Our functional definition of learning is: a change in an animal's behavioral responses as a result of a unique environmental stimulus. This broad definition is useful in that it encompasses various nonassociative forms of learning such as sensitization and habituation, but the breadth of the definition can be criticized. This can be illustrated by a consideration of "sensitization" in the Venus' flytrap plant.

Although plants are not thought of as expressing behavior in the same sense as an animal, plants can and do respond to environmental stimuli. We are all familiar with the phototactic responses of plants as, for example, they turn to follow the sun, their foliage changes in response to cooling weather, and the petals of certain flowers, close at night. These types of responses, however, are really more akin to reflexive, nonlearned behaviors in animals.

One intriguingly complex, multicom-ponent response of a plant to an environmental stimulus is exhibited by Dionaea muscipula, commonly known as the Venus' flytrap. This carnivorous plant, indigenous to the peat bogs of the Carolinas in the southeastern United States, supplements its nutrition by capturing and digesting insects. Dionaea trap insects when they land in one of the plant's V-shaped leaves, which closes on the hapless victim like a miniature steel bear trap.

The triggering mechanism for closure of the trap warrants our attention. Each half of the V-shaped trap has on its inward facing surface three trigger hairs. Mechanical stimulation of these hairs elicits closure of the trap. To eliminate "false alarms," Dionaea has evolved a mechanism whereby stimulation of a single trigger hair is insufficient to cause the trap to close. Two hairs must be


stimulated in succession (or simultaneously) to trigger a trapping response. Thus, in one circumstance, stimulating a particular trigger hair will give no response, whereas stimulating the same trigger hair will in another instance, depending on recent history, give trap closure. This is clearly an example of an altered response that depends on a prior environmental stimulus. In a sense, the mechanical stimulation of the first trigger hair could be viewed as analogous to "sensitizing" the plant, so that it will respond to the mechanical stimulation of the second hair. Photo courtesy of Muriel Weinerman, NY Botanical Gardens.

field agree, that many simple forms of behavioral modification qualify as learned responses. These forms of simple, nonas-sociative learning are described in the next section.

This broad, umbrella-like definition of learning covers so many different types of behavioral modifications that some sort of organizing principle and attendant nomenclature are necessary. Even though there is no universally accepted version of this at present, most contemporary experimentalists who work on learning utilize some variation of a system promulgated by Larry Squire and Eric Kandel (1-3). I will use their system as a starting point and would be remiss if I did not credit their many significant and influential contributions in this area. However, I note that there are some significant differences between their published framework and my own, so that I don't saddle them with responsibility for any inadequacies on the part of my framework.

A. Categories of Learning and Memory

I divide learning into two broad classes—unconscious learning and conscious learning. I also introduce a "recall"

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