Friday, August 3, 2012

Vehicularization: A Control Principle in a Complex Modal Animal

The following post consists of a section that David Hays and I removed from our article, Principles and Development of Natural Intelligence (downloadable PDF). The final draft was long and the journal editors asked that we cut it. This is much of what we cut.

I’ve interpolated some comments in italics and appended a later note. While the passage is best read in the context of the whole article—which explains how we used the mathematical notion of diagonalization and has a full discussion of behavioral mode (downloadable PDF)—it can be read independently. The general idea is of one system being nested within another such that the deepest, the innermost system, leads to the most immediate satisfactions, but also has the most restricted behavioral scope. A system with greater scope, while not capable of satisfying a basic need (e.g. for food, water, sex, companionship) is able to move the animal to a place in the environment where the innermost system can exact satisfaction. The outer system is thus serving as a vehicle for the inner system.

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This story can be brought to closure with the concept of vehicularization. Neurophysiologically, the concept is a generalization of Paul MacLean's (1978) familiar concept of the triune brain. In McLean's conception the mammalian brain consists of a complex of reptilian grade embedded within one of paleomammalian grade which is in turn embedded within one of neomammalian grade. Restating this general idea in our terms, with the emergence of the diagonaliztion principle the modal system becomes embedded within the emergent sensorimotor system. As each new principle is implemented in new tissue the previous system becomes embedded within it.

Behaviorally, a higher order system serves as a vehicle for moving the organism to a place in the environment where control can safely be transferred to a lower order system. Conversely, when a lower order system is blocked without having satisfied the exit requirement of the current mode, transfer can be given over to a higher level system, which will then transport the organism to a location in the environment where satisfaction of the current exit condition is more likely. The overall effect of behavioral vehicularization can be stated in terms of a hill-climbing search strategy.
For example, you’ve been working hard and, all of a sudden, you notice that you’re ravenously hungry. Your lowest level system, the one that is actually capable of satisfying your hunger, wants grab some food and start chewing. If a cheese burger or a head of lettuce is close at hand, you can see it and grab it, it does into action and your immediate hunger is sated. If no food is available, however, you turn control over to a higher-level system that then goes looking for food. If you are in your home, you’ll go to the kitchen or the pantry ans see what’s available, now.
In hill-climbing a gradient is placed on the environment, the search space, such that locations most likely to satisfy the search, to fulfill the system's current need, are higher than unpromising locations. [Don't confuse the physically real environment and the abstract search space. The hill being climbed is abstract.] The organism then climbs to the top of the nearest hill and, if all is well, is satisfied. However, hill-climbing has a weakness; the local maximum may not be the global maximum for the search space. When this is the case the system is stuck at the local peak with no way of moving down it and then over to the global maximum.
So, you’re hungry. There’s nothing immediately to hand, but you smell something potentially delicious. You follow your nose and it leads you to a window. There’s no food on the window sill and the window’s filled by a screen you can’t break through. What do you do? If you insist on following your nose, you’re stuck. That’s a local maximum. So you’ve got to stop following your nose and do something else to take to a place in your environment where following your nose will be more successful.
In such a situation a modal organism can only exit the current mode. That being done, the gradient which trapped it is lifted and it is now moving along a different gradient, quite possibly one defined by an exploratory or search mode. That is budgeting. Vehicularized organisms can deal with the situation by transferring control to a higher order system which can then move the lower order system away from its local maximum to a position in the environment closer to the global maximum for that lower system. When that location is reached control is then transferred to the lower system, which climbs the hill to satisfaction.

In such a vehicularized organism the modal system is stratified. The reorganizing mode [learning] is one which permits the resetting of the conditional elements of on-blocks [simple control triggers: ON Condition X DO Action Y], thus changing the coupling of the organism to its environment. The higher level modes, play, imitation, and language, permit a great deal of exploration and activity before the conditions and actions of on-blocks are committed. These modes are probably particularly important in building very complex conditional and actional elements.

This is particularly important in ontogeny. It is known that the human nervous system matures roughly in the phylogenetic order of its components (Milner 1976); that is the innermost vehicles mature first. This suggests that the tissue which will be implementing the higher level principles matures under the guidance of the electro-chemical gradients generated by the activity of the lower vehicle, which is controlling the behavior (cf. Edelman 1978). Thus, we know one feature of the development of the nervous system is the early proliferation of synapses in a region followed by the elimination of many of these synapses (Cowan 1979, Purves and Lichtman 1980). That elimination might, for example, be the primary method of diagonalizing in cortical tissue. Once tissue had been diagonalized its critical period would be over. After that it could only learn patterns within the types specified by the diagonalization. Once the new tissue had been diagonalized it would be ready for the implementation of higher level control organized into higher level modes.

At this point it is clear we are once again firmly within the region governed by the biological principles of epigenesis. Any full understanding of the nervous system requires a deep understanding of how these biological principles shape neural tissue. But those principles are outside our purview. Out point is simply that vehicularization is a critical link between the action of the information principles and their implementation in neural tissue. It is our suspicion that a deeper understanding of vehicularization will lead to, or perhaps follow from, an understanding of the particular adjustments of developmental sequences which follow from the ontogenetic recapitulation of phylogeny (Gould 1977).

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Note made on 8.29.2000:

I just read a bit on animal navigation that's relevant here. It's from David Gallistel's article in MIT's encyclopedia of cognitive science. Animal navigation is mostly dead reckoning. It's only beacon guided when the animal is close to the target. In Gallistel's words, “Beacon navigation is the following of sensory cues emanating from the goal itself or from its immediate vicinity until the source of the sensory beacon is reached. Widely diverse species of animals locate goals not by reference to the sensory characteristics of the goal of its immediate surroundings but rather by the goals’ position relative to the general framework provided by the mapped terrain.”

What this means is that beacon guidance is a different mode from long-range navigation. It’s a different mode and a different vehicle.

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