|PSY 101 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
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)
Energy Sources Perceived by the Senses
- Sense: Vision (= seeing)
- Electromagnetic energy in the form of photons which act both as a particle and a wave.
- The human eye sees only a small proportion of the electromagnetic spectrum
- Sense: Audition (= hearing)
- Pressure waves which require some type of medium to transmit (e.g., air, solid, liquid).
- The ear converts these pressure waves into the perception of sound
- Senses: Gustation (= tasting) and Olfaction (= smelling)
- These can either be
- (1) dissolved in some type of liquid or edible solid or
- (2) free-floating in the form of volatile molecules in the atmosphere.
- The taste buds of the human tongue detect #1 while the olfactory nerve cells in our nose detect #2.
- Mechanical Forces
- Sense: Cutaneous senses (= touch, temperature, pain)
- We are subject to a broad range of mechanical forces/energies. These include heat, cold, pressure, and pain.
- The skin has a variety of "cutaneous" sensory systems which detect these forces (touch is more than one sense!)
- Gravity & Forces of Acceleration/Deceleration
- Senses: Kinethesia (= feeling movement through space) & proprioception (= knowing position of body in space)
- Moving on the earth we experience the effects of gravity and acceleration.
- The body has developed a set of senses (vestibular, kinesthetic-proprioceptive) to detect how these forces act upon the body.
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
B. The Retina
- Light passes through the cells of the retina to two types of photorceptor cells: rods and cones
- Cones are sensitive to color, fine discrimination (high visual acuity) and well adapted to bright/strong light (daylight); located primarily in a small region around the fovea.
- There are three types of cones, each maximally sensitive to a specific range of light. These are
- "Blue" cones (short wavelength: ca. 450 nm)
- "Green" cones (medium wavelength: ca. 540 nm)
- "Red" cones (long wavelength: ca. 580 nm)
- Rods are sensitive to low levels of light (night vision). They do not distinguish among colors, but shades of gray. Outside the fovea, rods make up the vast majority of retinal photoreceptor cells.
- Dark Adaptation = under low illumination, the eye becomes more sensitive to light. It takes 15-30 minutes for the rods to become maximally sensitive
- Light Adapation = under high illumination (very bright light), the eye becomes less sensitive to light. This is why you have some difficulty seeing inside a tunnel when you drive from the very bright sun.
- Both types of photoreceptor cells convert light energy (photons) into neural impulses. This process is called transduction.
- The two photoreceptor cells (rods & cones) pass their impulses to a second layer of cells consisting of horizontal, bipolar, and amacrine cells. These, in turn, pass coded visual information to ganglion cells whose axions leave the eye and form the optic nerve. (See below for color perception.)
C. The Visual Pathways
- Information from the left visual field is transmitted to the right retina of both eyes and information from the right visual field is transmitted to the left retina of both eyes.
- The optic nerve from each eye meet at the optic chiasm. There, information from the right retina of the left eye and the left retina of the right eye cross over to the other side of the brain.
- Visual information converges at the lateral geniculate nucleus of the thalamus on each side of the brain. There the information is processed and transmitted to the visual cortex of the occipital lobe at the back of the brain.
D. Information Processing in the Visual Cortex
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
- Simple cell: cell responds to a particular line orientation (see diagram above) at a particular location in the receptive field
- Complex cell: cell responds about width, orientation, but at any location in the receptive field. Sensitive to movement of line
- Hypercomplex cell: cell is sensitive to the length of the particular line
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?
- 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 = 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 phenomenaNegative (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.
Contrast Effects =
Same wavelength is perceived differently depending upon contrasting colors
- 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
ReferencesColavita, 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.