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Rods and Cones Form the Basis for Scotopic and Photopic Vision

The human visual system has two subsystems that operate at different light energy levels. The scotopic, dark-adapted system operates at low levels of illumination, whereas the photopic, light-adapted system operates at high levels of illumination.

Rods are responsible for the initiation of the scotopic visual process. Rods

• contain the photopigment rhodopsin, which breaks down when exposed to a wide bandwidth of light (i.e., it is achromatic).
  
  • Rhodopsin is also more sensitive to light and reacts at lower light levels than the color sensitive (chromatic) cone pigments.
    
• have longer outer segments, more outer segment disks and, consequently, contain more photopigment.
  
• are more sensitive to light and function at scotopic (low) levels of illumination.
  
• dominate in the peripheral retina (Figure 14.21A), which is color insensitive, has poor acuity (Figure 14.21B), but is sensitive to low levels of illumination.
  

Cones are responsible for the initiation of the photopic visual process. Cones

• contain photopigments that breakdown in the presence of a limited bandwidth of light (i.e., cone photopigments are chromatic).
  
• are color sensitive.
  
• are less sensitive to light and require high (daylight) illumination levels.
  
• are concentrated in the fovea (Figure 14.21A)
  
• in the fovea have image of the central visual field projected on them.
  
• in the fovea are responsible for photopic, light-adapted vision (i.e., high visual acuity and color vision) in the central visual field (Figure 14.21B)

Biochemical processes in the photoreceptors participate in dark and light adaptation. Notice when you enter a darkened room after spending time in daylight, it takes many minutes before you are able to see objects in the dim light. This slow increase in light sensitivity is called the dark-adaptation process and is related to the rate of regeneration of photopigments and to the intracellular concentration of calcium³. A contrasting, but faster, process occurs in high levels of illumination. When you are fully dark-adapted, exposure to bright light is at first blinding (massive photopigment breakdown and stimulation of photoreceptors) and is followed rapidly by a return of sight. This phenomenon, light adaptation, allows the cone response to dominate over rod responses at high illumination.