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Brain mechanisms in motor control
Pergamon Press
Life Sciences Vol . 15, pp . 1393-1399 Printed in the U .S .A .
MINIREVIEiP
BRAIN MECHANI3!!3 IN MOTOR CONTROL Edward V . Evarta laboratory of Neurophysiology National Institute of Mental Benlth Bethesda, 1~laryland 20014
A central goal of currant research on brain mechanisms controlling movement is discovery of the way is which information flows between the three major subsystems of the brain's motor control system :
1) the cerebral motor
cortex, 2) the basal ganglia, 3) the cerebellum (1) .
Each of these structures
has long been known to have an importanq function in motor control, but only now are we coming to understand how the structures are interrelated in control of movement .
ûnderstanding of these interrelations has been advanced by devel-
opment of techniques which allow the activity of individual neurons to be recorded in the course of normal movement in monkeys .
The present review will
cover same of the observations which have emerged se a result of utilization of these techniques . 1.
The Cerebral Motoz Cortex
In primates, a zone of cerebral cortex lying anterior to the central fissure (precentral motor cortex) contains a net of neurone whose aeons pass without interruption to the spinal cord via a nerve bundle named the pyramidal tract ; the neurone of this tract are called pyramidal tract neurone (PTNs) .
In
the spinal cord the axon terminals of PTNe end both in the interneuronal pool and on alpha and gamma motoneurone . In considering the functional role of motor cortex P1Ns, we shall begin by comparing their activity with the activity of spinal cord a-motoneurons motoneurona innervata skeletal muscle, whereas r-motoneurone control the seneitivity of intramuscular stretch receptors) . 1393
The first point to nota in this
1394
Hrain Mechanisms in Motor Control
Vol . 15, No . 8
comparison is that while all ~-motonaurons are inactive during muscular inactivity, most FTNa are topically active even during total muscular quiaecence (2 ) .
Furthermore, recordings of PTN activity in awake monkeys make it
clear that discharge frequencies of PTNs are being modulated and controlled even in the absence of any movement (3) .
Studies of PTN discharge in monkeys
who are able to carry out differing movements depending upon the "instructions" they have received have revealed that one factor influencing motor cortex PTN discharge frequency in the absence of overt muscular contraction is the inten tion to
va (4,5) .
The variation in PTN activity in relation to intention to
move ie seen even before any muscular activity occurs .
Thus, when a monkey has
learned that a particular signal (e .g ., a red light) means he should flex his elbow, PTNe adopt a pattern of discharge corresponding to the intention to flex . A different pattern of PTN discharge frequency is adopted if the monkey receives a signal (e .g ., a green light) instructing him to extend .
These
patterns of PTN activity specific to the impending movement occur wall in advance of any overt muscular contraction, for in these experiments the monkeys were required to wait several seconds between the instruction and a second signal which told them that the time had come to carry out the instruction . Thus, motor cortex neurons show specific patterns of activity depending not only on a movement which is in progress but also depending upon a movement which is impending . In the experiment referred to above, the "instruction" (a red or green light) was followed by a second stimulus, which was a signal to the monkey to carry out the movement .
This second stimulus was a perturbation of the handle
grasped by the monkey, and such a stimulus activates receptors which send information both to the motor cortex and to the spinal cord .
A second striking
feature of motor cortex PTN activity which differentiates it from the activity of spinal cord motoneurona is the extent to which the reaponsivenesa of PTNa to such sensory inputs may be modified depending upon the movement which this sensory input will trigger .
Thus, the reaponsivenesa of FTNs to incoming sensory
Vol . 15, No . 8
1395
Hrain Mechanisms in Motor Control
stimuli was "gated" on or off depending upon the instruction which the monkey had received, and thus, depending on the movement he intended to make .
This
gating of PTN responsiveness to sensory inputs allons selectivity of motor behavior depending upon decisions made by the subject .
The motor tortes may be
thought of ere a flexibly controlled canter in which input-output relations can be changed . Another set of experiments on the relationship between motor cortex neuron discharge and muscular contraction has been carried out to determine the aevects of movement to which motor cortex activity might ba related (6,7,8) . Ia these experiments, the activity of motor cortex PTLQs aaa examined in relation to changea in joint position ere compared to changes in pattern of muscular activity .
