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Some we may overlook altogether; others we may be aware of very dimly. At some times we attend only to a very small section of the field of view, as when looking down a microscope, and notice little outside that area. At other times, we perceive a wider field; we look to and fro, noticing first one thing and then an other. Sometimes this is done rather inattentively; but on other occasions we may search the field of view eagerly and expectantly for a particular object or event. --- Magdalen D. Vernon: From Magdalen D. Vernon, "Perception, Attention, and Consciousness," Advancement of Science, 1960, 111-123. Reproduced with permission of the author and publisher. --- It appears that there are many degrees or levels in the clarity and detail with which we are aware of our surroundings, varying from a precise and accurate perception of that part of the environment upon which attention is concentrated and focused, to a very vague marginal awareness of its less important aspects, and even to a type of perception of which we are not directly aware, but which nevertheless affects our actions in some way. In some degree we can vary the direction and extent of our conscious awareness, making it more concentrated or more diffuse, or directing it to one part of the environment rather than to others. But the degree to which we can exercise this control is limited. It is greatly affected by the general state of alertness, or "vigilance" as it is sometimes called; by factors of motivation and interest; by learning and experience; and by features in the environment itself. But the totality of experiences of which we can be aware at any one moment seems to be limited. Thus if attention is closely concentrated on one task, irrelevant events are likely to be ignored. Again, if the eyes are fixated upon a single point in the field of vision in order that this area may be perceived accurately and in detail, then comparatively little of the surrounding area is perceived. It seems probable that we cannot attend to two events happening at one and the same moment, and perceive both of them clearly. Thus it was found that it was impossible to take in two pieces of information presented simultaneously, one visually and the other aurally (Mowbray, 1954). Unless the two different events can be combined in some way, one must be overlooked. But attention can alternate rapidly between two or more events. The time taken to switch attention from one thing to another is of the order of 1 / 5th of a second. Again, a figure can take on two different aspects, as in alternating perspective figures, but perception of one aspect precludes perception of the other. If we wish to obtain a general and not very detailed view of our surroundings, we can relax our concentration and look to and fro from one point to an other. Then no one part of the surroundings is very clearly perceived, and some may be overlooked altogether. Commonly, of course, we avail ourselves of a succession of such instantaneous impressions. If the field of view is a wide one, we look from one point to another, searching for what we want to see. Or with a narrower field, we can pick out more detail, or correct our first erroneous impressions by a further study. However, there are occasions, as in a motor accident, when we have no opportunity to look again, and we may not have time to see even the important features of the situation, if these exceed our capacity to perceive them instantaneously. Numerous experiments have been carried out to determine how much we can see at a single glance. If a number of black dots on a white background is exposed momentarily, an observer can estimate up to five or six accurately. With a larger number, he begins to guess and is often incorrect. He is particularly likely to overlook those which are furthest from the centre of the field of vision (Baker, 1958). But if the dots are arranged in groups, or combined together in some sort of pattern, he may perceive a much larger number. If he is shown, in stead of simple dots, more complex forms which he is required to describe or identify, then the number which he can perceive is reduced. But again, if the complex forms can be related together in some way, or "coded"-if they can be combined together like the parts of a pattern or like letters in a word-they may together produce corroborative or "redundant" information, and they will be perceived more readily. Recently a good deal of experimental investigation has been carried out (see, for instance, Anderson and Leonard, 1958) on the effects of "redundancy" on perception, particularly of shapes or patterns which have no ulterior meaning like that of letters, digits, diagrams or pictures. The observer has to perceive what their actual shape is, and perhaps discriminate it from other shapes. Sup pose that the shapes are symmetrical, or that the patterns consist of regularly repeated items. Then the same information about form is given twice over by a shape symmetrical in one direction, and four times over by a shape symmetrical in two dimensions. The knowledge as to its essential form can be gained more rapidly than from an asymmetrical shape, and for that reason it can be perceived more quickly. On the other hand, if the observer has to discriminate between similar shapes, his task may be harder when there is considerable