Behavioral neurophysiology of the motor cortex

The study of the motor cortex in behaving monkeys during the past 20 years has provided important information on the brain mechanisms underlying motor control. With respect to reaching movements in space, several aspects of motor cortical function concerning the specification of the direction of movement have now been elucidated and are reviewed in this article. The activity of single cells in the motor cortex is broadly tuned with respect to the direction of reaching, so that the discharge rate is highest with movements in a preferred direction and decreases progressively with movements made in directions more and more away from the preferred one. Thus the neural command for the direction of reaching can be regarded as an ensemble of cell vectors, with each vector pointing in the cell's preferred direction and having a length proportional to the change in cell activity. The outcome of this population code can be visualized...
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Movement Parameters and Neural Activity in Motor Cortex and Area 5

The relations of ongoing single-cell activity in the arm area of the motor cortex and area 5 to parameters of evolving arm movements in two-dimensional (2D) space were investigated. A multiple linear regression model was used in which the ongoing impulse activity of cells at time t + τ was expressed as a function of the (X, Y) components of the target direction and of position, velocity, and acceleration of the hand at time t, where τ was a time shift (-200 to +200 msec). Analysis was done on 290 cells in the motor cortex and 207 cells in area 5. The time shift at which the highest coefficient of determination (R2) was observed was determined and the statistical significance of the model tested. The median R2 was 0.581 and 0.530 for motor cortex and area 5, respectively. The median shift at which the highest R2 was observed was -90 and...
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Overlapping neural networks for multiple motor engrams

The hypothesis was tested that learned movement trajectories of different shapes can be stored in, and generated by, largely overlapping neural networks. Indeed, it was possible to train a massively interconnected neural network to generate different shapes of internally stored, dynamically evolving movement trajectories using a general-purpose core part, common to all networks, and a special-purpose part, specific for a particular trajectory. The weights of connections between the core units do not carry any information about trajectories. The core network alone could generate externally instructed trajectories but not internally stored ones, for which both the core and the trajectory-specific part were needed. All information about the movements is stored in the weights of connections between the core part and the specialized units and between the specialized units themselves. Due to these connections the core part reveals specific dynamical behavior for a particular trajectory and, as the result, discriminates different tasks. The...
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New concepts in generation of movement

The motor cortex has long been regarded as a key node in the generation of motor output, based on observations of patients with motor seizures (Jackson 1889) as well as results from various kinds of studies employing lesions and electrical stimulation (Porter and Lemon, 1993). Evarts (1966, 1968, 1969) recorded the impulse activity of single cells in the motor cortex of behaving monkeys. He showed that cells typically changed activity before the first changes in muscle activity and that the frequency of discharge varied during movement or production of isometric force. These studies initiated research into the role of the motor cortex in the specification and control of motor output, a subject that has been hotly debated and widely speculated upon and that remains under active investigation. Simplistic notions of motor cortical function have been exemplified by the view that motor cortical cells are simply "upper motorneurons." This view is clearly...
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Directional operations in the motor cortex modeled by a neural network of spiking neurons

A neural network with realistically modeled, spiking neurons is proposed to model ensemble operations of directionally tuned neurons in the motor cortex. The model reproduces well directional operations previously identified experimentally, including the prediction of the direction of an upcoming movement in reaching tasks and the rotation of the neuronal population vector in a directional transformation task.

Measuring Synaptic Interactions

A.P. Georgopoulos et al. present novel evidence which suggests that synaptic interactions between pairs of cortical neurons are directly related to the degree to which they fire together during directed limb movements. The cover for the issue of 2 April depicts synaptic interactions ranging form strongly excitatory (for cells with similiar direction preferences) to strongly inhibitory (for cells with opposite direcction preference). The calculation used by Georgopooulos et al. to document synaptic interaction differs from the cross correlation traditionally used to measure the effects of synaptic connections on firing probability. Instead, they "estimated the strength of presumed interaction (synaptic weight) from the i-th to the j-th neuron in a pair using an analysis based on waiting time probability density function...".

