetic” (PK) system. Podokinetic adaptation was induced in six subjects by stepping in-place over the axis of a horizontally rotating disc over a range of disc angular velocities (11.25–90°/s) and durations (7.5–60 min). After adaptation, subjects were blindfolded and attempted to step in-place on the floor without turning. Instead they all rotated relative to space. The rate of the “podokinetic afterrotation” (PKAR) was linearly related to stimulus amplitude up to 45°/s, and the ratio of initial PKAR velocity to that of the adaptive stimulus was approximately 1:3. PKAR exhibited exponential decay, which was composed of “short-” and “long-term” components with “discharging” time constants on the order of 6–12 min and 1–2 h, respectively. The effect of stimulus duration on PKAR revealed a “charging” time constant that approximated that of the short-term component. A significant suppression of PKAR occurred during the 1st min of the postadaptive response, suggesting functional interaction between the PK and vestibular systems during the period of vestibular stimulation. During PKAR subjects perceived no self-rotation, indicating that perception as well as locomotor control of spatial orientation were remodeled by adaptation of the PK system. Received: 4 August 1997 / Accepted: 19 November 1997     

Use of a Genetic Algorithm for the Analysis of Eye Movements from the Linear Vestibulo-Ocular Reflex

It is common in vestibular and oculomotor testing to use a single-frequency (sine) or combination of frequencies [sum-of-sines (SOS)] stimulus for head or target motion. The resulting eye movements typically contain a smooth tracking component, which follows the stimulus, in which are interspersed rapid eye movements (saccades or fast phases). The parameters of the smooth tracking — the amplitude and phase of each component frequency — are of interest; many methods have been devised that attempt to identify and remove the fast eye movements from the smooth. We describe a new approach to this problem, tailored to both single-frequency and sum-of-sines stimulation of the human linear vestibulo-ocular reflex. An approximate derivative is used to identify fast movements, which are then omitted from further analysis. The remaining points form a series of smooth tracking segments. A genetic algorithm is used to fit these segments together to form a smooth (but disconnected) wave form, by iteratively removing biases due to the missing fast phases. A genetic algorithm is an iterative optimization procedure; it provides a basis for extending this approach to more complex stimulus–response situations. In the SOS case, the genetic algorithm estimates the amplitude and phase values of the component frequencies as well as removing biases. © 2001 Biomedical Engineering Society.PAC01: 8719St, 8780Tq     

On the nature of the vestibular control of arm-reaching movements during whole-body rotations

Recent studies report efficient vestibular control of goal-directed arm movements during body motion. This contribution tested whether this control relies (a) on an updating process in which vestibular signals are used to update the perceived egocentric position of surrounding objects when body orientation changes, or (b) on a sensorimotor process, i.e. a transfer function between vestibular input and the arm motor output that preserves hand trajectory in space despite body rotation. Both processes were separately and specifically adapted. We then compared the respective influences of the adapted processes on the vestibular control of arm-reaching movements. The rationale was that if a given process underlies a given behavior, any adaptive modification of this process should give rise to observable modification of the behavior. The updating adaptation adapted the matching between vestibular input and perceived body displacement in the surrounding world. The sensorimotor adaptation adapted the matching between vestibular input and the arm motor output necessary to keep the hand fixed in space during body rotation. Only the sensorimotor adaptation significantly altered the vestibular control of arm-reaching movements. Our results therefore suggest that during passive self-motion, the vestibular control of arm-reaching movements essentially derives from a sensorimotor process by which arm motor output is modified on-line to preserve hand trajectory in space despite body displacement. In contrast, the updating process maintaining up-to-date the egocentric representation of visual space seems to contribute little to generating the required arm compensation during body rotations.     

Coding of self-motion signals in ventro-posterior thalamus neurons in the alert squirrel monkey

The firing behavior of 47 ventro-posterior thalamus neurons was studied in two alert squirrel monkeys during rotations of whole body, head and trunk. A total of 27 of these neurons (57%) were sensitive to spatial motion of the head irrespective of the mode of motion. These neurons responded similarly when the head moved simultaneously with the trunk, and when the head voluntarily or involuntarily moved on the stationary trunk. These neurons did not respond to rotation of the trunk when the spatial position of the head was fixed. Five neurons (11%) responded only to involuntary movement of the head produced by external force, but were insensitive to voluntary spatial head movement. They also did not respond to spatial motion of the trunk. Totally 15 neurons (32%) were sensitive to spatial motion, which included rotation of the trunk. These neurons responded when the trunk moved alone, and when the trunk moved simultaneously with the head, but were not responsive to spatial movement of the head while the trunk was stationary. We suggest that the vestibulo–thalamo–cortical pathway comprises two distinct functional channels. In one of these channels, cephalokinetic, spatial motion of the head is coded. In the other channel, somatokinetic, motion of the body in space is coded. Each of these channels further consists of two divisions. In the principal division the motion signal is conveyed continuously, irrespective of the behavioral context of motion. In the other auxiliary division the signal only codes movement caused by externally applied force.     

