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Neural Plasticity in Adult Somatic Sensory-Motor Systems

2005 Edition, May 26, 2005

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ISBN: 978-0-8493-1521-3
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Product Details:

  • Revision: 2005 Edition, May 26, 2005
  • Published Date: May 26, 2005
  • Status: Active, Most Current
  • Document Language: English
  • Published By: CRC Press (CRC)
  • Page Count: 311
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:

Preface

Neural plasticity is now well accepted as a universal property of multi-cellular nervous systems. Plasticity has been studied in particular detail in the mammalian cerebral cortex. The word "plasticity" has been applied to a wide variety of cortical changes, so an initial question is always: what metric has been used to conclude that a plastic event has occurred? The chapters in this book illustrate important examples in which the metric for plasticity is physiological alterations in neuronal response properties or changes in behavioral skills. The locus of these changes is in the somatic sensory pathways to and within sensory cortex or motor cortex in response to a variety of challenges. The initial chapters discuss issues relevant to modifications in sensory processing.

Although controversial and easy to ignore, an increasing number of investigators are convinced that silent neurons need further study. In somatic sensory cortex the silent neuron idea is linked to a 1988 paper by Robert Dykes and Yves Lamour in which they showed that a large fraction of cortical cells did not fire action potentials in response to tactile stimuli, even though the cells seemed healthy and responded vigorously to locally applied glutamate. Their hypothesis that the silent neurons become wired into cortical circuits during learning was too novel, and arrived too early, to be embraced by other workers in the field without additional lines of evidence. Strong evidence for the existence of silent neurons has since appeared, and the chapter by Michael Brecht and his colleagues in this book poses important questions about the silent neurons' role in cortical function. The specific contribution of these neurons to cortical plasticity is a particularly important ongoing idea that remains to be clarified.

Another fascinating dimension of sensory transduction is that rats may use the whiskers on their face to listen to vibrations in the world. Rats and mice are known to use their whiskers as a main source of sensory information. Christopher Moore and Mark Andermann describe how the resonance properties of the whiskers, like in the cochlea of the human ear, may allow rodents to amplify signals and help rats detect small vibrations present in the sensory world. These vibrations could be crucial to a rodent's ability to perceive the subtle texture properties of a solid surface, which generate these small vibrations when a whisker is swept across. They further provide evidence that rodent whiskers could even be used to "hear" sounds. Beyond just being an amplifier, the whiskers are organized in an orderly way, such that the shorter whiskers near the snout amplify higher frequency inputs than the longer whiskers further back. This arrangement of the whiskers, like the strings of a harp, creates a systematic map of tuning across the rat's face. This orderly map in the periphery creates an orderly neural representation in the primary somatic sensory cortex, a map of frequency embedded within the well-described body map representation. These authors also provide evidence for further subdivisions of this representation into isofrequency columns, modular groups of cells that all respond best to the same amplified frequency. These novel findings are considered with regard to classical theories of how resonance facilitates perception in other sensory systems, ranging from the cockroach to the human ear, and also consider how these principles of the biomechanical transduction of information may provide lessons for understanding the optimal use of tools by humans.

Continuing the coding theme more centrally, Mathew Diamond then discusses the role of modular, maplike cortical organization in the processing of sensory information, including the functional significance of cortical maps, as well as the individual modules that create the topographic framework for spatial coding in primary sensory cortex. These spatial rules for barrel cortex plasticity co-exist with temporal fluctuations in excitability (temporal coding), characterized in anesthetized rats by bursts of spikes that are synchronized across the entire barrel cortex. The bursts appear to briefly open a plasticity gate allowing incoming sensory inputs to modify the efficacy of the activated intracortical circuits. During the time between bursts the plasticity gate is closed and incoming inputs have no long-term effect on intracortical circuits. These modifications by sensory input patterns during discrete intervals provide a theoretical basis for understanding barrel cortex changes in awake, exploring rats because rhythmic oscillations occur in awake rat cortex as well.

