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PubMed2004

Motor coordination can be fully understood only by studying complex movements.

In this chapter, we use the sit-up to illustrate the complexity of coordination in movements that involve many muscles, joints, degrees of freedom, and high levels of muscle activity. Complex movements often involve the body axis. In addition to the intentional, focal part of any voluntary movement, complex movements also include "associated movements" that are not consciously controlled, but are necessary for the movement to succeed. Some associated movements serve a purpose, and others may not. During sitting up, the leg-lift is a purposive associated movement, whereas three-joint flexion is a non-purposive associated movement. The control of complex movements is also likely to be complex and, we argue, is hierarchically controlled. Associated movements may, themselves, be hierarchically organized and triggered by lower brain structures, local changes in neuronal excitability, and sensory feedback. Complex movements typically involve a high level of mobility. Because this mobility can lead to instability, anticipatory postural adjustments, a type of purposive associated movement, are commonly used to regulate posture. Thus, a number of important aspects of motor coordination can only be revealed by the study of complex movements.

Humans↗
PubMed1997

Changes in dorsal neck muscle activity related to imposed eye movement in the decerebrate pigeon.

Movements of the head and eyes are known to be intimately related. Eye position has also been shown to be closely related to the electromyographic activity of dorsal neck muscles; however, extraocular muscle proprioception has not generally been considered to play a part in the control of such movements. We have previously shown that, in the pigeon, imposed movements of one eye modify the vestibular responses of several dorsal neck muscles in ways that are dependent on stimulus parameters such as the amplitude and velocity of imposed eye movement. The present study examines more closely the interactions between imposed eye movements and different muscle pairs. The three neck muscle pairs studied each responded to afferent signals from the extraocular muscles in discrete and specific ways which appeared to be correlated with their different actions. Complementary effects of imposed eye movements in the horizontal plane were seen for both the complexus and splenius muscle pairs, with imposed eye movements in one direction producing the largest inhibition of the ipsilateral muscle's vestibular response and imposed eye movements in the opposite direction the largest inhibition of the contralateral muscle's vestibular response. During roll tilt oscillation (ear-up/ear-down) in the frontal plane, similar complementary effects of imposed eye movement were seen in the complexus muscle pair, but the splenius muscle pair showed little tuning, with similar inhibition for imposed eye movement directed either upwards or downwards. In contrast to these complementary effects, the biventer cervicis muscle pair showed no vestibular modulation during vestibular stimulation in the horizontal plane and their spontaneous activity was not altered by imposed eye movement. During roll-tilt oscillation (ear-up/ear-down) in the frontal plane imposed eye movement directed vertically upwards increased both muscles' vestibular responses and imposed eye movement directed vertically downwards inhibited both muscles' vestibular responses. Section of the ophthalmic branch of the trigeminal nerve (deafferenting the eye muscles) abolished the effects of imposed eye movement on the neck muscle pairs. In conjunction with further control experiments these results provide compelling evidence that proprioceptive signals from the extraocular muscles reach the neck muscles and provide them with a functionally significant signal. We have previously shown that signals from the extraocular muscles appear to be involved in the control of the vestibulo-ocular reflex. It follows from the experiments reported here that proprioceptive signals from the extraocular muscles are also likely to be involved in the control of gaze.

Animals↗
PubMed2000

Are periodic leg movements during sleep (PLMS) responsible for sleep disruption in insomnia patients?

On the basis of polygraphic findings, it has been suggested that periodic leg movements during sleep are not responsible for sleep impairment (Lugaresi et al., 1972). However, for some authors it is an important cause of insomnia (Guilleminault et al., 1975; Coleman, 1982). Thus, the relationship between periodic leg movements during sleep, sleep disruption and the complaint of patients is particularly complex. We investigated the macro- and micro-structure of sleep with and without leg movements in 10 PLMS patients complaining of insomnia to clarify whether periodic leg movements are responsible for sleep disruption. The total sleep time without periodic leg movements was significantly longer than sleep time with leg movements. Sleep time without leg movements was longer than sleep time with leg movements in stage 2, slow wave sleep (SWS) and rapid eye movement (REM) sleep. Short lasting awakenings were significantly more frequent during periodic leg movements associated sleep whilst long lasting awakenings were equally frequent during sleep with and without periodic leg movements. The percentage of the four electroencephalogram (EEG) activities (delta, theta, alpha and spindles) did not show any significant difference between periodic leg movements associated and not associated with sleep stages and total sleep time. The lack of significant differences in both the macro- and micro-structure of sleep and EEG activity content regarding the association with movements confirm the hypothesis that periodic leg movements did not primarily cause sleep disturbance.

