Human Motor Behavior: An Introduction

By J. A. Scott Kelso | Go to book overview

In fact, the highly coherent behavior of fish schooling can be accounted for with a fairly small set of key variables. One of these is "density" as defined through the metric of fish length. When the average distance between nearest neighbors is greater than one fish length, spacing between fish is random, not schooled ( Okubo, 1980). Observations of many fish schools reveal that "attraction " between fish is specified by a critical distance that seldom exceeds 16-25% of the mean body length of the fish.2 Under normal, stable conditions, individual fish rarely allow themselves to be separated from others by more than 40% of their body length. Interestingly, fish tend to form a "tighter" school (i.e., the critical distance decreases) when nearby shelter such as seaweed or rocks is absent. This type of gregarious behavior may have ecological significance, in that "close packing" reduces the chance of an individual fish being caught by a predator. (The predator, like Caesar of Gaul, seems to operate on the principle of divide and conquer.)

Now the details of collective fish behavior may seem far removed from the issues germane to the student of motor behavior, but I think there is a message here that is important. It has to do with the way we approach complex problems. In the case of the ecologist studying coherence phenomena such as fish schooling, we can see that considerable insights into the problem are gained by determining the necessary and sufficient conditions for organization to occur. Schooling is a consequence of certain crucial variables such as critical density and diffusion gradients. These variables are common in the analysis of physical systems and are constrained by a contextual occasion (are sharks nearby?; where can I hide?). The point is, and I apologize for belaboring it, there is no a priori prescription for the complex organization that we call schooling. Instead, this example motivates an alternative strategy for investigating action--one that rejects the introduction of special mechanisms before the laws of dynamics have been fully explored. Put another way, what can we, as students of movement, explain "for free" by considering the causal and logical support for action? And, by doing this, can we trim away the ghost in the machine? Therein lies the rub for the movement scientist, the brain theorist, the cognitive psychologist, the molecular biologist, and anyone else seeking to understand intelligent behavior.


REFERENCES

Dennett D. C. Brainstorms: Philosophical essays on mind and psychology. Montgomery, Vt.: Bradford Books, 1978.

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2
Note that the idea of a critical value is very different from the reference value in a servomechanism. Critical values are a common feature in natural systems with many degrees of freedom. For example, consider fluid phenomena in which water is enclosed between two cylinders that can be rotated in opposite directions. At slow rotation speeds the water flow is laminar and smooth; beyond a critical value of rotation, however, the flow becomes turbulent, progressing to still higher forms of organization such as vortices. (For further examples in natural systems, see Haken, 1977, and for examples in movement, see Kelso, 1981; Kugler, Kelso, & Turvey, 1982.)

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