Ординатура / Офтальмология / Английские материалы / books.google.com / Visual Perception Fundamentals of awareness multi-sensory integration and high-order perception_Martinez-Conde_2006

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Introduction

The detection of an object in a natural setting strongly depends on the extent to which a visual system can differentiate it from other objects, which make up the ‘‘background.’’ By ‘‘background’’ we mean the scene, at whatever level of visual complexity, in the absence of the object. So, for example, a hawk may not see a mouse on the ground if it is similar in color, texture, and lightness to its general surroundings.

The property of an object that renders it difficult to detect by virtue of its similarity to its environment is known as ‘‘camouflage.’’ An awareness of the rules of camouflage arose from attempts to apply knowledge from natural history and art to military applications at the beginning of the 20th

Corresponding author. Tel.: +44-117-928-8565;

Fax +44-117-928-8588; E-mail: tom.troscianko@bristol.ac.uk

century. Abbott Thayer is generally regarded as pivotal in the establishment of modern military camouflage research (Thayer, 1896, 1909). Prior to World War I, military uniforms were brightly colored, at least in part so that they might appear more menacing to the enemy (Scott, 1961). However, with the advent of long-range weapons, and guerrilla fighting tactics as employed, for example, against the British Army in the Boer War (1899–1902), the conspicuousness of these uniforms resulted in heavy losses and various armies began to consider the issue of camouflage seriously. Thayer was a painter, and the ‘‘Camouflage Units’’ set up by the French, British, and American armies brought together an ‘‘eclectic mix of artists, biologists, and military strategists’’ (Sherratt et al., 2005), thus being perhaps the earliest, and arguably one of the most important, collaborations between artists and scientists. World War II produced further examples of such collaboration. Of note here

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is the contribution of British zoologist and artist Hugh Cott who served as chief instructor to the British Army’s Middle East Camouflage School during that war (Cott, 1940).

Crypsis was realized by Darwin and many of his contemporaries to be an important factor in the apparent design, through evolution by natural selection, of biological systems as well. Many animals avoid predation by concealing themselves, and the benefits confirmed by camouflage survival provided some of the earliest and most convincing examples of Darwinism. In most studies of concealment or crypsis, the level of camouflage is measured by how well an animal represents a random sample of the background, in terms of shape and color, at the place and time of the highest predation risk, as Endler (1978, 1984, 1991) has frequently argued. This is termed ‘‘background matching’’ (Cott, 1940; Edmunds, 1974; Ruxton et al., 2004). Endler’s theory assumes that all random samples of the background will be equally cryptic. Given the widespread acceptance of background matching it is surprising that, until recently, there have been few empirical tests of the theory that both quantify background matching as perceived by the predator and measure its efficacy in terms of survival value (see Merilaita and Lind, 2005).

Although crypsis provides a powerful method of reducing the probability of detection of a target, it is clear that there are situations in which it cannot be totally effective. The problem is that the environment changes from location to location; a

static object designed to mimic the environment in one place may not be as effectively concealed in another place. For example, the left-hand part of Fig. 1 shows pebbles on a beach. The right-hand part of Fig. 1 shows the same image from which a rectangle has been copied and pasted into the center. In terms of crypsis, the rectangle in the righthand side of Fig. 1 is optimal because it is a perfect copy of part of the scene. However, even in the absence of relative motion, the object becomes visible because of the discontinuities in boundaries thus produced. Of course, relative motion would make the cryptic object even more visible.

From the above, we see that the property of an object that makes it detectable even in conditions of optimal background matching is the presence of extended contours, which give the object its shape. Arguably, this is where ‘‘recognition’’ rather than simple ‘‘detection’’ becomes the correct procedure, since the object is recognized (in this case, as a rectangle) by virtue of its occluding contour. We know that occluding contours are effective drivers of object recognition processes, e.g., from the study by Marr (1982). It is apparent that something more than just crypsis/background matching is required to prevent the object being recognized as an object of interest. Thayer (1909) realized that breaking up the contour of the object to be detected plays an important role in preventing the process of recognition of the target as such. One of the key arguments from Thayer’s (1909) theories was the concept of distracting or ‘‘dazzling’’ markings (termed ‘‘ruptive’’ by Thayer, but now

commonly called disruptive markings; Cott, 1940) (Fig. 2).

These markings are generally too small to perceive, until in near view, when their sharp, isolated, and ‘‘noncommittal conspicuousness’’ tend to draw and hold the attention of the receiver, so that the receiver less readily discerns the body of the animal bearing the markings (Thayer, 1909). Many of these markings are apparently found on lepidopteran wings, and break up or mask the contours of the body.

