Motor Cortex: Neural and Computational Studies

The motor cortex can be regarded as a network of neurons processing, interalia, spatial motor information. A basic component of this information is the direction of movement in space. Experimental studies in behaving monkeys have shown that the impulse activity of single motor-cortical cells relates to this component in an orderly fashion, such that the frequency of cell discharge is a sinusoidal function of the direction of movement, with the direction for which cell discharge is highest denoting the "preferred direction" of the cell. The neuronal ensemble of such directionally tuned cells can be regarded as a network in which each cell is represented as a vector pointing in the cell's preferred direction. The network operates to generate a signal in the direction of a desired movement. We regard this operation as the directorial summation of the cell vectors, weighted by a scalar measure of the intensity of cell activation. The...
read more

Mental transformations in the motor cortex

The behavioral and neural correlates of processing of motor directional information are described for two visuomotor tasks: mental rotation and context-recall. Psychological studies with human subjects suggested that these two tasks involve different time-consuming processes of directional information. Analyses of the activity of single cells and neuronal populations in the motor cortex of behaving monkeys performing in the same tasks provided direct insight into the neural mechanisms involved and confirmed their different nature. In the mental rotation task the patterns of neuronal activity revealed a rotation of the intended direction of movement. In contrast, in the context-recall task the patterns of neural activity identified a switching process of the intended direction of movement.

Arm movements in monkeys: behavior and neurophysiology

Reaching to objects of interest is very common in the behavioral repertoire of primates. Monkeys possess keen binocular vision and make graceful and accurate arm movements. This review focuses on behavioral and neurophysiological aspects of eye-hand coordination in behaving monkeys, including neural coding mechanisms at the single cell level and in neuronal populations. The results of these studies have converged to a common behavioral-neurophysiological ground and provided a springboard for studies of brain mechanisms underlying motor cognitive function.

A simulated actuator driven by motor cortical signals

One problem in motor control concerns the mechanism whereby the central nervous system translates the motor cortical command encoded in cell activity into a coordinated contraction of limb muscles to generate a desired motor output. This problem is closely related to the design of adaptive systems that transform neuronal signals chronically recorded from the motor cortex into the physiologically appropriate motor output of multijoint prosthetic limbs. In this study we demonstrated how this transformation can be carried out by an artificial neural network using as command signals the actual impulse activity obtained from recordings in the motor cortex of monkeys during the performance of a task that required the exertion of force in different directions. The network receives experimentally measured brain signals and recodes them into motor actions of a simulated actuator that mimics the primate arm. The actuator responds to the motor cortical commands with surprising fidelity, generating forces in...
read more

Intercepting real and path-guided apparent motion targets

Human subjects were instructed to intercept with a cursor real and apparent motion targets presented on a computer screen. Targets traveled counterclockwise (CCW) in a circle at one of five angular velocities (180, 300, 420, 480 and 540 deg/s), either smoothly (real motion) or in path-guided apparent motion. Subjects operated a computer mouse and were instructed to intercept targets at the 12 o'clock position; there were no constraints on when to initiate the response, which was a movement from the center of the screen towards and past 12 o'clock. We found the following: (a) for both motion conditions and all target velocities, subjects were late in intercepting the target, especially at higher target velocities; (b) for both motion conditions, the directional variability of the response increased as a linear function of the target velocity; (c) the directional variability of the response was systematically higher for the apparent than the real motion...
read more

On the relations between single cell activity in the motor cortex and the direction and magnitude of three-dimensional static isometric force

We examined the relations between the steady-state frequency of discharge of cells in the arm area of the motor cortex of the monkey and the direction and magnitude of the three-dimensional static force exerted by the arm on an isometric manipulandum. Data were analyzed from two monkeys (n=188 cells) using stepwise multiple linear regression. In 154 of 188 (81.9%) cells the regression model was statistically significant (P<0.05). In 121 of 154 (78.6%) cells the direction but not the magnitude of force had a statistically significant effect on cell activity; in 11 of 154 (7.1%) cells only the magnitude effect was significant; and in 22 of 154 (14.3%) cells both the direction and magnitude effects were significant. The same analysis was used to assess the effect of the direction and magnitude of force on the electromyographic activity of 9 muscles of the arm and shoulder girdle. The regression model was statistically significant....
read more

Neural computations underlying the exertion of force: a model

We have developed a model that simulates possible mechanisms by which supraspinal neuronal signals coding forces could converge in the spinal cord and provide an ongoing integrated signal to the motoneuronal pools whose activation results in the exertion of force. The model consists of a three-layered neural network connected to a two-joint-six-muscle model of the arm. The network layers represent supraspinal populations, spinal cord interneurons, and motoneuronal pools. We propose an approach to train the network so that, after the synaptic connections between the layers are adjusted, the performance of the model is consistent with experimental data obtained on different organisms using different experimental paradigms: the stiffness characteristics of human arm; the structure of force fields generated by the stimulation of the frog's spinal cord; and a correlation between motor cortical activity and force exerted by monkey against an immovable object. The model predicts a specific pattern of connections between supraspinal...
read more

