Perceptual Thresholds

     Gustav Fechner defined the absolute threshold as the lowest level of stimulation that a person can 
detect. This was redefined to the person having a 50% chance of detecting it.

Ψ  Examples of absolute thresholds: 
•  Vision - candle flame at 30 miles on a clear dark night 
•  Hearing - tick of a watch at 20 feet 
•  Taste - one teaspoon of sugar in two gallons of water 
•  Touch - wing of a fly falling onto your cheek from a height of 1 centimeter. 
•  Smell - one drop of perfume diffused throughout a three-room apartment. 
     A subliminal stimulus is one that a person has less than a 50% chance of detecting.
     Just-Noticeable Difference (JND) - the difference between two stimuli that (under properly controlled 
experimental conditions) is detected as often as it is undetected.
     Weber's law - the concept that a Just-Noticeable Difference (JND) in a stimulus is proportional to the 
magnitude of the original stimulus.

Sensation versus Perception

     Sensation - the faculty through which the external world is apprehended.
     Perception - The process of organizing information received through the senses and interpreting it. This 
is done by the conscious, mentally aware brain.
    Perception is only a special kind of knowledge, & sensation a special kind of feeling. . . . Knowledge and 
feeling, perception and sensation, though always coexistent, are always in the inverse ratio of each other. 
--Sir W. Hamilton.

     Steps in changing sensations into perceptions:

         1. Stimulus
         2. Transductions - physical to electrical signals
         3. Brain: primary area - nerve impulses into sensations.
         4. Brain: association area - sensations into perceptions.
         5. Perceptions become personalized.

Rules of Organization

     Gestalt psychologists believe that perception is governed by rules which allow the whole to be seen 
(the rules are self-evident by observation). The Gestalt psychologists won the debate with Structuralists who 
felt all the basic elements need to be assembled for perception to take place.
     Gestalt - a configuration or pattern of elements so unified as a whole that it cannot be described merely as 
a sum of its parts.

     Perceptual Rules of Organization

     Figure-ground rule - the figure with more detail stands out
      Similarity rule - similar objects are grouped together
       Closure rule - missing parts are filled in
        Proximity rule - if they are close they are grouped together
         Simplicity rule - organized as simply as possible
          Continuity rule - smooth paths 

Perceptual Constancy

    Perceptual Constancy - denotes the tendency of animals & humans to see familiar objects as having standard 
shape, size, color, or location regardless of changes in the angle of perspective, distance, or lighting. 

Types follow:
               -  Size constancy
                    -  Shape constancy
                        -  Brightness constancy
                            -  Color constancy 

Depth Perception - the ability to distinguish objects in a visual field.

Binocular depth cues depend on the movement of the both eyes.
                 1. Convergence
                     2. Retinal disparity

Monocular depth cues;
                 1. Linear perspective
                     2. Relative size
                         3. Interposition
                             4. Light & shadow
                                 5.Texture gradient
                                     6. Atmospheric perspective
                                         7. Motion parallax

ILLUSION - an erroneous mental representation.

An illusion is surely a strange perception.

A list of well known illusions follow:

     Impossible figure
         Moon illusion
             Ames room
                 Müller-Lyer illusion
                     Ponzo illusion 

    We can learn about perceptions & how to manipulate them from the study of illusions. 

                                                  Topics in Psychology
                                                       Robert C. Gates