RESUMEN
Avoiding detection can provide significant survival advantages for prey, predators, or the military; conversely, maximizing visibility would be useful for signalling. One simple determinant of detectability is an animal's colour relative to its environment. But identifying the optimal colour to minimize (or maximize) detectability in a given natural environment is complex, partly because of the nature of the perceptual space. Here for the first time, using image processing techniques to embed targets into realistic environments together with psychophysics to estimate detectability and deep neural networks to interpolate between sampled colours, we propose a method to identify the optimal colour that either minimizes or maximizes visibility. We apply our approach in two natural environments (temperate forest and semi-arid desert) and show how a comparatively small number of samples can be used to predict robustly the most and least effective colours for camouflage. To illustrate how our approach can be generalized to other non-human visual systems, we also identify the optimum colours for concealment and visibility when viewed by simulated red-green colour-blind dichromats, typical for non-human mammals. Contrasting the results from these visual systems sheds light on why some predators seem, at least to humans, to have colouring that would appear detrimental to ambush hunting. We found that for simulated dichromatic observers, colour strongly affected detection time for both environments. In contrast, trichromatic observers were more effective at breaking camouflage.
Asunto(s)
Mimetismo Biológico/fisiología , Aprendizaje Profundo , Modelos Biológicos , Pigmentación/fisiología , Percepción Visual/fisiología , Animales , HumanosRESUMEN
The Euclidean and MAX metrics have been widely used to model cue summation psychophysically and computationally. Both rules happen to be special cases of a more general Minkowski summation rule , where m = 2 and ∞, respectively. In vision research, Minkowski summation with power m = 3-4 has been shown to be a superior model of how subthreshold components sum to give an overall detection threshold. Recently, we have previously reported that Minkowski summation with power m = 2.84 accurately models summation of suprathreshold visual cues in photographs. In four suprathreshold discrimination experiments, we confirm the previous findings with new visual stimuli and extend the applicability of this rule to cue combination in auditory stimuli (musical sequences and phonetic utterances, where m = 2.95 and 2.54, respectively) and cross-modal stimuli (m = 2.56). In all cases, Minkowski summation with power m = 2.5-3 outperforms the Euclidean and MAX operator models. We propose that this reflects the summation of neuronal responses that are not entirely independent but which show some correlation in their magnitudes. Our findings are consistent with electrophysiological research that demonstrates signal correlations (r = 0.1-0.2) between sensory neurons when these are presented with natural stimuli.
Asunto(s)
Percepción Auditiva , Umbral Sensorial , Percepción Visual , Estimulación Acústica , Señales (Psicología) , Humanos , Modelos Biológicos , Modelos Estadísticos , Estimulación LuminosaRESUMEN
Juvenile cuttlefish (Sepia officinalis) camouflage themselves by changing their body pattern according to the background. This behaviour can be used to investigate visual perception in these molluscs and may also give insight into camouflage design. Edge detection is an important aspect of vision, and here we compare the body patterns that cuttlefish produced in response to checkerboard backgrounds with responses to backgrounds that have the same spatial frequency power spectrum as the checkerboards, but randomized spatial phase. For humans, phase randomization removes visual edges. To describe the cuttlefish body patterns, we scored the level of expression of 20 separate pattern 'components', and then derived principal components (PCs) from these scores. After varimax rotation, the first component (PC1) corresponded closely to the so-called disruptive body pattern, and the second (PC2) to the mottle pattern. PC1 was predominantly expressed on checkerboards, and PC2 on phase-randomized backgrounds. Thus, cuttlefish probably have edge detectors that control the expression of disruptive pattern. Although the experiments used unnatural backgrounds, it seems probable that cuttlefish display disruptive camouflage when there are edges in the visual background caused by discrete objects such as pebbles. We discuss the implications of these findings for our understanding of disruptive camouflage.
