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Experimental Psychology – PSY402 VU LESSON 05
ONE of the oldest problems in psychology is the relation between variations in physical stimulation and reported experience. Historically, this field of investigation has often been tied to such philosophical issues as the mind-body problem and the nature and meaning of "subjective experience:' Today psychologists are content to leave the solution of philosophical problems to the philosopher, but there still remains a large area of research of great theoretical as well as practical importance. What are the lawful relationships between the measurable characteristics of the stimulus, on the one hand, and the reportable attributes of sensory experience, on the other'? It is to this question that the division of experimental psychology known as psychophysics addresses itself. Psychophysics is indeed the earliest branch of experimental psychology. The theoretical value of psychophysics lies in the fact that it provides one important experimental approach to the study of the sensory processes and of judgment. As for its practical value, the knowledge gained by the methods of psychophysics has received increasingly wide application in such fields as personnel selection and equipment design.
History Many of the classical techniques and theory of psychophysics were formulated in 1860 when Gustav Theodor Fechner published Elemente der Psychophysik. He coined the term "psychophysics", and described research relating physical stimuli with how they are perceived and set out the philosophical foundations of the field. Fechner wanted to develop a theory that could relate matter to the mind, by describing the relationship between the world and the way it is perceived Fechner's work formed the basis of psychology as a science. Wilhelm Wundt, the founder of the first laboratory for psychological research, built upon Fechner's work. Psychophysicists usually employ experimental stimuli that can be objectively measured, such as pure tones varying in intensity, or lights varying in luminance. All the senses have been studied: vision, hearing, touch (including skin and enteric perception), taste, smell, and the sense of time. Regardless of the sensory domain, there are three main topics in the psychophysical classification scheme: absolute thresholds, discrimination thresholds, and scaling.
What is Psychophysics? Psychophysics is concerned with describing how an organism uses its sensory systems to detect events in its environment. This description is functional, because the processes of the sensory systems are of interest, rather than their structure (physiology). OR Study of quantitative relations between psychological events and physical events or, more specifically, between sensations and the stimuli that produce them. Physical science permits, at least for some of the senses, accurate measurement on a physical scale of the magnitude of a stimulus by determining the stimulus magnitude that is just sufficient to produce. Psychophysics is a sub discipline of psychology dealing with the relationship between physical stimuli and their subjective correlates, or percepts
The Basic Problems of Psychophysics In studying the relation between the characteristics of the stimulus and the attributes of experience, certain specific experimental questions are asked
Detection of minimal stimuli:What is the minimum of stimulation required for the detection of a stimulus? What kind of stimulus is needed? How intense must it be in order that a subject may reliably distinguish between its presence and absence? Obviously the minimum amount of stimulation required will vary with the conditions of testing. To be barely detectable, a tone has to be less intense in a sound-treated room than in a noisy one, and, similarly, a weaker light is needed in a dark room than in a well lit one. But for each condition of testing and for each subject, such a minimum value of a given stimulus can be estimated.
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Experimental Psychology – PSY402 VU Detection of minimal stimulus differences:that is the minimal difference, qualitative or quantitative needed between two stimuli so that they can be reliably recognized as different by a subject? For example, how different do two light stimuli have to be in wave length for a detectable difference in hue? How great a difference in intensity of light is required for a discrimination of brilliance? Again, the minimum value of the difference will vary from one testing situation to another and from one subject to another.
Judgment of relation among stimuli:The experimental problems of psychophysics are not limited to the study of stimuli and stimulus differences that are barely detectable. The judgment of stimuli well above the minimum needed for discrimination defines another important area of investigation. Under what conditions, for example, are two stimuli judged to be equal or as standing in a certain relation to one another? What is the extent of order when subjects attempt to equate two stimuli with respect to quality or quantity? How reliably can subjects respond to a stimulus as being half as intense or twice as intense as another stimulus? These are just a few illustrations of the problems which arise in connection with judgment of relations among stimuli
The Basic Concepts of Psychophysics Sensitivity The organism is equipped with a number of receptor organs specialized to respond to particular energy changes in the environment. The receptors of the eye are responsive to light within a certain range of wave lengths, the receptors of the ear to sound waves within a certain range of frequencies, and so on. The action of these receptor organs constitutes an important link in the chain of responses which occurs between the application of the stimulus and the subject's response. The capacity of the receptor organs and other reaction systems in the organism to respond selectively and differentially to physical stimulation we designate as sensitivity. The laws governing sensitivity are inferred, with the aid of psychophysical procedures, from the variations in response resulting from variations in stimulation. Our experimental measurements allow us to distinguish two types of sensitivity: absolute and differential.
