attention

—attentional, adj.

n. /euh ten"sheuhn/; interj. /euh ten'shun"/, n.

1. the act or faculty of attending, esp. by directing the mind to an object.

2. Psychol.

a. a concentration of the mind on a single object or thought, esp. one preferentially selected from a complex, with a view to limiting or clarifying receptivity by narrowing the range of stimuli.

b. a state of consciousness characterized by such concentration.

c. a capacity to maintain selective or sustained concentration.

3. observant care; consideration: Individual attention is given to each child.

4. civility or courtesy: attention to a guest.

5. notice or awareness: His deliberate cough caught the waiter's attention.

6. attentions, acts of courtesy or devotion indicating affection, as in courtship.

7. Mil. an erect position with eyes to the front, arms to the sides, and heels together (often used as a command).

[1325-75; ME attencioun < L attention- (s. of attentio). See ATTENT, -ION]

Syn. 1. awareness, consciousness, watchfulness, alertness, mindfulness, heed. 4. deference, politeness, regard; respect, homage.

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In psychology, the act or state of applying the mind to an object of sense or thought.

Wilhelm Wundt was perhaps the first psychologist to study attention, distinguishing between broad and restricted fields of awareness. He was followed by William James, who emphasized active selection of stimuli, and Ivan Pavlov, who noted the role attention plays in activating conditioned reflexes. John B. Watson sought to define attention not as an "inner" process but rather as a behavioral response to specific stimuli. Psychologists today consider attention against a background of "orienting reflexes" or "preattentive processes," whose physical correlates include changes in the voltage potential of the cerebral cortex and in the electrical activity of the skin, increased cerebral blood flow, pupil dilation, and muscular tightening. See also attention deficit disorder.

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▪ psychology

Introduction

      in psychology, the concentration of awareness on some phenomenon to the exclusion of other stimuli.

      Attention is awareness of the here and now in a focal and perceptive way. For early psychologists, such as Edward Bradford Titchener (Titchener, Edward Bradford), attention determined the content of consciousness and influenced the quality of conscious experience. In subsequent years less emphasis was placed on the subjective element of consciousness and more on the behaviour patterns by which attention could be recognized in others. Although human experience is determined by the way people direct their attention, it is evident that they do not have complete control over such direction. There are, for example, times when an individual has difficulty concentrating attention on a task, a conversation, or a set of events. At other times an individual's attention is “captured” by an unexpected event rather than voluntarily directed toward it.

      Attention has to do with the immediate experience of the individual; it is a state of current awareness. There are, of course, myriad events taking place in the world all the time, each impinging upon a person's senses. There are also events taking place within the body that affect attention, just as there are representations of past events stored in one's memory but accessible to awareness under appropriate circumstances.

      While it might be expected that current awareness is the totality of all those events at any given moment, clearly this is not the case. Within this vast field of potential experiences, an individual focuses upon—or attends to—some limited subset of the whole. This subset constitutes the subjective field of awareness. It is possible to determine the reason for this limitation. Control and coordination of the many inputs and stored experiences and the organization of appropriate patterns of response are the province of the brain. The brain has impressive processing capabilities, but it has a limited capacity. A person cannot consciously experience all the events and information available at any one time. Likewise, it is impossible to initiate, simultaneously, an unlimited number of different actions. The question becomes one of how an appropriate subset of inputs, intermediate processes, and outputs are selected to command attention and engage available resources.

      Attention, then, may be understood as a condition of selective awareness which governs the extent and quality of one's interactions with one's environment. It is not necessarily held under voluntary control. Some of the history of attention and the methods by which psychologists and others have come to characterize and understand it are presented in the discussion that follows.

Early views on attention

19th-century roots

      Psychologists began to study attention in the latter part of the 19th century. Before this time, philosophers had typically considered attention within the context of apperception (pedagogy) (the mechanism by which new ideas became associated with existing ideas). Thus Gottfried Wilhelm Leibniz (Leibniz, Gottfried Wilhelm) suggested that one's loss of awareness of the constant sound of a waterfall illustrates how events can cease to be apperceived (that is, represented in consciousness) without specific attention. He suggested that attention determines what will and will not be apperceived. The term apperception was still employed in the 19th century by Wilhelm Wundt (Wundt, Wilhelm), one of the founders of modern psychology. Wundt, however, was among the first to point out the distinction between the focal and more general features of human awareness. He wrote of the wide field of awareness (which he called the Blickfeld) within which lay the more limited focus of attention (the Blickpunkt). He suggested that the range of the Blickpunkt was about six items or groups. He also speculated that attention is a function of the frontal lobes of the brain.

