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Chapter 8.1-8.4

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PSYC 2410
Boyer Winters

Chapter 8 – The Sensorimotor System PSYC*2410 8.1 – Three Principles of Sensorimotor Function 1) The sensorimotor system is hierarchically organized. 2) Motor output is guided by sensory input. 3) Learning can change the nature and the locus of sensorimotor control. 1) Sensorimotor system is hierarchically organized  operation of both sensorimotor system and a large, efficient company is directed by commands that cascade down through the levels of a hierarchy from the association cortex or company president (highest levels) to the muscles or the workers (lowest levels)  hierarchical organization main advantage is that the higher levels can perform more complex functions  sensorimotor system and large, efficient companies are parallel hierarchical systems – signals flow between levels over multiple paths o ex. The association cortex may directly inhibit an eye blink reflex to allows insertion of a contact lens, and a company president may personally organize delivery to an important customer  functional segregation – each level fo the sensorimotor and company hierarchies tends to be composed of different units  sensory system information flows UP; sensorimotor system information flows DOWN 2) Motor output is guided by sensory input  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 o Case of G.O. the man with too little feedback  Infection had effected the somatosensory nerves in G.O.’s arms  He had difficulties performing intricate responses such as doing up buttons, or picking up coins, also inability to adjust his motor output in the light of unanticipated external disturbances  Greatest problem was inability to maintain a constant level of muscle contraction  G.O. would have to constantly make a visual check on his progress when performing tasks, however, sometimes this was not enough  Motor output in response to sensory feedback is controlled unconsciously by lower levels3) Learning changes the nature and locus of sensorimotor control  During initial stages of motor learning, each individual response is performed under conscious control; 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 8.2 – Sensorimotor Association Cortex Association cortex is at the top of sensorimotor hierarchy. Two major areas: 1. Posterior parietal association cortex 2. Dosolateral prefrontal association cortex 1. Posterior Parietal Association Cortex  portion of parietal neocortex posterior to the primary somatosensory cortex – plays an important role in integrating two kinds of information and in direction attention  posterior parietal cortex receives input from more than one sensory system. Receives information from three sensory systems that play a role in localization of the body and external object in space: the visual system, auditory system, and somatosensory system  output of the posterior parietal cortex goes to areas of motor cortex, which are located in the frontal cortex: to the dorsolateral prefrontal association cortex. To the various areas of secondary motor cortex and to the frontal eye field- area of prefrontal cortex that controls eye movements  fMRI and TMS studies shows that the posterior parietal cortex is comprised of a mosaic of small specialized areas  damages to the posterior parietal association cortex can produce deficits in perception and memory of spatial relationships, in accurate reaching and grasping, in control of eye movement and in attention o Apraxia - a disorder of voluntary movement that is not attributable to a simple motor deficit (e.g. not to paralysis or weakness) or to any deficit in comprehension or motivation  Patients have difficulty making specific movements when they are requested to do so, however they can readily perform movements when they are under natural conditions  Symtoms are bilateral, apraxia is often cause by unilateral damage to the left posterior parietal love or its connections o Contralateral neglect - a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite (contralateral) to the side of a brain lesion, in the absence of simple sensory or motor deficits  Patients often behave as if the left side of their world does not exist, and they often fail to appreciate that they have a problem  Disturbance is also associated with large lesions of the right posterior parietal lobe o Example: Case of Mrs. S. the Woman who turned in circles o Mrs. S’s stroke had left her unable to recognize or respond to things to the left – including external objects as well as parts of her own body o Left side contralateral neglect left problems mainly with eating o When a plate of food was put in front of her, she could only see food on the right half of the plate and thus ate only that much o She received a wheelchair for turning in place, and when food was put in front of her and she was still hungry she rotated sides in order to see the rest of her food  Patients with contralateral neglect have difficulty recognizing things to their 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  Neurons with egocentric receptive fields and others with object-based receptive fields have been found in primate parietal cortex  See figure 8.2 for contralateral neglect example  A study showed that in patients with contralateral neglect it is not a straightforward matter to define the field of neglect; when patients crossed their arms they neglected the left side still  Objects not noticed by patients with contralateral neglect is unconsciously perceived  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 unaware of the objects  secondly, patients could more readily identify fragmented (partial) drawings viewed to their right if complete version so if drawings had previously been presented to the left, where they were not consciously perceived 2. Dorsolateral Prefrontal Association Cortex  Other large area of association cortex – it receives projections from the posterior parietal cortex, and sends projections to areas of secondary motor cortex, to primary motor cortex, and to the frontal eye field (Figure 8.4)  Plays a role in evaluation of external stimuli and initiation of voluntary reactions to them  Activity of some neurons depends on characteristics of object; the activity of others depends on the locations of objects; and the activity of still others depends on a combination of both  Activity of other dorsolateral prefrontal neurons is related to the response, rather than to the object. These neurons typically begin to fire before the response and continue to fire until response is complete  The response properties of dorsolateral prefrontal cortex neurons suggest that decisions to initiate voluntary movements may be made in this area of the cortex, but depend on critical interactions with posterior parietal cortex 8.3 – Secondary Motor Cortex  Secondary motor cortex – areas that receive much of their input from association cortex and send much of their output to primary motor cortex o Two areas: 1) Supplementary motor area – wraps over the top of the frontal lobe and extends down
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