John F. Soechting

Early stages in a sensorimotor transformation

We present a model for several early stages of the sensorimotor transformations involved in targeted arm movement. In psychophysical experiments, human subjects pointed to the remembered locations of randomly placed targets in three-dimensional space. They made consistent errors in distance, and from these errors stages in the sensorimotor transformation were deduced. When subjects attempted to move the right index finger to a virtual target they consistently undershot the distance of the more distal targets. Other experiments indicated that the error was in the sensorimotor transformation rather than in the perception of distance. The error was most consistent when evaluated using a spherical coordinate system based at the right shoulder, indicating that the neural representation of target parameters is transformed from a retinocentric representation to a shoulder-centered representation. According to the model, the error in distance results from the neural implementation of a linear approximation in the algorithm to transform shoulder-centered target parameters into a set of arm orientations appropriate for placing the finger on the target. The transformation to final arm orientations places visually derived information into a frame of reference where it can readily be combined with kinesthetically derived information about initial arm orientations. The combination of these representations of initial and final arm orientations could give rise to the representation of movement direction recorded in the motor cortex by Georgopoulos and his colleagues. Later stages, such as the transformation from kinematic (position) to dynamic (force) parameters, or to levels of muscle activation, are beyond the scope of the present model.

Effect of Target Size on Spatial and Temporal Characteristics of a Pointing Movement in Man

SummaryThe effects of constraints related to movement accuracy on the spatial and temporal characteristics of pointing movements of the arm to a target were investigated. It was found that movement time increased, even at slow speeds, when target size decreased. Spatial variability of the trajectory of the index finger was also reduced, but only in proximity to the target, when higher accuracy was demanded while variability of motion at the wrist showed little change. The effect of varying the angular orientation of the target on the trajectories of the wrist and finger was also investigated. The data support the hypothesis that motion at the shoulder and elbow joints, which is closely linked, is determined primarily by target position while motion at the wrist joint, which is only loosely coupled to the motion at the more proximal joints, is related principally to the angular orientation of the target in space. The data also suggest that wrist motion is controlled separately from motion at the more proximal joints.

Does position sense at the elbow reflect a sense of elbow joint angle or one of limb orientation

Abstract The question is addressed of whether position sense at the elbow joint is best described as a sense of the elbow joint angle or as a sense of the orientation of the limb, limb orientation being defined as the angle the forearm makes with the vertical. When the upper arm is vertical, these two measures are equivalent; however, in the more general case of non-zero angles of forward flexion at the shoulder, they differ. Subjects were asked to match either joint angles or limb orientation of their two arms. It was found that the error in matching limb orientation was significantly smaller and that it was unaffected by an inequality in the loading on the two arms.

An algorithm for the generation of curvilinear wrist motion in an arbitrary plane in three-dimensional space

The elements of an algorithm are presented which predicts for some simple forms (circles and ellipses) the kinematic and figural aspects of the trajectories of the human wrist when these are drawn in any arbitrary plane of free, three-dimensional space. The algorithm is based on theoretical considerations and experimental data and specifies in a unique way the angular motion at the shoulder and elbow joints by utilizing a coordinate transformation, which is only approximate, between the chosen extrinsic (trajectory) and intrinsic (joint angles) parameters. A way to extend the use of this algorithm to generate any arbitrary complex movement in all possible planes of space is also suggested.

Psychophysical determination of coordinate representation of human arm orientation

The coordinate representation of the sense of limb orientation was investigated psychophysically by asking subjects to match the orientation of the arm or of the forearm in several different coordinate representations. Movement of all degrees of freedom of one arm was permitted while movement of the other limb was restricted to the degree of freedom investigated in that particular experiment. Performance on the tasks was assessed by calculating the standard deviation of the difference in the angles of the two limbs. According to this criterion, we suggest that limb orientation is represented by the angular elevation of the limb and by the yaw angle, referred to a spatial reference frame.

The mechanical behavior of the human forearm in response to transient perturbations

Static and dynamic components of mechanical impedance of human forearm were evaluated by applying two kinds of perturbations: 1) large viscoelastic loads, and 2) small pseudo-random perturbations. When the task involved the active resistance of the perturbations, both stiffness and viscosity increased relatively to their values in the passive task, the increment in stiffness being larger than that in viscosity. The time course of such changes was investigated during the transition between the two operating points defined by the instructions “do not resist” and “resist” the applied perturbations. The changes in stiffness and viscosity were relatively slow, those in the latter lagging behind those in the former.

