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Chapter 8

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PSYC 271
Richard Beninger

Page 1 of 12 Chapter 8: The Sensorimotor System 8.1 THREE PRINCIPLES OF SENSORIMOTOR FUNCTION The sensorimotor system is hierarchically organized • Hierarchical organization from association cortex to the muscles • The association cortex sends commands of general goals rather than specific plans of action, leaving it free to perform more complex functions. • Parallel structure: signals flow between levels over multiple paths, allowing association cortex to exert control over lower levels in more than one way. • Functional segregation: Each level composed of different units that each performs a different function. • Unlike sensory systems that have information mainly flow up, sensorimotor system has information mainly flowing down the hierarchy from association > primary > muscles Motor output is guided by sensory input • Sensory feedback: sensory input from the eyes, the organs of balance, and receptors in skin, muscles and joints monitor the body’s responses (the effects of the activity) and feed this back into the sensorimotor circuits to fine-tune its activity. Plays important role in direction continuation of the activities that produced those responses. • Ballistic movements: brief all-or-none high-speed movements not normally influenced by sensory feedback, such as swatting a fly. • Many adjustments in motor output that occur in response to sensory feedback are controlled unconsciously by the lower levels of the hierarchy without the involvement of higher levels. Learning changes the nature and locus of sensorimotor control • Initially, each individual response is performed under conscious control of the higher levels. • With learning through practice, more individual actions are coordinated into integrated sequences of prescribed procedures routinely carried out by the lower levels, without conscious regulation. • Learning minimizes discrepancy between predicted and actual feedback, based on past experience. 8.2 SENSORIMOTOR ASSOCIATION CORTEX Posterior Parietal Association Cortex • Posterior parietal association cortex: integrates information about starting position of body parts, and positions of external objects with which the body is going to interact – for directing behaviour with this spatial information, and directing attention. Page 2 of 12 • Receives information from visual system (dorsal stream: where/spatial) auditory system (posterior stream: spatial), and somatosensory system. • Outputs into the motor cortex (the dorsolateral prefrontal association cortex, and secondary motor cortex) and the frontal eye field, an area in the prefrontal cortex that controls eye movements. • Made of a mosaic of small areas, each specialized for guiding particular movement: anterior intraparietal (AIP) for grasping, caudal intraparietal (cIPS) for shape and orientation, lateral intraparietal (LIP) for eye saccades, parietal reach region (PRR) for reaching), and ventral intraparietal (VIP) for the mouth and head. • In one study with electrical stimulation of this area: at low stimulation patients experienced a conscious intention to perform an action, and at high stimulation felt they had actually performed it in absence of actual movement (illusory movement). • Apraxia: Disorder of voluntary movement: patients cannot make specific movements when requested to do so out of context, but can perform the same movement in context when not thinking about doing so – e.g. hammer a nail normally during course of work, but cannot demonstrate a hammering movement without a hammer. • Bilateral symptoms due to unilateral damage to left posterior parietal cortex. • Contralateral neglect: Disturbance of ability to respond to stimuli on the side of the body contralateral to the side of a brain lesion. Often due to lesions of right posterior parietal lobe, resulting in patient behaving as if the left side of their world does not exist. • The left is egocentric left, stimuli to the left of their own bodies (damage to neurons with egocentric receptive fields). Defined by gravitational coordinates: when patients tilt their heads, the field of neglect is not tilted with it. • Also tend to ignore the left sides of objects regardless of where the object is in their visual fields – object-based coordinates (damage to neurons with object-based receptive fields). • However, failure to perceive consciously does not mean object is not perceived: 1) When object repeatedly presented at same spot to left neglected side of visual field, patient tends to look to the same spot on future trials although unaware of it. 2) Can better identify fragmented drawings viewed to their right if the complete version of the drawing was previously presented to the neglected left and unconsciously perceived. Dorsolateral Prefrontal Association Cortex • Dorsolateral prefrontal association cortex: Receives from the posterior parietal cortex, and outputs to the secondary motor cortex, primary motor cortex, and frontal eye field • Plays role in evaluating external stimuli and the initiation of voluntary reactions to them. • Activity of some neurons depends on characteristics of objects, others on location of objects – some on a combination of both. Page 3 of12 • Other neurons fire before (in anticipation to) the response and continue until the response is complete; decisions to initiate voluntary movements may be made here (fires in monkeys when they make a correct decision for a reward, but not an incorrect decision). 