      One of the most influential psychologists at the turn of the century was William James (James, William). In his major work, The Principles of Psychology (1890), he says:

Every one knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things in order to deal effectively with others.

      James held that attention made humans perceive, conceive, distinguish, and remember more effectively and sped their reactions.

      In 1906 another prominent psychologist, W.B. Pillsbury, suggested three methods for measuring attention. The first relied upon tests that measured attention through performance of a task judged to require a high degree of attention; the second measured diminished attention through decreased performance; and the third gauged the strength of attention by the stimulus level required to distract the individual.

      As the 20th century progressed, psychology and the study of behaviour were subject to new influences that had far-reaching consequences for notions of attention. One such area of influence originated in the work of Russian physiologist Ivan Petrovich Pavlov (Pavlov, Ivan Petrovich), who reported what is now usually referred to as the orienting response. In dogs and other animals this includes such signs of attention as pricked-up ears, head turned toward the stimulus, increased muscular tension, and physiological changes detectable with instruments. Further influence came from work on reflexology by one of Pavlov's competitors, Russian Vladimir M. Bekhterev (Bekhterev, Vladimir). Many psychologists came to regard the conditioned reflex (an involuntary response conditioned by reward) as the basic building block of all human learning.

The influence of behaviourism

      During this period the development of the psychological school of behaviourism marginalized the study of attention. Behaviourism's principal advocate, John B. Watson (Watson, John B.), was interested primarily in stimulus–response relations. Attention seemed an unnecessary concept in a system of this kind, which rejected mentalistic notions, such as volition, free will, introspection, and consciousness. If used at all, the term attention was operationally defined in terms of discriminative responses to external stimuli. Ultimately, however, it became apparent that behaviourism failed to explain situations in which multiple stimuli compete with one another for attention. This led to a new emphasis on notions of attitude and expectancy and to a renewed interest in attention.

Relation to information theory

      Interest in attention revived in the 1940s, when engineers and psychologists became involved in problems of man-machine interaction in various military contexts. Faced with this new range of problems, such as helping soldiers stay alert when they were watching radar systems, applied psychologists found no help in existing academic theories and sought a new communications theory. As the occupational psychologist D.E. Broadbent expressed it, “attention had to be brought back into respectability.” Gradually the individual came to be viewed as a processor of information.

      Paradoxical as it may seem, attention appears to depend on both the unexpectedness of events and on their familiar association. information theory suggests that the significance of any event can only be estimated in terms of what else might have happened; hence, its tendency to attract attention is considered a function of its statistical improbability. The degree of novelty, which is estimated according to the number of times an event has been experienced previously, provides a measure of its surprise value. Thus an event that has never been experienced before has a high surprise value and should attract attention, even if it lacks any specific associations or consequences.

      The attempts to apply information theory to a diversity of psychological problems met in the end with limited success. Nevertheless, the view of the human brain as an information processor, a type of computer, was becoming more prevalent, and the notion that one might be able to quantify the gain or flow of information proved attractive. Information itself was defined as that which reduces or removes uncertainty. The process of removing uncertainty was seen as a series of binary (yes or no) choices. The unit of information that expressed this primitive choice between two alternatives, or halving the residual uncertainty, was called the bit (short for the term binary digit). In the terms of this theory, humans are seen as a communication channel, through which information is transmitted at the rate of so many bits per second. Attempts were made to measure the capacity of this communication channel in many areas of human activity, but the experimental results were found to be too inconsistent to be useful. Cognitive psychologists ultimately abandoned information theory, recognizing the incalculable effect of past experience on the information carried by any bit.

Aspects of attention

Selective attention

      Is an individual able to attend to more than one thing at a time? There is little dispute that human beings and other animals selectively attend to some of the information available to them at the expense of the remainder. One reason advanced for this is the limited capacity of the brain, which cannot process all available information simultaneously, yet everyday experience shows that people are able to do several things at the same time. When driving an automobile, they can apparently watch the road, turn the steering wheel, change gears, and apply the brakes simultaneously if necessary. This is not to say, however, that people attend to all these activities simultaneously. It may be that only one of them, such as the road or its traffic, is at the forefront of awareness, while the others are dealt with relatively automatically. Another kind of evidence indicates that when two stimuli are presented at the same time, often only one is perceived while the other is completely ignored. In those instances when both are perceived, the responses made to them tend to be in succession, not together.

