Evolutionary Adaptation in Vision
A fundamental assumption among vision theorists is that our visual mechanism evolved in response to environmental demands. Confirming this belief can be difficult, but two recent studies have made interesting inroads. The first used 5,000 high-resolution photographs of a tropical savanna habitat in Botswana similar to the one where humans are thought to have evolved. Analysis of the photos suggested why less than 10% of human cones are specialized for "blue" light: The scenes actually contained fewer short wavelengths, and the characteristics of the lens and aqueous humor further selected for the medium and long wavelengths. Red- and green-sensitive cones would be stimulated about equally, which is consistent with the fact that the proportion of these receptors is highly variable among individuals; there is no functional advantage to regulating them closely. PLoS Computational Biology, February, 2010.
The second study tackled the explanation for why our "off" ganglion cells—those that respond to a dark area on a light background—outnumber "on" cells two to one and have smaller receptive fields. This study also used photographs of the plains of Botswana, but it included others as different as scenes from downtown Philadelphia. The researchers found that real-world images contain more dark-on-light areas than light points. Mathematical modeling showed that the optimal ratio of "off" to "on" cells for processing these images corresponds to the ratio in our eyes. Proceedings of the National Academy of Sciences, 107, 17368-17373.
Restoring Vision
Efforts to restore vision are ramping up with advances on multiple fronts. Researchers at the University of Tübingen have developed a 1500-electrode retinal implant, far exceeding the capability of the device described in the text (p 300); with it, a patient can read large print and identify common objects, such as a knife and cup. Proceedings of the Royal Society B, Vol 278, 1489-1497 (click here for video).
Research also is continuing with rhodopsin genes obtained from algae; when inserted into ganglion cells, they create light-sensitive ion channels that convert the cells into functioning photoreceptors. This optogenetic procedure has demonstrated safety in mice and may enter human trials within two years. Molecular Therapy, doi:10.1038/mt.2011.69. You can see one of the mice locating the exit in a water maze here.
Finally, the biotechnology company Advanced Cell Technology has received Food and Drug Administration approval to begin clinical trials to determine the safety and tolerability of human embryonic stem cells in the treatment of macular degeneration. Advanced Cell Technology Press Release, May 16, 2011.