Chuan-Chin Chiao

Statistics of cone responses to natural images: implications for visual coding

We gathered hyperspectral images of natural, foliage-dominated scenes and converted them to human cone quantal catches to characterize the second-order redundancy present within the retinal photoreceptor array under natural conditions. The data are expressed most simply in a logarithmic response space, wherein an orthogonal decorrelation robustly produces three principal axes, one corresponding to simple changes in radiance and two that are reminiscent of the blue–yellow and red–green chromatic-opponent mechanisms found in the primate visual system. Further inclusion of spatial stimulus dimensions demonstrates a complete spatial decorrelation of these three cone-space axes in natural cone responses.

Cephalopod dynamic camouflage: bridging the continuum between background matching and disruptive coloration

Individual cuttlefish, octopus and squid have the versatile capability to use body patterns for background matching and disruptive coloration. We define—qualitatively and quantitatively—the chief characteristics of the three major body pattern types used for camouflage by cephalopods: uniform and mottle patterns for background matching, and disruptive patterns that primarily enhance disruptiveness but aid background matching as well. There is great variation within each of the three body pattern types, but by defining their chief characteristics we lay the groundwork to test camouflage concepts by correlating background statistics with those of the body pattern. We describe at least three ways in which background matching can be achieved in cephalopods. Disruptive patterns in cuttlefish possess all four of the basic components of ‘disruptiveness’, supporting Cotts hypotheses, and we provide field examples of disruptive coloration in which the body pattern contrast exceeds that of the immediate surrounds. Based upon laboratory testing as well as thousands of images of camouflaged cephalopods in the field (a sample is provided on a web archive), we note that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Mottle and disruptive patterns are frequently mixed, suggesting that background matching and disruptive mechanisms are often used in the same pattern.

Color signals in natural scenes: characteristics of reflectance spectra and effects of natural illuminants

Multispectral images of natural scenes were collected from both forests and coral reefs to represent typical, complex scenes that might be viewed by modern animals. Both reflectance spectra and modeled visual color signals in these scenes were decorrelated spectrally by principal-component analysis. Nearly 98% of the variance of reflectance spectra and color signals can be described by the first three principal components for both forest and coral reef scenes, which implies that three well-designed visual channels can recover almost all of the spectral information of natural scenes. A variety of natural illuminants affects color signals of forest scenes only slightly, but the variation in ambient irradiance spectra that is due to the absorption of light by water has dramatic influences on the spectral characteristics of coral reef scenes.

Spectral tuning of dichromats to natural scenes

Multispectral images of natural scenes were collected from both forests and coral reefs. We varied the wavelength position of receptors in hypothetical dichromatic visual systems and, for each receptor pair estimated the percentage of discriminable points in natural scenes. The optimal spectral tuning predicted by this model results in photoreceptor pairs very like those of forest dwelling, dichromatic mammals and of coral reef fishes. Variations of the natural illuminants in forests have little or no effect on optimal spectral tuning, but variations of depth in coral reefs have moderate effects on the spectral placement of S and L cones. The ratio of S and L cones typically found in dichromatic mammals reduces the discriminability of forest scenes; in contrast, the typical ratio of S and L cones in coral reef fishes achieves nearly the optimal discrimination in coral reef scenes.

Disruptive Body Patterning of Cuttlefish (Sepia officinalis) Requires Visual Information Regarding Edges and Contrast of Objects in Natural Substrate Backgrounds

Cuttlefish (Sepia officinalis Linnaeus, 1758) on mixed light and dark gravel show disruptive body patterns for camouflage. This response is evoked when the size of the gravel is equivalent to the area of the “White square,” a component of its dorsal mantle patterns. However, the features of natural substrates that cuttlefish cue on visually are largely unknown. Therefore, we aimed to identify those visual features of background objects that are required to evoke disruptive coloration. At first, we put young cuttlefish in a circular experimental arena, presented them with natural gravel and photographs of natural gravel, and established that the animals would show a disruptive pattern when presented either with three-dimensional natural gravel or its two-dimensional photographic representation. We then manipulated the digital photographs by applying (i) a low-pass filter to remove the edges of the fragments of gravel, and (ii) a high-pass filter to remove the contrast among them. The body patterns produced by the cuttlefish in response to these altered visual stimuli were then videorecorded and graded. The results show that, to evoke disruptive coloration in cuttlefish, visual information about the edges and contrast of objects within natural substrate backgrounds is required. Cephalopods have a remarkable ability to change the color and pattern of their skin, and research has demonstrated that visual input regulates these changes. Cuttlefish skin can show 20‐50 chromatophore patterns that are used

Hyperspectral imaging of cuttlefish camouflage indicates good color match in the eyes of fish predators

