Ch 8 - The Sensorimotor System

at least 8 different areas of secondary motor cortex, each w/ its own subdivisions - 3 supplementary motor areas (SMA, preSMA, supplementary eye field) - 2 premotor areas (dorsal and ventral) - 3 cingulate motor areas

How many areas of secondary motor cortex are there?

current view of primary motor cortex function

THEN primary motor cortex related to individual muscle contractions; each neuron was thought to encode the direct movement - evidence from finding that each neuron in the arm area of the primary motor cortex fires maximally when the arm reaches in a particular direction, and that each neuron has a different preferred direction NOW regions of the primary motor cortex support initiation of species-typical movements, which often involve several parts of the body rather than individual muscle contractions; sites that move a particular body part overlap greatly w/ sites that move other body parts; neurons direct to target of movement rather than a pre-coded direction - firing of neurons most closely related to the end point of a movement, not the direction of a movement signals from every site of the primary motor cortex diverge greatly, so each particular site has the ability to get a body part to a target location regardless of starting position - each location in the primary motor cortex can produce innumerable patterns of muscle contraction required to get a body part from any starting point to a target location

apraxia

a disorder in which patients have great difficult performing movements when asked to do so out of context, but can readily perform them spontaneously in natural situations, when they aren't thinking about what they're doing <-- definition a disorder of voluntary movement that isn't attributable to a simple motor deficit (e.g., paralysis or weakness) or to any deficit in comprehension or motivation symptoms are bilateral often caused by unilateral damage to the left posterior parietal lobe or its connections

posterior parietal association cortex definition

an area of association cortex that receives input from visual, auditory, and somatosensory systems, and is involved in the perception of spatial location and guidance of voluntary behavior - the portion of the parietal neocortex posterior to the primary somatosensory cortex

astereognosia

an inability to recognize objects by touch that isn't attributable to a simple sensory deficit or to a general intellectual impairment <-- definition deficits in stereognosis associated w/ large lesions to the primary motor cortex

advantage of hierarchical organization

higher levels are left free to perform more complex functions

development of central sensorimotor programs w/ practice

response chunking in the process of learning a central sensorimotor program, control is shifted from higher levels to lower levels - frees up the higher levels to deal w/ more esoteric/complex/abstract aspects of performance - permits greater speed because different circuits at the lower levels can act simultaneously w/out interfering w/ one another

sensory feedback

sensory signals that are produced by a response and are often used to guide the continuation of the response <-- definition the eyes, the organs of balance, and the receptors in skin, muscles, and joints all monitor the body's responses, and they feed their info back into sensorimotor circuits

cingulate motor areas

small areas of secondary motor cortex located in the cortex of the cingulate gyrus of each hemisphere

effects of primary motor cortex lesions

small lesions often have minimal effects large lesions may... - disrupt a patient's ability to move one body part independently of others - reduce speed, accuracy, and force of a patient's movements large lesions don't eliminate voluntary movement astereognosia

conventional view of primary motor cortex function

somatotopic stimulation of each particular cortical site activated a particular contralateral muscle and produced a simple movement more cortex devoted to body parts that make intricate, complex movements (e.g., hands and mouth_ motor homunculus each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints that site influences monkeys have at least 2 different hand areas in the primary motor cortex of each hemisphere --> one receives input from receptors in the skin rather than in the muscles and joints; presumably facilitates stereognosis - stereognosis requires interplay of sensory and motor systems

development of central sensorimotor programs w/out practice

species-specific behaviors - Fentress (1973) --> mice w/out forelimbs still make coordinated grooming motions

motor equivalency

the ability of the sensorimotor system to carry out the same basic movement in different ways that involve different muscles

supplementary motor area (SMA)

the area of secondary motor cortex that is within and adjacent to the longitudinal fissure - wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure

premotor cortex

the area of secondary motor cortex that lies between the supplementary motor area and the lateral fissure - runs in a strip

dorsolateral prefrontal association cortex definition

the area of the prefrontal association cortex that plays a role in the evaluation of external stimuli and the initiation of complex voluntary motor responses

primary motor cortex

the cortex of the precentral gyrus, which is the major departure for motor signals descending from the cerebral cortex into lower levels of the sensorimotor system, an major point of convergence of cortical sensorimotor signals - frontal lobe

response-chunking hypothesis

the idea that practice combines the central sensorimotor programs that control individual responses into programs that control sequences of responses (chunks of behavior) - chunks can themselves be combined into higher-order chunks

stereognosis

the process of identifying objects by touch

motor homunculus

the somatotopic map of the human primary motor cortex

contralateral neglect

a disturbance of the patient's ability to respond to visual, auditory, and somatosensory stimuli on the side of the body opposite to a site of brain damage, usually the left side of the body following damage to the right parietal lobe <-- definition occurs in the absence of simple sensory or motor deficits most patients behave as if the left side of their world doesn't exist and often don't acknowledge that they have a problem associated w/ large lesions to the right parietal lobe difficulty responding to stimuli to the left of their own body; referred to as egocentric left--partially defined by gravitational coordinates - when patients tilt their heads, their field of neglect isn't normally tilted w/ it tend not to respond tot he left side of objects, regardless of where the objects are in their visual fields - even when the objects are presented horizontally or upside down info about objects that aren't noticed may be unconsciously perceived - when objects were repeatedly presented at the same spot to the left of patients w/ this disorder, they tended to look to the same spot on future trials, even though they were unaware of the objects - 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 weren't consciously perceived

