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Abstract: When an observer moves through the world, the motions of images on the retina provide information about the direction of observer motion, or “heading”, and about the relative distance to surfaces in the scene. While it is simple to compute these quantities when the observer moves in a straight line, the computation becomes substantially more difficult when the observer also undergoes a rotation. Here, we examine a model that uses operators with spatially extended, direction-tuned, center-surround receptive fields to compute heading in the presence of rotations. We show that this model can simultaneously locate edges between surfaces and provide an estimate of the change in relative depth between those surfaces. We further show that the incorporation of speed tuning in this model does not appreciably affect the results. [Copyright &y& Elsevier]

Interphotoreceptor retinoid-binding protein (IRBP), which binds retinoids and fatty acids, is the major soluble protein in the interphotoreceptor matrix (IPM) but its role remains ambiguous. Using competitive fluorescence and tryptophan-quenching assays we found oleic acid and other cis-monounsaturated fatty acids bind much more strongly than does docosahexaenoic acid to bovine IRBP. IPM’s fatty acid composition was determined: it was richer in oleic acid than either the retinal pigment epithelium or rod outer segments. This may imply oleic acid has a key role in the balance and transport of retinoids and fatty acids in the retina. [Copyright &y& Elsevier]

Evolving 40 times independently, eyes are striking examples of convergent evolution in that 11-cis retinaldehyde is always used for photon capture, yet the mechanism for its regeneration may be dramatically different in between systems. In particular, insects, cephalopods and vertebrates show varying physical separation of the cis → trans photoisomerization and chromphore reisomerization. In the vertebrate retina, these two processes are actually distributed between different cells. This compartmentalization is made possible by the phylogenetic innovation of the two-layered optic cup of the vertebrate retina. This unprecedented design created the subretinal space as a novel anatomical compartment allowing photoreceptors access to the retinal pigment epithelium (RPE) and Mu¨ller cells, the two cell types which share the burden of 11-cis retinoid regeneration. To take advantage of this arrangement, early vertebrates appear to have recruited for retinoid binding, the ββα-spiral fold proven useful in enoyl-CoA isomerase/hydratases, and the carboxy-terminal proteases for stabilizing hydrophobic ligands. Quadruplication of this functional domain within a single polypeptide lead to the emergence of interphotoreceptor retinoid-binding protein (IRBP). IRBP is the main soluble component of the IPM, and is prevented from diffusing out of the subretinal space because its large size excludes it from the photoreceptor/Mu¨ller cell zonulae adheretes. Despite this physical entrapment, IRBP is rapidly turned over within the IPM through a process that coordinates secretion of the protein by the photoreceptors, and its removal from the matrix by RPE and photoreceptor endocytosis. The present review will summarize what is known about the structure and function of IRBP to anticipate future avenues of research. [Copyright &y& Elsevier]