In these experiments, monkeys sere trai~d to make a given type of
ariat movement under external loading conditions which at some times required one pattern of muscular activity and at other times required a different pattern of muscular activity .
In this study, a monkey sae required to lift and
then to lover a weight slowly .
First he had to flex his ariat, contracting his
flexor muscles in the act of lifting a load, and then sae required to extend his wrist to let the weight down slowly .
In letting the load dose slowly ha
had to maintain activity of flexor muscles so that the load did not fall too rapidly .
In this movement, the direction of joint displacement sae extension
but the flexor muscles rather than the extensor muscles sere the ones that sere action .
Monkeys sere also trained to make the same movement of the ariet, but
under conditions in which the load ass reversed, so that the extensor muscles sere required to bn active during both flnaion and extension movements . Recordings of motor cortex .naurons is these experiments showed that motor cortez activity is related to pattern of muscular contraction rather than to joint displacement .
Thus, the signals leaving the motor cortex and going to the
spinal cord appear to carry information about shat muscles should contract rather than about the position to which the moving limb should be brought .
139 6
Brain Mechanisms in Motor Control
Vol . 15, No . 8
Here, then, are some of the features of motor cortex activity which have been revealed in studies of single neuron discharge recorded from animals trained to make voluntary movement :
it has been found that motor cortex neu
rone are active prior to muscular contraction (3 ), that they era related to the pattern of muscular contraction associated with a movement, and that they shw changes in discharge during the period "getting set" prior to a movement . 2.
The Cerebellum and the Ba~al GanIIl ia
The anatomical origin of the fibers coming into motor cortex indicates that the activity of motor cortex neurons must depend to a very large extent on the impulses from the cerebellum and the basal ganglia which reach motor cortex via the thalamus (9,1) .
Unlike axons of motor cortex PTNs, axons of cerebellar
and basal ganglia neurone do not have direct access to the spinal cord .
Their
outputs must be relayed synoptically, either via cortex or via brain stem . Recordings of activity of single neurone in cerebellum and basal ganglia during movement have now led to reappraisal of former ideas as to the relationship of cerebellum and basal ganglia activity to motor cortex .
Formerly, it
was believed that the major role of the cerebellum was regulation of movement already
in
oroarese , this regulation being based on feedback from muscles after
they had begun their contraction .
It therefore came as a surprise when Thach
(10,11,12,13) found that changes in cerebellar activity occurred prior to movemeat .
Studies by DeLong (14,15,16) revealed that cells of basal ganglia also
became active prior to movement .
The finding that both basal ganglia and cere-
bellum are active prior to movement has led to a new notion of the functional relation of these two structures to motor cortex .
This notion, put forward by
Ramp and Powall (1), starts from the known anatomical fact that the entire cerebral cortex sends fibers to both the basal ganglia and the cerebellum .
The
cerebellum and basal ganglia in turn send massive connections back to the motor cortex by way of the thalamus .
Thus, the basal ganglia and the cerebellum
receive information from the whole brain, transform this information, and then sand a new pattern of signals back to the motor cortex .
Rather than being at a
Brain Mechanisms in Motor Control
Vol . 15, No . 8
139 7
loner level of the motor system (i .e ., closer to the motoneuron) and operating only to correct motor cortex discharge on the basis of feedback, both cerebellum and basal ganglia are now seen as structures which send commands to the motor cortex, whence signals pass down to the spinal cord . 3.
Aayecta of Movement
Given the notions referred to above in relation to the flow of signals from cerebellum and banal ganglia to motor cortex, it becomes of interest to discover the aspects of movement which may be selectively represented in the basal ganglia an compared to the cerebellum .
Studien of different aorta of
movements carried out in monkeys have given some answers ae to this selective representation of different aorta of movement in cerebellum an compared to banal ganglia .
A clan to the nature of selective motor representation camas
from the nature of the clinical disorder in Parkinson's dineane, where it is samntimea possible for patients to carry out high velocity movements though they have extreme difficulty in carrying out plow movements .
Cerebellar dis-
ease, on the other hand, produces particularly marked difficulties in execution of accurate rapid movements .
On the banaa of these and other observations,
Romhuber in 1971 (17) proposed that the cerebellum functions to program and initiate rapid (saccadic or ballistic) mavementn, whereon the major role of the banal ganglia in to generate slaw "ramp" movements .