Representations of movement and representations in movement

Marc Jeannerod champions the cognitive approach to motor control from an innovative perspective. In his target article he appraises the role of mental representations in the preparation and control of movement by considering studies on mental imagery. Although this may seem odd at first, this strategy provides significant advances into the understanding of the role of mental representation in the control of movement. In recent work we applied a strategy akin to the one advocated by Jeannerod, namely, we used mental imagery paradigms to study properties of representations of movement (Pellizzer & Georgopoulos 1993a). In this commentary we would like to present some studies on mental transformations of the intended direction of movement. Moreover, we would like to show that this strategy can give information on properties of motor representations not only at a "macroscopic" level (e.g., movement time), as in the examples presented in the target article, but also at...
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A Neural Network for Coding of Trajectories by Time Series of Neuronal Population Vectors

The neuronal population vector is a measure of the combined directional tendency of the ensemble of directionally tuned cells in the motor cortex. It has been found experimentally that a trajectory of limb movement can be predicted by adding together population vectors, tip-to-tail, calculated for successive instants of time to construct a neural trajectory. In the present paper we consider a model of the dynamic evolution of the population vector. The simulated annealing algorithm was used to adjust the connection strengths of a feedback neural network so that it would generate a given trajectory by a sequence of population vectors. This was repeated for different trajectories. Resulting sets of connection strengths reveal a common feature regardless of the type of trajectories generated by the network: namely, the mean connection strength was negatively correlated with the angle between the preferred directions of neuronal pair involved in the connection. The results are discussed...
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Mechanisms of Eye-Hand Coordination

We studied the capacities of human subjects to localize tactile stimuli in 3D space. For that purpose, five subjects (3 women and 2 men) were asked to make a pointing movement to a visual stimulus in otherwise complete darkness. At some random time before, during or after this initial movement, a tactile probe was presented to the tip of the subject's index finger. The probe (1-mm diameter, 0.75-mm extent, 5-ms duration) was applied by a lightweight (80 g) tactile stimulator worn on the subject's hand. To complete the task, the subject was required to point to the 3D spatial location at which the probe was applied. Hand position was monitored (200 Hz) by a video-based motion analysis system. In each subject, probes presented just before or during the initial movement were systematically mislocalized in the direction of that movement so that subjects perceived the probe to be at the location occupied...
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A dynamical neural network model for motor cortical activity during movement: population coding of movement trajectories

As a dynamical model for motor cortical activity during hand movement we consider an artificial neural network that consists of extensively interconnected neuron-like units and performs the neuronal population vector operations. Local geometrical parameters of a desired curve are introduced into the network as an external input. The output of the model is a time-dependent direction and length of the neuronal population vector which is calculated as a sum of the activity of directionally tuned neurons in the ensemble. The main feature of the model is that dynamical behavior of the neuronal population vector is the result of connections between directionally tuned neurons rather than being imposed externally. The dynamics is governed by a system of coupled nonlinear differential equations. Connections between neurons are assigned in the simplest and most common way so as to fulfill basic requirements stemming from experimental findings concerning the directional tuning of individual neurons and the...
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Motor cortical activity preceding a memorized movement trajectory with an orthogonal bend

Two monkeys were trained to make an arm movement with an orthogonal bend, first up and then to the left (◹), following a waiting period. They held a two-dimensional manipulandum over a spot of light at the center of a planar working surface. When this light went off, the animals were required to hold the manipulandum there for 600-700 ms and then move the handle up and to the left to receive a liquid reward. There were no external signals concerning the "go" time or the trajectory of the movement. It was hypothesized that during that period signs of directional processing relating to the upcoming movement would be identified in the motor cortex. Following 20 trials of the memorized movement trajectory, 40 trials of visually triggered movements in radially arranged directions were performed. The activity of 137 single cells in the motor cortex was recorded extracellularly during performance of the task....
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Functional Magnetic Resonance Imaging

Functional Magnetic Resonance Imaging (fMRI)

A functional neuroimaging procedure using MRI technology that measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.[citation needed] The primary form of fMRI uses the blood-oxygen-level dependent (BOLD) contrast, discovered by Seiji Ogawa. This is a type of specialized brain and body scan used to map neural activity in the brain or spinal cord of humans or other animals by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells. Since the early 1990s, fMRI has come to dominate brain mapping research because it does not require people to undergo shots, surgery, or to ingest substances, or be exposed to ionising radiation, etc.

of motor cortex: hemispheric asymmetry and handedness

A hemispheric asymmetry in the functional activation of the human motor cortex during contralateral (C) and ipsilateral (I) finger movements, especially in right-handed subjects, was documented with nuclear magnetic resonance imaging at high field strength (4 tesla). Whereas the right motor cortex was activated mostly during contralateral finger movements in both right-handed (C/I mean area of activation = 36.8) and left-handed (C/I = 29.9) subjects, the left motor cortex was activated substantially during ipsilateral movements in left-handed subjects (C/I = 5.4) and even more so in right-handed subjects (C/I = 1.3).

