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Biomedical subjects

J Hore

Publications and source records attributed to J Hore.

At least 37 records · Page 2Linked to original sources
PubMed1993

Cerebellar dysfunction of movement and perception.

This review describes some characteristics of patients with cerebellar lesions, including limb movements, changes in motor planning and disturbances in time-dependent perception. The delay in movement initiation can be explained by a delay in onset of movement-related discharge of neurons in motor cortex. Disorders of movement termination (hypermetria) are accompanied by asymmetric velocity profiles and by prolonged agonist and delayed antagonist EMG activity necessary to brake the movement. During complex movements in three-dimensional space, the cerebellum contributes to timing between single components of a movement, scales the size of muscular action, and coordinates the sequence of agonists and antagonists. The basic structure of motor programs is not generated exclusively within the cerebellum and patients with cerebellar lesions can use precuing information to improve their motor performance. Time-dependent perception in the auditory and visual domains are disturbed in patients with cerebellar lesions.

Cerebellar Diseases↗
PubMed1992

Constraints on arm position when pointing in three dimensions: Donders' law and the Fick gimbal strategy.

1. While making saccades between targets with the head stationary, eye positions are constrained to two of the possible three degrees of freedom. Classically this constraint has been described by Donders' and Listing's laws. The objective was to determine whether these laws also apply for the straight arm when pointing between different targets. Thus we determined whether the arm adopts only one angular position for every pointing direction (Donders' law) and whether these positions can be described by rotations from a reference position about axes that lie in a plane (Listing's law). 2. The angular positions (orientations) of the arm in three-dimensional space were studied as subjects pointed with a straight arm at different targets. Arm position was measured with the search coil technique by means of coils attached to the back of the hand. Pointing was studied over a range of +/- 45 degrees in all directions from a central target located 45 degrees to the right of the straight-ahead position. 3. The positions of the arm in space were described by quaternion vectors, i.e., a particular position was described in terms of the axis and amplitude of a rotation from a reference position to that position. Using this description, it was found that the straight arm adopted a similar orientation (standard deviations ranged from 2.8 to 4.8 degrees) when pointing at a particular target irrespective of which target from which it had moved. 4. The angular position vectors for arm positions associated with relatively small movements (e.g., less than +/- 30 degrees) lay in a flat surface with minimal torsion. At first sight, this surface appeared to be similar to Listing's plane of the eye. However, for positions associated with larger movements (e.g., +/- 45 degrees) it became apparent that, unlike the eye, the surface deviated from one obeying Listing's law, i.e., it was twisted and showed torsion like that produced by rotations around the horizontal and vertical axes of a Fick gimbal. (The characteristic of a Fick gimbal is that the vertical axis is fixed, whereas the horizontal axis moves with the gimbal.) 5. Although there were differences between subjects, all showed a twisted position vector surface. The twist was always in the same direction, and it was always less than that of a Fick gimbal. 6. This position vector surface had a similar shape whether the arm was stationary or was moving between targets, whether subjects pointed with or without vision, and whether the pointing arm had moved between targets or from a bent-elbow position on the lap.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1991

Cerebellar dysmetria at the elbow, wrist, and fingers.

1. The objective was to investigate in cerebellar patients with dysmetria the kinematic and electromyographic (EMG) characteristics of large and small movements at the elbow, wrist, and finger and thereby to determine the nature of cerebellar dysmetria at distal as well as proximal joints. Flexions were made as fast as possible by moving relatively heavy manipulanda for each joint to the same end position through 5, 30, and 60 degrees. 2. In normal subjects flexions at all joints were accompanied by similar triphasic EMG activity. Movements of increasing amplitude were made with increasing movement durations and increasing durations and magnitudes of initial agonist EMG activity. Antagonist activity often appeared to have two components: one coactive with the initial agonist burst but starting later, the other reaching its peak at about peak velocity. 3. Cerebellar patients with dysmetria showed hypermetria followed by tremor at all three joints when movements were made with the manipulanda. Hypermetria was most marked for aimed movements of small amplitude (5 degrees) at all joints. 4. A characteristic of cerebellar disordered movements, which could be present at all amplitudes and all joints, was an asymmetry with decreased peak accelerations and increased peak decelerations compared to normal movements. Both the asymmetry and the hypermetria for small amplitude movements could be used clinically as sensitive indicators of cerebellar dysfunction. 5. The EMG abnormalities accompanying hypermetria and asymmetry were a more gradual buildup and a prolongation of agonist activity and delayed onset of antagonist activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1990

Task-dependent changes in gain of the reflex response to imperceptible perturbations of joint position in man.

