Psychophysics: Threshold, Signal Detection Theory | Psychology for UPSC Optional (Notes) PDF Download

Introduction

Have you ever wondered how you perceive different degrees of sensation? When you experience a stimulus through your senses, you have a vague awareness of its presence, which is known as sensation. However, it is when you assign meaning to the stimulus and truly understand the object that it becomes perception. By now, you have gained an understanding of the processes involved in the conversion of sensation into perception. But how do you differentiate between various intensities of sensations?
Let's consider some examples: when you taste something and describe it as "bitter," feel pressure on your body and perceive it as "dull," or see a vivid red color and label it as "brightish red." How are you able to perceive these different levels of intensity? This is where the fascinating field of psychophysics comes into play. Psychophysics explores the relationship between the physical properties of stimuli and the resulting psychological sensations.

• It is important to note that the sensory system does not respond unless the stimulus energy surpasses a critical level of intensity. Stimulus energy below this critical level does not generate a sensation. Psychophysics encompasses various concepts that help us understand this phenomenon. These concepts include thresholds, such as the absolute threshold (the minimum intensity needed for a sensation to be detected) and the differential threshold (the smallest difference between two stimuli that can be perceived).
• To determine these thresholds, psychophysicists employ methods such as the method of limits and the method of constant stimuli. These methods enable us to experimentally measure and understand the thresholds of our sensations. Psychophysics is guided by several laws, including Weber's Law, which describes the relationship between the magnitude of a stimulus and the perceived change required to detect a difference. Fechner's Law explores the relationship between the physical intensity of a stimulus and the corresponding psychological sensation. Steven's Power Law, on the other hand, examines the subjective magnitude of a sensation in relation to the physical intensity of the stimulus.
• Response bias also plays a significant role in the determination and response to sensations. It refers to an individual's tendency to favor one type of response over others, even in the absence of a sensory stimulus. This bias influences our decision-making process in detecting and interpreting sensations. Signal detection theory provides a comprehensive framework for understanding the impact of response bias on our ability to detect signals amidst noise.
• By delving into the finer nuances of sensation and its determination, you will gain a clearer understanding of psychophysics. This knowledge will enable you to experimentally measure the intensities of sensations and make comparisons between them. Although it may initially seem daunting, this section aims to present these concepts in clear and simple terms, making them accessible and comprehensible.

Psychophysics

Psychophysics explores the relationship between physical stimuli and the resulting sensory experiences, despite variations in the details of the sequence for different senses. Whether it's vision, taste, or hearing, these senses share three common steps: the presence of a proximal stimulus, a neural chain of events (including nerve impulses and messages to the brain), and a psychological response or sensation.

• During this process, the neural message can undergo modifications by other parts of the nervous system. However, psychophysics focuses on the relationship between the characteristics of physical stimuli and the attributes of the sensory experience, disregarding the intermediate neural steps.
• The field of psychophysics aims to establish a connection between the properties of physical stimuli and the psychological consequences they produce, namely the sensory experiences. While it is relatively easy to measure the magnitude of a physical stimulus in terms of physical energy (such as kilos, degrees centigrade, inches, or decibels), quantitatively measuring the psychological intensity or magnitude of a sensation is challenging.
• To address this challenge, psychophysicists have developed various methods known as psychophysical methods. These methods are designed to quantitatively measure the magnitude of sensations and provide a means to understand and compare the intensity of sensory experiences.

