Until the middle of the nineteenth century the nature of the mind was solely the concern of philosophers. Indeed, there are a number of reasons why some have argued that the scientific investigation of the mind may prove to be an impossible undertaking. One objection is that thoughts cannot be measured; and without measurement, science cannot even begin. A second objection is to question how humans could objectively study their own thought processes, given the fact that science itself depends on human thinking. A final objection is that our mental life is incredibly complex and bound up with the further complexities of human social interactions; perhaps cognition is simply too complex to permit successful scientific investigation.
Despite these reasons for skepticism, scientific psychology emerged as a discipline separate from philosophy in the second half of the nineteenth century. A science depends on systematic empirical methods for collecting observations and on theories that interpret these observations. Beginning around 1850, a number of individuals, often trained in philosophy, physics, physiology, or neurology, began to provide these crucial elements.
The anatomist Ernst Heinrich Weber and the physicist and philosopher Gustav Fechner measured the relations between objective changes in physical stimuli, such as brightness or weight, and subjective changes in the internal sensations the stimuli generate. The crucial finding of Weber and Fechner was that subjective differences were not simply equivalent to objective differences. Rather, it turned out that for many dimensions, the magnitude of change required to make a subjective difference ("just noticeable difference," or "jnd") increased as overall intensity increased, often following an approximately logarithmic function, known as the Weber-Fechner Law. Weber and Fechner's contribution to cognitive psychology was much more general than identifying the law that links their names. They convincingly demonstrated that, contrary to the claim that thought is inherently impossible to measure, it is in fact possible to measure mental concepts, such as the degree of sensation produced by a stimulus. Fechner called this new field of psychological measurement PSYCHOPHYSICS: the interface of psychology and physics, of the mental and the physical.
A further foundational issue concerns the speed of human thought. In the nineteenth century, many believed that thought was either instantaneous or else so fast that it could never be measured. But HERMANN VON HELMHOLTZ, a physicist and physiologist, succeeded in measuring the speed at which signals are conducted through the nervous system. He first experimented on frogs by applying an electric current to the top of a frog's leg and measuring the time it took the muscle at the end to twitch in response. Later he used a similar technique with humans, touching various parts of a person's body and measuring the time taken to press a button in response. The response time increased with the distance of the stimulus (i.e., the point of the touch) from the finger that pressed the button, in proportion to the length of the neural path over which the signal had to travel. Helmholtz's estimate of the speed of nerve signals was close to modern estimates—roughly 100 meters per second for large nerve fibers. This transmission rate is surprisingly slow—vastly slower than the speed of electricity through a wire. Because our brains are composed of neurons, our thoughts cannot be generated any faster than the speed at which neurons communicate with each other. It follows that the speed of thought is neither instantaneous nor immeasurable.
Helmholtz also pioneered the experimental study of vision, formulating a theory of color vision that remains highly influential today. He argued forcefully against the commonsensical idea that perception is simply a matter of somehow "copying" sensory input into the brain. Rather, he pointed out that even the most basic aspects of perception require major acts of construction by the nervous system. For example, it is possible for two different objects—a large object far away, and a small object nearby— to create precisely the same image on the retinas of a viewer's eyes. Yet normally the viewer will correctly perceive the one object as being larger, but further away, than the other. The brain somehow manages to unconsciously perform some basic geometrical calculations. The brain, Helmholtz argued, must construct this unified view by a process of "unconscious inference"—a process akin to reasoning without awareness.
Helmholtz's insight was that the "reality" we perceive is not simply a copy of the external world, but rather the product of the constructive activities of the brain.
Another philosopher, HERMANN EBBINGHAUS, who was influenced by Fechner's ideas about psychophysical measurements, developed experimental methods tailored to the study of human memory. Using himself as a subject, Ebbinghaus studied memory for nonsense syllables—consonant-vowel-consonant combinations, such as "zad," "bim," and "sif." He measured how long it took to commit lists of nonsense syllables to memory, the effects of repetition on how well he could remember the syllables later, and the rate of forgetting as a function of the passage of time. Ebbinghaus made several fundamental discoveries about memory, including the typical form of the "forgetting curve"—the gradual, negatively accelerated decline in the proportion of items that can be recalled as a function of time. Like Weber, Fechner, and Helmholtz, Ebbinghaus provided evidence that it is indeed possible to measure mental phenomena by objective experimental procedures.
