Reliable identification by color under natural conditions

In order to recognize objects on the basis of the way in which they reflect different wavelengths of light, the visual system must deal with the different illuminant and background conditions under which the objects are seen. To test this ability under natural conditions, subjects were asked to name 6 uniformly colored papers. The experiment started by showing subjects six papers simultaneously in a normally illuminated room, and instructing them about how to name them. The papers were easy to differentiate when seen together but they were so similar that subjects only identified 87% correctly when they were presented in isolation under otherwise identical conditions to those during the instruction. During the main part of the experiment subjects walked between several indoor and outdoor locations that differed considerably in lighting and background colors. At each location subjects were asked to identify one paper. They correctly identified the paper on 55% of the trials (well above chance level), despite the fact that the variation in the light reaching their eyes from the same paper at different positions was much larger than that from different papers at the same position. We discuss that under natural conditions color constancy is probably as good as it can be considering the theoretical limitations. Keywords: color vision, color constancy, color naming, object recognition, natural environment Citation: Granzier, J. J. M., Brenner, E., & Smeets, J. B. J. (2009). Reliable identification by color under natural conditions. Journal of Vision, 9(1):39, 1-8, http://journalofvision.org/9/1/39/, doi:10.1167/9.1.39. Introduction The light that is reflected from an illuminated object depends both on its surfaces' reflectance properties and on the illumination of the scene. If we are interested in the object's reflectance properties the fact that the illumination can vary drastically over time and between locations raises a problem for our visual system, since the intensity and spectral distribution of the light that is reflected from the object in question onto the receptors in our eyes will also vary considerably Probably many factors are involved in achieving color constancy, including various kinds of spatial (e.g., The extent to which color constancy is achieved differs between studies, probably because factors such as overall scene complexity Methods Subjects 21 subjects (including two of the authors) with normal color vision Procedure For practical reasons, the experiment was performed in three groups of seven subjects. During an 'instruction phase' subjects were told how to name the colors of six different test papers that were presented simultaneously on a desk under daylight illumination (see Subjects were not told that each paper would be presented 4 times. They wrote the name of the color of the paper that they thought was being shown to them on an answer form ("white," "gray," "green," "red," "blue" or "yellow"). At each location, the experimenter presented the test paper separately to each subject. Subjects were allowed to hold the test paper in their hands and change its orientation. They were allowed to look around as they pleased, so they could compare the test paper's color to the colors of objects in the direct vicinity, but were not allowed to compare the color of the test paper with their white answer form, and they had to remain at the place at which the experimenter had given them the test paper. Subjects were not allowed to talk about the experiment during the tour and were instructed to keep their answer form hidden from the other participants. The locations We used both indoor and outdoor locations (see examples in Journal of Vision Baseline measurement Although the difference between the papers was very clear when they were presented simultaneously, identifying them in isolation was quite difficult. In a separate measurement, we tested our subjects' ability to identify the test papers at a fixed place under constant fluorescent illumination (Philips, 38 HF; 50 watt). Five subjects who also participated in the main experiment took part in this baseline measurement. The CIE xy coordinates of the light reflected by the test papers under these conditions, as measured with a Minolta CS-100A chroma meter, were (0.436, 0.404), (0.432, 0.406), (0.439, 0.402), (0.426, 0.401), (0.441, 0.411) and (0.436, 0.405), for the gray, green, red, blue, yellow and white test paper respectively. The procedure was similar to that of the main experiment, but the background was always the same (the gray surface of a table), the illumination did not change between the first simultaneous presentation and the subsequent test presentations, and subjects remained at the same place under constant illumination between the presentations. Thus, performance is unlikely to be limited by failures of color constancy. After presenting all six pieces of paper simultaneously, the experimenter placed one of the six test papers on the table every three minutes, and the subjects had to write down which paper they thought was being presented (i.e. its color). As in the main experiment, each test paper was presented four times, and the papers were presented in random order (24 trials). The three minutes waiting time was chosen to match the time between judgments in the main experiment. Subjects remained in the room during the 3 minutes between presentations. Analysis To illustrate the judgments that subjects made, pie charts of the groups of 7 subjects' responses were made per location and test paper. The corresponding color of the reflected light was indicated for each pie chart. Since subjects could move the papers around and the illumination could change slightly while the members of the group sequentially made their judgments, we measured the color of the reflected light several times at each location for each group (while the subjects were making their decisions) and calculated the average CIE xyY values. These averages are shown together with the abovementioned pie charts. The variability between these repeated measurements turned out to be quite small (the median standard deviations while a paper was shown to the 7 subjects of the second group were 0.002, 0.001 and 12.3% for CIE x, y and Y, respectively). That performance would not be perfect is obvious because we chose shades of colors that were difficult to distinguish. The question is to what extent performance is worse when the papers are shown at various locations with different kinds of illuminations than when the papers were shown at a single location under a fixed illumination (baseline). To find out, we plotted the percentage of correct responses as a function of the distance in CIE color space between the test papers' CIE xy coordinates during the experiment and during the corresponding instruction phase. We averaged consecutive groups of 6 presentations (a presentation is a set of 7 responses for a given combination of paper and illumination) after sorting the presentations in terms of the above-mentioned distance. Results The average number of correct responses during the main experiment was 55.8% (ranging between 37.5% and 79.2% for individual subjects; 16.6% is chance level). During the baseline measurement, in which there was no change in illumination or in background color and the subjects were fully adapted to the illumination, 87.5% of the responses were correct. That subjects made error

