PubMed · 14296430
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W A RUSHTON. 1965-03-16. VISUAL ADAPTATION.. https://doi.org/10.1098/rspb.1965.0024
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We describe a neuromorphic retina that signals a luminance edge as a spike. In a fast process, the luminance profile of the receptor layer determines the membrane potential of the ganglion cells and their individual, adjustable spiking thresholds. In a slower process, a wave-propagation process, the charge of ganglion cells with high membrane potential will propagate toward neighboring cells with low membrane potential and low spiking threshold, thus signaling the edge as a spike. Following that, the signaled edge (or contour) actively propagates across the retinal map. The retinal signal can be used for a contour-integration or a contour-propagation approach.
The ability of cone photoreceptors to adapt to light is extraordinary. In this study we evaluated two biochemical processes, visual pigment phosphorylation and transducin translocation, for their ability to contribute to light adaptation in zebrafish cones. Since cytoplasmic Ca2+ regulates light adaptation, the sensitivities of these processes to both light and Ca2+ were examined. Cytoplasmic Ca2+ regulates the sites of light-stimulated phosphorylation. Unexpectedly, we found that Ca2+ also regulates the extent of phosphorylation of unbleached cone pigments. Immunocytochemical analyses revealed that neither light nor cytoplasmic Ca2+ influences the localization of transducin in zebrafish cones.
Nonlinear regression analysis (NLR) is applied to quantify the dynamic response of non-photochemical fluorescence quenching (NPQ) of Trifolium repens cv. Regal upon dark to light transition. Commonly, only steady-state levels of NPQ are evaluated, ignoring transient kinetics. Experimental NPQ kinetics are fitted best with a sum of two functions: a sigmoidal Hill function plus a transient logarithmic normal function. It is shown that not only steady-state level of NPQ, but also the speed at which steady state is reached, increased with light intensity. The question is raised which biological processes cause the induction of the components of NPQ kinetics. The NPQ kinetics are found to resemble the kinetics of antheraxanthin and zeaxanthin formation during a dark to light transition. Furthermore, both molecules are known to induce NPQ. The hypothesis is put forward that a transient phase of NPQ (0-2 min after transition) is dependent upon concentrations of antheraxanthin, whereas the saturating phase corresponds with the production of zeaxanthin. A mathematical model, based on the presented hypothesis, predicts the effect of increasing light intensity on concentrations of antheraxanthin and zeaxanthin which correspond with experimental results. Implications of the hypothesis are discussed as well as the role of NLR in evaluating chlorophyll a fluorescence kinetics.