2015 Vision

Example Test QuestionPhotoreceptors

Critical Fact Icon
    Rods are responsible for SCOTOPIC vision (the monochromatic vision that occurs in low light).  The three types of cones (blue, green and red; or Short, Medium and Long wavelength) have better temporal and spatial resolution than rods, making PHOTOPIC VISION better for discrimination of surfaces and movement under bright light conditions.

Rods

  • more photopigment
    (~108 rhodopsins/outer segment)

  • single pigment type = rhodopsin
    • spectral sensitivity falls between blue and green cones

  • high sensitivity (single photon)
    • saturate in daylight
    • smaller dynamic range

  • lower temporal resolution
    • slow response
    • signal integration

  • poor spatial resolution
    • not in fovea
    • respond to scattered light
    • high degree of convergence onto bipolar cells

Absorbance spectra

 

Distribution of rods & cones

Cones
  • less photopigment

  • 3 pigment types (red, blue, green)
    • absorbance of the pigments overlap, so that different colours produce different patterns of activity
    • extreme red wavelengths don't overlap with rod spectral sensitivity

  • lower sensitivity (multiple photons)
    • saturate only in intense light
    • larger dynamic range

  • high temporal resolution
    • faster response
    • less signal integration

  • good spatial resolution
    • concentrated in fovea
    • respond best to narrow axes of light
    • low degree of convergence onto bipolar cells
Email: Dr. Janet Fitzakerley | ©2015 University of Minnesota Medical School Duluth | Last modified: 12-feb-15 6:05 AM