The Behavior of Animals. Группа авторов

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      Table 2.3 Feature discrimination causes dishabituation.

      For example, the prey-catching activity in the common toad habituates when the animal is repeatedly offered a small, orthogonal, triangular piece of black cardboard moved with its small side leading and the tip trailing (a):

      If immediately after habituation, the toad is offered the triangle’s mirror image (b), the prey-capture responses return immediately (Ewert & Kehl 1978; cit. Ewert 1984; see also Further Reading, Movie A3). Another example: when young gallinaceous birds are exposed to any medium-sized silhouette from a bird flying above, they exhibit escape behavior. However, over time, young turkeys, Gallopavo meleagris, become habituated to goose-like birds (long neck, short tail) flying overhead recurrently, whereas the less frequently seen birds of prey (short neck, long tail) continue to be avoided (Schleidt 1961). This explains the goose/hawk discrimination (Figure 2.3c).

      The phenomenon of stimulus-specific habituation also occurs in human perception. During exercise in a fitness center we habituate rapidly to the smell of our own sweat but readily detect the smell of another person. We habituate to the ticking of an old-fashioned clock. If the clock is replaced by another one, ticking somewhat differently, we will notice this sound—until habituation.

       Search images facilitate stimulus recognition

      When birds discover a tasty cryptic prey in their environment, for example, a type of insect difficult to detect because it is embedded in masking distractors, they employ a search image. A predator using a search image takes one type of prey and neglects others, even if the types—e.g., investigated in a choice procedure—appear equally attractive (Langley et al. 1996). A search image neglects certain cues from the complete image of the object being sought, but rather focuses attention on particular cues of the search object. The discrimination principle is in some ways opposite to stimulus-specific habituation, since the searcher tends to see what it expects to see.

      There are parallels in human perception. Suppose we want to pick blueberries in the forest. At first glance the bushes seem to be empty since the dark-green leaves distract from the berries. By concentrating on the dark-blue coloration of the berries—and “printing” a search image—suddenly it seems quite easy to collect them. Expecting a visitor at an airport, not seen for a long time, we have a search image in mind of what the visitor will look like, based upon experience or a photograph—and this works. But if the visitor has changed his image, e.g., wearing a beard, we might have been more successful in identifying him without search image.

       The behavioral meaning of stimuli can depend on motivation

Responses to particular stimuli are also subject to internal (e.g., hormonal) and external (e.g., photoperiod) factors (Figure 2.2; see also Chapters 3–8). In spring, when nesting motivation in birds is high, pieces of branch are attractive for nest building. After hatching, a juicy piece of branch may be chosen as food for the nestlings. The sharpness of stimulus identification, too, may depend on the level of motivation. During the mating season, if in a pond there is no female toad rapidly available to a highly motivated conspecific male, the male may clasp even a piece of bark it encounters—somewhat reminiscent of Goethe’s Mephisto who promises Faust “with this drink in your body, soon you’ll greet a Helena in every girl you meet.”

      Analyzing Neural Processes that Underlie Perception of Sign-stimuli

      The conclusion that a sign-stimulus is configurational is merely a provisional way of describing the complexity of the sensory stimulating process. It is thus a challenge rather than a solution, a challenge to analyse the complex system of processes denoted by the convenient collective name ‘stimulus’. […] Accepting a mere descriptive term as a causal explanation […] causes a false satisfaction which is a hindrance to further research.(Tinbergen 1951; reissued 1989, p. 79 top)

      Here, Tinbergen is stressing the need to analyze the processes underlying ethological concepts—down to the neuronal level—by means of a broad spectrum of physiological/anatomical methods. He introduced the discipline of this causal-analytical research as “ethophysiology,” today called neuroethology or behavioral neurobiology (Ewert 1976; Carew 2004; Zupanc 2019).

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