The Senses

  • Psychophysics studies the relationship between the physical properties of stimuli and people’s experience of stimuli.
  • Psychologists assess the acuity of our senses by measuring the absolute threshold and the difference threshold and by applying signal detection theory.
  • Sensory adaptation is the decrease in sensitivity to an unchanging stimulus.
  • Babies are born with all the basic sensory abilities and some perceptual skills, which develop and become more sensitive over time.


  • The sense of vision depends on light, which is a kind of electromagnetic radiation emitted by the sun, stars, fire, and lightbulbs.
  • We experience light as color, brightness, and saturation, which depend respectively on wavelength, amplitude, and complexity of light waves.
  • The eye is composed of the cornea, the iris, the pupil, the lens, the retina, and the fovea. The lens adjusts its shape to focus light from objects that are near or far away in a process called accommodation.
  • Dark and light adaptation are processes by which receptor cells sensitize and desensitize to light, respectively.
  • The retina has millions of photoreceptor cells called rods and cones. Rods and cones connect via synapses to bipolar neurons, which connect to ganglion cells. The axons of the ganglion cells make up the optic nerve, which connects to the eye at the optic disk, also called the blind spot.
  • After being processed in the brain, visual signals reach the primary visual cortex, where feature detectors respond to the signals.
  • Color is a psychological experience created when the eyes and the brain interpret light.
  • Trichromatic theory, or the Young-Helmholtz theory, states that there are three types of cones in the retina, which are sensitive to light of different wavelengths corresponding to red, green, or blue. This theory accounts for color blindness.
  • The opponent process theory states that receptors act in opposite ways to wavelengths associated with three pairs of colors: red vs. green, blue vs. yellow, and black vs. white. The theory accounts for the perception of four primary colors. It also accounts for afterimages, or colors perceived after other complementary colors are removed.
  • Gestalt psychology proposes that the perceived whole sometimes has properties that didn’t exist in the parts that make it up. An example is the phi phenomenon, in which an illusion of movement occurs when images are presented in a series, one after another.
  • Gestalt psychologists describe principles people use to organize vision into units that make sense, including: figure and ground, proximity, closure, similarity, continuity, and simplicity.
  • Binocular and monocular cues enable people to determine distance from an object.
  • Perceptual constancy is the ability to recognize that an object is the same when it produces different images on the retina. Visual constancies relate to shape, size, brightness, color, and location.
  • Visual illusions are misinterpretations of visual stimuli.
  • Selective attention is the ability to focus on some pieces of sensory information and ignore others.


  • Hearing depends on sound waves. Sound has three features: loudness, pitch, and timbre, which depend respectively on wave amplitude, frequency, and complexity.
  • The ear comprises the outer ear, the middle ear, and the inner ear. These parts contain the pinna, the eardrum, ossicles, oval window, cochlea, and cilia.
  • Neurons in the ear form the auditory nerve, which sends impulses from the ear to the brain. The thalamus and auditory cortex receive auditory information.
  • Place theory and frequency theory explain how people distinguish the pitch of different sounds.

Taste and Smell

  • The stimuli for taste and smell are chemicals.
  • Taste occurs when chemicals stimulate receptors in the tongue and throat.
  • The five tastes are salty, sweet, sour, bitter, and umami.
  • Smell occurs when chemicals in the air are inhaled into the nose. Smell receptors send impulses along the olfactory nerve to the brain.

Position, Movement, and Balance

  • Kinesthesis is the sense of the position and movement of body parts.
  • The sense of balance gives information about where the body exists in space and involves the vestibular system.
  • The main structures of the vestibular system are the semicircular canals.


  • The sense of touch encompasses pressure, pain, cold, and warmth.
  • Pressure has specific receptors.
  • The gate-control theory of pain proposes that pain signals traveling from the body to the brain pass through a gate in the spinal cord. This gate is a pattern of neural activity that prevents pain signals or admits them.

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