redundancy, or repetition of pattern, because there are fewer differences between the shapes for him to discriminate. These facts are sometimes important in practical tasks involving discrimination of patterns, for instance, in radar displays. In ordinary perception of meaningful objects, however, there is much redundancy of information, that is to say, a large number of corroborative impressions which enable us to identify these objects very rapidly; and to continue to do so in conditions, for instance, of brief exposure or dim illumination, when some of the impressions are scarcely perceptible. However, whatever the nature of the shapes or objects we wish to perceive, the total amount perceived at any one moment and the information derived from it are strictly limited. Broadbent (1958) has recently put forward the hypothesis that there is some type of "filter" operating in the central nervous system which allows certain information to penetrate to consciousness, while preventing the ac cess of other kinds of information. That part of the information which cannot be attended to immediately may, however, be put into temporary storage. When we have disposed of what immediately concerns us, we may then either turn deliberately to consider the stored information; or it may enter consciousness spontaneously; or it may affect our subsequent actions without becoming fully conscious at any time. This storage is only a short-term affair, and the information stored may soon disappear if it is debarred from consciousness for any length of time. There are, however, certain situations in which we are unable to maintain concentration indefinitely upon the matters which most concern us; and others which we temporarily keep in the background at length insist on intruding and distracting our awareness from the task in hand. We shall return to these later. Let us consider first the nature of the selective processes which determine what will be most readily and accurately perceived and will tend to be focal in consciousness. And here we encounter what at first sight appears to be a paradox-that sometimes we perceive most readily familiar events to which we are well accustomed, while on other occasions it is the unusual and unexpected which erupts in consciousness. Much experimental work has demonstrated that in our normal everyday life, in circumstances which we encounter frequently, we come to expect the appearance of certain familiar objects and events; and we perceive what has the greatest probability of occurring in these circumstances. Now I think that this statement can be held to be valid in the following sense: At any one moment, the visual impressions on the retina and in the central nervous sys tem are often too limited, vague and ambiguous to define exactly the objects to which they refer. Nevertheless, they provide cues which insofar as they corroborate one another may lead to inferences as to the nature of what is present--inferences based on expectations derived from past experiences which have taught us what it is most probable that we should perceive. Thus in fact we may believe ourselves to be aware of something on the basis of very inadequate sensory data-and perhaps of something which is not there at all. In an often quoted demonstration by Ames (1946), a trapezoidally-shaped window revolving about a vertical axis was perceived, especially in monocular vision, as a rectangular window oscillating to and fro. In another demonstration Ames showed what was called the distorted room, in which walls, floor and ceiling were slanted at various angles, instead of being at right angles to one another. But if viewed with one eye through a small hole in a screen, the sensory impressions were similar to those produced by a normal rectilinear room, and that in fact was what the observer perceived. There seems thus to be a tendency to "make sense" of what is perceived only partially, vaguely or momentarily; and "making sense" means that we perceive what may reasonably be inferred from our general knowledge of the situation to be the source of our sensory impressions. Indeed, it may be that we hardly perceive at all in the ordinary sense of the word; but we infer from the vague sensory impressions that what is there is a rectangular window or a rectilinear room, because there is a greater probability that we are being con fronted with these objects than with a trapezoidal window or a distorted room. But there are several ways in which the expectations of an observer may be altered, so that he no longer perceives what is most probable or most likely to be before him. His expectations may be modified if he is told to look out for certain things, or certain aspects of the situation. Numerous experiments have shown that an observer may be "set" to perceive by the instructions given him by the experimenter. When observers were told that they were going to be shown in a brief exposure words relating to "animals," and were then shown groups of letters such as "seal" and "wharf," they often reported words such as "seal" and "whale" (Siipola, 1935). But when they were told that the words would be related to "boats," they reported "sail" and "wharf." If observers are instructed beforehand to attend particularly to one aspect of a complex field-a field, for instance, which contains numerous types of shape of different colors-they will report this aspect with