Human perception of horizontal trunk and head rotation in space during vestibular and neck stimulation

Summary The vestibular signal of head motion in space must be complemented by a neck signal of the trunk-to-head excursion in order to provide the individual with information on trunk motion in space. This consideration led us to study psychophysically the role of vestibular-neck interaction for human self-motion perception. Subjects (Ss) were presented with passive horizontal rotations of their trunk and/or head (sinusoidal rotations, f=0.025 –0.4 Hz) in the dark for vestibular and neck stimulation, as well as for combinations of both. Ss' perception was evaluated in terms of gain (veridical perception of stimulus magnitude, G=1), phase, and detection threshold. (1) Perception of trunk rotation in space. During vestibular stimulation (whole-body rotation) and neck stimulation (trunk rotation with the head kept stationary) the frequency-transfer characteristics underlying this perception were very similar. The gain fell short; it was only about 0.7 at 0.4 and 0.2 Hz stimulus frequency and was further attenuated with decreasing frequency. In contrast, the phase was close to that of actual trunk position. The gain attenuation was found to be a function of the peak angular velocity of the stimulus, a fact, which we related to a velocity threshold of the order of 1 deg/s. During the various vestibular-neck combinations used, Ss' perception was again erroneous, reflecting essentially the sum of its two non-ideal constituents. However, there was one noticeable exception; during the combination head rotation on stationary trunk, Ss veridically perceived their trunk as stationary (compatible with the notion that the sum yielded zero). (2) Perception of head rotation in space. During vestibular stimulation, Ss' estimates showed the same non-ideal gain-vs.-frequency characteristics as described above for the trunk. Neck stimulation induced an illusion as if the head had been rotated in space. This neck contribution was such that, when it was combined with its vestibular counterpart during head rotation on stationary trunk, the perception became almost veridical. On closer inspection, however, this neck contribution was found to reflect the sum of two components; one was the non-ideal neck signal contributing to the perception of trunk in space, the other was an almost ideal neck signal of head-on-trunk rotation. (3) The results could be described by a simple model. In this model, the erroneous vestibular signal head in space is primarily used to create an internal representation of trunk in space. To this end, it is combined with the closely matching neck signal of trunk to head. The perception of head rotation in space is achieved by summing this trunk in space signal with the almost ideal head on trunk signal, again of nuchal origin. These seeming complex interactions have two implications: (i) the head is referred to trunk coordinates, whereas the trunk is referred to space coordinates; (ii) there is at least one condition in the dark where orientation is correct (despite an erroneous vestibular signal), i.e., during head rotation on stationary trunk.Supported by Deutsche Forschungsgemeinschaft, SFB 325     

The suppression of cervico-ocular response by the haptokinetic information about the contact with a rigid,immobile object

Horizontal eye movements were recorded in eight healthy subjects during super-slow trunk rotation with respect to the space-stationary head. In some trials, subjects simultaneously indicated their perception of selfmotion by means of a joystick. Over the frequency range employed (0.007–0.05 cycles per second, ±20°), all subjects perceived the relative motion of head and trunk as a head rotation with respect to the stationary trunk. Eye movements were observed which were in phase with imaginary head rotation; their amplitude exceeded the amplitude of actual body rotation. The grasping of a rigid ground-based handle (1) produced a sensation of trunk rotation in space, (2) suppressed the sensation of imaginary head rotation in space and (3) gave rise to a significant decrease in amplitude of eye movements. The grasping of a stiff rod with non-zero compliance did not produce these effects. It is concluded that eye movements in response to body rotation with respect to the fixed head are not purely reflex reactions, but are influenced by the internal representation of body motion.     