central sensorimotor programs are capable of motor equivalency

a given movement can be accomplished in various ways using different muscles illustrates the inherent plasticity of the sensorimotor system suggests that general programs are stored higher than muscles in the hierarchy and then are adapted to the situation as required - may be stored in secondary motor cortex

frontal eye field

a small area of prefrontal cortex that controls eye movements

secondary motor cortex

areas of the cerebral cortex that receive much of their input from association cortex (i.e., posterior parietal association cortex and dorsolateral prefrontal association cortex) and send much of their output to primary motor cortex <-- definition subject of ongoing research may be involved in programming movements in response to input from dorsolateral prefrontal cortex - active during imaging or planning of movements supplementary motor area premotor cortex both the supplementary motor area and premotor cortex are visible on the lateral surface of the frontal lobe, just anterior to the primary motor cortex to be considered this, an area must be appropriately connected w/ association and secondary motor areas electrical stimulation typically elicits complex movements, often involving both sides of the body neurons often become more active just prior to the initiation of a voluntary movement and continue to be active throughout the movement

hierarchically organized

association cortex at highest levels, muscles at lowest - commands that emerge from association cortex specify general goals rather than specific plans of action; doesn't routinely get involved in the details parallel system functional segregation info mainly flows down

general model of sensorimotor system function

begins at the level of association cortex and traces major motor signals as they descend the sensorimotor hierarchy to the skeletal muscles that ultimately perform the movements

ballistic movements

brief, all-or-none, high-speed movements - only responses that aren't normally influenced by sensory feedback

motor output is guided by sensory input

continuously monitor the effects of their own activities and use this info to fine-tune the activities sensory feedback many adjustments in motor output that occur in response to sensory feedback are controlled unconsciously by the lower levels of the sensorimotor hierarchy w/out involvement of the higher levels

damage to posterior parietal cortex

deficits in... - perception and memory of spatial relationships - control of eye movement - attention apraxia contralateral neglect

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 w/out conscious regulation i.e., conscious to automatic

functional segregation

each level of the sensorimotor hierarchy tends to be composed of different units (neural structures) that perform different functions

sensory info that controls central sensorimotor programs isn't necessarily conscious

evidence that patients could respond to visual stimuli of which they had no conscious awareness evidence that patients couldn't effectively interact w/ objects that they consciously perceived Ebbinghaus Illusion --> conscious perception of disk size differs from motor response

parallel hierarchical system

hierarchical system in which signals flow between levels over multiple paths

three principles of the sensorimotor system

hierarchically organized motor output is guided by sensory input learning changes the nature and locus of sensorimotor control

posterior parietal association cortex

integrates info about original positions of the parts of the body that are to be moved and the positions of any external objects w/ which the body is going to interact plays important role in directing behavior by providing spatial info and in directing attention outputs to motor cortex, which are located in the frontal cortex - dorsolateral prefrontal association cortex - various areas of secondary motor cortex - frontal eye field contains a mosaic of smaller areas, each specialized for guiding particular movements of eyes, head, arms or hands; evidence from... - electrophysiological studies in monkeys - fMRI and TMS studies in humans electrical stimulation in inferior parts... - low current levels --> patients experienced an intention to perform a particular action - high current levels --> felt they had actually performed it - action didn't occur in either case

social cognition

knowledge of others' mental processes (e.g., perception, ideas, and intentions)

mirror neurons

neurons that fire both when a person makes a particular movement, and when the person observes somebody else making the same movement <-- definition possible neural basis of social cognition - mapping the actions of others onto one's own action repertoire would facilitate social understanding, cooperation, and imitation - respond to the understanding of the purpose of an action - respond fully to a particular action and its goal regardless of how it was detected likely to be found in humans - indirect evidence from fMRI studies - active when a person performs, watches, or imagines a particular action in monkey studies, neurons fire while... - grasping or watching another grasp a particular object, but not others - grasping or watching another grasp an object for a specific purpose, but not for another purpose

What characterizes most sensorimotor learning?

organization of individual responses into continuous motor programs and the transfer of their control to lower levels

somatotopic

organized according to a map of the surface of the body

central sensorimotor programs

patterns of activity that are programmed into the sensorimotor system <-- definitions perhaps all but highest levels have patterns of activity programmed into them, and complex movements are produced by activating the appropriate combos of these programs once activated, each level is capable of operating on the basis of current sensory feedback w/out direct control of higher levels - highest levels have the option of directly controlling activities, but most responses are performed w/out direct cortical involvement, and you are often barely aware of them basal ganglia and cerebellum coordinate the various programs - make sure the programs are well-coordinated and effective - modify and regulate them

dorsolateral prefrontal association cortex

receives input from posterior parietal association cortex output to areas of secondary motor cortex, primary motor cortex, and frontal eye field activity of neurons depends on either... - the characteristics of the objects - locations of the objects - combo of both characteristics and locations of objects activity of other neurons is related to the response rather than the object - typically begin to fire before the response, and continue to fire until the response is complete - suggests that decisions to initiate voluntary movements may be made in this area of the cortex, but these decisions depend on critical interactions w/ posterior parietal cortex an other areas of frontal cortex