Learned attachment. Imprinting is particularly widespread in birds. Filial imprinting determines the preferred target for the following response of young birds. Sexual imprinting determines mate preference in birds. Filial imprinting can be reliably produced in the laboratory. Typically, young birds are dark-reared, then they are exposed to a conspicuous object. Several hours or days later, birds are given a preference test. The choice is between the object to which birds were exposed and a novel object. Young birds prefer to approach the object they have seen previously. In sexual imprinting experiments, young birds are cross-fostered with another species. Cross-fostered males exhibit a greater preference for females from the foster species than from their own species. The song of some cross-fostered males (for example, zebra finches) also resembles those of their foster-parent species (see BIRD-SONG).
Much of the early research focused on the validity of the definition of imprinting proposed by Konrad Lorenz (1903–1989). Lorenz made four important claims concerning imprinting. (1) Imprinting was confined to a CRITICAL PERIOD early in development. During the critical period, a single exposure to a biologically relevant stimulus activated this process. (2) After the critical period, imprinting was resistant to reversal. (3) Imprinting influenced behaviours that may not be exhibited until later in life (such as SEXUAL BEHAVIOUR). (4) Imprinting differed from other forms of LEARNING Such as PAVLOVIAN CONDITIONING and OPERANT CONDITIONING. Imprinting occurs in the absence of any external REINFORCEMENT, is resistant to EXTINCTION and there is a long delay between ACQUISITION and the exhibition of behaviour.
Empirical investigations have extended the original more narrow concept of imprinting proposed by Lorenz. Findings indicate that imprinting is not an all-or-nothing process. Rather, it is characterized by a more gradual diminishment of imprintablity. Hence, the term critical period has been replaced by SENSITIVE PERIOD. Similar to other forms of learning, amount of exposure and the nature of the stimuli used influence sexual imprinting. Under certain conditions, imprinting does appear to be reversible. In general, however, phase specificity and selectivity about the stimuli that are most potent in attaining stable results differentiates imprinting from other types of learning. Neurobiological research on imprinting in birds has shown that the IMHV (intermediate and medial part of the HYPERSTRIATUM VENTRALE) is involved. Damage to the IMHV prior to imprinting prevents the acquisition of preferences. Damage to the area after imprinting, renders the bird amnesic. Following imprint-ing, changes in POSTSYNAPTIC density and density of postsynaptic receptors are observed. There is a positive correlation between the number of RECEPTOR sites and the degree to which the bird prefers the familiar stimulus at testing. Increase in density could not be attributed to visual stimulation, AROUSAL or locomotor activity but only to how much the chick had learned. Electrophysiological recordings from the left IMHV have shown a positive correlation between neuronal activity and preference tests. Biochemical analysis of TESTOSTERONE levels have shown the influence of hormonal levels on stimulus-dependent effects on imprinting. IMHV is also involved in another type of learning, PASSIVE AVOIDANCE learning. In the passive avoidance task, young birds must learn to recognize the characteristics of particular coloured stimulus and to associate it with an aversive taste. Lesions to the IMHV inhibit successful avoidance learning. Both imprinting and the passive avoidance learning task may require the formation of a representation of a specific visual object. Individual stimulus recognition may then be the common characteristic that is critical for the involvement of the IMHV in both types of learning.
Further research has revealed other developmental processes besides filial and sexual imprinting in birds. These include the development of host preferences in parasitic species and the establishment of preference for food, habitat and locality. In addition, imprinting occurs in other animals besides birds. Imprinting to olfactory stimuli plays a crucial role in spawning MIGRATION of salmonoids such as the Pacific salmon. After hatching, juvenile fish mature in their natal fresh water streams. At about 18 months of age the fish undergo morphological changes called MOLTIFICATION that prepares them for life in salt water. At this time the fish also imprint to the olfactory bouquet of their natal streams. The hormone THYROXINE seems to be involved in both moltification and home stream odour imprinting. The salmon then swim downstream to the ocean where there live for 1–4 years before migrating back to their natal stream to spawn and die. During this return migration the salmon appear to swim up a concentration gradient of home steam odours. Imprinting may have an adaptive benefit for any organism characterized by rapid ontogenetic development and by early dispersal of young. The time course of the sensitive phase also appears to have a selective advantage. It is geared to the age requirement when information is needed for the first time and capitalizes on the opportunity to acquire that information under favourable learning conditions.