The isolation of neural codes related to perception and learning is another important issue discussed in this series by Ranulfo Romo and his colleagues. The underlying premise is that unraveling the sensory code from the periphery to cortical processing is key to understanding initial perceptual processes. They use the ideas of Vernon Mountcastle and colleagues who quantified the relationship between action potentials in cutaneous, primary afferents and mechanical (especially flutter) stimuli applied to the skin. By combining human psychophysics with single unit analysis in monkeys, they looked for the psychophysical link between stimulus and sensation. Using this approach, it should be possible to identify neural codes for simple stimuli in early stages of cortical processing that can be compared with the psychophysical responses. However, even the simplest cognitive task may engage many cortical areas, and each one might represent sensory information using a different code, or combine new inputs with stored signals representing past experience. Romo and and his colleagues explore these ideas in primary somatic sensory (SI) cortex of primates. Starting with optimal conditions for flutter discrimination, they studied the neuronal responses in SI cortex, and correlated them with psychophysical performance. The evoked neuronal responses in SI could be shown to correlate well with correct or incorrect responses, even when they bypassed the usual sensory pathway by electrical activation of neuronal clusters in SI to produce an artificial perceptual input to SI cortex that could be used by the animals to guide their behavior.

In Krish Sathian's studies on human perception, he and his colleagues used a variety of stimuli and tasks to study the transfer of perceptual learning between fingers and hands. They employed periodic gratings actively stroked by the subjects where the task was to discriminate between gratings that varied either in their groove width or in their ridge width. Initial training was carried out with one index finger, and progressed to the index or middle finger of the other hand. Learning was reflected in improved performance, and transfer of learning occurred between fingers, and was substantial between the two hands, presumably based on interhemispheric connections. In subsequent studies, these findings were extended to a variety of tactile stimuli and tasks leading to the conclusion that transfer of tactile learning appears to be a general rule. It is interesting to speculate that interhemispheric transfer of tactile learning may relate to intermanual referral of tactile sensations following amputation or stroke. The mechanisms of perceptual learning are relevant to the perceptual improvements that are observed in spared modalities following sensory deprivation in a particular modality, such as improved tactile skills in people with very low vision.

Examples of somatic sensory processing after early postnatal sensory deprivation has identified a number of ways in which activity is needed to develop normal sensory processing in cortex. Ford Ebner and Michael Armstrong-James describe the nature of cortical impairments induced by low activity during the early postnatal period in the somatic sensory system in rats and mice after they mature to normallooking adults. The literature shows that both excitatory and inhibitory processes are affected by sensory deprivation, with the severity of effects depending upon the time of onset, the duration of the deprivation, and the length of the recovery period after deprivation ends. Intracortical circuit dynamics are most severely affected. Neural transmission from cortical layer IV to more superficial layers II/III is a major site of synaptic dysfunction. Trimming all whiskers produces a more uniform downregulation of sensory transmission than trimming a subset of whiskers presumably because restricted deprivation creates competition between active and inactive interconnected cell groups. Activity-based changes in function can be induced by altered tactile experience throughout life, but early postnatal deprivation degrades neuronal plasticity, and interferes with the animal's ability to learn subtle tactile discriminations throughout life.

The remaining chapters deal with the motor side of sensory-motor transformations.

John Chapin and his colleagues discuss the mechanisms by which the brain transforms sensory inputs into motor outputs. The rules for such sensory-motor conversions have proven elusive, and the authors suggest that this is due to the multiplicity of "bridges" between these systems in the CNS. Moreover, while the development and maintenance of the sensorimotor transformation machinery must involve some sort of plasticity, it is not yet clear how or where this plasticity occurs. They then offer specific recommendations for studying these issues in awake animals performing behaviors that involve sensory-motor transformations, an area in which they have made significant contributions.

The plastic responses of neurons in motor cortex after stroke-like lesions have clinical as well as basic science relevance. Randy Nudo and his colleagues have been studying the mutability of sensory, motor and premotor maps of the mature cerebral cortex following experimental lesions of cortex to document the mechanisms of neuroplasticity in the adult brain. They use direct brain stimulation (ICMS) in layer V of motor cortex to elicit muscle or joint movement before and after motor skill training. The maps are composed of various digit and arm movements. An initial result was that monkeys trained to use their digits to retrieve food pellets from a food board showed an increase in the size of representations of the digits used in the task. Further, multijoint responses to ICMS were infrequent before training, but were found in abundance after digit training. The implication is that simultaneous movements may become associated in the cortex through Hebbian synaptic mechanisms in which horizontal fibers connecting two areas become strengthened through associated repetitive activation. When spontaneous recovery was studied at 3 to 5 months after a hand area motor cortex lesion, skilled use of the hand returned, but roughly half of the digit movement representation was still replaced by shoulder and elbow. However, if squirrel monkeys were trained to retrieve food pellets from food wells, and then re-trained after a motor cortex lesion using the less affected hand (ipsilateral to a small infarct), the monkeys returned to baseline levels on the most difficult food-well task. In this case, motor skill training saved the remaining preinfarct distal hand representation from the expected takeover by surrounding inputs. The implication of these results is that physical rehabilitation after stroke can drive physiological changes in the cortex associated with recovering skilled hand use, if the conditions are optimized.

