PSYC 351A (Chapter 2B)

CogLab: Signal Detection:

[ How do we detect stimuli in our environment? ]

Task: Detect Target (10 dots in a diagonal row) among Noise (star field) 

• Two Choice Decision: Target Present or Absent 

• Independent Variable: Amount of noise (144,400 or 1,000 dots) 

(presence (50%) or absence (50%) of target) 

• Dependent Variable: response of “present” or “absent”

Hit Rate :

Hits / Hits + Misses

False Alarm Rate:

False Alarms / False Alarms + Correct Rejections

Sensitivity: (0= no discrimination, larger value = better discrimination) 

• Ability to discriminate signal from noise 

• Depends on stimulus, sensory system, attention, etc….

[d’ = Z(hit rate) - Z(false alarm rate) ]

Bias: (<0=bias for “present”, 0=no bias, >0 = bias for “absent”) 

• Tendency to provide one answer over the other 

• A feature of decision making, can depend on relative cost of misses versus false alarms

[C = Z(hit rate) + Z(false alarm rate) / 2]

Direct Perception Theories : Bottom-up processing

• Perception comes from stimuli in the environment 

• Parts are identified and put together, and then recognition occurs

Constructive Perception Theories: Top-Down processing 

• People actively construct perceptions using information based on expectations 

Bottom-Up Processing: Recognition by-components theory (RBC) - Irving Biederman (1987) 

• We perceive objects by perceiving elementary features 

• Geons: three-dimentional volumes 

• Objects are recognized when enough information is available to identify objects geons 

[ Parts are identified and put together, and then recognition occurs ]

Geons: Three-dimentional volumes 

• Discriminability: geons can be distinguished from other geons from almost all viewpoints 

• Resistance to Visual noise: geons can be perceived in “noisy” conditions 

• Invariance: recognizable no matter the illumination direction, surface markings, and texture

• Distinctiveness: 36 different geons have been identified 

[ objects are recognized by identifying geons and their relationships ]

Geons & RBC:

 we can recognize objects from a subset of geons 

• Airplane composed of 9 geons

[ Recognized correctly: 96% of the time from 6 geons // 78% of the time from 3 geons ]

Principal of Componential Recovery:

 The key to object recognition is not the amount of information, but the ability to identify its components (geons)

Top-Down Processing: Perception Involves: 

• Inferences based on context (surrounding elements of the visual scene) 

• Guessing from experience (knowledge and expectations based on the past)

Perceiving Size: bottom-up processing 

the size of the image on the retina

Perceiving Size: top-down processing 

the perceived distance of the object // the size of the object relative to other objects in the environment

Job of Perception:

Infer the distal stimulus given only the proximal stimulus

Helmholtz’s Theory of Unconscious Inference (~1860) : Top-Down Theory 

• Our perceptions result from unconscious assumptions we make about the environment (we use knowledge to inform our perceptions) 

• We infer much of what we know about the world

Likelihood Principle….

Likelihood Principle:

 we perceive the world in the way that is “most likely” based on our past experiences

Gestalt Laws: Gestalt Psychology (~1900) 

• The mind groups patterns according to laws of perceptual organization 

• These “laws” are actually heuristics based on what usually happens in the environment 

[ Heuristic = “rule of thumb” ]

• Provides best-guess solution to a problem 

• Fast

• Often correct 

“The whole is greater than the sum of its parts”

Gestalt: Law of Good continuation:

 lines tend to be seen as following the smoothest path

Gestalt: Laws of good figure

(simplicity or pragnanz) : every stimulus is seen so the resulting structure is as simple as possible

Gestalt: Law of similarity:

similar things appear grouped together

Gestalt: Law of Proximity:

nearby objects appear grouped together

Gestalt: Law of Closure:

separate elements will tend to be grouped to form closed figures

Gestalt: Law of familiarity:

things are more likely to form groups if the groups appear familiar or meaningful