1. Cognition – 2/e Dr. Daniel B. Willingham
Chapter 8:
Motor Control
PowerPoint by Glenn E. Meyer, Trinity University

2. How Do We Select a Movement?
Efficiency Theories
Synergy Theories
The Mass Spring Model
Which Model is Right?
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3. Efficiency Theories Important Terms:
Degrees of Freedom Problem The problem of how the mind selects which way to execute a movement, given that there are many ways to make any given movement
Trajectory: The path of a movement
Effector: Part of the body that you use to have an effect on the environment (e.g., the hand, the foot).
Efficiency Theory: A solution to the degrees of freedom problem in motor control, which claims that movements are evaluated for their efficiency and the most efficient movement, is selected.
Problem: What is best measure of efficiency?
Joint space: In motor control, a representation for planning movements that uses joint angles
Cartesian Space
Minimum Joint Torque
Minimizing Jerk: Rate of acceleration. Used as a measure of efficiency in one theory addressing the degrees of freedom problem
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4. Synergy Theories
Synergy Biases for joints or muscle groups to work together in a particular way
Evidence for:
Santello, et al (1998) as seen in Fig. 8.2, argue for two basic postures and synergies that explain a grasping situation
Anticipatory Postural Adjustment Muscle contractions that counteract changes in the center of gravity that occur due to other movements (e.g., reaching movements) – seen in reaching behavior, counteracting what makes you unstable
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5. The Mass Spring Model Which Model is Right?
A model addressing the degrees of freedom problem that capitalizes on a biomechanical property of the way our muscles and limbs are designed. It proposes that endpoints are selected for movements, but trajectories are not planned – as seen in Fig 8.3
Evidence: Polit and Bizzi (1978) as seen in Fig. 8.4 , use and non-use of proprioceptive information by monkey
Proprioception A sense of the body’s location generated by any of a number of special receptors in the joints, skin, and muscles
Graziano, et al. (2002) – electrical simulation of motor cortices in monkey – stimulated predicted endpoints
Which Model is Right?
Correct model needs to incorporate many of the models
Rosenbaum, et al. (1993, 1995, 2001) combines many factors:
Endpoints selected
Trajectory develops naturally as joint angles go to target posture
Movement selected by criteria: accuracy and energy
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6. How are the Movements Sequenced?
Response Chaining
Motor Program Theories
Hierarchical Control in Motor Programs
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7. Response Chaining
Response chaining A theory of how sequences are organized in motor control. A movement Glossary triggers a proprioceptive feeling of having completed the movement, which triggers the next movement, and so on.
Suggested by James – based on proprioceptive feedback as seen in Fig. 8.6
Lashley (1951) – argued against as speed of skilled movements argues against use of proprioception
Experiments severing dorsal roots of spinal cord don’t support the theory
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8. Motor Program Theories
Motor program Theory (Keele, 1981)
A representation supporting movement that has three key features:
it contains a set of commands for movement
peripheral feedback is not needed
the commands can be applied to different effectors
Supporting evidence –
Henry and Dodd (1960) – time it takes to initiate a series of commands depends on the number of movements in the series
Taub shows monkeys can produce actions without propioception
You can write your name with the nondominate hand or a foot – as seen in Fig. 8.7
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9. Hierarchical Control in Motor Programs
Motor sequences are organized hierarchically as seen in Fig Hierarchical schemes contain:
Movement nodes: In a hierarchical sequencing representation, the movement nodes control muscles
Control nodes: In a hierarchical sequencing representation, the control nodes tell the movement nodes what to do.
Supporting Evidence:
Rosenbaum, et al. (1983) as seen in Fig Time between key presses depends on number of nodes traversed
MacKay and Bowman (1969) as seen in Table 8.1 – argues for two levels of speech production
Control of articulators
Abstract Control of words to be said
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10. How Is Perceptual Information Integrated into Ongoing Moments?
Vision
Propioception
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11. Vision
Questions:
What is the relation of the visual system’s “How” system to motor control
Is visual feedback needed for motor control, as in reaching?
Zelaznik et al. (1983), as in Fig. 8-10, found for fastest movements vision did not contribute but when movements took > 150 ms, feedback made them more accurate
Constant visual feedback not needed, periodic feedback can be substituted as seen in Fig (Elliot, et al., 1994)
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12. Propioception
Proprioception : A sense of the body’s location generated by any of a number of special receptors in the joints, skin, and muscles:
Muscle receptors:
Muscle Spindles: Receptors in the fleshy part of muscles that detect muscle stretch. They are important for proprioception
Golgi Tendon Organs: Receptors located where the muscles and tendons join that are active when muscles stretch. They are important for proprioception
Cutaneous receptors: Receptors in and under the skin. Some of these respond when the skin is displaced by pressure. This is important in detecting the pressure exerted by muscle contraction, as when you grip a glass
Joint Receptors: located in joints and fire for extreme joint angles
Loss of propioception is devastating and vision cannot substitute in some tasks (Cole, 1995) but can in others (Sanes et al., 1985 as seen in Fig, 8-12)
Propioception contributes egocentric spatial representation: A spatial representation in which objects are located relative to part of the body as seen in Fig. 8-13
Evidence suggest egocentric spatial representation is used to support movement as seen in Fig (Blouin, et al., 1993)
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13. How Are Motor Skills Learned?
Three Obvious Properties of Motor Skill Learning
Two Approaches to Motor Skill Learning
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14. Three Obvious Properties of Motor Skill Learning
Motor skill learning Increasing accuracy (either spatial or temporal accuracy) of motor acts that occur as a result of practice
Three Properties
Generalization
Transfer depends not just on overall similarity but on representation of the skill (Willingham, 1997,1998)
Long-Term Retention
As seen in Fig 8.18 (Fleishman and Parker, 1962)
Automaticity:
When motor skills are practiced to sufficient degree, the attentional demand for the action is reduced as is the felt need for conscious direction of the action. Consistent feature of well trained skills (Wulf and Prinz, 2001)
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15. Two Approaches to Motor Skill Learning
Generalized Motor Programs:
A motor program that can produce not just a specific movement, but a whole class of movements
Motor skill learning is matter of acquiring generalized motor programs – Schmidt’s (1975) schema theory
Can be thought of as a function that relates some input parameters to a pattern of movements that will produce a desired outcome as seen in Fig 8.18
Ability to generalize will be better if you have trained in a broader set of circumstances
There is ongoing debate about the predictions of the theory
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16. Two Approaches - Continued
Multiple Processes - Willingham
Many brain areas involved, suggesting several systems
Motor skill learning is the adjustment of motor control processes to work more effectively in particular environments
Suggested Processes to Support Motor-Skill Learning (as seen in Table 8.2)
Strategic: Selects goal of movement in environment coordinates.
Perceptual-motor integration: Selects spatial targets for movement that will fulfill environmental goal; represented in egocentric space.
Sequencing: Orders spatial targets in the correct sequences
Dynamic: Translates egocentric spatial targets and a pattern of muscle firing
Processes can operate in conscious or unconscious mode, rely on different neural structures
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