Biomedical subjects

W A MacKay

Publications and source records attributed to W A MacKay.

At least 19 recordsLinked to original sources

PubMed1999

Spinal cord-evoked potentials and muscle responses evoked by transcranial magnetic stimulation in 10 awake human subjects.

Transcranial magnetic stimulation (TCMS) causes leg muscle contractions, but the neural structures in the brain that are activated by TCMS and their relationship to these leg muscle responses are not clearly understood. To elucidate this, we concomitantly recorded leg muscle responses and thoracic spinal cord-evoked potentials (SCEPs) after TCMS for the first time in 10 awake, neurologically intact human subjects. In this report we provide evidence of direct and indirect activation of corticospinal neurons after TCMS. In three subjects, SCEP threshold (T) stimulus intensities recruited both the D wave (direct activation of corticospinal neurons) and the first I wave (I1, indirect activation of corticospinal neurons). In one subject, the D, I1, and I2 waves were recruited simultaneously, and in another subject, the I1 and I2 waves were recruited simultaneously. In the remaining five subjects, only the I1 wave was recruited first. More waves were recruited as the stimulus intensity increased. The presence of D and I waves in all subjects at low stimulus intensities verified that TCMS directly and indirectly activated corticospinal neurons supplying the lower extremities. Leg muscle responses were usually contingent on the SCEP containing at least four waves (D, I1, I2, and I3).

Adult↗

PubMed1997

Movement-induced gain modulation of somatosensory potentials and soleus H-reflexes evoked from the leg. I. Kinaesthetic task demands.

Movement-related gating of cerebral somatosensory evoked potentials (SEPs) occurs during active and passive movements of both the upper and the lower limbs. The general hypothesis was tested that the brain participates in setting the gain of the ascending path from somatosensory receptors of the human leg to the somatosensory cortex. In experiment 1, SEPs from Cz' and soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa during passive movement about the right ankle. Early SEPs and H-reflexes sampled during simple passive movement were significantly attenuated when compared with stationary controls (P<0.05). The additional requirement of tracking the passive ankle movement with the other foot led to a significant relative facilitation of mean SEP, but not H-reflex amplitude, compared with means from passive movement alone (P<0.05). In experiment 2, SEPs were evoked in the active (tracking) leg during a forewarned reaction-time task. Subjects were required to move in a preferred direction or to track the passive movement of their right foot with their left. Significant attenuation of early SEP components occurred 100 ms prior to EMG onset (P<0.05), with no apparent effect due to tracking. In the 3rd experiment, SEPs and H-reflexes were evoked in the passively moved leg (the target for active movement of the left leg) during the same forewarned reaction-time task. During the warning period, SEPs were significantly attenuated compared with stationary controls for non-tracking movements, but not for movements involving tracking (P<0.05). It is concluded that centrifugal factors are important in modulating SEP gain required by the kinaesthetic demands of the task.

Adult↗

PubMed1996

Cooperative structures for visually guided reach.

In this introductory commentary, it is argued that the many areas in the cerebral cortex, cerebellum, and elsewhere that are active during reaching movements must interact to generate the appropriate neuronal drive for any given reach trajectory. Cooperative interaction among dispersed neuronal groups may be facilitated by synchronization of intrinsic oscillatory cycles of excitability. In sensorimotor cortex, intervals of oscillatory activity in relatively high frequency bands have been linked to the preparation phase of limb movements. Moreover, synchronization of oscillatory cycles in different loci of frontal and parietal cortex has been reported. Oscillation frequencies change abruptly at the onset of movement, which could reflect altered cerebellar influences on motor thalamocortical circuits. Since relaxation coupling within the thalamus is a probable mechanism of cortical synchronization, it is postulated that cerebellar output contributes to synchronization patterns.

Animals↗

PubMed1995

Field potential oscillatory bursts in parietal cortex before and during reach.