To test this hypothesis,
DaIang (15) carried out studies both on basal ganglia and on cerebellum in association with ramp and ballistic movements .
It was found that there was a
differential representation for thane two north of mavamnntn in the two atructares, with the cerebellur neurons in the basal ganglia being more i~olved in slar, ramp movements .
Theca studies on differential representation of differ-
ent ports of movements in the tvo structures point to the possibility of localicing specific features of motor activity in particular cull groups . 4.
Outlook for Future Reenarçh
An understanding of the physiological mechanisms underlying motor control is important for several raaeona .
First of all, development of pharmacological
139 8
Brain Mechanisms in Motor Control
Vol . 15, No . 8
agents for the treatment of motor disorders in man requires that we understand the neural circuits that the drugs will act on, and a better understanding of the pathophyaiology of motor disorders in man will be had if the basic animal physiology is understood .
Knowledge of the pathophyaiology of motor disorders
will also lead to more effective ways of assessing dose-affect relationships in the case of drugs already available for treating neurological disorders .
For
example, we need better measures of motor function for use in adjusting the dosage of L-dope in patients with parkineoniam . A particularly premising area of imeatigation for the near future is the analysis of experimentally produced motor disturbances in monkeys, disturbances that are similar to the motor disturbances found in man, for example, Parkin son's disease .
Mecovery of the naurophyaiological errors in such experimental
models of motor disturbances will be of value in developing and testing therapeutic drugs .
Many patients with Parkinson's disease have shown a dramatic
improvement after taking the drug L-dope .
With naurophysiological studies of
parkinsoniam in monkeys, it may be possible to detezmine exactly how L-dope works .
Analogous studies should be feasible for diseases involving the cere-
bellum . People suffering from Parkinson's disease exhibit emotional as well as muscular disorders .
Other basal ganglia diseases, some of them genetically
determined, are associated with psychological disorders .
This indicates that
the basal ganglia may be the region that provides the major associative link between thn more specialised aenaory division of the nervous system and the motor division . The implications of theca studies also extend into the areas of psychology and psychiatry .
Indeed, it seems possible that understanding of the human
nervous system, even its moat complex intellectual functions, may be enriched if the operation of the brain ie analyzed in texma of its motor output rather than in terms of its aenaory input .
Vol. 15, No . 8
Hrain Mechanisms in Motor Control
1399
References 1.
J. M. KEMP and'T, P, S . POWELL, Phil . Trane . R. Soc. fond . B. 262, 441457 (1971) .
2.
E . V, EVARTS, J. Neurophvsiol . 28, 216-228 (1965) .
3.
E . V. EVARTS, J. Neurophvsiol . 29, 1011-1027 (1966) .
4.
E . V. EVARTS, Science 179, 501-503 (1973) .
5.
E, V. EVARTS and J . TANJI, Brain Research 71, 479-494 (1974) .
6.
E, V, EVARTS, in NeurophvsioloRical Beeis of Normal and Abnormal Motor Activities (M, D, Yahr and D, P, Purpurs, eds .), pp . 215-251, Raven Press, Hewlett, New York (1967) .
7.
E . V . EVARTS, J, Neurophyaiol . 31, 14-27 (1968) .
8.
E, V. EVARTS, J. Neurophvsiol . 32, 375-385 (1969) .
9.
E . V. EVARTS and W, T, THACH,
in Annual Review of Phyaiolosty (V, E, Hall,
A, C, Giese and R. R. Sonnenschein, eda .), pp . 451-498, Annual Reviews, Inc ., Palo Alto, California
(1969) .
10 .
W . T. THACH, J . Neuro hvsiol . 31, 785-797 (1968) .
11 .
W, T, TNACH,
in The Cerebellum in Health and Disease (W, S . Fields and
W. D, Willie, Jr ., eda .), pp . 217-230, W. H, Green,
St . Louis (1970) .
12 .
W. T. THACH, J . Neurophvsiol . 33, 527-536 (1970) .
13 .
W. T. THACH, J . Neurophvsiol . 33, 537-547 (1970) .
14 .
M, R. DeIANG, Brain Research 40, 127-135 (1972) .
15 .
M, R . De LONG, Science 179,
16 .
M. R. De1.ONG and P, L. STRICK, Brain Research 71, 327- 335(1974) .
17 .
H, H. KORNHABER Kybernetik 8,
1240-1242 (1973) .
