The Problem of the “Stimulus”

Abstract

The term “stimulus” is often associated with an extremely broad concept subsuming almost every external influence that could elicit an observable response from any organic system—from a protozoan, from a whole multicellular organism, from a single nerve cell or even from an isolated neurite. We describe as a “key stimulus” that complex configuration of many stimuli to which the IRM of a higher animal responds selectively. A key stimulus elicits a very special and teleonomic response. We make this descriptive distinction in spite of our awareness that the selectivity is due to a most complex “filtering” mechanism through which any stimulation must pass—a filtering mechanism which evaluates a multiplicity of stimuli and reports nothing but a single, reliable signal to the higher loci within the animal’s central nervous system. At the same time we speak of a “stimulus” when we cause a single, quantifiable electric shock of a few millivolts to excite a single neuron, or even an isolated nerve fiber, and occasionally the term “stimulus” is used even when an unspecific neural response is released by some change in the environment not “provided for” by phyletic adaptation—when, for example, a deficiency of calcium ions in the blood or in the nutrient solution causes some neurons to fire spontaneously.