greater accuracy than if they had not been so instructed. But they will be more likely to overlook the other aspects, and be able to report very little about them. If observers are asked to discriminate shapes, certain characteristics of which are irrelevant to their judgments, these judgments will also be slower and less accurate if the observers think that these characteristics may sometimes be relevant to their judgments; but there is no such effect if the observers are instructed beforehand that these characteristics can be disregarded as altogether irrelevant (Henneman, 1957). However, another experiment (Lawrence and Laberge, 1956) showed that in fact observers may perceive something of the characteristics of figures to which they have not been directed to attend. For a short period of time after the figures have been presented, the observers may be able to store in their memories these secondary aspects, but they soon forget them. In several experiments it has been found that similar results are obtained if the observer is told to respond to a particular shape shown with a number of other shapes, and is informed beforehand as to how many different shapes there will be. The smaller the number of possible alternatives from which he has to choose, the quicker and more accurate will be his perceptions. Thus these experiments all show that a "set" can be established to attend to some particular aspect of the field which "filters" perception and concentrates it upon one aspect rather than others; but nevertheless the latter need not be completely overlooked. There is a number of situations in which expectation of what is most probable has been modified by learning, and the observer has been "set" by training to perceive aspects of the environment which he would otherwise have over looked. We have all heard stories of native trackers who can perceive the spoors of wild animals in the jungle which are invisible to Europeans. In our own society, tea-tasters and wine-tasters learn to perceive qualities in tea and wine which are not apparent to the ordinary person. In many industrial occupations, people learn to grade materials from small, not easily perceptible characteristics. Now, there are two types of processes which may take place in perceptual learning: (1) improvement in the perceptual discrimination of particular forms or objects, or the differences between these; (2) a more general type of judgment based upon inferences made from what is perceived. Practice in the recognition and discrimination of particular forms can undoubtedly improve these processes. The observer may get to know the shapes of particular forms (see, for instance, Bevan and Zener, 1952) ; or his actual acuity may appear to improve because he learns to find and utilize finer and more accurate cues to discrimination (Bruce and Low, 1951). But it may be necessary with complex material to point out to the observer just what he is to notice. For instance, in learning to identify air craft, observers will improve most quickly if they are shown just what are the significant points of difference between different aircraft. But this type of training is usually specific to a particular situation or class of material; it does not produce any general improvement in perception. It has sometimes been claimed that practice in the discrimination of shapes will improve the ability to read letters; but there is no reliable evidence for this. Again, practice in the rapid perception of letters and words exposed momentarily may improve this ability, but have little effect on ordinary reading. However, people may also be trained to utilize their percepts efficiently in making judgments or inferences from them. Thus it has been shown that absolute judgments of distance could be improved when the observers were taught how to break up long distances into smaller units which they then judged directly in yards (Gibson et al., 1955). Such judgments can usually be improved by teaching observers to note certain cues in the situation against which they can match their percepts; and the method of judgment can often be generalized to a variety of similar situations. In all these forms of learning, the essential feature is that something is perceived which was not noticed before; and insofar as the observer practices concentrating his attention upon that characteristic, so in time he will come to notice it automatically without further difficulty. There are circumstances, however, in which people spontaneously perceive certain things or certain aspects of the field of view particularly readily. If they have strong feelings about what is shown them or if they desire to perceive or to avoid perceiving something, then not only is the speed of perceiving altered; they may even think they perceive what is not actually there-or if they don't want to see it, they may fail to do so when it is, as we say, staring them in the face. Experiments have been carried out (Sanford, 1936, and Levine, Chein and Murphy, 1942) in which hungry observers were shown pictures of food, or objects related to food, for brief periods of time, or partially obscured. They perceived the food objects more quickly than observers who were not hungry; and also thought they saw food when no pictures of food were shown them. However, in some cases, as they became hungrier and hungrier, they identified fewer of the pictures, presumably