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Figure 2 Normalized absorption spectra of the three cone mechanisms of human vision plotted against the fourth root of wavelength, which tends to make these curves independent of their position on the abscissa. An equivalent wavelength abscissa is shown above. Short-wave- (S), middle (M), and long-wavelength (L)-sensitive cones. The colors perceived at different points in the spectrum are shown below.

organized in an identical way, although there may be slightly more L than M cones. The central fovea contains only L and M and no S cones. L and M cones in the fovea are slender, optimizing their ability to sample small areas of visual space.

Each L and M cone synapses with at least two different bipolar cells. One is an on bipolar that is excited (depolarized) whenever its cone or cones absorb light; the other is an off bipolar, which is excited (depolarized) whenever light absorption by its cone or cones decreases (Fig. 3). This is a push-pull system that provides excitatory signals for increments (lightness) in one channel and excitatory signals for decrements (darkness) of light energy in a parallel channel. These signals are used in visual cortex to sense both energy (achromatic) and wavelength (chromatic) contrasts.

In the fovea, each L and M cone on and off bipolar synapses with a single cone (Fig. 3, left). Each single bipolar cell synapses with a single on or off ganglion cell. This is the so-called ''midget cell'' system discovered by Stephen Polyak. The signals from single L or M cones are transmitted by relay cells in the parvo cellular layers of the lateral geniculate nucleus of the thalamus to striate cortex mediating the high visual resolution of the fovea.

Away from the fovea, the midget arrangement ceases and several L and/or M cones synapse with single on or off bipolar cells. The breakdown of the midget system creates a problem for transmitting information about both energy and wavelength con-

Figure 3 The retinal circuitry of the parvocellular L and M cone system. The midget system on the left is characteristic of the fovea, where each L and M cone has a private on and off bipolar and ganglion cell. Away from the fovea, this private line breaks down. Some cells, midget-like, are thought to preserve the selectivity for only one type of cone, a prerequisite for chromatic contrast.

Figure 3 The retinal circuitry of the parvocellular L and M cone system. The midget system on the left is characteristic of the fovea, where each L and M cone has a private on and off bipolar and ganglion cell. Away from the fovea, this private line breaks down. Some cells, midget-like, are thought to preserve the selectivity for only one type of cone, a prerequisite for chromatic contrast.

trast in the same neural channel. For energy contrast, it is best to minimize the area within which the cones are selected. Therefore, selecting both L and M cones would be preferable (Fig. 3, right). For wavelength contrast it is best to select only L or only M cones in any one area. This tends to expand the area from which cones were selected and therefore decrease spatial resolution. It is not clear how this situation is handled.

1. The Parvo System: L and M Cone Antagonism

The selectivity for either an L or M cone input (Fig. 3) exposes antagonistic cone interaction in these retinal ganglion cells that depends on the wavelength of stimulation and is mediated by amacrine and horizontal cell interneurons. Ganglion cells transmitting signals of L cones are antagonized by stimuli that affect M cones and vice versa. This antagonism enhances both achromatic and chromatic contrast.

It enhances achromatic spatial contrast by reducing a ganglion cell's response to large but not small stimuli covering the center of the receptive field of the ganglion cell. It enhances chromatic contrast by reducing a ganglion cell's spectral response to green light if it is transmitting signals of L cones or reducing the cell's response to red light if it is transmitting the signals of M cones. It also enhances successive chromatic contrast, which occurs when a stimulus moves over the retina. The removal of green (antagonizing) light as red light enters the receptive field of an on ganglion cell transmitting the signals of L cones will enhance its response to the following red light. The removal of red (antagonizing) light as green light enters the receptive field of an on ganglion cell transmitting the signals of M cones will enhance its response to the green light. Off cells will also exhibit successive color contrast in the opposite way.

Retinal ganglion cells showing this behavior, called ''cone or color opponency,'' are a system of small cells relatively concentrated at the fovea, including the ''midget system.'' These cells transmit their signals to the parvocellular layers of the lateral geniculate nucleus of the thalamus. This system seems to be involved in both achromatic and chromatic contrast and is responsible for the high spatial resolution of the fovea. These cells receive no input from S cones.

2. The Magno System: L and M Cone Synergism

There is a parallel system of on- and off-bipolars and ganglion cells transmitting signals from L and M cones to striate cortex via relay cells in the magnocellular layers of the lateral geniculate nucleus. These are larger cells, which have faster conduction velocities and respond phasically to maintained stimuli. They mix synergistic signals of L and M cones. They are only involved in achromatic contrast, showing no color opponency (Fig. 4). This phasic magnocellular system is relayed to layer 4C alpha of striate cortex. The system is not as foveally oriented as the parvosystem. It is not involved in high spatial resolution and color vision; it receives no input from S cones. It seems to play a role in the detection of movement and body orientation and perhaps brightness perception.

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