Spatial and physical frames of reference in positioning a limb

Gravitational torque partially accounts for proprioceptive acuity

Proprioception of the upper extremity is commonly measured using joint position sense tasks. Recent evidence suggests heightened position sense at elevation angles in the shoulder and elbow near 90° in the sagittal plane. The influence of external torque has been suggested to play a pivotal role in the heightened acuity in elevated positions due to increased moment arm with respect to gravitational vectors. We hypothesized that the addition of a buoyance vector in opposition to this gravitational vector would reduce the influence of torque on proprioceptive acuity, resulting in consistent position sense errors with respect to elevation angle. Joint position sense was measured using an apple iPod touch using a custom application. Participants elevated their arm to 50, 70 and 90° of elevation in the sagittal plane in the absence of visual feedback. Data were collected in three conditions, normal (control) and submerged and weighted. We found angular differences between control and submerged conditions, but not between control and weighted conditions. When the arm was elevated to 90° in the submerged condition, we found participants undershot the target position by approximately -0.5° with the addition of the buoyancy force vector. Participants without this buoyancy vector at the same target position consistently overshot the target by approximately 2.0°, which suggests that external torque may be more involved in the direction of proprioceptive errors more than the magnitude of the error as the magnitude of the difference was relatively small (2.5°).

Physics and Psychology of the Muscle Sense

The sensibility associated with muscles contributes little to the conceptual content of current psychology. In part, this is because perceptions achieved through the muscle sense typically go unnoticed. Nonetheless, researchers have discovered a rich variety of muscle-based perceptual capabilities—such as those relating to held objects, probed objects, and body segments—that seem to depend on quantities well known in physics, quantities that reflect how the mass of an object or limb is distributed. The conceptual and technical issues posed by these capabilities warrant study by psychologists interested in the general problems of space perception, action, and selective attention.

Obtaining information by dynamic (effortful) touching

Dynamic touching is effortful touching. It entails deformation of muscles and fascia and activation of the embedded mechanoreceptors, as when an object is supported and moved by the body. It is realized as exploratory activities that can vary widely in spatial and temporal extents (a momentary heft, an extended walk). Research has revealed the potential of dynamic touching for obtaining non-visual information about the body (e.g. limb orientation), attachments to the body (e.g. an object's height and width) and the relation of the body both to attachments (e.g. hand's location on a grasped object) and surrounding surfaces (e.g. places and their distances). Invariants over the exploratory activity (e.g. moments of a wielded object's mass distribution) seem to ground this 'information about'. The conception of a haptic medium as a nested tensegrity structure has been proposed to express the obtained information realized by myofascia deformation, by its invariants and transformations. The tensegrity proposal rationalizes the relative indifference of dynamic touch to the site of mechanical contact (hand, foot, torso or probe) and the overtness of exploratory activity. It also provides a framework for dynamic touching's fractal nature, and the finding that its degree of fractality may matter to its accomplishments.

Unconstrained shoulder joint position sense does not change with body orientation

Our knowledge of the role of muscle activation on proprioception is incomplete. Previous work has either focused on comparing active and passive motions or manipulated both muscle activation and joint angles simultaneously. We conducted an experiment at the shoulder in which subjects' trunks were tilted backwards to decouple joint angle from joint torque. Twenty three healthy subjects underwent testing in an unconstrained joint position sense task. Kinematics were measured with a magnetic tracking device. The joint position sense task consisted of subjects moving their arms to a predetermined orientation in space with the help of visual feedback from the magnetic tracking device presented to the subjects through a head-mounted display. Subjects were then asked to reproduce the presented joint position in the absence of visual feedback. The protocol was performed under two tilts: upright and trunk tilted back 45 degrees. This allowed for a comparison of joint position sense at different joint angles (at the same resistive torque) and at different resistive torques (at the same joint angles). When comparing these two tilts, we found that matching based on elevation angle demonstrated no significant difference, while matching based on torques did find differences. These results implicate elevation angle at the shoulder as playing a more important role in modulating joint position sense than joint torque.