Interestingly, and detrimentally for the credibility of his theories to many at the time, Thayer went to extremes with some of his ideas. For example, he argued that every aspect of a peacock’s coloration was used in concealment (Fig. 3), and that the eyespots on the tail blended into the background vegetation, with the smallest and dimmest spots close to the body, growing bigger and brighter toward the end of the tail, leading attention away from the body. This was, and is, in direct contrast to the idea that many apparently conspicuous markings, such as those of the peacock, are used in sexual selection.

Disruptive coloration is used in military camouflage as well. Fig. 4 shows the ‘‘dazzle’’ coloration of HMS Belfast, which prevents its outline resembling that of a large warship, and supposedly made it difficult to assess its range and speed (Berhens, 2002). This camouflage worked so well that it led to accidental collisions between ships of the Royal Navy (Scott, 1961).

Most of the references to disruptive coloration quote Cott’s classic 1940 book. This is, in part,

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because Cott set out a detailed discussion of the theory, with a series of testable hypotheses. Cott (1940), like Thayer (1909), stated that most animals are active and their movements bring them over a range of changing backgrounds, which themselves are rarely uniform in color or pattern. In addition, the light falling on the background and the animals changes in its intensity and composition. Therefore, even with the processes of countershading (Thayer, 1896; Kiltie, 1988; Edmunds and Dewhirst, 1994), a uniformly colored animal presents to the receiver

Fig. 3. An artistic representation of a peacock in its natural habitat, appearing well concealed. (From Thayer, 1909.)

Fig. 5. A drawing of a snake, showing how the continuous outline of the animal’s body reveals its presence and form. (from Cott, 1940.)

a continuous patch or area, which would stand out strongly from the background. It is the outline of the body that enables a receiver to identify the presence of an animal (Fig. 5). Therefore, for concealment to be effective, the perception of the animal’s form must be prevented.

The function of disruptive coloration is to prevent or delay the visual recognition of the object. When the surface of an animal is covered with irregular patches of contrasting colors or tones, the patches catch the eye of the receiver and draw their attention away from the outline of the animal that bears them (Cott, 1940). The patches concentrate the attention on themselves, while also passing for part of the general environment. This is coupled to the additional effect of breaking up the continuity of the body outline so that it is difficult to determine the form of the animal possessing the markings.

Differential blending

The first key component of effective disruptive coloration that Cott (1940) identified is the process of differential blending, where the patterns that break up the outline of the body, into a series of unrecognizable units, consist of darker and lighter colors (Fig. 6). Some of these patches blend into the background, while others stand out strongly from it. With this arrangement, some pattern elements will fade into the background while others stand out from it strongly. Provided that the animal is seen against a broken background, it is probably true that any pattern of darker and lighter colors and tones will tend to hinder recognition by destroying the animal’s outline, though certain colors, arrangements, and degrees of contrast will be more effective than others. Some of the pattern elements should match the colors found in the general background environment.

Maximum disruptive contrast

The second key component outlined by Cott (1940) is termed maximum disruptive contrast. In this instance, Cott hypothesized that the most effective disruptive patterns would be those that contain elements where the adjacent patterns contrast strongly in tone, since it is important to realize the role played by tone as well as that played by color. The strongly contrasting tones of adjacent patches cause the receiver to distinguish the

two patches as separate objects. Generally, light markings on an otherwise dark object, and dark markings on an otherwise light object will be most effective in creating a disruptive effect, provided that the patterns broadly conform to the background environment (Fig. 7). Cott (1940) also argued that distractive markings need to be used in moderation in relation to the whole area of the body, or else they will fail in their effect. Overall, maximally contrasting pattern elements’ essential function is to break up the continuity of the surface by means of strongly contrasting tones.

Coincident disruptive coloration

A third component proposed by Cott (1940) is termed coincident disruptive coloration. This describes the continuous patterns that range over the appendages and the main body, masking the telltale presence of the otherwise conspicuous appendages. This technique could join separate parts of the body, and unite what are actually discontinuous surfaces (Fig. 8). Cott (1940) argued that coincident markings are particularly common in insects, such as moths, where the patterns span the forewings, hind wings, and the body of the individual when at rest.

Differences between background matching and disruptive coloration

Merilaita (1998) used Cott’s (1940) theorem to compose a set of testable predictions about the

Fig. 7. On the left, an Oleander hawk moth (Daphnis nerii) (top) showing apparent differential blending. Image courtesy of David L. Mohn, copyright Light Creations 1993–2005. On the right, a lime hawk moth (Mimas tiliae) (bottom) showing maximum disruptive contrast. Image courtesy of Mike J. Rubin, copyright M. J. Rubin.

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Fig. 8. An illustration of coincident disruptive coloration in the tree frog Hyla leucophyllata (from Cott, 1940).

distribution, the geometry, and the colors of patterns in disruptive coloration:

1.The elements of background matching patterns should be distributed in a manner that matches the distribution of the pattern elements as found in the background environment. In comparison, a higher number of markings would be expected to touch the outline of the body if forming part of a disruptive effect.

