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Chapter 8

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 8: The Sensorimotor System THREE PRINCIPLES OF SENSORIMOTOR FUNCTION • The Sensorimotor System Is Hierarchically organized • Operation of the sensorimotor system is directed by commands that cascade down through the levels of a hierarchy from the association cortex to the muscles • The commands that emerge from the association cortex specify general goals rather than specific plans of action • The main advantage of this hierarchical organization is that the higher levels of the hierarchy are left free to perform more complex functions • The sensorimotor system are parallel hierarchical systems; that is, they are hierarchical systems in which signals flow between levels over multiple paths • This parallel structure enables the association cortex to exert control over the lower levels of the hierarchy in more than one way • The sensorimotor hierarchies are also characterized by functional segregation • That is, each level of the sensorimotor and company hierarchies tends to be composed of different units each of which performs a different function • Motor Output Is Guided by Sensory Input • In most instances, sensory feedback plays an important role in directing the continuation of the responses that produced it • The only responses that are not normally influenced by sensory feedback are ballistic movements - brief, all-or-none, high speed movements • Many adjustments in motor output that occur in response to sensory feedback are controlled unconsciously by the lower levels of the sensorimotor hierarchy without the involvement of the higher levels • Learning Changes the Nature and Locus of Sensorimotor Control • During the initial stages of motor learning, each individual response is performed under conscious control; then, after much practice, individual responses become organized into continuous integrated sequences of action that flow smoothly and are adjusted by sensory feedback without conscious regulation • A General Model of Sensorimotor System Function • Association cortex --> secondary cortex --> primary motor cortex --> brain stem motor nuclei SENSORIMOTOR ASSOCIATION CORTEX • There are two major areas of sensorimotor association cortex: the posterior parietal association cortex and the dorsolateral prefrontal association cortex • Posterior parietal cortex and the dorsolateral prefrontal cortex are each composed of several different areas, each with different functions • Posterior Parietal Association Cortex • The posterior parietal association cortex plays an important role in integrating these two kinds of information, in directing behaviour by providing spatial information and in directing attention • It receives information from the three sensory systems that play roles in the localization of the body and external objects in space: the visual system, the auditory system, and the somatosensory system • Much of the output of the posterior parietal cortex goes to areas of motor cortex, which are located in the frontal cortex: to the dorsolateral prefrontal association Chapter 8: The Sensorimotor System cortex, to the various areas of secondary motor cortex, and to the frontal eye field - a small area of prefrontal cortex that controls eye movements • Damage to the posterior parietal cortex can produce a variety of deficits, including deficits in the perception and memory of spatial relationships, in accurate reaching and grasping, in the control of eye movement, and in attention • Apraxia is a disorder of voluntary movement that is not attributable to a simple motor deficit or to any deficit in comprehension or motivation • Apraxic patients have difficulty making specific movements when they are requested to do so, particularly when the movements are out of context • Although its symptoms are bilateral, apraxia is often caused by unilateral damage to the left posterior parietal lobe or its connections • Contralateral neglect is a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite to the side of a brain lesion, in the absence of simple sensory or motor deficits • Most patients with contralateral neglect often behave as if the left side of their world does not exist, and they often fail to appreciate that they have a problem • The disturbance is often associated with large lesions of the right posterior parietal lobe • The deficits in responding occur for stimuli to the left of their own bodies, referred to as egocentric left • Egocentric left partially defined by gravitational coordinates: When patients tilt their heads, their field of neglect is not normally tilted with it • Many patients tend not to respond to the left sides of objects, regardless of where the objects are in their visual fields • Patients with contralateral neglect had deficits in responding to the right of an experimenter who was facing them, regardless of its specific location in the patients visual field • Two types of evidence suggest information about objects that are not noticed by patients with contralateral