A Dynamic Model for Limb Selection

Two experiments and a model on limb selection are reported. In Experiment 1 left-handed and right-handed participants (N = 36) repeatedly used one hand for grasping a small cube. After a clear switch in the cube’s location, perseverative limb selection was revealed in both handedness groups. In Experiment 2 the cubes were presented in a clockwise and counter-clockwise sequence to right-handed participant (N = 15). A spatial delay in the switch point between right-hand use and left-hand use was observed. The model simulates the experiments, by implementing the multiple-timescale dynamics of the action-selection process underlying limb selection. It integrates two mechanisms that were earlier proposed to underlie this selection aspect of manual activity: limb dominance and attentional information. Finally, the model is used to simulate Gabbard et al.’s (1997) experiment, offering a concise coupling of strength and direction of handedness.

Reflexive Limb Selection and Control of Reach Direction to Moving Targets in Cats, Monkeys, and Humans

When we reach for an object, we have to decide which arm to use and the direction in which to move. According to the established view, this is voluntarily controlled and programmed in advance in time-consuming and elaborate computations. Here, we systematically tested the motor strategy used by cats, monkeys, and humans when catching an object moving at high velocity to the left or right. In all species, targets moving to the right selectively initiated movement of the right forelimb and vice versa for targets moving to the left. Movements were from the start directed toward a prospective target position. In humans, the earliest onset of electromyographic activity from start of motion of the target ranged from 90 to 110 ms in different subjects. This indicates that the selection of the arm and specification of movement direction did not result from the subject's voluntary decision, but were determined in a reflex-like manner by the parameters of the target motion. As a whole the data suggest that control of goal-directed arm movement relies largely on an innate neuronal network that, when activated by the visual signal from the target, automatically guides the arm throughout the entire movement toward the target. In the view of the present data, parametric programming of reaching in advance seems to be superfluous.

Fine Structure of Photospheric Faculae

Properties of the photospheric bright points associated with magnetic flux tubes are reviewed both in faculae (facular points) and in the photospheric network (network bright points - NBPs) out of active regions. A special attention is given to their size distribution, to their location relative to the granular, mesogranular and supergranular patterns, and to their relation with the small scale magnetic features, both in active and quiet regions. In particular a new granulation movie reveals that NBPs form in large intergranular spaces, compressed by the surrounding granules.At the center of the solar disk, bright points are much brighter than the mean photosphere; their contrast increases toward the limb up to μ = 0.3 − 0.2 and then decreases to the limb, as it is now widely accepted. But, all the published contrasts are of little significance because of center-to-limb selection effects. New center-to-limb contrast variations of individual network bright points are presented, which take into account the selection effects.