Cortical representation of whole-body movement is modulated by proprioceptive discharge in humans

Previous studies have revealed the influence of ongoing sensory discharge on modulating the central representation of muscle afferents from individual limbs. In the present study, we explored the potential for such modulatory influence on the afferent discharge arising from induced whole-body movement. Vestibular and somato-sensory inputs arise from such whole-body movement. The convergence of these two modalities is important in motor control, especially for the maintenance of postural stability. We hypothesised that transmission of proprioceptive and vestibular information to the cortex would be reduced as a result of muscle-spindle discharge in knee extensor muscles. Perturbation-evoked responses (PERs), recorded from central scalp electrodes (C3, CZ, C4), were evoked through rapid translations of subjects who were seated in a chair on a movable platform. PERs were recorded during passive linear translations alone and preceded by vibration of the patellar tendon. The PER was characterised by a slow, negative potential peaking at approximately 150 ms (N150) following displacement of the chair. The amplitude of the PER was reduced following vibration to 56% of the control. Such reduction of PERs was comparable to the attenuation of somatosensory evoked potentials and soleus H-reflex magnitudes from tibial-nerve stimulation. We conclude that muscle-spindle discharge in knee extensor muscles leads to gating of both of these afferent pathways. These results have potential implications to the understanding of the CNS control of stability during ongoing movement.     

Vestibular nuclei activity in the alert monkey during suppression of vestibular and optokinetic nystagmus

Summary Single neurons were recorded in the vestibular nuclei of monkeys trained to suppress nystagmus by visual fixation during vestibular or optokinetic stimulation. During optokinetic nystagmus vestibular nuclei neurons exhibit frequency changes. With the suppression of optokinetic nystagmus this neuronal activity on average is attenuated by 40% at stimulus velocities of 40 °/s. At a stimulus velocity of 5 °/s responses are, under both conditions, close to threshold. For steps in velocity, suppression of vestibular nystagmus shortens the time constants of the decay of neuronal activity from 15–35 s to 5–9 s, while the amplitude of the response remains unchanged. The results are discussed in relation to current models of visual-vestibular interaction. These models use a feedback mechanism which normally operates during vestibular and optokinetic nystagmus. Nystagmus suppression interrupts this feedback loop.Supported by the Swiss National Foundation for Scientific Research (SNF 3.233.77) and the Deutsche Forschungsgemeinschaft (U.W. Buettner, Bue 379/2)     

A quantitative [14C]-2-deoxy-D-glucose study of brain stem nuclei during horizontal nystagmus induced by lesioning the lateral crista ampullaris of the rat

Summary A study of the brainstem of the rat during horizontal nystagmus using the quantitative 2-deoxy-D-glucose technique reflected changes in the functional activity of cell groups based on their glucose utilization rates. Horizontal nystagmus was induced by unilateral crista ampullectomy of the horizontal canal. Comparisons of glucose utilization rates were made between experimental and control groups as well as from side to side within each group. There was a decrease of the ipsilateral medial and superior vestibular nuclei with a concomitant increase in the contralateral medial vestibular nucleus when compared to control. The medial rectus motor division of the ipsilateral oculomotor nucleus showed an increase whereas the ipsilateral abducens and the ipsilateral nucleus prepositus hypoglossi exhibited a decline in their utilization rates. The extra ocular motor nuclei responsible for the excitatory fast phase of nystagmus utilizes more substrate than those involved in the slow phase. An increase was also measured in the ipsilateral lobule of the cerebellar nodulus. The lateral reticular nucleus showed a bilateral decrease in its glucose utilization rate when compared to control.     

Influence of otolithic stimulation by horizontal linear acceleration on optokinetic nystagmus and visual motion perception

Summary Several studies in the past have demonstrated the existence of an Otolith-Ocular Reflex (OOR) in man, although much less sensitive than canal ocular reflex. The present paper 1 confirms these previous results. Nystagmic eye movements (L-nystagmus) appear in the seated subject during horizontal acceleration along the interaural axis in the dark for an acceleration level (1 m/s²) about ten times the perception threshold with a sensitivity of about 0.035 rad/m.When sinusoidal linear acceleration is combined with optokinetic stimulation, the recorded nystagmus slow phase velocity exhibits strong periodic modulation related to subject motion. This marked effect of linear acceleration on the optokinetic nystagmus (OKN) appears at a level (0.1 m/s²) close to the acceleration perception threshold and has a 4-fold higher sensitivity than L-nystagmus. Modulation of OKN can reach a peak-to-peak amplitude as great as 20 °/s; for a given optokinetic field size it increases with the velocity of the optokinetic stimulus, i.e. with the slow phase eye velocity. In parallel with changes in OKN slow phase velocity, linear acceleration induces a motion related decrease in the perceived velocity of the visual scene and modifications in selfmotion perception.The results are interpreted in terms of a mathematical model of visual-vestibular interaction. They show that sensory interaction processes can magnify the contribution given to the control of eye movements by the otolithic system and provide a way of exploring its function at low levels of acceleration.The present work has been presented at III European Neurosciences Meeting, Rome, September 1979     

