Course Handout - The Motor Hierarchy
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written and published in Wales by Derek J. Smith (Chartered Engineer). It forms
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First published online 07:38 GMT 27th March 2003,
Copyright Derek J. Smith (Chartered Engineer). This version [2.0 - copyright] 09:00 BST 4th July
2018.
Earlier versions of this material appeared in
Smith (1997; Chapter 6). It is simplified here and supported with hyperlinks.
1 - The Concept
of "Motor Hierarchy"
The ability to initiate voluntary physical behaviour is known as "praxis", and (because anything voluntary involves the will) praxis has been a traditionally difficult area for the cognitive theorist. For one thing, there is the philosophical problem that nobody knows what the will actually is, and for another, there is also the technical problem of explaining how ideas (ie. thoughts, images, or intentions) might be retrieved from some initially timeless representational state - a structural memory trace of some sort - and converted into a time-sequenced succession of behaviours. This latter is the problem of motor sequence, and it has been around for some time, having been stated very forcefully by Lashley (1951) in a paper entitled "The Problem of Serial Order in Behaviour". The standard explanation is that the motor memory for a particular piece of behaviour is capable (a) of being reactivated as a single unit whenever its performance is required, and (b) of having its component movements reactivated one by one. This sort of motor memory is conventionally referred to as a "motor schema" .....
Key Concept - Motor Schemas: A motor
schema is a long term memory structure capable of being retrieved as a whole,
and then executed in parts. It is the "representation of a to-be-performed
movement" (Gallistel, 1980, p368). This implies
that the memory trace has a start and a finish, so to speak, unlike the memory
traces for visual form, say, where reactivation is all or nothing at any given
point in time. The term originated with Head (1926), was refined by Bartlett
(1932), and was made popular within motor theory by Schmidt (1975). Drawing on
earlier work by Pew (1966, 1974), Schmidt saw schemas as bringing together four
different types of information into a single motor memory, namely (a) the
current state of one's body in space, (b) what is to be achieved by a given
movement, (c) what feedback is to be expected during its execution, and (d) how
successfully it meets its aim.
Now the point about motor schemas is that by definition they are organised hierarchically. There are at least two layers of control in this hierarchy, because it must always start with the act of volition, and always end with the muscles. Additional layers of organisation can then be inserted between the top and the bottom, according to the demands of the task at hand, and in Figure 1 we show one of the "classics" of motor theory, Weiss's (1941) six-"level" model.
Figure 1 - Weiss's (1941) Six-"Level"
Motor Hierarchy: This block diagram is schematised from Weiss (1941,
as reproduced in Gallistel, 1980). Messages to and
from the skeletal muscles are controlled by "behaviour sequences"
initiated and integrated at the level of the behavioural "act"
(Level 6). The motor units themselves are fired by activity at Level 1.
Levels 5 to 2 cascade control from the top of the hierarchy to the bottom,
gradually implementing additional mechanisms of shaping given muscle group
contractions. Note the problems which arise from having to coordinate lots of
functionally opposed paired muscle groups. Note also that in Gallistel's view Weiss's hierarchy of action "has
never been improved upon" (Gallistel, 1980,
p275). If
this diagram fails to load automatically, it may be accessed separately at |
Redrawn from a black-and-white original in Smith (1997; Figure 6.3). After Weiss (1941), via Gallistel (1980). This version Copyright © 2003, Derek J. Smith. |
See now the paper on Motor Programming.
References
Bartlett, F.C. (1932). Remembering.
Cambridge: Cambridge University Press.
Gallistel, C.R. (1980). The Organisation of Action: A New
Synthesis. Hillsdale, NJ: Erlbaum.
Head, H. (1926). Aphasia
and Kindred Disorders of Speech. Cambridge: Cambridge University
Press.
Lashley, K.S. (1951). The problem of serial
order in behaviour. In Jeffress, L.A. (Ed.), Cerebral
Mechanisms in Behaviour, the Hixon Symposium. New York: Wiley.
Pew, R.W. (1966). Acquisition of hierarchical control
over the temporal organisation of a skill. Journal of Experimental
Psychology, 71:764-771.
Pew, R.W. (1974). Human perceptuo-motor
performance. In Kantowitz, B.H. (Ed.), Human
Information Processing: Tutorials in Performance and Cognition. New York:
Erlbaum.
Schmidt, R.A. (1975). A
schema theory of discrete motor skill learning. Psychological Review,
82:225-260.
Smith, D.J. (1997). Human
Information Processing. Cardiff: UWIC. [ISBN: 1900666081]
Weiss, P. (1941). Self-differentiation
of the basic patterns of coordination. Comparative
Psychology Monographs, 17(4). [Page numbering from
the reprint in Gallistel, C.R. (1980), The
Organisation of Action. Hillsdale, NJ: Erlbaum.]
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