neglect may be unconsciously perceived • First, when objects were repeatedly presented at the same spot to the left of patients with contralateral neglect, they tended to look to the same spot on future trials, although they were unaware of the objects • Second, patients could more readily identify fragmented drawings viewed to their right if complete versions of the drawings had previously been presented to the left, where they were not consciously perceived • Dorsolateral Prefrontal Association Cortex • The dorsolateral prefrontal association cortex receives projections from the posterior parietal cortex, and it sends projections to areas of secondary motor cortex, to primary motor cortex, and to the frontal eye field • Dorsolateral prefrontal cortex seems to play a role in the evaluation of external stimuli and the initiation of voluntary reactions to them • The activity of some neurons depends on the characteristics of objects; the activity of others depends on the locations of objects; and the activity of still others depends on a combination of both • The activity of other dorsolateral prefrontal neurons is related to the response, rather than to the object Chapter 8: The Sensorimotor System • These neurons typically begin to fire before the response and continue to fire until the response is complete • The response properties of dorsolateral prefrontal neurons suggest that decisions to initiate voluntary movements may be made in this area of cortex, but these decisions depend on critical interaction with posterior parietal cortex SECONDARY MOTOR CORTEX • Areas of secondary motor cortex are those that receive much of their input from association cortex and send much of their output to primary motor cortex • Two areas of secondary motor cortex • Both of these large areas are clearly visible on the lateral surface of the frontal lobe, just anterior to the primary motor cortex • The supplementary motor area wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure • The premotor cortex runs in a strip from the supplementary motor area to the lateral fissure • Identifying the Areas of Secondary Motor Cortex • At least eight areas of secondary motor cortex • Three different supplementary motor areas, two premotor areas, and three small areas - the cingulate motor areas - in the cortex of the cingulate gyrus • To qualify as secondary motor cortex, an area must be appropriately connected with association and secondary motor areas • In general, areas of secondary motor cortex are thought to be involved in the programming of specific patterns of movements after taking general instructions from dorsolateral prefrontal cortex • Mirror Neurons • Mirror neurons are neurons that fire when an individual performs a particular goal- directed hand movement or when she or he observes the same goal-directed movement performed by another • Mirror neurons provide a possible mechanism for social cognition (knowledge of the perceptions, ideas, and intentions of others) • Mirror neurons have also been found in the inferior portion of the posterior parietal lobe • Some of these respond to the purpose of an action rather than to the action itself • Although it has not been possible to directly demonstrate the existence of human mirror neurons, indirect lines of evidence suggest that mirror neurons do exist in the human brain PRIMARY MOTOR CORTEX • The primary motor cortes is located in the precentral gyrus of the frontal lobe • It is the major point of convergence of cortical sensorimotor signals, and it is the major, but not only, point of departure of sensorimotor signals form the cerebral cortex • Conventional View of Primary Motor Cortex Function • The primary motor cortex is organized somatotopically - that is, according to a map of the body Chapter 8: The Sensorimotor System • The somatotopic layout of the human primary motor cortex is commonly referred to as the motor homunculus • Each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints that the site influences • One interesting exception to this general pattern of feedback that has been described in monkeys • Monkeys have at least two different hand areas in the primary motor cortex of each hemisphere, and one receives input from receptors in the skin rather than from receptors in the muscles and joints • Presumably, this adaptation facilitates stereognosis - the process of identifying objects by touch • Each neuron has a different preferred direction • Current View of Primary Motor Cortex Function • Rather than stimulating with with brief pulses investigators have used longer bursts of current which are more similar to the duration of a motor response, at sllightly higher intensities • These currents elicited complex natural looking sequences • Stimulation in the face area tended to elicit face movements • If a monkey reached toward a particular location, primary motor cortex neurons sensitive to that target location tended to become active regardless of the direction of the movement that was needed to get to the target • The finding means that the signals from