Local field potentials were recorded in parietal cortex, areas 5, 7a and 7b, of a macaque monkey to determine if oscillatory bursts occurred in an observable relationship to behavioral events. The monkey performed a visually-guided reaching task to targets displayed on a touch-sensitive video monitor. The task was pre-cued with a 1.6 s preparatory period. Intracortical recordings were made with a microelectrode or epidural recordings with silver ball electrodes. Compared to the relaxed state, task performance was distinguished by a drop in power for frequencies below 20 Hz (most prominent in area 7), and an increase for frequencies above 20 Hz. For the beta frequency band 20-25 Hz, maximal power occurred during the preparatory periods, and minimal power during reach performance. Above 30 Hz, reach preparation and performance episodes did not differ significantly in spectral power, except in parts of area 5 where 40 Hz activity was observed to increase during movement. The spatial extent of the beta preparatory activity was monitored using an array of 15 epidural electrodes, positioned in 2 rows stretching from the arcuate sulcus to the lunate sulcus. During each preparation, premotor cortex was found to be the major focus of increased power at 20 Hz, whereas posterior parietal cortex was the dominant focus of increased power in the 21-29 Hz band. Although beta frequencies were most prevalent during early stages of motor preparation, the oscillatory bursts were not tightly time-locked to the visual signals. beta Frequencies may be associated with an internally-triggered process to prepare the upcoming movement.

Animals↗

PubMed1994

Spatially modulated touch responses in parietal cortex.

Cortical neurons with low-threshold, cutaneous receptive fields on the fingers were recorded in areas 5 and 7b of the parietal lobe in two awake monkeys, trained in a visually guided reach task. 72% (81/113) of the cells responded when targets displayed on a videomonitor were actively touched. Of these, 20 neurons discharged preferentially when target contact was made on one side of the screen compared with the other. This spatial modulation of the cutaneous modality may have originated in neighboring joint-related neurons which were directionally selective.

Animals↗

PubMed1994

Are extent and force independent movement parameters? Preparation- and movement-related neuronal activity in the monkey cortex.

Movement extent and movement force can be independently controlled in motor performance. Therefore, independent representations of extent and force should exist in the central nervous system (CNS). To test this hypothesis, microelectrode recordings were made in sensorimotor cortex of monkeys trained to perform visually cued wrist flexion movements of two extents, against two levels of frictional resistance. An initial preparatory signal (PS) provided complete, partial or no information about extent and/or force of the movement, which had to be performed in response to a second, response signal (RS). The activity of 511 neurons of the primary motor cortex (MI), the premotor cortex (PM), the postcentral cortex (PC), and the posterior parietal cortex (PA) was recorded in two monkeys. Both reaction time (RT) and neuronal data suggest that there exist independent neuronal mechanisms responsible for the programming of either parameter. On the one hand, partial information about either movement parameter shortened RT when compared with the condition of no prior information. On the other hand, there were, among others, two discrete populations of neurons, one related only to extent, the other only to force. Preparatory changes in activity related to either movement parameter were mainly located in the frontal cortex, especially in the PM. After occurrence of the RS, the percentage of selective changes in activity increased and tended to extend to the parietal cortex. In particular during the movement, force-related changes in activity have been encountered in PA. Furthermore, we conducted trial-by-trial correlation analyses between RT and preparatory neuronal activity for all conditions of prior information. The mean correlation coefficient was significantly higher in the condition of information about movement extent than of information about movement force and it was significantly higher in MI/PM than in PC/PA.

Animals↗

PubMed1994

EEG rhythms of the sensorimotor region during hand movements.

The aim of this study was to determine what motor behaviors or conditions were associated with an increased occurrence of beta activity in the sensorimotor region of human subjects. EEG recordings were obtained from 8 electrodes symmetrically arranged around C3, with 3 cm interelectrode spacing. The electrode montage allowed calculation of the Laplacian operator at two positions, C3r and C3c, overlying the hand area of the motor cortex and of the somatosensory cortex, respectively. A variety of tasks involving right-hand movements of different levels of complexity, attention and preparation were performed. The corresponding EEG power spectra were subsequently computed for frequencies between 7 and 50 Hz. Repetitive hand movements alone (either drawing circles or writing one's signature) did not result in significantly increased beta activity in the sensorimotor region compared to relaxed conditions. However, both motor preparations and focused attention, whether movements were performed or not, were associated with an increase of high frequency beta activity (30-50 Hz) in the sensorimotor region. Therefore, the facilitatory effect of attention and motor preparation and not the functional activation of the sensorimotor cortex by hand movements was associated with an increase in synchronized fast beta activity.