because their need for food was not in fact satisfied by merely perceiving pictures of it. But the fact that we perceive more quickly something we are motivated to see does not mean that we necessarily perceive more correctly. In everyday life, if we find that in such circumstances we are mistaken, we may be able to rectify our mistakes by looking again. But one experiment showed that observers who were rewarded for guessing the identity of a single shape picked out from a large scattered group of shapes did not become more accurate at this task; they merely made more incorrect guesses (Boynton, 1957). The influence of emotions and desires on perceiving also appears in our everyday life perceptions of other people. We perceive most clearly and remember in most detail the faces of those in whom we are interested, and notice less clearly the faces of those to whom we are indifferent. A group of white American students who were favorably disposed towards Negroes perceived and recognized photographs of Negroes more correctly than did students who were unfavorable in their attitudes (Seeleman, 1940). The former perceived the faces in the photo graphs as those of particular individuals, with clearly differentiated individualities. The latter lumped them together as "niggers," and therefore saw little recognizable difference between them. But in another experiment (Secord et al., 1956) it was shown that a group of people prejudiced against Negroes accentuated the differences between Negroes and whites in characteristics such as width of nose, fullness of lips, etc., to a greater extent than did a group of non-prejudiced people . . . . Sometimes . . . perception appears to be retarded by unpleasantness or anxiety associated with the material which is being perceived. . . . Several experiments have been devoted to what has been termed "perceptual defense." Observers were shown in brief exposure sexual words which are taboo in polite conversation; and it appeared that many of the observers were slower to perceive these than to perceive neutral words which have no such connotation (see Bruner and Postman, 1947). It was hypothesized that these taboo words were perceived without their reaching consciousness, and were then refused entry to full consciousness because they would provoke disgust or anxiety. But it may be that the observers were unfamiliar with some of the words; or else they guessed what they might be, but rejected their guesses on the score that it was unlikely that such words would have been shown them; or else they knew what they were, but were shy of uttering them. When these various possibilities were eliminated, it was often found that the taboo words were perceived as quickly as the neutral ones. Indeed, in some cases observers exhibited "perceptual sensitization," and perceived the taboo words more readily than the neutral ones. There are other situations in which "perceptual sensitization" seems to occur; when highly significant events appear to have a "prior entry" and to force themselves into consciousness without their being any preliminary expectancy or state of desire in the observer. We all know the rapidity with which we perceive a sudden bright light or a loud noise, a blow or other painful stimulus; and how we immediately take action, by starting, jumping aside, shielding our eyes or ears. Now it might be argued that it is the great intensity of these stimuli which ensures that they shall blot out everything else and rapidly enter consciousness. Intensity may indeed be one of the factors concerned, but there is evidence to show that it is not the only one, and that these stimuli are significant also be cause they constitute a potential threat to us, and it is important that we take immediate action to avoid them. Thus intense stimuli, if repeated or prolonged, may cease to be central in consciousness. It is possible to ignore the loud noise of traffic in the street outside, and even sleep in spite of it, but to awake immediately at the sound of someone opening the bedroom door. Therefore it appears that suddenness and unexpectedness may be the important factors in bringing these events to consciousness. Again, we may quickly become aware of a sudden change in the perceptual field, and especially of rapid movement, without at first perceiving what is changing or moving. But once it is realized that there is some thing unexpected or incongruous in the field of view, then the tendency is to de vote more attention to that part of the field than to its more familiar and hum drum aspects (see Berlyne, 1957, 1958). From these observations we may then predict what are the situations and events of which we are least likely to become aware, and which we find most hard to attend to over any length of time. These are, first, stimuli which are very brief, very dim (because they are blurred or dimly illuminated) or situated at the margin of the field of vision. Secondly, they are situations which remain comparatively unchanged over long periods of time. We all know that if the intensity of a light or a sound is reduced sufficiently we cease to be able to perceive them. If we have cause to make a considerable effort, we may continue for a time to perceive them; but no effort nor desire, however strong, will make them perceptible below a certain minimum intensity--which is termed the "threshold intensity." Much experiment