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Biomedical subjects

A B Schwartz

Publications and source records attributed to A B Schwartz.

At least 19 recordsLinked to original sources
PubMed2003

Training in cortical control of neuroprosthetic devices improves signal extraction from small neuronal ensembles.

We have recently developed a closed-loop environment in which we can test the ability of primates to control the motion of a virtual device using ensembles of simultaneously recorded neurons /29/. Here we use a maximum likelihood method to assess the information about task performance contained in the neuronal ensemble. We trained two animals to control the motion of a computer cursor in three dimensions. Initially the animals controlled cursor motion using arm movements, but eventually they learned to drive the cursor directly from cortical activity. Using a population vector (PV) based upon the relation between cortical activity and arm motion, the animals were able to control the cursor directly from the brain in a closed-loop environment, but with difficulty. We added a supervised learning method that modified the parameters of the PV according to task performance (adaptive PV), and found that animals were able to exert much finer control over the cursor motion from brain signals. Here we describe a maximum likelihood method (ML) to assess the information about target contained in neuronal ensemble activity. Using this method, we compared the information about target contained in the ensemble during arm control, during brain control early in the adaptive PV, and during brain control after the adaptive PV had settled and the animal could drive the cursor reliably and with fine gradations. During the arm-control task, the ML was able to determine the target of the movement in as few as 10% of the trials, and as many as 75% of the trials, with an average of 65%. This average dropped when the animals used a population vector to control motion of the cursor. On average we could determine the target in around 35% of the trials. This low percentage was also reflected in poor control of the cursor, so that the animal was unable to reach the target in a large percentage of trials. Supervised adjustment of the population vector parameters produced new weighting coefficients and directional tuning parameters for many neurons. This produced a much better performance of the brain-controlled cursor motion. It was also reflected in the maximum likelihood measure of cell activity, producing the correct target based only on neuronal activity in over 80% of the trials on average. The changes in maximum likelihood estimates of target location based on ensemble firing show that an animal's ability to regulate the motion of a cortically controlled device is not crucially dependent on the experimenter's ability to estimate intention from neuronal activity.

Action Potentials↗
PubMed2001

Extraction algorithms for cortical control of arm prosthetics.

Now that recordings of multiple, individual action potentials are being made with chronic electrodes, it seems that previous work showing simple encoding of movement parameters in these spike trains can be used as a real-time control signal for prosthetic arms. Efficient extraction algorithms can compensate for the limited ensemble sample acquired with this emerging technology.

Action Potentials↗
PubMed2001

On the relationship between joint angular velocity and motor cortical discharge during reaching.

Single-unit activity in area M1 was recorded in awake, behaving monkeys during a three-dimensional (3D) reaching task performed in a virtual reality environment. This study compares motor cortical discharge rate to both the hand's velocity and the arm's joint angular velocities. Hand velocity is considered a parameter of extrinsic space because it is measured in the Cartesian coordinate system of the monkey's workspace. Joint angular velocity is considered a parameter of intrinsic space because it is measured relative to adjacent arm/body segments. In the initial analysis, velocity was measured as the difference in hand position or joint posture between the beginning and ending of the reach. Cortical discharge rate was taken as the mean activity between these two times. This discharge rate was compared through a regression analysis to either an extrinsic-coordinate model based on the three components of hand velocity or to an intrinsic-coordinate model based on seven joint angular velocities. The model showed that velocities about four degrees-of-freedom (elbow flexion/extension, shoulder flexion/extension, shoulder internal/external rotation, and shoulder adduction/abduction) were those best represented in the sampled population of recorded activity. Patterns of activity recorded across the cortical population at each point in time throughout the task were used in a second analysis to predict the temporal profiles of joint angular velocity and hand velocity. The population of cortical units from area M1 matched the hand velocity and three of the four major joint angular velocities. However, shoulder adduction/abduction could not be predicted even though individual cells showed good correlation to movement on this axis. This was also the only major degree-of-freedom not well correlated to hand velocity, suggesting that the other apparent relations between joint angular velocity and neuronal activity may be due to intrinsic-extrinsic correlations inherent in reaching movements.