Coordination of arm movements in three-dimensional space. Sensorimotor mapping during drawing movement

In this paper data are presented concerning the motion of limb segments during drawing movements executed in different planes in free space. The technique used allows the determination of the wrist and elbow positions in space as well as the measurement of the elbow angle of extension. Other kinematic variables are determined trigonometrically. Elbow and shoulder torque is also calculated. For circles and ellipses, it was found that the motion at the wrist is sinusoidal in two orthogonal directions in the plane of motion. Angular motion, when described by a set of angles previously identified psychophysically as constituting an appropriate coordinate system, is also sinusoidal. Although the number of degrees of freedom of the arm affords many possible ways of performing the task, there is a fixed phase relation between the angles of elevation of the upper arm and forearm for naturally executed movements in all planes of space. Also, the phase of the yaw angles of the upper arm and forearm relative to the angles of elevation are related to the plane of motion and to the slant of ellipses in a fixed manner. There is a simple mapping between angular motion and intended wrist trajectory. Because this mapping is not valid for all planes of space, the actual trajectory can deviate from the intended one. However, the subject has no cognizance of the distortion. The calculated torque deviates substantially from sinusoidal and does change significantly when the same movement is executed in different planes. Results of simulations and mathematical analysis indicate that the fixed phase relationship between angles of elevation leads to a minimal distortion from sinusoidal motion at the wrist in an average sense and that the characteristic distortions observed in the sagittal plane result inevitably from this constraint on the phase relations. The results support the assumption that the topology of the sensorimotor map used for the production of the movement and for its perception is the same. The problem of invariant relationships between kinematic parameters is discussed and the suggestion is made that they represent a general constraint, leading through learning and practice to an optimal solution in an average sense.

Path constraints on point-to-point arm movements in three-dimensional space

In this paper data are presented concerning the kinematic and dynamic characteristics of point-to-point arm movements which are inwardly or outwardly directed in three-dimensional space. Elbow and wrist position as well as elbow angle of extension were measured. From these data, other angles were computed trigonometrically and elbow and shoulder torques were calculated. Some of the angles describing arm and forearm motion were found to be linearly related for any given movement. Changes in shoulder and elbow torque were found to be similar to those described for movements restricted to one degree of freedom. Shoulder and elbow motions were not affected when it was required that the orientation of the hand in space remain constant. These observations were taken to indicate that shoulder and elbow motions are tightly coupled for movements in three-dimensional space and that wrist motion has no influence on this coupling. Linear relations between angles express such coupling. They are taken to result from functional constraints and may facilitate the mapping between extrinsic and intrinsic coordinate systems. Some of the observations pertaining to the torque lead to the hypothesis of a further constraint limiting the number of possible trajectories in a point-to-point movement.

EMG responses to load perturbations of the upper limb: effect of dynamic coupling between shoulder and elbow motion

SummaryLoad perturbations were applied to the arm of human subjects under conditions where both limb segments (upper arm and forearm) were free to move. The perturbations consisted of pulses of torque 50 ms in duration and of pseudo-random sequences of such pulses. They were applied to either the forearm or the upper arm. Under all conditions, the perturbations resulted in angular motion at the shoulder and elbow joints and evoked consistent responses in muscles acting about these joints (biceps, triceps, anterior and posterior deltoid). Activity in biceps and triceps was not related simply to angular motion at the elbow joint. For example, activation of biceps could be evoked during elbow flexion (by applying a torque perturbation at the shoulder) as well as during elbow extension (by applying a torque perturbation at the elbow). The effect of varying degrees of dynamic coupling between upper arm and forearm on EMG responses was investigated by applying torque perturbations to the upper arm over a wide range of elbow angles. When the forearm is extended, such a perturbation induces a greater amount of elbow flexion than when the forearm is in a flexed position. The results of these experiments showed that the larger was the amount of flexion of the forearm induced by the perturbation, the larger was the activation of biceps. The results are incompatible with the notion of a negative feedback of total muscle length as being responsible for the EMG activity following the load perturbations. It is suggested that the EMG responses can best be interpreted functionally in terms of parameters more global than muscle length. Among such global parameters, changes in net torque at a joint resulting from the perturbation gave the best correlation with the pattern of EMG activities observed.

Some factors pertinent to the organization and control of arm movements

Trajectories of arm movement during a pointing task were shown to be unaffected by large loads. Moreover, when the effective arm length was changed, the target was still approached along a direct trajectory. It is concluded that: (1) compensation for load occurs automatically without affecting those constraints on the basis of which movements are organized; (2) that the effective lengths of body segments as well as joint angles constitute variables utilized in the organization of trajectories.