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Research paper thumbnail of Research Note The Effects of Muscle Vibration on the Attainment of Intended Final Position During Voluntary Human Arm Movements

Research paper thumbnail of Neuromuscular Models for Locomotion

Elsevier eBooks, 2017

Abstract Nature has solved the problem of controlling legged locomotion many thousands of times i... more Abstract Nature has solved the problem of controlling legged locomotion many thousands of times in animals exhibiting an enormous variety of neuromechanical structures. Control systems features that are common to nearly all species include feedback control of limb displacement and force and feedforward generation of motor patterns by neural networks within the central nervous system (Central Pattern Generators: CPGs). Here we review the components of locomotor control systems, with a focus on mammalian animals including humans. We then propose a generalized model of reflex control and discuss the mathematical functions that describe the properties of the components of a spinal stretch reflex model. Reflex model of neuromuscular control. Here we provide and discuss the mathematical functions for the components in the reflex model presented in Subchapter 6.6 . These include functions that describe muscle intrinsic properties (excitation–contraction coupling, force–velocity and orce–length curves) and sensory receptor models (muscle spindle responses to length changes and fusimotor drive, Golgi tendon organ responses to changes in muscle force).

Research paper thumbnail of Task-dependent changes of motor cortical network excitability during precision grip compared to isolated finger contraction

Journal of Neurophysiology, Mar 1, 2012

Research paper thumbnail of Motor Cortex: Evidence for a Recurrent Neural On the Nature of the Intrinsic Connectivity of the Cat

Research paper thumbnail of Correction to: On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental brain research, 2021

Research paper thumbnail of Correction to: On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, Oct 11, 2021

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, 2021

The purpose of this study was to determine the form of the relation between the mean amplitude an... more The purpose of this study was to determine the form of the relation between the mean amplitude and variance of motor-evoked potentials (MEP). To this end, single-pulse transcranial magnetic stimulation (TMS) was applied over the motor cortex of seventeen neurologically normal adult human subjects. The coil was positioned at a locus on the scalp that elicited an MEP in the first dorsal interosseous (FDI) at the lowest stimulus intensity. The subjects were instructed to maintain tonic activity in the FDI of 5 or 10% of the maximum voluntary contraction (MVC). The relation between MEP variance and amplitude was found to have an inverted parabolic shape, with maximal variance occurring near the half-maximal MEP amplitude. The coefficient of variation CV of MEPs decreased approximately as a rectangular hyperbolic function of MEP amplitude (i.e. ~ 1/MEP). A probabilistic model is proposed to explain the inverted parabolic relation between MEP variance and MEP amplitude, as well as the sigmoid shape of the MEP input-output relation (i.e. stimulus-response curve). The model is based on a description of α-motoneurons as binary threshold units, with unit thresholds distributed according to a positively skewed probability density function. The units are driven by noisy synaptic input currents having a Gaussian distribution. The model predicts an inverse parabolic relation between MEP variance and amplitude and a sigmoid input-output relation, as experimentally observed. Furthermore, increasing model motoneuron excitability by increasing the background synaptic drive increases MEP variability independently of MEP size, a surprising prediction. The model also explains the approximately rectangular hyperbolic relation between CV and MEP amplitude. The implications of these results for the interpretation of neurophysiological experiments and the statistical analysis of MEPs are discussed.