Adult↗
PubMedDate not supplied

Effects of aging on linear and curvilinear aiming arm movements.

This research was designed to test the hypothesis that seniors may have deficits in their movement planning and execution of rapid target oriented arm movements. Twenty senior adults (M = 81.2 years, SD = 1.8) and 20 young adults (M = 25.2 years, SD = 2.5) performed two types of fast aiming arm movements (linear and curvilinear) on a digitizer in a counterbalanced order. Age effects on the variables of movement time, movement jerk, as well as on the profiles of movement displacement, velocity, and acceleration were examined. The outcomes of analysis of variance (ANOVA) with repeated measures indicated that the seniors executed both movement tasks more slowly and had higher movement jerk than the young adults. This suggests that aging is not only associated with slower but less smooth movements. In addition, the examination of movement profiles indicated that the young adults implemented the curvilinear task as one smooth and integrated motion, whereas the seniors executed the task in two segments. One possible reason of this "segmental" execution of curvilinear movement is that seniors may compensate for their deficits in movement planning by adopting a control strategy that reduces movement complexity. The aging effects on rapid aiming arm movements are discussed from the perspectives of motor planning, the "speed-accuracy trade-off," and task complexity.

Adult↗
PubMed2001

Replication and discrimination of limb movement velocity.

It is well known that proprioception is composed of the senses of movement and position. Whereas tests of position sense are quite commonly used, tests of the acuity in perception of movement velocity are scarce. In the present study we examined some novel tests for assessing the sense of limb movement velocity, involving replication and discrimination of single-joint movement velocity. Specifically, we investigated: (1) whether replication of limb movement velocity is more accurate following active criterion movements as compared to passive; (2) whether antagonist muscle contraction during passive limb movement enhances velocity discrimination; (3) how criterion movement velocity influences response accuracy; (4) the relationship between movement velocity and movement extent during velocity replication; and (5) whether subjects really base discrimination of velocities on perceived velocity. Sixteen healthy subjects participated in four tests (I-IV). For each test, horizontal abductions were performed about the right glenohumeral joint from the sagittal plane. The subjects were required to actively replicate the velocity of either an active (Test I) or passive (Test II) criterion movement, or judge whether a passive/semipassive (passive during antagonist muscle contraction) movement was faster or slower than a previous passive/semipassive criterion movement (Test III/IV). The results revealed higher response accuracy for Test I compared to Test II and for slower movements compared to faster, but no difference in response accuracy between Test III and IV. For velocity discrimination, the analysis revealed that the subjects based their judgment on the difference between criterion and comparison velocity rather than time or extent cues.

Adult↗
PubMed1986

Performance of simultaneous movements in patients with Parkinson's disease.

Ten right-handed patients with Parkinson's disease and 9 normal subjects performed five different types of movements as rapidly as possible in their own time: isotonic elbow flexion through an angle of 15 deg ('flex'); isometric squeezing of a force transducer between thumb and fingers ('squeeze'); isotonic finger flexion ('cut'); simultaneous performance of both 'flex' and 'squeeze'; simultaneous performance of 'flex' and 'cut'. The patients performed the separate movements of 'flex', 'squeeze' and 'cut' more slowly than normals. However, a more striking deficit was seen when a 'flex' and a 'squeeze' had to be performed at the same time, and with the same arm. There was an additional increase in movement times over and above that seen in the separate movements alone. If the patients used both arms ('flex' with the right, 'squeeze' with the left), rather than one, or when a 'flex' and a 'cut' had to be combined in the same arm, only a slight increase in movement times was observed. In normals, however, the speed of individual movements of 'flex', 'squeeze' or 'cut' was the same irrespective of whether they were performed separately or simultaneously. In any one subject, movement times for the separate components of 'flex' and 'squeeze' varied independently during the performance of the simultaneous movement. Because they remain independent, we suggest that when 'flex' and 'squeeze' are performed at the same time, two separate motor programmes are superimposed to produce the combined movement. In Parkinson's disease there may be a deficit in superimposing two separate motor programmes which leads to the pronounced slowness of simultaneous movements with the same arm. Comparison of movement times for a 'flex' (but not for a 'squeeze') in the separate and simultaneous movements showed that the degree of clinical akinesia was more closely related to the additional slowness in simultaneous movements than to the slowness seen when the movements were performed separately. The degree of disturbance in superimposing separate motor programmes may determine the amount of clinical akinesia in patients with Parkinson's disease.