1. It has been demonstrated recently that, when suitably instructed, subjects could alter the stiffness at the elbow in response to a slowly and imperceptibly changing elastic load. Although evidence was provided in favour of this occurring via changes in gain of the reflex response to stretch, changes in the degree of co-contraction could not be entirely ruled out. The major objective of the present experiments was to determine if subjects could alter stiffness at the wrist in a similar task, and then to determine whether they retained this ability when co-contraction was made impossible by anaesthetizing the nerve to the wrist extensors. A second objective was to determine if changes in stiffness could be controlled independently at the wrist and elbow. 2. Subjects, with eyes closed, initially held position constant against a constant force that loaded the flexors. For the wrist, they were instructed: (i) to keep the hand as still as possible (keep position constant) or (ii) to let the hand be moved by the perturbation (keep force constant). The perturbation was an initially imperceptible elastic load whose direction (loading or unloading) could not be predicted. Subjects were also asked to indicate when the perturbation was first perceived. 3. When asked to hold position constant or force constant at the wrist, subjects demonstrated task-dependent changes in stiffness prior to perception of the perturbation. These changes in stiffness were still achieved when the nerve to the wrist extensors was anesthetized and thus co-contraction was prevented. 4. Five subjects demonstrated the ability to control stiffness independently at the wrist and the elbow although most subjects had difficulty with the task we employed to demonstrate this. 5. The results demonstrate: (i) that for the wrist, set-dependent changes in stiffness that occur prior to perception of a slowly developing perturbation can be mediated by changes in gain of reflex responses to those perturbations, and (ii) that stiffness can be controlled independently at the wrist and elbow, presumably in part by changes in gain of stretch reflexes.

Elbow Joint↗
PubMed1990

Behavior of human muscle receptors when reliant on proprioceptive feedback during standing.

1. This study investigated the muscle-spindle discharge from the pretibial flexor muscles of standing human subjects while they performed maneuvers that altered their reliance on proprioceptive feedback to control balance. Single-unit recordings were made from 100 identified muscle afferents, 81 from muscle-spindle endings and 19 from Golgi tendon organs. 2. With 49 spindle endings the subjects stood on a horizontal platform and with 32, on a platform tilted in dorsiflexion (4 degrees) to ensure that the pretibial muscles were active to maintain balance. When standing freely on a horizontal platform without support or vision, there was little or no electromyographic (EMG) activity in the pretibial muscles, and spindle discharge rates were low (55% active; mean rate for all 49 endings, 4.1 Hz). When standing similarly on the tilted platform, 69% of the spindle afferents were active, and the mean discharge rate was 5.4 Hz. The greater number of actively discharging spindle afferents and the preservation of mean discharge rate despite muscle shortening indicates that the pretibial muscles are subjected to increased fusimotor drive when they are tonically active to maintain balance. 3. The effects of small degrees of body sway induced voluntarily or by an external stimulus were studied with 41 afferents (29 spindles; 12 tendon organs). Activation of the pretibial muscles to compensate for backward sway was accompanied by a spindle discharge that usually exceeded the discharge produced by comparable passive movement. This indicates that the pretibial muscles are subjected to increased fusimotor drive when they are phasically active to maintain balance. 4. To vary the reliance placed on the feedback from proprioceptive inputs, the subjects abruptly opened and shut their eyes, took and released support, or tilted their heads. There were no detectable changes in afferent activity unless the maneuver produced a change in EMG activity in the pretibial muscles and/or body sway. Thirty afferents (26 of 46 spindles; 4 of 7 tendon organs) underwent a change in discharge rate associated with a transient change in posture, as recorded by the force platform, or a change in EMG activity in the receptor-bearing muscle. The discharge pattern of 23 afferents did not show any clear change with these maneuvers. 5. It is concluded that maneuvers that increase the reliance on proprioceptive feedback when subjects are standing quietly do not significantly alter the fusimotor drive to the pretibial muscles in the absence of muscle contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1990

Reflex activation of muscle spindles in human pretibial muscles during standing.

