The Centrally Coordinated Movement or Fixed Motor Pattern

Abstract

To give an exact definition of what we mean when we speak of a fixed motor pattern or a fixed action pattern is difficult because one should not include any working hypotheses in the definition of any biological function or structure that has not already been thoroughly analyzed. Still, it is possible to confess that we believe that the hard core of what we call fixed motor patterns consists in centrally coordinated sequences of endogenously generated impulses, and that this coordination has evolved phylogenetically and is very resistant to any individual modification. A fixed motor pattern’s most important distinction from motor patterns that are not fixed and, simultaneously, the cogent argument for its being genetically programmed, consists in its taxonomic distribution. Fixed motor patterns show, from species to species and from genus to genus, similarities and dissimilarities that concur strictly with those of morphological characters. The discovery of these facts of similarity and dissimilarity is due to the work done by C. H. Whitman and O. Heinroth. They discovered that the concept of homology could be applied to motor patterns in certain “ritualized” movements of courtship in pigeons (Columbidae) and in waterfowl (Anatidae). It is typical for discoveries constituting major breakthroughs that they are made with objects in which the newly found laws of nature are represented in the simplest possible ways. The classic example of this, as already mentioned, is Gregor Mendel’s hitting upon monohybrids, that is, hybrids between races of plants which differ in only one gene. The courtship movements in question also obey the simplest possible laws. They obey the so-called all-or-nothing law of nervous discharge; in other words, their identity is not veiled by the great variability of the forms in which other motor patterns appear in correlation with the varying intensity of their specific excitation. As will be discussed in the following section, even a low excitation can elicit barely noticeable “intention movements,” and with rising excitation, a gradual scale of transitions leads from this “intention” to the fully intensified motor pattern achieving its teleonomic function. In some particular motor patterns which function as signals, this intensity-correlated variability has been abolished in the interest of unambiguity: when they are performed at all, it is always with the same “typical intensity,” as Desmond Morris (1957), who discovered the phenomenon, has described it.