College of Education

School of Continuing and Distance Education2014/2015 – 2016/2017

PSYC 441

Cognitive Psychology II

Session 6 – Psychophysics

Lecturer: Dr. Benjamin Amponsah, Dept., of Psychology, UG, LegonContact Information: bamponsah@ug.edu.gh

Session Overview

• We are going to look at measurement of sensory capacities or how we measure and make decisions in psychology. We look at how we sense the world or detect stimuli and how these are measured. We shall look at the earlier attempts at measurement in the area of psychophysics. This constitutes the first part of psychological measurements.

Slide 2

Session Objectives

• At the end of the session, the student will be able to

• Describe the concepts of sensation and perception

• Discuss the basic concepts used in earlier psychophysical measurement.

• Describe the representative values suggested for defining sensory limits

Slide 3

Session Outline

The key topics to be covered in the session are as follows:

• Topic One: Measuring Sensory Capacity

• Topic Two: Conditions in Order to Experience Sensation

• Topic Three: Psychophysical Measures

• Topic Four: Measuring Sensory Intensity

Slide 4

Reading List

• Ashcraft, M. H. (2013). Cognition (6th edn.), London: Pearson Education Int.

• Eysenck, M. W. (2012). Fundamentals of Cognition (2ed). Psychology Press, Sussex.

• Galotti, K. M. (2017). Cognitive Psychology: In and out of the laboratory (6th edn.). Belmont, CA: Wadsworth.

• Glietman, H. (2011). Psychology (8ed). Norton, New York.

• Hunt, R. R. & Ellis, H. C. (2004). Fundamentals of Cognitive Psychology (7th edn.), New York: McGraw-Hill.

• Willingham, D, B. (2013). Cognition: The thinking animal (International edn. NJ: Prentice-Hall).

Slide 5

MEASURING SENSORY CAPACITIESTopic One

Slide 6

Psychophysics - Measuring Sensory Capacities

• We are about to explore how we sense, experience or interpret the world. We will look at measurement of sensory capacities (eye, nose, ear, skin, tongue) or specifically how we measure and make decisions in psychology.

• The sense organs are our link to reality. But what are the limits of the sense organs?

• An approach known as psychophysics will help us gain insight into such measurements.

• In psychophysics - physical changes in stimuli are measured and related to psychological sensations such as brightness, loudness, taste etc. This is the subject matter of Psychophysics.

Slide 7

Sensation and Perception

• The human brain is insulated from the outside world by layers of tissue. Knowledge from the outside world is carried into the brain through the sensory systems of the eye, nose, tongue, skin and the ear.

• Sensation refers to the initial detection of energy from the physical world. Sensation then deals with the structure and processes of the sensory mechanism and the stimuli that affect those mechanisms.

• Sensory systems (eye, nose, ear etc) respond to certain aspects of the environment to produce sensations such as light, sound, taste etc.

Slide 8

– We could consider Sensation as the psychological experience of a stimulus.

Sensations as we experience them are private or subjective events.

You may describe your sensations to others, but no one else can directly experience them.

Example:

If a particular change in the physical environment evokes a similar response from many people, we can then assume that we have experienced similar sensations (e.g., turning up the volume of a stereo may cause most listeners to describe the sound as loud).

Slide 9

• This shows that there is a consistent relationship between a physical stimulus (amount of energy coming from the environment e.g., speakers) and the listener’s sensations (descriptions of what they hear – sound).

• These relations are referred to as psychophysical relationsbecause they express a relationship between physical and psychological variables (stimuli and sensations).

• We can therefore say that the relationship between the physical changes of the world and the psychological experience associated with these changes is called psychophysics.

Slide 10

Perception

• Perception, on the other hand, involves higher-order

interpretation of the sensory information.

• Sensation refers to the initial detection of stimuli;

• Perception to an interpretation of the things we sense.

• We receive so many sensations. Each sensory event is

processed within the context of our knowledge of the world.

Our previous experiences give meaning to simple sensory

experiences and that is perception.

Slide 11

Sensory Systems

• Our body is equipped with sensory systems (ear, tongue, eyes etc) and these systems are sensitive to some form of physical energy.

• For example - our Auditory System responds to rapid variations in air pressure we call sound.

• Our Visual System responds to specific forms of electromagnetic energy we call light.

Slide 12

CONDITIONS IN ORDER TO EXPERIENCE SENSATION

Topic Two

Slide 13

Conditions in Order to Experience

Sensation

• 1. First as human beings, we can only respond to stimuli in the environment of which we have “receivers” or technically the receptor organs (ears, nose, eyes etc).