Camouflage is a widespread phenomenon throughout nature and an important antipredator tactic in natural selection. Many visual predators have keen color perception, and thus camouflage patterns should provide some degree of color matching in addition to other visual factors such as pattern, contrast, and texture. Quantifying camouflage effectiveness in the eyes of the predator is a challenge from the perspectives of both biology and optical imaging technology. Here we take advantage of hyperspectral imaging (HSI), which records full-spectrum light data, to simultaneously visualize color match and pattern match in the spectral and the spatial domains, respectively. Cuttlefish can dynamically camouflage themselves on any natural substrate and, despite their colorblindness, produce body patterns that appear to have high-fidelity color matches to the substrate when viewed directly by humans or with RGB images. Live camouflaged cuttlefish on natural backgrounds were imaged using HSI, and subsequent spectral analysis revealed that most reflectance spectra of individual cuttlefish and substrates were similar, rendering the color match possible. Modeling color vision of potential di- and trichromatic fish predators of cuttlefish corroborated the spectral match analysis and demonstrated that camouflaged cuttlefish show good color match as well as pattern match in the eyes of fish predators. These findings (i) indicate the strong potential of HSI technology to enhance studies of biological coloration and (ii) provide supporting evidence that cuttlefish can produce color-coordinated camouflage on natural substrates despite lacking color vision.

The scaling effects of substrate texture on camouflage patterning in cuttlefish

Camouflage is the primary defense in cuttlefish. The rich repertoire of their body patterns can be categorized into three types: uniform, mottle, and disruptive. Several recent studies have characterized spatial features of substrates responsible for eliciting these body patterns on natural and artificial backgrounds. In the present study, we address the role of spatial scales of substrate texture in modulating the expression of camouflage body patterns in cuttlefish, Sepia officinalis. Substrate textures were white noise patterns first filtered into various octave-wide spatial frequency bands and then thresholded to generate binary (black/white) images. Substrate textures differed in spatial frequency but were identical in all other respects; this allowed us to examine the effects of spatial scale on body patterning. We found that as the spatial scale of substrate texture increased, cuttlefish body patterns changed from uniform, to mottle, to disruptive, as predicted from the camouflage mechanism of background matching. For substrates with spatial scales larger than skin patterning components, cuttlefish showed reduced disruptive patterning. These results are consistent with the idea that the body pattern deployed by a cuttlefish attempts to match the energy spectrum of the substrate, and underscore recent reports suggesting that substrate spatial scale is a key determinant of body patterning responses in cuttlefish.

Starburst cells nondirectionally facilitate the responses of direction-selective retinal ganglion cells

The mechanism of direction selectivity in retinal ganglion cells remains controversial. An important issue is how the starburst amacrine cells, which are known to provide a major synaptic input to the direction-selective ganglion cells, participate in the directional discrimination. Here, we present evidence that the cholinergic outputs of the starburst cells affect the responses of the ganglion cells symmetrically; they provide a feedforward excitation that facilitates the response of the ganglion cells to movement in both the preferred and null directions. This seems to place a constraint on models of the directional discrimination in which the starburst cells participate, namely, that their cholinergic synapses be nondirectional in their effects on the ganglion cells.

Effect of visual experience on the maturation of ON-OFF direction selective ganglion cells in the rabbit retina.

Activity-dependent neural plasticity is well known in the development of the visual cortical circuitry. However, the role of neural plasticity in the developing retina is less well understood. In the light of recent findings that light deprivation alters the development of synaptic pathway in the mouse and turtle retinas, we studied whether visual experience is required for the maturation of the ON-OFF direction selective ganglion cells (DSGCs) in the rabbit retina. The DSGCs of rabbits raised under a normal light-dark cycle and in the constant darkness were recorded extracellularly at various postnatal stages. Receptive field properties, such as direction selectivity, velocity tuning, classical center-surround interaction and motion-induced surround inhibition were examined. Recorded cells were subsequently injected with Neurobiotin in order to characterize their morphological features and tracer coupling patterns. Our results revealed that visual experience is not critical for the maturation of the classical receptive field properties of the DSGCs, such as direction selectivity and velocity tuning. However, the dark-reared rabbits showed altered surround inhibition, which is mediated by the amacrine cells of the inner retina. In addition, the DSGCs of both normal- and dark-reared rabbits showed similar dendritic features and tracer coupling patterns. Taken together, this study indicates that visual experience plays a less significant role on the DS circuitry maturation in the retina than in the cortex.

Characterization of natural illuminants in forests and the use of digital video data to reconstruct illuminant spectra

We describe illumination spectra in forests and show that they can be accurately recovered from recorded digital video images. Natural illuminant spectra of 238 samples measured in temperate forests were characterized by principal-component analysis. The spectra can be accurately approximated by the mean and the first two principal components. Compared with illumination under open skies, the loci of forest illuminants are displaced toward the green region in the chromaticity plots, and unlike open sky illumination they cannot be characterized by correlated color temperature. We show that it is possible to recover illuminant spectra accurately from digital video images by a linear least-squares-fit estimation technique. The use of digital video data in spectral analysis provides a promising new approach to the studies of the spatial and temporal variation of illumination in natural scenes and the understanding of color vision in natural environments.