of eggs of different sizes placed outside the nest, the largest one will be preferred, even if it is twice the size of its own egg and thus difficult to handle. Egg retrieval by the brooding herring gull, Larus argentatus, has been studied in even more detail by Baerends and his colleagues (Baerends & Drent 1982). In hundreds of experiments carried out over many years, they showed that a green, speckled egg the size of a football was preferred over the gull’s own brown egg, which is the size of a large hen’s egg.
Humans, too, take advantage of exaggerated stimuli for various communicative purposes. Messages can be made appealing by stressing features. In cartoons, caricatures, and graphic icons certain cues are accentuated in order to abstract and emphasize the expression of a subject or an object. Women’s eyes and lips are underlined cosmetically in order to make the face attractive and distinctive from other faces.
Behavioral Ways of Stimulus Selection
Stimulus-specific habituation implies stimulus discrimination
Hinde (1954) showed that, in many cases, the responsiveness of animals to the same repeatedly presented stimulus passes three stages: an initial increase (warming-up), a plateau of maximal activity, and a subsequent decrease until the stimulus is neglected (habituation) (see Chapter 8). There are instances showing that habituation can be stimulus-specific, such that a small change in the stimulus to which an animal has become habituated can produce dishabituation: a sudden increase in response compared to prehabituation levels. The phenomena of stimulus-specific habituation and dishabituation provide a method to investigate distinctive features of stimuli (Table 2.3).
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).
The classical concept of innate releasing mechanism
What mechanism “translates” a sign-stimulus into the adequate behavior (Figure 2.2)? The concept of innate releasing mechanism (IRM), introduced by Konrad Lorenz and Niko Tinbergen, concerns the observation that some organisms are apparently able to recognize behaviorally meaningful stimuli never before experienced in their environment and to respond to them in a predictable manner (Tinbergen 1951). A sign-stimulus—also called “key-stimulus” was thought to activate the IRM much like a safe is unlocked by a key. The comparison with a key-lock principle may be misleading, however, since it suggests that the IRM is just responsive to one specific stimulus. This is not the case. More likely a sort of “group key” works, since the mechanism responds to a category of stimuli that share a set of defining features. Controversy also surrounds the term “innate” (Chapter 7). A revised concept considers that both experience and genetic factors contribute to behavior, and most authors now refer to the concept as a “releasing mechanism” (RM), or a “releasing system” (Ewert 1987, 1997).
Toward neuronal correlates of releasing systems
The concept of “releasing system” suggests a neuronal sensorimotor interface translating perception into action (Figure 2.2). At its afferent (input) side this interface has stimulus recognition and localization properties; its efferent (output) side runs (“commands”) the corresponding motor pattern generating system (Table 2.4). Theoretically, the simplest structure of a ballistic releasing system would entail a “command neuron,” CN, operating in a processing stream that—once triggered—proceeds