every site in the primary motor cortec diverge greatly do each particular site has the ability to get a body part to a target location regardless of the starting position • Second, it means that the sensorimotor system is inherently plastic • The route that neural signals follow from a given area of primary motor cortex is extremely plastic and is presumably determined at any point in time by somatosensory feedback • The neurons of the primary motor cortex play a major role in initiating body movements • Effects of Primary Motor Cortex Lesions • Large lesions to the primary motor cortex may disrupt a patient’s ability to move one body part independently of others, may produce astereognosia, and may reduce the speed, accuracy, and force of a patient’s movements • Such lesions do not however, eliminate voluntary movement, presumably because there are parallel pathways that descend directly from secondary motor areas to subcortical motor circuits without passing through primary motor cortex CEREBELLUM AND BASAL GANGLIA • The cerebellum and the basal ganglia are neither a major part of the pathway by which signals descend through the sensorimotor hierarchy • Both the cerebellum and the basal ganglia interact with different levels of the sensorimotor hierarchy and, in so doing, coordinate and modulate its activities • The interconnections between sensory and motor areas via the cerebellum and basal ganglia are thought to be one reason why damage to cortical connections between visual cortex and frontal motor areas does not abolish visually guided responses Chapter 8: The Sensorimotor System • Cerebellum • The cerebellum recieves information from primary and secondary motor cortex, information about descending motor signals from brain stem motor nuclei, and feedback from motor responses via the somatosensory and vestibular systems • The cerebellum is thought to compare these three sources of input and correct ongoing movements that deviate from their intended course • It is believed to play a major role in motor learning, particularly in the learning of sequences of movements in which timing is a critical factor • Consequences of diffuse cerebellar damage for motor function loses the ability to control precisely the direction, force, velocity, and amplitude of movements and the ability to adapt patterns of motor output to changing conditions • There are also several disturbances in balance, gait, speech, and the control of eye movement • Basal Ganglia • The basal ganglia do not contain as many neurons as the cerebellum, but in one sense they are more complex • The basal ganglia are a complex heterogeneous collection of interconnected nuclei • They contribute few fibers to descending motor pathways; instead they are part of neural loops that receive cortical input from various cortical areas and transmit it back to the cortex via the thalamus • Many of these loops carry signals to and from the motor areas of the cortex • The basal ganglia are thought to be involved in a variety of cognitive functions in addition to their role in the modulation of motor output DESCENDING MOTOR PATHWAYS • Two pathways descend in the dorsolateral region of the spinal cord, and two descend in the ventromedial region of the spinal cord • Signals conducted over these pathways act together in the control of voluntary movement • Dorsolateral Corticospinal Tract and Dorsolateral Corticorubrospinal Tract • One group of axons that descends from the primary motor cortex does do through the medullary pyramids - two bulges on the ventral surface of the medulla - then decussates and continues to descend in th econtralateral dorsolateral spinal white matter • This group of axons constitutes the dorsolateral corticospinal tract • Most notable among its neurons are the Betz cells - extremely large pyramidal neurons of the primary motor cortex • Most axons of the dorsolateral corticospinal tract synapse on small interneurons of the spinal gray matter, which synapse on the motor neurons of distal muscles of the wrist, hands, fingers, and toes • A second group axons that descends from the primary motor cortex synapses in the red nucleus of the midbrain • The axons of neurons in the red nucleus then decussate and descend through the medulla, where some of them terminate in the nuclei of the cranial nerves that control the muscles of the face • The rest continue to descend in the dorsolateral portion of the spinal cord Chapter 8: The Sensorimotor System • This pathway is called the dorsolateral corticorubospinal tract • The axons of the dorsolateral corticorubrospinal tract synapse on interneurons that in turn synapse on motor neurons that project to the distal muscles of the arms and legs • Ventromedial Corticospinal Tract and Ventromedial Cortico-brainstem-spinal Tract • There are two major division of the ventromedial motor pathway, one direct and one indirect • The direct ventromedial pathway is the ventromedial corticospinal tract, and the indirect one is the ventromedial cortico-brainstem-spinal tract
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