Edge enhancement improves disruptive camouflage by emphasising false edges and creating pictorial relief

Abstract

Disruptive colouration is a visual camouflage composed of false edges and boundaries. Many disruptively camouflaged animals feature enhanced edges; light patches are surrounded by a lighter outline and/or a dark patches are surrounded by a darker outline. This camouflage is particularly common in amphibians, reptiles and lepidopterans. We explored the role that this pattern has in creating effective camouflage. In a visual search task utilising an ultra-large display area mimicking search tasks that might be found in nature, edge enhanced disruptive camouflage increases crypsis, even on substrates that do not provide an obvious visual match. Specifically, edge enhanced camouflage is effective on backgrounds both with and without shadows; i.e. this is not solely due to background matching of the dark edge enhancement element with the shadows. Furthermore, when the dark component of the edge enhancement is omitted the camouflage still provided better crypsis than control patterns without edge enhancement. This kind of edge enhancement improved camouflage on all background types. Lastly, we show that edge enhancement can create a perception of multiple surfaces. We conclude that edge enhancement increases the effectiveness of disruptive camouflage through mechanisms that may include the improved disruption of the object outline by implying pictorial relief.

A Pixel-Encoder Retinal Ganglion Cell with Spatially Offset Excitatory and Inhibitory Receptive Fields

“…Many RGCs show attenuated responses to large stimuli ( Kuffler, 1953 ; Demb and Singer, 2015 ). This enhances edges in their representation of the retinal image, which in turn facilitates pattern and object recognition in the brain ( Egan et al, 2016 ; Biederman and Ju, 1988 ). To explore the stimulus size tuning of Pix ON -RGCs, we presented spots of varying diameter (20–1,200 µm) in pseudorandom sequences.…”

Disruptive coloration and binocular disparity: breaking camouflage

“…We determined that to detect an effect of edge enhancement similar in magnitude to that reported previously [13], with 90% power, we required eight participants. However, to detect an interaction between edge enhancement and disparity would require 26 observers (M ore P ower 6.0; [34]) under the hypothesis that the presence of disparity halves the effect of edge enhancement.…”

Camouflage

“…Visual search experiments with humans shows that such edge enhancement is highly effective (Egan et al, 2016). It is the salience of the false edges and the contrasting surfaces within those boundaries that creates the 'apparent but unreal configuration' which Cott referred to.…”

“…As such, disruptive coloration is likely to work best in environments which possess differently coloured objects with clearly defined edges; possessing colour patches that match different objects enhances the likelihood that each patch on the target segregates with a different background object rather than as a coherent whole (Espinosa & Cuthill, ). Having surfaces that appear to lie at different distances from the viewer (different depth planes) is also effective (Egan et al ., ), because surfaces that lie in different depth planes are less likely to ‘belong together’ than those in the same plane. Cott (), as Thayer () before him, drew attention to what appear to be false depth cues in animal camouflage, something he called ‘pictorial relief’.…”