Proceedings of the Royal Society of London. Series B: Biological Sciences, 1990
The rhabdoms of the eyes of butterflies can support one, two, or more waveguide modes, depending ... more The rhabdoms of the eyes of butterflies can support one, two, or more waveguide modes, depending on their diameter. The pupil mechanism consists of pigment granules that migrate towards the rhabdom in the light, and absorb the modal light that travels outside the rhabdom. Because there is relatively more extra-rhabdomal light in the higher-order modes, these are shed earlier and at lower light intensities, than the first mode. The effect of this is to reduce the acceptance angle of each ommatidium. We have measured the extent and timecourse of this change by both optical and electrophysiological methods. In small butterflies whose rhabdoms support only one mode there is no change. In Nymphalids such as Vanessa itea, where there are two modes, the reduction is by ca. 30 % and in the crepuscular Satyrid Melanitis leda where three or more modes are present the reduction is almost 50%, from 2.9° to 1.5°. These pupil-induced acuity changes will occur towards the lower end of the range of environmental luminances seen by the butterfly during natural flight.
Cognitive dimensions of predator responses to imperfect mimicry Article (Published Version) http:... more Cognitive dimensions of predator responses to imperfect mimicry Article (Published Version) http://sro.sussex.ac.uk Chittka, Lars and Osorio, Daniel (2007) Cognitive dimensions of predator responses to imperfect mimicry. PLoS Biology, 5 (2). e339.
Animals use colour vision in a range of behaviours. Visual performance is limited by thresholds, ... more Animals use colour vision in a range of behaviours. Visual performance is limited by thresholds, which are set by noise in photoreceptors and subsequent neural processing. The receptor noise limited (RNL) model of colour discrimination is widely used for modelling colour vision and accounts well for experimental data from many species. In one of the most comprehensive tests yet of colour discrimination in a non-human species, we used Ishihara-style stimulus patterns to examine thresholds for 21 directions at five locations in colour space for the fish Rhinecanthus aculeatus. Thresholds matched RNL model predictions most closely for stimuli near the achromatic point, but exceeded predictions (indicating a decline in sensitivity) with distance from this point. Thresholds were also usually higher for saturation than for hue differences. These changes in colour threshold with colour space location and direction may give insight into photoreceptor non-linearities and post-receptoral mech...
Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foragin... more Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, even though animal visual systems differ from our own. Behavioural tests of non-human animals are required to understand how colour signals are perceived by them. Here we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. These charts consist of dots that vary in colour, brightness and size, and are designed so that a numeral or letter is distinguishable from distractor dots for humans with normal colour vision. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide Matlab code for creating these stimuli, which can be modified for use with different animals. We demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, and highlight behavioural parameters that can be measured, including success of finding the target dot, time to detect dot and error rate. Triggerfish quickly learnt to select target dots that differed from distractors dots regardless of the particular hue or saturation, and proved to use acute colour vision. We measured discrimination thresholds by testing the detection of target colours that were of increasing colour distances (∆S) from distractor dots in different directions of colour space. At least for some colours, thresholds indicated better discrimination than expected from the Receptor Noise Limited (RNL) model assuming 5% Weber fraction for the long-wavelength cone. This methodology seems to be highly effective because it resembles natural foraging behavior for the triggerfish and may well be adaptable to a range of other animals, including mammals, birds, bees and freshwater fish. Other questions may be addressed using this methodology, including luminance thresholds, sensory bias, effects of sensory noise in detection tasks, colour categorization and salienc
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (Δ<i>S</i>) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or JND, just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed 'suprathreshold' colour differences—often assumes that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated...
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (ΔS) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or JND, just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed ‘suprathreshold’ colour differences—often assumes that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between Δ...
Proceedings of the Royal Society B: Biological Sciences, 2020
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (Δ S ) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed ‘suprathreshold’ colour differences—often assume that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between Δ S a...
Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foragin... more Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, and behavioural tests of non-human animals are required to understand how colour signals are perceived. Here, we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide MATLAB code for creating these stimuli, which can be modified for use with different animals. We demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, which quickly learnt to select target dots that differed from distractor dots, and highlight behavioural parameters tha...
