Fine structural dependence of ultraviolet reflections in the King Penguin beak horn.

The visual perception of many birds extends into the near-ultraviolet (UV) spectrum and ultraviolet is used by some to communicate. The beak horn of the King Penguin (Aptenodytes patagonicus) intensely reflects in the ultraviolet and this appears to be implicated in partner choice. In a preliminary study, we recently demonstrated that this ultraviolet reflectance has a structural basis, resulting from crystal-like photonic structures, capable of reflecting in the near-UV. The present study attempted to define the origin of the photonic elements that produce the UV reflectance and to better understand how the UV signal is optimized by their fine structure. Using light and electron microscopic analysis combined with new spectrophotometric data, we describe here in detail the fine structure of the entire King Penguin beak horn in addition to that of its photonic crystals. The data obtained reveal a one-dimensional structural periodicity within this tissue and demonstrate a direct relationship between its fine structure and its function. In addition, they suggest how the photonic structures are produced and how they are stabilized. The measured lattice dimensions of the photonic crystals, together with morphological data on its composition, permit predictions of the wavelength of reflected light. These correlate well with experimentally observed values. The way the UV signal is optimized by the fine structure of the beak tissue is discussed with regard to its putative biological role.

Long-range spatial integration across contrast signs: a probabilistic mechanism?

It is widely assumed that the integration of orientation contrast across spatial gaps within the long-range regime is not selective to the contrast sign of the individual stimuli. Probabilistic models of perceptual integration, however, suggest that long-range spatial integration should be, if not selective, at least sensitive to local contrast signs. To clarify this issue, we tested predictions of a model based on conditional probabilistic weights of identical and opposite contrast signs in a simple spatial configuration of two co-linear lines. Contrast detection thresholds of the target line presented either by itself (control condition) or simultaneously with the co-linear inducer (test condition) were measured. The contrast sign of targets and inducers was varied so that all four possible combinations of signs were produced in the test conditions: (1) dark target with dark inducer, (2) dark target with bright inducer, (3) bright target with bright inducer and (4) bright target with dark inducer. The contrast intensity (Weber ratio) of dark and bright inducers was identical. The coaxial distance between target and inducer was constant in each of two experiments, testing for two distances that corresponded to an angular separation within the long-range domain of spatial integration as defined previously. It is found that targets and inducers with identical contrast signs produce significantly stronger facilitating effects on detection than stimuli with opposite signs. The data closely match predictions consistent with those of a probabilistic model of line contrast integration across spatial gaps and contrast signs within the long-range regime.

Short- and long-range effects in line contrast integration.

Brincat and Westheimer [Journal of Neurophysiology 83 (2000) 1900] have reported facilitating interactions in the discrimination of spatially separated target orientations and co-linear inducing orientations by human observers. With smaller gaps between stimuli (short-range effects), facilitating interactions were found to depend on the contrast polarity of the stimuli. With larger gaps (long-range effects), only co-linearity of the stimuli seemed necessary to produce facilitation. In our study, the dependency of facilitating interactions on the intensity (luminance) of line stimuli is investigated by measuring detection thresholds for a target line separated from the end of an inducing line by co-axial gaps ranging from 5 to 200 min of visual arc. We find facilitating interactions between target and inducing orientations, producing short-range and long-range effects similar to those reported by Brincat and Westheimer. In addition, detection thresholds as a function of the co-axial separation between target and inducing line reveal an interaction between the spatial regime of facilitating effects and the luminance of the stimuli. Short-range effects are found to be sensitive to changes in local intensity while long-range effects remain unaffected.

Which geometric model for the curvature of 2-D shape contours?