Can illumination estimates provide the basis for color constancy?

Objects hardly appear to change color when the spectral distribution of the illumination changes: a phenomenon known as color constancy. Color constancy could either be achieved by relying on properties that are insensitive to changes in the illumination (such as spatial color contrast) or by compensating for the estimated chromaticity of the illuminant. We examined whether subjects can judge the illuminant's color well enough to account for their own color constancy. We found that subjects were very poor at judging the color of a lamp from the light reflected by the scene it illuminated. They were much better at judging the color of a surface within the scene. We conclude that color constancy must be achieved by relying on relationships that are insensitive to the illumination rather than by explicitly judging the color of the illumination. © ARVO

Atomically sharp non-classical ripples in graphene

A fundamental property of a material is the measure of its deformation under applied stress. After studying the mechanical properties of bulk materials for the past several centuries, with the discovery of graphene and other two-dimensional materials, we are now poised to study the mechanical properties of single atom thick materials at the nanoscale. Despite a large number of theoretical investigations of the mechanical properties and rippling of single layer graphene, direct controlled experimental measurements of the same have been limited, due in part to the difficulty of engineering reproducible ripples such that relevant physical parameters like wavelength, amplitude, sheet length and curvature can be systematically varied. Here we report extreme (>10%) strain engineering of monolayer graphene by a novel technique of draping it over large Cu step edges. Nanoscale periodic ripples are formed as graphene is pinned and pulled by substrate contact forces. We use a scanning tunneling microscope to study these ripples to find that classical scaling laws fail to explain their shape. Unlike a classical fabric that forms sinusoidal ripples in the transverse direction when stressed in the longitudinal direction, graphene forms triangular ripples, where bending is limited to a narrow region on the order of unit cell dimensions at the apex of each ripple. This non-classical bending profile results in graphene behaving like a bizarre fabric, which regardless of how it is pulled, always buckles at the same angle. Using a phenomenological model, we argue that our observations can be accounted for by assuming that unlike a thin classical fabric, graphene undergoes significant stretching when bent. Our results provide insights into the atomic-scale bending mechanisms of 2D materials under traditionally inaccessible strain magnitudes and demonstrate a path forward for their strain engineering.Comment: 22 pages, 4 figure

