Perception - the Basics

perception - "conscious sensory experience"

Perception involves many interrelated hidden perceptual processes that occur "behind the scenes" (Goldstein, 2002, p. 2); analogy to watching a play in a theater (i.e., the difference in "reality" between the audience perception and what goes on backstage.

• "[P]erception does not just happen but is the end result of complex processes, many of which are not available to your awareness." (Goldstein, 2002, p. 2)

It is these processes we will be learning more about.

The importance of perception

• inform us about properties of the environment that are important for our survival, creating a likeness of the environment (cognitive map) in our minds
• help us act (appropriately) in relation to our environment

environmental stimulus - all of the things in our environment that we can potentially perceive

Goldstein's model of the perceptual process (2002, p. 4)

(roll mouse over image below)

[Consider the instance of entering a circus "big top" and focusing attention on one of the many events taking place inside.]

Visual perception results from the sensation of an inverted image on the retina; why is the upside-down character not so much of a concern?

• we don't perceive the image on the retina; this is just an early step in the process
• the "upside-downness" of the image is such a small transformation compared to what happens next ...

Types of Perceptual Processing

• bottom-up - processing that begins with the information received by the receptors; e.g., tracking the perceptual process from sensory stimulation to transduction, to neural processing is an example of bottom-up processing
• top-down - processing that begins first by considering the effect of the knowledge a person brings to a perceptual situation; includes not only an individual's fact-based knowledge, but also memories and expectations

Excellent example: my sloppy writing!!

Perception often involves both bottom-up and top-down processing

Levels of Analysis

• psychophysical level - relationship between the stimulus and perception (physical to perceptual)
  • psychoacoustics - present a stimulus and determine the subject's response
• physiological level - relationship between the stimulus and measurable correlate physiological changes
  • physiological - present a stimulus and measure the electrical response in the nervous system
• combined psychophysical and physiological level - measure both in response to the same stimuli and determine relationships
  • physiological and psychoacoustic - measure physiological and perceptual responses to the same stimuli and determine relationship(s)

To truly understand perception, we must study the process at all three levels, since each one provides information about a different aspect of the processes involved. Each level is intimately related to the others and there is a significant amount of "cross-talk" between them.

The Psychophysical (psychoacoustical) Approach

Types of measurement:

• phenomenological (descriptive) - listing specific perceptual qualities of a stimulus; can be at a very basic level (color, size, [apparent] motion, etc.)
• recognition - (categorization) - placing the stimulus into a preconceived category of like items

[Do the above two types of measurement using musical examples]

Classical Psychophysical Methods (Fechner, 1860)

• detection - establishes a quantitative relationship between the stimulus and perception (Gustav Fechner's Elemente der Psychophysik [1860] laid the groundwork for the following classical psychophysical methods):
• absolute threshold - the smallest amount of stimulus energy necessary to detect a stimulus
  • method of limits - stimuli at discrete levels of intensity are presented in ascending (increasing) or descending (decreasing) order to determine threshold; e.g., hearing test
  • method of adjustment - stimulus intensity is adjusted in a continuous manner (either by the experimenter or subject) until the observer can just barely detect the stimulus
  • method of constant stimuli - stimuli with five to nine intentisy levels are presented in a random order; threshold is typically identified as that intensity that is detected on 50% of the trials.
• difference threshold (differenze Limen or "DL")- the smallest different between two stimuli that a person can detect

[distribute various items to allow students to determine threshold]

[frequency discrimination; ASA #17]
answers (standard f = 1000 Hz):
1. up, down, up, up (10 Hz)
2. up, down, down, down (9 Hz)
3. down, up, up, down (8 Hz)
4. down, up, up, down (7 Hz)
5. up, down, up, down (6 Hz)
6. up, down, up, up (5 Hz)
7. down, down, up, up (4 Hz)
8. up, down, up, down (3 Hz)
9. down, down, down, up (2 Hz)
10. down, up, up, down (1 Hz)

• as the magnitude of the stimulus increases so does the DL; Weber's Law (true for most senses) - the DL is a constant (K) function (i.e., 5% of standard weight), as long as the stimulus intensity is not too close to the absolute threshold

Other Psychophysical Measurements

• magnitude estimation - subject is asked to compare the intensity of a series of stimuli to that of a standard stimulus by assigning a number that expresses the perceived relationship; e.g., if the standard stimlus is arbitrarily assigned the value of "10," a stimulus perceived as twice as intense would be rated as "20"
  • the relationship between the intensity of a stimlus and our perception of its magnitude follows the same general equation for each sense
  • power function - the relationship is logarithmic, not linear (compare Fig. 1.15 to 1.16); Stevens' power law
• searching - the process of locating a known object in a novel environment (e.g., looking for a face in a crowd or presenting interleaved melodies)

[Where's Waldo?]

[interleaved melodies]

Important Outcomes of Perception

• recognition - ability to place an object in a category; perception and recognition are distinct processes
  • visual form agnosia (like "Dr. P" from Sacks' The Man Who Mistook His Wife for a Hat) - the inability to recognize objects
• action - motor activities (e.g., moving head and eyes or moving through the environment) intended to keep object of attention within visual/aural field
• knowledge - the information a person brings to a sensory situation; plays an important role in determining both recognition and perception

[Rat-Man demo]
[optical illusions - and a surprise]