1. Experiments were performed in standing subjects to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the human foot reflexly influence fusimotor neurons innervating pretibial flexor muscles. Recordings were made from 30 identified muscle-spindle afferents, four tendon-organ afferents, and one alpha-motor axon innervating the pretibial flexor muscles. The subjects stood without support or vision on a force platform while trains of electrical stimuli (5 stimuli, 300 Hz) were delivered at nonpainful intensities to the sural nerve or to the posterior tibial nerve at the ankle. 2. Seventeen of the 30 spindle endings had no background discharge, and none was activated by the sural or posterior tibial stimuli. Five silent afferents were given a background discharge by sustained pressure on the relevant tendon, but with two the discharge was dominated by a tremor rhythm obscuring any reflex response to the stimuli. Based on peristimulus time histograms (PSTHs), the sural stimuli then produced increases in discharge of two of the remaining three endings at latencies of 84 and 90 ms. These effects could not be explained by muscle stretch and are presumed to have been fusimotor mediated. 3. When the subjects stood freely without support or vision, 13 muscle-spindle endings had a background discharge, but with three endings tremor developed at the ankle and dominated the spindle discharge. Sural stimuli affected the discharge of five of nine endings unaffected by tremor. With three of these endings, there were changes in discharge that could be explained by muscle stretch.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Comparison of cerebellar intention tremor under isotonic and isometric conditions.

The characteristics of cerebellar intention tremor were investigated by comparing the properties of tremor following movements about the elbow with that following isometric contractions of biceps and triceps brachii. Cerebellar dysfunction was produced by local, reversible cooling of the dentate and interposed nuclei in three Cebus monkeys. Cerebellar nuclear cooling disrupted isometric contractions to the aimed target and produced oscillations in torque. Whereas the cerebellar tremor that followed movements was regular and had a frequency of 3-5 Hz, the oscillations in the isometric task were irregular and were often of a lower frequency. In addition, cycles of tremor following movements were in phase from trial to trial, while the oscillations that occurred following isometric contractions did not show this phase-relationship. It is concluded that a cerebellar tremor can occur in an isometric situation but that movement about a joint is required for development of a rhythmic 3-5-Hz cerebellar intention tremor.

Animals↗
PubMed1988

Relations of motor cortex neural discharge to kinematics of passive and active elbow movements in the monkey.

1. Bedingham and Tatton recently reported that in cats trained not to resist imposed limb perturbations, some motor cortex (area 4) neurons responded predominantly to acceleration or jerk (the third derivative of position). The questions arose whether motor cortex neurons responding to higher derivatives of limb displacement exist in the primate in a resist-perturbation task and, if so, whether discharge of such neurons responds to the same kinematics in active (voluntary) movements. 2. To answer these questions we studied the discharge patterns of 203 motor cortex neurons that responded to torque pulse perturbations about the elbow and fired during active elbow flexions and extensions in four monkeys. Detailed analysis was performed on 66 neurons that responded reciprocally in both situations. 3. Reciprocal neurons discharged at short latency (20-40 ms) for one direction of arm perturbation. For the opposite direction they were initially silent or inhibited and then discharged at a variety of latencies but in apparent relation to limb kinematics. Based on the timing and overall pattern of their discharge the majority of neurons (68%) were classified as being acceleration-like. 4. Twenty-four (36%) of these reciprocal neurons had only sensory (kinematic)-like properties in active movements, i.e., they discharged after (and not before) movement onset. Discharge of these neurons followed the timing, but not the magnitude, of acceleration (20 neurons) or velocity (4 neurons). The discharge of these neurons also had a static component as the arm was held stationary. 5. Twenty-nine (44%) of reciprocal neurons commenced firing before movement onset for one direction of active movement, while for the opposite direction their discharge occurred after movement onset. Thus their discharge appeared to be muscle-related: both when the muscle was contracting as an agonist and stretched as an antagonist. 6. Although in these tasks discharge of MCNs could be generated either by sensory feedback or by motor responses, the strong response sensitivity of many neurons to acceleration supports the hypothesis that feedback based on higher derivatives of limb displacement could represent a "predictive" control system for accurate regulation of limb motion.