– Some of the stimuli we are unable to respond to. E.g., electromagnetic energy in the form of radio and television waves - our bodies cannot receive them unless we have a radio and a television.

• 2. Second, the energy that we are receiving must be intense enough (sensitivity) to produce a noticeable sensation. (If the energy is low in radio frequency we get poor reception)

Slide 14

The Five Senses

1. Receptors for hearing (Ear) – these are the hair cells in the inner ear

that responds to rapid vibration of air pressure.

Sense modalities and where their receptors are located

2. Receptors of touch, pain, warmth, cold and other skin senses

(Tactile senses) – These are mostly found at the tip of the fingers.

A B

Receptors for Touch

3. Receptors for odors (sense of smell). The receptors are located in

olfactory epithelium. It is estimated that humans have over 40 million

olfactory receptors.

Receptors for Smell

4. Receptors for taste (sense of taste – gustation) these are taste

buds and are grouped into structures called papillae (small

projections of tissue on the tongue).

Receptors for Taste

5. Receptors for the eye – retina (these receptors respond to

electromagnetic energy we call light.

Receptors for the Eye

PATHWAYS FROM STIMULUS TO RESPOND

Sensory nerves carry inbound information about the

body's movements and sensations.

Motor nerves carry impulses outward to the muscles and

glands.

Mixed nerves perform both functions.

PSYCHOPHYSICAL MEASURESTopic Three

Slide 22

Psychophysical Measurements

Psychophysical Measurements

• How did the early psychophysicists measure stimuli and sensation?

Measurement of Sensory Limits

• Early experimental psychologists who engaged in psychological

measurements tried to assess our sensory limits (the point at which one

can experience sensation) by varying the strength or intensity of a stimulus

in order to determine the minimum level capable of evoking a sensation.

– Two types of sensory limits :

• Absolute threshold and

• Differential threshold.

Slide 23

1. In psychophysics, Absolute Threshold refers to the hypothetical barrier which incoming stimuli must cross before they can be perceived. It is the lowest intensity of some stimulus that produces a response.

2. Another kind of Sensory Limit is defined by our ability to notice a change in sensation. E.g.,, a source of light has to be increased or decreased in some intensity for someone to notice that change.

• Early experimental psychologists tried to measure exactly: what is the minimal amount by which the original physical stimulus must be increased so that the difference between the old and new intensities would be noticed.

Slide 24

• Stated in another way, we are interested in finding out the smallest difference in a physical stimulus that would be noticed.

• This amount is referred to as the Difference threshold (the level of stimulus change necessary in order for a person to perceive a difference.

• This is also known as just noticeable difference – JND).

Defined as the amount of stimulation necessary to detect a difference between two stimuli or the smallest change from the standard stimulus that an observer can notice.

Slide 25

• In other words, the difference threshold produces a just noticeable difference - a psychological entity which describes the subject’s ability to discriminate.

• With difference threshold, we are simply asking - how much must a stimulus change (increase or decrease) before it becomes just noticeably different?

• These methods Absolute threshold and Difference threshold were among the earliest methods developed to attempt precise measurement in psychology.

Slide 26

• They are still of interest, not only on historical grounds, but also because they are simple and adequate as rough measures.

• The study of just noticeable difference (JND) led to one of psychology’s first natural “laws”.

This is the Weber’s law, stated as: the amount of change needed to produce a JND is a constant proportion of the original stimulus intensity.

Slide 27

• Absolute Threshold and Difference Threshold

• It must be understood that the absolute threshold may be considered as a special case of a difference threshold.

• For example, if we ask a question. How much stimulus energy must be added to zero stimulus before the subject can tell the difference between the old stimulus “I see nothing” and the new “Now I see it”?

• In this situation, the absolute threshold and the differential threshold will be the same.

Slide 28

• Absolute Threshold

• By this we are asking - How much stimulus energy is needed to trigger a conscious perceptual experience?

• Think about the following questions:

What is the quietest sound that can be heard?

The weakest light that can be seen?

The lightest touch that can be felt?

• The minimum detectable amount of light, sound, pressure or other physical energy is the absolute threshold. For normal human beings these limits can be amazingly low.

