Search Results

This chapter discusses the emergence of dominant ideas about motor cortex. It traces motor cortex research from its beginning to motor maps of Penfield and Boldrey (1937) and Woolsey et al. (1952). This segment of the history is mainly about electrical stimulation applied to the surface of the cortex. Using this technique, researchers drew motor maps of greater and greater elaboration. After Penfield and Woolsey, more fine-grained techniques such as microstimulation and single-neuron recording were used to probe the details and reopened all the same questions and debates.

This chapter begins by reviewing the sequence of events that results in neuronal injury after stroke. It then reviews the advantages and disadvantages of various animal models used to model stroke and ischemic cortical injury. It considers the relationship of various aspects of behavioral assessment and the understanding of post-stroke plasticity and recovery. After a brief review of the organization of the motor cortex, the chapter reviews the evidence that neurophysiological and neuroanatomical plasticity occurs after cortical injury, emphasizing the role of postinjury behavior in the modulation of injury-induced changes. The cellular and synaptic basis for postinjury plasticity is briefly reviewed. Finally, the role of the intact hemisphere in recovery of function after unilateral cortical injury is discussed.

This chapter proposes that the motor cortex does not encode movement as any one simple Newtonian parameter of motion. Rather, if motor cortical neurons actually encode anything at all, they represent movement fragments. The chapter begins by defining the meaning of “encoding” in the nervous system. It then provides evidence against the contention that motor cortical neurons encode any simple movement parameter, and argues for the hypothesis that motor cortex may form a substrate where elementary movement fragments are assembled into motor behaviors. In the last part of the chapter, it is argued that encoding is only part of a full explanation of motor cortical functioning. The motor cortex not only encodes information but also transforms information by processing its inputs to generate its output, much like any information processing system.

This chapter reviews evidence that motor cortex in primates is not organized according to topographic maps related to the body surface, but is organized according to species-typical motor behavior. Microstimulation with behaviorally relevant time courses evokes basic movements such as bringing food to the mouth, climbing, or defensive responses. It is argued that primate motor cortex serves as an interface functionally specialized for producing species-typical actions.

This chapter reviews the results of a large body of studies that examined the cerebral plasticity in the primary motor cortex (M1) using neurophysiological approaches in monkeys and rodents, or behavioral experiments using modern brain mapping techniques such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and transcranial magnetic stimulation (TMS) in human adults. It describes the plastic changes that occur within M1 over the course of learning in order to determine their role in the acquisition, consolidation, and long-term retention of new motor skilled behaviors. It discusses the physiological and neurobiological correlates of such plasticity to give insights into the underlying mechanisms for the representational functional changes associated with the learning and retention of a motor skill.

This chapter discusses neural remapping after a stroke with respect to maximal recovery, considering the effectiveness of current therapies, including TMS, physical therapy, pharmacological therapies and their own work on inhibition therapies, in purposefully generating a new map following stroke-induced damage.

This chapter focuses on the differential involvement of multiple areas of the lateral frontal cortex in rule‐based behavior. It presents evidence obtained from physiological and anatomical studies of monkeys and discusses the specific role played by each area from the viewpoint of a hierarchical network within the lateral frontal cortex. It introduces several key types of neuronal activity found in the prefrontal, premotor, and primary motor cortices of macaque monkeys performing a variety of rule‐based behaviors, such as following location‐matching rules or shape‐matching rules.

Two main types of experiment have addressed the question of how neurons in motor cortex control movement. One type focuses on the descending pathways that map specific points in motor cortex to specific muscles. A second type of experiment focuses on the activity of single neurons in motor cortex while the animal, usually a monkey, performs a complex task. These two approaches have resulted in contrasting descriptions of motor cortex. This chapter reviews these two major approaches to motor cortex. The first part summarizes experiments on the direct pathways from motor cortex, through the spinal cord, to the muscles, and how those pathways might control movement. The second part summarizes experiments on correlations between the activity of neurons in motor cortex and a variety of control variables related to the arm.

The hypothesis of hierarchy in the cortical motor system has a long history of conflicting opinions. Whether any premotor cortex existed was initially controversial. Researchers then began to describe not one but at least six premotor areas, and the exact properties of these areas are still debated. This chapter reviews the emergence of ideas on these many premotor areas.

This chapter considers human hand function from a neurophysiological perspective and begins by highlighting several areas of controversy in this research area. The structure and function of the different types of sensory receptors embedded in the hand's skin, muscles, and joints are then described, together with the role of centrally generated corollary discharges in perception. The various classes of mechanoreceptors found in the glabrous and hairy skin of the hand are detailed and the neurophysiological functions of these tactile receptors are described. The properties of cutaneous thermoreceptors and muscle and joint receptors are reviewed and the sensory cortical projections of all these various sensory receptors are described. Finally, this chapter considers the areas in the human cerebral cortex that contribute directly to the control of hand movements.