Colour patterns displayed by animals frequently comprise multiple elements, including hue, patter... more Colour patterns displayed by animals frequently comprise multiple elements, including hue, pattern, luminance and texture. Predators' perception of and learning about visual stimuli has important implications for the evolution of animal coloration, including aposematism and mimicry. This study investigated how a coral reef fish, the triggerfish Rhinecanthus aculeatus, learnt different elements of colour patterns. Fish trained to associate a food reward with blue, yellow and green patterns on a grey background selected novel stimuli by chromaticity, rather than pattern or luminance contrast. By comparison, when presented with small orange spots the fish appeared to learn luminance, which is consistent with findings in other animals, including bees, birds and humans, that for small objects the achromatic component of the signal is more salient than chromaticity. While internal pattern did not appear to be learnt in our first two experiments, a subsequent test showed that fish could distinguish between spotted and striped patterns over various sizes, up to the limits of their visual acuity. These results are discussed in relation to visual processing of colour patterns and the evolution of visual signals in the marine environment.
Proceedings of the Royal Society B: Biological Sciences, 2016
Humans use shading as a cue to three-dimensional form by combining low-level information about li... more Humans use shading as a cue to three-dimensional form by combining low-level information about light intensity with high-level knowledge about objects and the environment. Here, we examine how cuttlefishSepia officinalisrespond to light and shadow to shade the white square (WS) feature in their body pattern. Cuttlefish display the WS in the presence of pebble-like objects, and they can shade it to render the appearance of surface curvature to a human observer, which might benefit camouflage. Here we test how they colour the WS on visual backgrounds containing two-dimensional circular stimuli, some of which were shaded to suggest surface curvature, whereas others were uniformly coloured or divided into dark and light semicircles. WS shading, measured by lateral asymmetry, was greatest when the animal rested on a background of shaded circles and three-dimensional hemispheres, and less on plain white circles or black/white semicircles. In addition, shading was enhanced when light fell ...
Colour vision and colour signals are important to aquatic animals, but light scattering and absor... more Colour vision and colour signals are important to aquatic animals, but light scattering and absorption by water distorts spectral stimuli. To investigate the performance of colour vision in water, and to suggest how photoreceptor spectral sensitivities and body colours might evolve for visual communication, we model the effects of changes in viewing distance and depth on the appearance of fish colours for three teleosts: a barracuda, Syphraena helleri, which is dichromatic, and two damselfishes, Chromis verater and C. hanui, which are trichromatic. We assume that photoreceptors light-adapt to the background, thereby implementing the von Kries transformation, which can largely account for colour constancy in humans and other animals, including fish. This transformation does not however compensate for light scattering over variable viewing distances, which in less than a metre seriously impairs dichromatic colour vision, and makes judgement of colour saturation unreliable for trichrom...
Proceedings of the Royal Society of London. Series B: Biological Sciences, 1990
The rhabdoms of the eyes of butterflies can support one, two, or more waveguide modes, depending ... more The rhabdoms of the eyes of butterflies can support one, two, or more waveguide modes, depending on their diameter. The pupil mechanism consists of pigment granules that migrate towards the rhabdom in the light, and absorb the modal light that travels outside the rhabdom. Because there is relatively more extra-rhabdomal light in the higher-order modes, these are shed earlier and at lower light intensities, than the first mode. The effect of this is to reduce the acceptance angle of each ommatidium. We have measured the extent and timecourse of this change by both optical and electrophysiological methods. In small butterflies whose rhabdoms support only one mode there is no change. In Nymphalids such as Vanessa itea, where there are two modes, the reduction is by ca. 30 % and in the crepuscular Satyrid Melanitis leda where three or more modes are present the reduction is almost 50%, from 2.9° to 1.5°. These pupil-induced acuity changes will occur towards the lower end of the range of environmental luminances seen by the butterfly during natural flight.
Cognitive dimensions of predator responses to imperfect mimicry Article (Published Version) http:... more Cognitive dimensions of predator responses to imperfect mimicry Article (Published Version) http://sro.sussex.ac.uk Chittka, Lars and Osorio, Daniel (2007) Cognitive dimensions of predator responses to imperfect mimicry. PLoS Biology, 5 (2). e339.
Animals use colour vision in a range of behaviours. Visual performance is limited by thresholds, ... more Animals use colour vision in a range of behaviours. Visual performance is limited by thresholds, which are set by noise in photoreceptors and subsequent neural processing. The receptor noise limited (RNL) model of colour discrimination is widely used for modelling colour vision and accounts well for experimental data from many species. In one of the most comprehensive tests yet of colour discrimination in a non-human species, we used Ishihara-style stimulus patterns to examine thresholds for 21 directions at five locations in colour space for the fish Rhinecanthus aculeatus. Thresholds matched RNL model predictions most closely for stimuli near the achromatic point, but exceeded predictions (indicating a decline in sensitivity) with distance from this point. Thresholds were also usually higher for saturation than for hue differences. These changes in colour threshold with colour space location and direction may give insight into photoreceptor non-linearities and post-receptoral mech...
Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foragin... more Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, even though animal visual systems differ from our own. Behavioural tests of non-human animals are required to understand how colour signals are perceived by them. Here we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. These charts consist of dots that vary in colour, brightness and size, and are designed so that a numeral or letter is distinguishable from distractor dots for humans with normal colour vision. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide Matlab code for creating these stimuli, which can be modified for use with different animals. We demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, and highlight behavioural parameters that can be measured, including success of finding the target dot, time to detect dot and error rate. Triggerfish quickly learnt to select target dots that differed from distractors dots regardless of the particular hue or saturation, and proved to use acute colour vision. We measured discrimination thresholds by testing the detection of target colours that were of increasing colour distances (∆S) from distractor dots in different directions of colour space. At least for some colours, thresholds indicated better discrimination than expected from the Receptor Noise Limited (RNL) model assuming 5% Weber fraction for the long-wavelength cone. This methodology seems to be highly effective because it resembles natural foraging behavior for the triggerfish and may well be adaptable to a range of other animals, including mammals, birds, bees and freshwater fish. Other questions may be addressed using this methodology, including luminance thresholds, sensory bias, effects of sensory noise in detection tasks, colour categorization and salienc
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (Δ<i>S</i>) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or JND, just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed 'suprathreshold' colour differences—often assumes that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated...
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (ΔS) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or JND, just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed ‘suprathreshold’ colour differences—often assumes that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between Δ...
Proceedings of the Royal Society B: Biological Sciences, 2020
To be effective, animal colour signals must attract attention—and therefore need to be conspicuou... more To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (Δ S ) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed ‘suprathreshold’ colour differences—often assume that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between Δ S a...
Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foragin... more Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, and behavioural tests of non-human animals are required to understand how colour signals are perceived. Here, we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide MATLAB code for creating these stimuli, which can be modified for use with different animals. We demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, which quickly learnt to select target dots that differed from distractor dots, and highlight behavioural parameters tha...
Colour patterns displayed by animals frequently comprise multiple elements, including hue, patter... more Colour patterns displayed by animals frequently comprise multiple elements, including hue, pattern, luminance and texture. Predators' perception of and learning about visual stimuli has important implications for the evolution of animal coloration, including aposematism and mimicry. This study investigated how a coral reef fish, the triggerfish Rhinecanthus aculeatus, learnt different elements of colour patterns. Fish trained to associate a food reward with blue, yellow and green patterns on a grey background selected novel stimuli by chromaticity, rather than pattern or luminance contrast. By comparison, when presented with small orange spots the fish appeared to learn luminance, which is consistent with findings in other animals, including bees, birds and humans, that for small objects the achromatic component of the signal is more salient than chromaticity. While internal pattern did not appear to be learnt in our first two experiments, a subsequent test showed that fish could distinguish between spotted and striped patterns over various sizes, up to the limits of their visual acuity. These results are discussed in relation to visual processing of colour patterns and the evolution of visual signals in the marine environment.
Proceedings of the Royal Society B: Biological Sciences, 2016
Humans use shading as a cue to three-dimensional form by combining low-level information about li... more Humans use shading as a cue to three-dimensional form by combining low-level information about light intensity with high-level knowledge about objects and the environment. Here, we examine how cuttlefishSepia officinalisrespond to light and shadow to shade the white square (WS) feature in their body pattern. Cuttlefish display the WS in the presence of pebble-like objects, and they can shade it to render the appearance of surface curvature to a human observer, which might benefit camouflage. Here we test how they colour the WS on visual backgrounds containing two-dimensional circular stimuli, some of which were shaded to suggest surface curvature, whereas others were uniformly coloured or divided into dark and light semicircles. WS shading, measured by lateral asymmetry, was greatest when the animal rested on a background of shaded circles and three-dimensional hemispheres, and less on plain white circles or black/white semicircles. In addition, shading was enhanced when light fell ...
Colour vision and colour signals are important to aquatic animals, but light scattering and absor... more Colour vision and colour signals are important to aquatic animals, but light scattering and absorption by water distorts spectral stimuli. To investigate the performance of colour vision in water, and to suggest how photoreceptor spectral sensitivities and body colours might evolve for visual communication, we model the effects of changes in viewing distance and depth on the appearance of fish colours for three teleosts: a barracuda, Syphraena helleri, which is dichromatic, and two damselfishes, Chromis verater and C. hanui, which are trichromatic. We assume that photoreceptors light-adapt to the background, thereby implementing the von Kries transformation, which can largely account for colour constancy in humans and other animals, including fish. This transformation does not however compensate for light scattering over variable viewing distances, which in less than a metre seriously impairs dichromatic colour vision, and makes judgement of colour saturation unreliable for trichrom...
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