Adult↗
PubMed1987

Programming and execution of movement in Parkinson's disease.

Programming and execution of arm movements in Parkinson's disease were investigated in choice and simple reaction time (RT) situations in which subjects made aimed movements at a target. A no-aiming condition was also studied. Reaction time was fractionated using surface EMG recording into premotor (central) and motor (peripheral) components. Premotor RT was found to be greater for parkinsonian patients than normal age-matched controls in the simple RT condition, but not in the choice condition. This effect did not depend on the parameters of the impending movement. Thus, paradoxically, parkinsonian patients were not inherently slower at initiating aiming movements from the starting position, but seemed unable to use advance information concerning motor task demands to speed up movement initiation. For both groups, low velocity movements took longer to initiate than high velocity ones. In the no-aiming condition parkinsonian RTs were markedly shorter than when aiming, but were still significantly longer than control RTs. Motor RT was constant across all conditions and was not different for patient and control subjects. In all conditions, parkinsonian movements were around 37% slower than control movements, and their movement times were more variable, the differences showing up early on in the movement, that is, during the initial ballistic phase. The within-subject variability of movement endpoints was also greater in patients. The motor dysfunction displayed in Parkinson's disease involves a number of components: (1) a basic central problem with simply initiating movements, even when minimal programming is required (no-aiming condition); (2) difficulty in maintaining computed forces for motor programs over time (simple RT condition); (3) a basic slowness of movement (bradykinesia) in all conditions; and (4) increased variability of movement in both time and space, presumably caused by inherent variability in force production.

Adult↗
PubMed1988

Co-ordination of a simple voluntary multi-joint movement with postural demands: trunk extension in standing man.

The interaction between primary movements and associated postural adjustments was studied during trunk extension movements in standing man. Six healthy male subjects performed one series of six consecutive fast trunk extensions. Movements were recorded with an optoelectronic system (Selspot). Angular displacements, velocities and accelerations at the ankle, knee, hip and trunk were calculated. Trunk inclination was measured as the sum of pelvic tilt and spine flexion. The spatial and temporal coordination patterns chosen by the subjects to complete the task were compared. The self-selected trunk movement amplitude ranged 14-30 degrees for all subjects. The individual amplitude was maintained within 4-7 degrees over the six consecutive trials. Peak velocity of the primary trunk movement was highly correlated with movement amplitude (r = 0.73, P less than 0.01), whereas peak acceleration was not. Trunk extension, was the result of a simultaneous backward pelvic tilt and spine extension, together with hip extension and knee flexion. Knee flexion became increasingly important at larger movement amplitudes since hip extension as well as spine extension was limited. Associated postural adjustments occurred mainly as ankle flexion. In conclusion, individuals can accurately reproduce the multi-joint co-ordination pattern seen during a voluntary trunk extension movement. Movements at the ankle appear to be controlled to counteract the backward shift of the centre of gravity caused by the primary movement. Knee and ankle movements seem to be independently controlled in the preparatory phase, whereas they are closely linked later during the primary movement. It is suggested that the interaction between knee and ankle movements is necessary for optimal equilibrium control during trunk extension movements in standing.

Adult↗
PubMed1983

Response of the normal human ankle joint to imposed sinusoidal movements.

Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made of electromyograms (e.m.g.s) in calf muscles, and of the forces at the joints. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus e.m.g.s. The e.m.g. response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as one could expect of a reflex pathway that involves a delay. The results suggest that this delay is between 50 and 60 ms, and we conclude that under these circumstances spinal stretch reflexes are playing the important part. The relation of the resisting force to the movement has been displayed as a vector. As the frequency changes, this vector describes the circular path that is characteristic of a system which includes delays or lags; this path enables one to draw conclusions about the amplitude and timing of the reflex resistance to the movement. When a subject exerts a moderate flexing force against the sinusoidal movement for some minutes, the reflex response becomes progressively potentiated. A subject whose reflex responses are normally slight may then exhibit a vigorous reflex response to the movement of that ankle. This enhancement of spinal reflex activity was accompanied by an increase in the myotatic reflex response at the ankle. Reflex responses to sinusoidal movement were most clearly seen when the subject exerted a mean flexing force that amounted to about one-fifth of his maximum. Very small movements (+/- 0.5 degrees) generated little or no reflex response. With large amplitudes of movement there was more reflex activity, but at some amplitude (which varied from subject to subject and from time to time) the reflex mechanism appeared to 'saturate', and further increases in amplitude were not accompanied by comparable increases in the reflex response. With movements at 10-15 Hz the e.m.g. response often became large in alternate cycles, with less activity in the intervening cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1985

Magnocellular red nucleus activity during different types of limb movement in the macaque monkey.