Threshold

• Threshold refers to the minimum level of stimulation required for a sensation to be detected. Our sensory system does not respond to all stimuli, particularly those that are low in intensity. Different types of sensations have different thresholds of activation. For instance, we can hear the tick of a watch from twenty feet away in a quiet room, detect the taste of a teaspoon of sugar in two gallons of water, smell a drop of perfume in an empty three-room apartment, and see a dim candle thirty miles away on a clear dark night. This raises the question of how much stimulation is necessary to experience a sensation.
• Contrary to the belief that the relationship between stimulus intensity and sensation is always direct, our sensitivity to external stimuli fluctuates, and our sensory organs vary in their sensitivity to maintain the body's internal environment at an optimal level. The absolute threshold refers to the minimum amount of stimulus that can be detected 50% of the time. It represents the point at which a person is able to detect the presence of a stimulus in half of the trials.
• Another important aspect of sensation is our ability to discriminate between two sensations. The difference threshold, also known as the just noticeable difference (j.n.d.), is the minimal amount by which the intensity of a stimulus needs to be increased in order for a changed sensation to be perceived. For example, in the case of visual brightness, the minimal increase in the intensity of a light patch is required to perceive a sensation of brightness just greater than the previous one. The difference threshold is determined when the changed sensation is experienced in 50% of the presentations, indicating the subject's capacity to discriminate between sensations.

Psychophysical Methods

Psychophysical methods refer to a set of procedures used to determine sensory thresholds. One such method is the method of limits, which involves using an aesthesiometer with two points that can be adjusted in length. The subject's hand is enclosed, and the two points of the apparatus are applied with equal pressure. The subject, blindfolded, must report whether they feel one point or two points. Trials are conducted in both ascending and descending order, starting with the minimum distance between the points and gradually increasing or decreasing it. Multiple trials are performed to obtain an accurate threshold estimate, accounting for habituation and anticipation errors. The point at which the sensation changes from one point to two points is the absolute threshold.

• Another method is the method of constant stimuli, which determines the just noticeable difference (j.n.d.) or difference threshold. In this method, a set of weights is used, with one weight as the standard stimulus and others as comparison stimuli. The subject is asked to judge whether the comparison weight is lighter, equal to, or heavier than the standard weight. The comparison stimuli are presented in a specific order, and the amount of weight needed to create a just noticeable difference is determined. Care is taken to minimize errors during the experiment. These methods enable the assessment of psychological intensity and the magnitude of sensations.
• According to Fechner, the founder of psychophysics, sensations cannot be compared directly to physical stimuli, but they can be compared to each other. Individuals can compare their own sensations and judge whether they are the same or different. This concept allows for the comparison of sensations and facilitates the study of psychophysics.

The Weber's Law

Weber's Law, formulated by Gustav Theodor Fechner, the founder of psychophysics, is based on an empirical generalization proposed by E.H. Weber, a German physiologist, in 1834. According to Weber, the size of the difference threshold, known as the just noticeable difference (j.n.d.), is proportional to the intensity of the standard stimulus. This ratio remains constant and is referred to as the Weber Fraction.

An example clarifies this law: If you can differentiate between 100 grams and 104 grams, you will also be able to distinguish between 200 grams and 208 grams, 400 grams and 416 grams, and so on. Fechner named this observation Weber's law and expressed it algebraically as DI/I = C, where DI represents the increment in stimulus intensity (j.n.d.), I represents the stimulus intensity (standard stimulus), and C is a constant.

Numerous studies have examined whether Weber's law applies to all sensory modalities. In most cases, it has been verified, except for a few where the nervous system is more attuned to relative differences rather than absolute ones. Weber's law enables the comparison of sensitivities across different sensory modalities. For instance, by using Weber's law, it is possible to determine whether the eye is more sensitive than the ear. A smaller Weber's ratio indicates greater discriminative power in a sensory modality, while a larger ratio indicates the opposite. This law helps us understand the distinctive characteristics of various sensory modalities. Research has shown that humans are better at discriminating brightness compared to loudness, with Weber's fractions of 1/62 and 1/11, respectively.

Fechner's Law

• Fechner's Law builds upon Weber's law and provides a broader relationship between sensory and physical intensity. It states that the strength of a sensation grows as the logarithm of stimulus intensity. The formula representing Fechner's Law is S = K log I, where S represents the psychological magnitude (subjective experience), I represents the stimulus intensity, and K is a constant.
• Fechner's Law is biologically sensible because it reflects how our nervous system compresses a wide range of sensations into a more manageable scope. The logarithmic transformation employed in Fechner's Law helps in this compression process.