Animals↗
PubMed2000

Arm trajectory and representation of movement processing in motor cortical activity.

We review experiments in which single-cell primary motor cortical activity was recorded from Rhesus monkeys (Macaca mulatta) while they performed reaching and drawing tasks. The directional tuning curves generated during reaching were modulated by the speed of movement and this was reflected in the magnitude of population vectors calculated from firing rates of a large population of cells. Directional and speed representation in the firing rates of these cells is robust across both reaching and drawing. Several behavioural invariants related to the speed of drawing were represented in the time-series of population vectors. This high fidelity neural representation of velocity found in motor cortex can be used to visualize the dynamics of motor cortical activity during drawing and suggests that the cost function governing the rate of drawing is bound by neural processing.

Animals↗
PubMed2000

Work toward real-time control of a cortical neural prothesis.

Implantable devices that interact directly with the human nervous system have been gaining acceptance in the field of medicine since the 1960's. More recently, as is noted by the FDA approval of a deep brain stimulator for movement disorders, interest has shifted toward direct communication with the central nervous system (CNS). Deep brain stimulation (DBS) can have a remarkable effect on the lives of those with certain types of disabilities such as Parkinson's disease, Essential Tremor, and dystonia. To correct for many of the motor impairments not treatable by DBS (e.g. quadriplegia), it would be desirable to extract from the CNS a control signal for movement. A direct interface with motor cortical neurons could provide an optimal signal for restoring movement. In order to accomplish this, a real-time conversion of simultaneously recorded neural activity to an online command for movement is required. A system has been established to isolate the cellular activity of a group of motor neurons and interpret their movement-related information with a minimal delay. The real-time interpretation of cortical activity on a millisecond time scale provides an integral first step in the development of a direct brain-computer interface (BCI).

Animals↗
PubMed1999

Motor cortical representation of speed and direction during reaching.

The motor cortical substrate associated with reaching was studied as monkeys moved their hands from a central position to one of eight targets spaced around a circle. Single-cell activity patterns were recorded in the proximal arm area of motor cortex during the task. In addition to the well-studied average directional selectivity ("preferred direction") of single-cell activity, we also found the time-varying speed of movement to be represented in the cortical activity. A single equation relating motor cortical discharge rate to these two parameters was developed. This equation, which has both independent (speed only) and interactive (speed and direction) components, described a large portion of the time-varying motor cortical activity during the task. Electromyographic activity from a number of upper arm muscles was recorded during this task. Muscle activity was also found to be directionally tuned; however, the distributions of preferred directions were found to be significantly different from cortical activity. In addition, the effect of speed on cortical and muscle activity was also found to be significantly different.

Animals↗
PubMed1999

Motor cortical activity during drawing movements: population representation during spiral tracing.

Monkeys traced spirals on a planar surface as unitary activity was recorded from either premotor or primary motor cortex. Using the population vector algorithm, the hand's trajectory could be accurately visualized with the cortical activity throughout the task. The time interval between this prediction and the corresponding movement varied linearly with the instantaneous radius of curvature; the prediction interval was longer when the path of the finger was more curved (smaller radius). The intervals in the premotor cortex fell into two groups, whereas those in the primary motor cortex formed a single group. This suggests that the change in prediction interval is a property of a single population in primary motor cortex, with the possibility that this outcome is due to the different properties generated by the simultaneous action of separate subpopulations in premotor cortex. Electromyographic (EMG) activity and joint kinematics were also measured in this task. These parameters varied harmonically throughout the task with many of the same characteristics as those of single cortical cells. Neither the lags between joint-angular velocities and hand velocity nor the lags between EMG and hand velocity could explain the changes in prediction interval between cortical activity and hand velocity. The simple spatial and temporal relationship between cortical activity and finger trajectory suggests that the figural aspects of this task are major components of cortical activity.

Animals↗
PubMed1999

Motor cortical activity during drawing movements: population representation during lemniscate tracing.

