Task combination and selective intake of information

Acta Psychologica 50 (1982) 253-290 North-Holland Publishing Company

TASK COMBINATION INFORMATION Donald Unioersity

Accepted

E. BROADBENT of O.xjord,

January

253

AND SELECTIVE

INTAKE

OF

*

UK

1982

The topic of this paper is one that is often termed ‘attention’, and it may seem unduly artificial to have given it a more cumbrous title. ‘Attention’ is a word in ordinary language, that can reasonably be used as a label for experiments in a particular area. Yet it has also been used on occasion as a theoretical concept, a mysterious asset or energy which is sometimes attached to human functions and sometimes not. This use of attention as a theoretical concept is perhaps more common in recent years than it used to be; it is not very helpful, and avoiding the word in the title is a step towards clarity. A good many readers however can be reassured that the paper is indeed going to be about ‘attention’, in one sense of that word. The topic is one that has been important in the recent history of psychology, of Acta Psychologica, and indeed specifically of Netherlands psychology. So far as the discipline in general is concerned an active interest in this field encourages a cognitive or informationprocessing approach, and vice versa. With the increasing popularity of the information-processing approach there has been a large amount of experimentation and theory in this field. Much of this work has come from and been published in the Netherlands. One need only consider the series of conferences on ‘Attention and Performance’, which were at first held locally, and published in the journal (Sanders 1967). They have now become inter* Author’s address: D.E. Broadbent, Dept. of Experimental South Parks Road, Oxford. OX1 3UD, UK.

0001-69 1S/82/0000-0000/$02.75

Psychology,

0 1982 North-Holland

University

of Oxford.

national and so large that any journal would hesitate to publish them; but their origins were in Acta Psyhologiw, and they are not the least of national contributions to psychology. One needs no more reasons therefore for considering progress in this area; but there is still a problem of selection. There is too much to cover, and the choice is bound to be arbitrary; we cannot be exhaustive. and readers must accept an apology if their favourite literature is not mentioned. There will for instance be no neuropsychology (Geschwind 1982; Shallice 1982). no auditory streaming (Bregman 1978). nor many other important topics. In what follows, the first section will deal with the 1950s and the second with the 1960s. These periods will doubtless be familiar to many readers though they have to be covered for the others, so some attempt will be made to keep them interesting for the experts by emphasising points that may have been forgotten. The 1970s however form the greater part of the field to be examined; the third section will look at some areas of factual or empirical study in that decade and the fourth section at some theoretical thrusts. To make exposition smoother. comment and criticism will be kept to a minimum as we go through these areas (though doubtless original sin will ensure some intrusions at certain points). In the final section we shall try to sum up what conclusions can be drawn at this time. The 1950s From stimulation

to informution

The interest in attention arose from the practical problems of skilled performance and its breakdown. It is manifest to anybody who watches a pilot or a tennis player that many activities are in progress simultaneously; one hand is adjusting the throttle while the pilot speaks to the control tower, the player’s eyes are detecting the position of the opponent even while the ball is being flung into the air to be struck. and so on. Formal measurement confirmed this ability to ‘do two things at once’. Yet such measurement also showed cases of interference: the skilled pilot controlled the aircraft less well when the incoming speech messages were faulty, or the laboratory subject carried out eye-hand tracking less efficiently if given difficult acoustic signals to receive

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verbal response. The problem was therefore to determine the limits on the ability to overlap tasks. To fulfil our promise to take relatively unfamiliar illustrations from this early period, let us take two papers by Broadbent (1956a, b) that have not been much cited since that time, but that illustrate conclusions common to a number of better known studies of the period. In the first, people were asked to reply to a rapid series of spoken questions about a visual display, all of them requiring a brief answer which could be scored objectively. Occasionally a buzzer would sound, and in the control group this produced only a slight disruption in correct response to the questions. One of the experimental groups was asked to tap a key when the buzzer sounded, and this gave rise to even worse performance on the questions, both when the buzzer was sounding and when it was not. Practice on the buzzer task abolished the effect between buzzes: that is, when one task needed no action the other task was normal. There was still some interference (within the limits of practice in this study) at the actual time of the buzz. In another group, the subjects were told that the response to a simple buzz would be a tap; but if the experimenter put his hand over the key, the response should in that case be a stamp of the foot. Originally this extra complication caused still further interference at the moment of the buzz. With practice the subjects reduced the interference at the moment of the buzzer to the same as that for the simple task. There were signs however that they were adopting a strategy of reacting to the buzzer in any case and then inhibiting the reaction in cases where the stamp was required. So, this experiment showed that interference did not occur just because stimuli or responses were occurring together. Interference only appeared at moments when, in both tasks, some event had to be noted and a decision taken to launch an action. Other experiments had shown that long sequences of movements could be emitted without interference with other activities, once the decision to launch them had been taken. The key variable was not the stimulus being processed but the information, in the technical sense, which it represented. If it could be predicted it did not interfere: the less predictable it was, the more it interfered. Sometimes however two highly unpredictable events do seem to be handled even though they occur together. How could that be? The experiment of Broadbent (1956b) was one of those answering this point. In it, people were presented with six digits as if in a test of and memory span. However, three of the digits arrived acoustically

