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Chapter 9.pdf

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University of Toronto Scarborough

Chapter 9 – Motor Systems Why Studying Motor System is Difficult - To stimulate it – have to know what is appropriate pattern of stimulation to apply in order to get a responsnd o 2 - Sensory system – is largely straightforward way from level to level but a characteristic of motor control is that every action necessarily results in sensory feedback (from muscle, join and vision and other special senses) - Better approach – apply real stimuli to the senses & trace the resultant pattern as they penetrate deeper & deeper through levels of NS & emerge at motor end as movements o Not possible w complex system ( e.g. movement of hand) but w number of levels is much smaller ( i.e. primate brain of insects or in simpler sub-system of mammalian brain like eye movements ) – this is feasible - Generally – have learned lot by studying complete system w output compared with purely sensory system Motor control & Feedback - Feedback – using info about result to improve performance  is important in control of movement No Feedback: Ballistic control - Many circumstances where motor system is forced to act blindly b/c for one reason or another they are deprived of normal sensory feedback  e.g. tossing orange peel into a waste bin – once it leaves hand & no sensory feedback helps w/ trajectory o So have to work it before hand (e.g. the muscular contraction) o classic e.g. is nest-building behaviour of brown rat (by Lorenz)  once it decided to build , it goes through stereotypic movements even if there is no material left to finish it - Motor type of this act called ballistic (‘thrown’)  can be drawn in diagram o Desired results (or goal) - e.g. orange peel into basket  this is translated by controller into appropriate pattern of commands o These commands then produce the actual result through their effect on plant (i.e. muscles) o How well actual result compare with desired results depends on how good the controller is  more it knows about how plant will behave in response to any particular command , the better it will perform - it needs a library of motor programs  E.g. ballistic missiles - But the this system has flaw – it vulnerable to “noise”  noise is any kind of unpredictable disturbance that makes actual result differ from what the controller expects (e.g. wind) - World is unpredictable  particular pattern of activation in motor nerves will produce diff movements of limbs depending on host of internal factors (e.g. temperature, fatigue etc.) - More important –effect of load  when using limbs , given degree of muscle activity will generate quite different movements depending what you have (e.g. feather vs. rock) o Most of lower levels that control limbs are devoted to solving noise introduced by unpredictable load Parametric adjustment: Feed-forward & Feedback - One way of dealing w/ noise to have some kind of sensor that monitors the noise before it affects system & to use this info to adjust parameters of controller to allow for it o Is called parametric feed-forward – controller parameters to anticipate the effects of noise  e.g. adjusting for wind before launching the missile - Much of info is used by the brain in this way especially for allowing different load  e.g. neural circuits controlling muscle length use info from force-detectors in the skin & tendons that make appropriate adjustments of motor commands - This is system also has flaw – there are infinite of things can cause perturbations & brain cannot have plan for every one of them in advance  so approach is to have system that learn from its own mistakes – parametric feedback - Need comparator  compares actual result w desired results by subtracting one from another  if generates error singal that is used to modify controller’s parameter o Advantage to feedback is its flexibility  rather than having stored program ready in advance for anything – can have rather simple all-purpose programs that maybe redefine via trial & error for needed task that are actually encountered - such that great deal of learning motor skills can be thoughts as parametric feedback in which errors is used to modify our stored motor programs - e.g. playing darts or cricket o specific e.g. is vestibule-ocular reflex (VOR) – move head signals from semicircular canal are used to drive eyes by equal amount in the opposite directions to maintain the direction of gaze in space and retinal image of outside fixed = ballistic system o reflex is continually adjusted to ensure that eye movement is equal to head movement & error signal comes form neurons that respond to movements of visual image across the retina (retinal slip occurs when head & eye movements are not matched) - Overall – feedback & feedforward still not ideal  need calculation before action takes place are in general complex & don’t have vast library of motor commands & only corrects after the event Direct feedback - Here error signal produced by comparing desired w/ actual results at every movement is used directly as the input to the controller ( not to tweak parameters) o So error immediately generate motor commands that reduce diff b/w desired vs. actual - Similar to lot of homeostatic mechanism & guided missiles are controlled by this system - In this system – have specific what you want not how to do (‘servent’)  so called servo system & computation used to correct is generally much simpler - E.g. domestic heater system - thermostat is comparator & it generates error which just comparing actual temperature with desired temperature  makes change by either switching on or off boiler - Physiological e.g. – converging of the eyes  velocity of converges is high at first & declines exponentially as eyes gets closer to its target ( eyes driven by error singal) o Disparity b/w 2 retinal images sensed by cortical receptors & error signal generated by which convergence is driven o Exp show velocity of eyes is proportional to size of error  as eyes approach target , error signal is reduced & so is the rate of movement (
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