      The conclusion reached and embodied in theories of the 1950s was that somewhere in the system was a bottleneck. Views differed as to where the bottleneck occurred. One of the most influential of the psychological models of selective attention was that put forward by Broadbent in 1958. He postulated that the many signals entering the central nervous system in parallel with one another are held for a very short time in a temporary “buffer.” At this point the signals are analyzed for features such as their location in space, their tonal quality, their size, their colour, or other basic physical properties. They then pass through a selective “filter” that allows only those signals with the appropriate properties to proceed along a single channel for further analysis. Part of the lower-priority information held in the buffer will fail to pass this stage before the time limit on the buffer expires. Items lost in this way have no further effect on behaviour. The original theory held that signals from only one source at a time could proceed. Subsequent work cast doubt on this explanation, and it was later modified by Anne Treisman, to suggest that the filter does not completely block, but simply attenuates, the nonattended signals.

      With the notion of attenuation, rather than exclusion, of nonattended signals came the idea of the establishment of thresholds. Thus threshold sensitivity might be set quite low for certain priority classes of stimuli, which, even when basically unattended and hence attenuated, may nevertheless be capable of activating the perceptual systems. Examples would be the sensitivity displayed to hearing one's own name spoken or the mother's sensitivity to the cry of her child in the night. This latter example demonstrates how processing at some level occurs even in sleep. Before attention can be said to be deployed on the activating event, however, the brain must return to a state of wakefulness. Some theorists have considered that there is no real need to postulate an early filter at all. They suggest that all signals reach central brain structures, which are, according to current circumstances, weighted to take account of particular properties. Some have a high weighting, for example, in response to one's own name; others are weighted according to the immediate task or interest. Among the concurrently active structures, that with the highest weighting gains awareness and is most directly responded to.

      Some critics of the above theories consider that they overemphasize the serial elements in attention. Apart from the everyday instances of tasks performed in parallel, as in the example of driving, they point to experimental evidence for highly demanding combinations of concurrent activities. As early as 1887 the French philosopher Frédéric Paulhan reported the ability to write one poem while reciting another. More recently it has been shown that some music students can sight read and play piano music while at the same time repeating aloud a prose passage. Of course it can still be held that, when two such tasks are being performed together, one of them is being done automatically and essentially without direct attention. An alternative explanation might be that attention alternates between them in a rapid, and frequently imperceptible, way. An analogous situation occurs when many users access a mainframe computer simultaneously. In practice, the computer is servicing their demands in very rapid alternation, yet each user remains relatively unaware that the interactive process is not absolutely continuous.

The intensity of attention

      These theories have been criticized for dealing with only the passive aspects of attention—certainly there is more to attention than mere selection. Such critics point out that there is also the question of the degree or intensity with which attention is applied to a particular task or situation. These “intensive” aspects of attention may be regarded as a subset of the broader dimension of arousal (activation); that is to say, they relate to the continuum of awareness that extends from sleep (or even coma) to alert wakefulness. The topic of arousal is discussed later; for the present it is sufficient to note that the level of arousal can be determined by the demands of the task or activity in which the individual is engaged or by internal states; these are sometimes manifested as instinctive drives and frequently accompanied by high emotions, ranging from keen excitement to unpleasant stress. In the case of some drive states, the high arousal may be directed to the satisfaction of a particular need. The consequences for attention can be the allocation of a high priority, or weighting, to all stimuli that relate to satisfaction of the need.

      By contrast, the level of arousal associated with a particular task varies from moment to moment as the task demands change; in other words, it is very much dependent upon overall stimulus load. One of the consequences of high-demand tasks is that spare capacity decreases. At full load, virtually all attention must be concentrated on the main task, leaving little attention available for perceptual monitoring of the surroundings.

      In recent years the direction of attention in response to task demands has often been spoken of in terms of the deployment of mental effort. The implication is that the intensive aspects of attention correspond to effort rather than just wakefulness. Effort, like arousal, is subject to task demands and available capacity. It is regarded as being mobilized in response to such demands, although the degree of voluntary control of effort is limited. Effort is not simply to be equated with the amount of work required by a task. Much mental activity takes place without the investment of a large amount of conscious effort.