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, 2021

The purpose of this study was to determine the form of the relation between the mean amplitude an... more The purpose of this study was to determine the form of the relation between the mean amplitude and variance of motor-evoked potentials (MEP). To this end, single-pulse transcranial magnetic stimulation (TMS) was applied over the motor cortex of seventeen neurologically normal adult human subjects. The coil was positioned at a locus on the scalp that elicited an MEP in the first dorsal interosseous (FDI) at the lowest stimulus intensity. The subjects were instructed to maintain tonic activity in the FDI of 5 or 10% of the maximum voluntary contraction (MVC). The relation between MEP variance and amplitude was found to have an inverted parabolic shape, with maximal variance occurring near the half-maximal MEP amplitude. The coefficient of variation CV of MEPs decreased approximately as a rectangular hyperbolic function of MEP amplitude (i.e. ~ 1/MEP). A probabilistic model is proposed to explain the inverted parabolic relation between MEP variance and MEP amplitude, as well as the sigmoid shape of the MEP input-output relation (i.e. stimulus-response curve). The model is based on a description of α-motoneurons as binary threshold units, with unit thresholds distributed according to a positively skewed probability density function. The units are driven by noisy synaptic input currents having a Gaussian distribution. The model predicts an inverse parabolic relation between MEP variance and amplitude and a sigmoid input-output relation, as experimentally observed. Furthermore, increasing model motoneuron excitability by increasing the background synaptic drive increases MEP variability independently of MEP size, a surprising prediction. The model also explains the approximately rectangular hyperbolic relation between CV and MEP amplitude. The implications of these results for the interpretation of neurophysiological experiments and the statistical analysis of MEPs are discussed.

Research paper thumbnail of Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories

Frontiers in Computational Neuroscience, 2022

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (... more The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABAA receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal bursts were also highly correlated with those obtained by ICMS at the same cortical point. Varying the initial position of the forelimb did not change the MAPs evoked from a cortical point. Consequently, the evoked movements reached nearly the same final end point and posture, with variability. However, the movement trajectories were quite different depending on the initial limb configuration and starting position of the paw. The evoked movement trajectory was most natural when the forelimb lay pendant ∼ perpendicular to the ground (i.e., in equilibrium with the gravitational force). From other starting positions, the movements did
not appear natural. These observations demonstrate that while the output of the cortical point evokes a seemingly coordinated limb movement from a rest position, it does not specify a particular movement direction or a controlled trajectory from other initial positions.

Research paper thumbnail of Joint afferents subserve human proprioception that is interhemispherically transferred

Research paper thumbnail of Amelioration Des Protheses et Aides Orthotiques Pour Les Handicapes Au Moyen De Stimulation Electrique et D'Enregistrement De Signaux Bioelectriques

Research paper thumbnail of Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories

Frontiers in Computational Neuroscience

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (... more The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABAA receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal ...

Research paper thumbnail of On the potential role of motor cortical GABA neurons in releasing inter-joint muscle synergies

The Society for Neuroscience Abstracts, 1999

Research paper thumbnail of Neural mechanism of activity spread in the cat motor cortex and its relation to the intrinsic connectivity

The Journal of Physiology, 2011

Research paper thumbnail of Amplitude modulation of the soleus H-reflex in the human during walking and

Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the... more Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the soleus muscle at various phases of the step cycle, using a computer-based analysis procedure. In all subjects tested the amplitude of the H-reflex was strongly modulated in amplitude during the walking cycle and was highest during the stance phase. In many subjects the peak reflex amplitude occurred at about the same time as the peak soleus electromyographic (EMG) activity, but in others it occurred earlier. The form of the reflex variation (i.e., envelope of H-reflex amplitude versus phase in cycle) during the step cycle could also be quite different from that of the EMG produced during stepping. At an equal stimulus strength and EMG level, the H-reflex was always much larger, up to 3.5 x, during steadily maintained contractions while standing than during walking. The large reflexes when subjects were standing are consistent

Research paper thumbnail of On the functional organization and operational principles of the motor cortex