Damage to peripheral nerves or the central axons of those nerves in the spinal cord disrupts the normal flow of information to the brain, and the output of the brain via motor neuron projections to muscles. These lesions have immediate consequences for sensory processing and motor control, and start a process of compensation first involving rebalancing of inhibition and excitation in central circuits, followed by an array of changes in gene expression, neurotransmitters and receptor expression, synaptic adjustments and neuron growth and retraction. The changes, commonly referred to as reorganizations, often promote partial recoveries, but they can lead to perception errors and movement disorders. This chapter focuses on reorganizations that have been experimentally studied in monkeys, and compares the results with those that have been obtained in noninvasive studies in humans. The emphasis is on sensory and motor cortex, where reorganizations have most extensively been explored, but subcortical alterations related to cortical reorganizations are also reviewed.

This chapter begins by discussing the question of whether the musculature of the hand is controlled in a decomposed or in an integrated manner in the primary motor cortex. It then discusses integration among other body-part representations, and how this integration may develop through experience with complex actions.

The primary motor cortex (M1) maintains a dynamic representation of higher-order features of movement, most notably the direction of reaching. In fact, almost half of the cells in the arm region of the motor cortex show an orderly variation in activity as a function of the movement direction, with a peak of activity in their preferred direction (PD), and progressively lower rates for movements farther and farther away from the PD. This orderly variation of cell activity is characterized by the directional tuning curve that can be approximated by a cosine function. This chapter addresses the question: What are the anatomical bases for directional tuning? The micro- and macro-anatomical architecture of directional tuning in the motor cortex are discussed.

This introductory chapter begins with a backstory on the experiments that led to the proposition of two principles to explain the basic properties of motor cortex. One principle concerned the topographic layout of motor cortex, and the other concerned the neuronal mechanism by which motor cortex caused movement. A theoretical framework for understanding at least the outlines of motor cortex is described. An overview of the subsequent chapters is presented.

Neuromodulatory substances evoke beta2 oscillations in motor and secondary somatosensory cortex, that depend on gap junctions. In the latter case, the oscillations are only weakly dependent on synaptic transmission. Beta2 is most prominent in intrinsically bursting layer 5 pyramidal cells (some of which are expected to contribute to the pyramidal tract, at least in primates). The oscillation is an emergent phenomenon, in that individual neurons are not oscillators at beta2 frequency. The period is determined in part by the “M” type of K⁺ current. Oscillations in deep and superficial cortical layers interact with one another. Gap junctions mediating beta2 are probably located on axons.

This chapter summarizes electrical stimulation studies in the macaque motor cortex, in which stimulation on a behaviorally relevant time scale evoked complex movements that resembled behaviorally meaningful actions. It begins with a brief summary of methods followed by a summary of two sets of findings: first the quantitative profile of the evoked arm and hand movements, and second the manner in which evoked movements resemble actions in a monkey's natural behavioral repertoire. The chapter ends with a review of other experiments using similar stimulation techniques to study motor areas in a range of animal species.

This chapter uses functional magnetic resonance imaging (fMRI) to study reorganization following motor cortex damage resulting from a stroke. It describes high activity soon after the stroke followed by a gradual decrease, and activity patterns correlated to an increased or decreased behavioral outcome.

Although the function of a particular cortical area is determined not so much by its intrinsic structure as by its extrinsic connections, its inputs and outputs, it is important to review the intrinsic structure of the motor cortex with which these inputs, and the determinants of its functions, must interact. Intrinsic networks are available for selection by different inputs and are utilized in the organization of cortical outputs to the multiple sites which receive these projections. This chapter concentrates on the features of the motor cortex which distinguish it from other cortical areas. It describes in detail the sources of the inputs which operate on its intrinsic networks and gives special attention to the organization of its corticofugal outputs. In hierarchical models of mammalian motor systems, the motor cortex, and in particular its corticospinal output, are seen as executive structures. Nevertheless, it is now abundantly clear that the motor cortex is not independent and self-sufficient in these executive functions: it is itself under the influence of a large variety of different inputs, including some originating in the periphery.

The seventh part of this book deals with possible neurophysiological mechanisms involved in motor synergies. It starts with a discussion of the structure-function controversy in neurophysiology. Then, it presents reviews of the literature on the role in synergy formation of different structures within the central nervous system such as the spinal cord, the cerebellum, the basal ganglia, and the cortex of the large hemispheres. The review covers animal studies, studies of patients with different neurological disorders, and studies of healthy persons. Studies using transcranial magnetic stimulation are reviewed with respect to the equilibrium-point hypothesis of movement control. Relations between neuronal population vectors and performance variables are discussed. There is one more Digression in this part addressing the issue of localization of functions within brain structures.