Vestibular and vestibulo-proprioceptive perception of motion in the horizontal plane in blindfolded man--I. Estimations of linear displacement

Finding Home: Landmark Ambiguity in Human Navigation

Memories of places often include landmark cues, i.e., information provided by the spatial arrangement of distinct objects with respect to the target location. To study how humans combine landmark information for navigation, we conducted two experiments: To this end, participants were either provided with auditory landmarks while walking in a large sports hall or with visual landmarks while walking on a virtual-reality treadmill setup. We found that participants cannot reliably locate their home position due to ambiguities in the spatial arrangement when only one or two uniform landmarks provide cues with respect to the target. With three visual landmarks that look alike, the task is solved without ambiguity, while audio landmarks need to play three unique sounds for a similar performance. This reduction in ambiguity through integration of landmark information from 1, 2, and 3 landmarks is well modeled using a probabilistic approach based on maximum likelihood estimation. Unlike any deterministic model of human navigation (based e.g., on distance or angle information), this probabilistic model predicted both the precision and accuracy of the human homing performance. To further examine how landmark cues are integrated we introduced systematic conflicts in the visual landmark configuration between training of the home position and tests of the homing performance. The participants integrated the spatial information from each landmark near-optimally to reduce spatial variability. When the conflict becomes big, this integration breaks down and precision is sacrificed for accuracy. That is, participants return again closer to the home position, because they start ignoring the deviant third landmark. Relying on two instead of three landmarks, however, goes along with responses that are scattered over a larger area, thus leading to higher variability. To model the breakdown of integration with increasing conflict, the probabilistic model based on a simple Gaussian distribution used for Experiment 1 needed a slide extension in from of a mixture of Gaussians. All parameters for the Mixture Model were fixed based on the homing performance in the baseline condition which contained a single landmark. from the 1-Landmark Condition. This way we found that the Mixture Model could predict the integration performance and its breakdown with no additional free parameters. Overall these data suggest that humans use similar optimal probabilistic strategies in visual and auditory navigation, integrating landmark information to improve homing precision and balance homing precision with homing accuracy.

Does the human odometer use an extrinsic or intrinsic metric?

It is commonly assumed that path integration is based on an extrinsic measure of the objective distance traversed during locomotion. In contrast, biological odometers may rely on embodied intrinsic measures, such as idiothetic information specific to an action mode. We investigated this question using a distance reproduction task in which participants traveled an outbound distance and then reproduced that distance using the same or a different action mode. The extrinsic model predicted that distance reproduction should be invariant across action modes, whereas the intrinsic model predicted invariance only within an action mode. In Experiment 1, we held the outbound mode constant while varying the response mode (walk-walk, walk-throw) and corrected for response production error (view-walk, view-throw). In Experiment 2, we crossed different gaits in the outbound and response modes (walk, gallop). In both cases, we found that distance reproduction was significantly more accurate when the outbound and response modes matched, consistent with the intrinsic model. The results indicate that the human odometer preferentially relies on an intrinsic, rather than an extrinsic, metric. This solution is sufficient to support successful path integration within an action mode (but not across action modes), without the difficulties of objective distance estimation.

Path integration: effect of curved path complexity and sensory system on blindfolded walking

Path integration refers to the ability to integrate continuous information of the direction and distance traveled by the system relative to the origin. Previous studies have investigated path integration through blindfolded walking along simple paths such as straight line and triangles. However, limited knowledge exists regarding the role of path complexity in path integration. Moreover, little is known about how information from different sensory input systems (like vision and proprioception) contributes to accurate path integration. The purpose of the current study was to investigate how sensory information and curved path complexity affect path integration. Forty blindfolded participants had to accurately reproduce a curved path and return to the origin. They were divided into four groups that differed in the curved path, circle (simple) or figure-eight (complex), and received either visual (previously seen) or proprioceptive (previously guided) information about the path before they reproduced it. The dependent variables used were average trajectory error, walking speed, and distance traveled. The results indicated that (a) both groups that walked on a circular path and both groups that received visual information produced greater accuracy in reproducing the path. Moreover, the performance of the group that received proprioceptive information and later walked on a figure-eight path was less accurate than their corresponding circular group. The groups that had the visual information also walked faster compared to the group that had proprioceptive information. Results of the current study highlight the roles of different sensory inputs while performing blindfolded walking for path integration.