157-162 (1971) .
Life Sciences Vol . 15, pp . 1393-1399 Printed in the U .S .A .
MINIREVIEiP
BRAIN MECHANI3!!3 IN MOTOR CONTROL Edward V . Evarta laboratory of Neurophysiology National Institute of Mental Benlth Bethesda, 1~laryland 20014
A central goal of currant research on brain mechanisms controlling movement is discovery of the way is which information flows between the three major subsystems of the brain's motor control system :
1) the cerebral motor
cortex, 2) the basal ganglia, 3) the cerebellum (1) .
Each of these structures
has long been known to have an importanq function in motor control, but only now are we coming to understand how the structures are interrelated in control of movement .
ûnderstanding of these interrelations has been advanced by devel-
opment of techniques which allow the activity of individual neurons to be recorded in the course of normal movement in monkeys .
The present review will
cover same of the observations which have emerged se a result of utilization of these techniques . 1.
The Cerebral Motoz Cortex
In primates, a zone of cerebral cortex lying anterior to the central fissure (precentral motor cortex) contains a net of neurone whose aeons pass without interruption to the spinal cord via a nerve bundle named the pyramidal tract ; the neurone of this tract are called pyramidal tract neurone (PTNs) .
In
the spinal cord the axon terminals of PTNe end both in the interneuronal pool and on alpha and gamma motoneurone . In considering the functional role of motor cortex P1Ns, we shall begin by comparing their activity with the activity of spinal cord a-motoneurons motoneurona innervata skeletal muscle, whereas r-motoneurone control the seneitivity of intramuscular stretch receptors) . 1393
The first point to nota in this
1394
Hrain Mechanisms in Motor Control
Vol . 15, No . 8
comparison is that while all ~-motonaurons are inactive during muscular inactivity, most FTNa are topically active even during total muscular quiaecence (2 ) .
Furthermore, recordings of PTN activity in awake monkeys make it
clear that discharge frequencies of PTNs are being modulated and controlled even in the absence of any movement (3) .
Studies of PTN discharge in monkeys
who are able to carry out differing movements depending upon the "instructions" they have received have revealed that one factor influencing motor cortex PTN discharge frequency in the absence of overt muscular contraction is the inten tion to
va (4,5) .
The variation in PTN activity in relation to intention to
move ie seen even before any muscular activity occurs .
Thus, when a monkey has
learned that a particular signal (e .g ., a red light) means he should flex his elbow, PTNe adopt a pattern of discharge corresponding to the intention to flex . A different pattern of PTN discharge frequency is adopted if the monkey receives a signal (e .g ., a green light) instructing him to extend .
These
patterns of PTN activity specific to the impending movement occur wall in advance of any overt muscular contraction, for in these experiments the monkeys were required to wait several seconds between the instruction and a second signal which told them that the time had come to carry out the instruction . Thus, motor cortex neurons show specific patterns of activity depending not only on a movement which is in progress but also depending upon a movement which is impending . In the experiment referred to above, the "instruction" (a red or green light) was followed by a second stimulus, which was a signal to the monkey to carry out the movement .
This second stimulus was a perturbation of the handle
grasped by the monkey, and such a stimulus activates receptors which send information both to the motor cortex and to the spinal cord .
A second striking
feature of motor cortex PTN activity which differentiates it from the activity of spinal cord motoneurona is the extent to which the reaponsivenesa of PTNa to such sensory inputs may be modified depending upon the movement which this sensory input will trigger .
Thus, the reaponsivenesa of FTNs to incoming sensory
Vol . 15, No . 8
1395
Hrain Mechanisms in Motor Control
stimuli was "gated" on or off depending upon the instruction which the monkey had received, and thus, depending on the movement he intended to make .
This
gating of PTN responsiveness to sensory inputs allons selectivity of motor behavior depending upon decisions made by the subject .
The motor tortes may be
thought of ere a flexibly controlled canter in which input-output relations can be changed . Another set of experiments on the relationship between motor cortex neuron discharge and muscular contraction has been carried out to determine the aevects of movement to which motor cortex activity might ba related (6,7,8) . Ia these experiments, the activity of motor cortex PTLQs aaa examined in relation to changea in joint position ere compared to changes in pattern of muscular activity .
In these experiments, monkeys sere trai~d to make a given type of
ariat movement under external loading conditions which at some times required one pattern of muscular activity and at other times required a different pattern of muscular activity .