three others at the same time visually. Memory for the six items was just as good as the average of ordinary visual and auditory memory spans, at least once people had found the right strategy to do the task. That strategy is to recall first all the items on one sense, and then subsequently all the items on the other; attempts to alternate items lead to inferior performance. In other words, provided that two simultaneous stimuli are well separated in sensory terms, they can both be taken in, with absolutely no loss of data, and form some kind of representation internally. (This particular experiment is taken here as an illustration instead of the better-known case using the two ears because there was poorer performance in the dichotic case than there was in the case of sequential stimuli. There were however reasons for thinking that this was purely due to confusion between two sensory messages of the same type and that is why the bisensory study was done.) However, there is then clearly some problem about further translation of the information from the first memory-stage into another form; this translation cannot be carried out in rapid alternation for the eye and ear, but an efficient optional strategy is for the selection of items for the next stage to be based on sensory properties of the input. If the items possessing the common property of arriving at the eye are selected for further processing, those on the ear can stay in buffer storage until they can be handled. So we reach the position that there is after the more peripheral some kind of limited system, occurring memory stage. The limit is, from the first experiment. one of information rather than stimulation; so two stimuli could both be handled at once by the limited system if they represented little information and were highly predictable, but not if they were chosen from a large vocabulary. Even if there was a temporary overload of information, however, it could be handled by leaving some items in temporary storage for later processing, as long as a suitable pause occurred to allow that. There will therefore be many apparently complex tasks that can be performed simultaneously: interference will only occur, and can only be studied, with stimuli and responses chosen from large sets, with inputs arriving at times that do not allow alternation or preview of input; and when long motor programs of predictable responses cannot be left to run off while fresh inputs are processed. Only such conditions will overload the stage of limited capacity, that follows the first stage of

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memory and leads on to a later stage of running off motor programs. If overload is occurring, however then its bad consequences can be relieved by ‘filtering’ the transfer of information from the buffer memory to later stages; that is, selecting events that share some basic physical property, such as arriving at the eye, or coming from a certain location in space. This ‘filter’ theory represented a reasonable consensus amongst those working in the area at the time (Broadbent 1958); it also had features that made it intelligible to a wider group of psychologists, sometimes with unfortunate results.

The l%Os Breakthrough

of the unattended

A dominant theme of the next decade was therefore the attempt to find phenomena that could not be handled by filter theory. Of these, the most successful (or unfortunate if one believed in filter theory), was a series of studies by Anne Treisman. She showed that even if a person was attending to one stream of events, some of the happenings in another stream might be reported. This breakthrough depended upon the meaning of the unattended events, and that is the crucial point. Random awareness of a few things other than the main focus of attention could simply mean that the main task did not fully occupy capacity. But if unattended events of particular types did break through, this appeared to suggest that the type had been recognised. If so, some translation from the first memory stage to a later process must have been achieved, and on the face of it everything that arrived at the senses might have been analysed in a similar way. In one of her best-known studies (Treisman 1960), people were told to listen to one ear and repeat what they heard. Without warning the messages at the two ears were switched; if the next word or two on the unattended ear were highly probable in the context that the person had just been saying aloud, then those words might appear in the response. Yet they did not appear (within the limits of the experiment) if the probable words did not occur on the unattended ear. It was not the context alone that produced the perception; it merely helped a breakthrough for the stimulation on the unattended ear.

This led a number of people to suggest that the selection studied in the 1950s was occurring very late in the internal processes. after all the stimuli reaching the senses had been fully processed. Deutsch and Deutsch (1963) put forward a view of that kind. It became a prototype of ‘late selection’ theories, contrasting with the ‘early selection’ theories of which filter theory is a corresponding example. It is probably fair to say that from that time onwards late selection theories of one form or another have been supported by a slight majority of authors. Significantly, however, Treisman herself did not share this view, and carried out a number of experiments that raise difficulties for it also. For example (Treisman 1964) she made a close analysis of the breakthroughs that occur when listening to a passage of highly constrained or less constrained context, with a constrained or less constrained passage on the ear being ignored. The probability of staying on the correct message is higher if the context is more constrained; but is unaffected by the amount of constraint on the other message. Once one has got onto the wrong message, the probability of staying there is high if the wrong message (that is, the one now being repeated) is constrained. The probability is unaffected by the degree of constraint on the correct channel (that is, the one not now being repeated). So when two words are competing to be heard, what counts is their probability in the context of what has been responded to previously; not the context of what has arrived but received no response. Rather similar results have been found in the visual domain by Underwood (1977). If everything is analysed and selection is late, how could it happen that unselected items have no effect on subsequent selection? It cannot be that they are forgotten; we know that they persist for much longer than is necessary. Some other explanation is needed. Treisman provided a possible one, taking into account not only this but many other results. One can explain her view as yet another application of the ‘information, not stimulation’ principle. She pointed out that the response of saying a particular word does not necessarily imply that the corresponding stimulus has been totally processed. It is quite possible for the response, or its internal equivalent, to be triggered by detection of one or two features appropriate to the word; we know indeed that in a strong context people will make appropriate but false percepts when a word similar to the correct one is presented, provided there is suitable blurring or accompanying noise to lose a good few of the sensory features (Broadbent and Broadbent 1975, 1980). So her idea was that