Activity was recorded extracellularly from single cells in motor and premotor cortex as monkeys traced figure-eights on a touch-sensitive computer monitor using the index finger. Each unit was recorded individually, and the responses collected from four hemispheres (3 primary motor and 1 dorsal premotor) were analyzed as a population. Population vectors constructed from this activity accurately and isomorphically represented the shape of the drawn figures showing that they represent the spatial aspect of the task well. These observations were extended by examining the temporal relation between this neural representation and finger displacement. Movements generated during this task were made in four kinematic segments. This segmentation was clearly evident in a time series of population vectors. In addition, the (2)/(3) power law described for human drawing was also evident in the neural correlate of the monkey hand trajectory. Movement direction and speed changed continuously during the task. Within each segment, speed and direction changed reciprocally. The prediction interval between the population vector and movement direction increased in the middle of the segments where curvature was high, but decreased in straight portions at the beginning and end of each segment. In contrast to direction, prediction intervals between the movement speed and population vector length were near-constant with only a modest modulation in each segment. Population vectors predicted direction (vector angle) and speed (vector length) throughout the drawing task. Joint angular velocity and arm muscle EMG were well correlated to hand direction, suggesting that kinematic and kinetic parameters are correlated in these tasks.

Animals↗
PubMed1997

Self-organization of firing activities in monkey's motor cortex: trajectory computation from spike signals.

The population vector method has been developed to combine the simultaneous direction-related activities of a population of motor cortical neurons to predict the trajectory of the arm movement. In this article, we consider a self-organizing model of a neural representation of the arm trajectory based on neuronal discharge rates. As self-organizing feature map (SOFM) is used to select the optimal set of weights in the model to determine the contribution of an individual neuron to an overall movement representation. The correspondence between movement directions and discharge patterns of the motor cortical neurons is established in the output map. The topology-preserving property of the SOFM is used to analyze the recorded data of a behaving monkey. The data used in this analysis were taken while the monkey was tracing spirals and doing center-->out movements. The arm trajectory could be well predicted using such a statistical model based on the motor cortex neuronal firing information. The SOFM method is compared with the population vector method, which extracts information related to trajectory by assuming that each cell has a fixed preferred direction during the task. This implies that these cells are acting along lines labeled only for direction. However, extradirectional information is carried in these cell responses. The SOFM has the capability of extracting not only direction-related information but also other parameters that are consistently represented in the activity of the recorded population of cells.

Algorithms↗
PubMed1996

Neural signals for command control and feedback in functional neuromuscular stimulation: a review.

In current functional neuromuscular stimulation systems (FNS), control and feedback signals are usually provided by external sensors and switches, which pose problems such as donning and calibration time, cosmesis, and mechanical vulnerability. Artificial sensors are difficult to build and are insufficiently biocompatible and reliable for implantation. With the advent of methods for electrical interfacing with nerves and muscles, natural sensors are being considered as an alternative source of feedback and command signals for FNS. Decision making methods for higher level control can perform equally well with natural or artificial sensors. Recording nerve cuff electrodes have been developed and tested in animals and demonstrated to be feasible in humans for control of dorsiflexion in foot-drop and grasp in quadriplegia. Electromyographic signals, being one thousand times larger than electroneurograms, are easier to measure but have not been able to provide reliable indicators (e.g., of muscle fatigue) that would be useful in FNS systems. Animal studies have shown that information about the shape and movement of arm trajectories can be extracted from brain cortical activity, suggesting that FNS may ultimately be directly controllable from the central nervous system.

Electric Stimulation Therapy↗
PubMed1995

A method for detecting the time course of correlation between single-unit activity and EMG during a behavioral task.

The chance that a change in excitability of one neuron leads to a change in excitability of another is likely to vary within a single volitional act. This temporal variability in functional connectivity is impossible to assess with standard analytical procedures to accurately that measure the correlation between such elements. This reports describes a technique designed to overcome this limitation by expressing a correlation measure calculated repeatedly in short epochs throughout a behavioral trial. The activity of two elements, a motor cortical neuron and a shoulder muscle, that might take place during a drawing task was first simulated so that the correlation could be manipulated. Various correlation algorithms (standard cross-correlation, spike-triggered average, impulse-response function, impulse-response surface) were tested with these data. Spike trains from a monkey's motor cortex and rectified EMG from its posterior deltoid muscle were compared using the same techniques and shown to have a correlation that changed in a characteristic manner throughout a task that required the monkey to draw a sinusoid.