has been devoted to measuring the physical values of the threshold intensities for light and sound. It has been shown that these values are comparatively constant, in favorable conditions, but not completely so. It seems that awareness of such stimuli fluctuates; at one moment they can be perceived, but not at the next. The cause of this fluctuation lies probably in the nature of the physiological mechanisms of sensation. However, it is also true that different observers vary in the consistency with which they report that a stimulus of low intensity has appeared or disappeared; experience in making these judgments usually produces an increase in consistency. Nevertheless, evidence has been obtained in recent years to show that events of which observers are not consciously aware may yet influence their thoughts and actions. An instance of this may be the phenomenon of "perceptual defence" which we have already considered. One experiment (McGinnies, 1949) appeared to show that before the observer became sufficiently aware of taboo words to be able to report them, he yet gave a psychogalvanic reflex response to them. (This is a reflex decrease in the resistance of the skin to an electric current, which occurs as part of the response of the autonomic nervous system to painful and emotional stimuli.) In another experiment, nonsense syllables, some of which had previously been associated with electric shocks, were presented for perception during brief intervals of time (Lazarus and McCleary, 1951). Psychogalvanic re flex responses occurred when the previously shocked nonsense syllables were shown, before the observer perceived what the nonsense syllables were. It was claimed that a process called "subception" was taking place, setting up responses of the autonomic nervous system to stimuli with painful associations, even when these were not consciously perceived. However, it is possible that the observers did in fact see parts of the nonsense syllables, though not enough to report them correctly. But evidence of a similar kind has been obtained by Dixon (1955), that is to say, of psychogalvanic reflex responses to sexual words which were presented below threshold intensity, and were therefore never consciously perceived. Now it has been shown that even in sleep psychogalvanic responses may appear not only with loud noises, but also with faint but significant sounds such as the whispering of the observer's name (Jung, 1954). This may happen without the observer waking; or if he does wake, he does not remember what stimulated him. The psychogalvanic response is nevertheless accompanied by changes in the natural brain rhythms of sleep, as shown in the electroencephalogram, changes such as characterize the transition from sleep to wakefulness. Thus clearly there is a mechanism in the brain which can respond to certain types of stimulation, of potential importance to the individual, and although he does not become aware of their precise nature, yet his autonomic nervous system may react to them as to an alarm or emotional threat. But Dixon (1955) also obtained evidence as to other effects produced by "subliminal stimulation," that is to say, by stimuli below threshold intensity, but only just below. His observers were asked to say the first words that came into their minds when each of the stimulus words was presented subliminally; and often they responded with words which had some meaningful association with the stimulus words. With the sexual words, this sometimes had a Freudian character. Again, the observers were frequently able to associate their responses to the corresponding stimulus words when these were shown them subsequently. In other experiments Dixon found that such associations occurred only when they had been long established by use and familiarity. All these effects were more likely to occur if the observer had been instructed beforehand to expect that he was being stimulated in this way. Even when there was no such instruction, the observers were concentrating upon the task of guessing, so that there was little competition from other forms of conscious perception. There are other situations in which the observer has apparently not been fully conscious of an event at the time of its original occurrence; but he has in fact perceived and remembered it, and it has reappeared more or less consciously somewhat later. Thus in the phenomenon called "incidental memory," an ob server perceives events, is not directly conscious at the time that he has done so, but nevertheless acts in such a way later as to show that he had in fact registered them. In an experiment by Belbin (1956), road safety propaganda posters were hung on the walls of a waiting room in which observers sat for three minutes be fore they went into another room to be tested. Although they all seemed to have realized that there were posters in the waiting room, they neither perceived nor recalled them as such with any accuracy. But they were then tested to discover if they could apply the warnings given in these posters by picking out from photo- graphs of traffic scenes any faults they could see in relation to road safety-such as pedestrians failing to give way to traffic which had been signaled on by a policeman. Now it was found that car-drivers picked out from the photographs significantly more faults suggested by the posters than did non-drivers. This was not due