The following factors underlying behavioral plasticity are discussed: (1) reflex adaptability and its role in the voluntary control of movement, (2) degrees of freedom and motor equivalence, and (3) the problem of the discrete organization of motor behavior. Our discussion concerns a variety of innate motor patterns, with emphasis on the wiping reflex in the frog.It is proposed that central regulation of stretch reflex thresholds governs voluntary control over muscle force and length. This suggestion is an integral part of the equilibrium-point hypothesis, two versions of which are compared.Kinematic analysis of the wiping reflex in the spinal frog has shown that each stimulated skin site is associated with a group of different but equally effective trajectories directed to the target site. Such phenomena reflect the principle of motor equivalence -the capacity of the neuronal structures responsible for movement to select one or another of a set of possible trajectories leading to the goal. Redundancy of degrees of freedom at the neuronal level as well as at the mechanical level of the body's joints makes motor equivalence possible. This sort of equivalence accommodates the overall flexibility of motor behavior.An integrated behavioral act or a single movement consists of dynamic components. We distinguish six components for the wiping reflex, each associated with a certain functional goal, specific body positions, and motor-equivalent movement patterns. The nervous system can combine the available components in various ways in forming integrated behavioral sequences. The significance of command neuronal organization is discussed with respect to (1) the combinatory strategy of the nervous system and (2) the relation between continuous and discrete forms of motor control. We conclude that voluntary movements are effected by the central nervous system with the help of the mechanisms that underlie the variability and modifiability of innate motor patterns.
This study investigated the influence of different modalities of target information (visual, kinesthetic) on the accuracy, kinematics, and interjoint coordination of pointing movements to remembered targets. The targets were presented by a robot arm in five locations in three-dimensional (3D) space, either as a point of light in a dark room ("visual" condition), or kinesthetically. Relative pointing accuracy in the visual compared with kinesthetic conditions was influenced by the target location: pointing errors were the largest for the visual targets most eccentric relative to the subject's head. In addition, for the two most lateral targets, the final arm positions were, on average, closer to the center than the targets in the visual condition and farther from the center than the targets in the kinesthetic conditions. This result suggests that the pattern of errors in the visual condition described elsewhere ("range effect") may derive from visual processing rather than motor planning and implementation. Two modes of kinesthetic target presentation were utilized. During "passive" kinesthetic presentation of the target, the experimenter moved the subject's relaxed arm. Alternately, in "active" kinesthetic presentation of the target, the subject actively (with minimal help from the experimenter) moved his arm. No visual feedback was allowed in either kinesthetic condition. The variability in the final fingertip position was significantly smaller in the active condition than in the passive condition. In contrast, variability in the final values of arm orientation angles did not differ significantly in the active and passive conditions. This apparent contradiction may be resolved by the fact that, for the given target location, the influence of the deviation of these angles in the given trial from their average values on the position of the fingertip tended to be mutually compensated, and this tendency was stronger in the active condition. Our analysis of the correlations among the arm orientation angles and of the relationship between the initial and final arm configurations suggests that the kinesthetic conditions enabled the implementation of a mixture of strategies for achieving accuracy. The first strategy is to use a specific memory of an adequate arm configuration (that assumed during target presentation), such that accuracy is achieved by using this memory as a template. The second strategy is to use synergistically coordinating joint angles, such that accuracy is achieved by focusing on a specific endpoint that can be reached by a range of equivalent arm positions. The latter strategy was better utilized in the active condition. In conclusion, our results indicate that human subjects can use diverse sensory information to achieve comparable final accuracy, but that the details of the strategies employed differ with the kind of information available.
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