Psychophysics: The Fundamental Science

Marc Green

The primary question is whether the brain receives or makes sensations. When we look at grass: is the sensation of green picked up by the eyes, from light reflected from the grass, or are the sensations, the qualia of green, created in our brains? It is now as certain as anything-as Sir Isaac Newton appreciated three centuries ago-that light itself has no colour. Light evokes colour in suitable eyes and brains, which is very different. And violins have no sounds without ears and brains to create sound qualia. (Gregory, 1998)

Psychophysical methods are a cornerstone of psychology, cognitive science, and neuroscience (Straub & Rothkpf, 2022)

2.1 Introduction This chapter covers perhaps the two most basic areas of experimental psychology, psychophysics, and operant learning. They are fields that examine deeply hard-wired, primitive attributes of the human (and animal) organism: sensing and adapting to the world. The two disciplines also share other attributes. They are the most rigorously scientific areas of psychology. By this, I mean that they examine a direct relationship between the physical world and behavior. They posit no intervening variables in the mind (i.e. cognitive, inferred mental states), although modern psychophysics recognizes that decision processes affect even the simplest sensory judgment. Further, they are both largely unknown by people outside of experimental psychology. Psychophysics can seem dry and esoteric at first glance while operant learning can appear overly deterministic and unpleasantly authoritarian. Perhaps the main reason is that neither involves cognition, the paradigm which has dominated psychology since the overthrow of behaviorism 60 years ago. However, this and future chapters also explain why knowledge of psychophysics and operant learning are necessary for understanding road user behavior and for promoting safety. The many examples will make this clear.

Psychophysics is the chapter's primary focus. This chapter should probably be entitled, What is psychophysics, and why should I care? The answer is that you should care very much if you study matters where perception and sensory judgments of visibility, motion, distance, speed, time-to-collision, etc., are important. In order to act, the viewer must first decide. In order to decide, he must first perceive. In order to perceive, he must first sense. Psychophysics¹ is the science of measuring sensation. A more technical definition of psychophysics is "an exact theory of the functionally dependent relations of body and mind or, more generally, of the material and the mental, of the physical and the psychological worlds" (Fechner, 1860).

Moreover, its methods are the basis of many other types of measurement used in a wide variety of experimental and practical research studies. In learning about the methods of psychophysics, the reader is learning about the methods of most behavioral research. Always remember that the science is in the method and not in the data.

The importance of psychophysics is best illustrated in the answer to the familiar question, "If a tree falls in the forest and there is no one there to hear it, does it make a sound?" It is usually posed as a philosophical conundrum.² However, it is not. As the Gregory quote³ above notes, it is a scientific question of the most fundamental importance, and it has a definite answer: No! The reason for this answer is the key to understanding human sensory and ultimately perceptual processing. In short, there is a critical distinction between physical events, such as air vibrations caused by a falling tree and psychological events, such as hearing a sound. Similarly, there is a critical distinction between the physical properties of a light and the sensations of its brightness and color. Despite its centrality for many sensory and nonsensory judgments, psychophysics remains largely anonymous, and few people outside the discipline even know that the science exists.

The distinction between physical and perceptual domains has major implications: 1) perception is a psychological, private event that occurs in the brain and cannot be observed publicly, 2) physical measurements with light meters, sound meters, measuring tapes, stopwatches, etc. do not necessarily bear any simple relationship to perception, and 3) there is a need for some scientific, behavioral method to specify the functional relationships between the physical and perceptual worlds. Psychophysics (i.e., psycho-physics) is the scientific field that specifies those relationships, making it the most fundamental of all sciences. It is the starting point for answering Helmholtz's fundamental question, "In what way do our ideas correspond to reality?" Psychophysics has shown that the answer is: not all that closely.

Psychophysics is both a set of methodologies and the data that those methodologies generate. The earliest "classical" methods, as well as the more recent techniques, are described below. These methods measure the functional relationships between the physical and sensory worlds. This is why psychophysics is important as the bedrock science for any understanding of vision, audition, smell, taste, etc. Whether he knows it or not, anyone giving a scientific opinion on these issues is doing psychophysics.

For present purposes, psychophysics is important for three main reasons that form the focus of this chapter. First, it is the tool that specifies the limits of human ability in tasks such as contrast detection and motion perception. If we want to know whether the pedestrian was visible or the looming was perceptible, then psychophysical thresholds are necessary to answer the question. Second, the ability of eyewitnesses or anyone else to estimate speed, size, distance, etc., depends on the relationship between the perceived and physical quantities. Psychophysics also establishes this relationship and highlights where distortions are likely to occur. Third, psychophysics shows how the method used to measure these functions and the nature of the judgment being made affect the answer, both in the lab and in the real-world.