Absolute sensitivitydefines the limits of the organism's capacity to respond to stimulation. It is inversely related to the minimum stimulus which can be detected reliably by a subject. Differential sensitivity defines the organism’s capacity to respond to differences, both qualitative and quantitative, between stimuli. It is inversely related to the minimum difference between stimuli needed for reliable discrimination Thresholds. Some stimuli are so weak that they always fail to evoke an effective response in the organism; others are so intense that they never fail to produce a reaction. The line separating these two kinds of stimuli-those never yielding responses and those always yielding responses-can never be sharply drawn; rather, the transition from one to the other is gradual and continuous. Suppose we wish to measure a subject’s absolute sensitivity to sound. We begin with a very weak sound which the subject fails to hear on repeated trials. We then increase the intensity of the sound. At this second level of stimulation the subject may sometimes hear the sound and sometimes fail to hear it. When we increase the intensity even further, the subject may hear the sound more frequently than before but still miss it part of the time. Finally, we may increase the intensity of stimulation to a level at which the subject never fails to report the presence of the sound. Clearly, then, there is no one stimulus value which represents the minimum necessary for a response. For purposes of measurement, it is generally agreed to consider as the absolute threshold that stimulus value which yields a response 50 percent of the time, i.e. on half the test trials. It is essential to understand that the absolute threshold is not a fixed point on the stimulus scale but rather is inherently variable in time. A single value representing the absolute threshold must necessarily be a statistical concept Similar considerations apply to estimates of a subject's differential sensitivity? The differential threshold is defined as that stimulus difference which gives rise to a judgment of different 50 percent of the time. For example if we present a subject with two tones differing only very little in intensity he will fail to report a difference most of the time. As we increase the intensity difference between the two sounds so as to obtain a judgment of different on half the trials this difference defines the differential threshold. There are many variations in the experimental and statistical procedures for the determination of the absolute and differential thresholds, but they an have the same general purpose: to make as good as possible an estimate
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Experimental Psychology – PSY402 VU of that stimulus value which will yield a given judgment presence vs. absence, same vs. different on half the trials of a series.
Point of Subjective Equality One fundamental category of relational judgment is sameness vs. difference. Sometimes stimuli whose physical characteristics are identical may give rise to a judgment of different, and stimuli which differ physically may be judged same. Thus, there is no necessary correspondence between physical equality of stimuli and judgments of sameness, nor is there a necessary correspondence between physical differences and judgments of different. For this reason, experiments on discrimination often include an estimate of the point of subjective equality. Suppose we present a subject with pairs of stimuli, one member of the pair being fixed and the other member varying from trial to trial, sometimes being equal to the first stimulus, sometimes larger sometimes smaller. The subject is required to make a judgment of same or different in response to each pair. In such all experiment the point of subjective equality is defined by that comparison stimulus which is most likely to result in a judgment of same. Under many experimental conditions the stimuli most likely to be judged same are physically equal ones. Sometimes, however, two stimuli which differ by a certain amount are more likely to be judged same than physically equal ones,
Thresholds A threshold (or limen), is the point of intensity at which the participant can just detect the presence of, or difference in, a stimulus. Stimuli with intensities below the threshold are considered not detectable, however stimuli at values close to threshold will often be detectable some proportion of the time. Due to this, a threshold is considered to be the point at which a stimulus, or change in a stimulus, is detected some proportion p of the time. There are two kinds of thresholds: absolute and difference. An absolute threshold is the level of intensity of a stimulus at which the subject is able to detect the presence of the stimulus some proportion of the time (a p level of 50% is often used). An example of an absolute threshold is the number of hairs on the back of one's hand that must be touched before it can be felt - a participant may be unable to feel a single hair being touched, but may be able to feel two or three as this exceeds the threshold. A difference threshold is the magnitude of the difference between two stimuli of differing intensities that the participant is able to detect some proportion of the time (again, 50% is often used). To test this threshold, several difference methods are used. The subject may be asked to adjust one stimulus until it is perceived as the same as the other, may be asked to describe the magnitude of the difference between two stimuli, or may be asked to detect a stimulus against a background. Absolute and difference thresholds are sometimes considered similar because there is always background noise interfering with our ability to detect stimuli, however study of difference thresholds still occurs, for example in pitch discrimination tasks.
