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Sensation and Perception





3.1 Sensation and Perception

The difference between sensation and perception could be described with the picture below.  The sensation is the stimuli you and me receive.  In this example we all see a beautiful beach,  However, the perception that we each have could be quite different. For me, this picture evokes happy childhood memories.  For some of you, this picture could remind you of a friend, or maybe that you are afraid of sharks…etc.  The point is that even though we are all looking at the same picture, we could perceive it differently. 




  • Process by which physical energy from the environment is detected by sense receptors and then encoded as neural signals


  • Process by which we organize and interpret the sensory information, allowing us to give meaning to objects and events

There are a few ways researcher have develop to measure how we sense and perceive stimuli.

Absolute threshold

  • The minimum stimulation required to detect a particular stimulus 50% of the time.
  • They are not “absolute”. Some people are more sensitive than others.
      • Examples of some measures of the absolute thresholds for humans:
        • Vision: a candle flame viewed from about 30 miles on a clear dark night
        • Hearing: a watch ticking from about 20 feet away in a quiet room

Difference threshold (aka just noticeable difference) – the minimum difference a person can detect between any two stimuli 50% of the time.

Weber’s Law – to be perceived as different, two stimuli must differ by a constant proportion of that stimulus (the particular proportion depends on the particular type of stimulus!)

  • Proportion depends on type of stimulus:
    • Weights- must differ by about 2%
    • Sound intensity – must differ by about 4%



3.2a Light

The Visible Spectrum

  • Hue (color)
    • Determined by wavelength – the distance from one wave peak to the next
    • Visible light ranges from approx. 400-700nm
  • Intensity
    • Determined by wave’s amplitude
    • Determines brightness of the light



  • Cornea: Transparent eye cover.
  • Pupil: Small, adjustable opening. It is sensitive to light and emotion.
  • Iris: Colored muscles surrounding pupil. It adjusts amount of light entering eye.
  • Lens: Focuses incoming light rays onto retina and it adjusts by changing its thickness.
  • The retinais the light-sensitive inner surface onto which light rays focus.
    • It consists of tow types of Photoreceptors:
      • Rods:
        • black and white vision
        • Night Vision (low level of illumination)
      • Cones:
        • Color Vision
      • Daytime Vision (high illumination levels
    • bipolar cells– neurons that conduct neural impulses from rods and cones to ganglion cells
    • ganglion cells– neurons whose axons form the optic nerve
    • optic nerve– the nerve that transmits sensory information from the eye to the brain
    • Fovea
      • Central focal point in the retina
      • Contains only cones, no rods
      • The periphery contains all of the rods, but many cones as well
      • Cones are capable of higher resolution than rods
      • Overall, there are FAR more rods than cones in the retina

Visual Acuity:

  • Sharpness of vision is affected by small distortions in the eye’s shape.
  • Nearsightedpeople have to be close to an object to discriminate its details: eyeball too long causing distant objects to be focused in front of the retina.
  • Farsightedpeople face difficulty in focusing on nearby objects: eyeball too short causing nearby objects to be focused behind of the retina.
  • Presbyopia
    • Makes it difficult to perceive nearby visual stimuli
    • Caused by loss of elasticity of the lens



3.4 Hearing

  • Sound waves require a medium, such as air or water, to travel.
  • Human ear is sensitive to sound waves with frequencies of 20 to 20,000 cycles per second.

3.4a Pitch and Loudness

  • Pitch
      • Determined by a sound’s frequency
      • Frequency – Number of cycles per second expressed in hertz (Hz)
        • 1 Hz =1 cycle per second
      • Greater the frequency, the higher the pitch of the sound
    • Loudness
        • Height (amplitude) of sound waves
        • Frequency and amplitude are independent
        • Expressed in decibels (dB)



3.4b The Ear

  • The ear is shaped and structured to:
      • Capture sound waves
      • Vibrate in sympathy with the waves
      • Transmit auditory information to the brain