Perception of limb orientation in the vertical plane depends on center of mass rather than inertial eigenvectors

We performed two experiments to test the hypothesis that the perception of limb orientation depends on inertial eigenvectors (e_i) against the alternative hypothesis that it depends on the center of mass vector (CM). Whereas e_i constrains the dynamic torques involved in angular rotation, CM constrains the static torque necessary to keep the limb aloft in the gravitational field. Hence, possible effects of e_i and CM on kinesthetic judgments must be related to the dynamic and static torques, respectively, involved in moving and positioning a limb. In the first experiment, blindfolded participants matched, with upper arms supported, the orientation of their forearms while the forearms’ e_i and CM were manipulated relative to the elbow. The manipulation of the vector CM alone induced a matching bias, as did the combined manipulation of e_i and CM, whereas the manipulation of e_i alone did not. In the second experiment, participants positioned their unseen and unsupported right arm at an indicated spatial configuration while e_i and CM of the right forearm were manipulated as in Experiment 1. As in the first experiment, forearm positioning was affected by the independent manipulation of CM and the combined manipulation of e_i and CM, but not by the independent variation of e_i. Moreover, none of the manipulations affected upper arm positioning. These results refute the claim that the perception of limb orientation (in the vertical plane) is based on e_i and demonstrate, for the first time, the implication of a limb segment’s CM in the perception of its orientation.

Role of the inertial eigenvectors in proprioception near the limits of arm adduction range of motion

Common everyday activities such as reaching for a cup, combing one's hair, or positioning a tool involve the accurate registration of the limb's orientation relative to the body and the environment. Past work has demonstrated that the ability of humans to perceive the spatial orientation of an occluded arm is tied to the arm's inertial eigenvectors, invariant mechanical parameters corresponding to a limb's axes of rotational symmetry. In a recent study participants pointed to visible targets while the arms' inertial eigenvectors were displaced by asymmetrically weighted splints. The lack of an effect of those splints on pointing error led them to conclude that inertial eigenvectors are not the primary invariants used for proprioception. However, their data suggested an effect of the eigenvectors, but only for conditions in which pointing did not require placing the arm near the limits of shoulder adduction range of motion. In the present study we found a significant effect of displacing the eigenvector for all tested arm angles, regardless of degree of shoulder adduction. Overall, the results supported the hypothesis that proprioception is based on sensitivity to the limb's inertial eigenvectors, with no reduction of the eigenvector effect when adducting the arm so as to reach far across the body midline.

Differential exploitation of the inertia tensor in multi-joint arm reaching

The identification of the kinaesthetic information used for directing 3D multi-joint arm movements toward a target remains an open question. Several psychophysical studies have suggested that the ability to perceive and control the spatial orientation of our limbs depends on the exploitation of the eigenvectors (e (3)) of the inertia tensor (I ( ij )), which correspond to the arm rotational inertial axes. The present experiment aimed at investigating whether e (3) was used as a collective variable to direct the masses toward the target and hence to control the spatial accuracy of the final hand position. Natural, unconstrained, three-dimensional multi-joint reaching movements were submitted to alterations of forearm mass distribution. Given the existence of several "sensorimotor strategies" for the control of arm movements, the participants were a priori contrasted and ranged in groups according to their reliance on either visual or kinaesthetic information. The results indicated (1) the dependency of the arm's directional control on I ( ij ) parameters, (2) a non-linear relationship between the performance predicted by the inertia tensor and the observed performance, depending on the deviation amplitude and (3) the presence of a large inter-individual variability suggesting the existence of different strategies, including proprioceptive compensation mechanisms. This study validates in unconstrained multi-joint arm movements the exploitation of the inertia tensor by the central nervous system, thus simplifying the coordination of the segments' masses during reaching. The results also provide evidence for the existence of motor alternatives in exploiting proprioceptive information that may depend on spatial referencing modes.

