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Motion Dazzle and the Effects of Target Patterning on Capture Success Anna E Hughes1*, Jolyon Troscianko2 and Martin Stevens2

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Motion Dazzle and the Effects of Target Patterning on Capture Success Anna E Hughes1*, Jolyon Troscianko2 and Martin Stevens2 Hughes et al. BMC Evolutionary Biology 2014, 14:201 http://www.biomedcentral.com/1471-2148/14/201 RESEARCH ARTICLE Open Access Motion dazzle and the effects of target patterning on capture success Anna E Hughes1*, Jolyon Troscianko2 and Martin Stevens2 Abstract Background: Stripes and other high contrast patterns found on animals have been hypothesised to cause “motion dazzle”, a type of defensive coloration that operates when in motion, causing predators to misjudge the speed and direction of object movement. Several recent studies have found some support for this idea, but little is currently understood about the mechanisms underlying this effect. Using humans as model ‘predators’ in a touch screen experiment we investigated further the effectiveness of striped targets in preventing capture, and considered how stripes compare to other types of patterning in order to understand what aspects of target patterning are important in making a target difficult to capture. Results: We find that striped targets are among the most difficult to capture, but that other patterning types are also highly effective at preventing capture in this task. Several target types, including background sampled targets and targets with a ‘spot’ on were significantly easier to capture than striped targets. We also show differences in capture attempt rates between different target types, but we find no differences in learning rates between target types. Conclusions: We conclude that striped targets are effective in preventing capture, but are not uniquely difficult to catch, with luminance matched grey targets also showing a similar capture rate. We show that key factors in making capture easier are a lack of average background luminance matching and having trackable ‘features’ on the target body. We also find that striped patterns are attempted relatively quickly, despite being difficult to catch. We discuss these findings in relation to the motion dazzle hypothesis and how capture rates may be affected more generally by pattern type. Keywords: Motion dazzle, Vision, Animal coloration Background these periods. It is therefore important to ask whether Camouflage can be defined as “strategies involved in some types of patterning can provide protection from concealment, including prevention of detection and predator attack when in motion. recognition” [1], and offers an important anti-predator Recent research into this area has focused specifically defence for many species. One common type of camou- on concept of ‘motion dazzle’, where high contrast stripe flage is crypsis, which helps to prevent initial detection and zigzag markings are hypothesised to ‘dazzle’ an ap- of an object through a variety of mechanisms, from proaching predator, making it difficult for them to judge simple background matching through to disruptive cam- the speed and direction of the animal’s movement [1]. ouflage and self-shadow concealment [2]. Many studies This concept was proposed by Thayer and others and of camouflage have considered the case in which a prey was applied in World War I and II to the painting of item is stationary on its background (e.g. [3-9]). How- some Allied ships [10]. It was theorised that using strik- ever, most animals cannot remain still indefinitely and ing geometric patterns would make it difficult to target often need to move to find food and mates, becoming a moving ship accurately by making the trajectory or especially vulnerable to detection and attack during speed of the vessel difficult to estimate, although evi- dence obtained at the time was inconclusive about the * Correspondence: [email protected] effectiveness of the dazzle markings [11]. From an eco- 1 Department of Physiology, Development, and Neuroscience, University of logical perspective, several studies have suggested that the Cambridge, Downing Street, Cambridge CB2 3EG, UK Full list of author information is available at the end of the article high contrast markings seen on animals such as zebras and © 2014 Hughes et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Hughes et al. BMC Evolutionary Biology 2014, 14:201 Page 2 of 10 http://www.biomedcentral.com/1471-2148/14/201 some snakes may create visual illusions, distorting speed or camouflage markings may offer good protection when in direction perception in some manner [12-16]. motion as well as when stationary. However, other results The first experimental test of the motion dazzle hy- have suggested that background matching may not be as pothesis was conducted by Stevens and colleagues, who effective as ‘motion dazzle’ patterning; in one study, back- ran human artificial ‘prey’ capture experiments where ground matching camouflaged stimuli were found to be subjects were required to attempt to capture moving tar- harder to detect than high contrast striped stimuli when gets in a computer game. They found that some targets they were stationary, but were caught more often than the with highly conspicuous patterns, such as bands and same stimuli when they were moving [21]. It is therefore zigzags, were more difficult to capture than uniformly also unclear to what extent cryptic camouflage strategies coloured, luminance matched conspicuous stimuli [17]. offer protection against capture in motion. Scott-Samuel and colleagues investigated the potential It is not fully understood what aspects of a target’s mechanisms behind dazzle camouflage, in a task where pattern are most critical for making capture difficult, human subjects were asked to judge which of two tar- and by what mechanism motion dazzle could work. One gets appeared to be moving faster, in order to determine phenomenon which may be of critical importance is the point at which they were subjectively perceived to be “aperture problem”, which occurs because the cells travelling at the same speed. They found that dazzle pat- involved in the early stages of the human visual system terning could affect speed perception in humans, with in processing motion information have small, local re- high contrast patterns causing a reduction in perceived ceptive fields. The motion of a line through these recep- speed [18]. However, they did not find a significant effect tive fields is ambiguous for motion parallel to the line for striped stimuli, with only some high contrast 2D pat- itself, with only movement perpendicular to the line terns (such as zigzags and checks) causing perceived being detectable [22]. Unambiguous ‘features’ in the speed reductions. image such as corners, contour end points or points of However, not all research has supported the motion high curvature do not suffer from the aperture problem dazzle hypothesis. One study combined a number of and therefore may be used to estimate the true speed approaches, asking human participants to both attempt and direction of motion [23-28]. These features may be to capture moving stimuli and also to make perceptual detected by end stopped cells in primary visual cortex in judgements of speed in separate experiments [19]. This primates [29], which allows segmentation of the visual challenged the finding that high contrast patterning scene [30] and then tracking of the features over time causes a reduction in perceived speed, suggesting that [31,32]. It has been hypothesised that the terminator striped targets are in fact perceived as moving faster signals may then propagate along the ambiguous seg- than non-patterned targets and also arguing that striped ments of contours, constraining the velocity and direc- targets are no more difficult, or perhaps sometimes eas- tion signals, allowing recovery of object motion [31]. It ier, than non-patterned targets to capture [19]. A further may therefore be the case that targets without obvious study used cuttlefish (Sepia officinalis) as a means to test features may be most difficult to catch. In addition, there whether dazzle patterning is observed in a more natural may be some feature types that are better at preventing system [20]. As cuttlefish are able to change their body the aperture effect and allowing the true motion to be patterns rapidly in response to their surroundings, the determined than others. Recent research involving mod- researchers asked whether they would be more likely elling potential motion detection mechanisms in vision to use low contrast mottled patterns or high contrast showed that the stripes on zebra create erroneous infor- ‘dazzle’ patterns when in motion. They found that cuttle- mation about direction of movement, much more so fish reduced the amount of high contrast patterns they than unpatterned horses [16]. The researchers hypothe- displayed when in motion, and concluded that they did sised that these erroneous motion signals may be caused not use motion dazzle patterning for the purpose of both by the aperture effect and the wagon-wheel effect, motion camouflage. Similarly, several human studies where motion is perceived to be inverted by spatio- have found that low contrast striped targets are more temporal aliasing (although there is still debate as to difficult to capture than high contrast ones [17,21]. There
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