The process in which an individual learns, during a sensitive period usually early in life, to restrict its social preferences to an object or to a class of objects.
When we select a partner, we often proudly consider it to be an epitome of free choice of a mature human being. Well, we may be wrong. Members of many species learn very early in life to restrict their social preferences to a specific class of objects. This acquired "bias comes to restrict the choice of the mate much later on. It is called 'sexual imprinting'. In other words, there are always traces of one's parents in one's spouse. The basic phylogenetic function of sexual imprinting is considered to ensure that in due time, individuals do not waste their energy in courting the wrong species. Most readers of both the scientific and the general literature are much more aware of another type of imprinting, filial imprinting, in which a juvenile of the species forms attachment to the mother immediately after hatching. Here the idea is to provide the newborn with much needed food and care.1
Different types of imprinting, even in the same species, are established in different time windows and last for different times (Hess 1959; Bateson 1966; Immelmann 1972; Vidal 1980; Leon 1992; Hudson 1993; ten Cate and Vos 1999). Multiple sensory modalities are involved, depending on the species, the object to be imprinted, and the occasion. Indeed, the world of imprinting is astonishingly rich: imprinting navigates Salmons across oceans and rivers to their natal habitat (Dittman and Quinn 1996), socializes dogs with humans (Pfaffenberger and Scott 1959), and can turn a laboratory mouse into a Mozart fan (doesn't work with
Schoenberg; Cross et al. 1967). Although usually characteristic of the young, learning to restrict social preferences during a sensitive time window could occur in the adult (Hudson 1993). For example, reciprocal ewe-lamb bonding is established by olfactory imprinting in parturition. Imprinting also plays a part in the mother-infant bonding in humans (Kennell et al. 1979), and here, again, olfactory cues are important (Leon 1992). As even days-old babies readily recognize their mother's perfume (Schleidt and Genzel 1990), we might expect to find out that a nursing mother who switches too many fragrances cultivates a potential neurotic.
Filial imprinting is especially striking in precocial birds and was the subject of systematic observations already during the early days of experimental psychology (Spalding 1873). A few hours after hatching, the chick approaches and follows a conspicuous moving object, which may or may not be the mother. In nature it usually is; in the laboratory it may actually be an earnest scientist (Lorenz 1937). 'After hatching and becoming able to look around, a greyleg gosling utters its lost piping, to which... its mother answers with a rhythmic cackle. To this the gosling responds by greeting. The mutually releasing sequence of piping-cackle-greeting is predictable with a high degree of probability... The program timing the period of sensibility of that irreversible learning process for this particular moment is obviously adaptive. The built-in mechanism conveys to the gosling information which, if verbalized, would say: 'When you first feel lonely, utter your lost piping, then look for somebody who moves and says 'gang, gang, gang' and never, never forget who that is, because it is your mother'' (Lorenz 1981). This imprinting process involves at least two distinct, interactive components. One is the innate ("a priori) predisposition to approach "stimuli that have the characteristics of the natural mother. The other is the learning to approach a stimulus to which the chick is exposed during the sensitive period—which may be the natural mother, Lorenz, or an artificial stimulus in the lab.
The approach behaviour of the domestic chick, although not as long-lasting as the one in Lorenz's narration, became a useful "system for investigating the behavioural, neuroanatomical, cellular, and molecular substrates of filial imprinting (Horn 1985; Rogers 1994; Bolhuis 1999). The chick is imprinted on a combination of visual and auditory "cues, which are "bound synergistically (Smith and Bird 1963; Bolhuis 1999). Candidate forebrain substrates of both visual and acoustic imprinting have been identified (Maier and
Scheich 1983; Horn 1985; Bolhuis 1999; Bock and Braun 1999a) (Figure 35). Noteworthy among those regions that subserve visual imprinting is the intermediate and medial part of the hyperstriatum ventrale (IMHV), lesions in which impair the learning process but not the innate predisposition (Johnson and Horn 1987; Bolhuis 1999). The IMHV may hence be a critical station in the circuit that enables the chick to "recognize individual birds.
The molecular and cellular mechanisms unveiled in the brain regions that subserve imprinting in the chick are similar to those identified in other types of "acquisition and "consolidation. They involve, among
Fig. 35 (a) Apparatus and stimuli used in the study of filial imprinting in the domestic chick. Imprinting is quantified by measuring the approach toward the imprinted object. The chick is placed in a training wheel, which is here drawn with one of its opaque sides exposed.The *subject runs on the mesh towards a visually conspicuous object, such as those depicted in the inset. As the wheel rotates on its axle, the distance to the object remains constant. The rate of rotation is an index of the approach *performance. Acoustic *stimuli could be combined with the visual ones. (b) A diagrammatic longitudinal section in the brain of a 2-day-old chick, cut laterally to the mid-line; h.a., hyperstriatum accessorium; h.d., hyperstriatum dorsale; h.i., hyperstriatum intercalatus; h.v., hyperstriatum ventrale; IMHV, intermediate and medial part of the hyperstriatum ventrale, delineated ventrally and dorsally by heavy lines; n., neostriatum; o.n., optic nerve; o.t., optic tectum; W, visual Wulst. The IMHV has been particularly implicated in the process of learning to *recognize individual members of the species in imprinting. (Adapted from Bateson and Wainwright 1972; Horn 1985.)
Infantile amnesia others, "glutamatergic N-methyl-o-aspartate "receptors, other "neurotransmitter systems, "protein kinases, "immediate early genes, cell adhesion molecules, and morphological synaptic "plasticity (Horn and McCabe 1990; Sheu et al. 1993; Solomonia et al. 1998; Ambalava-nar et al. 1999; Bock and Braun 1999a,fc).
Imprinting could be considered a subclass of 'prepared learning' processes, in which brain maturation ("development) is modified by sensory experience during a sensitive period. This sensitive period is constrained, among others, by the growth potential of the neuronal circuits and by hormonal states. Other examples are visual (Wiesel 1982; Crair et al. 1998) and auditory (Knudsen 1998) sensory learning early in life, and the learning of "birdsong. Such learning could leave in the young brain dormant "engrams, which, given the appropriate conditions and cues, are capable of being reactivated after many years in the adult (Knudsen 1998).
Selected associations: A Priori, Acquisition, Birdsong, Development
1in biology, the term 'imprinting' is also used in an utterly different *context. 'Gametic' or 'genomic imprinting' is the process whereby the expression of certain genes in the embryo is restricted to either the maternal or paternal allele, although both are present.
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