脊椎動物網膜の視覚センサ機能と構造 : その時空間・色覚過程

For image sensing, the solid-state image sensor is now the most popular device. However, such devices, including the videcon tube, merely transfer the optical image into an electrical signal with corresponding intensity and color profiles of the image without processing or discriminating signals for higher level uses, such as pattern recognition or robot eyes. In addition, the sensitivity and the aperture size must be adjusted according to the external optical environment. In the vertebrate retina, on the other hand, the receptor can adapt its sensitivity automatically to an appropriate level. In the outer plexiform layer, receptors (rods and cones), horizontal cells, and bipolar cells make complex circuits including feedback mechanisms and center/surround organizations. These circuits and their functionings are studied widely in physiology and morphology by utilizing glass micro-electrode and intracellular dying techniques. The basic features of the outer plexiform layer are in (band-pass type) spatial filtering, which improves the contrast sensitivity, and in color-coding mechanisms. This paper discusses spatial characteristics and the opponent color mechanisms in this layer. Spatial characteristics: Three types of cone with a peak action spectrum at blue, green, and red were electrically connected by type, producing a spatial summation within 100 to 200μm ranges. Enlarging the spot diameter caused increasing response amplitude within this range. However, further enlargement did not produce a more enlarged response, but a decrease due to the feedback signal from the horizontal cells. The horizontal cell exhibited tremendously wide spatial summation over a few mm due to the electrical coupling through the gap junctions. A two-dimensional discrete horizontal cell network model was proposed and analyzed. The model interpreted the experimental data with good results. Receptor and horizontal cell signals input to the bipolar cell with an opposite sign and thus form the center/surround organization at this stage. This was confirmed by matching the subtraction of the two experimental data (R and H) with the bipolar cell area effect characteristics. Opponent color mechanism: The three primary color signals were converted to the opponent color signals through the feedback path (H to cones) in the outer plexiform layer, that is, R/G/B to R+G-and Y+B-, in the carp retina horizontal cell. According to the center/surround organization described above and these chromatic type horizontal cells with the ON and OFF bipolar channels, all the opponent color signals are established at this level. Four types of opponent color receptive fields are found in the carp retina: center opponent, surround opponent, spatially segregated opponent, and double opponent. The possible wiring diagrams for each type are presented.