Feedback from horizontal cells to cones mediates color induction and may facilitate color constancy in rainbow trout

Infrastructure to Support Accrual of Older Adults to National Cancer Institute Clinical Trials

As part of ongoing efforts to meaningfully improve recruitment, enrollment, and accrual of older adults into cancer clinical trials, the National Cancer Institute (NCI) sponsored a workshop with experts across the country entitled Engaging Older Adults in the NCI Clinical Trials Network: Challenges and Opportunities. Three working groups, including Study Design, Infrastructure, and Stakeholders, were formed, who worked together to offer synergistic improvements in the system. Here, we summarize the workshop discussions of the Infrastructure Working Group, whose goal was to address infrastructural challenges, identify underlying resources, and offer solutions to facilitate accrual of older adults into cancer clinical trials. Based on preconference work and workshop discussions, four key recommendations to strengthen NCI infrastructure were proposed: 1) further centralize resources and expertise; 2) provide training for clinical research staff; (3) develop common data elements; and 4) evaluate what works and does not work. These recommendations provide a strategy to improve the infrastructure to enroll more older adults in cancer clinical trials.

Underwater Image Enhancement Using Adaptive Retinal Mechanisms

We propose an underwater image enhancement model inspired by the morphology and function of the teleost fish retina. We aim to solve the problems of underwater image degradation raised by the blurring and nonuniform color biasing. In particular, the feedback from color-sensitive horizontal cells to cones and a red channel compensation are used to correct the nonuniform color bias. The center-surround opponent mechanism of the bipolar cells and the feedback from amacrine cells to interplexiform cells then to horizontal cells serve to enhance the edges and contrasts of the output image. The ganglion cells with color-opponent mechanism are used for color enhancement and color correction. Finally, we adopt a luminance-based fusion strategy to reconstruct the enhanced image from the outputs of ON and OFF pathways of fish retina. Our model utilizes the global statistics (i.e., image contrast) to automatically guide the design of each low-level filter, which realizes the self-adaption of the main parameters. Extensive qualitative and quantitative evaluations on various underwater scenes validate the competitive performance of our technique. Our model also significantly improves the accuracy of transmission map estimation and local feature point matching using the underwater image. Our method is a single image approach that does not require the specialized prior about the underwater condition or scene structure.

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

The effect of luminance differences on color assimilation

The color appearance of a surface depends on the color of its surroundings (inducers). When the perceived color shifts towards that of the surroundings, the effect is called "color assimilation" and when it shifts away from the surroundings it is called "color contrast." There is also evidence that the phenomenon depends on the spatial configuration of the inducer, e.g., uniform surrounds tend to induce color contrast and striped surrounds tend to induce color assimilation. However, previous work found that striped surrounds under certain conditions do not induce color assimilation but induce color contrast (or do not induce anything at all), suggesting that luminance differences and high spatial frequencies could be key factors in color assimilation. Here we present a new psychophysical study of color assimilation where we assessed the contribution of luminance differences (between the target and its surround) present in striped stimuli. Our results show that luminance differences are key factors in color assimilation for stimuli varying along the s axis of MacLeod-Boynton color space, but not for stimuli varying along the l axis. This asymmetry suggests that koniocellular neural mechanisms responsible for color assimilation only contribute when there is a luminance difference, supporting the idea that mutual-inhibition has a major role in color induction.

Specific connectivity between photoreceptors and horizontal cells in the zebrafish retina

The functional and morphological connectivity between various horizontal cell (HC) types (H1, H2, H3, and H4) and photoreceptors was studied in zebrafish retina. Since HCs are strongly coupled by gap junctions and feedback from HCs to photoreceptors depends strongly on connexin (Cx) hemichannels, we characterized the various HC Cxs (Cx52.6, Cx52.7, Cx52.9, and Cx55.5) in Xenopus oocytes. All Cxs formed hemichannels that were conducting at physiological membrane potentials. The Cx hemichannels differed in kinetic properties and voltage dependence, allowing for specific tuning of the coupling of HCs and the feedback signal from HCs to cones. The morphological connectivity between HC layers and cones was determined next. We used zebrafish expressing green fluorescent protein under the control of Cx promoters. We found that all HCs showed Cx55.5 promoter activity. Cx52.7 promoter activity was exclusively present in H4 cells, while Cx52.9 promoter activity occurred only in H1 cells. Cx52.6 promoter activity was present in H4 cells and in the ventral quadrant of the retina also in H1 cells. Finally, we determined the spectral sensitivities of the HC layers. Three response types were found. Monophasic responses were generated by HCs that contacted all cones (H1 cells), biphasic responses were generated by HCs that contacted M, S, and UV cones (H2 cells), and triphasic responses were generated by HCs that contacted either S and UV cones (H3 cells) or rods and UV cones (H4 cells). Electron microscopy confirms that H4 cells innervate cones. This indicates that rod-driven HCs process spectral information during photopic and luminance information during scotopic conditions.

Modelling colour constancy in fish: implications for vision and signalling in water

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, Sphyraena helleri, which is dichromatic and two damselfishes, Chromis verater and Chromis hanui, which are trichromatic. We assume that photoreceptors light-adapt to the background, thereby implementing the von Kries transformation, which can largely account for observed 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 trichromats. The von Kries transformation does substantially offset colour shifts caused by changing depth, so that from depths of 0 to 30 m modelled colour changes (i.e. failures of colour constancy) are sometimes negligible. However, the magnitudes and directions of remaining changes are complex, depending upon the specific spectral sensitivities of the receptors and the reflectance spectra. This predicts that when judgement of colour is important, the spectra of signalling colours and photoreceptor spectral sensitivities should be evolutionarily linked, with the colours dependent on photoreceptor spectral sensitivities, and vice versa.