Algorithms↗
PubMed1995

The effects of recombinant human erythropoietin on mean corpuscular volume in patients with the anemia of chronic renal failure.

In historical studies erythropoietin stimulated bone marrow was shown to produce less stable, macrocytic, "stress erythrocytes". Recent work from our lab suggests that erythropoietin serves as both a growth factor and as a survival factor. To investigate the effects of recombinant human erythropoietin (rHuEPO) on development of red blood cell size of these longer lived erythrocytes, rHuEPO in 50-150 U/kg/dose was administered to patients with the anemia of chronic renal failure (CRF). Mean corpuscular volume (MCV) was determined at control, short term (n = 117, avg. 53 d), intermediate term (n = 73, avg. 136 d) and at long term (n = 66, avg. 221d) for effects of rHuEPO. Statistical evaluation at these time points was made comparing all patients to themselves as their own controls and using contingency tables for distribution of RBC size change. MCV at both short term (p = .02) and intermediate-term (p < .01) was decreased; there was no change (p = .71) at the long term. Analysis of distribution showed a significant (p < .01) trend toward microcytosis at short- and intermediate terms. This decrease of MCV and trend toward microcytosis is consistent with iron deficiency secondary to the early, rapid increase in bone marrow iron utilization and early increased reticulocytosis. Previous reports from our laboratory coupled with data presented in this report refute earlier findings that rHuEPO creates a "stress" mechanism producing less stable macrocytes.

Anemia, Hypochromic↗
PubMed1994

Direct cortical representation of drawing.

How the intention to act results in movement is a fundamental question of brain organization. Recent work has shown that this operation involves the cooperative interaction of large neuronal populations. A population vector method, by transforming neuronal activity to the spatial domain, was used to visualize the motor cortical representation of the hand's trajectory made by rhesus monkeys as they drew spirals. Hand path was accurately reflected by a series of population vectors calculated throughout the task. A psychophysical rule relating speed to curvature, the "power law," was found in this cortical representation. The relative timing between each population vector and the corresponding portion of the movement was variable. The population vectors only preceded the movement in a predictive manner in portions of the spiral where the radius of curvature was greater than 6 centimeters. These results show that the movement trajectory is an important determinant of motor cortical activity and that this aspect of motor cortical activity may contribute only to discrete portions of the drawing movement.

Algorithms↗
PubMed1994

Distributed motor processing in cerebral cortex.

Responding to a visual cue requires processing throughout many areas in the brain. The anatomical pathways connecting these diffuse areas are numerous. One way to study the cognitive processing associated with volitional movement is to identify common physiological properties in each area. Recently, the discovery that neuronal activity is broadly tuned in many of these cortical areas has led to new insights into the physiological structure of the process underlying cognition in this distributed system.

Animals↗
PubMed1993

Motor cortical activity during drawing movements: population representation during sinusoid tracing.

1. Monkeys were trained to trace sinusoids with their index fingers on a planar surface. During this task, both the direction and speed of movement varied continuously. Activity of individual units in the precentral gyrus contralateral to the moving arm was recorded as the task was performed. These cells responded to passive movement of the shoulder and/or elbow. The relation between discharge rate and movement direction for these individual cells could be described with a cosine tuning function. 2. Data recorded as the sinusoid was traced were divided into 100 bins as each cell was studied during the experiment. In each bin, the activity of a particular cell was represented by a vector. The vector ("cell vector") pointed in the direction of finger movement that corresponded to the highest rate of neuronal discharge. This direction, referred to as the preferred direction, corresponded to the peak of the cosine tuning function. The direction of the vector was constant between bins, but the magnitude of this cell's vector was a function of the instantaneous discharge rate. 3. This cell vector is a hypothetical contribution of a single cell to the population response comprised of 554 similarly derived vectors from different cells. The population response was represented as the vector that resulted from forming the sum of the vector contributions from the individual cells. A separate calculation was made for each bin, resulting in 100 population vectors for each sinusoid. 4. Within a given time series of population vectors, their lengths and directions varied in a consistent relation to the tangential velocity of the drawing movement.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMed1992

Motor cortical activity during drawing movements: single-unit activity during sinusoid tracing.