Frontiers in Neural Circuits, Apr 18, 2013

Recent studies on the functional organization and operational principles of the motor cortex (MC... more Recent studies on the functional organization and operational principles of the motor
cortex (MCx), taken together, strongly support the notion that the MCx controls the
muscle synergies subserving movements in an integrated manner. For example, during
pointing the shoulder, elbow and wrist muscles appear to be controlled as a coupled
functional system, rather than singly and separately. The recurrent pattern of intrinsic
synaptic connections between motor cortical points is likely part of the explanation for
this operational principle. So too is the reduplicated, non-contiguous and intermingled
representation of muscles in the MCx. A key question addressed in this article is whether
the selection of movement related muscle synergies is a dynamic process involving the
moment to moment functional linking of a variety of motor cortical points, or rather the
selection of fixed patterns embedded in the MCx circuitry. It will be suggested that both
operational principles are probably involved. We also discuss the neural mechanisms by
which cortical points may be dynamically linked to synthesize movement related muscle
synergies. Separate corticospinal outputs sum linearly and lead to a blending of the
movements evoked by activation of each point on its own. This operational principle may
simplify the synthesis of motor commands. We will discuss two possible mechanisms
that may explain linear summation of outputs. We have observed that the final posture of
the arm when pointing to a given spatial location is relatively independent of its starting
posture. From this observation and the recurrent nature of the MCx intrinsic connectivity
we hypothesize that the basic mode of operation of the MCx is to associate spatial location to final arm posture. We explain how the recurrent network connectivity operates to generate the muscle activation patterns (synergies) required to move the arm and hold
it in its final position.

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research

Research paper thumbnail of Proprioceptive Acuity is Enhanced During Arm Movements Compared to When the Arm is Stationary: A Study of Young and Older Adults

Research paper thumbnail of Pointing to One's Moving Hand: Putative Internal Models Do Not Contribute to Proprioceptive Acuity

Frontiers in human neuroscience, 2018

We can easily and without sight bring our fingertip to our nose, or swat a mosquito on our arm. T... more We can easily and without sight bring our fingertip to our nose, or swat a mosquito on our arm. These actions rely on proprioception, also known as kinesthesia, which classically has been attributed to processing of sensory inflow by the CNS. However, internal model theories of sensorimotor neuroscience propose that proprioceptive localization also involves a contribution from estimates of limb kinematics derived from motor commands. We tested this prediction in 19 subjects who moved the right index finger tip to touch the moving left index finger tip under three conditions: (1) vision allowed, active movement of the left hand (2) vision blocked, active movement of the left hand, and (3) vision blocked, passive movement of the left hand imposed by the experimenter. The target left index finger tip was moved in a wide range of directions by unrestricted movements of the arm. Mean errors in apposition of the right to the left index finger tips were small, averaging <2 cm between se...

Research paper thumbnail of Linear summation of outputs in a balanced network model of motor cortex

Given the non-linearities of the neural circuitry's elements, we would expect cortical circuits t... more Given the non-linearities of the neural circuitry's elements, we would expect cortical circuits to respond non-linearly when activated. Surprisingly, when two points in the motor cortex are activated simultaneously, the EMG responses are the linear sum of the responses evoked by each of the points activated separately. Additionally, the corticospinal transfer function is close to linear, implying that the synaptic interactions in motor cortex must be effectively linear. To account for this, here we develop a model of motor cortex composed of multiple interconnected points, each comprised of reciprocally connected excitatory and inhibitory neurons. We show how non-linearities in neuronal transfer functions are eschewed by strong synaptic interactions within each point. Consequently, the simultaneous activation of multiple points results in a linear summation of their respective outputs. We also consider the effects of reduction of inhibition at a cortical point when one or more surrounding points are active. The network response in this condition is linear over an approximately two-to threefold decrease of inhibitory feedback strength. This result supports the idea that focal disinhibition allows linear coupling of motor cortical points to generate movement related muscle activation patterns; albeit with a limitation on gain control. The model also explains why neural activity does not spread as far out as the axonal connectivity allows, whilst also explaining why distant cortical points can be, nonetheless, functionally coupled by focal disinhibition. Finally, we discuss the advantages that linear interactions at the cortical level afford to motor command synthesis.