2.The size and shape of pattern elements should be similar to those found in the background environment when forming part of a background matching pattern. In contrast, disruptive elements should be complex and highly variable in size to give the impression of separate objects.

3.The colors of background matching patterns should be similar to those found in the background environment, whereas in disruptive patterns, the elements should also have patterns that are highly contrasting.

Evidence for and against the disruptive coloration hypothesis

Cott described disruptive coloration as ‘‘certainly the most important set of principles relating to

concealment’’, yet in truth, experimental support has been scarce since its proposal.

Invertebrates

The neotropical nymphalid butterfly Anartia fatima exhibits strong polymorphism in the wing banding color on the dorsal surface of males (Emmel, 1973). According to Emmel (1973), mate selection experiments have indicated that both males with the white and males with the yellow bands prefer females with a white band in a ratio of about 2:1, and so the elimination of the yellow band in females must be prevented by some other counterbalancing selective force. That force could be disruptive coloration.

Silberglied et al., (1980) conducted a field study and eliminated the apparently disruptive wing stripe in some palatable Anartia fatima butterflies found in central America and then compared their survival to unaltered individuals in the field. Since the study was conducted on an island, it was possible to capture and mark the majority of the population of butterflies, with high recapture rate, often several times per individual. It was therefore possible to determine the minimum age of every butterfly at each capture and a minimum longevity for each individual in the population. Butterflies were also measured for general condition and a score of wing damage. They found that butterflies without the wing stripe survived equally well as those with the stripe over a 21-week period of testing, and the condition of individuals in the two groups did not differ. However, the vertical wing stripe was obliterated by applying black felt-tip marking pens — these could have influenced the palatability of modified individuals, so that even though they no longer had a vertical wing stripe, they may have had reduced palatability. It should also be noted that there is no evidence that the wing stripes are disruptive in function, and so it is quite possible that they have another unrelated function. A further problem is that the methods may have unintentionally made the butterflies more similar to a cooccurring unpalatable species (Waldbauer and Sternburg, 1983). Additionally, Endler (1984) suggested that obliterating the white stripe simply converted the pattern from

one background matching type to another background matching type of equal crypsis.

In other butterfly species, the presence of wing stripes has also been assumed to be disruptive in function (e.g. Limenitis arthemus; Platt and Brower, 1968; Platt, 1975), adding to the concealment of the individuals by breaking up the outline of the butterfly, especially in flight, but again, with no evidence as support for their claims, especially given that the stripe only touches the body outline at two locations.

Merilaita (1998) appears to have undertaken the only study that directly tests for the presence of disruptive coloration in an animal, as opposed to simple background matching. Merilaita (1998) found that spots mimicking the background were more likely to be found at the edge of the body than would be expected by chance in the whitespotted phenotype albafusca of the polymorphic isopod Idotea baltica. The isopods appear to be cryptic on the brown alga Fucus vesiculosus with its white-colored epizooties Electra crustulenta and Balanus imporvisus. Crypsis via the process of background matching would require that the shapes and sizes of the pattern elements should closely match those of the background, whereas disruptive coloration would involve more marginal elements than expected by the pattern element distribution in the background. Merilaita (1998) recorded for each individual the total number of spots, the number of marginal spots, plus the size and shape of the spots. An index of roundness was also created IR ¼ (P2/4pA) 1, where P is the perimeter and A the area of the spot. When the index equals zero the spot is a perfect circle, and the more complex the shape is and the longer the perimeter in relation to the area, the larger are the values produced. Merilaita (1998) used a random distribution as a null hypothesis for the distribution of the spots on the isopod, which is debatable since the distribution of the spots in the background environment may not have been random, but rather could be clumped. Additionally, the isopod markings may have an additional function, such as providing structural support.

Merilaita (1998) found that spots on the isopods were significantly smaller than E. crustulenta colonies and B. improvisus individuals, plus the

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shape of the spots on the males and females was significantly more complex than the shape of the

E. crustulenta colonies and B. improvisus individuals. The spots on the isopods were also on average more variable than E. crustulenta colonies and B. improvisus individuals in terms of shape and size. Importantly, the number of marginal spots found on the isopods was significantly more than would be expected by chance. Therefore, evidence indicates that I. baltica achieves camouflage based on disruptive coloration, though background matching may also be important.