simply to their driving experience, since a control group who had not seen the posters did not show the effect. But apparently the drivers, set by their driving experience to assimilate warnings of this kind, did in fact register them and act on them though they were not fully conscious of having perceived them . . . . We must now consider the situations in which perception is at first reasonably clear and accurate, but later ceases to be so. This seems to occur particularly when the observer is required to perceive a long series of very similar events, and when there is comparatively little change in the external situation over a long period of time. Such a situation was that in the "clock test" designed by Mackworth (1950). A pointer rotated in successive small jumps, one every second, round a dial like that of a clock; and at irregular and comparatively infrequent intervals it made a double jump. Observers had to notice and signal each of these double jumps by depressing a key. After only about half-an-hour, they began to miss the double jumps; and the number missed increased, throughout a two-hour period. Efficiency could be maintained, however, by increasing motivation; and by signaling to the observer every time a double jump had occurred, telling him whether or not he had responded to it. However, the perceptual characteristics of such tasks are also important in maintaining efficiency of response. In an experiment in which observers had to report the occurrence of the 'echo' on a mock-up radar screen, there was a sharp decline in efficiency when the echo was dim and difficult to see, but relatively little decline when it was bright and clear (see Broadbent, 1958). Lengthening the time over which a signal is visible and increasing its frequency and regularity of appearance may also prevent decline, because the observer can expect when it will occur. But a simultaneous loud continuous noise enhances the decline. These experiments thus demonstrate that a decrease in "vigilance" and awareness of repeated signals may occur in a monotonous serial task, but this decrease may be prevented by certain methods of emphasizing the signals or making them clearer. Broadbent (1958) has explained these effects as being due to an inability to maintain any concentration of awareness on a relatively unimpressive and un changing situation over a long period of time. Sooner or later there occur block ages in perception, and a wandering of attention to other features of the environment or to the observer's own thoughts, and an event occurring during this period goes unnoticed-though this is less likely to happen if the event is expected. The shorter the duration of the signal, the more likely is it to occur in one of these blockages, and therefore to be overlooked. But presumably the wandering is prevented by more frequent and more intense stimuli. Again, a paced task such as that of the clock test is more affected than is an un-paced task in which signals remain on view for a considerable period of time. Thus in another experiment (Broadbent, 1958), an observer had to make a response to any one of twenty dials which showed a "danger" reading. The "danger" reading continued to be visible until the observer had responded to it, that is to say, until any temporary lapse of attention had disappeared. There was some oscillation of vigilance, but no overall decrease. Again, we know from earlier experiments on the deterioration of performance of aircraft pilots during long periods of work (see Bartlett, 1943), that in these circumstances attention wanders to an increasing extent from signals on instrument dials. But it is important to note that this occurs not only as the result of general fatigue, but also from repetitiveness and lack of variation in perceptual stimulation. The application of these findings to the performance of monotonous tasks is obvious; and Colquhoun (1957) has begun a study of the factors which are most likely to produce loss of vigilance in industrial inspection tasks. . . . The psychological evidence as to variations in conscious awareness which occur in various types of perceptual situations and with varying degrees of attention has been related in recent years to certain physiological processes occurring in the central nervous system; and especially to the functions of a particular type of nerve tissue in the sub-cortical region of the brain called the "reticular formation." The activities of this appear to be associated with the arousal of awareness, the maintenance of vigilance and the direction of attention to specific events (see Jasper, 1957, and Samuels, 1959). The functions of the reticular formation seem to be two-fold. In the first place, one part of it appears to be concerned with general arousal and wakefulness. Impulses from this part inhibit the spontaneous activity of the cortex which occurs in the long, slow, synchronous rhythms appearing in the electroencephalogram during sleep; and these rhythms are replaced by the more rapid alpha rhythm which characterizes waking states in which attention is relatively relaxed; and which in turn is blocked by direct sensory stimulation or when attention is aroused. The reticular formation is itself stimulated to action both by impulses arriving through collateral fibers from the sensory nerve tract; and also by impulses coming from the cortex. The latter arise particularly