Research paper thumbnail of Research Note The Effects of Muscle Vibration on the Attainment of Intended Final Position During Voluntary Human Arm Movements

Research paper thumbnail of Neuromuscular Models for Locomotion

Elsevier eBooks, 2017

Abstract Nature has solved the problem of controlling legged locomotion many thousands of times i... more Abstract Nature has solved the problem of controlling legged locomotion many thousands of times in animals exhibiting an enormous variety of neuromechanical structures. Control systems features that are common to nearly all species include feedback control of limb displacement and force and feedforward generation of motor patterns by neural networks within the central nervous system (Central Pattern Generators: CPGs). Here we review the components of locomotor control systems, with a focus on mammalian animals including humans. We then propose a generalized model of reflex control and discuss the mathematical functions that describe the properties of the components of a spinal stretch reflex model. Reflex model of neuromuscular control. Here we provide and discuss the mathematical functions for the components in the reflex model presented in Subchapter 6.6 . These include functions that describe muscle intrinsic properties (excitation–contraction coupling, force–velocity and orce–length curves) and sensory receptor models (muscle spindle responses to length changes and fusimotor drive, Golgi tendon organ responses to changes in muscle force).

Research paper thumbnail of Task-dependent changes of motor cortical network excitability during precision grip compared to isolated finger contraction

Journal of Neurophysiology, Mar 1, 2012

Research paper thumbnail of Motor Cortex: Evidence for a Recurrent Neural On the Nature of the Intrinsic Connectivity of the Cat

Research paper thumbnail of Correction to: On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental brain research, 2021

Research paper thumbnail of Correction to: On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, Oct 11, 2021

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, 2021

The purpose of this study was to determine the form of the relation between the mean amplitude an... more The purpose of this study was to determine the form of the relation between the mean amplitude and variance of motor-evoked potentials (MEP). To this end, single-pulse transcranial magnetic stimulation (TMS) was applied over the motor cortex of seventeen neurologically normal adult human subjects. The coil was positioned at a locus on the scalp that elicited an MEP in the first dorsal interosseous (FDI) at the lowest stimulus intensity. The subjects were instructed to maintain tonic activity in the FDI of 5 or 10% of the maximum voluntary contraction (MVC). The relation between MEP variance and amplitude was found to have an inverted parabolic shape, with maximal variance occurring near the half-maximal MEP amplitude. The coefficient of variation CV of MEPs decreased approximately as a rectangular hyperbolic function of MEP amplitude (i.e. ~ 1/MEP). A probabilistic model is proposed to explain the inverted parabolic relation between MEP variance and MEP amplitude, as well as the sigmoid shape of the MEP input-output relation (i.e. stimulus-response curve). The model is based on a description of α-motoneurons as binary threshold units, with unit thresholds distributed according to a positively skewed probability density function. The units are driven by noisy synaptic input currents having a Gaussian distribution. The model predicts an inverse parabolic relation between MEP variance and amplitude and a sigmoid input-output relation, as experimentally observed. Furthermore, increasing model motoneuron excitability by increasing the background synaptic drive increases MEP variability independently of MEP size, a surprising prediction. The model also explains the approximately rectangular hyperbolic relation between CV and MEP amplitude. The implications of these results for the interpretation of neurophysiological experiments and the statistical analysis of MEPs are discussed.