Multisensory integration in the estimation of walked distances

When walking through space, both dynamic visual information (optic flow) and body-based information (proprioceptive and vestibular) jointly specify the magnitude of distance travelled. While recent evidence has demonstrated the extent to which each of these cues can be used independently, less is known about how they are integrated when simultaneously present. Many studies have shown that sensory information is integrated using a weighted linear sum, yet little is known about whether this holds true for the integration of visual and body-based cues for travelled distance perception. In this study using Virtual Reality technologies, participants first travelled a predefined distance and subsequently matched this distance by adjusting an egocentric, in-depth target. The visual stimulus consisted of a long hallway and was presented in stereo via a head-mounted display. Body-based cues were provided either by walking in a fully tracked free-walking space (Exp. 1) or by being passively moved in a wheelchair (Exp. 2). Travelled distances were provided either through optic flow alone, body-based cues alone or through both cues combined. In the combined condition, visually specified distances were either congruent (1.0×) or incongruent (0.7× or 1.4×) with distances specified by body-based cues. Responses reflect a consistent combined effect of both visual and body-based information, with an overall higher influence of body-based cues when walking and a higher influence of visual cues during passive movement. When comparing the results of Experiments 1 and 2, it is clear that both proprioceptive and vestibular cues contribute to travelled distance estimates during walking. These observed results were effectively described using a basic linear weighting model.

Integration of vestibular and proprioceptive signals for spatial updating

Spatial updating during self-motion typically involves the appropriate integration of both visual and non-visual cues, including vestibular and proprioceptive information. Here, we investigated how human observers combine these two non-visual cues during full-stride curvilinear walking. To obtain a continuous, real-time estimate of perceived position, observers were asked to continuously point toward a previously viewed target in the absence of vision. They did so while moving on a large circular treadmill under various movement conditions. Two conditions were designed to evaluate spatial updating when information was largely limited to either proprioceptive information (walking in place) or vestibular information (passive movement). A third condition evaluated updating when both sources of information were available (walking through space) and were either congruent or in conflict. During both the passive movement condition and while walking through space, the pattern of pointing behavior demonstrated evidence of accurate egocentric updating. In contrast, when walking in place, perceived self-motion was underestimated and participants always adjusted the pointer at a constant rate, irrespective of changes in the rate at which the participant moved relative to the target. The results are discussed in relation to the maximum likelihood estimation model of sensory integration. They show that when the two cues were congruent, estimates were combined, such that the variance of the adjustments was generally reduced. Results also suggest that when conflicts were introduced between the vestibular and proprioceptive cues, spatial updating was based on a weighted average of the two inputs.

Self-motion signals in vestibular nuclei neurons projecting to the thalamus in the alert squirrel monkey

Sixty vestibular nuclei neurons antidromically activated by electrical stimulation of the ventroposterior thalamus were recorded in two alert squirrel monkeys. The majority of these neurons were monosynaptically activated by vestibular nerve electrical stimulation. Forty-seven neurons responded to animal rotations around the earth-vertical axis; 16 of them also responded to translations in the horizontal plane. The mean sensitivity to 0.5-Hz rotations of 80 degrees /s velocity was 0.40 +/- 0.31 spikes.s(-1).deg(-1).s(-1). Rotational responses were in phase with stimulus velocity. Sensitivities to 0.5-Hz translations of 0.1 g acceleration varied from 92.2 to 359 spikes.s(-1).g(-1) and response phases varied from 10.1 degrees lead to -98 degrees lag. The firing behavior in 28 neurons was studied during rotation of the whole animal, of the trunk, and voluntary and involuntary rotations of the head. Two classes of vestibulothalamic neurons were distinguished. One class of neurons generated signals related to movement of the head that were similar either when the head and trunk move together or when the head moves on the stationary trunk. A fraction of these neurons fired during involuntary head movements only. A second class of neurons generated signals related to movement of the trunk. They responded when the trunk moved alone or simultaneously with the head, but did not respond to head rotations while the trunk was stationary.

Direct evidence for distance measurement via flexible stride integration in the fiddler crab

While on foraging excursions, fiddler crabs track their burrow location despite having no visual contact with it . They do this by path integration, a common navigational process in which motion vectors (the direction and distance of animals' movements) are summed to form a single "home vector" linking the current location with the point of origin. Here, we identify the mechanism by which the integrator measures distance, by decoupling motor output from both inertial and visual feedback. Fiddler crabs were passively translated to a position such that the home vector lay across an acetate sheet on the ground. After being frightened, crabs tried to escape but slipped as they did so. Detailed high-speed video analysis reveals that crabs measure distance by integrating strides, rather than linear acceleration or optic flow: the number of steps they took depended on both the length of the home vector and how large their steps were, whether they slipped and fell short or not. This is the most direct evidence to date of a stride integrator that is flexible enough to account for significant variation in stride length and frequency.