Steven's Power Law

Steven's Power Law, proposed by S.S. Stevens, explores the relationship between stimulus intensity and subjective sensory magnitude directly. While Fechner believed in a logarithmic relationship, Stevens conducted experiments where subjects were asked to assign numbers proportional to their subjective impressions of stimuli.

• Stevens found that the relationship between the subjective magnitude scale and physical intensity was not logarithmic. Instead, he formulated a power function represented by the formula S = kI^N, where S represents subjective magnitude, I represents stimulus intensity, and k and N are constants.
• According to Steven's Power Law, the intensity of a sensation is proportional to the stimulus intensity raised to a certain power. The value of N determines how sensation grows in relation to stimulus intensity. When N is smaller than I, sensation grows more slowly than stimulus intensity, and when N is larger than I, sensation grows more rapidly than stimulus intensity. The nervous system plays a crucial role in compressing or expanding this relationship.
• For example, in the sensation of pain caused by an electric shock, the subjective scale benefits from expansion rather than compression. Even a small increase in stimulus intensity can make a significant difference between survival and harm. Expanding the subjective scale prompts the individual to take action and escape before serious harm occurs.
• The development of quantitative measurement methods for sensation, from Weber to Stevens, has greatly contributed to the field of psychology. Fechner paved the way for measuring psychological processes, and Stevens' work allowed for more accurate and quantitative measurement of sensation.

Response Bias

Response bias refers to the systematic tendency of an individual to consistently respond in a certain way, regardless of the actual stimulus magnitude or sensory sensitivity. It occurs when attitudes, beliefs, or personal inclinations influence the individual's responses rather than an accurate perception of the stimuli.

For example, if a person is asked to compare the weights of different objects with very small differences in magnitude, they may have difficulty accurately determining whether the weights are equal or if one is heavier or lighter than the other. In such cases, the person's attitudes and beliefs may lead them to consistently respond with "equal" or "heavier" or "lighter" regardless of the actual stimulus magnitude. This response bias prevents the accurate determination of the true threshold.

• Similarly, in the case of presenting a very weak tone to a blindfolded individual and asking if it is audible, the individual's response may be influenced by their attitude or inclination. They may freely and easily respond "yes" or adopt a conservative approach and respond "no" whenever in doubt. The response will depend on the person's attitude rather than an objective perception of the stimulus. This response bias hinders the accurate determination of the threshold.
• Early psychophysicists attempted to address this problem by using highly trained observers as subjects. To detect and counterbalance response bias, occasional catch trials were introduced, where no stimulus was presented. These catch trials helped identify and account for errors in response due to bias.

In summary, response bias refers to the consistent and systematic tendency of individuals to respond in a certain way, influenced by attitudes, beliefs, or personal inclinations, rather than the true perception of the stimuli. Counterbalancing techniques and catch trials can be employed to identify and account for response bias in psychophysical studies.

Signal Detection Theory

Signal detection theory is a theoretical framework developed to address the issue of response bias and the detection of stimuli in psychophysical experiments. It suggests that there are no absolute thresholds for sensations and that the detection of stimuli depends on various factors, including their physical energy and external factors such as the perceived costs and benefits associated with detecting their presence.

• In signal detection theory, catch trials are incorporated as a regular part of the experimental procedure rather than occasional checks. To determine if a subject can detect the presence of a stimulus, a relatively weak stimulus is presented on half of the trials, while no stimulus is presented on the other half. Both types of trials are randomized.
• Subjects can make two types of errors in this context. A "miss" occurs when they fail to report a stimulus that is actually present, while a "false alarm" happens when they report a stimulus that is not present. Additionally, there are two types of correct responses: a "hit" occurs when they correctly report a stimulus that is present, and a "correct negative" happens when they correctly do not report a stimulus when none is present.
• The detection experiment provides a basis for investigating the non-sensory factors that contribute to response bias. One such factor is the differential payoff based on a "payoff matrix." In this approach, subjects are rewarded for hits and correct negatives, while being penalized for misses and false alarms, according to a predetermined schedule of gains and losses.