Jon Kaas then discusses how motor experience rebalances dynamic systems to reveal latent neural circuit properties. Short term changes emerge over a time period ranging from seconds to hours due to a range of activity-dependent cellular mechanisms that affect synaptic strengths. Over somewhat longer periods of days to weeks, anatomical circuits may be lost or gained as local circuits grow and rearrange. Over a time period of weeks to months, considerable new growth of axons and synapses can occur that considerably alter the functional organization of sensory and motor systems, sometimes in ways that promote behavioral recovery, and sometimes in ways that do not promote such recovery.. One goal of research on sensorymotor plasticity is to understand the mechanisms of change and how to manipulate them in order to maximize recovery after sensory and motor loss. This chapter focuses on changes in the motor system that are the result of a particularly severe type of motor system damage— the loss of an entire forelimb or hindlimb. In humans, badly damaged limbs might require amputation, and it is important to determine what happens to the somatosensory and motor systems as a result of the loss of both the sensory afferents from the limb and the motor neuron outflow to the muscles of that limb.

Leonardo Cohen and colleagues focus on central nervous system adaptations to environmental challenges or lesions. Understanding the mechanisms underlying cortical plasticity can provide clues to enhance neurorehabilitative efforts. Upper limb amputation (e.g., at the elbow level) results in an increase in the excitability of body part representations in the motor cortex near the deafferented zone in the form of decreased motor thresholds, larger motor maps and a lateral shift of the center of gravity with transcranial magnetic stimulation. This increased excitability appears to be predominantly cortical in origin. The mechanisms underlying these reorganizational changes are incompletely understood, however, intracortical inhibition in the motor cortex contralateral to an amputated limb is decreased relative to healthy subjects suggesting that GABAergic inhibition may be reduced. Another issue is phantom limb pain, a condition characterized by the presence of painful perceptions referred to the missing limb. Phantom limb pain is associated with profound changes in cortical and subcortical organization. Reorganization in the primary somatosensory cortex has been demonstrated to be strongly correlated with the magnitude of phantom limb pain. Interestingly, phantom pain was more prominent in patientsin whom the motor representations of face muscles were displaced medially, possibly reflecting an invasion of the face motor representation in motor cortex.

In the last chapter the behavioral basis of focal hand dystonia is discussed by Nancy Byl as a form of aberrant learning in the somatic sensory cortex. The cause of this disabling movement disorder has remained elusive. It is common in productive, motivated individuals, such as musicians, who perform highly repetitive, intensive hand tasks., Their studies document degradation of the cortical somatosensory representation of the hand characterized by large receptive fields overlapped across adjacent digits, overlap of glabrous-hairy surfaces, persistence of digital receptive fields across broad cortical distances, high ratio of amplitude to latency in somatic sensory evoked field responses, and abnormal digit representation. Challenging, rewarded, repetitive behavioral tasks that require high speed, high force, precision and intense work cycles with minimal breaks accelerate the onset and severity of dystonia. The development of dystonia may be minimized if individuals use the hands in a functional, mid-range position, take frequent breaks, work at variable speeds for short durations, attend to sensory-motor feedback, and initiate digital movements with the intrinsic muscles. The central theme is that attended, progressive, rewarded, learning-based sensory-motor training consistent with the principles of neuroplasticity, can facilitate recovery of task-specific motor control.

All of the examples in this book suggest that our understanding of neural plasticity and its mechanisms is increasing at a rapid rate, and that the knowledge will modify many of the procedures now in place to improve perceptual and motor skills after brain damage.