Vestibular syndrome: a change in internal spatial representation

The vestibular system contributes to a wide range of functions from reflexes to spatial representation. This paper reviews behavioral, perceptive, and cognitive data that highlight the role of changes in internal spatial representation on the vestibular syndrome. Firstly, we review how visual vertical perception and postural orientation depend on multiple reference frames and multisensory integration and how reference frames are selected according to the status of the peripheral vestibular system (i.e., unilateral or bilateral hyporeflexia), the environmental constraints (i.e., sensory cues), and the postural constraints (i.e., balance control). We show how changes in reference frames are able to modify vestibular lesion-induced postural and locomotor deficits and propose that fast changes in reference frame may be considered as fast-adaptive processes after vestibular loss. Secondly, we review data dealing with the influence of vestibular loss on higher levels of internal representation sustaining spatial orientation and navigation. Particular emphasis is placed on spatial performance according to task complexity (i.e., the required level of spatial knowledge) and to the sensory cues available to define the position and orientation within the environment (i.e., real navigation in darkness or visual virtual navigation without any actual self-motion). We suggest that vestibular signals are necessary for other sensory cues to be properly integrated and that vestibular cues are involved in extrapersonal space representation. In this respect, vestibular-induced changes would be based on a dynamic mental representation of space that is continuously updated and that supports fast-adaptive processes.

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.

Activity of Ventroposterior Thalamus Neurons During Rotation and Translation in the Horizontal Plane in the Alert Squirrel Monkey

The firing behavior of 107 vestibular-sensitive neurons in the ventroposterior thalamus was studied in two alert squirrel monkeys during whole body rotation and translation in the horizontal plane. Vestibular-sensitive neurons were distributed primarily along the anterior and posterior borders of ventroposterior nuclei; three clusters of these neurons could be distinguished based on their location and inputs. Eighty-four neurons responded to rotation; 66 (78%) of them responded to rotation only and 18 (22%) to both rotation and translation. Forty-one neurons were sensitive to linear translation; 23 (56%) of them responded to translation only. The population rotational response to 0.5-Hz sinusoids with a peak velocity of 40°/s showed a gain of 0.23 ± 0.15 spike·s−1·deg−1·s−1 and phase lagging behind the angular velocity by −9.3 ± 34.1°. Although rotational response amplitude increased with the stimulus velocity across the range 4–100°/s, the rotational sensitivity decreased with and was inversely proportional to the stimulus velocity. The rotational response amplitude and sensitivity increased with the stimulus frequency across the range 0.2–4.0 Hz. The population response to sinusoidal translation at 0.5 Hz and 0.1 g amplitude had a gain of 111.3 ± 53.7 spikes·s−1· g−1 and lagged behind stimulus acceleration by −71.9 ± 42.6°. Translational sensitivity decreased as acceleration increased and this was inversely proportional to the square root of the acceleration. Results of this study imply that changes in the discharge rate of vestibular-sensitive thalamic neurons can be approximated using power functions of the angular and linear velocity of spatial motion.

Direction and distance deficits in path integration after unilateral vestibular loss depend on task complexity

The effects of peripheral vestibular disorders on the direction and distance components of the internal spatial representation were investigated. The ability of Menière's patients to perform path integration was assessed in different situations aimed at differentiating the level of spatial processing (simple versus complex tasks), the available sensory cues (proprioceptive, vestibular, or visual conditions), and the side of the path (towards the healthy versus the lesioned side). After exploring two legs of a triangle, participants were required either to reproduce the exploration path, to follow the reverse path, or to take a shortcut to the starting point of the path (triangle completion). Patients' performances were recorded before unilateral vestibular neurotomy (UVN) and during the time-course of recovery (1 week and 1 month) and were compared to those of matched control subjects tested at similar time intervals. Both the angular and linear path components of the trajectory were impaired for patients compared to controls. However, deficits were restricted to the complex tasks, which required a higher level of spatial processing. Most deficits were maximal 1 week after UVN, and some remained up to the first post-operative month. Spatial representation was differentially impaired according to the available sensory cues: deficits were absent in active locomotor blindfolded condition, appeared in conditions involving visual and vestibular information, and were maximal when visual cues alone were available. Finally, concerning the side of the path, unilateral vestibular loss led to global impairment of the internal spatial representation, yet some asymmetrical spatial performances were observed 1 week after UVN. On the whole, results suggest that the environment experienced by the patients is different after UVN and that a different internal spatial representation is constructed, especially for tasks requiring high levels of spatial processing.