The horizontal vestibulo-ocular reflex during linear acceleration in the frontal plane of the cat

Summary Horizontal and vertical eye movements were recorded in alert, restrained cats that were subjected to whole-body rotations with the horizontal semicircular canals in the plane of rotation and the body centered on the axis or 45 cm eccentric from the axis of rotation. Changes in the horizontal vestibulo-ocular reflex (HVOR) due to the resultant of the linear forces (i.e., gravity and linear acceleration) acting on the otolith organs were examined during off-axis rotation when there was a centripetal acceleration along the animal's interaural axis. The HVOR time constant was slightly shortened when the resultant otolith force was not parallel to the animal's vertical axis. This effect was independent of the direction of the otolith force relative to the direction of the slow phase eye velocity. No effect on the HVOR amplitude was observed. In addition to changes in the HVOR dynamics, a significant vertical component of eye velocity was observed during stimulation of the horizontal canals when the resultant otolith force was not parallel with the animal's vertical axis. The effect was greater for larger angles between the resultant otolith force and gravity. An upward or downward component was elicited, depending on the direction of the horizontal component of eye velocity and the direction of the resultant otolith force. The vertical component was always in the direction that would tend to align the eye velocity vector with the resultant otolith force and keep the eye movement in a plane that had been rotated by the angle between the resultant otolith force and gravity.     

Reorientation of a visually evoked postural response during passive whole body rotation

Visually evoked postural responses (VEPR) to a roll-motion rotating disk were recorded from normal subjects standing on a yaw axis motorised rotating platform. The disk was fluorescent so that subjects could be tested in an otherwise dark room. Movements of the head and centre of foot pressure were measured while subjects looked at the disk with their eyes and head in the primary position and while the rotating platform moved the subjects randomly to 0, +/-45 degrees and +/-90 degrees angles from the visual stimulus. Subjects were instructed to maintain fixation on the centre of the rotating disk but the amount of horizontal eye and head movement used was not specified. Platform rotational velocity was set near threshold values for perception of self-rotation (approximately 2 degrees/s) so that subjects would find it difficult to reconstruct the angle travelled. The data showed that the VEPR occurred in the plane of disk rotation, regardless of body position with respect to the disk, and despite the subjective spatial disorientation induced by the experiment. Averages of the response revealed a good match (gain=0.95) between disk orientation and sway direction. The horizontal gaze deviation required to fixate the centre of the disk was largely achieved by head motion (head 95%, eye 5%). The results confirm previous results that VEPRs are reoriented according to horizontal gaze angle. In addition, we show that the postural reorientation is independent of cognitively or visually mediated knowledge of the geometry of the experimental conditions. In the current experiments, the main source of gaze position input required for VEPR reorientation was likely to be provided by neck afferents. The results support the notion that vision controls posture effectively at any gaze angle and that this is achieved by combining visual input with proprioceptively mediated gaze-angle signals.     

Visual and non-visual cues in the perception of linear self motion

Surprisingly little is known of the perceptual consequences of visual or vestibular stimulation in updating our perceived position in space as we move around. We assessed the roles of visual and vestibular cues in determining the perceived distance of passive, linear self motion. Subjects were given cues to constant-acceleration motion: either optic flow presented in a virtual reality display, physical motion in the dark or combinations of visual and physical motions. Subjects indicated when they perceived they had traversed a distance that had been previously given to them either visually or physically. The perceived distance of motion evoked by optic flow was accurate relative to a previously presented visual target but was perceptually equivalent to about half the physical motion. The perceived distance of physical motion in the dark was accurate relative to a previously presented physical motion but was perceptually equivalent to a much longer visually presented distance. The perceived distance of self motion when both visual and physical cues were present was more closely perceptually equivalent to the physical motion experienced rather than the simultaneous visual motion, even when the target was presented visually. We discuss this dominance of the physical cues in determining the perceived distance of self motion in terms of capture by non-visual cues. These findings are related to emerging studies that show the importance of vestibular input to neural mechanisms that process self motion.     

Getting the measure of vergence weight in nearness perception

Combining multiple sources of information allows the human nervous system to construct an approximately Euclidean representation of near (personal) space. Within this space, binocular vergence is an important source of egocentric distance information. We investigated how the nervous system determines the significance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy between vergence information and other cues and (2) reduced vergence demand. The results also provided evidence that the nervous system represents vergence related distance information in units of nearness (the reciprocal of distance).     