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p.eople who are told to attend to one ear, to ‘filter’ in the older theoretical term, still allow response to be influenced by a few features arriving at the other ear. If the words previously heard are biasing the listener towards a certain word, then these few features from the wrong ear may trigger that word and so give rise to breakthrough. Getting a few features however is something very different from total analysis of the unattended ear; the latter has not indeed been blocked out altogether, but it is ‘attenuated’, in Treisman’s term. On this view, apparent breakthrough from the unattended channel ought to be associated with a fair number of misperceptions, or false alarms. Treisman herself performed experiments showing that indeed this was so, and so did others; Broadbent (1971) reviewed a large amount of data from many fields to demonstrate that her theory gave a better account of them than the popular ‘late selection’ views. It is fair to say that this review was badly written and over-packed with detail: one comment by a sympathetic reviewer was that it described a slow, tortured and laborious process. It may be helpful therefore to summarise some of the broad distinctions that seemed reasonable at the end of the 1960s. As before, filtering was conceived as a strategy that could allow satisfactory performance in cases when interference would otherwise occur. That is, if a large number of complex events are occurring, a person who selects those events possessing a particular physical feature (such as location in a particular point in space) will be able to cope adequately with those events at the cost of knowing less about the remainder of the things that are happening. Filtering was not however now imagined to block out everything about the unattended parts of the surroundings; some features would break through from other places, perhaps enough to trigger some later process. Filtering was also more clearly accompanied by other strategies that were different in operation but would also reduce interference. The second strategy of that type was called categorising. This meant processing only those aspects of an event that show it to belong to a particular class, and distinguish it from another class which might also occur. The most obvious example in everyday life is that a letter E may take a number of different detailed shapes, but all of them are identified as instances of the same letter, and are distinguished from instances of the letter F. More subtle instances of categorising had however appeared during the 1960s; if a person is asked to find a target

letter from a prescribed set amongst a number of irrelevant letters. early in practice the time taken increases as the number of irrelevant letters increases. As practice goes on with the same set of letters, the effect of the irrelevant ones decreases; they no longer interfere so much. If the irrelevant letters are now changed, the interference comes back; unless one is careful to make the new irrelevant letters share some visual feature with the old ones. For instance, if one has been searching amongst AEFHIKL for target letters C and 0, substituting new irrelevant letters made up of straight lines is less disruptive than using a new set such as BDGQ. These and similar findings had been produced by Rabbitt (1964, 1967), and made it clear that the person was learning to identify members of the class of irrelevant letters by certain key features. The person need not, of course know which irrelevant letters they were, merely that they belong to that class. and are not in the target set. So only certain features need be detected. Categorising therefore is simply an extension of the idea that only information the person finds essential is processed: but there is one important difference from the filtering strategy. Changing the setting of the filter is fairly quick; you can say to somebody ‘Listen to that man’ in a crowd, and almost immediately the words produced by the man will have a priority over those produced by others. Changing the setting of the categorising system is slow. It needs repeated practice at the task before a person can detect the presence of a member of a category with minima1 interference from non-members of the category. Once a category has been set up, however, a third strategy becomes possible. This is called pigeon-holing, and is like filtering in that it selects some events for further processing. Instead of selecting the events by a unique feature, however, this strategy operates by applying a bias to certain categories, so that they will be triggered off by less evidence than they would normally need. Like filtering, this strategy can have its setting changed quickly; you can say to somebody ‘Listen in case somebody in this crowd mentions a kind of animal’ and almost immediately words in that class (whoever speaks them) will have a priority over others. All three strategies operate, as before, after an unselective memory that receives all the input from the senses; but they are not a ‘late selection’ theory of the type of Deutsch and Deutsch (1963). That is, they do not suppose that the complex of features in memory has been identified as an instance of some category before the selective operation

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starts. If there is a pigeon-holing bias in favour of some category, that bias pre-exists the stimulus, and results in false alarms as well as correct perceptions and reactions. It is these misperceptions that Broadbent (1971) regarded as the key test of the difference between late and early selection theories. The reader should remember that many investigators did nevertheless hold a late selection theory by the end of the 1960s. The essence of this view was that all stimuli are fully analysed (categorised, in the above terms) before selection takes place. Since their identity is then known it can enter the calculation which decides whether any further reaction shall follow. Instructions to listen to a particular voice can form part of such a calculation and account for the fact that words from that source have an advantage; but there is clearly no problem in explaining breakthrough. Any stimulus possessing an identity that makes it temporarily of higher priority will overcome those stimuli that are coming from the prescribed source. No problem of explaining breakthrough; just a problem of explaining all the other phenomena of task combination. Why do not unattended events provide a context for other events? Why cannot simultaneous items on the eye and ear be in visual alternated in response ? Why is the reduction of interference search reversed by removing the crucial features that distinguish the irrelevant ones? Why the false percepts provoked by events in unattended streams? A late selection theory needs ad hoc postulates to explain all these and many, many other findings that fall naturally out of an early selection theory such as Treisman’s. Perhaps somewhere there is a formulation that has seriously attempted to meet all these difficulties; mostly however late selection theories are driven by the need to explain breakthrough, and have not developed explanations for these other phenomena. Some of the experimental work of the 1970s therefore, has been attempting to explore ways in which late-selection theories can be extended to cover other aspects of the two-task situation. These make a natural starting place for a survey of the more recent work.