Many key ideas about possible components of cognition were systematically presented by the American philosopher WILLIAM JAMES in the first great psychology textbook, Principles of Psychology, published in 1890. His monumental work included topics that remain central in psychology, including brain function, perception, attention, voluntary movement, habit, memory, reasoning, the SELF, and hypnosis. James discussed the nature of "will," or mental effort, which remains one of the basic aspects of attention. He also drew a distinction between different memory systems: primary memory, which roughly corresponds to the current contents of consciousness, and secondary memory, which comprises the vast store of knowledge of which we are not conscious at any single time, yet continually draw upon. Primary memory is closely related to what we now term active, short-term, or WORKING MEMORY, while secondary memory corresponds to what is usually called long-term memory.
James emphasized the adaptive nature of cognition: the fact that perception, memory, and reasoning operate not simply for their own sake, but to allow us to survive and prosper in our physical and social world. Humans evolved as organisms skilled in tool use and in social organization, and it is possible (albeit a matter of controversy) that much of our cognitive apparatus evolved to serve these basic functions (see EVOLUTIONARY PSYCHOLOGY). Thus, human cognition involves intricate systems for MOTOR CONTROL and MOTOR LEARNING; the capacity to understand that other people have minds, with intentions and goals that may lead them to help or hinder us; and the ability to recognize and remember individual persons and their characteristics. Furthermore, James (1890:8) recognized that the hallmark of an intelligent being is its ability to link ends with means—to select actions that will achieve goals: "The pursuance of future ends and the choice of means for their attainment are thus the mark and criterion of the presence of mentality in a phenomenon." This view of goal-directed thinking continues to serve as the foundation of modern work on PROBLEM SOLVING, as reflected in the views of theorists such as ALAN NEWELL and Herbert Simon.
Another pioneer of psychology was Sigmund Freud, the founder of psychoanalysis, whose theoretical ideas about cognition and consciousness anticipated many key aspects of the modern conception of cognition. Freud attacked the idea that the "self' has some special status as a unitary entity that somehow governs our thought and action. Modern cognitive psychologists also reject (though for different reasons) explanations of intelligent behavior that depend upon postulating a "homunculus"— that is, an internal mental entity endowed with all the intelligence we are trying to explain. Behavior is viewed not as the product of a unitary self or homunculus, but as the joint product of multiple interacting subsystems. Freud argued that the "ego"—the information-processing system that modulates various motivational forces—is not a unitary entity, but rather a complex system that includes attentional bottlenecks, multiple memory stores, and different ways of representing information (e.g., language, imagery, and physiognomic codes, or "body language"). Furthermore, as Freud also emphasized, much of information processing takes place at an unconscious level. We are aware of only a small portion of our overall mental life, a tip of the cognitive iceberg. For example, operating beneath the level of awareness are attentional "gates" that open or close to selectively attend to portions of the information that reaches our senses, memory stores that hold information for very brief periods of time, and inaccessible memories that we carry with us always but might never retrieve for years at a time.
Given the breadth and depth of the contributions of the nineteenth-century pioneers to what would eventually become cognitive science, it is ironic that early in the twentieth century the study of cognition went into a steep decline. Particularly in the United States, psychology in the first half of the century came to be dominated by BEHAVIORISM, an approach characterized by the rejection of theories that depended on "mentalistic" concepts such as goals, intentions, or plans. The decline of cognitive psychology was in part due to the fact that a great deal of psychological research had moved away from the objective measurement techniques developed by Fechner, Helmholtz, Ebbinghaus, and others, and instead gave primacy to the method of INTROSPECTION, promoted by WILHELM WUNDT, in which trained observers analyzed their own thought processes as they performed various cognitive tasks. Not surprisingly, given what is now known about how expectancies influence the way we think, intro-spectionists tended to find themselves thinking in more or less the manner to which they were theoretically predisposed. For example, researchers who believed thinking always depended on IMAGERY usually found themselves imaging, whereas those who did not subscribe to such a theory were far more likely to report "imageless thought."