1. This study examines the neuronal activity of motor cortical cells associated with the production of arm trajectories during drawing movements. Three monkeys were trained to perform two tasks. The first task ("center----out" task) required the animal to move its arm in different directions from a center start position to one of eight targets spaced at equal angular intervals and equal distances from the origin. Movements to each target were in a constant direction, and the average rate of neuronal discharge with movements to different targets varied in a characteristic pattern. A cosine tuning function was used to map each cell's discharge rate to the direction of arm movement. This function spanned all movement directions, with a peak firing rate in the cell's preferred direction. 2. The second task ("tracing" task) required the animal to trace curved figures consisting of sine waves of different spatial frequencies and amplitudes. Both the speed and direction changed continuously throughout these movements. The cosine tuning function derived from the center----out task was used to model the activity of the cell during the tracing of sinusoids in the second task. Sinusoidal data were divided into 20-ms bins; instantaneous direction, speed, and discharge rate were analyzed bin by bin. This provided a way to compare directly the tuning parameters during a task with constant direction to a task where the direction varied continuously. 3. Movement direction as it changed during the tracing task was an important factor in the discharge pattern of cells that had discharge patterns that could be represented by the cosine tuning function. 4. The modulation of discharge rate during figure tracing depended on both the cell's preferred direction and the orientation of the figure. The activity of cells with preferred directions perpendicular to the axis of the sinusoidal figure was most modulated, whereas the activity of those cells with preferred directions aligned to the figure's axis was least modulated. 5. The cells with modulated activity tended to have firing rates that differed from the predicted cosine tuning function during the sinusoidal movements for those portions of the trajectory where the movement direction was in the cell's preferred direction. 6. Finger speed during figure tracing varied inversely with path curvature with the same relation that has been found during human drawing. To assess the relation of instantaneous speed to discharge rate, the component of the discharge pattern related to direction was subtracted from the total discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

RBC improved survival due to recombinant human erythropoietin explains effectiveness of less frequent, low dose subcutaneous therapy.

Effectiveness of less frequent, once weekly, low dose subcutaneous recombinant human erythropoietin (rHuEPO) in maintaining 35% hematocrit in patients with chronic renal failure, predialysis and ESRD receiving dialysis, is dependent on rHuEPO induced prolonged RBC survival. One year of weekly rHuEPO doses to 7 patients originally part of the National Cooperative Protocol were evaluated for a total of 372 weeks for an average of 53 weeks per patient. The original 8 to 12 week dosage was directed by protocol for units per dose at 3 doses per week (4 IV, 3 subcutaneous). Thereafter, all doses were subcutaneous. Units/dose and doses/week were titrated to keep hematocrit at 35-38%. Dosage reduction of rHuEPO was determined by two investigators at the time of each examination. Statistical correlation was performed on effect of rHuEPO on 51Cr T1/2 RBC survival changes and changes of rHuEPO weekly doses. Patients evaluated at specific time points in the study were compared to themselves as their own controls by paired t-test analysis. The long-term increased RBC count correlated with prolonged RBC survival by 51Cr T1/2 rather than reticulocytosis. A relatively increased ease of sustaining the target hematocrit of 35% was demonstrated from the 8th week to 1 year. Thirty-two percent of the expanded RBC mass was older at 12 weeks and 22% was older at 1 year. rHuEPO dosage was reduced to 27% at weeks 8-12, to 21% at weeks 20-24, and to 38% at 1 year corresponding to prolonged RBC survival. 51Cr T1/2 increased from 21.6 days control to 28.6 days at 12 weeks and 26.3 days at 1 year.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