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The 1970s Cross-talk

between tasks

In this section, we shall outline a line of enquiry that looks at the effects on a task either of particular stimuli that are supposed to be unattended; or of particular events in another task. One example is the Stroop interference situation. In this task the colour of some ink is to be named despite the fact that the ink is used to print a colour name different from its own colour. A typical paper is that of Keele (1972). point is that From a late-selection point of view, the interesting interference depends on the nature of the word that is printed; a colour name interferes, but a word unconnected with colour has little and often even no effect at all. The argument is therefore that the word has always been recognised in some sense, but that interference only arises at a late stage when the response to the word has to be inhibited in favour of that to the ink (Keele and Neil1 1978). Treisman’s theory on the other hand explains the effect by saying that the colour names are in a state of readiness to trigger off even if a few relevant features are detected. Thus the wrong one can be unleashed by the presence of some appropriate stimulus even though most of the details of that stimulus have not been analysed (Broadbent 197 1). Interference can result from letter sequences that are not colour names but possess some similarity to colour names. Indeed most of the interference is due to the first two letters alone (Singer et al. 1975). In general the results obtained with the Stroop phenomenon do not therefore distinguish the two theories, though all sides are agreed that the interference produced by a word does depend on the ‘priming’ of that word. A preceding sentence containing a word with which the interference word is associated can make it interfere even if it is not a colour word (Warren 1972, 1974). A particularly interesting effect is that an ambiguous word in the sentence can in some conditions produce later interference from words connected with both meanings of the priming word (Conrad 1974; Killion 1982). This does not always happen, however, and the effect seems to depend on the strategy being adopted by the reader when the prime word is read (Schvaneveldt et al. 1976; Killion 1982). For instance. it may be particularly likely to occur if the prime is pattern masked and the reader is unaware of it (Marcel 1980). Turning away from the Stroop, let us consider tasks where the

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relevant stimuli are separated from irrelevant ones by being on separate ears, or in separate locations of visual space. In these cases, there is detectable facilitation or interference from the presence amongst irrelevant events of stimuli whose meaning is related to that of the relevant ones happening at that time. A good recent example is the study by Underwood (198 l), which measured response time to a word presented tachistoscopically. The time was affected by the presence, in another part of the visual field, of semantically related rather than unrelated words obscured by a pattern mask. This general type of phenomenon is now well-established; late selection theorists would argue that it should be interpreted as the result of all stimuli being analysed to determine their meaning, with some later interference if and only if an irrelevant stimulus is found to be in the same general class of meaning as the relevant one. A theory of the Treisman type, on the other hand, would argue that there are no research findings showing complete analysis of unrelated words; rather, when the evidence for the target or relevant word is nearing the criterion of identification, it partially excites associated words, which in turn then tend to be triggered by any relevant stimulus features that may be present, so that the final stages of identification are slower than would otherwise be. Thus, only words related to the attended words are analysed, and then only partially. Again, experiments of this type seem to be consistent with either type of theory. Another variety of experiment in this tradition attempts to pin down the point at which interference is occurring. They find the time at which interference is maximal, to see if it is close to the response or to the stimulus. Alternatively, they may vary one of the tasks, to see if the interference is reduced by minimising response competition. As an example of the first type, Posner and Boies (1971) used as one task a matching problem, in which a stimulus has to be judged as same as or different from an earlier stimulus. A second probe task was simply to react to a signal arriving at various points in time during the performance of the first task. At first it seemed that this technique pointed only to the overt response as the cause of interference, for the probe reaction was chiefly delayed when the overt reaction of matching was due; and a probe at the moment of arrival of the first stimulus showed reaction time little slower than a probe in the interval between trials. We now know however that the large delay at the moment of the overt reaction in the first task was because the response to the probe was a