Disruptive coloration has also been proposed in the gyrodactylid monogenean parasite Macrogyrodactylus polypteri, of the African freshwater fish Polypterus senegalus (Cable et al., 1997). The black-banded gut of the parasite is due to melanin derived from the host epithelial cells in the diet and may be disruptive in making the individual more difficult to detect against the pigmented scales of the host (Cable et al., 1997). Otherwise, the gut of the parasite is colorless and pigmentation of the gut is lost after 5 days without feeding (Cable et al., 1997). The most likely explanation for the banding, suggested by Cable et al., (1997), could be in disruptive concealment from visually hunting predators, though it should be noted that selection pressure on the visual appearance of the parasites in the murky waters may be limited.

Disruptive coloration has been hypothesized in some species of snails, such as the dark stripe down the mid-dorsal part of the body of Clavator moreleti (Emberton, 1994). Again, evidence for a role of disruptive coloration was limited, with little correlation between shell disruptive coloration and aestivation site, with exposure rate to visual predators not investigated (Emberton, 1994). Again, there is no evidence that the stripes are used in concealment, and that they are disruptive, rather than a form of background matching.

Cephalopods have on various occasions been described as possessing disruptive patterns at times (since they are often capable of rapid adaptive color changes by neurally controlled chromatophores). For instance, the squid Loligo pealei is capable of producing a ‘‘banded bottom sitting’’ pattern that apparently produces disruptive coloration against the substrate (Hanlon et al., 1999).

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The pattern is commonly produced and consists of bands with dorsal iridophore ‘‘splotches,’’ and serves to break up the longitudinal shape of the squid (Hanlon et al., 1999). Hanlon and Messenger (1988) proposed that disruptive effects were one of the main types of patterns observed in cuttlefish and are characterized by bold transverse or longitudinal components that can be expressed at different strengths according to the level of contrast between the dark and light patches. Hanlon and Messenger (1988) stated that the principles of disruptive coloration are excellently demonstrated in young cuttlefish, in breaking up the outline of the body. These markings, especially prevalent when the substrate particle size is large, obliterate the appearance of a body well, making concealment highly effective to the human eye (Hanlon and Messenger, 1988). According to Hanlon and Messenger (1988), Cott’s (1940) principle of differential blending is achieved well in the cuttlefish Sepia by some chromatic components blending with the substrate, while others contrast sharply with it, allowing some body parts to stand out strongly, while others fade into the background. Maximum disruptive contrast can also occur when the adjacent components have a high contrast in terms of tone, and there exists a set of patterns of irregular shapes and sizes that are highly contrasting (Hanlon and Messenger, 1988). Even the process of coincident disruptive coloration appears to be present in Sepia, with the eyes concealed with a dark anterior head bar, and the mantle and head can be joined optically by other bars and bands, plus a general color resemblance (Hanlon and Messenger, 1988). Hanlon and Messenger’s study is important since it is a rare example of a detailed consideration of Cott’s (1940) hypotheses, with explanations as to why the authors believe the subjects to be demonstrating disruptive coloration and not just background matching. Chiao and Hanlon (2001) tested Sepia for various forms of camouflage on various sizes of black and white checkerboard squares as substrate. They argued that the results they found supported Edmunds’ (1974) definition of crypsis, in that the animals resembled specific parts of their background, and also that the disruptive patterns fitted into Endler’s (1978) definition that an animal is cryptic if it

resembles a random sample of the background. Chiao and Hanlon (2001) stated that achievement of a disruptive effect is brought about by making chromatic components of their body appear as random samples of the background. Unfortunately this illustrates that Chiao and Hanlon have misunderstood the key principles about disruptive coloration: that disruptive patterns do not represent random samples of the background, but employ a series of adjacent highly contrasting shapes, which are more likely to be found at the margins of the body than would be expected if the animal were actually matching the background. Disruptive coloration and background matching are two logically distinct methods of concealment.

A more recent study in Sepia officinalis by Chiao et al., (2005) extended previous research by investigating camouflage of the cuttlefish on both natural substrate of rocks and laminated images of the substrate. As with the previous studies, the authors failed to show that the markings of the cuttlefish were definitively disruptive as opposed to background matching, but the general findings of the study are interesting for a variety of reasons. Firstly, the animals displayed the disruptive pattern on both the true three-dimensional substrate but also on the laminated images, showing that visual cues are the main (or at least one of the main) cues for producing the appropriate color pattern. Perhaps more interesting, however, was the experiment in which Chiao et al., (2005) manipulated the digital images by removing certain spatial frequencies (such as low-pass and high-pass filterings) to remove certain information about the background, such as edges between objects. Results showed that the cuttlefish did not show disruptive patterns on the high-pass, low-pass, or contrast-enhanced images, leading the authors to conclude that cuttlefish requires both edges and local contrast to recognize the objects. However, it should not be overlooked that the images may have looked strange to the subjects, and that the sample size here was only two.

Fish

Disruptive coloration has also been documented in fish. For instance, the southern mouth brooder

Pseudocrenilabrus philander, a type of cichlid, is