Processes controlling visually evoked movements.
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Afferent Pathways↗
Biomedical subjects
A Polit
Publications and source records attributed to A Polit.
Characteristics of motor programs underlying arm movements in monkeys.
1. The experiments described here are addressed at identifying some of the processes underlying arm movements in monkeys. 2. We used three adult monkeys that were trained to point to a target light with the forearm and hold at that position for about 1 s in order to obtain a reward. During the experimental sessions the monkey was seated in a primate chair and its forearm was fastened to an apparatus that permitted flexion and extension of the forearm about the elbow in the horizontal plane. 3. We tested their performance prior to and after bilateral dorsal rhizotomy (C2--T3). Forearm movements were performed without the sight of the arm both before and after the surgical intervention. In intact animals we unexpectedly displaced the arm prior to movement initiation (150--200 ms) and observed the outcome of this displacement on movement termination. Our results indicated that the arm moved accurately to the target. The same procedure was used in the deafferented monkeys, yielding qualitatively the same results; i.e., a displacement of the initial position did not affect the attainment of the intended final position. 4. These results are relevant to the question of what is being controlled by motor commands. It appears that the controlled variable is an equilibrium point resulting from the interaction of agonist and antagonist muscles. Consequently, a change in the equilibrium leads to movement and the attainment of a new posture. The fact that both intact and deafferent monkeys display essentially similar motor behavior in our highly practiced task should not obliterate the dramatic difference in motor performance that exists between intact and rhizotomized animals. In fact, the successful execution of the learned motor performance in the deafferented animal is contingent on the animal's body being in a fixed relation to the arm apparatus. Whenever we changed the usual spatial relationship between the monkey's body and the arm apparatus, the animal's pointing response to the target was inaccurate. All of our intact monkeys, in contrast, were able to compensate quickly for any variations in their accustomed position with respect to the arm apparatus. The dramatic inability of the deafferented monkey to execute accurate pointing responses in an unusual postural setting underscores the great importance of the afferent monkey to execute accurate pointing responses in an unusual postural settiing underscores the great importance of the afferent feedback. These findings suggest that, in the performance of visually evoked learned movements, one of the major functions of the afferent feedback is in the adaptive modifications of learned motor programs.
Afferent Pathways↗
Processes controlling arm movements in monkeys.
The experiments identify some of the processes underlying arm movements in rhesus monkeys. Three monkeys were trained to point to a target with the hand and forearm and to hold that position for about 1 second to obtain a reward. Forearm movements were performed without sight of the arm before and after bilateral dorsal rhizotomy. In both intact and deafferented animals, we unexpectedly displaced the forearm prior to movement initiation and observed that the arm moved accurately to the target. These results are relevant to the question of what is being controlled by motor commands. The controlled variable appears to be an equilibrium point between agonist and antagonist muscles. The findings suggest that the feedback system plays a major role in updating and adjusting the central programs subserving the execution of learned motor patterns.
Afferent Pathways↗
Effect of load disturbances during centrally initiated movements.
1. We have investigated the relative contributions of mechanical and reflex mechanisms in generating the forces produced by the neck muscles when loads were unexpectedly applied during centrally programmed head movements in monkeys. These movements, subserved by muscles well endowed with muscle spindles, are part of the coordinated eye-head response to the appearance of a stimulus in the animal's visual field. Our preparation was a chronically vestibulectomized monkey trained to make a visual discrimination. 2. Two procedures were used to evaluate the torque generated by the neck musculature when an unexpected load disturbance was applied: first, by surgically interrupting the afferent loop subserving the reflex action (section of cervical dorsal roots) and second, by building a mathematical model of the head-neck system and carrying out a process of simulation. 3. Our results indicated that the compensatory torque of reflex origin stimulated by the application of an opposing force was less than 10--30% of that required for perfect compensation, and the larger fraction of the observed compensation was due to the mechanical properties (inertial, viscous, and elastic) of the neck musculature. The combined action of reflex and mechanical processes never completely compensated for the disturbance.
Animals↗
Mechanisms underlying achievement of final head position.
The studies reported here are directed toward understanding some of the mechanisms whereby the central nervous system terminates a given phase in a motor sequence and maintains a newly acquired position. In particular, we investigated the extent to which the termination of a centrally initiated head movement in monkeys and the subsequent maintenance of posture depend on a readout of proprioceptive afferent input generated during the movement itself or are instead centrally programmed. We approached this question in two ways: first, using vestibulectomized, but otherwise intact monkeys, we applied load disturbances unexpectedly at the beginning and throughout centrally initiated head movements with the aim of provoking a proprioceptive response in all types of neck receptors and to observe the outcome of this stimulation on the head final position. In a second set of experiments, we interrupted the flow of afferent input by cutting cervical and upper thoracic dorsal roots and observed how the absence of proprioceptive feedback affects the achievement of final head position. The results indicated that the central pattern of neural impulses establishing final head position is preprogrammed and it is not reset by the afferent proprioceptive impulses generated during the intended movement. In addition, our findings are consistent with the view that final head position is an equilibrium point dependent on a number of factors, such as the firing rate and the recruitment of the alpha motoneurons, the length-tension properties of the muscles involved in posture, and passive elastic properties of external loads.
Animals↗