Modeling of directional operations in the motor cortex: A noisy network of spiking neurons is trained to generate a neural-vector trajectory

A fully connected network of spiking neurons modeling motor cortical directional operations is presented and analyzed. The model allows for the basic biological requirements stemming from the results of experimental studies. The dynamical evolution of the network's output is interpreted as the sequential generation of neuronal population vectors representing the combined directional tendency of the ensemble. Adding these population vectors tip-to-tail yields the neural-vector trajectory that describes the upcoming movement trajectory. The key point of the model is that the intra-network interactions provide sustained dynamics, whereas external inputs are only required to initiate the population. The network is trained to generate neural-vector trajectories corresponding to basic types of two-dimensional movements (the network with specified connections can store one trajectory). A simple modification of the simulated annealing algorithm enables training of the network in the presence of noise. Training in the presence of noise yields robustness of the learned dynamical behaviors. Another...
read more

Visuo-manual Aiming Movements in 6- to 10-Year-Old Children: Evidence for an Asymmetric and Asynchronous Development of Information Processes

Sixty children from 6 to 10 years old participated in an open-loop visuo-manual aiming task (Experiment 1). They were asked to point as fast and accurately as possible toward lateralized visual targets. Responses were wrist flexion-extension movements. Results showed non-monotonic changes with age of constant error, reaction time, and movement time. Constant error for targets presented in the right visual field increased between 6 and 8 years and decreased afterward. Reaction time and movement time decreased with age except at 8 years where they tended to increase. The same subjects participated in two control tasks. One task was designed to test the spatial localization of the lateralized visual targets (Experiment 2). Results showed that subjects localized very accurately the targets at all ages. The second control task was designed to test simple reaction time to the same visual stimuli used in the previous tasks (Experiment 3). Results indicate that reaction time...
read more

On the translation of directional motor cortical commands to activation of muscles via spinal interneuronal systems

I discuss in this paper some of the neural mechanisms by which directional motor cortical commands could be potentially translated into multi-muscle activations to generate a directed force (and initial movement) in space. Specifically, I review the results of recent studies in the motor cortex of monkeys and the spinal cord of the frog, and propose a possible mechanism by which these results could be formally connected. It is suggested that spinal mechanisms of the kind described in the spinal frog could serve as substrates for the operation of directionally tuned motor cortical activity to produce an appropriately directed motor output by the limb.

Modeling motor cortical operations by an attractor network of stochastic neurons

Understanding the neural computations performed by the motor cortex requires biologically plausible models that account for cell discharge patterns revealed by neurophysiological recordings. In the present study the motor cortical activity underlying movement generation is modeled as the dynamic evolution of a large fully recurrent network of stochastic spiking neurons with noise superimposed on the synaptic transmission. We show that neural representations of the learned movement trajectories can be stored in the connectivity matrix in such a way that, when activated, a particular trajectory evolves in time as a dynamic attractor of the system while individual neurons fire irregularly with large variability in their interspike intervals. Moreover, the encoding of trajectories as attractors ensures high stability of the ensemble dynamics in the presence of synaptic noise. In agreement with neurophysiological findings, the suggested model can provide a wide repertoire of specific motor behaviors, whereas the number of specialized cells and specific...
read more

Quantitative relations between parietal activation and performance in mental rotation

The quantitative relationships between functional activation of the superior parietal lobule (SPL) and performance in the Shepard-Metzler mental rotation task were investigated in 16 human subjects using magnetic resonance (MR) imaging at high field (4 Tesla). Subjects were shown pairs of perspective drawings of three-dimensional objects and asked to judge whether they were the same or mirror images. Increased SPL activation was associated with a higher proportion of errors in performance. The increase in errors, and the concomitant increase in SPL activation, could be due to an increased difficulty in, and therefore increased demands for, information processing at several stages involved in making a decision, including encoding of the visual images shown, mentally rotating them, and judging whether they are the same or mirror images.

Changing the intended direction of movement

The behavioral and neural correlates of processing of motor directional information are described for two different visuomotor tasks: mental rotation and context-recall. Psychological studies in human subjects suggested that these two tasks may involve different mechanisms processing directional information. Analyses of the activity of single cells and neuronal populations in the motor cortex of behaving monkeys performing in the same tasks provided direct insight into the neural mechanisms involved, and confirmed their different nature.