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research, 2021

The purpose of this study was to determine the form of the relation between the mean amplitude an... more The purpose of this study was to determine the form of the relation between the mean amplitude and variance of motor-evoked potentials (MEP). To this end, single-pulse transcranial magnetic stimulation (TMS) was applied over the motor cortex of seventeen neurologically normal adult human subjects. The coil was positioned at a locus on the scalp that elicited an MEP in the first dorsal interosseous (FDI) at the lowest stimulus intensity. The subjects were instructed to maintain tonic activity in the FDI of 5 or 10% of the maximum voluntary contraction (MVC). The relation between MEP variance and amplitude was found to have an inverted parabolic shape, with maximal variance occurring near the half-maximal MEP amplitude. The coefficient of variation CV of MEPs decreased approximately as a rectangular hyperbolic function of MEP amplitude (i.e. ~ 1/MEP). A probabilistic model is proposed to explain the inverted parabolic relation between MEP variance and MEP amplitude, as well as the sigmoid shape of the MEP input-output relation (i.e. stimulus-response curve). The model is based on a description of α-motoneurons as binary threshold units, with unit thresholds distributed according to a positively skewed probability density function. The units are driven by noisy synaptic input currents having a Gaussian distribution. The model predicts an inverse parabolic relation between MEP variance and amplitude and a sigmoid input-output relation, as experimentally observed. Furthermore, increasing model motoneuron excitability by increasing the background synaptic drive increases MEP variability independently of MEP size, a surprising prediction. The model also explains the approximately rectangular hyperbolic relation between CV and MEP amplitude. The implications of these results for the interpretation of neurophysiological experiments and the statistical analysis of MEPs are discussed.

Research paper thumbnail of Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories

Frontiers in Computational Neuroscience, 2022

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (... more The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABAA receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal bursts were also highly correlated with those obtained by ICMS at the same cortical point. Varying the initial position of the forelimb did not change the MAPs evoked from a cortical point. Consequently, the evoked movements reached nearly the same final end point and posture, with variability. However, the movement trajectories were quite different depending on the initial limb configuration and starting position of the paw. The evoked movement trajectory was most natural when the forelimb lay pendant ∼ perpendicular to the ground (i.e., in equilibrium with the gravitational force). From other starting positions, the movements did
not appear natural. These observations demonstrate that while the output of the cortical point evokes a seemingly coordinated limb movement from a rest position, it does not specify a particular movement direction or a controlled trajectory from other initial positions.

Research paper thumbnail of Joint afferents subserve human proprioception that is interhemispherically transferred

Research paper thumbnail of Amelioration Des Protheses et Aides Orthotiques Pour Les Handicapes Au Moyen De Stimulation Electrique et D'Enregistrement De Signaux Bioelectriques

Research paper thumbnail of Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories

Frontiers in Computational Neuroscience

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (... more The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABAA receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal ...

Research paper thumbnail of On the potential role of motor cortical GABA neurons in releasing inter-joint muscle synergies

The Society for Neuroscience Abstracts, 1999

Research paper thumbnail of Neural mechanism of activity spread in the cat motor cortex and its relation to the intrinsic connectivity

The Journal of Physiology, 2011

Research paper thumbnail of Amplitude modulation of the soleus H-reflex in the human during walking and

Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the... more Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the soleus muscle at various phases of the step cycle, using a computer-based analysis procedure. In all subjects tested the amplitude of the H-reflex was strongly modulated in amplitude during the walking cycle and was highest during the stance phase. In many subjects the peak reflex amplitude occurred at about the same time as the peak soleus electromyographic (EMG) activity, but in others it occurred earlier. The form of the reflex variation (i.e., envelope of H-reflex amplitude versus phase in cycle) during the step cycle could also be quite different from that of the EMG produced during stepping. At an equal stimulus strength and EMG level, the H-reflex was always much larger, up to 3.5 x, during steadily maintained contractions while standing than during walking. The large reflexes when subjects were standing are consistent

Research paper thumbnail of On the functional organization and operational principles of the motor cortex