• Estimates for the absolute threshold:

Slide 29

Absolute Thresholds

for Human Vision,

Hearing, Taste,

Smell and Touch

MEASURING SENSORY INTENSITYTopic Four

Slide 31

Measuring Sensory Intensity

Measuring Sensory Intensity

• How do we measure psychological intensity?

• For humans, measuring the magnitude of a stimulus is in principle

very easy. E.g., we measure physical stimulus energy in pounds or

kilograms, in degrees centigrade, Fahrenheit, and measure of sound

in decibels etc.

• This is not so with sensory intensity and it is extremely difficult when

we try to assess psychological intensity – the magnitude of a

sensation rather than that of a stimulus.

Measurement

• The founder of psychophysics Gustav Theodor Fechner (1801-

1887), believed that sensations cannot be measured directly. Slide 32

• He believed sensations and the stimuli that produced them belong to two different realms. In philosophical terms this is referred to as that of the body (stimuli) and that of the mind (sensation).

• Fechner, believed that while sensations can’t be compared to physical stimuli, at least, they can be compared to each other.

• Example, we can compare two of our sensations and judge whether the two are the same or are different.

Slide 33

Weber’s Fraction

• The Weber Fraction

• Measuring the j.n.d was just a means to a larger goal – that is, to formulate a general law relating stimulus intensity to sensory magnitude.

• Fechner believed such a law could be built upon the empirical generalizations which were first proposed by the German physiologist Ernst H. Weber (1795-1878) in 1834.

• Weber believed that the:

• “size of the difference threshold (JND) is a constant ratio of the standard stimulus”.

Slide 34

• Example

Let us assume that we can tell the difference between 100 and 102 candles burning in a dark room.

If Weber is right, then we would be able to distinguish between:

200 and 204 candles,

400 and 408 candles,

800 and 816 candles and so on.

• Weber’s claim is that the difference threshold (JND) between two stimulus magnitudes is a certain constant fraction of the total magnitude.

• He was so impressed with this relationship that he referred to it as Weber’s law, which is written as:

Slide 35

Slide 36

Where I is the increment in stimulus intensity (that is j.n.d). I is the

magnitude of the standard stimulus and C is the constant.

The fraction I / I = C is often referred to as the -

Weber’s Fraction.

Example:

If 1 kg must be added to a 10 kg standard weight for a j.n.d, then 10

kg would have to be added to a 100 kg standard weight to produce a j.n.d.

Example: 2

• If the shoppers might note the change; similarly, it might

take a 5 million cedis price hike in a 50 million cedis

Mercedes-Benz to raise the eyebrows of its potential

buyers.

• In both cases, the price went up by 10 percent.

Slide 37

Estimates for Weber’s Fraction

Slide 38

Teghtsoonian, 1971; Cain, 1977

Table 6.2

Significance of Weber’s Fraction

Weber’s law is important in several ways.

• It allows us to compare the sensitivity of different sensory modalities. Consider you want to know whether the eye is more sensitive than the ear.

• How can we determine this? Obviously, we cannot compare the JND for brightness and JND for loudness.

• The values will certainly be different – e.g., in different units. For brightness we measure in millilamberts and decibels for sound.

• This difficulty is circumvented by using the Weber fractions for the two sensory modalities. Fractions are dimensionless and can therefore be used to compare the sensitivity of different senses.

Slide 39

• When the fraction (I / I ) is small, the discriminating

power of the sense modality is great – that is

proportionately only little must be added to the standard

for a difference to be noticed.

• The opposite holds true when the fraction is large. That

is, when the fraction is large, the discriminating power of

the sensory modality is weak.

• Based on this analysis we can now refer to Table 6.2

above and determine which sensory modality is sensitive

or you may be able to compare the senses.

Slide 40

• While reading these important contributions from Weber on psychological measurements, have it in mind that more recent research indicates that Weber’s law should be viewed as only rough characterization of our sensitivity to changes in stimulation.

• It fails in the case of very weak or very intense stimuli and is only approximately true for the middle range of stimuli. Nevertheless, it is a useful general approximation of human sensitivity to stimulus differences.

Slide 41

Sample Questions

1. Explain the Weber’s law and cite your own examples to illustrate how it is used.

2. Explain how we compare the various sensitivities on the sensory system.

3. Each of our senses is limited in the stimulation to which is sensitive. For example, our eyes perceive only light in the visible spectrum, but the visible spectrum is only a fraction of the entire electromagnetic spectrum. Why do you suppose we cannot directly sense the TV and radio waves that surround us all the time?

Slide 42

THANK YOU

Slide 43