8.3 SECONDARY MOTOR CORTEX • Receive input from association cortex and output to the primary motor cortex. • Two areas on the lateral surface of the frontal lobe, anterior to the primary motor cortex: the supplementary motor area and the premotor cortex. Identifying the areas of Secondary Motor Cortex • Many other areas of 2° motor cortex proposed: 3 supplementary motor areas, 2 premotor areas, and 3 cingulate motor areas in cortex of the cingulate gyrus. • What qualifies as a secondary motor cortex? Connected to association and secondary motor areas, electrical stimulation elicits bilateral complex movements, neurons activate before initiation of a voluntary movement and continue to be active throughout the movement. • Secondary motor cortex involved in programming of specific patterns of movements after receiving general instructions from dorsolateral prefrontal cortex. • When electrically stimulated, generates complex movement for which the patient is not consciously aware. Mirror neurons • Mirror neurons: neurons that fire when an individual performs a particular goal-directed movement (e.g. picking up a red ball), or when s/he observes the same goal-directed movement performed by another – but not when the goal-directed movement is different (e.g. picking up a blue cube). Discovered by Rizzolatti et al. in early 1990s. • Found in the ventral premotor cortex, and the inferior posterior parietal cortex. • This provides a possible mechanism for social cognition: knowledge of the perception, ideas, and intentions of others. Mapping actions of others onto oneself would facilitate social understanding, cooperation and imitation. • Also related to theory of mind: inferring someone’s thoughts based on observing their actions. • Mirror neurons respond to the understanding of an action: neurons fire to the sight of an action that makes a sound (e.g. cracking a peanut) as robustly as to the sound alone; they respond fully to the particular action regardless of how it was detected. They even fire when clues are sufficient to create a mental representation of the action (you can imagine a red ball being picked up even if you lose sight right before the action occurs). • Mirror neurons in the posterior parietal lobe respond to the purpose of an action, rather than to the action itself – e.g. grasping a piece of food would elicit response only if food was planned to be eaten, not if it was just being moved (and when seeing someone else pick up the food to eat it). 8.4 PRIMARY MOTOR CORTEX Page 4 of 12 • Primary cortex: located in the precentral gyrus of the frontal lobe, it is the major point of convergence of cortical sensorimotor signals, and the major point of departure of sensorimotor signals from the cerebral cortex. Conventional view of primary motor cortex function • Penfield’s motor homunculus: motor cortex is somatotopic (according to a map of the body) – electrical stimulation at cortical site activates that particular contralateral muscle. More cortical area is dedicated to parts of the body that control intricate movements, like the hands and mouth. • Each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints which the site influences. • Exception: an additional area of the hand that receives input from the skin rather than muscles and joints. Presumed to facilitate stereognosis, process of identifying objects by touch by this interplay between motor responses and somatosensory stimulation. • Each neuron thought to encode the direction of movement: based on evidence that each neuron in the arm area fired maximally when the arm reaches a particular direction (the preferred direction). Current view of primary motor cortex function • Current views informed by new stimulation technique of using longer (0.5 second) burst of current at higher intensities, rather than brief pulses of just-above-threshold intensities: more similar to natural motor responses. • Result: eliciting complex natural-looking response sequences, not just individual muscle contractions. For example, eliciting a feeding response with arm reaching forward, closing as if to clasp, bringing hand to mouth, and opening the mouth. • Responses are more complex species-typical movements that often involve several parts of the body and not just individual muscle contractions. • There is a crude somatotopic organization (stimulation in face area elicits facial movement), with a great deal of overlap