The 13 th AVA Christmas Meeting Bristol 19 th December , 2008

Getting distracted: camouflage and attention Innes C. Cuthill*, Tom Troscianko & Emma Howell* (*School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK † Department of Experimental Psychology, Woodland Road, Bristol BS8 1TN, UK) Camouflage, in animals or in military applications, comes in many forms. Background matching, or crypsis, minimises detection through the target falling (well) within the sampling distribution of background colours, textures, or features. Alteratively, or in addition, camouflage can impair target recognition, either through direct mimicry of an irrelevant background object, or the use of disruptive coloration to break up shape and form. In fact, several phenomena are grouped under the heading 'disruptive' coloration and these may exploit different perceptual mechanisms. For example, high contrast colour patches at the body’s edge exploit edge detectors in the viewer’s visual system to create false contours of higher salience than the weak (true) contours at the target’s periphery (Stevens & Cuthill 2006, Proc R Soc B, 273, 2141–2147). Here, low-level visual processes are fooled. However, the pioneer of camouflage theory, Abbott Thayer (1909; Concealing-Coloration in the Animal Kingdom. Macmillan, New York) also used the term ‘dazzle coloration’ with respect to disruptive markings, and John Endler (2006, Proc R Soc B 273, 24252426), like many before, considered that “conspicuous elements distract the predator’s attention”. The counter-intuitive proposal is that conspicuous markings can aid concealment precisely because they attract attention (away from features that might identify the target). We review the literature on disruptive camouflage and present data from human visual search experiments that test the proposition of ‘distractive camouflage’. We show that the effects depend crucially on the prevalence of both similar objects and features in the background, and the prior expectation of subjects undertaking the search task. What is binocular vision for? Clues from birds Graham Martin (School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK; e-mail: [email protected]) A large frontal binocular field, produced by widely-spaced forward-facing eyes with parallel axes and conjugate movements, is a significant feature of vision in only some vertebrates. In the majority of vertebrates binocular fields are quite different. In birds, for example, in which the eyes placed close together and project laterally from the skull, binocular fields are typically small, vertically long and narrow (Martin, 2007, J. Ornithol 148, 547-562). Furthermore, there seems to be an evolutionary trade-off between frontal binocularity and comprehensive vision and, due to eye movements, binocular fields can be spontaneously abolished or made asymmetric. What are the functions of binocular vision in such species? Stereopsis seems an unlikely function since gaining relative depth from small disparity cues is probably too slow a process to guide a fast manoeuvring animal (Davies & Green, 1994, Perception and motor control in birds: an ecological approach, 339-356. Springer). The spontaneous abolition of binocularity in many birds would suggest that each eye functions independently. It is proposed here that in the majority of birds, and perhaps in many other vertebrates, binocularity does not have a specific or special function. Binocularity is best viewed as a by product of the need to have a portion of each eye’s visual field projecting in the direction of head movement (during