Course Handout - The Motor Hierarchy
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First published online 07:38 GMT 27th March 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 12:00 13th January 2010
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).
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.
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.]