between sites that move certain body parts. • Neurons not tuned to movement in a particular direction, but to a particular end point of the movement (reaching towards a particular target location regardless of direction of movement that got you there). • Signals from every site in the primary motor cortex diverge greatly: innumerable patterns of muscle contraction to get a body part to a target location regardless of starting position. • The sensorimotor system is also inherently plastic: route the neural signals follow from given area is determined at any point in time by somatosensory feedback. Effects of primary motor cortex lesions • Large lesions disrupt ability to move one body part independently of others (e.g. a finger independent of others), may produce astereognosia (deficit in stereognosis), and reduce the speed, accuracy, and force of a movement. • They do not, however, eliminate voluntary movements: many parallel pathways from secondary motor areas to subcortical motor circuits do not pass through the primary motor cortex. 8.5 CEREBELLUM AND BASAL GANGLIA • Not a major part of the pathway by which signals descend through the hierarchy, but instead interact with different levels to coordinate and modulate its activities. Cerebellum • 10% of brain mass, 50% of neurons. 3 cell layers (molecular, Purkinje, granular) with parallel and climbing fibres. Page 5 of12 • Receives information from primary and secondary motor cortex, info about descending motor signals from motor nuclei in the brain stem, and feedback from motor responses via somatosensory and vestibular systems. • Coordinates motor movements by excitatory output: by contracting those muscles needed for the movement. • Cerebellum compares these 3 sources of input and corrects ongoing movements that deviate from their intended course (the predictor); major role in motor learning, especially for sequences in which timing is important. • Diffuse cerebellar damage: Losing ability to precisely control the direction, force, velocity and amplitude of movements, and ability to adapt patterns of motor output to changing conditions. There can also be disturbances in balance, gait, speech, and control of eye movement. Learning new motor sequences also becomes difficult. • Cerebellum may also be involved in non-motor cognitive tasks: may participate in the fine-tuning and learning of cognitive responses in that same way it does for motor responses. Basal Ganglia • Unlike the systematically organized cerebellum, the basal ganglia is a complex heterogeneous collection of interconnected nuclei. • Performs a modulatory function, part of neural loops that receive cortical input from various areas and transmit it back to the cortex via the thalamus, including to and from the motor areas. • Coordinated motor movements by inhibitory output: inhibiting antagonist muscles of those needed. • Participates in learning to respond correctly to obtain reward and avoid punishment. • Impairment related to many disorders: Parkinson’s, Schizophrenia, Tourette’s, Huntington’s (uncontrollable motor drive, involuntary jerking of hands and legs, lack of coordination) 8.6 DESCENDING MOTOR PATHWAYS • Conduct neural signals from 1° motor cortex to motor neurons of the spinal cord. Control voluntary movement. • There are 4 different pathways, 2 descending in the dorsolateral region of the spinal cord, and 2 descending in the ventromedial region of the spinal cord. Dorsolateral Corticospinal and Dorsolateral Corticorubrospinal Tracts Page 6 of 12 Dorsolateral Corticospinal: (DIRECT) Dorsolateral Corticorubrospinal: (INDIRECT) 1. Descend from Betz cells in the primary motor cortex 1. Descend from the primary motor cortex through 2. Decussates in medulla at the medullary pyramids the red nucleus of the mid brain (bulges on ventral surface of medulla, large fiber 2. Decussates at the red nucleus bundles of axons) 3. Descend through the medulla 3. Descends in the contralateral dorsolateral spinal4A. Terminate in nuclei of cranial nerves that control white matter tract facial muscles (more diffuse) 4. Synapse onto interneurons in the spinal gray matter 4B. Descend in the dorsolateral corticorubrospinal tract 5. Synapse onto motor neurons of distal muscles of the 5. Synapse onto interneurons in the spinal grey wrist, hands, fingers and toes. matter 6. Synapse onto motor neurons that project to Ventromedial Corticospinal and Ventromedial Cortico-brainstem-spinal Tracts Page 7 of12 Ventromedial Corticospinal: Ventromedial cortico-brainstem-spinal: (INDIRECT) (DIRECT) 1. Descend from the primary motor cortex into a complex network of brain 1. Descend from the primary stem structures motor cortex ipsilaterally directly • Tectum (superior colliculus): auditory and visual info about through the ventromedial areas of the spinal white matter spatial location • Vestibular nucleus: info about balance from semicircular 2. As each axon discends, it canals in the ears branches d
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