Perceiving a stable world during active rotational and translational head movements

When a person moves through the world, the associated visual displacement of the environment in the opposite direction is not usually seen as external movement but rather as a changing view of a stable world. We measured the amount of visual motion that can be tolerated as compatible with the perception of moving within a stable world during active, sinusoidal, translational and rotational head movement. Head movements were monitored by means of a low-latency, mechanical head tracker and the information was used to update a helmet-mounted visual display. A variable gain was introduced between the head tracker and the display. Ten subjects adjusted this gain until the visual display appeared stable during sinusoidal yaw, pitch and roll head rotations and naso-occipital, inter-aural and dorso-ventral translations at 0.5 Hz. Each head movement was tested with movement either orthogonal to or parallel with gravity. A wide spread of gains was accepted as stable (0.8 to 1.4 for rotation and 1.1 to 1.8 for translation). The gain most likely to be perceived as stable was greater than that required by the geometry (1.2 for rotation; 1.4 for translation). For rotational motion, the mean gains were the same for all axes. For translation there was no effect of whether the movement was inter-aural (mean gain 1.6) or dorso-ventral (mean gain 1.5) and no effect of the relative orientation of the translation direction relative to gravity. However translation in the naso-occipital direction was associated with more closely veridical settings (mean gain 1.1) and narrower standard deviations than in other directions. These findings are discussed in terms of visual and non-visual contributions to the perception of an earth-stable environment during active head movement.

Self-Motion Perception During Locomotor Recalibration: More Than Meets the Eye

Do locomotor aftereffects depend specifically on visual feedback? In 7 experiments, 116 college students were tested, with closed eyes, at stationary running or at walking to a previewed target after adaptation, with closed eyes, to treadmill locomotion. Subjects showed faster inadvertent drift during stationary running and increased distance (overshoot) when walking to a target. Overshoot seemed to saturate (i.e., reach a ceiling) at 17% after as little as 1 min of adaptation. Sidestepping at test reduced overshoot, suggesting motor specificity. But inadvertent drift effects were decreased if the eyes were open and the treadmill was drawn through the environment during adaptation, indicating that these effects involve self-motion perception. Differences in expression of inadvertent drift and of overshoot after adaptation to treadmill locomotion may have been due to different sets of ancillary cues available for the 2 tasks. Self-motion perception is multimodal.

Multifactorial interactions involved in linear self-transport distance estimate: a place for time

As the vestibular system is the only sensory organ whose primary function is self-motion detection, we examined the conditions under which the otoliths, which detect the linear acceleration of the head, could be used to estimate traveled distance. In order to isolate the contribution of the otoliths (with the somatosensory system) from contributions of the visual and motor systems subjects were transported in darkness. We initially hypothesized that self-transport with continuously varying linear velocity should facilitate distance computation by continuously stimulating the otoliths, and that active control of self-motion should also help subjects estimate the distance traveled. However, it was found that the distance covered during self-motion is actually better estimated when transport velocity is quasi-constant. Nevertheless, such estimates strongly depend upon velocity magnitude; subjects show an idiosyncratic preferred self-motion velocity for which distance measurements are most accurate. Furthermore, the active control of self-transport improves estimates of self-motion mainly because the subjects can then adopt a constant velocity, and more precisely their preferred one. It was finally found that subjects mentally count in order to assess their displacement length, and that time perception is indeed disturbed by varying self-motion velocity.

Does mental simulation of following a path improve navigation performance without vision?

We recorded the paths of subjects who walked along a memorized hexagonal route without vision, and studied the impact of previous mental simulation of this activity on how well the path could be reproduced from memory. We compared two kinds of mental simulation to actual physical practice, rest, or simple memorization of the path. The results indicated that mental simulation led to better reproduction of the global shape of a path than rest, and in fact mental simulation was as effective as actual physical practice. However, this result occurred only for 'simple shapes' when the lengths of the sides of the path were kept constant. Nevertheless, this level of performance was not reached when the complexity of the shape was increased by altering the lengths of sides, even when keeping constant the angles between consecutive sides. This finding indicates that the internal representation of space depends on geometric properties of the environment in which the subjects operate. Mental simulation appears to affect both a map-like and route-like representation of the environment.