6.3

• The first problem of the inside-out arrangement of the retina is minimized by the fovea.
• An indentation, about 0.33 centimeter in diameter, at the center of the retina it is the area of the retina that is specialized for high-acuity vision (for seeing fine details).

  Fovea
• When we look at big, plain areas, our brain doesn't see the color and brightness directly. Instead, it fills them in using a process called surface interpolation. This means our brain guesses what the whole surface looks like based on the edges and other clues it sees.
• The process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large surfaces.

  Surface Interpolation
• Two different types of receptors in the human retina:

  1. Cones
  2. Rods
• Species active only in the day tend to have cone-only retinas, and species active only at night tend to have rod-only retinas.
• Species active only in the day tend to have cone-only retinas, and species active only at night tend to have rod-only retinas.
• This theory states that cones and rods mediate different kinds of vision: Photopic vision and Scotopic Vision

  Duplexity Theory of Vision
• Photopic vision (cone-mediated vision) predominates in good lighting and provides high-acuity (finely detailed) colored perceptions of the world.
• Scotopic Vision predominates in dim illumination because there is not enough light to reliably excite the cones. The cost of this vision is that it lacks both the detail and the color of photopic vision.
• In the scotopic system, the output of several hundred rods converges on a single retinal ganglion cell.
• In the photopic system, only a few cones converge on each retinal ganglion cell.
• The convergent scotopic system pays for its high degree of sensitivity with a low level of acuity.
• There are no rods at all in the fovea.
• At the boundaries of the foveal indentation, the proportion of cones declines.
• At the boundaries of the foveal indentation, there is an increase in the number of rods.
• There are more rods in the nasal hemiretina than in the temporal hemiretina.
• A graph of the relative brightness of lights of the same intensity presented at different wavelengths is called a spectral sensitivity curve.
• The most important thing to remember about spectral sensitivity curves is that humans and other animals with both cones and rods have two of them:

  1. photopic spectral sensitivity curve
  2. scotopic spectral sensitivity curve
• This can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the fovea.

  photopic spectral sensitivity curve