Spatiotemporal motor processing

Many studies of motor function deal with essentially static aspects of motor control; for example, the representation of motor output in maps, the encoding of motor parameters in the discharge of single cells, and the effect of behavioral context on neuronal activity. These questions do not encompass time as a crucial variable. However, actual motor performance always evolves in time. Moreover, daily activities, from eat to playing, depend critically on efficient processing of sensorimotor information. The devastating effect of a general slowness of this processing can be seen in patients with Parkinson's disease, who, without medication, are quite incapacitated in practically all everyday activities. In this paper we focus on the spatiotemporal processing of sensory-motor information in the motor cortex and the basal ganglia within the context of simple motor acts, such as movements to a target, as well as in more complicated tasks, such as mental rotation. Moreover, we treat...
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Cortical cell types from spike trains

The patterns of cell activity recorded extracellularly in the motor cortex of behaving monkeys were classified into the following three groups using a combination of cluster and discriminant analyses of 1925 spike trains: (a) cells with low discharge rate and low bursting (67.1%), (b) cells with low discharge rate but bursting (20.2%), and (c) cells with high discharge rate and low bursting (12.7%). The percentage of directionally tuned cells and of cells engaged during a memorized delay task were very similar in all three cell groups.

Structural models of captivity trauma, resilience, and trauma response among former prisoners of war 20 to 40 years after release

Long-term responses to captivity trauma were measured in a national sample of American former prisoners of war. Their responses included negative affect, positive affect, and somatic symptoms as assessed by the Cornell Medical Index in 1967 and the Center for Epidemiological Study Depression Scale in 1985. These responses were strongly associated with captivity trauma (as indexed by captivity weight loss, torture, and disease) and resilience (as indexed by age and education at capture). Symptoms reported in 1967 were related to symptoms reported in 1985, suggesting symptom stability. These results are consistent with a model of trauma response that incorporates both trauma exposure and individual resilience. The findings are interpreted within a theoretical view of trauma response as adaptive when viewed from an evolutionary perspective.

Cognitive neurophysiology of the motor cortex.

A major challenge of current neuroscience is to elucidate the brain mechanisms that underlie cognitive function. There is no doubt that cognitive processing in the brain engages large populations of cells. This article explores the logic of investigating these problems by combining psychological studies in human subjects and neurophysiological studies of neuronal populations in the motor cortex of behaving monkeys. The results obtained show that time-varying psychological processes can be visualized in the time-varying activity of neuronal populations. Moreover, the functional interactions between cells in the motor cortex are very similar to those observed in a massively interconnected artificial network performing the same computation.

Mental rotation of the intended direction of movement

Reviews studies of mental rotation (MR), neural coding of the direction of movement, and neural activity during a mental transformation of the intended direction of movement. In a study of similarities between MR of the direction of movement and mental images, the authors (in press) compared the performances of human Ss in a visuomotor MR and a visual MR task. The processing rates (PRs) in both tasks were correlated, but neither rate correlated significantly with the PR in a visuomotor memory scanning task. These results suggest that visuomotor and visual possess common processing constraints that could not be ascribed to general PR performances. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

Common processing constraints for visuomotor and visual mental rotations

Naive human subjects were tested in three different tasks: (1) a visuomotor mental rotation task, in which the subjects were instructed to move a cursor at a given angle from a stimulus direction; (2) a visual mental rotation task, in which the subjects had to decide whether a displayed letter was normal or mirror image regardless of its orientation in the plane of presentation; and (3) a visuomotor memory scanning task, in which a list of two to five stimuli directions were presented sequentially and then one of the stimuli (test stimulus), except the last one, was presented again. Subjects were instructed to move a cursor in the direction of the stimulus that followed the test stimulus in the previous sequence. The processing rate of each subject in each task was estimated using the linear relation between the response time and the angle (mental rotation tasks) or the list length (memory...
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Functional imaging of human motor cortex at high magnetic field

1. We used conventional gradient echo magnetic resonance imaging (MRI) at high field strength (4 Tesla) to functionally image the right motor cortex in six normal human subjects during the performance of a sequence of self-paced thumb to digit oppositions with the left hand (contralateral task), the right hand (ipsilateral task), and both hands (bilateral task). 2. A localized increase in activity in the lateral motor cortex was observed in all subjects during the task. The area of activation was similar in the contralateral and bilateral tasks but 20 times smaller in the ipsilateral task. The intensity of activation was 2.3 times greater in the contralateral than the ipsilateral task.

Cortical Representation of Intended Movements

The neural representation of reaching movements in the motor cortex of the monkey is discussed with respect to the coding of the direction of movement in the activity of single cells and neuronal populations. This code is then used to monitor the processing of directional information in various contexts involving delayed movements or directional transformations. Finally, some implications of these findings for the role of the motor cortex in planning and execution of reaching movements are discussed.