Memory and habituation

      Attempts to accommodate the selective and intensive aspects of attention and its links with both awareness and more automatic processes have led to the formulation of a number of “two-process” theories of attention. One of the most influential was that advanced by the American psychologists Richard M. Shiffrin and Walter Schneider in 1977 on the basis of experiments involving visual search. Their theory of detection, search, and attention distinguishes between two modes of processing information: controlled search and automatic detection. Controlled search is highly demanding of attentional capacity and is usually serial in nature. It is easily established and is largely under the individual's control in that it can be readily altered or even reversed. It is strongly dependent on the stimulus load. It has been suggested that it uses short-term memory. By contrast, automatic detection, or automatic processing, operates in long-term memory and is dependent upon extensive learning. It comes into operation without active control or attention by the individual, it is difficult to alter or suppress, and it is virtually unaffected by load.

      The vast subject of memory is beyond the scope of this survey of attention, but a few pointers to the interactions that take place between what is attended to, how it is perceived and recognized, and factors that govern its subsequent recall are relevant. Memorizing is not simply a matter of repetition; attention plays a role in organizing material in ways that can influence its later recall. One example, known as the Von Restorff effect, is that, in any given number of items to be learned, an item that is notably different from the rest in size, colour, or other basic characteristics will be more readily recalled than the others. Unfortunately there is a price to be paid for this improvement; other “standard” items will be less well-recalled than they otherwise would have been.

      It is also important to realize that what is actually perceived is not a neutral, objective representation of what exists in the external world. It is coloured by past experiences and current expectations, to the extent that substantial distortions can occur to make a perceived item fit those experiences and expectations. Perceptions (perception) are frequently formed on the basis of quite limited cues; the art of camouflage utilizes this characteristic to the benefit of both humans and other animals in certain situations. It seems that even the culture within which a person lives determines the way he perceives the world. Following a study of the Hopi and Shawnee languages, the linguist Benjamin Whorf concluded that what these Native American peoples perceived was itself different from the perceptions of English-speaking Americans, by virtue of the way their languages were structured.

      Broadly speaking, the two types of attention can be characterized as focal and automatic. Someone who is focally attentive is highly aware, consciously in control, and selective in handling sensory phenomena. A person in such a state also uses the brain for short-term storage. (Indeed, some focal attention is almost certainly necessary for storing information in the memory at all.) Focal attention is flexible but makes great demands on brain capacity. Automatic attention makes fewer demands but is relatively inflexible, as it cannot cope with the unexpected. The focal and automatic modes may be illustrated by a driving example: a new driver has to attend to gear shifting in a focal way (actively thinking about it), while an experienced driver changes gears automatically (not needing to think about it).

      An important aspect of the control process in many circumstances is rehearsal. In this sense rehearsal means the mental repetition of incoming information. One consequence of rehearsal is that input items spend an extended period of time in the short-term memory store. It is also generally the case that what is attended to and rehearsed eventually ends up being stored in long-term memory. This suggests a close relationship between the conditions for awareness and those for storage in memory. Evidence for learning during sleep has sometimes been cited as contradicting this assertion that people remember only those things of which they were consciously aware at the time they occurred. It is now generally accepted, however, that the original evidence for sleep learning was suspect. In subsequent studies, when more stringent electrophysiological measures were made to ensure that individuals were in fact asleep, no clear evidence for learning during sleep could be found. There has been an indication that some type of conditioning may be possible during sleep, but, generally, awareness appears to be necessary for learning to take place.

      As already noted, one of the conditions for becoming aware, or selectively engaged, is when current expectations are violated. Just as people learn skills to the point where they become automatic, they also encode current experience into patterns of expectation that, as long as they continue to be fulfilled, need not engage focal processing resources. On entering a room, a person may be aware of the regular one-second tick of a grandfather clock, but the ticking soon fades from awareness as other things command attention. One is likely to remain unaware of it unless it stops (meaning that established expectations are violated) or unless other demands upon attention drop to the point where the person has sufficient spare focal capacity to become at least partially aware of the sound.

