1. The aim of this study was to describe the time-varying changes in the mechanical parameters of a multijointed limb. The parameters we considered are the coefficients of stiffness, viscosity, and inertia. Continuous pseudorandom perturbations were applied at the elbow joint during a catching task. A modified version of an ensemble technique was used for the identification of time-varying parameters. Torques at the elbow and wrist joints were then modeled with a linear combination of the changes in angular position and velocity weighted by the matrix of angular stiffness and the matrix of angular viscosity, respectively. Control experiments were also performed that involved the stationary maintenance of a given limb posture by resisting actively the applied perturbations. Different limb postures were examined in each such experiment to investigate the dependence of the mechanical parameters on limb geometry. 2. The technique for the identification of limb mechanical parameters proved adequate. The input perturbations applied at the elbow joint elicited angular oscillations at the wrist essentially uncorrelated with those produced at the elbow. The frequency of oscillation is much higher at the wrist than at the elbow, mainly because of the smaller inertia. The variance accounted for by the model was approximately 80% under both stationary and time-varying conditions; in the latter case the value did not vary significantly throughout the task. In addition, the model predicted values of the inertial parameters that were close to the anthropometric measures, and it reproduced the stepwise increase in limb inertia that occurs at the time the ball is held in the hand. 3. The values of angular stiffness and viscosity estimated under stationary conditions did not vary significantly with joint angle, in agreement with previous results obtained under quasi-static postural conditions. The matrix of the coefficients of angular stiffness was not symmetrical, indicating a prominent role for nonautogenic reflex feedbacks with unequal gains for elbow and wrist muscles. 4. A complex temporal modulation of angular stiffness and viscosity was observed during the catching task. The changes in the direct coefficients of angular stiffness tended to covary with those in the coupling coefficients from trial start up to approximately 30 ms before impact time. Around impact time, however, there was a complete dissociation: the direct terms peaked, whereas the coupling terms dropped. The direct terms of angular viscosity also increased before impact, whereas the viscosity coupling terms remained close to zero throughout.(ABSTRACT TRUNCATED AT 400 WORDS)

To intercept a fast target at destination, hand movements must be centrally triggered ahead of target arrival to compensate for neuromechanical delays. The role of visual-motion cortical areas is unclear. They likely feed downstream parietofrontal networks with signals reflecting target motion, but do they also contribute internal timing signals to trigger the motor response? We disrupted the activity of human temporoparietal junction (TPJ) and middle temporal area (hMT/V5ϩ) by means of transcranial magnetic stimulation (TMS) while subjects pressed a button to intercept targets accelerated or decelerated in the vertical or horizontal direction. Target speed was randomized, making arrival time unpredictable across trials. We used either repetitive TMS (rTMS) before task execution or doublepulse TMS (dpTMS) during target motion. We found that after rTMS and dpTMS at 100 -200 ms from motion onset, but not after dpTMS at 300 -400 ms, the button-press responses occurred earlier than in the control, with time shifts independent of target speed. This suggests that activity in TPJ and hMT/V5ϩ can feed downstream regions not only with visual-motion information, but also with internal timing signals used for interception at destination. Moreover, we found that TMS of hMT/V5ϩ affected interception of all tested motion types, whereas TMS of TPJ significantly affected only interception of motion coherent with natural gravity. TPJ might specifically gate visual-motion information according to an internal model of the effects of gravity.

It has been hypothesized that the end-point position of reaching may be specified in an egocentric frame of reference. In most previous studies, however, reaching was toward a memorized target, rather than an actual target. Thus, the role played by sensorimotor transformation could not be disassociated from the role played by storage in short-term memory. In the present study the direct process of sensorimotor transformation was investigated in reaching toward continuously visible targets that need not be stored in memory. A virtual reality system was used to present visual targets in different three-dimensional (3D) locations in two different tasks, one with visual feedback of the hand and arm position (Seen Hand) and the other without such feedback (Unseen Hand). In the Seen Hand task, the axes of maximum variability and of maximum contraction converge toward the mid-point between the eyes. In the Unseen Hand task only the maximum contraction correlates with the sight-line and the axes of maximum variability are not viewer-centered but rotate anti-clockwise around the body and the effector arm during the move from the right to the left workspace. The bulk of findings from these and previous experiments support the hypothesis of a two-stage process, with a gradual transformation from viewer-centered to body-centered and arm-centered coordinates. Retinal, extra-retinal and arm-related signals appear to be progressively combined in superior and inferior parietal areas, giving rise to egocentric representations of the end-point position of reaching.

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