in sudden and unexpected stimulation of the cortex, and their effect appears to be relatively temporary. After partial arousal to wakefulness, repetition of stimulation produces habituation, arousal is inhibited and sleep re stored. Injuries to the reticular formation produce a condition of lethargy or coma in animals, which cannot then be aroused. But another part of the reticular formation appears to be concerned with more specific alerting to particular sensory stimuli. Impulses from this part may interrupt and re-set the general pattern of cortical excitation; enhance or recruit discharges in specific areas of the cortex; and inhibit discharges in other areas. In visual stimulation, they block the alpha rhythm of the visual areas of the cortex. This part of the reticular formation is also stimulated by impulses through collaterals from the specific sensory pathways. It is affected to the greatest extent by pain impulses; and auditory impulses produce more effect than do visual ones. These differences would appear to reflect the relative degree of attention paid to these different sensory modes. The activities of this part of the reticular formation are also very closely geared to and dependent upon impulses from the cortex. Sensory impulses passing up the direct pathways to the cortex travel at higher speeds than those proceeding through collaterals to the reticular formation. Thus there is time for the cortex to evaluate the former and to discharge downwards to the reticular formation, regulating its reactions to the sensory impulses it received through collaterals. Its facilitatory activities may then be directed towards percepts significant to the individual, and its inhibitory activities towards irrelevant percepts, producing an enhancement of discrimination. Such an enhancement has been demonstrated in an experiment in which people were required to distinguish between two successive flashes of light (Lindsley, see Jasper, 1957). It was found that the temporal interval necessary for discrimination between the flashes was shortened by direct stimulation of the reticular formation. We may then infer that although the direct sensory pathways transmit the information which forms the actual content of consciousness, the degree of awareness of this and the manner in which it is discriminated depend on the activities of the reticular formation. This is demonstrated by the effects of stimulation by barbiturate drugs. The activities of the reticular formation are depressed and inhibited by concentrations of these drugs which are insufficient to affect direct responses of the sensory areas of the cortex. Thus an individual may continue to be aware of sensory stimulation after he has lost the power to attend or discriminate. The functions of the reticular formation have also been shown to be affected by impulses from those areas of the cortex specifically involved in motivational and emotional states. General arousal, specific anticipatory "set" and attentive searching with the eyes, followed by exploratory behavior, are set up through the activity of the reticular formation. Similar effects have been demonstrated by direct electrical stimulation of the reticular formation in monkeys. Thus mild stimulation produced the arousal of sleeping monkeys and the attention and alerting of waking animals, with inhibition of voluntary movement. Higher intensities of stimulation produced fear reactions of cowering and avoiding; still higher intensities, panic flight. Strong electric shocks to the area of the cortex associated with motivational and emotional functions may, however, result in a complete blockage of activity in that part of the reticular formation concerned with specific alerting and attention. This effect seems to parallel the breakdown of discrimination which takes place in states of violent emotion; it may even occur in a mild form in "perceptual defense." . . . This evidence as to the functions of the reticular formation appears to show that there are two separate and distinct processes which can occur in our reactions to the environment: first, a general and non-focal perception or registration of the environmental field or some part of it, which may include little or even no conscious awareness; and secondly, a focalized attention to some particular part of the field, accompanied by maximal conscious awareness. The latter process has been likened to a "spotlight" focused upon a small area of the field, the remain der of which is only dimly lit. The greater the degree of attention, the greater the clarity in the awareness of that part which is attended to. But although in such conditions perception is generally rapid and accurate, it can nevertheless take place also when there is little or no attention; and the precise relationship be tween perception and attention is difficult to define or determine. The concept of a spotlight focused upon a particular area of the field is obviously applicable to situations such as those in which an observer is required to perceive and report the number and nature of objects exposed in a small area for a brief period of time. In such circumstances, the amount perceived is certainly limited by the maximal attentive and receptive capacity of the observer. But whether this capacity is exercised to the full is partly a function of physical conditions such as the brightness, contrast, heterogeneity