Is perception of upper body orientation based on the inertia tensor? Normogravity versus microgravity conditions

During lateral leg raising, a synergistic inclination of the supporting leg and trunk in the opposite direction to the leg movement is performed in order to preserve equilibrium. As first hypothesized by Pagano and Turvey (J Exp Psychol Hum Percept Perform, 1995, 21:1070-1087), the perception of limb orientation could be based on the orientation of the limb's inertia tensor. The purpose of this study was thus to explore whether the final upper body orientation (trunk inclination relative to vertical) depends on changes in the trunk inertia tensor. We imposed a loading condition, with total mass of 4 kg added to the subject's trunk in either a symmetrical or asymmetrical configuration. This changed the orientation of the trunk inertia tensor while keeping the total trunk mass constant. In order to separate any effects of the inertia tensor from the effects of gravitational torque, the experiment was carried out in normo- and microgravity. The results indicated that in normogravity the same final upper body orientation was maintained irrespective of the loading condition. In microgravity, regardless of loading conditions the same (but different from the normogravity) orientation of the upper body was achieved through different joint organizations: two joints (the hip and ankle joints of the supporting leg) in the asymmetrical loading condition, and one (hip) in the symmetrical loading condition. In order to determine whether the different orientations of the inertia tensor were perceived during the movement, the interjoint coordination was quantified by performing a principal components analysis (PCA) on the supporting and moving hips and on the supporting ankle joints. It was expected that different loading conditions would modify the principal component of the PCA. In normogravity, asymmetrical loading decreased the coupling between joints, while in microgravity a strong coupling was preserved whatever the loading condition. It was concluded that the trunk inertia tensor did not play a role during the lateral leg raising task because in spite of the absence of gravitational torque the final upper body orientation and the interjoint coupling were not influenced.

Haptic alignment of the hands in three dimensions

The problem of constancy of perception over variations in sensory input applies as much to proprioception as it does to vision and audition. The authors conducted an experiment in which blindfolded participants (N = 6) attempted to produce a constant relation between the left and the right hands' attitudes (roll, pitch, and yaw) at different distances and directions relative to the body. The measure of perceptual constancy over the changing 3-dimensional configurations of the arms' 14 joint degrees of freedom was the hands' deviations from the required attitudes when participants perceived that they satisfied the required attitudes. Two slant spinors quantified the attitudes of the hands relative to the body or the laboratory reference frame. A vergence spinor quantified the attitude of the right hand relative to that of the left hand. The scalar components of the spinors were found to be idiosyncratic to, and reasonably constant for, individual participants, as were the bivector components (revealed through spherical graphical and statistical procedures). Achieving particular attitudinal relations to the body and between contralateral limb segments are individual-specific perception-action skills. They are constrained by haptic invariants approximated by the slant and vergence spinors.

Inertial constraints on limb proprioception are independent of visual calibration

When the coincidence of a limb's spatial axes and inertial eigenvectors is broken, haptic proprioception of the limb's position conforms to the eigenvectors. Additionally, when prisms break the coincidence between an arm's visual and actual positions, haptic proprioception is shifted toward the visual-spatial direction. In 3 experiments, variation of the arm's mass distribution was combined with prism adaptation to investigate the hypothesis that the proprioceptive effects of inertial and visual manipulations are additive. This hypothesis was supported across manipulations of plane of motion, body posture, proprioceptive target, and proprioceptive experience during prism adaptation. Haptic proprioception seems to depend on local, physical reference frames that are relative to the physical reference frames for the body's environmental position and orientation.

Dynamics of effortful touch and interlimb coordination

In biological movement systems, the level of muscular-articular links is responsible for assembling highly reliable, stable, and reproducible coordination patterns involving very many joints and muscles. Research shows that the important perceptual capabilities of this level arise from the bulk sensitivity of muscles and tendons, so-called effortful or dynamic touch, to the quantities of rotational dynamics that remain invariant (such as the inertia tensor) over variations in rotational forces and motions. The power laws characterizing this sensitivity point to underlying fractal (self-similar) processes. Other research shows that the hallmark ability of this level to produce repetitive interlimb coordinations can be addressed through a dynamics of coordination in which equations express the time-evolution of collective neuromuscular states. This research also suggests that the assembled rhythms exploit the unique blend of stability and variability characteristic of low-dimensional chaotic motion on strange attractors. In overview, research into the capabilities of the level of muscular-articular links highlights the importance of applying classical and modern (nonlinear) dynamics to understanding the assembly and perceptual control of biological movements.