The apparent subjectivity and inconstancy of the introspective method encouraged charges that all cognitive theories (rather than simply the method itself, as might seem more reasonable) were "unscientific." Cognitive theories were overshadowed by the behaviorist theories of such leading figures as John Watson, Edward Thorndike, Clark Hull, and B. F. Skinner. Although there were major differences among the behavior-ists in the degree to which they actually avoided explanations based on assumptions about unobservable mental states (e.g., Hull postulated such states rather freely, whereas Watson was adamant that they were scientifically illicit), none supported the range of cognitive ideas advanced in the nineteenth century.
Cognitive psychology did not simply die out during the era of behaviorism. Working within the behaviorist tradition, Edward Tolman pursued such cognitive issues as how animals represented spatial information internally as COGNITIVE MAPS of their environment. European psychologists were far less captivated with behaviorism than were Americans. In England, Sir FREDERICK BARTLETT analyzed the systematic distortions that people exhibit when trying to remember stories about unfamiliar events, and introduced the concept of "schema" (see SCHEMATA) as a mental representation that captures the systematic structural relations in categories of experience. In Soviet Russia, the neuropsychologist Aleksandr LURIA provided a detailed portrait of links between cognitive functions and the operation of specific regions of the brain. Another Russian, LEV VYGOTSKY, developed a sociohistorical approach to cognitive development that emphasized the way in which development is constructed through social interaction, cultural practices, and the internalization of cognitive tools. Vygotsky emphasized social interaction through language in the development of children's concepts. The Swiss psychologist JEAN PIAGET spent decades refining a theory of cognitive development. Piaget's theory emphasizes milestones in the child's development including decentration, the ability to perform operations on concrete objects, and finally the ability to perform operations on thoughts and beliefs. Given its emphasis on logical thought, Piaget's theory is closely related to SCIENTIFIC THINKING AND ITS DEVELOPMENT.
In addition, the great German tradition in psychology, which had produced so many of the nineteenth-century pioneers, gave rise to a new cognitive movement in the early twentieth century: GESTALT PSYCHOLOGY. The German word Gestalt translates roughly as "form," and the Gestalt psychologists emphasized that the whole form is something different from the mere sum of its parts, due to emergent properties that arise as new relations are created. Gestalt psychology was in some ways an extension of Helmholtz's constructivist ideas, and the greatest contributions of this intellectual movement were in the area of GESTALT PERCEPTION. Where the behaviorists insisted that psychology was simply the study of how objective stimuli come to elicit objective responses, the Gestaltists pointed to simple demonstrations casting doubt on the idea that "objective" stimuli—that is, stimuli perceived in a way that can be described strictly in terms of the sensory input—even exist. Figure 1 illustrates a famous Gestalt example of the constructive nature of perception, the ambiguous Necker cube. Although this figure is simply a flat line drawing, we immediately perceive it as a three-dimensional cube. Moreover, if you look carefully, you will see that the figure can actually be seen as either of two different three-dimensional cubes. The same objective stimulus—the two-dimensional line drawing—gives rise to two distinct three-dimensional perceptions.
Although many of the major contributions by key Gestalt figures such as Max Wertheimer were in the area of perception, their central ideas were extended to memory and problem solving as well, through the work of people such as Wolfgang Köhler and Karl Duncker. Indeed, one of the central tenets of Gestalt psychology was that highlevel thinking is based on principles similar to those that govern basic perception. As we do in everyday language, Gestalt psychologists spoke of suddenly "seeing" the solution to a problem, often after "looking at it" in a different way and achieving a new "insight." In all the areas in which they worked, the Gestalt idea of "a whole different from the sum of parts" was based on the fundamental fact that organized configurations are based not simply on individual elements, but also on the relations between those elements. Just as H2O is not simply two hydrogen atoms and one oxygen atom, but also a particular spatial organization of these elements into a configuration that makes a molecule of water, so too "squareness" is more than four lines: it crucially depends on the way the lines are related to one another to make four right angles. Furthermore, relations can take on a "life of their own," separable from any particular set of elements. For example, we can take a tune, move it to a different key so that all the notes are changed, and still immediately recognize it as the "same" tune as long as the relations among the notes are preserved. A focus on relations calls attention to the cen-trality of the BINDING PROBLEM, which involves the issue of how elements are systematically organized to fill relational roles. Modern work on such topics as ANALOGY and SIMILARITY emphasizes the crucial role of relations in cognition.
