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Each person possesses 23 pairs of chromosomes that encode the formation of proteins in sequences of DNA. The sequence for a particular protein is called a gene. In recent years, researchers have identified the location and chemical sequence of the genes that encode the photopigments in the rods and cones. This figure shows the structure of the rhodopsin molecule.

The molecule forms 7 columns that are embedded in the disk membrane. Although not shown in this schematic, the columns are arranged in a circle like the planks of a barrel. (Another molecule called a chromophore binds within this barrel.) Each circle is an amino-acid which are the building blocks of proteins. Each amino acid is encoded by a sequence of three nucleic acids in the DNA. Before identifying the genetic sequence of human rhodopsin, it was sequences in other animals. Here is shown the comparison between the bovine (cow) sequence and the human sequence. They are very similar with only a small number of differences (the dark circles). Even when there is a difference it may not be functionally significant.

The gene for human rhodopsin is located on chromosome 3. This figure shows the sequence for the S-cone pigment compared to that of rhodopsin. The S-cone pigment gene is located on chromosome 7. This figure shows the sequence of the L- and M-cone pigments compared to each other. Only those differences within the cell membrane can contribute to the differences in their spectral sensitivity. The M- and L- cone pigments are both encoded on the X chromosome in tandem. For females this pair is XX and for males this pair is XY. We will return to this later on when we discuss color vision and color blindness. This figure shows how the three cone types are arranged in the fovea. Currently there is a great deal of research involving the determination of the ratios of cone types and their arrangement in the retina. This diagram was produced based on histological sections from a human eye to determine the density of the cones. The diagram represents an area of about 1° of visual angle. The number of S-cones was set to 7% based on estimates from previous studies. This is a reasonable number considering that recent studies have shown wide ranges of cone ratios in people with normal color vision. In the central fovea an area of approximately 0.34° is S-cone free. The S-cones are semi-regularly distributed and the M- and L-cones are randomly distributed. Throughout the whole retina the ratio of L- and M- cones to S-cones is about 100:1. From the cone mosaic we can estimate spatial acuity or the ability to see fine detail. In the central fovea, there are approximately 150,000 cones/ sq. The distance between cone centers in the hexagonal packing of the cones is about 0.003 mm. To convert this to degrees of visual angle you need to know that there are 0.29 mm/deg so that the spacing is 0.003/0.29 = 0.013° between cone centers.

The Nyquist frequency, f , is the frequency at which aliasing begins. That is a grating pattern of cos(2*pi(N/2+ f )) above the Nyquist frequency is indistinguishable from the signal cos(2*pi(N/2- f )) below the Nyquist frequency where N is the number of sample points per unit distance. In actuality, the foveal Nyquist limit is more like 60 cycles per degree. This may be a result of the hexagonal rather than the rectangular packing of the cone mosaic. The optics of the eye blur the retinal image so that this aliasing is not produced. Using laser interferometry, the optics of the eye can be bypassed so we can reveal this aliasing. We will discuss this in more detail in the chapter on visual acuity. The mosaic of the retina in addition to the processing in the visual system produces another ability to see fine resolution and ascertain alignment of object called hyperacuity . People have the ability to see misalignment of objects of 5 seconds of arc (which is 1/5 of a cone width).

This corresponds to seeing the misalignment in headlights 39 miles away. Maybe you can try working this out to see if I am exaggerating.


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