Light Sensing and Vision

The brittlestar in the upper row does not respond to changes in light The brittlestar in the upper row does not respond to changes in light. The individual on the second row darkens at night and has microscopic lenses of calcite covering the body surface. Nature, August 23, 2001

Color Vision According to the trichromatic theory of color vision, humans can distinguish about 1500 hues of color based on only three different photopsins in three different types of cone cells. The retinal is the same as in rhodopsin, but he opsins have slightly different amino acid structure. Blue cones absorb maximally at 445nm, green at 530 nm and red at 625. This compares with the absorption of rhodopsins at 498. There are 6 million cones in the retina concentrated in the fovea, in contrast there are 120 million rods distributed outside the fovea. Other animals also have color vision. Birds and reptiles have 4 color pigments, flies and butterflies have 5 and the mantis shrimp uses 10 different opsins.

In humans, genes for the red and green opsins differ by only 15 of 348 amino acids. These genes are carried on the X chromosome. Males with only one X can express a recessive defective gene and have red/green color blindness. The red/green divergence first appeared in primates.

There are three types of color sensitive cones that are maximally stimulated at red (625), green(530) and blue (455) wavelengths Information leaving the eye is encoded in three channels -- one for intensity, one for red/green and one for blue/yellow. One red/green cell would increase activity when presented with red and decrease it with green. Another red/green cell would increase activity with green and decrease with red. When you stare at a red stimulus, the cell signaling the presence of red with start to fatigue. When you switch to a blank page these cells will fire very little -- an you interpret the lack of red firing as green.

What Do You See ??

Visual Processing in the Eye

Horizontal cells provide connections between rods and cones and bipolar cells. They provide “cross talk” between different fields .

Bipolar cells connect the rods and cones with the ganglionic cells Bipolar cells connect the rods and cones with the ganglionic cells. These cells may either polarize or depolarize when stimulated.

Amacrine cells provide cross links between ganglionic cells.

Ganglionic cells form the optic nerve Ganglionic cells form the optic nerve. Action potentials are generated here.

In the fovea there are approximately 4,000 cones and they are connected to 4,000 ganglionic cells. Each ganglionic cell receives input from one cone or about 2 micometers of the visual field. Outside the fovea many rods synapse with a single bipolar cell and many bipolar cells with a single ganglionic cell so a single ganglionic neuron covers one mm2. In response to light, the rhodopsin of the rods and cones dissociates into retinal and opsin, this is called a bleaching reaction. When a light adapted person enters a dark room their sensitivity to light is low. Receptor sensitivity increases gradually over about 20 minutes in dark adaptation. Rods don’t absorb red light but cones do, so if a dark adapted person turned on a red light to read, the rods would remain dark adapted.

Optical Processing Level one processing occurs in the retina; the rods and cones are the first order cells, bipolar cells are 2nd order and ganglionic cells are third. Amacrine cells and horizontal cells create both converging and diverging pathways. Rods and cones hyperpolarize when stimulated and Na channels close Horizontal cells also produce graded hyperpolarizations. Some bipolar cells hyperpolarize and some depolarize Some ganglionic cells hyperpolarize and some depolarize Ganglionic cells are matched to their biplolar cells, if the bipolar cells hyperpolarize then the ganglionic cells they are connected to also hyperpolarize and visa versa. Amacrine cells respond transiently when the stimulus changes.