manual one and so was the reaction to the probe signal. If a verbal reaction is used in one case and a manual response in the other. the interference shifts away from the moment of overt response (McLeod 1978). We also know now that the interval between trials is the wrong time to measure the control speed of reaction, because subjects expecting a trial on the primary task are already slowed down at that point: if one uses, as the control measure. reaction time from a situation with no primary task, then there is interference even when the stimuli of the primary task are merely arriving to be matched (Paap and Ogden 1981). It appears that McLeod and Posner have now reached agreement on the existence of interference that does not depend upon response identity (Posner 1982). An example of the second type of attempt to show that interference arises at the response stage is provided by a series of studies by Trumbo and associates (e.g., Noble et al. 1967). In these it was reported that no interference was caused to eye-hand tracking by merely learning verbal material without overt response; only by having to make responses. Yet, McLeod (1973) demonstrated that this was true only in the case when the level of learning was extremely slight, so that the people were taking in rather little from the learning task while tracking. If care was taken to ensure that genuine learning was taking place, then tracking was impaired even without overt response occurring in the interfering task. One can summarise the conclusions of experiments of this type as follows. The main interference between two tasks occurs at the point where they compete most for the same functions. Thus in the ludicrous extreme, if one asks for performance of two tasks that require the same hand to be in two places a metre apart, that will pose such serious problems that competition for central decision-making will be unimportant. There will be ample time to take decisions in both tasks while the hand is moving from one place to the other, as we know that decision and movement can overlap (Leonard 1953). Such motor interference can be more subtle than competing requirements for the same limb; using a particular finger on one hand may alter the relative difficulty of moving various fingers on the other hand (Broadbent and Gregory 1967). Similarly, if one task requires the eye to point in one direction, and a second task requires it to point in another, other limits on performance would be unimportant. The limitations of the transfer system between sensory buffer and output program only become rele-

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vant when the tasks are such as to minimise problems at these initial and final stages. If that is done however interference can be shown to change using the same stimuli and responses, but merely altering the central mapping from one to the other. Thus Shaffer (1975) gave a typist visual and auditory language inputs, while requiring her to type and to speak. It was considerably harder to speak the seen words and type the heard ones than vice versa. A number of similar studies exist, and show that the problem of transferring the input information to the stage of launching an action can indeed be the locus of interference. As Wickens (1980) shows by surveying a number of studies, one must distinguish sources of interference at the input, output, and also in the central mechanisms. The notion that all events can happily be handled at the same time until the output stage receives little support. That particular form of late selection theory is no longer tenable. One illustration sums up work in this sub-area: the results of Long (1975) on simultaneous judgements of the pitch of a tone and of the brightness of a light. Long found that a judgement of ‘bright’ increased the bias of the person towards also saying ‘high-pitched’, given similar sensory evidence. Since performance was imperfect, he could also analyse what happened when the stimulus was in fact ‘bright’, though the judgement was ‘dim’. The nature of the actual stimulus still affected the other sensory judgement. It was not only the outcome of the decision therefore but the data on which it was based, that was producing cross-talk. Furthermore, the efficiency of one task was impaired primarily on the occasions when the judgement on the other task was difficult and the response was made with less than perfect confidence; clear decisions on one channel had little impact on the other. Interference could occur at any stage; and the attempt to confine it to the output is over-simple. The search for minimal interference From the point of view of late-selection theory, it becomes important to find cases in which the interference between two tasks disappears if steps are taken to avoid sensory and motor interference. There are several studies that have attempted to show that two tasks can be carried on with success when this is done. The position about these studies is a little odd; their authors make very strong claims for their

results, apparently thinking that they are in some way contrary to the expectations of, say, 1950s skill theory. On the other hand. those brought up in the latter tradition find nothing surprising in the results at all. It may be worth going into the results in a little detail to see what is happening. Let us take three papers, and label each for easy reference; A involves reading while simultaneously writing to acoustic dictation (Hirst et al. 1980). B used piano playing from seen music while repeating heard speech (Allport et al. 1972). C used typing while repeating heard speech (Shaffer 1975). In every case high levels of practice are involved, either before or during the experiment. In each case, the task (reading, piano playing, or typing) was compared on the same practiced subject with and without the other task. The features of the results that are usually interpreted as meaning that the tasks were succesfully combined are that, in A, subjects read no faster without dictation than with it: and their scores on comprehension of their reading (about 50%) were equally unchanged. In B, tape recordings of the piano playing, when assessed by somebody who did not know the condition of playing, gave only about two timing or wrong-note errors per trial in the dual task condition. In C, typing 512 symbols produced 3 errors when only one task was being done, but only 1 when both were performed. These results do indeed show that people are making a good shot at the dual task, just as pilots do speak while adjusting the throttle. There are on the other hand some signs of remaining interference, agreed by the authors but regarded as minor. Thus in C, the median speed of typing was about 10% slower in the dual task case. and another 10%) if the typing was of random words rather than prose. Intrusion errors also occurred from one task to the other. In B, no measures of performance at the verbal task alone were taken during the experimental session; but all subjects had achieved an error-free criterion of performance previously, and they averaged two errors per trial even under the best of the dual-task conditions. (This rise was not significant; with five subjects it would of course be stopped from being so by one lucky session by one subject.) Comprehension levels for the speech are reported, are low and increase with the skill of the pianist. Similarly timing errors in piano playing are numerically more than twice as common in the dual task case even though this is not significant in the second session when the dual task preceded the task of playing alone. It is significant in the first session, where the order