In this study, a monkey sae required to lift and
then to lover a weight slowly .
First he had to flex his ariat, contracting his
flexor muscles in the act of lifting a load, and then sae required to extend his wrist to let the weight down slowly .
In letting the load dose slowly ha
had to maintain activity of flexor muscles so that the load did not fall too rapidly .
In this movement, the direction of joint displacement sae extension
but the flexor muscles rather than the extensor muscles sere the ones that sere action .
Monkeys sere also trained to make the same movement of the ariet, but
under conditions in which the load ass reversed, so that the extensor muscles sere required to bn active during both flnaion and extension movements . Recordings of motor cortex .naurons is these experiments showed that motor cortez activity is related to pattern of muscular contraction rather than to joint displacement .
Thus, the signals leaving the motor cortex and going to the
spinal cord appear to carry information about shat muscles should contract rather than about the position to which the moving limb should be brought .
139 6
Brain Mechanisms in Motor Control
Vol . 15, No . 8
Here, then, are some of the features of motor cortex activity which have been revealed in studies of single neuron discharge recorded from animals trained to make voluntary movement :
it has been found that motor cortex neu
rone are active prior to muscular contraction (3 ), that they era related to the pattern of muscular contraction associated with a movement, and that they shw changes in discharge during the period "getting set" prior to a movement . 2.
The Cerebellum and the Ba~al GanIIl ia
The anatomical origin of the fibers coming into motor cortex indicates that the activity of motor cortex neurons must depend to a very large extent on the impulses from the cerebellum and the basal ganglia which reach motor cortex via the thalamus (9,1) .
Unlike axons of motor cortex PTNs, axons of cerebellar
and basal ganglia neurone do not have direct access to the spinal cord .
Their
outputs must be relayed synoptically, either via cortex or via brain stem . Recordings of activity of single neurone in cerebellum and basal ganglia during movement have now led to reappraisal of former ideas as to the relationship of cerebellum and basal ganglia activity to motor cortex .
Formerly, it
was believed that the major role of the cerebellum was regulation of movement already
in
oroarese , this regulation being based on feedback from muscles after
they had begun their contraction .
It therefore came as a surprise when Thach
(10,11,12,13) found that changes in cerebellar activity occurred prior to movemeat .
Studies by DeLong (14,15,16) revealed that cells of basal ganglia also
became active prior to movement .
The finding that both basal ganglia and cere-
bellum are active prior to movement has led to a new notion of the functional relation of these two structures to motor cortex .
This notion, put forward by
Ramp and Powall (1), starts from the known anatomical fact that the entire cerebral cortex sends fibers to both the basal ganglia and the cerebellum .
The
cerebellum and basal ganglia in turn send massive connections back to the motor cortex by way of the thalamus .
Thus, the basal ganglia and the cerebellum
receive information from the whole brain, transform this information, and then sand a new pattern of signals back to the motor cortex .
Rather than being at a
Brain Mechanisms in Motor Control
Vol . 15, No . 8
139 7
loner level of the motor system (i .e ., closer to the motoneuron) and operating only to correct motor cortex discharge on the basis of feedback, both cerebellum and basal ganglia are now seen as structures which send commands to the motor cortex, whence signals pass down to the spinal cord . 3.
Aayecta of Movement
Given the notions referred to above in relation to the flow of signals from cerebellum and banal ganglia to motor cortex, it becomes of interest to discover the aspects of movement which may be selectively represented in the basal ganglia an compared to the cerebellum .
Studien of different aorta of
movements carried out in monkeys have given some answers ae to this selective representation of different aorta of movement in cerebellum an compared to banal ganglia .
A clan to the nature of selective motor representation camas
from the nature of the clinical disorder in Parkinson's dineane, where it is samntimea possible for patients to carry out high velocity movements though they have extreme difficulty in carrying out plow movements .
Cerebellar dis-
ease, on the other hand, produces particularly marked difficulties in execution of accurate rapid movements .
On the banaa of these and other observations,
Romhuber in 1971 (17) proposed that the cerebellum functions to program and initiate rapid (saccadic or ballistic) mavementn, whereon the major role of the banal ganglia in to generate slaw "ramp" movements .
To test this hypothesis,
DaIang (15) carried out studies both on basal ganglia and on cerebellum in association with ramp and ballistic movements .