Assessment of

Posttraumatic Stress Disorder

Posttraumatic Stress Disorder (PTSD)

A complex psychiatric syndrome that develops in response to trauma exposure. Individuals with PTSD experience intrusive recollections or reexperiencing of the traumatic event, avoidance of trauma reminders, emotional numbing, and hyperarousal. In addition, PTSD is associated with high rates of concomitant physical and mental health problems, increased health care use, and impairment in social and occupational functioning. Almost 7% of the general population and up to 30% of veterans meet lifetime criteria for PTSD. Indeed, PTSD is one of the most common psychiatric disorders, representing a significant and costly public health concern.

in World War II Veterans

Four scales were compared in a community sample of 330 American former prisoners of war and combat veterans of World War II. The Mississippi Scale for Combat-Related (M-), the Minnesota Multiphasic Personality Inventory-2 Pk scale, and the Impact of Event Scale (IES) all demonstrated moderate relationships with as defined by the Structured Clinical Interview for DSM-III-R. Comparative validities were similar to those observed in Vietnam veteran samples. Confirmatory factor analysis indicated that the 3 scales loaded significantly on 1 factor. The impact of Diagnostic and Statistical Manual of Mental Disorders (4th ed.) criteria changes was examined and found to be minimal. Implications for the use of the M-, Pk, and IES in combat-related assessment are discussed.

The mental and the neural: Psychological and neural studies of mental rotation and memory scanning

In this article we review studies pertaining to psychophysical measurements and neural correlates of tasks requiring the processing of directional information in spatial motor tasks. The results of psychological studies in human subjects indicate that time-consuming processes underlie mental rotation and memory scanning. Other studies have suggested that these processes may rely on different basic mechanisms. A direct insight into their neural mechanisms was obtained analyzing the activity of single cells and neuronal populations in the brain of behaving monkeys performing the same tasks. These studies revealed the nature of the neural processes underlying mental rotation and memory scanning and confirmed their different nature.

System for projection of a three-dimensional, moving virtual target for studies of eye-hand coordination

Eye-hand tracking of moving visual objects in three-dimensional (3D) space is common in the behavioral repertoire of primates. However, behavioral and/or neurophysiological studies of this function are lacking mainly because devices do not exist that allow its investigation. We describe a device by which a spot of light can be presented in the immediate extrapersonal space of a subject and can be moved in various trajectories in 3D space. The target is a real image of a circular aperture produced by a system consisting of a light source, aperture, filters, several lenses and fold mirros, and a large concave mirror to focus the final real image. Rapid, computer-controlled movement of the image is obtained by tilting a gimbal-mounted guide mirror (for x and y motion) and by translating a lens (for motion in the z direction). A second configuration of the system allows movement of a 3D image in the 3D...
read more

Current issues in directional motor control

Many studies during the past 15 years have shown that the direction of motor output (movement or isometric force) is an important factor for neuronal activity in the motor cortex, both at the level of single cells and at the level of neuronal populations. Recent studies have investigated several new aspects of this problem including the effect of posture, the relations to time-varying movement parameters (for example, position, velocity and acceleration) and the cortical representation of memorized simple movements and complex-movement trajectories. Furthermore, the neural correlates of directional operations, such as mental rotation and memory-scanning of visuomotor directions, have also been investigated. In addition, neural networks have been used to model dynamic, time-varying, spatial motor trajectories.

Colon cancer: Personality factors predictive of onset and stage of presentation

This study examined premorbid personality correlates of colon cancer and stage of presentation of colon cancer to health care providers. Sixty-one male veterans who completed the MMPI between 1947 and 1975 and were then diagnosed with colon cancer between 1977 and 1988 were matched with control patients. A 21-factor solution of the MMPI [1] was used to seek potential personality differences between colon cancer cases and their controls in terms of presence of colon cancer and stage of presentation for this disease. A stepwise conditional regression analysis found significant differences between the colon cancer and control groups on the Aggressive Hostility variable (p<0.018). A multivariate analysis of variance conducted across the stages of colon cancer presentation found that patients who presented later on for colon cancer had higher Phobia scores (p<0.05). Religious Fundamentalism who also related to presentation (p<0.05), but in a nonlinear manner. Discussion is related to previous findings regarding...
read more

Motor cortical activity in a context-recall task

A monkey was trained to respond on the basis of the serial position of a test stimulus in a sequence. First, three stimuli were presented successively on a circle. Then one of them (except the last) changed color (test stimulus) and served as the go signal: The monkey was required to produce a motor response in the direction of the stimulus that followed the test stimulus. When the test stimulus was the second in the sequence, there was a change in motor cortical activity from a pattern reflecting the direction of this stimulus to the pattern associated with the direction of the motor response. This change was abrupt, occurred 100 to 150 milliseconds after the go signal, and was evident both in the activity of single cells and in the time-varying neuronal population vector. These findings identify the neural correlates of a switching process that is different from a mental rotation...
read more

Motor cortex and cognitive processing

summarizes key observations and concepts concerning the involvement of the motor cortex in [the] motor cognitive processes operating on the direction of movement in space / large populations of neurons in motor cortex are engaged with reaching movements / single cells are directionally broadly tuned, but the neuronal population carries an unambiguous directional signal / the outcome of this population code can be visualized as a vector that points in the direction of the upcoming movement during the reaction time, during an instructed delay period, and during a memorized delay period / moreover, when a mental transformation is required for the generation of a reaching movement in a direction different from a reference direction, the population vector provides a direct insight into the nature of the cognitive process by which the required transformation is achieved (PsycINFO Database Record (c) 2019 APA, all rights reserved)