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was the other way round. In A, the efficiency of dictation is not reported for the first experiment; in a second, one of the two subjects was tested at dictation without reading, and performed very substantially and significantly better than when reading at the same time. (26 errors against 58 for one kind of material, and 7 against 27 for another.) It would be fair to sum up the attitude of the authors of these papers by quoting B: “the question whether the time-sharing is complete or not is secondary”. The performance is better than they feel earlier workers would have expected, and in particular is better than performance at comprehending two simultaneous speech messages. They also place great importance on the fact that performance on one task does not vary with changes in the difficulty of the other; speech errors in B are no more common when playing a more difficult piano piece than for an easier one, and in A practice at reading stories during dictation gives immediate equality of reading speed and comprehension of reading of an encyclopaedia with and without simultaneous dictation. Those who are unsurprised by these findings would rather emphasise the timing of the stimuli in the tasks and their statistical structure. In A and B method of presentation of the visual stimuli is undescribed and they were presumably available in the normal manner for reading. That is, the reader could choose when to take in the visual information and prepare response, at times left convenient by the auditory task. In C, 40 items ahead of the one being keyed could be seen, which is effectively similar. The acoustic materials did indeed allow convenient times for looking at the visual stimuli; the materials were either prose, or (in A) isolated words given only every ten seconds. Prose emphatically does not require detailed processing of every word; in A, the authors checked the ability of subjects to replace words deleted from the prose they were using as reading text, and even when every fourth word was totally deleted the missing words could be guessed perfectly more than 40% of the time in the hardest condition. Music similarly is highly predictable: a random sequence of notes would not be presented by the Royal School of Music as an examination piece! From the point of view of skill theorists, statistically structured tasks such as typing, speaking, or playing the piano involve the intake of large units of stimulus information well ahead of response, and the decision to output similar large units of motor sequence; which in turn can be overlapped with intake and decision for the same or another task. To study task interference, one must control the timing of stimulus input and output and one must

use statistically independent stimuli. These characteristics of skill were well established by experiments in the 1950s; from this point of view. the surprising feature of A, B and C is the fairly clear evidence that even under these very favourable conditions some interference still continues. It suggests that learning to select the right information at the right time is harder than we used to think. The question of effects on one task from changes of difficulty in another had better be left till a later section: it has been debated a good deal. For the moment, readers can take their choice. If they thought people could only do one task at a time, they should be surprised by the degree of success in these studies. If they thought that two tasks could be done with redundant stimuli and ample preview. then they should be surprised by the size of the remaining interference. The data do not seem to let us come to any theoretical conclusions. Simple

sensory

detection

One particular form of task combination that has aroused a good deal of interest is the measurement of simple detection at threshold, when there are two or more places or sense-organs that may be stimulated. Thus a person might be told to detect a tone that would certainly be on the right ear; or one that might be on either ear. There is no doubt that detection of two tones that arrive simultaneously is worse than it would be for a single tone; it is even the case that detection is worse on one ear when one falsely thinks that there is a target on the other. But the point that has aroused a good deal of recent interest is that, provided nothing happens in one task, detection in the other is often as good as it would be if only one task was being performed (e.g., Moray 1975; Shiffrin 1975). There are some complications concerning the particular stimulus dimensions involved, and the statistical independence of the channels is also important; in some cases there is a small effect of uncertainty. But equal performance in certain and uncertain conditions occurs sufficiently often to be welcome to those who think interference occurs only between response processes. They can then argue that the events on both sensory locations are being encoded in parallel, and that it is only when two responses are being made, as in the work of Long, that impairment arises. There are also however a number of counter points that can be raised by early-selection theorists. First, we saw that in Long’s case it is not

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only the overt response that produces interference, but the nature of the physical stimulus being analysed. An overt response is not necessary for interference to occur between two inputs, merely internal detection. Duncan (1980) has found that noting the presence of two stimuli for later response does impair performance on either, and there is a similar result in one of the conditions of Schneider and Shiffrin (1977: exp. 3a). Second, it has always been held by early selection theorists that two tasks would not interfere in the absence of any event in one of them; as in the study of Broadbent (1956a) with which we began. Formally, the proposal was that the sensory field was first scanned for elementary properties such as presence of any change at all, and that a novel event would capture the selective system in the absence of any other. Third, however, such capture might well take time; in the study by Broadbent (1956b), it was urged that separated sensory inputs could both receive accurate response, but at the cost of delay, due to the use of peripheral buffer memory. Yet most of the studies reporting accurate sensory detection have not reported latency, even though Shiffrin (1975) specifically warns that he believes there to be latency effects from the monitoring of several potential sources of signals. When latency has been measured, as by Dennis (1977) this belief has turned out to be correct. Older studies of dichotic listening, such as that of Lawson (1966), have shown that accurate response to a target even on an unattended channel may occur; but at the cost of delay. More modern scepticism about the equivalence of accuracy and latency (Miller 1981b; Santee and Egeth 1982) points in the same direction. Thus, quite apart from the question of capture of the selective system. accurate monitoring of several sensory inputs tells us merely that the buffer memories of the inputs are well separated. Latency measures are needed. The best conclusion is that this is one of the areas in which the data from the 1970s seem to add only details to the facts available in the 1950s and do not help to distinguish theories. Analysis

of sensory properties

In this section we come to lines of study that are more removed from the interests of the 1960s. They show that selectivity is linked to particular properties of the stimulus, that it has a development in time, and that it can be an early stage of response to an input. As a starting