Action Potentials↗
PubMed1988

Changes in motor cortex neural discharge associated with the development of cerebellar limb ataxia.

1. The relation between changes in motor cortex neural (MCN) discharge and the development of limb ataxia during cerebellar dysfunction was studied in 4 Cebus monkeys. Elbow movements with decreased accelerations or with tremor were produced by reversible cerebellar nuclear cooling. Discharge from 160 neurons was analyzed in detail. 2. During cerebellar cooling 37 of 80 neurons that discharged before movement onset decreased their phasic, but not their tonic, activity. This could not be explained by decreased peak velocities during cerebellar cooling. It is suggested that this decreased phasic discharge is related to the less abrupt onset and smaller magnitude of agonist EMG activity, and to the decreased initial accelerations, without decreased peak velocities, observed in limb movements during cerebellar dysfunction. This view implies that the cerebellum is involved in some way in the generation of commands to agonist alpha-motoneurons. 3. No evidence was found that 3- to 4-Hz cerebellar intention tremor is driven by a purely central oscillator. All 28 neurons that discharged strongly in relation to cerebellar tremor in movements responded strongly and reciprocally to limb perturbations. 4. A number of changes were observed during cerebellar nuclear cooling in kinematically related neural discharge associated with disordered elbow movements: an increase in discharge of some velocity- and acceleration-like neurons, a decrease in (reciprocal) inhibition, and a shift from an acceleration-like to a velocity-like discharge in some neurons. 5. Fourteen of 29 neurons with muscle-like discharge patterns discharged in a servoassistance-like manner during cerebellar dysfunction that was consistent with them contributing to tremor. 6. The results indicate that a variety of disorders, i.e., in the generation of central commands that initiate movements and in the regulation of the gain and phase of proprioceptive feedback, contribute to the development of limb ataxia during cerebellar dysfunction.

Action Potentials↗
PubMed1988

Temperature of the hyperemic bulbar conjunctiva.

The relationship between bulbar conjunctival hyperemia and temperature was investigated. Conjunctival redness was induced in eighteen volunteers by instilling hypertonic saline into the conjunctival sac. The degree of redness was estimated using a subjective grading scale. The subsequent changes in temperature of the nasal bulbar conjunctiva were monitored using an infra-red bolometer. Conjunctival hyperemia was significantly correlated with conjunctival temperature; the maximum response of a 3-grade change in redness was accompanied by an increase of 0.5 degrees C in temperature. These findings confirm the classic association between inflammation, rubor and calor.

Adult↗
PubMed1986

Movement and electromyographic disorders associated with cerebellar dysmetria.

The objective of these experiments was to determine whether dysmetric elbow flexions, which occurred during cerebellar dysfunction, had the same kinematic and electromyographic characteristics as movements of the same amplitude and velocity performed under normal conditions. Reversible cerebellar lesions were produced by cooling through two probes implanted on either side of the dentate nucleus in five Cebus albifrons monkeys. Normal, fast, and accurate elbow flexions had single-peaked velocities and a bi- or triphasic EMG pattern in agonist and antagonist muscles. During cerebellar dysfunction movements became ataxic. Ataxic movements were classified into two categories: those with oscillations (tremor) during the movement and those without oscillations. A terminal tremor occurred after both types of movements. Oscillations during movements were more likely to occur when a constant force loaded the antagonist. Addition of mass to the handle attenuated or abolished the oscillations. Movements with oscillations reached the target with increased variability of end position, whereas movements without oscillations were often hypermetric. The movement parameters and EMG patterns associated with flexions without oscillations during the movement were studied in detail. A characteristic of these movements was that the acceleration and deceleration phases were asymmetric. Compared with control movements of the same peak velocity, they had smaller magnitudes of acceleration and larger magnitudes of deceleration. The large deceleration was abnormal because it initiated the terminal tremor. The disorder in acceleration was associated with agonist EMG activity that was less abrupt in onset, smaller in magnitude, and more prolonged in duration. The disorder in deceleration was associated with delayed onset of phasic antagonist EMG activity. The results show that hypermetric arm movements without oscillations have different properties than those of normal movements of similar velocity and amplitude. Thus it is unlikely that dysmetria results from inappropriate selection or triggering of an otherwise normal motor program. We conclude that normal function of the cerebellum is necessary for the generation of agonist and antagonist muscle activity that is both of the appropriate magnitude and timing to control the dynamic phase of arm movements.

