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                Images] Sensation & Perception: Vision I

The World of Sensations (after Kardas)

Imagine what the world is like if you are not a human being?
(after Colavita, 2006)

Cats: Do not perceive color  Humans: Perceive color

Bees and Butterflies: See ultraviolet light  Humans: Do not see ultraviolet light
Visible vs. Ultraviolet Light

Snakes: See infrared light  Humans: Do not see infrared light
Dogs: Recognize smells 1/1000 weaker than humans do
(Smell receptors in nose =
200,000 in dogs vs. 20,000 in humans)

Bats: Hear ultrasonic sound (> 20 kH)  Humans: Cannot hear ultrasonic sound (< 20 kH)

[Human vs. Dog Vision]

Energy Sources Perceived by the Senses



There is an orderly sequence by which light in the external world is transformed by the eye into neural signals and transmitted to the brain for perception. This process, as a whole, is what we mean by vision.

This orderly sequence codes the visual field in such a way that, 125 million photoreceptor cells (cones & rods) send their information over the 1 million axons of the optic nerve.

A. The Structure of the Eye

[The Human Eye]

B. The Retina

C. The Visual Pathways

D. Information Processing in the Visual Cortex

[Hubel-Wiesel Procedure] [Hubel
                    & Wiesel\

In the early 1960s, Hubel & Wiesel discovered how the cortex (occipital lobe) begins to process visual information. Experiments with cats showed three types of cells:

Feature Detection: neurons in the visual cortex respond only selectively to very specific features of a stimulus

Beyond the occipital cortex, visual information is further processed in the ventral & dorsal streams or pathways (see diagram on right).

E. Seeing Color

What are other qualities of light and its perception?

                - Physical & Perceptual Qualities]
  • Human beings can perceive approximately 200 different hues in the visual spectrum between 380 and 700 nm (see diagram above).
  • Yet, because each hue can be altered along two further dimensions -- brightness and saturation (purity) -- humans can probably discriminate among about 2 million different colors.

E. Color Perception

1. Trichromatic Color Perception (Retina) is an additive process

  • Blue
  • Green
  • Red
  • Blue + Green = Marine Blue or Cyan
  • Green + Red = Yellow
  • Blue + Green + Red = White

2. Opponent-Process Color Perception (Ganglion Cells) in an excitation/inhibition process

  • The Trichromatic Theory does not explain three phenomena
Negative (Inverted) Afterimages =
After long stimulation of looking at colors, we tend to see a continuing image in reversed color when we look at a blank page.

[Inverted US Flag]

Contrast Effects =
Same wavelength is perceived differently depending upon contrasting colors

[Contrast Effects]

Red-Green & Blue-Yellow

{Color vision: normal & red-green colorblind]

[Blue-Yellow & Achromatopsia]

  • How do we account for these? A second level of color processing occurs among the ganglion cells and in the lateral geniculate nucleus which can account for these phenomena
    • Red/Green ganglion cells: signal red when stimulated and green when inhibited (and vice versa)
    • Blue/Yellow ganglion cells: signal blue when stimulated and yellow when inhibited (and vice versa)
    • All colors together brightly sum up to white while low levels of all colors give shades of grey

[Trichromatic-Opponent-process Vision]


This page originally posted on 02/02/04 and updated on 09/20/2011


Colavita, F. (2006). Sensation, Perception, and the Aging Process: Guidebook, Pt. 1. The Teaching Company.

Kardas, E. Physical forces and associated sensory systems. Retrieved February 1, 2004 from the World Wide Web: http://peace.saumag.edu/faculty/kardas/Courses/GPWeiten/C4SandP/Forces.html.

WebExhibits.Com. Causes of Color.