Adolescent↗

PubMed1992

Visual responses to reward-related cues in inferior parietal lobule.

A sample of 263 neurones was recorded in area 7a of the parietal lobe, in a monkey performing a reach task to visual targets displayed on a touch-sensitive videomonitor. The task had been operantly conditioned on food or juice rewards, and 78 (30%) of the units showed activity changes linked in some way to the reward. For most of these cells, the response was to the approach of the trainer's hand with the food reward. This specific visual response was similar irrespective of the direction of approach. Six cells increased discharge as soon as the task was completed in apparent anticipation of the reward. Another two neurones responded to missing a reward: they fired vigorously if the videoscreen was blanked in mid-trial because a target was not correctly touched. In many cases (40/78) the same cells responding to some aspect of the reward also responded to visual cues given during the task, especially the presentation of the target location. Reward-related activity in area 7a probably results from an integration of the visual and limbic inputs to this region, such that visual information which foretells behaviourally important events is emphasized.

Animals↗

PubMed1992

Properties of reach-related neuronal activity in cortical area 7A.

1. In protocol 1, two macaque monkeys were trained to reach to illuminated buttons with the right arm as reach-related unit activity was monitored in area 7a of the left hemisphere. 2. Of 402 neurons recorded in area 7a, 109 changed their discharge rates during the reach task. The change could occur early or late in the trajectory, or during the return movement of the arm to the rest plate. Spatial preferences were seen in 59/109 reach-related cells, usually for the right or center buttons. 3. In protocol 2, another monkey was trained to reach with either arm to targets displayed on a touch-sensitive video monitor. Of 273 neurons sampled in area 7a (both hemispheres) during the bilateral task performance, 84 were reach-related: 33 responded similarly to reaches of either arm. Most of the rest had a contralateral arm preference. When bilateral reach-related cells had a spatial preference, that preference was the same for both arms. 4. With the use of two target sequences in either protocol, it was found that spatial preferences were observable only for primary reaches from the side of the body up to the target. Relatively few cells responded to other trajectories, and those that did usually failed to discriminate movement direction. Movement extent did not influence discharge rates. 5. Although a total of 125/270 reach cells had observable visual responses, only 4 out of 18 cells tested in both dark and light conditions showed a significant drop in reach-related activity in the dark. Thus visual input from the moving hand probably is responsible for only part of the reach activity in area 7a. 6. Reach-related activity in area 7a appears to signal specific phases of the motor performance and is often restricted to distinct spatial regions. As such, it could be used by the frontal lobe to facilitate upcoming elements of a motor sequence, including terminal corrections.

Animals↗

PubMed1990

CNV, stretch reflex and reaction time correlates of preparation for movement direction and force.

Ten human subjects were tested in a pre-cued choice reaction time (RT) paradigm in which the warning stimulus gave a varying amount of prior information regarding the direction (flexion or extension) or force level (weak or strong) for an impending right forearm movement. During the preparatory period (PP), either CNV was monitored from 8 scalp leads, or elbow stretch reflexes were tested at selected times using mechanical torque steps as stimuli. Mean RTs increased as the amount of prior information decreased. The locus of maximal rate of increase of scalp negativity migrated from the frontal lobe to the parietal lobe during the PP, under all conditions. Using laplacian derivations, it was found that the CNV at Cz did not distinguish among the different information conditions, whereas the CNV over the somatosensory arm area was greatest when direction information was given in advance. The CNV over the arm motor region was greatest when force information was available. The last 100 msec of the PP was characterized by the development of current sources over the premotor region for full information, and over the somatosensory region for the other conditions. This coincided with the appearance of very large late stretch reflexes or triggered reactions in the prepared agonist for full information, indicating that the intended movement had been fully processed by this time and awaited only a sensory trigger. The data support a parametric model of motor preparation, with direction and force being processed by at least partially independent networks.