By considering response bias and using techniques such as the payoff matrix, signal detection theory allows researchers to examine and account for the influence of decision-making processes and external factors on the detection of stimuli, providing a more comprehensive understanding of sensory perception.

The Receiver Operating Characteristic (ROC) curve

The Receiver Operating Characteristic (ROC) curve is a graphical representation that helps separate sensitivity and response bias in psychophysical experiments. It involves several steps to assess and measure these factors accurately.

• The first step in the experiment is to manipulate the response bias while keeping sensitivity constant. This can be achieved by changing the payoff matrix, which determines the rewards and penalties for different types of responses. Another approach is to vary the proportion of trials in which a "no stimulus" is presented. Increasing the number of such trials leads to a greater bias towards responding "yes." When a subject becomes more conservative in their responses, there is a decrease in the proportion of false alarms (incorrectly reporting a stimulus) and hits (correctly reporting a stimulus). These proportions can be plotted against each other.
• The next step involves obtaining an index of sensitivity that is unaffected by response bias. To achieve this, a separate ROC curve is plotted for the detection experiment. The ROC curve represents the relationship between the hit rate (proportion of actual stimuli correctly detected) and the false alarm rate (proportion of no stimuli incorrectly reported). A stronger stimulus typically produces an ROC curve that is more bowed away from the main diagonal. The displacement of the ROC curve from the main diagonal provides a pure measure of sensitivity for the stimulus being tested. This displacement is measured along the second diagonal of the ROC space.

By using the ROC curve and analyzing its shape and displacement, researchers can quantify the sensitivity of a stimulus independent of the individual's response bias. This allows for a more accurate assessment of the ability to detect stimuli and the impact of biases on decision-making processes.

Signal Detection and Decision Process

• The difficulty in distinguishing between the presence and absence of a stimulus can be explained by signal detection theory. This theory suggests that psychophysical judgments are not solely based on the presence or absence of an external stimulus, but rather on the underlying neural activity within the sensory system, which can vary in magnitude.
• According to signal detection theory, there is no such thing as a "zero stimulus." Sensations and perceptual experiences can occur even in the absence of an actual external stimulus. This is because there is spontaneous activity in the nervous system that can generate sensations. For example, the sensation of hearing can occur even if no sound is present, due to the background activity in the auditory system.
• In this view, sensation is considered a combination of the presentation of an external stimulus and the influence of background factors, such as mental states and neural activity. Signal detection theory provides a more comprehensive explanation of the sensory process and its measurement by acknowledging the role of both external stimuli and internal factors in shaping our perceptual experiences. It recognizes that sensory judgments involve not only detecting actual stimuli but also evaluating the presence or absence of sensory information in the presence of background neural activity.

Conclusion

Psychophysics is a fascinating field that explores the relationship between physical stimuli and the resulting psychological sensations. It involves understanding thresholds, such as the absolute threshold and the difference threshold, which determine the minimum levels of stimulation required for sensations to be detected and discriminated.

• Psychophysical methods, such as the method of limits and the method of constant stimuli, are used to experimentally measure and understand these thresholds. Laws like Weber's Law, Fechner's Law, and Steven's Power Law provide insights into the relationship between stimulus intensity and perceived sensory magnitude.
• Response bias is an important factor to consider in psychophysics, as it can influence our ability to detect and interpret sensations. Signal detection theory provides a framework for understanding how response bias affects our perception of signals amidst noise.
• The Receiver Operating Characteristic (ROC) curve is a graphical representation that helps separate sensitivity and response bias in psychophysical experiments. It allows researchers to accurately measure sensitivity independent of the individual's bias.
• By delving into the concepts of psychophysics, we gain a clearer understanding of how we perceive different levels of sensation and the factors that influence our sensory experiences. This knowledge enables us to experimentally measure and compare the intensities of sensations, leading to a deeper understanding of human perception.