Modern cognitive psychology emerged in the second half of this century. The "cognitive revolution" of the 1950s and 1960s involved not only psychology but also the allied disciplines that now contribute to cognitive science. In the 1940s the Canadian psychologist DONALD HEBB began to draw connections between cognitive processes and neural mechanisms, anticipating modern cognitive neuroscience. During World War II, many experimental psychologists (including JAMES GIBSON) were confronted with such pressing military problems as finding ways to select good pilots and train radar operators, and it turned out that the then-dominant stimulus-response theories simply had little to offer in the way of solutions. More detailed process models of human information processing were needed. After the war, DONALD BROADBENT in England developed the first such detailed model of attention. Even more importantly, Broadbent helped develop and popularize a wide range of experimental tasks in which an observer's attention is carefully controlled by having him or her perform some task, such as listening to a taped message for a particular word, and then precisely measuring how quickly responses can be made and what can be remembered. In the United States, William K. Estes added to the mathematical tools available for theory building and data analysis, and Saul Sternberg developed a method for decomposing reaction times into component processes using a simple recognition task.
Meanwhile, the birth of computer science provided further conceptual tools. Strict behaviorists had denounced models of internal mental processes as unscientific. However, the modern digital computer provided a clear example of a device that took inputs, fed them through a complex series of internal procedures, and then produced outputs. As well as providing concrete examples of what an information-processing device could be, computers made possible the beginnings of artificial intelligence— the construction of computer programs designed to perform tasks that require intelligence, such as playing chess, understanding stories, or diagnosing diseases. Herbert Simon (1978 Nobel Laureate in Economics) and Allan Newell were leaders in building close ties between artificial intelligence and the new cognitive psychology. It was also recognized that actual computers represent only a small class of a much larger set of theoretically possible computing devices, which had been described back in the 1940s by the brilliant mathematician ALAN TURING. Indeed, it was now possible to view the brain itself as a biological computer, and to use various real and possible computing devices as models of human cognition. Another key influence on modern cognitive psychology came from the field of linguistics. In the late 1950s work by the young linguist Noam Chomsky radically changed conceptions of the nature of human language by demonstrating that language could not be learned or understood by merely associating adjacent words, but rather required computations on abstract structures that existed in the minds of the speaker and listener.
The collective impact of this work in the mid-twentieth century was to provide a seminal idea that became the foundation of cognitive psychology and also cognitive science in general: the COMPUTATIONAL THEORY OF MIND, according to which human cognition is based on mental procedures that operate on abstract mental representations. The nature of the COGNITIVE ARCHITECTURE has been controversial, including proposals such as PRODUCTION SYSTEMS and NEURAL NETWORKS. In particular, there has been disagreement as to whether procedures and representations are inherently separable or whether procedures actually embody representations, and whether some mental representations are abstract and amodal, rather than tied to specific perceptual systems. Nonetheless, the basic conception of biological information processing as some form of computation continues to guide psychological theories of the representation and processing of information.
See also ANALOGY; BARTLETT, FREDERICK; BEHAVIORISM; BINDING PROBLEM; BROADBENT, DONALD; COGNITIVE ARCHITECTURE; COGNITIVE MAPS; COMPUTATIONAL THEORY OF MIND; EBBINGHAUS, HERMANN; EVOLUTIONARY PSYCHOLOGY; GESTALT PERCEPTION; GESTALT PSYCHOLOGY; GIBSON, JAMES; HEBB, DONALD; HELMHOLTZ, HERMANN VON; IMAGERY; INTROSPECTION; JAMES, WILLIAM; LURIA, ALEXSANDR ROMANOVICH; MOTOR CONTROL; MOTOR LEARNING; NEURAL NETWORKS; NEWELL, ALAN; PIAGET, JEAN; PROBLEM SOLVING; PRODUCTION SYSTEMS; PSYCHOPHYSICS; SCHEMATA; SCIENTIFIC THINKING AND ITS DEVELOPMENT; SELF; SIMILARITY; TURING, ALAN; VYGOTSKY, LEV; WORKING MEMORY; WUNDT, WILHELM
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