Adolescent↗

PubMed1990

Reaching in the dark: enhanced responses in posterior parietal cortex.

In order to assess the relative importance of visual input to area 7 reach-related neuronal activity, a monkey was trained to reach to visual targets displayed on a video-monitor, both with and without visual feedback. Visual feedback was removed by having the monkey reach in darkness to a previously illuminated target. Of 19 reach-related cells recorded in area 7 both in the light and the dark, ten showed an enhancement of discharge in the dark. These included area 7b cells sensitive to screen contact and area 7a cells active during reach. Dark enhancement of active somatic responsiveness may partially compensate for the loss of visual guidance.

Animals↗

PubMed1989

Changes in contingent-negative variation and reaction time related to precueing of direction and force of a forearm movement.

Scalp-recorded contingent-negative variation was analyzed in a reaction time paradigm with full, partial, or no prior information regarding two dimensions of a forearm response: direction (flexion/extension) and force level (weak/strong). Visual cues (light-emitting diodes) were used for the warning and response signals. The reaction time was shorter when direction, rather than force, was known in advance. Source derivation techniques revealed that the somatosensory arm area was more 'activated' by direction than force information, whereas the precentral cortex seemed to be more strongly influenced by force information. Partial advance information was sufficient to trigger preparatory activities specific for the revealed dimension of the ensuing movement.

Adult↗

PubMed1988

Unit activity in the cerebellar nuclei related to arm reaching movements.

Single units in the fastigial, interpositus and dentate nuclei of two stump-tail macaque monkeys were studied in relation to a right arm, visually guided reaching task. Of 638 recorded cells, 149 showed activity changes correlated to the task, including 24 in the contralateral fastigial and interpositus. Reach-related discharge patterns fell into two broad categories, tonic and phasic. Tonic responses were maintained throughout the reach with no observable relation to kinematic parameters. Most of the task-related activity occurred during the upward lift of the arm toward the target button, with a drop-off as the arm was lowered toward the rest plate. Phasic response cells fired bursts (or suppressed discharge) at specific points in the arm trajectory, most commonly during the lift phase. Many had a sharp drop in discharge when the shoulder flexion torque was transiently reversed to decelerate the arm. For either type, restricted directional specificity was rarely seen in any nucleus, and correlations with recorded EMGs were weak. Visual responses to target button illumination were observed in both the fastigial and dentate nuclei, but did not necessarily correspond with the button giving the best movement-related response. Task-related activity changes started earliest in the fastigial nuclei and latest in the interpositus nuclei. The data suggested that cerebellar output facilitates motor centers in a rather general manner, but at precisely determined times.

Animals↗

PubMed1988

Cerebellar nuclear activity in relation to simple movements.

Single unit activity in the fastigial, interpositus and dentate cerebellar nuclei was recorded in relation to simple elbow flexion and extension movements in two macaque monkeys. In common with proximal muscle activity, 94% of the task-related neurons had qualitatively similar discharge patterns for both directions of forearm movement. In many cases the flexion and extension discharge was virtually identical, but some cells had a distinct directional bias. The very few neurons which were directionally specific were located in the dentate and interpositus. Two had tonic activity well correlated to elbow angle. Task-related changes in discharge rate occurred earliest in dentate and latest in fastigial, but almost always during the period of concomitant proximal and elbow EMG changes. Correlations of phasic activity with movement velocity were uniformly weak. Many eye movement-related neurons were encountered in the fastigial, dentate and y-group nuclei. Fastigial eye cells, both bursting and tonic, tended to be highly direction specific, whereas dentate eye cells were usually omnidirectional and variable. For both arm and eye cerebellar cells, the directional preferences of phasic and tonic discharge, in the same neuron, could be opposed to one another.

Action Potentials↗

PubMed1987

Neuronal correlates in posterior parietal lobe of the expectation of events.