Frontiers in Neural Circuits, Apr 18, 2013

Recent studies on the functional organization and operational principles of the motor cortex (MC... more Recent studies on the functional organization and operational principles of the motor
cortex (MCx), taken together, strongly support the notion that the MCx controls the
muscle synergies subserving movements in an integrated manner. For example, during
pointing the shoulder, elbow and wrist muscles appear to be controlled as a coupled
functional system, rather than singly and separately. The recurrent pattern of intrinsic
synaptic connections between motor cortical points is likely part of the explanation for
this operational principle. So too is the reduplicated, non-contiguous and intermingled
representation of muscles in the MCx. A key question addressed in this article is whether
the selection of movement related muscle synergies is a dynamic process involving the
moment to moment functional linking of a variety of motor cortical points, or rather the
selection of fixed patterns embedded in the MCx circuitry. It will be suggested that both
operational principles are probably involved. We also discuss the neural mechanisms by
which cortical points may be dynamically linked to synthesize movement related muscle
synergies. Separate corticospinal outputs sum linearly and lead to a blending of the
movements evoked by activation of each point on its own. This operational principle may
simplify the synthesis of motor commands. We will discuss two possible mechanisms
that may explain linear summation of outputs. We have observed that the final posture of
the arm when pointing to a given spatial location is relatively independent of its starting
posture. From this observation and the recurrent nature of the MCx intrinsic connectivity
we hypothesize that the basic mode of operation of the MCx is to associate spatial location to final arm posture. We explain how the recurrent network connectivity operates to generate the muscle activation patterns (synergies) required to move the arm and hold
it in its final position.

Research paper thumbnail of On the variability of motor-evoked potentials: experimental results and mathematical model

Experimental Brain Research

Research paper thumbnail of Proprioceptive Acuity is Enhanced During Arm Movements Compared to When the Arm is Stationary: A Study of Young and Older Adults

Research paper thumbnail of Pointing to One's Moving Hand: Putative Internal Models Do Not Contribute to Proprioceptive Acuity

Frontiers in human neuroscience, 2018

We can easily and without sight bring our fingertip to our nose, or swat a mosquito on our arm. T... more We can easily and without sight bring our fingertip to our nose, or swat a mosquito on our arm. These actions rely on proprioception, also known as kinesthesia, which classically has been attributed to processing of sensory inflow by the CNS. However, internal model theories of sensorimotor neuroscience propose that proprioceptive localization also involves a contribution from estimates of limb kinematics derived from motor commands. We tested this prediction in 19 subjects who moved the right index finger tip to touch the moving left index finger tip under three conditions: (1) vision allowed, active movement of the left hand (2) vision blocked, active movement of the left hand, and (3) vision blocked, passive movement of the left hand imposed by the experimenter. The target left index finger tip was moved in a wide range of directions by unrestricted movements of the arm. Mean errors in apposition of the right to the left index finger tips were small, averaging <2 cm between se...

Research paper thumbnail of Linear summation of outputs in a balanced network model of motor cortex

Given the non-linearities of the neural circuitry's elements, we would expect cortical circuits t... more Given the non-linearities of the neural circuitry's elements, we would expect cortical circuits to respond non-linearly when activated. Surprisingly, when two points in the motor cortex are activated simultaneously, the EMG responses are the linear sum of the responses evoked by each of the points activated separately. Additionally, the corticospinal transfer function is close to linear, implying that the synaptic interactions in motor cortex must be effectively linear. To account for this, here we develop a model of motor cortex composed of multiple interconnected points, each comprised of reciprocally connected excitatory and inhibitory neurons. We show how non-linearities in neuronal transfer functions are eschewed by strong synaptic interactions within each point. Consequently, the simultaneous activation of multiple points results in a linear summation of their respective outputs. We also consider the effects of reduction of inhibition at a cortical point when one or more surrounding points are active. The network response in this condition is linear over an approximately two-to threefold decrease of inhibitory feedback strength. This result supports the idea that focal disinhibition allows linear coupling of motor cortical points to generate movement related muscle activation patterns; albeit with a limitation on gain control. The model also explains why neural activity does not spread as far out as the axonal connectivity allows, whilst also explaining why distant cortical points can be, nonetheless, functionally coupled by focal disinhibition. Finally, we discuss the advantages that linear interactions at the cortical level afford to motor command synthesis.