      The process of habituation occurs when a person's response to novelty wanes with the repeated and regular presentation of the same signal. Habituation represents a progressive loss of behavioral responsivity to a stimulus as its lack of adaptive significance is recognized. The unchanging repetition of the signal facilitates this recognition and confirms the inappropriateness of deploying further attention upon the signal. Generally, a shorter time interval between signals means a more rapid drop in responsiveness. If, however, the signals hold special significance for the individual, they will continue to be attended to and responded to even though they may be repetitive. For example, a person who counts the ticks of the clock to check its accuracy will not become habituated to the ticking sound. In other circumstances where stimuli have special signal properties, habituation may take place but only very slowly. Other factors, such as loudness, brightness, or intensity, can affect the magnitude of response to a signal and the rate at which habituation takes place. Although response enhancement and resistance to habituation are associated with increased stimulus intensity, they can also occur in reaction to faint signals. These observations of changes in attention with time and signal properties raise the wider question of how attention behaves over long periods of time.

Sustained attention: vigilance

      Sustained attention, or vigilance, as it is more often called, refers to the state in which attention must be maintained over time. Often this is to be found in some form of “watchkeeping” activity when an observer, or listener, must continuously monitor a situation in which significant, but usually infrequent and unpredictable, events may occur. An example would be watching a radar screen in order to make the earliest possible detection of a blip that might signify the approach of an aircraft or ship. It is especially difficult to detect infrequent signals of this nature.

      Vigilance is difficult to sustain. No single theory explains vigilance satisfactorily, probably because of its complexity. In the first place, there is a distinction between sustaining attention in a detection task, where the overall workload is high, and sustaining it when little is happening except for the occasional looked-for events. Under both conditions performance can decline over time. Much depends on the allocation of neural resources to deal with the task. These resources are somewhat limited by the processing capacity already mentioned. When the task is complex, detection difficult, time limited, and a series of decisions required using variable data, the brain may not succeed in coping. Long, boring, and for the most part uneventful tasks result in lowered performance with regard to both speed and accuracy in detecting looked-for events. If the task is interesting or is taking place in a stimulating environment, the individual will be better able to sustain attention and maintain performance.

      The frequency of task-relevant events holds a significant influence on vigilance performance. Generally speaking, the more frequent the events are, the better the performance, while long periods of inactivity constitute the worst case for performance. Surprisingly, the ratio of signals to nonsignal stimuli makes little difference to performance. The magnitude of the signal, however, is significant. During the course of a watch, expectancies develop about the frequency with which signals appear. If a signal occurs after an atypical interval, it is less likely to be detected. Performance can be improved (up to a point) by increasing task complexity, and in some vigilance situations the introduction of a secondary task can actually improve performance on the primary task. Performance is also enhanced when the individual receives feedback on the vigilance effort. Performance tends to dwindle in a noisy environment, particularly if the noise is high-pitched and loud and the task is difficult. Lack of sleep also impairs performance. Conversely, vigilance can be improved—or at least lapses prevented—by short periods of rest or by conversation or other mild forms of diversion. Monetary or other rewards tend to improve performance, as do some stimulant drugs.

The neurophysiology of attention

Physiological (physiology) changes

 The external manifestations of attention are accompanied by physiological changes, particularly within the brain and nervous system. These physiological changes can be studied by examining responses to novel stimuli. Growing out of Pavlov's research, the orienting response to novel stimuli has come to be characterized by a broad complex of physiological changes. These include changes in heart rate, in the electrical conductivity of the skin, in the size of the pupils of the eyes, in the pattern of respiration, and in the level of tension in the muscles. If the novel signal is an interesting one, the heart transiently slows down; if it is startling, the heart transiently speeds up. Most of the other types of change reflect similar reactions. Thus, the startling signal increases the level of skin conductance and the size of the pupils of the eyes, causes respiration to pause or briefly become irregular, and increases tension in certain muscles. Closer inspection reveals many more changes: for example, in the size of blood vessels and consequently in blood circulation, in digestive processes, and in other bodily functions. The majority of these changes are regulated by the autonomic nervous system. They prepare the individual to respond to new and potentially threatening situations. Senses become temporarily more responsive to signals from the outside world. Overall the pattern is one of preparing the individual to take in information rapidly and efficiently and of giving priority to those systems that might need to respond promptly to that information. The endocrine system will release hormonal agents that further facilitate the preparatory process. Once the novel signal has been fully assessed and classed as nonthreatening or of no continuing importance, the defenses are “stood down.” As might be predicted from the behavioral evidence, repetitive signals lead to habituation of the physiological responses as novelty dissipates.

      One of the crucial factors in this process is the evaluation of the signal and the assessment of its significance. Physiologically this entails shifting the level of arousal and focusing available resources (attention) on the demands the signal makes.

