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; 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 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.