and form qualities of the field; and also of factors in the observer himself related to his expectations as to what will appear and his ability to "code" the information supplied him and to assimilate its meaning. But also it seems that no one is capable of exercising his full capacities for attention and perception over more than a limited period of time in a relatively unchanging environment, apparently through some process of habituation or self-inhibition in the functions of the reticular formation. However, we have seen that perception is by no means limited to that part of the field upon which attention is maximally focused. Observers are certainly aware of something in the surrounding areas of the field, though it is difficult to determine at all exactly what and how much is perceived; how much is remembered and for how long; and what effect it has on the observer's thoughts and behavior. But we have noted that in some cases aspects of the field appear to be registered in such a way as to produce effects even when the observer is barely conscious of having perceived them, or is not conscious of them at all. Again, parts of the field may be perceived in varying amounts and with varying correctness when attention is not focused-when it is extremely diffuse, and the ob server allows it to wander in an undirected fashion over the field. It seems that in these circumstances certain objects or events in the field may "arrest" attention, which will then be focused upon them. These events may possess characteristics such as intensity, vividness or sudden appearance which cause this effect. Or they may appeal to the observer's desires or interests, or may be a potential threat of danger to him. But we also have the paradoxical situation in which certain events which might cause anxiety or unpleasantness are apparently excluded from consciousness. Possibly, in the first case the general arousal function of the reticular formation is stimulating a general alerting of consciousness to the situation as a whole; whereas in the second case specific attentive discrimination is inhibited by a downward discharge from the cortex in response to emotional shock. However, in all these cases some process of "filtering" appears to take place below the level of consciousness, which allows certain percepts to penetrate to full consciousness, and directs attention upon them; whereas other percepts are marginally conscious and are stored for a while until we can attend to them; yet others never become more than vaguely conscious. But the marginal percepts may in fact have considerable effect on behavior, particularly upon the more automatic types of behavior. The type of "filtering" and the direction of attention vary according to the significance of the situation to the observer, and are subject to the effects of learning. It has been hypothesized that the functions of the reticular formation itself may be modified considerably by learning; but more evidence is required to determine the manner in which this takes place. REFERENCES Ames, A. (1946): Some Demonstrations Concerned with the Origin and Nature of our Sensations. A Laboratory Manual. Dartmouth Eye Institute. ANDERSON, N. S. & LEONARD, J. A. (1958): J. Exp. Psychol. 56, 262. BAKER, C. H. (1958): Brit. J. Psychol. 49, 279. BARTLETT, F. C. (1943): Proc. roy. Soc. B, 131, 247. BEI.RIN, E. (1956): Brit. J. Psychol. 47, 259. BERLYNE, D. E. (1957): J. Exp. Psychol. 53, 399. BERLYNE, D. E. (1958): J. Exp. Psychol. 55, 289. BEVAN, W. J. & Z ENER, K. (1952): Amer. J. Psychol. 65, 434. BOYNTON, R. M. (1957): In Form Discrimination as Related to Military Problems. Ed. by J. W. Wulfeck & J. H. Taylor. Washington: Nat. Acad. Sci.-Nat. Res. Counc., Publication 561. BROADBENT, D. E. (1958): Perception and Communication. London: Pergamon Press. BRUCE, R. H. & L ow, F . N. (1951): J. Exp. Psychol. 41, 275. BRUNER, J. S . & P OSTMAN, L. (1947): J. Personality, 16, 69. COLQ UHOUN, W. P. (1957): Nature, London, 180, 1331. DIXON, N. F. (1955): The Effect of Subliminal Stimulation upon Cognitive and Other Processes. Unpublished Ph.D. Thesis, University of Reading. GIBSON, E. J ., BERGMAN, R. & PURDY, J. (1955): J. Exp. Psychol. 50, 97. HENNEMAN, R. H. (1957): In Form Discrimination as Related to Military Problems. Ed. by J. W. Wulfeck & J . H. Taylor. Nat. Acad. Sci.-Nat. Res. Counc., Publication 561. JASPER, H. H. et al., Editors (1957): Reticular Formation of the Brain. Henry Ford Hospital International Symposium. London: Churchill. JUNG, R. (1954) : In Brain Mechanisms and Consciousness. A symposium organized by the Council for International Organizations of Medical Sciences. Oxford: Blackwell. LAWRENCE, D. A. & LABERGE, D. L. (1956): 1. Exp. Psychol. 51, 12. LAZARUS, R. S. & McazsaY, R. A. (1951) : Psych. Rev. 58, 113. LEVINE, R., CHEIN, I. & MURPHY, G. (1942) : J. Psychol. 13, 283. MACKWORTH, N. H. (1950): Researches on the Measurement of Human Performance. M.R.C. Spec. Rep. Ser., No. 268. McGiNnies, E. (1949) : Psych. Rev. 56, 244. MOWBRAY, G. H. (1954) : Quart. J. exper. Psychol. 6, 86. SAMUELS, I. (1959) : Psychol. Bull. 56, 1. SANFORD, R. N. (1936) : J. Psychol. 2, 129. SECORD, P. I., BEVAN, W. & KATZ, B. (1956): J. abnorm. Psychol. 53, 78. SEELEMAN, V. (1940): Arch. Psychol., No. 258. SUPOLA, E. (1935): Psychol. Monogr. 46, No. 210, 27. Also in Part 3:
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