      Sensory inputs travel to the brain via primary sensory pathways that converge on a central relay structure, the thalamus, from which they are sent to relatively specific and localized receiving areas in the higher (cortical) levels of the brain. On their way from the sensory receptors to the thalamus, the signals pass an area of the brainstem and midbrain to which the sensory pathways have lateral connections. This area, called the reticular formation, is important in changing the overall level of arousal. When it is damaged, the individual may be unarousable. It has interconnections with the higher brain centres, and it projects pathways to the cerebral cortex. Unlike the primary sensory projections, which are limited to specific sensory modalities, many of the reticular formation cells respond to signals from any of the sensory modalities.

      When this ascending reticular activating system is operating, the individual is alert, aroused, and attentive. Reduction of its activity results in somnolence or inattentiveness; extreme reduction (for example, by anesthesia or concussion) may lead to confusion or unconsciousness, even though the senses still pass messages to the brain over the direct pathways. The reticular system seems to account physiologically for the sustained, tonic shifts in an individual's level of involvement with the environment, including the control of sleep-wakefulness. One nonspecific route to the cerebral cortex via the thalamus, the diffuse thalamic projection system, appears concerned with moment-to-moment fluctuations in the focus of attention. Collectively, the primary sensory pathways, associated areas of the cerebral cortex, and the more diffuse projection systems cooperate in the process of registering the incoming sensory signal, evaluating its contents, and mobilizing brain resources in response to the demands made.

Electrical changes

      Inevitably this account is an oversimplification. In the human brain, other structures, particularly the hypothalamus, are involved in regulating states of sleep and wakefulness, while limbic structures, such as the hippocampus, take part in arousal when rewards, punishments, or other emotional factors are involved. Much of our understanding of these systems and their interactions comes from the study of animal brains and from observing what happens in the human brain when things go wrong. There is, however, another important source of information about what is taking place in the healthy human brain when it processes incoming information. This is through the associated electrical changes that take place within the brain; these changes can be recorded from electrodes attached to the scalp. Such recording, known as electroencephalography, involves amplification of the very weak neuroelectric signals, often followed by computer analysis and display. Electroencephalography enables observation of the minute patterns of voltage fluctuation that take place as the brain cells process information and relay messages.

      Often the patterns of these intrinsic brain rhythms are modified by attention to external events or by thinking and other internal activity. The clearest effect of this kind is the inhibition (blocking) of so-called alpha rhythms, usually when the eyes are opened or when the person is thinking about a task, especially one involving visual imagery. Alpha rhythms, or waves, are more or less regular electric oscillations at a frequency of about 10 cycles per second (hertz), seen principally in the hindmost (visual) part of the brain. They tend to be most prominent when the mind is relatively blank and the eyes are closed. Absence of rhythmic features in the electroencephalogram (EEG) is generally regarded as evidence of arousal as long as there are signs of less rhythmic (asynchronous) activity. A total lack of electric discharge is a serious sign of brain morbidity.

      Buried within the fluctuating pattern of voltage changes are more consistent patterns that accompany the registration and evaluation of each discrete piece of sensory information. These changes are referred to as evoked potentials or, more precisely, as event-related potentials (ERPs). They extend over the period of half a second or so immediately following the onset of the signal concerned. ERPs are composed of a relatively consistent pattern of positive and negative electrical peaks that vary systematically when the properties of the signal that elicits them change. The whole waveform is divided into components, which are roughly approximate to its peaks and troughs, though not exactly, because there is overlap between adjacent components. Each component has its own pattern of distribution over the brain and varies according to the properties of the eliciting signal.

      This succession of ERP components constitutes a convenient and meaningful indicator of the various aspects of information processing being carried out on the signal. Moreover, because recording of such electrical potentials offers no insult to the brain or serious interference with the performance of a wide range of tasks, many aspects of processing can be studied in healthy as well as disordered brains. Among the components of the ERP are several related to attention in its various forms. For example, about 100 milliseconds after a novel sound is heard, a prominent negative component is produced, which, if the sound becomes repetitive, diminishes (habituates). The closer together in time the sounds occur, the smaller the component becomes. Components with similar characteristics, but varying slightly in time, are found following novel visual and tactile stimuli. In situations where the individual must pay particular attention to a signal, these electrically negative components become larger. Conversely, if the individual is not paying attention—but is, perhaps, reading a book when the sound occurs—the component is smaller. This physiological sign of selective attention can be shown to be larger to all stimuli in an attended channel than in a nonattended channel. For example, if an individual who is hearing different voices speaking simultaneously in each ear is told to listen for a particular word spoken by one voice only, all words spoken by the “attended” voice elicit a larger 100-millisecond component than those spoken by the other voice. Only the designated word, however, elicits a later prominent, electrically positive component, occurring about 300 milliseconds after it is spoken. These responses appear to offer physiological support for the behavioral view that there is an early filtering for broad characteristics, followed by a later one of the more complex task-relevant properties.

