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Biomedical subjects

W A MacKay

Publications and source records attributed to W A MacKay.

At least 37 records · Page 2Linked to original sources
PubMed1985

Evoked potentials from passive elbow movements. I. Quantitative spatial and temporal analysis.

Evoked potentials following perturbations of the forearm were recorded monopolarly at 8 scalp sites. Successive 10 msec bins of individual EPs were compared across subjects to determine intervals of consistent potential change. From this analysis it was possible to objectively sort subjects into two subpopulations with different common wave forms. Spatial vectors (potential gradient between two adjacent leads) were computed as well as Laplacian derivatives, to identify electrodes closest to source activity. Subsequently, temporal derivatives of the vectors were computed to define the timing of statistically significant response phases. To more precisely localize the largest potential gradients, component vectors (or their derivatives) in the Laplacians were resolved algebraically on a scaled representation of the scalp. Convergence of these resultant vectors from adjacent areas, identified zones of significant potential change which corresponded to known somatosensory areas. The two subpopulations had initial responses of similar topography localized to the central sulcal region. Subsequently, for one group the area of activated cortex expanded to include posterior parietal and more frontal areas. Prestimulus negative potential shifts had different distributions for the two groups and are described in relation to the poststimulus differences.

Adult↗
PubMed1984

Resonance properties of the human elbow.

In normal human subjects, elbow resonance was measured by driving the forearm over a range of frequencies. The relaxed forearm, in a manipulandum with a low inertial moment, had a resonance of 0.068 +/- 0.06 Hz which was remarkably consistent for either sex and over a wide variety of arm sizes. Elbow stiffness, therefore, appears to be well adapted to the forearm moment of inertia. The resonant frequency increased as the elbow muscles were cocontracted, but the amplitude of resonance dropped exponentially because of increased damping. By 25% of maximum electromyogram levels the elbow was critically damped. Comfortable voluntary motion exhibited an amplitude-period relation very similar to that of forearm resonance, suggesting that significant use was made of resonance properties. It was found that the forearm was voluntarily oscillated at its resonant frequency within a specific range, characteristic for the individual. Depending on movement frequency, resonance can provide anything from a major to a negligible contribution to voluntary oscillatory motion. It would greatly increase efficiency of performance when antagonist muscles are kept as relaxed as possible to minimize damping.

Adult↗
PubMed1983

Stretch reflex modulation during a cyclic elbow movement.

Small torque pulses were delivered to the forearm in order to test the stretch reflex of the brachialis and triceps arm muscles in 11 normal subjects performing a cyclic movement about the elbow in the horizontal plane. The flexion-extension movement was paced by a metronome and performed under various loading conditions. Reflexes for each muscle were tested either in each 50 msec segment of the 2 sec cycle period, or in a smaller number of selected phases. A late reflex, appearing at a latency of about 60 msec (measured from the onset of the torque increment), was modulated extensively during the movement cycle. The amplitude of the late reflex increased markedly at the onset of a muscle contraction. In many of the subjects reflex responsiveness began to increase as early as 200 msec prior to the onset of voluntary muscle activity. Peak reflex responses were elicited by stimuli delivered 100-150 msec prior to the peak rate of increase of dynamic load (composed of inertial, viscous and elastic forces). The increase in responsiveness was followed by a drop which was generally coincident in time with the peak rate of increase of the load opposing muscle contraction. The modulation of the late reflex is appropriately timed for reflex-generated tension to help counteract dynamic loads, intrinsic to the movement.

Adult↗
PubMed1982

Activity of primate precentral neurons during voluntary movements triggered by visual signals.

Awake, intact monkeys were trained to perform discrete flexion or extension movements of the hand about the wrist in response to visual signals. The object of the movement was to align a cursor, coupled to a manipulandum, on a target line. Cursor and target lines are displayed on a video monitor placed in front of the monkey. The target line was stepped to the right or left, randomly with regard to direction and timing, with each step implying an instruction for the monkey to make a voluntary movement for alignment. Single unit recording was made in the forelimb area of contralateral precentral cortex. Neurons were classified by their responses to passive sensory stimulation and the effects of local intracortical microstimulation into two populations; wrist flexion-extension (F-E) neurons, and all other forelimb neurons (non-wrist (F-E)). A significantly higher proportion of wrist (F-E) neurons as compared to non-wrist (F-E) neurons were task-related. Moreover the wrist (F-E) neurons exhibited exclusively reciprocal responses to the oppositely directed visual signals, whereas the non-wrist (F-E) neurons showed both reciprocal and bidirectional responses. No significant differences in mean latencies of responses, either in respect to the visual signals or to movement onset, were observed between the two populations of neurons. However the range of latencies in both instances was greater in the non-wrist (F-E) populations. The wrist (F-E) population showed significantly less response variability than the non-wrist (F-E) population with regard to response latencies to visual signals and movement onsets, and the degree of correlation between duration of response and reaction time.

Animals↗
PubMed1981

Distribution of responses to visual cues for movement in precentral cortex or awake primates.

Unit recordings were made from areas 4 and 6 monkeys after they were trained to align a cursor over a vertical target line on a video screen by control of a manipulandum with wrist flexion or extension movement. The appearance of the cursor and line on the screen was the visual cue for movement. Responses were observed 150 (+/- 40) msec after cue presentation. The responses were found only in the forelimb area of precentral cortex, which was most immediately involved in the control of the task, and the majority of them were uncorrelated with either the specific details of the visual cue, or with the direction of the subsequent wrist movement.

Animals↗
PubMed1976

Integrative versus delay line characteristics of cerebellar cortex.

In order to determine which of two general models ("tapped delay line" or "integrator") provides a more accurate desciption of mammalian Purkinje cell (P-cell) activation by natural stimulation, the spatial and temporal characteristics of a population of neurons in cerebellar cortex responsive to small controlled stretches of forelimb muscles were examined in awake, locally anesthetized cats. Stretch of a single wrist muscle excited P-cells over a distance of about 1 mm in the long axis of a folium, a span which is at most half the length of parallel fibers. Both granule cells and molecular layer interneurons were excited over a wider zone than P-cells. Furthermore, P-cells across a response zone all fired on the average at the same time, as determined by computing peristimulus cross-interval histograms from pairs of simultaneously recorded neurons. Consistent delays could only be demonstrated in the minimal response latencies as measured from peristimulus time histograms. These delays, however, were longer than could be ascribed to parallel fiber conduction velocity. No evidence, therefore, was found in cat cerebellum to support the "tapped delay line" model, which postulates the successive activation of P-cells as an excitatory volley travels along a parallel fiber beam. Instead, an integrative mode of operation seems to predominate: a relatively wide substratum of activated granule cells simultaneously activates a narrower focus of P-cells centrally situated with respect to the granule cell population. The role of inhibitory interneurons in promoting the "integrator" model is discussed.

Animals↗
PubMed1975

Physiological basis of cerebellar dysmetria.

A primary control system for the arm position is formulated. The hypothesis that the cerebellum is a part of the system controller is checked by studying the nerve cells responses in the cerebellum, and motor cortex, to natural activation of muscular receptors. The results show that the cerebellum receives feedback information related to the speed of these receptors. The discussion concentrates on how the interruption of this feedback may result in excessive oscillations to instability. These observations are the base for evaluating how the cerebral lesions produce dismeasurements.

Animals↗