Absolute Thresholds Just when does a stimulus become strong enough to be detected by our sense organs? The answer to this question requires an understanding of the concept of absolute thresholds. An absolute threshold is the smallest intensity of a stimulus that must be present for it to be detected. Consider the following examples of absolute thresholds for the various senses:
Hearing: The licking of a watch can be heard 20 feet away under quiet conditions Taste: Sugar can be discerned when 1 teaspoon is dissolved in 2 gallons of water. Smell: Perfume can be detected when one drop is present in a three-room apartment. Touch: A bee's wing falling from a distance of 1 centimeter can be felt on the cheek Such thresholds permit our sensory apparatus to detect a wide range of sensory stimulation. In fact, the capabilities of our senses are so fine-tuned that we might have problems if they were any more sensitive. For instance, if our ears were just slightly more acute, we would be able to hear the sound of air molecules in our ears knocking into our eardrum-a phenomenon that would surely prove distracting and might even prevent us from hearing sounds outside our bodies.
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Experimental Psychology – PSY402 VU Of course, the absolute thresholds we have been discussing are measured under ideal conditions. Normally our senses cannot detect stimulation quite as well because of the presence of noise. Noise as defined by psychophysicists, is background stimulation that interferes with the perception of other stimuli. Hence, noise refers not just to auditory stimuli the most obvious example, but also to stimuli that affect the other senses. Picture a talkative group of people crammed into a small, crowded, smoke~ filled room at a party. The din of the crowd makes it hard to hear individual voices; and the smoke makes it difficult to see, or eyen taste, the food. In this case, the smoke and crowded
Signal detection theory (SDT) Detection theory, or signal detection theory, is a means to quantify the ability to discern between signal and noise. Much of the early work in detection theory was done by radar researchers. Detection theory was used in 1966 by John A. Swets and David M. Green for Psychophysics. Green and Swets criticized the traditional methods of Psychophysics for their inability to discriminate between the real sensitivity of subjects and their (potential) response biases. Detection theory has applications in many fields such as diagnostics of any kind, quality control, telecommunications, and psychology. The concept is similar to the signal to noise ratio used in the sciences, and it is also usable in alarm management, where it is important to separate important events from background noise. According to the theory, there are a number of psychological determiners of how we will detect a signal, and where our threshold levels will be. Experience, expectations, physiological state (e.g. fatigue) and other factors affect thresholds. For instance, a sentry in wartime will likely detect fainter stimuli than the same sentry in peacetime. Signal detection theory (SDT) is used when psychologists want to measure the way we make decisions under conditions of uncertainty, such as how we would perceive distances in foggy conditions. SDT assumes that the decision maker is not a passive receiver of information, but an active decision-maker who makes difficult perceptual judgments under conditions of uncertainty. In foggy circumstances, we are forced to decide how far an object is away from us based solely upon visual stimulus which is impaired by the fog. Since the brightness of the object, such as a traffic light, is used by the brain to discriminate the distance of an object, and the fog reduces the brightness of objects, we perceive the object to be much further away than it actually is .To apply signal detection theory to a data set where stimuli were either present or absent, and the observer categorized each trial as having the stimulus present or absent, the trials are sorted into one of four categories:
Respond "Absent" Respond "Present"
Stimulus Present Miss Hit
Stimulus Absent Correct Rejection False Alarm
Based on the proportions of these types of trials, numerical estimates of sensitivity can be obtained with statistics like the sensitivity index d' and A', and response bias can be estimated with statistics like β.