Asunto(s)
Sepia/fisiología , Percepción Visual , Animales , Ecosistema , Humanos , Análisis de Componente Principal , Sepia/anatomía & histologíaRESUMEN
Sensory generalization influences animals' responses to novel stimuli. Because color forms a perceptual continuum, it is a good subject for studying generalization. Moreover, because different causes of variation in spectral signals, such as pigmentation, gloss, and illumination, have differing behavioral significance, it may be beneficial to have adaptable generalization. We report on generalization by poultry chicks following differential training to rewarded (T(+)) and unrewarded (T(-)) colors, in particular on the phenomenon of peak shift, which leads to subjects preferring stimuli displaced away from T(-). The first three experiments test effects of learning either a fine or a coarse discrimination. In experiments 1 and 2, peak shift occurs, but contrary to some predictions, the shift is smaller after the animal learned a fine discrimination than after it learned a coarse discrimination. Experiment 3 finds a similar effect for generalization on a color axis orthogonal to that separating T(+) from T(-). Experiment 4 shows that generalization is rapidly modified by experience. These results imply that the scale of a "perceptual ruler" is set by experience. We show that the observations are consistent with generalization following principles of Bayesian inference, which forms a powerful framework for understanding this type of behavior.
Asunto(s)
Pollos/fisiología , Visión de Colores/fisiología , Generalización del Estimulo/fisiología , Modelos Biológicos , Animales , Teorema de Bayes , MasculinoRESUMEN
Low-level mechanisms in vertebrate vision are sensitive to line orientation. Here we investigate orientation sensitivity in the cuttlefish Sepia pharaonis, by allowing animals to settle on stripe patterns. When camouflaging themselves cuttlefish are known to be sensitive to image parameters such as contrast and spatial scale, but we find no effect of background orientation on the patterns displayed. It is nonetheless clear that the animals see orientation, because they prefer to rest with the body-axis perpendicular to the stripes. We consider three possible mechanisms to account for this behaviour. Firstly, that the body patterns are themselves oriented, and that the cuttlefish align themselves to aid static camouflage. This is unlikely, as the patterns displayed have no dominant orientation at any spatial scale. A second possibility is that motion camouflage favours alignment of the body orthogonal to background stripes, and we suggest how this alignment can minimise motion signals produced by occlusion. Thirdly we show that cuttlefish prefer to rest with their body-axis parallel to the water flow, and it is possible that they use visual patterns such as sand ripples to determine water flow.
Asunto(s)
Conducta Animal/fisiología , Actividad Motora/fisiología , Sepia/fisiología , Pigmentación de la Piel/fisiología , Percepción Visual/fisiología , AnimalesRESUMEN
Flounders and cuttlefish have an impressive ability to change colouration, for camouflage and, in the case of cuttlefish, for communication. We pursue the hypothesis that these diverse patterns are created by combining a small number of distinct pattern modules. Independent component analysis (ICA) is a powerful tool for identifying independent sources of variation in linear mixtures of signals. Two versions of ICA are used, one assuming that sources have independence over time, and the other over space. These reveal the modularity of the skin colouration system, and suggest how the pattern modules are combined in specific behavioural contexts. ICA may therefore be a useful tool for studying animal camouflage and communication.
Asunto(s)
Adaptación Fisiológica/fisiología , Comunicación Animal , Lenguado/fisiología , Modelos Neurológicos , Moluscos/fisiología , Animales , Color , Reconocimiento Visual de Modelos/fisiología , Pigmentación de la Piel/fisiologíaRESUMEN
Sensory-motor integration has frequently been studied using a single-step change in a control variable such as prismatic lens angle and has revealed human visuomotor adaptation to often be partial and inefficient. We propose that the changes occurring in everyday life are better represented as the accumulation of many smaller perturbations contaminated by measurement noise. We have therefore tested human performance to random walk variations in the visual feedback of hand movements during a pointing task. Subjects made discrete targeted pointing movements to a visual target and received terminal feedback via a cursor the position of which was offset from the actual movement endpoint by a random walk element and a random observation element. By applying ideal observer analysis, which for this task compares human performance against that of a Kalman filter, we show that the subjects' performance was highly efficient with Fisher efficiencies reaching 73%. We then used system identification techniques to characterize the control strategy used. A "modified" delta-rule algorithm best modeled the human data, which suggests that they estimated the random walk perturbation of feedback in this task using an exponential weighting of recent errors. The time constant of the exponential weighting of the best-fitting model varied with the rate of random walk drift. Because human efficiency levels were high and did not vary greatly across three levels of observation noise, these results suggest that the algorithm the subjects used exponentially weighted recent errors with a weighting that varied with the level of drift in the task to maintain efficient performance.