      Although such an explanation is plausible, the indications are that selection is not a simple serial process taking place at two discrete stages. When the task is relatively simple, looked-for properties can be distinguished substantially earlier than 100 milliseconds. There is also evidence of a more sustained, electrically negative change that can begin before 100 milliseconds and continue for perhaps several hundred milliseconds. This overlaps several components, supporting the idea that much processing must take place in parallel. Another component with attentional properties occurs just after 200 milliseconds when the incoming signal and current expectations are mismatched.

      In addition to the transient electrical links that are associated with selective attention, there are longer electrical changes that occur during preparatory states when one anticipates making a rapid response to an expected signal. When the signal that requires the rapid response (the imperative stimulus) is preceded at a short but fixed interval by a warning or “get-ready” signal, the warning signal triggers in the cerebral cortex a slowly rising negative voltage, which reaches its maximum by the time the imperative signal arrives. (A race starter saying “on your marks, get set,” may be a warning signal; the following pistol shot, an imperative stimulus.) When the response has been made, the voltage returns to its normal level. This slow potential change, contingent on the association of the two stimuli and the individual's intended response to the imperative second stimulus, has been termed the contingent negative variation (CNV). It appears as a correlate of focal attention, and it has been suggested that one of its functions may be to prime the appropriate areas of the cerebral cortex for the expected stimuli. The expectation must be focal—i.e., in the forefront of conscious awareness. If the preparation and response are well learned or the individual is distracted, the CNV is reduced. Conversely, if the individual concentrates on the task or is highly motivated to perform it well, the CNV increases in size. The CNV offers links with two-process theories of attention in that it seems to provide a physiological distinction between the more-demanding focal, flexible mode (large CNV) and the less-demanding automatic mode (small CNV). If the priming view of the CNV is correct, it may well indicate recruitment of the necessary resources to deal with the (imperative) task.

      These descriptions of a physiological basis for attention constitute an important first step toward the integration of behavioral and neurophysiological evidence and theory. Evidence already links the cortical CNV with similar processes taking place in the brainstem reticular formation of humans and animals. Several theoretical models of selective attention based on ERP evidence have been advanced. They have in common an attempt to use systematic patterns of ERP change as an index of the cerebral mechanisms underlying cognitive processes.

      All the electrical changes considered are in practice electrochemical. That is, complex chemical changes underlie the electrical correlates of attention. To take just one instance, the passage of electrical signals from nerve cell to nerve cell is dependent upon a range of neurotransmitter substances. Each of the neural systems already discussed is dependent upon the action of one—or sometimes combinations—of these neurochemicals. One transmitter substance, noradrenaline, is particularly prominent in alerting processes, along with its close relative dopamine. The total amount of another transmitter substance, acetylcholine, in the brain is found to be inversely related to the level of central nervous system activity. For example, if an individual is anesthetized, the electrical activity of the brain is reduced, and the content of acetylcholine is found to be increased. Direct electrical stimulation of the brain, or the convulsant action of certain drugs, tends to decrease brain levels of acetylcholine. This transmitter seems to be involved in a wide range of behaviour and functions. Among those related to attentional and arousal states are stress, awakening from sleep, and exploring behaviour. Certain amino acids, such as gamma-aminobutyric acid (GABA) and glycine, appear to play an inhibitory role in the brain and nervous system. Hence they, too, may be involved in reciprocal inhibitory processes accompanying some attentional states.

Lack of attention

Inattention and distractibility

      Limited processing capacity invariably implies a competition for attention. Humans spend their waking hours attending to one thing or another. The term inattention usually implies that, at a given moment, the thing being attended to is either not what it was intended to be or not what adaptively it ought to be. People will often report, “I was present, but I was not taking in what was happening.” On many such occasions, internal preoccupations become the object of current attention at the expense of sensory information from the external world. Alternatively, an internal stimulus, such as a pain or hunger, might capture attention. It is also possible for irrelevant sensory information from the external world to distract individuals from their current focus of attention. When this happens, it could be because the intrusive stimulus has a high priority (such as the ringing of a telephone) or perhaps because the task engaged in is simply uninteresting.