Three hundred and thirty-four neurones located in the magnocellular division of red nucleus (r.n.m.) were studied in three alert macaque monkeys. These cells had low discharge rates at rest and produced high frequency bursts during movement. Single cells were selectively active for movement of one body part, and a motor somatotopy was evident. From dorsal to ventral we encountered cells related to movements of the face, contralateral upper limb, contralateral lower limb and, in one case, the tail. Free-form tests indicated that 76% of upper-limb cells were preferentially related to hand and finger movements, and 84% of lower limb cells were preferentially related to foot and toe movements. Quantitative tests of movement relations were based on depths of modulation in discharge rate recorded while the monkeys operated several devices that served as manipulanda in a tracking task; each device elicited a different movement. We conducted 220 tests on eighty-one cells using eleven devices. The modulation in discharge rate exceeded a 50 pulses/s criterion level in seventy-nine cases; eleven were well related to proximal movements, twenty-two to movements of the digits and forty-six to a co-ordinated hand movement elicited by a device called the twister. Both unidirectional and bidirectional patterns of bursting were frequent. A few cells showed reciprocal patterns consisting of a large increase in rate for one direction of movement and a small decrease for the other. The bursts in discharge preceded movement onset (97% of 132 cases) by an average of 135 ms. Electromyographic activity in forearm muscles preceded movement by about 55 ms. In some cases we recorded from a single cell while the monkey operated two, three or four devices. Depth of modulation on the twister device was twice that on a proximal device in nine cases whereas one case showed a proximal device preference; five cases showed overlap. Comparison between twister and digits yielded ten cases of twister preference, four of digit preference and twenty cases of overlap. A directional preference was found for ten out of eleven cells responsive during active movement restricted to the metacarpo-phalangeal joints, and in all cases the preferred direction was extension. Responses to natural somatosensory stimulation were weak or absent for the majority of cells. When present, sensory fields were confined to the same limb but were frequently out of register with motor fields.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed2002

Cerebellar damage impairs automaticity of a recently practiced movement.

It has been suggested that the cerebellum plays a critical role in learning to make movements more "automatic" (i.e., requiring less attention to the details of a movement). We hypothesized that cerebellar damage compromises learning of movement automaticity, resulting in increased attentional demands for movement control. The purpose of our study was to determine whether cerebellar damage disrupts the ability to make a practiced movement more automatic. We developed a dual task paradigm using two tasks that did not have overlapping sensory or motor requirements for execution. Our motor task required subjects to maintain an upright posture while performing a figure-8 movement using their arm. This motor task was chosen to simulate requirements of everyday movements (e.g., standing while reaching for objects), but it was novel enough to require practice for improvement. Our secondary task was an auditory vigilance task where subjects listened to letter sequences and were asked to identify the number of times a target letter was heard. We tested controls and people with cerebellar damage as they practiced the movement task alone and then performed it with the auditory task. We recorded 3D position data from the arm, trunk, and leg during the movement task. Errors were recorded for both the movement and the letter tasks. Our results show that cerebellar subjects can improve the movement to a very limited extent with practice. Unlike controls, the motor performance of cerebellar subjects deteriorates to prepractice levels when attention is focused away from the movement during dual task trials. Control subjects' insensitivity to dual task interference after practice was due to learned movement automaticity and was not a reflection of better dual task performance generally. Overall, our findings suggest that the cerebellum may be important for shifting movement performance from an attentionally demanding (unpracticed) state to a more automatic (practiced) state.

Adult↗
PubMed2004

Alternate leg movement amplifies locomotor-like muscle activity in spinal cord injured persons.