Asunto(s)
Algoritmos , Desempeño Psicomotor , Visión Ocular , Adulto , Retroalimentación , Femenino , Mano/fisiología , Humanos , Cinética , Masculino , Modelos Teóricos , MovimientoRESUMEN
Spectral stimuli form a physical continuum, which humans divide into discrete non-overlapping regions or categories that are designated by colour names. Little is known about whether non-verbal animals form categories on stimulus continua, but work in psychology and artificial intelligence provides models for stimulus generalization and categorization. We compare predictions of such models to the way poultry chicks (Gallus gallus) generalize to novel stimuli following appetitive training to either one or two colours. If the two training colours are (to human eyes) red and greenish-yellow or green and blue, chicks prefer intermediates, i.e. orange rather than red or yellow and turquoise rather than green or blue. The level of preference for intermediate colours implies that the chicks interpolate between the training stimuli. However, they do not extrapolate beyond the limits set by the training stimuli, at least for red and yellow training colours. Similarly, chicks trained to red and blue generalize to purple, but they do not generalize across grey after training to the complementary colours yellow and blue. These results are consistent with a modified version of a Bayesian model of generalization from multiple examples that was proposed by Shepard and show similarities to human colour categorization.
Asunto(s)
Pollos/fisiología , Percepción de Color , Animales , Generalización del Estimulo , Humanos , Aprendizaje , Visión OcularRESUMEN
Recently, it has been proposed that all suppressive phenomena observed in the primary visual cortex (V1) are mediated by a single mechanism, involving inhibition by pools of neurons, which, between them, represent a wide range of stimulus specificities. The strength of such inhibition would depend on the stimulus that produces it (particularly its contrast) rather than on the firing rate of the inhibited cell. We tested this hypothesis by measuring contrast-response functions (CRFs) of neurons in cat V1 for stimulation of the classical receptive field of the dominant eye with an optimal grating alone, and in the presence of inhibition caused by (1) a superimposed orthogonal grating (cross-orientation inhibition); (2) a surrounding iso-oriented grating (surround inhibition); and (3) an orthogonal grating in the other eye (interocular suppression). We fitted hyperbolic ratio functions and found that the effect of cross-orientation inhibition was best described as a rightward shift of the CRF ('contrast-gain control'), while surround inhibition and interocular suppression were primarily characterised as downward shifts of the CRF ('response-gain control'). However, the latter also showed a component of contrast-gain control. The two modes of suppression were differently distributed between the layers of cortex. Response-gain control prevailed in layer 4, whereas cells in layers 2/3, 5 and 6 mainly showed contrast-gain control. As in human observers, surround gratings caused suppression when the central grating was of high contrast, but in over a third of the cells tested, enhanced responses for low-contrast central stimuli, hence actually decreasing threshold contrast.
Asunto(s)
Inhibición Neural/fisiología , Corteza Visual/fisiología , Potenciales de Acción , Animales , Mapeo Encefálico , Gatos , Reconocimiento Visual de Modelos/fisiología , RotaciónRESUMEN
A neural net method is used to extract principal components from real-world images. The initial components are a Gaussian followed by horizontal and vertical operators, starting with the first derivative and moving to successively higher orders. Two of the components are 'bar-detectors'. Their measured orientation selectivity is similar to that suggested by Foster & Ward (Proc. R. Soc. Lond. B 243, 75 (1991] to account for brief-exposure psychophysical data. In tests with noise images, the ratio of sensitivity between the two components is controlled by the degree of anisotropy in the image.