Short Wavelength Cone System

The S cones are only involved in chromatic contrast, which has a lower spatial resolution than achromatic contrast. Chromatic aberration makes the short wavelength image out of focus when the image that the long-wave cone system sees is in focus. Therefore, the S cones have been excluded from high-resolution achromatic vision and the central fovea. S cones transmit their signals to the brain by a unique system of retinal ganglion cells. S cones synapse on S cone-specific on bipolars, which excite a bistratified ganglion cell that is also excited by L and M cone off bipolars (Fig. 5). Such a ganglion cell cannot mediate energy contrasts, being excited by both increments and decrements of light energy. It is designed for successive chromatic contrast, being excited when short wavelengths enter and long wavelengths leave its receptive field. The S cone channel is transmitted by relay cells in the parvo and/

Figure 4 The retinal circuitry of the magnocellular L and M cone system. Here, the L and M cone act synergistically on both on and off channels.

Figure 5 The retinal circuitry of the S cone channel. There are two inputs to a bistratified retinal ganglion cell. One is from on bipolars from S cones and the other is from off bipolars from L and M cones. Both bipolars excite the ganglion cell. The S cones excite their bipolars when there is an increment and the L and M cones excite their bipolar when there is a decrement of light absorption in the corresponding cone. The existence of an antagonistic amacrine cell that interacts off with on bipolars can explain transient tritanopia, a phenomenon of S cone vision.

Figure 4 The retinal circuitry of the magnocellular L and M cone system. Here, the L and M cone act synergistically on both on and off channels.

Figure 5 The retinal circuitry of the S cone channel. There are two inputs to a bistratified retinal ganglion cell. One is from on bipolars from S cones and the other is from off bipolars from L and M cones. Both bipolars excite the ganglion cell. The S cones excite their bipolars when there is an increment and the L and M cones excite their bipolar when there is a decrement of light absorption in the corresponding cone. The existence of an antagonistic amacrine cell that interacts off with on bipolars can explain transient tritanopia, a phenomenon of S cone vision.

or the intercalated layers of the lateral geniculate nucleus of the thalamus to striate cortex, layer 4C beta, where it is used to distinguish chromatic contrasts.

There is agreement that this S cone channel, consisting of an on-off ganglion cell, is a major route, possibly the only route for information from S cones to reach striate cortex and visual perception. This channel exhibits relatively little cone opponent behavior. Both white and blue lights excite this cell. Long wavelengths excite the cell when they go off, producing strong responses to successive color contrast. It responds best to white or blue after yellow. This cell informs the brain that the S cone system is or is not absorbing significant light in a particular area of visual space.

Whether there is also an S cone off channel is unclear. Few investigators have detected it. The strongest evidence for its existence comes from measurements with stimuli changing along the tritanopic axis of color space. It is possible that some of the more numerous tonic L and M cone opponent cells could also respond uniquely to such a stimulus. Because S cones are not involved in achromatic contrast, an off channel may not be necessary. Evidence suggesting the absence of an S cone off channel is the subjective phenomenon of transient titanopia, which involves a brief weakening of the appearance of blue whenever a long-wavelength field is turned off. There is no corresponding weakening of yellow when a short-wavelength field is turned off. S cones appear to lack the ability to antagonize the other cone channels at the retinal level, presumably because their image is so out of focus that it would interfere with achromatic vision.

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