It is now well recognized that muscle activity can be induced even in the paralyzed lower limb muscles of persons with spinal cord injury (SCI) by imposing locomotion-like movements on both of their legs. Although the significant role of the afferent input related to hip joint movement and body load has been emphasized considerably in previous studies, the contribution of the "alternate" leg movement pattern has not been fully investigated. This study was designed to investigate to what extent the alternate leg movement influenced this "locomotor-like" muscle activity. The knee-locked leg swing movement was imposed on 10 complete SCI subjects using a gait training apparatus. The following three different experimental conditions were adopted: 1) bilateral alternate leg movement, 2) unilateral leg movement, and 3) bilateral synchronous (in-phase) leg movement. In all experimental conditions, the passive leg movement induced EMG activity in the soleus and medial head of the gastrocnemius muscles in all SCI subjects and in the biceps femoris muscle in 8 of 10 SCI subjects. On the other hand, the EMG activity was not observed in the tibialis anterior and rectus femoris muscles. The EMG level of these activated muscles, as quantified by integrating the rectified EMG activity recorded from the right leg, was significantly larger for bilateral alternate leg movement than for unilateral and bilateral synchronous movements, although the right hip and ankle joint movements were identical in all experimental conditions. In addition, the difference in the pattern of the load applied to the leg among conditions was unable to explain the enhancement of EMG activity in the bilateral alternate leg movement condition. These results suggest that the sensory information generated by alternate leg movements plays a substantial role in amplifying the induced locomotor-like muscle activity in the lower limbs.

Adult↗
PubMed1984

Integration of retinal and motor signals of eye movements in striate cortex cells of the alert cat.

Responses of saccade-depressed (SD) and saccade-excited (SE) cells in the striate cortex to eye movements of alert cats under presentation of a visual pattern were studied under reinforcement of the eye movements with rewards of water. These responses were compared to those on passive displacement of the visual pattern reproducing the movements of the retinal image occurring during eye movements while eye movements were suppressed by withdrawal of reinforcement. Passive displacement of the visual pattern produced in the SD cells depression closely resembled the depression occurring during eye movements under presentation of the visual pattern, in time course as well as in amplitude. Both the saccade depression and the depression due to passive movement of the visual pattern were nonselective to the direction of eye movements. Saccade excitation of the SE cells frequently contained two components occurring at 20 and 80 ms after the onsets of eye movements. Passive displacement of the visual pattern produced in the SE cells excitation comparable with the early component of the saccade excitation. These findings suggest that saccade depression in the SD cells and the early component of the saccade excitation in the SE cells are related to retinal reafference of eye movement. During presentation of visual patterns, saccade excitation in the SE cells was closely related to parameters of eye movements, such as direction, amplitude, duration, and velocity. The correlations were completely lost or strongly reduced in darkness. Lines of evidence were provided that the saccade excitation of the SE cells in darkness or the later component of the saccade excitation under presentation of a visual pattern represents efference copy signals of eye movement transferred to the striate cortex through the Clare-Bishop (CB) cortex. Excitation comparable with saccade excitation in darkness occurred in synchrony with activities of the oculomotor nuclei even after retrobulbar paralysis of eye movement, indicating that the excitation is related to efference copy signals rather than proprioceptive reafference of eye movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Basal ganglia motor control. III. Pallidal ablation: normal reaction time, muscle cocontraction, and slow movement.

1. Inactivation of the portions of globus pallidus pars interna (GPi) containing the greatest concentration of wrist-related neurons was achieved in two rhesus monkeys with microinjections of muscimol (temporary) and kainic acid (permanent). 2. After muscimol injection, there was onset within 30 s of 1) tonic and phasic coactivation of wrist flexors and extensors; 2) slightly greater activation of the flexors, giving a flexor bias in postural holds and the endpoint of movements; and 3) slowness of all movements with a prolonged movement time. Nevertheless, 4) movements made by lessening prior loaded muscle activity (to move in the direction of the load) were slower than movement made by increasing muscle activity (to move against the direction of the load). Despite marked slowing of all movements, there was 5) a normal reaction time for movement onset. Finally, there was 6) a reduced amplitude of most movements. Open room behavior included 7) spiraling contralateral to the lesion while walking. Effects were reproducible (12 injections), were apparent for 7-8 h and were usually completely gone by the next day's testing. 3. After kainic acid injection, there was a period of mixed effects, followed by a period of permanent defects (observed for up to 24 days) that duplicated the temporary effects of muscimol. 4. By contrast, muscimol inactivation of the cerebellar dentate nucleus resulted in 1) a prolonged reaction time and 2) an increased variability of movement trajectory, but 3) without change in movement time or peak velocity. Open room behavior included overshoot in reaching for fruit with the forelimb ipsilateral to the injection. 5. From the facts that normal pallidal neurons fire constantly, that pallidal neurons inhibit their target neurons, and that the muscimol effect was immediate, we conclude that the release of the target neurons from the tonic inhibition allowed them to fire in patterns that promoted a maintained state of cocontraction of agonist and antagonist muscles. From the fact that movement time was prolonged, we conclude that the maintained state of neural activity that caused the muscle cocontraction interfered with the commands for voluntary movement, which were generated by other mechanisms. From the fact that reaction time for movement onset was normal, we conclude that the pallidal neurons may play little or no role in the voluntary initiation of these movements, which are instead generated by other structures that include the anterior cerebral cortex and the lateral cerebellum.