Animals↗
PubMed1986

Evidence that a disordered servo-like mechanism contributes to tremor in movements during cerebellar dysfunction.

The characteristics of discontinuities and tremor that occurred in elbow flexions during cooling of the lateral cerebellar nuclei were investigated in five Cebus monkeys. Discontinuities in movements appeared as rhythmical oscillations (kinetic tremor) when movements were slow or when movements were made with a constant force that loaded the antagonist. These oscillations had similar properties to cerebellar terminal tremor following movements; e.g., their amplitude and frequency were decreased by addition of mass to the handle and they occurred in the absence of visual feedback. The abnormal initial decrease in velocity that initiated oscillations in flexion movements was associated with abnormally early or large antagonist (triceps) electromyogram (EMG) activity. This abnormal EMG activity did not follow the normal inverse relation between initial velocity and antagonist latency from onset of movement. The initial deflection from the expected trajectory was opposed by a second burst of EMG activity in the agonist (biceps). This second burst was not the continuation of a step of EMG activity because its amplitude was often larger than the amplitude of the first agonist burst. The second agonist burst had the properties of a servo-like response: it occurred when biceps shortening was slowed (but biceps was not stretched), its magnitude was proportional to the magnitude or the deflection in velocity, its latency was 50-80 ms from onset of the abnormal decrease in velocity, and it occurred in the absence of visual feedback. However, this servo-like response was disordered because it did not return the limb accurately to the expected trajectory. The servo-like mechanism was studied further by applying torque pulse perturbations during elbow flexions. When the cerebellar nuclei were cooled, agonist responses to the perturbation were proportional to the size of the velocity deflection, but they were prolonged and onset of antagonist activity was delayed. It is suggested that discontinuities and tremor in movements during cerebellar dysfunction result from the same mechanism: alternation between disordered stretch reflexes and disordered servo-assistance mechanisms, both partly involving transcortical pathways.

Animals↗
PubMed1985

Adaptation of saccadic and vestibulo-ocular systems after extraocular muscle tenectomy.

Adaptation of saccadic magnitude and vestibulo-ocular reflex gain (VOR) was examined in six monkeys that had undergone a tenectomy of the medial and lateral recti of one eye. After the tenectomy, when the normal eye was patched, a conjugate increase in saccadic magnitude and VOR was observed in both eyes. Subsequent unpatching resulted in selective (nonconjugate) changes in the two eyes such that control values of saccadic magnitude and VOR gain were eventually reestablished in both eyes. Evidence that this return to normal was mediated by a selective adaptation of the neural innervation to one eye was provided by the following observations: (1) saccades in the normal eye were of a lower peak velocity and longer duration after normalization than prior to the tenectomy; and (2) repatching the tenectomized eye after normalization produced a selective return of deficits in that eye over a period of a few days. Thus, the saccadic and vestibulo-ocular systems can be adapted in both a conjugate and a selective or nonconjugate fashion.

Animals↗
PubMed1984

Dependence of cerebellar tremor on proprioceptive but not visual feedback.