During studies of response properties of single units in the posterior parietal cortex of 6 awake monkeys, 168 neurons were encountered (7.1% of examined units) which showed anticipatory types of activity. These neurons were found on either side of the intraparietal sulcus. In area 5, this expectation activity was expressed as a change in discharge rate whenever a specific body part (e.g. hand or shoulder) was approached by the investigator as though contact would be made. Invariably the neurons also responded to cutaneous and/or proprioceptive stimulation of the target body area. In area 7a the same type of response was also found but not always with a corresponding somatosensory receptive field. In addition, many neurons increased discharge rates (or rarely, decreased them) immediately prior to the expected occurrence of a reward, a visual task cue, or on hearing the approaching footsteps of a familiar person. None of these responses were correlated to eye movements, nor could they be attributed to any other body movement.

Action Potentials↗

PubMed1986

Evoked potentials from passive elbow movements. II. Modification by motor intent.

Subjects were instructed to remain passive or to react to a forearm perturbation by opposing the imposed movement. Evoked potentials (EPs) were recorded at 8 scalp sites in both conditions. In the React condition, reflexes were observed in the EMG (mean onsets of 67 and 81 msec) and the EP was modified. Source derivation techniques revealed that the earliest cortical response (31 msec) across the central sulcus was not changed. Therefore the intention to react did not seem to affect afferent transmission to the primary sensorimotor cortex. Two periods of modulation were observed. In both, parietal and frontal potentials were modulated together, prior to the reflex components. After 70 msec, the pattern of potential gradients which occurred in the Passive case was accelerated and intensified in the React condition. The overall effect was to focus a larger zone of negativity over motor cortex at the time of triggered EMG output (109 msec). The earlier changes in cortical activity could be causally related to the appearance of the late stretch reflex. Since parietal and frontal areas were principally involved and not the motor area, it is suggested that the former exert a modulatory influence on spinal and brain-stem reflex centres.

Elbow↗

PubMed1986

Measurements of human forearm viscoelasticity.

In human subjects, stiffness of the relaxed elbow was measured by three methods, using a forearm manipulandum coupled to a.d.c. torque motor. Elbow stiffness calculated from frequency response characteristics increased as the driving amplitude decreased. Step displacements of the forearm produced restoring torques linearly related to the displacement. The stiffness was very similar to that calculated from natural frequencies at amplitudes above 0.1 rad. Thirdly, elbow stiffness was estimated from brief test pulses, 120 ms in duration, by mathematically simulating the torque-displacement functions. Stiffness values in the limited linear range (under +/- 0.1 rad) were higher than in the linear range of the first two methods. A major component of elbow stiffness appears to decay within 1 s. The coefficients of viscosity determined from the simulation were, however, very similar to those calculated from the frequency response. Test pulse simulation was then used to determine joint impedance for different, actively maintained elbow angles. Joint stiffness and viscosity increased with progressive elbow flexion.

Adult↗

PubMed1985

Properties of visual cue responses in primate precentral cortex.

Monkeys were trained to perform a visuomotor task involving the alignment of a cursor over a vertical target line on a videomonitor by flexion or extension movements of the wrist. The forelimb area of the contralateral precentral cortex was thoroughly explored during the task. Intracortical microstimulation was employed to classify the forelimb region into wrist flexion--extension and non-wrist flexion--extension populations. Unit recording revealed an initial response to the cue for movement, viz. the appearance of the cursor and target line on the videomonitor, while visual signals not related to the task failed to evoke any response. The mean latency of these visual cue responses was approximately 150 ms. A great majority of the responses (96%) were bidirectional in character, in that they did not correlate with the directional information embedded in the visual cue, nor were they good predictors for the direction or timing of the subsequent movement. They were uniformly distributed in both the wrist and non-wrist regions of the forelimb area; the non-forelimb areas were devoid of the cue response. Further, when the variability of response to the visual cue for the wrist and non-wrist populations was compared, no significant difference was observed. These observations are consistent with an interpretation that the visually triggered cue responses provide a generalized activation over the task-related area of precentral cortex, paving the way for later and more specific activations leading to the execution of the task.

Animals↗