point, WC may take the studies of Garner and Felfoldy ( 1970) and of Lockhead (1972) on the ‘integrality’ of stimuli. Two events that possess integrality tend to get processed together regardless of the person’s intentions; it is hard to judge position of a dot in the vertical dimension if it is varying irrelevantly in position in the horizontal dimension. But it is easy to sort cards by the value of one colour chip on the card even if the chroma of another chip on the same card is varied; they do not possess integrality. Keele and Neil1 have objected to this generalisation, on the grounds that Stroop interference can be obtained when the irrelevant word is in a different place from the colour to be named (Dyer 1973). On the other hand, Dyer’s subjects did not know in advance which location was to hold the relevant stimulus and which the irrelevant one. So they had to take in information from the whole display to find out where the colour was and where the word. It is known that advance uncertainty does affect the interaction between relevant and irrelevant stimuli (Underwood 1976). Two stimuli can well interfere even though non-integral, particularly when the selective system is not operating. The key point is that two stimuli can only be stopped from interfering if the irrelevant features are non-integral with the relevant ones. A difference in spatial location is a good way of keeping the selected and unselected apart. As we said in the last section. speed, rather than accuracy of response, is a good way of demonstrating selective intake from one location in space. But selective intake takes time to develop. Eriksen and Collins (1969) showed that reaction to a stimulus at a position indicated by a cue was faster than uncued reaction, but only if 150 msec or so were allowed to intervene between presentation of the cue and the reaction stimulus. A large series of studies by Posner and associates (summarised by Posner 1980) show amongst other points that the best time interval between cue and reaction signal is longer if the reaction signal is spatially further from the point currently selected. The point of maximum selectivity moves through space as if it was a material object. It is also a genuine improvement in intake of information, not a criterion shift; filtering rather than pigeon-holing. Once selectivity has settled on a point in space, there is still some breakthrough from other points; another major series of studies by Eriksen and his colleagues (summarised by Eriksen and Schultz 1979) show that reaction time to the prepared position still depends on the nature of other stimuli elsewhere in the visual field. On the one hand.

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irrelevant stimuli that possess some features in common with the stimulus for action A will slow down action B; on the other hand the interference does reduce as the irrelevant stimuli are further in space from the relevant one. A good rule is that events more than lo from the target are unlikely to interfere, but this depends on whether or not the person is allowed to use fixation to help control the selectivity of intake from different spatial positions. If fixation is allowed, events as close as ! ’ may in some cases occur without interference (Humphreys 1981). Although eye-movements reinforce the selectivity, it can occur without them. One could summarise the data thus far as making us think of selectivity as like the beam of a searchlight, with the option of altering the focus. When it is unclear where the beam should go, it is kept wide. When something seems to be happening, or a cue indicates one location rather than another, the beam sharpens and moves to the point of maximum importance. Whatever the size of the beam at any instant, everything within it obtains access to a further processing system, so that if there are stimuli inconsistent with the message conveyed by the target one, they will interfere. The motion and width of the focus of selectivity is dependent on the events already detected. There is however more to the basis of visual selection than a simple question of visual angle. For example, Kahneman and Henik (1977) required subjects to select red items from among black ones; the selectivity was more efficient when the relevant items formed spatially adjacent groups than when they were scattered. A region of the visual field can possess the property of forming a unit whose properties are analysed integrally; and the factors that make it more likely that this will happen are similar to the classic Gestalt principles of good figure. Thus Banks and Prinzmetal (1976) found that a target item suffered more interference from noise elements if the latter formed a ‘good figure’ with it than if they did not; and Prinzmetal and Banks (1977) found similar results for the detection of a target amongst irrelevant stimuli. It would seem that the first stage of visual processing performs a segmentation of the field into regions on the basis of broad characteristics of each region. After this segmentation, properties of one region may have a selective advantage, dependent on the results of the earlier analysis. Thus Kinchla (1977) used large letter stimuli each of which was made up of a number of different close-packed small letters, so that the large letter

was a good figure. The task was to look for a target small letter. and this target might be either in the right-hand or left-hand of the two large figures presented. If a particular shape of large letter was associated with a higher probability of presence of the target letter in that large letter, then the target was better detected than it was in the other large figure. Incidentally, the benefit was of the type that involves both a rise in detections and a drop in false alarms; that is, it was a change in the weight given to information coming from that region of space, not in the response criterion. In a similar way, Murrell (1977) found that information raising the probability of a stimulus in one part of the visual field increased hits without false alarms; whereas raising the probability with no spatial guidance, raised false alarms as well as hits. Navon (1977) also used large letters made of small ones for a two-choice reaction task, in which the large and small stimuli might either point to the same or to opposite responses. People were instructed to react to one size of stimulus and ignore the other. The large stimuli showed no interference from the small ones, but the small ones showed interference from the large ones. This effect does not occur if the whole size of the display is increased, or if the large letter is made up of a few small letters (Kinchla and Wolfe 1979; Martin 1979). Thus one should not think of the largest stimuli in the field as necessarily being the first to be encoded, as is pointed out by Navon (1981). When there are large gaps between each of the small letters, then they rather than the large ones form the basic segments of the visual field. B. Smith (personal communication) has found that ratings of goodness of figure in the materials used in these papers predict the order of reaction times. The searchlight analogy needs therefore to be treated with care: the most useful element in it is the interactive relation between processes of selection and the results of previous elementary analysis. The picture of selection that emerges from these studies is like the ‘verification’ model of word recognition put forward by Becker (1976). In that model an earlier partial analysis of the input gives rise to further tests directed at checking the presence or absence of sensory details relevant to possible words. A similar approach, is adopted by Broadbent and Broadbent (1980), though with important differences. Thus Becker puts the effect of word probability in the later verification stage, while the Broadbents regard it as coming in the earlier, because of the false alarms associated with word frequency. They show however that if the experiment contains many pairs of highly associated words, this reliable priming