Most introductory sensation and perception texts begin with a chapter on psychophysics because it is the foundation of all behavioral research on sensory abilities. It is an essential science because the relationship between sensation and perception and the physical world is both fundamental and complex. For example, suppose someone illuminates a lamp, and light energy reaches a viewer's eye. The viewer experiences an increase in the sensation of brightness. The amount of light reaching the eye is a physical variable while apparent brightness is a psychological variable. Suppose we want to know the minimum amount of light needed to create an increase in apparent brightness. Suppose we want to know whether a fluorescent bulb would create a bigger brightness increase. Suppose we want to know how much brighter the room becomes when the lamp output is doubled. Physics alone cannot answer these questions. There is no simple one-to-one relationship between light intensity and brightness. Doubling intensity will not double brightness. A meter can measure light intensity, but no meter can measure brightness. Moreover, no meter can measure any sensation or perception-they are private, internal experiences. However, psychophysics can and does measure experience. It provides practical methods and theoretical analyses for measuring these private experiences and for answering questions about the senses. The task of performing a visibility analysis, as described in Chapter 4 and Chapter 5, is a good example of psychophysics at work. Only psychophysical measurement and analysis can determine whether a given level of contrast would be detectable.

Psychophysics also has numerous scientific and non-scientific applications that are not covered here. While a topic of study in itself, psychophysics is also used as a tool in many other domains. Clinicians use psychophysics to diagnose and monitor eye and ear diseases. Visual scientists use psychophysics for "dry dissection," as a surrogate for cutting into the brain to discover the mechanisms that subserve sensation and perception. In both of these usages, psychophysics is very closely related to physiology. However, psychophysics provides tools that are used in a wide range of applied domains beyond simple sensory judgments. It is commonly used to judge the strength of beliefs in fields such as politics and economics. Market researchers use psychophysical methods extensively to create new products, to measure consumer response, and to determine prices (e.g., Poundstone, 2010).

This chapter provides a general overview of psychophysical theory and practice. There are two vantage points for explaining psychophysics. The first treats psychophysics as a set of specialized fields, each devoted to a different sense modality: visual psychophysics (vision), auditory psychophysics (hearing), olfactory psychophysics (smelling), gustatory psychophysics (taste), etc. Each has its own scientific literature and specific knowledge base.

The second vantage point takes psychophysics as a single field, regardless of sensory modality. All psychophysicists share a common body of scientific knowledge, basic methodology, and core set of assumptions, which were originally set out by Gustav Fechner who formally founded psychophysics in 1860 with Elements of Psychophysics. Fechner's book was a major landmark in establishing psychology as a science. Until then, the major psychology research paradigm was introspection where subjects looked inward and made qualitative statements about what they were experiencing. Psychophysics provided the first means to quantitatively measure human experience. The history of psychophysics is briefly described in Poundstone (2010).

While much of this book takes the first approach and discusses only visual psychophysics, this chapter explains the general psychophysical analyses and methods used by scientists in all sense modalities. It is highly quantitative and describes sensory relationships in concepts such as JNDs, Weber's Law, Stevens's Law, Fechner's Law, the "classical psychophysical" methods, and Signal Detection Theory. Still, since vision is the main sense used in driving, the discussion leans heavily toward visual psychophysics. By necessity, the chapter also includes some discussion of cognitive factors in decision-making. Cognitive factors, including an array of innate biases and heuristics, influence fundamental sensory judgments as well as other decisions. There is no avoiding cognition, even in the simplest sensory judgments. The history of psychophysics consists of two parts. The first is the creation of "classical" methodologies to measure sensations while the second is the development of methods to separate the purely sensory (sensitivity) from contaminating cognitive (criterion) factors in sensory judgments. This eventually led to the development of Signal Detection Theory and forced-choice methodology, as described below. However, cognition inevitably intrudes into judgments, whether sensory or otherwise.

Endnotes

¹ A note on terminology. Psychologists sometimes call the field "sensory psychology." When speaking of visual psychophysics, ophthalmologists, and especially optometrists may refer to it as "physiological optics." Lastly, some restrict the term "vision" to mean "visual psychophysics" only and treat perception as cognition, which it is.

² Similarly, the other popular philosophical question, "Which came first, the chicken or the egg?" has a definite answer. Anyone who doesn't know the answer and why it is correct needs to read Darwin.

³ Gregory mentions "qualia," a term used by philosophers for instances of subjective experience.

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