Psychology Chapter 5: The Visual System

General Principles of Perception

• Each of our senses has specialized receptors that are sensitive to a particular kind of energy.
• Receptors for vision are sensitive to light.
• Receptors "transduce" (convert) energy into electrochemical patterns so that the brain can perceive sights, sounds, smells, etc.
• The law of specific nerve energies states that activity by a particular nerve always conveys the same type of information to the brain.
• Which neurons respond, the amount of response, and the timing of response influence what we perceive.

The Eye and Its Connections to the Brain

• Light enters the eye through the pupil.
• Light is focused by the lens and the cornea onto the rear surface of the eye, known as the retina.
• The retina is lined with visual receptors.
• Light from the left side of the world strikes the right side of the retina and vice versa.
• Visual receptors send messages to neurons called bipolar cells.
• Bipolar cells send messages to ganglion cells that are even closer to the center of the eye.
• The axons of ganglion cells join one another to form the optic nerve that travels to the brain.
• Amacrine cells are additional cells that receive information from bipolar cells and send it to other bipolar, ganglion, or amacrine cells.
• Amacrine cells control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli.

Color Vision

• The trichromatic theory explains that:
  • Each cone responds to a broad range of wavelengths, but some more than others.
  • The ratio of activity across the three types of cones determines the color.
  • More intense light increases the brightness of the color but does not change the ratio.
• The opponent-process theory suggests that:
  • We perceive color in terms of paired opposites.
  • The brain has a mechanism that perceives color on a continuum from red to green and another from yellow to blue.
  • A possible mechanism for the theory is that bipolar cells are excited by one set of wavelengths and inhibited by another.
• The Retinex theory suggests that the cortex compares information from various parts of the retina to determine the brightness and color for each area.
• Color vision deficiency is an impairment in perceiving color differences, and the gene responsible is contained on the X chromosome (~8% of men).

The Optic Nerve

• The optic nerve consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain.
• The point at which the optic nerve leaves the back of the eye is called the blind spot because it contains no receptors.