J.T. Murphy

Correlation of Peak Systolic Velocity and Angiographic Measurement of Carotid Stenosis Revisited

BACKGROUND AND PURPOSE Recent observations from the North American Symptomatic Carotid Endarterectomy Trial (NASCET) questioned the reliability of peak systolic velocity (PSV) criteria for grading carotid stenosis. We compared PSV and angiographic measurements at our center together with known physiological relationships to investigate the accuracy of ultrasound. METHODS Consecutive patients who underwent both color-coded duplex ultrasound and intra-arterial digital subtraction angiography were studied. PSV was determined with angle correction at the site of the tightest internal carotid artery narrowing. Carotid stenosis was measured on angiograms with the North American (N) and common carotid (C) methods. Variables for the stepwise multiple linear regression analysis were selected from an axisymmetrical flow model. RESULTS Eighty bifurcations were imaged in 40 patients. PSV did not exceed 140 cm/s in normal vessels. In diseased arteries, PSV increased proportionally with increasing stenosis and decreased to 0 cm/s at occlusion. In stepwise selection of polynomial terms, the linear, quadratic, and cubic correlations of .38, .17, and .22 for N and .45, .24, and .03 for C were found to be significant (P < .02). When only stenosed vessels were evaluated, PSV increase was found with greater scatter for the N measurement: r2 = .73 for N and r2 = .85 for C (n = 50; P = .03 for the difference between two correlated correlation coefficients). CONCLUSIONS At our laboratory PSV consistently correlates well with N and C angiographic measurements, as determined with a simple flow model. The complex nature of these correlations and greater variability of the N measurement should be taken into account when data from different centers are compared.

The role of the basal ganglia in controlling a movement initiated by a visually presented cue

This study examines the effects on single neurons in caudate nucleus and globus pallidus of a visual stimulus, which is a cue to begin a tracking movement involving the forelimb on the side contralateral to the recording site. The experiments were designed to examine the relationship between activity of single neurons in these structures and movement initiation. Four monkeys were prepared for chronic recording of single neurons. The animals were trained to perform a tracking task that involved flexion and extension of the wrist at the onset of visual cue. The activities of a large proportion of neurons examined were correlated with the task, the proportion being greater in globus pallidus than in caudate nucleus. The majority of correlated neurons were activated after the onset of muscle activity and subsequent movement. It is thus unlikely that these cells were involved in movement initiation. However a role in movement execution is not excluded by the results. The fact that many cells were responsive only to certain specific conditions of the visual cue trial suggests an influence on these cells by highly processed visual information from other brain areas.

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.

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.

Interaction between neurons in precentral cortical zones controlling different joints

The relationship of the strength of interaction between precentral cortical neurons and their distance of separation during active reaching movements was studied in adult primates. Chronic unit recording experiments with two independent microelectrodes were performed in the left precentral forearm area of monkeys trained to execute reaching movements with the right arm in response to a visual cue. Neurons were identified by the joint actions produced by intracortical microstimulation. Cross-correlation analysis was employed to assess the strength of interaction between units. Unit pairs which exhibited the highest strength were recorded by the same electrode. For unit pairs derived from separate electrodes, the incidence and strength of interaction fell as the separation between the units was increased. Neurons identified by intracortical microstimulation as controlling the same or contiguous joints tended to interact with each other with much higher probability than did those neurons identified as controlling non-contiguous joints. When the direction of flow of information was assessed, these was a preferential flow from neurons controlling proximal joints to those controlling distal ones. These results are consistent with recent findings of tight kinematic coupling between contiguous joints and the observation of proximal-to-distal sequence of activation at the neuronal and electromyogram levels during voluntary movement.

Differences between cerebellar mossy and climbing fibre responses to natural stimulation of forelimb muscle proprioceptors

Abstract Forelimb muscle proprioceptors were activated by passive stretch in intact, regionally anaesthetized cats. Input from single muscles provided an adequate stimulus to activate both granule cells and mossy fibre responses in Purkinje cells in the anterior lobe of the cerebellar cortex. The zone of cells responsive to mossy fibre inputs was sharply localized within lobule V of the ipsilateral pars intermedia. The responses within the zone were excitatory, and inputs activated from synergistic and antagonistic muscles acting upon the same joints converged upon single granule and Purkinje cells to produce similar excitatory responses. Climbing fibre responses in Purkinje cells were also activated by proprioceptive inputs from single muscles. The threshold for these responses was slightly higher than for responses carried by mossy fibre pathways, and the spatial extent of the cortical climbing fibre response zone coincided with and extended beyond that of the mossy fibre response zone. Relatively few purely inhibitory Purkinje cell responses were found with these single inputs in this preparation, and latency measurements indicated that they were mediated by mossy fibre pathways. The inhibited cells were found in narrow strips on the rostral and caudal sides of the zone of excited Purkinje cells, affording an example of lateral inhibition in the cerebellar cortex in a physiological context. The results are discussed in terms of anatomical and functional distinctions between the two afferent systems.

Cross correlation studies in primate motor cortex: synaptic interaction and shared input

Awake, unrestrained monkeys were trained to reach out with the forelimb and touch a button. Extracellular spike trains were recorded from pairs of neurons in contralateral precentral cortex with the same or separate microelectrodes. The neurons were located in the same or different functional columns as defined by intracortical microstimulation and passive sensory stimulation. Cross correlation analysis showed patterns consistent with synaptic excitation and/or inhibition between members of the cell pairs during the voluntary movement. The strength of correlation was inversely related to distance between columns, with the strongest correlations found between cells within the same column. Inhibitory correlations were virtually restricted to cell pairs within a single column. Temporal analysis showed that direct synaptic interaction and shared input patterns could be clearly distinguished in this physiologic setting. Spatial analysis indicated that shared input was concentrated among columns in the same and adjacent joint controlling zones as well as within a single column. No directional preference of shared input was present, a finding which was consistent with the observed nested organization of the forelimb area.

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.

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

Electrical conductivity in cat cerebellar cortex

Abstract Electrical conductivity in molecular and granular layers of cat cerebellar cortex is studied, taking into account anisotropy. A suitable mathematical analysis involving a Greens function approach gives an equation relating voltage to three principal variables: the magnitude of the point current source, the distance from the source, and the conductivities in each of three orthogonal directions. Voltage is measured experimentally as a function of distance in the molecular and granular layers of the cerebellum. The results of these measurements are used to provide an empiric fit to the derived equations. From a set of parameters obtained from the fitted curves, the electrical conductivities are calculated. Sample values of the conductivities are used for three dimensional plots of voltage as a function of two space coordinates, to show the degree of anisotropy in two of the three directions. Results indicate that the conductivities for each direction in the granular layer are all of the order of 2.0 mmho/cm. In the molecular layer, the conductivity in two directions is about 3.0 mmho/cm, but in the other direction it is 1.7 mmho/cm. The granular layer is more isotropic than the molecular layer due to structural differences between the two layers.