Animals↗
PubMed1991

Regional cerebral blood flow during voluntary arm and hand movements in human subjects.

1. Regional cerebral blood flow (rCBF) was measured using positron emission tomography in six normal volunteers while at rest and while performing four different repetitive movements of the right arm. 2. The four movements were performed in random order and consisted of abduction of the index finger, making a fist, sequential thumb to digit opposition, and shoulder flexion. All the movements were done at the same rate, using an auditory cue and involved displacements through similar amounts of the physiological range at each joint. 3. Increases in rCBF were interpreted as evidence of local neural activation and all four movements were associated with significant increases in CBF in the contralateral sensorimotor and premotor areas and in the supplementary motor area (SMA). 4. The average increase in blood flow in the contralateral sensorimotor cortex was significantly greater for the shoulder movement (31%) than for the three other movements. The increases with finger opposition (21%) and fist-making (24%) were not significantly different, and both were significantly greater than with index finger movement (13%). These data indicate that neither "fractionation" nor distal movement per se cause selective activation of sensorimotor cortex. 5. Significantly greater increases in blood flow in both the contralateral premotor cortex and the SMA ("nonprimary motor areas") occurred with shoulder movement than with the other movements. Because this difference may be related to the significantly greater activation occurring concurrently in the sensorimotor cortex, this finding does not prove unequivocally a "selective" role of the nonprimary motor areas in proximal movement. 6. Neither of the two nonprimary motor areas showed selective activation when a simple sequence of finger movements was performed compared with repetitive contractions of the same fingers. 7. Shoulder movement alone was associated with significant increases in rCBF in the ipsilateral sensorimotor cortex (10%), the superior vermis of the cerebellum (19%), and Brodmann areas 5 and 40 in the contralateral hemisphere. 8. The average location of the center of excitation in the sensorimotor cortex and SMA differed for the four movements and was interpreted as evidence of within-limb somatotopy. The shoulder focus lay highest in the sensorimotor cortex and lowest in the SMA.

Adult↗
PubMed1992

Two components of muscle activation: scaling with the speed of arm movement.

1. The temporal waveform of muscle activity was related to the speed of arm movement. Speed was expressed in terms of the duration of a fixed amplitude movement or the "movement time." 2. Human subjects moved their arms to targets in three-dimensional space. The right arm started at a standard initial position and moved directly to the target in a single stroke. The targets were placed in various directions in a vertical plane. The arm movements consisted of shoulder and elbow rotations. 3. Subjects were required to vary the speed of their movements. In most of the experiments, trials with different movement times were randomly ordered. One of the experiments also included randomly interspersed static trials, in which the subject held the arm still at the initial posture, the final posture, or halfway between the two extremes. 4. Electromyographic (EMG) activity was recorded from several superficial elbow and/or shoulder muscles. The time base of rectified EMG records was normalized for movement time such that records from movements with various speeds were compressed to align the ending times of the movements. 5. A principal component (PC) analysis revealed that the compressed EMG waveforms could be described by a summation of PC1 and PC2 waveforms; each individual EMG waveform was approximated by a weighted sum of these two components. 6. The PC1 weighting coefficients scaled down in a monotonic relationship with movement time such that the fastest movement corresponded to a large positive weighting coefficient and the slowest movement corresponded to a small positive weighting coefficient. The PC2 weighting coefficients exhibited a similar monotonic scaling, but the values ranged from positive to negative. Further analysis demonstrated that these two components can be mathematically transformed into a tonic waveform with a constant mathematically transformed into a tonic waveform with a constant weighting coefficient and a phasic waveform with positive weighting coefficients that scale down with movement time. 7. The amplitude scaling of EMG records cannot be described by a single component, but instead requires a summation of two separate components. The tonic component may correspond to the force element needed to counteract gravity, because the magnitude of this element does not scale with movement speed. The phasic component may correspond to the force element that scales quadratically to produce a linear increase in velocity.