      Some individuals are more easily distracted than others, but for everyone distractibility varies with circumstances. When motivation and the level of involvement are high, an individual may totally disregard intense and persistent “outside” signals. Such inputs are either heavily filtered or dealt with only at an automatic level. Even when the competing stimulus is pain from an injury sustained, say, by an athlete in the early stages of a game, it is often scarcely noticed until the game ends and attention is no longer absorbed by the game. Nevertheless, because people's ability to focus attention varies, some report “difficulties of concentration” and may find themselves so easily distracted that they can scarcely read a book. There are indications that persons who are chronically anxious may be among those whose attention can readily be distracted by quite modest and irrelevant levels of stimuli. This feature has been noted in a number of psychiatric disorders (mental disorder), such as attention-deficit/hyperactivity disorder, and it has been suggested that one cause of these disorders may be a flaw in the mechanisms of attention.

Lapses of attention

      It has been established that, to conserve limited resources, whole sequences and hierarchies of actions can apparently be elicited without focal attention when they have been well learned or executed many times. There is reason, however, to suppose that at least a minimum level of focal attention may be necessary, if only to ensure that the correct sequence or hierarchy is initiated. Failures of this minimal monitoring can result in the phenomena usually classed as lapses of attention. For example, most people have experienced trivial behavioral slips such as finding themselves taking a regularly used route when they had meant to go in a different direction; attempting to switch off a light when leaving a room in daylight; or, perhaps, pouring tea into the sugar bowl instead of a cup. In each case a well-established action has been inappropriately triggered by partial cues and has slipped past the attentional monitor. Such errors typically occur because a person was “thinking about something else” or was not paying attention to the activity at hand. In many circumstances it is advantageous that automatic sequences of behaviour should be executed with only very limited reference to conscious attention. Musicians, typists, and other skilled persons are well aware that too much attention devoted to the execution of a well-learned skill can disrupt performance. Nevertheless, people cannot dispense entirely with some degree of attentional monitoring if they are to avoid errors. Another kind of lapse entails being unable to remember whether one has performed a particular action as part of a highly automated sequence: “Did I or did I not put the sugar in my tea?” Yet another occurs when an automatic action triggers another unwanted or inappropriate action: “I meant to take off only my shoes but took off my socks as well.” Most lapses have in common that they occur when attention has been claimed by an internal preoccupation or external distraction.

Conclusion

      Attention has sometimes been described not as a single concept but as the name of a complex field of study. This is true only to the extent that around it have grown up a multitude of peripheral (if not poorly defined) constructs. Some, like consciousness and awareness, are related to subjective mental states. Others, like arousal, activation, and orientation, are more representative of physiological terms. Still others, like alertness and expectancy, are characterized in terms of behaviour and performance. Another dimension considers attention in terms of effort, intention, drive, motivation, or automaticity. If a single definition could be derived from this, it would refer primarily to that state of the individual which represents the shifting, selective focus of consciousness. This is the state through which learning takes place. It makes heavy demands upon the brain's processing capacity. While individuals have always been able to recognize it in themselves, attention is becoming increasingly recognizable in others through indications of neurophysiological activity as well as by individual behaviour. Attention is a state of awareness that subserves the more flexible and directable aspects of a person's transactions with the environment.

W. Cheyne McCallum

Additional Reading

Harold E. Pashler, The Psychology of Attention (1998), is an overview of the subject; D.E. Broadbent, Perception and Communication (1958, reprinted 1987), presents an approach to the study of attention that uses communication theory; Carl M. Stroh, Vigilance: The Problem of Sustained Attention (1971), is an account of the factors influencing vigilance, its physiological correlates, and theories of vigilance performance; Raja Parasuraman (ed.), The Attentive Brain (1998) and John E. Richards (ed.), Cognitive Neuroscience of Attention: A Developmental Perspective (1998) discuss research issues in cognitive science; Nelson Cowan, Attention and Memory: An Integrated Framework (1995, reissued 1997), propounds a theory on the processes of working memory.W. Cheyne McCallum Ed.

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