Just Noticeable Differences Suppose you wanted to choose the six best apple from a supermarket display-the biggest, reddest, and sweetest apples. One approach would be to systematically compare one apple with another until you were left with a few so similar that you could not tell the difference between them. At that point it wouldn't matter which ones you chose. Psychologists have discussed this comparison problem in terms of the difference threshold, the smallest detectable difference between two stimuli, also known as a just noticeable difference. They have found that the stimulus value that constitutes a just noticeable difference depends on the initial intensity of the stimulus for instance, you may have noticed that the light change when you switch a three way bulb from 75 - to 100 watts appears greater than when you switch from IOO to 125 watts, even though the voltage increase is the same in both cases. Similarly, when the moon is visible during the late afternoon, it appears relatively dim-yet against a dark night sky, it seems quite bright. The relationship between changes in the original value of a stimulus and the degree to which the change will be noticed forms one of the basic laws of psychophysics: Weber's law. Weber's law states that a just noticeable difference is a constant proportion of the intensity of an initial stimulus. Therefore, if a I-pound increase in a l0 pound weight
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Experimental Psychology – PSY402 VU produces a just noticeable difference, it would take a 10-pound increase to produce a noticeable difference if the initial weight were 100 pounds. In both cases, the same proportional increase is necessary to produce a just noticeable difference-I: 10 = 10:100. (Actually, Weber found the true proportional increase in weight that produces a just notice- able difference to be between 2 and 3 percent.) Similarly, the just noticeable difference distinguishing changes in loudness between sounds is larger for sounds that are initially loud than for sounds that are initially soft. This principle explains why a person in a quiet room is more apt to be startled by the ringing of a telephone than a person in a room that is already noisy. In order to produce the same amount of reaction in a noisy room, a telephone ring might have to approximate the loudness of cathedral bells Weber's law seems to hold up for all sensory stimuli, although its predictions are less accurate at extremely high or extremely low levels of stimulation (Sharpe et al, 1989; MacLeod & Willen, 1995). Moreover, the law helps explain psychological phenomena that lie beyond the realm of the senses. For example, imagine that you own a house you would like to sell for $150,000. You might be satisfied if you received an offer of $145,000 from a potential buyer, even though the offer is $5,000 less than the asking price. On the other hand, if you were selling your car and asking $10,000 for it, an offer of $5,000 less than your asking price would probably not make you happy,. Although the absolute amount of money is the same in both cases, the psy- chological value of the $5.000 is very different.
Classic Methods of experimentation Psychophysical experiments have traditionally used three methods for testing subjects' perception in stimulus detection and difference detection experiments: the method of limits, the method of constant stimuli, and the method of adjustment.
Method of limits Wilhelm Wundt invented the method of limits. The subject reports whether he or she detects the stimulus. In ascending method of limits, some property of the stimulus starts out at a level so low that the stimulus could not be detected, and then this level is gradually increased until the participant reports that they are aware of it. For example, if the experiment is testing the minimum amplitude of sound that can be detected, the sound begins too quietly to be perceived, and is made gradually louder. In the descending method of limits, this is reversed. In each case, the threshold is considered to be the level of the stimulus property at which the stimuli is just detected. In experiments, the ascending and descending methods are used alternately and the thresholds are averaged. A possible disadvantage of these methods is that the subject may become accustomed to reporting that they perceive a stimulus and may continue reporting the same way even beyond the threshold (the error of habituation). Conversely, the subject may also anticipate that the stimulus is about to become detectable or undetectable and may make a premature judgment (the error of expectation). To avoid these potential pitfalls, Georg von Bekesy introduced the staircase method in 1960 in his study of auditory perception. In this method, the sound starts out audible and gets quieter after each of the subject's responses, until the subject does not report hearing it. At that point, the sound is made louder at each step, until the subject reports hearing it, at which point it is made quieter in steps again. This way the experimenter is able to "zero in" on the threshold.
Method of constant stimuli Instead of being presented in ascending or descending order, in the method of constant stimuli the levels of a certain property of the stimulus are not related from one trial to the next, but presented randomly. This prevents the subject from being able to predict the level of the next stimulus, and therefore reduces errors of habituation and expectation. The subject again reports whether he or she is able to detect the stimulus.
Method of adjustment Also called the method of average error, the method of adjustment asks the subject to control the level of the stimulus, instructs them to alter it until it is just barely detectable against the background noise, or is the same as the level of another stimulus.
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