  • Outer ear
    • Funnels sound waves to the eardrum, which vibrates in response to sound waves
  • Middle ear
    • Contains the eardrum, hammer, anvil, and stirrup
    • Acts as an amplifier
    • Oval window and round window work in conjunction, which balance the pressure in the inner ear
  • Inner ear
    • Oval window transmits vibrations into the cochlea
      • Basilar membrane lies coiled within the cochlea
      • Organ of Corti is attached to the basilar membrane
    • Neural impulses created by movement of hair cells are transmitted to the brain via the auditory nerve
  • Locating Sounds
    • Loudness and the sequence in which sounds reach the ears provide directional cues
      One may turn his or her head to locate the sound

3.4d Perception of Loudness and Pitch

Research suggest that it takes three processes to perceive sound

Place theory

  • Pitch is sensed according to the place along the basilar membrane that vibrates in response
  • It only applies to higher frequencies over 4000 Hz

Frequency theory

  • Frequency of the sound waves needs to match with one’s neural impulses in order to perceive lower pitches
  • For 20 to a few hundred Hz per second, hair cells on the basilar membrane fire at the same frequencies as the sound waves

Volley Principle

  • For frequencies between a few hundred and 4000 Hz
  • In response to sound waves of these frequencies, group of neurons take turns at firing



3.3 Visual Perception

Visual Perception:

Process used to organize sensory impressions caused by the light that strikes one’s eyes. It is an active process.  Involves knowledge, expectations, and motivations

  • Sensationis a mechanical process

3.3a Perceptual Organization

Gestalt psychologists noted that we our perceptions are guided by certain rules.

  • Perceptual Organization: Gestalt psychologists noted that we organize information in a certain way consistently: integrating bits of information into meaningful wholes. 
    • Figure-Ground Relation:( The Rubin Vase)

Top Down versus Bottom up Processing

  • Top Down: Use of contextual information or knowledge of a pattern in order to organize parts of the pattern
  • Bottom up: Organization of the parts of a pattern to recognize, or form an image of, the pattern they compose


3.3c Depth Perception

Monocular cues (cues we can receive with one eye, usually used by artists in pictures and paintings):

    • Perspective– convergence of parallel lines as they recede into the distance

Relative size – distant objects look smaller than nearby objects of the same size


  • Clearness– we see more details in nearby objects


  • Overlapping– nearby objects can block more distant objects


  • Shadowing– shadows could make us perceived objects as 3-d


  • Texture gradient– Closer objects are expected to have rougher textures



Motion parallax – a monocular cue for depth based on movement objects in relation to our motion.

    • Objects closer than the fixation point seem to move backward (they move through your visual field in the direction opposite to your direction of travel)
    • The closer objects are to you (i.e. the further the object is from your fixation point), the faster they move through your visual field
    • Objects beyond the fixation point seem to move WITH you (in the same direction as your direction of motion)


Binocular cues: Involve both eyes and help perceive depth

    • Types include:
    • Binocular cues:Involve both eyes and help perceive depth
    • Retinal Disparity– refers to the difference between where an object’s image falls on the left retina compared to the right retina


3.3d Perceptual Constancies

  • Acquired through experience and creates stability
      • Size Constancy–  the tendency to perceive an object as being the same size, even though its size varies depending on its distance.
        • Color Constancy– the tendency to perceive an object as being the same color, even though lighting conditions change its appearance
          • Brightness Constancy– the tendency to perceive an object as being just as bright even though lighting conditions change its intensity.


      • Shape Constancy– the tendency to perceive an object as being the same shape although the image varies in shape as it rotates



3.5 The other senses


  • Contributes to the flavor of foods
  • Odors are sample molecules of substances in the air
  • Trigger firing of receptor neurons in the olfactory membrane
  • Sensory information about odors is sent to the brain through the olfactory nerve


  • Sensed through taste cells
  • Taste cells: Receptor neurons on taste buds
  • Taste qualities
  • Sweet, sour, salty, and bitter
  • Umami (fifth basic taste) – Savory
  • Flavor of food depends on odor, texture, temperature, and taste
  • Some people have superior taste sensitivity than others


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