Visual and graviceptive influences on lower leg EMG activity in humans during brief falls

Summary Human subjects were suspended in a safety harness 28 cm above the floor by a steel cable connected to a computer controlled force generator (electromagnetic brake). After the subjects were unexpectedly released, various controlled patterns of downward acceleration (less than 1 g) could be produced. During the falls, EMG activity was recorded simultaneously from the gastrocnemius, soleus, tibialis anterior, rectus femoris, and biceps femoris, along with knee and ankle joint angle in one leg. Subjects were tested eyes closed and also eyes open, both in darkness and in light using a wide field visual display. The display scene could be moved downwards at exactly the same velocity as the moving subject, left fixed with respect to the laboratory (normal visual field), or moved upwards at a speed equal to the subject's falling speed (upward moving visual field). Ten vestibularly normal subjects each underwent a total of 45 drops, experiencing three replications of each vision/motion combination used. Under normal visual field conditions, both short and long latency postural responses were seen, which were dependent on the magnitude of the acceleration stimulus. Several of the visual conditions significantly altered both the short and the long latency responses in most of the muscles tested. Effects were particularly prominent in the gastrocnemius and soleus, and were also more pronounced during slow (0.5 g) falls. The upward moving visual field condition increased the short latency EMG reaction in gastrocnemius and soleus for 0.5 g falls. A preliminary scheme for visual-vestibular interaction in short latency EMG responses is presented. Long latency responses are more variable and not conducive to a simple interpretation.R.W. Wicke was supported by NASA NSG-2032 and by the Veterans AdministrationC.M. Oman was also supported by NASA NSG-2032     

Dynamics of the Human Head-Neck System in the Horizontal Plane: Joint Properties with Respect to a Static Torque

The vestibular system has often been studied by perturbing the position of the head. This study was conducted to identify the dynamic properties of the head-neck system in response to horizontal plane perturbations. A quasilinear approach was used to quantify the dynamics of the head-neck system at different levels of static torque. An operating point was established by applying a static torque to the head with a helmet-based perturber. The head-neck dynamics were then probed with a rich spectrum, stochastic, torque perturbation. Impulse response functions (IRFs) were estimated from correlation measures, and parametric models were fit to the IRFs. The results indicated that when the mean torque was held constant, the head-neck system behaved like a second-order, underdamped, passive system between 0.5 and 10.0 Hz. The system was not strictly linear, however. The properties of the system were sensitive to the static component of the torque. As the mean torque increased, the effective stiffness and damping progressively increased, and did so such that the systems damping ratio remained essentially constant. The findings of the study will assist in designing stimuli that are well tolerated by subjects and can induce head motions that span the performance capabilities of the vestibular system. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr, 8710+e     

Heaviness perception. I. Constant involvement of haptically perceived size in weight discrimination

With visual input blocked, subjects in this study utilized fingertips only to investigate the involvement of haptically perceived size in heaviness perception among humans. The objects used for testing consisted of three sets – copper (CP), aluminum (AL), and plastic (PL) – of ten cubes of various weights (0.05–0.98 N). All of the cubes were covered with a smooth vinyl material to eliminate any extraneous input concerning the actual composition. Screens enclosed the working space to eliminate any possible visual cues. Each comparison was between a pair of cubes of the same material to eliminate the effect of density. Fifteen subjects (M=19.2, SD=0.68 years) attempted to judge differences in heaviness between the first and second cube in each trial that had been handed to them by the experimenter and were grasped between the thumb and the index finger. A total of 340 trials with 70 combinations of weight composed of 160 ascending trials (heavier), 160 descending trials (lighter), and 20 identical weight trials were pseudo-randomly presented to each subject for each material. Combinations of difference in weight and the number of trials were identical for all materials so that haptic size was regarded as the single independent factor. Accuracy of the subjects' responses for identical weight differences that resulted from placing a pair of cubes of the same combination was compared among the three materials. It was observed that a material like CP that had a lesser size effect facilitated significantly more accurate discrimination of the identical weight differences than PL with its greater size effect. This suggests that small changes in haptic size by the fingertips have a direct influence on heaviness perception when comparing objects of equal density. This finding, therefore, can be considered analogous to the size-weight illusion when comparing objects of unequal density. The findings of this study also suggest the constant involvement of haptic size in heaviness perception by humans along with the existence of a processing mechanism that integrates the factors of weight and haptic size in which heaviness increases either as weight increases or as size decreases, and vice versa. Electronic Publication