We studied the influence of proprioceptive and visual feedback on cerebellar tremor which occurred after arm perturbations and after voluntary elbow flexions. Cerebellar tremor was produced in monkeys by reversibly cooling through two probes implanted lateral and medial to the dentate nucleus. Cerebellar tremor was synchronized in different trials to torque pulse onset and to the end, but not the start, of voluntary movements. Addition of loads to the handle held by the monkey (increases in spring stiffness, viscosity, constant torque, and inertial load) changed the amplitude and frequency of tremor that follows arm perturbations or voluntary movements in the same way. In both situations EMG activity in each cycle of tremor followed stretch of its own muscle and attained a peak near peak velocity irrespective of the mechanical load. Removal of visual feedback did not alter the characteristics of the tremor or the associated EMG activity. We concluded that cerebellar intention tremor, which occurs when attempting to hold the arm in an intended position, is driven by stretch-evoked peripheral feedback and not by voluntary corrections based on vision.

Animals↗
PubMed1984

Braking of fast and accurate elbow flexions in the monkey.

The processes responsible for braking fast and accurate elbow movements were studied in the monkey. The movements studied were made over different amplitudes and against different inertias . All were made to the same end position. Only fast movements that showed the typical biphasic or triphasic pattern of activity in agonists and antagonists were analysed in detail. For movements made over different amplitudes and at different velocities there was symmetry between the acceleration and deceleration phases of the movements. For movements of the same amplitude performed at different velocities there was a direct linear relation between peak velocity and both the peak acceleration (and integrated agonist burst) and peak deceleration (and integrated antagonist burst). The slopes of these relations and their intercept with the peak velocity axis were a function of movement amplitude. This was such that for large and small movements of the same peak velocity and the same end position (i) peak acceleration and phasic agonist activity were larger for the small movements and (ii) peak deceleration and phasic antagonist activity were larger for the small movements. The slope of these relations and the symmetry between acceleration and deceleration were not affected by the addition of an inertial load to the handle held by the monkey. The results indicate that fast and accurate elbow movements in the monkey are braked by antagonist activity that is centrally programmed. As all movements were made to the same end position, the larger antagonist burst in small movements, made at the same peak velocity as large movements, cannot be due to differences in the viscoelastic contribution to braking (cf. Marsden, Obeso & Rothwell , 1983).(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗
PubMed1984

Loss of set in muscle responses to limb perturbations during cerebellar dysfunction.

The properties of electromyograph (EMG) responses that enabled the arm to return accurately to target following limb perturbations were investigated in five Cebus monkeys. In particular, factors that affected the timing and magnitude of an early antagonist response that occurred prior to stretch of the antagonist muscle were examined. The early antagonist response was large and early (latency, 60 ms) when the perturbation was brief and a constant force assisted the return movement. In this situation, early contraction of the antagonist muscle was required to prevent the return movement from overshooting the target. To determine whether this early antagonist response was influenced by prior instruction (which in this case was the type of perturbation the monkey had previously received), two types of perturbations requiring different EMG responses were studied. When torque steps (duration, 2,000 ms) were expected and were applied, monkeys generated M1, M2, and M3 responses and later activity only in the agonist (the initially stretched) muscle. When torque pulses (duration, 40 ms) were expected and were applied, monkeys generated M1 and M2 responses in the agonist and an early antagonist response. EMG responses to torque pulses and steps were then compared when the type of perturbation was expected and when it was unexpected. These comparisons revealed that the early antagonist response only occurred when the monkey expected a torque pulse. Therefore, this response was dependent on set. Expectation of a torque step caused enhancement of the agonist M2 and M3 responses. These agonist and antagonist EMG responses that were dependent on set were also influenced by changes in afferent drive. Cerebellar nuclear cooling through probes implanted lateral and medial to the dentate abolished that component of EMG responses attributed to set. The residual EMG responses in agonists and antagonists appeared to be driven by stretch of their respective muscles. The results suggest that when the nature of an arm perturbation is correctly predicted, the cerebellum provides accuracy in repositioning the limb a) by adjusting the magnitude of the M2 agonist response and b) by enabling activity after a latency of 60 ms (e.g., the M3 and early antagonist response) to be switched to the agonist or antagonist as appropriate, irrespective of which muscle is being stretched. This latter mechanism provides the motor system with predictive ability.

Animals↗