Arm↗
PubMed1998

Optokinetic eye movements elicited by radial optic flow in the macaque monkey.

We recorded spontaneous eye movements elicited by radial optic flow in three macaque monkeys using the scleral search coil technique. Computer-generated stimuli simulated forward or backward motion of the monkey with respect to a number of small illuminated dots arranged on a virtual ground plane. We wanted to see whether optokinetic eye movements are induced by radial optic flow stimuli that simulate self-movement, quantify their parameters, and consider their effects on the processing of optic flow. A regular pattern of interchanging fast and slow eye movements with a frequency of 2 Hz was observed. When we shifted the horizontal position of the focus of expansion (FOE) during simulated forward motion (expansional optic flow), median horizontal eye position also shifted in the same direction but only by a smaller amount; for simulated backward motion (contractional optic flow), median eye position shifted in the opposite direction. We relate this to a change in Schlagfeld typically observed in optokinetic nystagmus. Direction and speed of slow phase eye movements were compared with the local flow field motion in gaze direction (the foveal flow). Eye movement direction matched well the foveal motion. Small systematic deviations could be attributed to an integration of the global motion pattern. Eye speed on average did not match foveal stimulus speed, as the median gain was only approximately 0.5-0.6. The gain was always lower for expanding than for contracting stimuli. We analyzed the time course of the eye movement immediately after each saccade. We found remarkable differences in the initial development of gain and directional following for expansion and contraction. For expansion, directional following and gain were initially poor and strongly influenced by the ongoing eye movement before the saccade. This was not the case for contraction. These differences also can be linked to properties of the optokinetic system. We conclude that optokinetic eye movements can be elicited by radial optic flow fields simulating self-motion. These eye movements are linked to the parafoveal flow field, i.e., the motion in the direction of gaze. In the retinal projection of the optic flow, such eye movements superimpose retinal slip. This results in complex retinal motion patterns, especially because the gain of the eye movement is small and variable. This observation has special relevance for mechanisms that determine self-motion from retinal flow fields. It is necessary to consider the influence of eye movements in optic flow analysis, but our results suggest that direction and speed of an eye movement should be treated differently.

Animals↗
PubMed1998

Off-centric rotation axes in natural head movements: implications for vestibular reafference and kinematic redundancy.

Until now, most studies concerning active head movements in three dimensions have used the classical rotation vector description. Although this description yields both the orientation of the head rotation axis and the amount of rotation, it is incomplete because it cannot specify the location of this rotation axis in space. The latter is of importance for a proper picture of the vestibular consequences of active head movements and has relevance for the problem of how the brain deals with the inherent kinematic redundancy of the multijoint head-neck system. With this in mind, we have extended the rotation vector description by applying the helical axes approach, which yields both the classical rotation vector as well as the location of the rotation axis in space. Subjects (n = 7), whose head movements were recorded optically, were instructed to shift gaze naturally to targets in 12 different directions at an eccentricity of 40 degrees. The results demonstrate that the axes for these head movements occupy consistently different spatial locations. For purely horizontal movements, the rotation axis is located near a point midway between the two ear canals. For gaze shifts in other directions, the rotation axes are located below the ear canals along two circles, one for movements with an upward component (up circle), the other (typically larger in size) for movements with a downward component (down circle). Purely vertical movement (up and down) axes were located on the lower pole of the up and down circles, respectively. It was found that both circles, the upper poles of which coincided, became larger in size as movement amplitude increased, which means that the axis location shifts to lower and more eccentric locations with respect to the skull for larger flexion and extension movements. Although this pattern could be recognized in most subjects, there were consistent intersubject differences in the absolute size of the circles, their increase with movement amplitude, and in the relative sizes of the up and down circles. Because multiple vertebrae are involved in head movements, there are theoretically many possibilities to execute a certain head movement. The differences in circle patterns among subjects indicate different strategies in resolving this kinematic redundancy problem, a fact that was not apparent from the classical rotation vector part of our description, which yielded a rather uniform picture. A simple model suggests that the downward shift of the location of the rotation axis requires a modulation in vestibulo-ocular reflex gain of </=10% to maintain fixation of a near target during vertical head movement. The involvement of the otolith system in this process remains to be determined.

Adult↗