Objectives Chapter 1, Part 1• Illustrate and describe the perceptual cycle• Explain the difference between top-down and

bottom-up processing• Compare and contrast psychophysical and

physiological approaches to studying perceptual processes

Why Study Perception?

• Understanding how you perceive the world

– Language processing

– Color vision

– Depth perception

• Future careers in Research

– Multitasking - Driving and cell Phones

– Optimizing Computer Monitor Displays

• Medical applications

– Assisting people with vision and hearing losses by understanding their needs

Shall we play a game?

Perception is cyclical. It never ends!

The Perceptual Process - continued

• Transduction– Change from environmental energy to

electrical energy in the nervous system

• Neural processing

– Interconnected neurons that propagate the electrical signal from receptor cells throughout the brain

Two Interacting Aspects of Perception

• Bottom-up processing

– Processing based on incoming stimuli from the environment

– Also called data-based processing

• Top-down processing

– Processing based on the perceiver’s previous knowledge

– Also called knowledge-based processing

Approaches to the Study of Perception

• Levels of Analysis

– Observing perceptual processes at different scales

• Psychophysical level - the stimulus-perception relationship (Line A)

• Physiological level - the stimulus-physiology relationship (Line B)

– These levels are interconnected, but we usually have to focus on specific parts in controlled experiments

Objectives Chapter 1, Part 2• Define psychophysics.

• Describe ways to measure perception.

• Explain mental processes with mathematical laws.

• Compare and contrast Absolute Threshold and Difference Threshold.

Psychophysics - Qualitative Methods

• Description

– Basic description of what a person perceives

– First step in studying perception

– Called phenomenological method

• Recognition

– Categorization of stimuli

Psychophysics - Quantitative Methods

• Absolute threshold - smallest amount of energy needed to detect a stimulus

– Method of limits

• Stimuli of different intensities presented in ascending and descending order

• Observer responds to whether she perceived the stimulus

• Cross-over point is the threshold

Quantitative Methods - continued

• Absolute threshold (cont.)

– Method of adjustment

• Stimulus intensity is adjusted continuously until observer detects it

• Repeated trials averaged for threshold

Quantitative Methods - continued

• Absolute threshold (cont.)

– Method of constant stimuli

• 5 to 9 stimuli of different intensities are presented in random order

• Multiple trials are presented

• Threshold is the intensity that results in detection in 50% of trials

Method of Constant Stimuli

Quantitative Methods - continued

• Difference Threshold (DL) - smallest difference between two stimuli a person can detect

– Same methods can be used as for absolute threshold

– As magnitude of stimulus increases, so does DL

– Weber’s Law explains this relationship

• DL / S = K

Table 1.3 Weber fractions for a number of different sensory dimensions

Quantitative Methods - continued

• Magnitude estimation

– Stimuli are above threshold

– Observer is given a standard stimulus and a value for its intensity

– Observer compares the standard stimulus to test stimuli by assigning numbers relative to the standard

Quantitative Methods - continued

• Magnitude estimation (cont.)

– Response compression

• As intensity increases, the perceived magnitude increases more slowly than the intensity

– Response expansion

• As intensity increases, the perceived magnitude increases more quickly than the intensity

Actual Intensity of the stimulus.

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Quantitative Methods - continued

• Magnitude estimation (cont.)

– Relationship between intensity and perceived magnitude is a power function

– Steven’s Power Law• P = KSn

Other Measurement Methods

• Searching for stimuli

– Visual search - observers look for one stimulus in a set of many stimuli and compares it to stored data (memory)

• Reaction time (RT) - time from presentation of stimulus to observer’s response is measured

Objectives Chapter 1, Part 2• Define psychophysics.

• Describe ways to measure perception.

• Explain mental processes with mathematical laws.

– Weber’s law

– Steven’s Power Law

• Compare and contrast Absolute Threshold and Difference Threshold.

• Reading2 and 3 (synesthesia: Hard-Wired or Learned?)– What impact would synesthesia have on visual search?