Strain Modulated Superlattices in Graphene

Strain engineering of graphene takes advantage of one of the most dramatic responses of Dirac electrons enabling their manipulation via strain-induced pseudo-magnetic fields. Numerous theoretically proposed devices, such as resonant cavities and valley filters, as well as novel phenomena, such as snake states, could potentially be enabled via this effect. These proposals, however, require strong, spatially oscillating magnetic fields while to date only the generation and effects of pseudo-gauge fields which vary at a length scale much larger than the magnetic length have been reported. Here we create a periodic pseudo-gauge field profile using periodic strain that varies at the length scale comparable to the magnetic length and study its effects on Dirac electrons. A periodic strain profile is achieved by pulling on graphene with extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap pulled taut at its edges. Combining scanning tunneling microscopy and atomistic calculations, we find that spatially oscillating strain results in a new quantization different from the familiar Landau quantization observed in previous studies. We also find that graphene ripples are characterized by large variations in carbon-carbon bond length, directly impacting the electronic coupling between atoms, which within a single ripple can be as different as in two different materials. The result is a single graphene sheet that effectively acts as an electronic superlattice. Our results thus also establish a novel approach to synthesize an effective 2D lateral heterostructure - by periodic modulation of lattice strain.Comment: 18 pages, 5 figures and supplementary informatio

Modular proteins from the Drosophila sallimus (sls) gene and their expression in muscles with different extensibility

The passive elasticity of the sarcomere in striated muscle is determined by large modular proteins, such as titin in vertebrates. In insects, the function of titin is divided between two shorter proteins, projectin and sallimus (Sls), which are the products of different genes. The Drosophila sallimus (sls) gene codes for a protein of 2 MDa. The N-terminal half of the protein is largely made up of immunoglobulin domains and unique sequence; the C-terminal half has two stretches of sequence similar to the elastic PEVK region of titin, and at the end of the molecule there is a region of tandem Ig and fibronectin domains. We have investigated splicing pathways of the sls gene and identified isoforms expressed in different muscle types, and at different stages of Drosophila development. The 5’ half of sls codes for zormin and kettin; both proteins contain Ig domains and can be expressed as separate isoforms, or as larger proteins linked to sequence downstream. There are multiple splicing pathways between the kettin region of sls and sequence coding for the two PEVK regions. All the resulting protein isoforms have sequence derived from the 3’ end of the sls gene. Splicing of exons varies at different stages of development. Kettin RNA is predominant in the embryo, and longer transcripts are expressed in larva, pupa and adult. Sls isoforms in the indirect flight muscle (IFM) are zormin, kettin and Sls(700), in which sequence derived from the end of the gene is spliced to kettin RNA. Zormin is in both M-line and Z-disc. Kettin and Sls(700) extend from the Z-disc to the ends of the thick filaments, though, Sls(700) is only in the myofibril core. These shorter isoforms would contribute to the high stiffness of IFM. Other muscles in the thorax and legs have longer Sls isoforms with varying amounts of PEVK sequence; all span the I-band to the ends of the thick filaments. In muscles with longer Ibands, the proportion of PEVK sequence would determine the extensibility of the sarcomere. Alternative Sls isoforms could regulate the stiffness of the many fibre types in Droso phila muscles

An inverse problem approach to identify the internal force of a mechanosensation process in a cardiac myocyte

Mechanosensation and mechanotransduction are fundamental processes in understanding the link between physical stimuli and biological responses which currently still remain not well understood. The precise molecular mechanism involved in stress and strain detection in cells is unclear. Sarcomeres are the contractile machines of a cardiac myocyte and two main sarcomeric components that are directly involved in the sensation and transmission of mechanical stimuli are titin and filaments (thin and thick). Titin is known as the largest protein in biology with a mass of up to 4.2 MDa. Its flexible region (I-band region) may function as a length sensor (ε=l/l0) while its Z-disc domain may be involved in the sensation of tension and stress (σView the MathML source). Filaments act as contractile machineries by converting biochemical signals into mechanical work which in response cells either shorten or relax. Based on these considerations and a qualitative understanding of the maladaptation contribution to the development of heart failure, an inverse problem approach is taken to evaluate the contractile force in a mathematical model that describes mechanosensation in normal heart cells. Different functional forms to describe the contractile force are presented and for each of them we study the computational efficiency and accuracy of two numerical techniques