It was shown within a few years of Darwin’s death that even single-cell organisms like protozoa could exhibit a range of adaptive responses. In particular, Herbert Spencer Jennings showed that the tiny, stalked, trumpet-shaped unicellular organism Stentor employs a repertoire of at least five different responses to being touched, before finally detaching itself to find a new site if these basic responses are ineffective. But if it is touched again, it will skip the intermediate steps and immediately take off for another site. It has become sensitized to noxious stimuli, or, to use more familiar terms, it “remembers” its unpleasant experience and has learned from it (though the memory lasts only a few minutes). If, conversely, Stentor is exposed to a series of very gentle touches, it soon ceases to respond to these at all—it has habituated.
Jennings described his work with sensitization and habituation in organisms like Paramecium and Stentor in his 1906 book Behavior of the Lower Organisms. Although he was careful to avoid any subjective, mentalistic language in his description of protozoan behaviors, he did include an astonishing chapter at the end of his book on the relation of observable behavior to “mind.”
He felt that we humans are reluctant to attribute any qualities of mind to protozoa because they are so small:
The writer is thoroughly convinced, after long study of the behaviour of this organism, that if Amoeba were a large animal, so as to come in the everyday experience of human beings, its behaviour would at once call forth the attribution to it of states of pleasure and pain, of hunger, desire, and the like, on precisely the same basis as we attribute these things to the dog.
Jennings’s vision of a highly sensitive, dog-size Amoeba is almost cartoonishly the opposite of Descartes’s notion of dogs as so devoid of feelings that one could vivisect them without compunction, taking their cries as purely “reflex” reactions of a quasi-mechanical kind.
Sensitization and habituation are crucial for the survival of all living organisms. These elementary forms of learning are short-lived—a few minutes at most—in protozoa and plants; longer-lived forms require a nervous system.
- Oliver Sacks
Jennings described his work with sensitization and habituation in organisms like Paramecium and Stentor in his 1906 book Behavior of the Lower Organisms. Although he was careful to avoid any subjective, mentalistic language in his description of protozoan behaviors, he did include an astonishing chapter at the end of his book on the relation of observable behavior to “mind.”
He felt that we humans are reluctant to attribute any qualities of mind to protozoa because they are so small:
The writer is thoroughly convinced, after long study of the behaviour of this organism, that if Amoeba were a large animal, so as to come in the everyday experience of human beings, its behaviour would at once call forth the attribution to it of states of pleasure and pain, of hunger, desire, and the like, on precisely the same basis as we attribute these things to the dog.
Sensitization and habituation are crucial for the survival of all living organisms. These elementary forms of learning are short-lived—a few minutes at most—in protozoa and plants; longer-lived forms require a nervous system.
- Oliver Sacks
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