It was found that there was a
differential representation for thane two north of mavamnntn in the two atructares, with the cerebellur neurons in the basal ganglia being more i~olved in slar, ramp movements .
Theca studies on differential representation of differ-
ent ports of movements in the tvo structures point to the possibility of localicing specific features of motor activity in particular cull groups . 4.
Outlook for Future Reenarçh
An understanding of the physiological mechanisms underlying motor control is important for several raaeona .
First of all, development of pharmacological
139 8
Brain Mechanisms in Motor Control
Vol . 15, No . 8
agents for the treatment of motor disorders in man requires that we understand the neural circuits that the drugs will act on, and a better understanding of the pathophyaiology of motor disorders in man will be had if the basic animal physiology is understood .
Knowledge of the pathophyaiology of motor disorders
will also lead to more effective ways of assessing dose-affect relationships in the case of drugs already available for treating neurological disorders .
For
example, we need better measures of motor function for use in adjusting the dosage of L-dope in patients with parkineoniam . A particularly premising area of imeatigation for the near future is the analysis of experimentally produced motor disturbances in monkeys, disturbances that are similar to the motor disturbances found in man, for example, Parkin son's disease .
Mecovery of the naurophyaiological errors in such experimental
models of motor disturbances will be of value in developing and testing therapeutic drugs .
Many patients with Parkinson's disease have shown a dramatic
improvement after taking the drug L-dope .
With naurophysiological studies of
parkinsoniam in monkeys, it may be possible to detezmine exactly how L-dope works .
Analogous studies should be feasible for diseases involving the cere-
bellum . People suffering from Parkinson's disease exhibit emotional as well as muscular disorders .
Other basal ganglia diseases, some of them genetically
determined, are associated with psychological disorders .
This indicates that
the basal ganglia may be the region that provides the major associative link between thn more specialised aenaory division of the nervous system and the motor division . The implications of theca studies also extend into the areas of psychology and psychiatry .
Indeed, it seems possible that understanding of the human
nervous system, even its moat complex intellectual functions, may be enriched if the operation of the brain ie analyzed in texma of its motor output rather than in terms of its aenaory input .
Vol. 15, No . 8
Hrain Mechanisms in Motor Control
1399
References 1.
J. M. KEMP and'T, P, S . POWELL, Phil . Trane . R. Soc. fond . B. 262, 441457 (1971) .
2.
E . V, EVARTS, J. Neurophvsiol . 28, 216-228 (1965) .
3.
E . V. EVARTS, J. Neurophvsiol . 29, 1011-1027 (1966) .
4.
E . V. EVARTS, Science 179, 501-503 (1973) .
5.
E, V. EVARTS and J . TANJI, Brain Research 71, 479-494 (1974) .
6.
E, V, EVARTS, in NeurophvsioloRical Beeis of Normal and Abnormal Motor Activities (M, D, Yahr and D, P, Purpurs, eds .), pp . 215-251, Raven Press, Hewlett, New York (1967) .
7.
E . V . EVARTS, J, Neurophyaiol . 31, 14-27 (1968) .
8.
E, V. EVARTS, J. Neurophvsiol . 32, 375-385 (1969) .
9.
E . V. EVARTS and W, T, THACH,
in Annual Review of Phyaiolosty (V, E, Hall,
A, C, Giese and R. R. Sonnenschein, eda .), pp . 451-498, Annual Reviews, Inc ., Palo Alto, California
(1969) .
10 .
W . T. THACH, J . Neuro hvsiol . 31, 785-797 (1968) .
11 .
W, T, TNACH,
in The Cerebellum in Health and Disease (W, S . Fields and
W. D, Willie, Jr ., eda .), pp . 217-230, W. H, Green,
St . Louis (1970) .
12 .
W. T. THACH, J . Neurophvsiol . 33, 527-536 (1970) .
13 .
W. T. THACH, J . Neurophvsiol . 33, 537-547 (1970) .
14 .
M, R. DeIANG, Brain Research 40, 127-135 (1972) .
15 .
M, R . De LONG, Science 179,
16 .
M. R. De1.ONG and P, L. STRICK, Brain Research 71, 327- 335(1974) .
17 .
H, H. KORNHABER Kybernetik 8,
1240-1242 (1973) .
157-162 (1971) .