We investigated the geometric representations underlying the perception of 2-D contour curvature. 88 arcs representing lower and upper halves of concentric circles, or halves of ellipses derived mathematically through planar projection by affinity with the circles, a special case of Newton's transform, were generated to produce curved line segments with negative and positive curvature and varying sagitta (sag) and/or aspect ratio. Aspect ratio is defined here as the ratio between the sagitta and the chord-length of a given arc. The geometric properties of the arcs suggest a regrouping into four structural models. The 88 stimuli were presented in random order to 16 observers eight of whom were experienced in the mathematical and visual analysis of 2-D curvature ('expert observers'), and eight of whom were not ('non-expert observers'). Observers had to give a number, on a psychophysical scale from 0 to 10, that was to reflect the magnitude of curvature they perceived in a given arc. The results show that the subjective magnitude of curvature increases exponentially with the aspect ratio and linearly with the sagitta of the arcs for both experts and nonexperts. Statistical analysis of the correlation coefficients of linear fits to individual data represented on a logarithmic scale reveals significantly higher correlation coefficients for aspect ratio than for sagitta. The difference is not significant when curves with the longest chords only (7 degrees -10 degrees ) are considered. The geometric model that produces the best psychometric functions is described by a combination of arcs of vertically and horizontally oriented ellipses, indicating that perceptual sensations of 2-D contour curvature are based on geometric representations that suggest properties of 3-D structures. A 'buckled bar model' is shown to optimally account for the perceptual data of all observers with the exception of one expert. His perceptual data can be linked to a more analytical, less 'naturalistic' representation originating from a specific perceptual experience, which is discussed. It is concluded that the structural properties of 'real' objects are likely to determine even the most basic geometric representations underlying the perception of curvature in 2-D images. A specific perceptual learning experience may engender changes in such representations.

Ultraviolet reflecting photonic microstructures in the King Penguin beak.

King and emperor penguins (Aptenodytes patagonicus and Aptenodytes forsteri) are the only species of marine birds so far known to reflect ultraviolet (UV) light from their beaks. Unlike humans, most birds perceive UV light and several species communicate using the near UV spectrum. Indeed, UV reflectance in addition to the colour of songbird feathers has been recognized as an important signal when choosing a mate. The king penguin is endowed with several highly coloured ornaments, notably its beak horn and breast and auricular plumage, but only its beak reflects UV, a property considered to influence its sexual attraction. Because no avian UV-reflecting pigments have yet been identified, the origin of such reflections is probably structural. In an attempt to identify the structures that give rise to UV reflectance, we combined reflectance spectrophotometry and morphological analysis by both light and electron microscopy, after experimental removal of surface layers of the beak horn. Here, we characterize for the first time a multilayer reflector photonic microstructure that produces the UV reflections in the king penguin beak.

Interaction of color and geometric cues in depth perception: when does "red" mean "near"?

Luminance and color are strong and self-sufficient cues to pictorial depth in visual scenes and images. The present study investigates the conditions under which luminance or color either strengthens or overrides geometric depth cues. We investigated how luminance contrast associated with the color red and color contrast interact with relative height in the visual field, partial occlusion, and interposition to determine the probability that a given figure presented in a pair is perceived as "nearer" than the other. Latencies of "near" responses were analyzed to test for effects of attentional selection. Figures in a pair were supported by luminance contrast (Experiment 1) or isoluminant color contrast (Experiment 2) and combined with one of the three geometric cues. The results of Experiment 1 show that the luminance contrast of a color (here red), when it does not interact with other colors, produces the same effects as achromatic luminance contrasts. The probability of "near" increases with the luminance contrast of the color stimulus, the latencies for "near" responses decrease with increasing luminance contrast. Partial occlusion is found to be a strong enough pictorial cue to support a weaker red luminance contrast. Interposition cues lose out against cues of spatial position and partial occlusion. The results of Experiment 2, with isoluminant displays of varying color contrast, reveal that red color contrast on a light background supported by any of the three geometric cues wins over green or white supported by any of the three geometric cues. On a dark background, red color contrast supported by the interposition cue loses out against green or white color contrast supported by partial occlusion. These findings reveal that color is not an independent depth cue, but is strongly influenced by luminance contrast and stimulus geometry. Systematically shorter response latencies for stronger "near" percepts demonstrate that selective visual attention reliably detects the most likely depth cue combination in a given configuration.

Depth perception from pairs of overlapping cues in pictorial displays.

The experiments reported herein probe the visual cortical mechanisms that control near-far percepts in response to two-dimensional stimuli. Figural contrast is found to be a principal factor for the emergence of percepts of near versus far in pictorial stimuli, especially when stimulus duration is brief. Pictorial factors such as interposition (Experiment 1) and partial occlusion (Experiments 2 and 3) may cooperate, as generally predicted by cue combination models, or compete with contrast factors in the manner predicted by the FACADE model. In particular, if the geometrical configuration of an image favors activation of cortical bipole grouping cells, as at the top of a T-junction, then this advantage can cooperate with the contrast of the configuration to facilitate a near-far percept at a lower contrast than at an X-junction. Varying the exposure duration of the stimuli shows that the more balanced bipole competition in the X-junction case takes longer exposures to resolve than the bipole competition in the T-junction case (Experiment 3).