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produces a rise in the hits on the associated words without a corresponding rise in false alarms. This is true only if the stimuli contain a good deal of detail, being lightly degraded by blurring: if there is heavy blurring then the effect of context is a ‘pigeon-holing’ type of change in false alarms as well as hits. In suitable conditions however there is evidence for a change in the intake of information rather than in response bias. This interactive or verification approach challenges one of the most dubious assumptions of the 1950s and 1960s the idea that events unroll from stimulus to response without any counterflow back towards the input. As Kahneman (1973) discusses, early partial analysis of the input may lead to extra intake of evidence from regions showing some sign of interesting activity. Rapid reallocation of resources in this way could explain phenomena surprising on the traditional one-way analysis of flow of information (McLeod 1977). For example, it might explain the fact that a word associated with shock, and thus giving a galvanic skin response, may give rise to just as large a GSR when presented to the unattended ear as it does to the attended one (Von Wright et al. 1975). This is embarrassing to the Treisman theory, because most other evidence of breakthrough can be explained by triggering on the basis of a few features; but there would be expected to be at least some reduction in GSR in that case. On an interactive theory, detecting a few features from a shock-related stimulus could cause a swing in the intake of information towards the previously unattended ear, and this in turn increase the size of the response. Memory One less common approach deserves mention at this stage. This is the view that interference between tasks does not occur in any stage of transfer of information from input to output, but rather because each task places demands upon some central memory system. This is for example suggested by Logan (1980); the argument for it stems from the fact that two tasks may well be done less efficiently if both are required, and yet the impairment of one does not get any worse when the difficulty of the other is changed. By the ‘additive factor’ logic of Sternberg (1969), this means that the impairment due to simultaneous performance cannot lie in any of the stages being affected by the variables chosen. Logan suggests therefore that the interference is

rather with something outside the stages of processing altogether, such as memory for instructions in the task. The immediate objection to this argument is that the additive factor logic is highly open to question. When one variable makes a task harder by the same amount whether or not a second factor is operative, it is nevertheless perfectly possible that both variables are affecting the same process. In filling a reservoir, the depth required will alter the time taken; and so will the original depth of the water. But these two variables are additive in their effects, not multiplicative. In the same way it is perfectly possible that simultaneous performance may affect some stage of processing without the impact of that effect being greater under conditions that make the stage harder. We shall look more generally at this question of interactions between tasks in the next section.

General theoretical attacks Shured and unshared resources We have noted several times the fact that investigators have found inferior performance when two tasks are combined, but because they have failed to get differences in one task as the difficulty of the other is varied, they have drawn strong conclusions against the idea that the two tasks share some system of limited capacity. The general notion of plotting changes in success on one task as a function of success on the other was put forward by Norman and Bobrow (1976) in one kind of application, by Sperling and Melchner (1979) in another. and by Kinchla (1980) in another. In the extreme case instructions to abandon one task, to abandon the other, or to combine the two, form three points on a Performance Operating Characteristic or Attention Operating Characteristic. One can perfectly well introduce more points by changing instructions or other variables to give a range of levels of performance on one task and see what happens to the other. There have been a number of points of view about the conclusions to be drawn from different shapes of operating characteristics. First, the view already mentioned appears to be that a rise in difficulty on one task should give impairment of the other. The logic of this would be that a harder task demands more of the limited system.

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that this leaves less for the second task, and that accordingly the second task should suffer. The argument is not explicitly stated by the authors we have met already, but appears for instance in Kantowitz and Knight (1976). Second, Norman and Bobrow ( 1975,1976) challenge the foregoing view on the basis that change in difficulty need not logically lead to reduction of resources available for a second task. The change in difficulty may simply leave the first task performed less efficiently with the same resources; depending on the priority the person gives to one task rather than the other. Furthermore change in difficulty may result from factors that make no demand on the shared part of the system. They particularly distinguish the case of data-limitation; if somebody is reading and listening at the same time, turning out the lights will make the visual task impossible, but that will not reduce the resources available for the second task. Their suggestion is therefore that one should not examine POCs produced by changes in difficulty, but only by changes of priority. If one tells a person to work harder at task A, without changing difficulty, any change in task B must be the result of shifting some resource from one task to the other. If the limit on task A is in the data available to it, then changes in effort applied to B will change performance on B but not on A; if the lights are out it makes no difference how hard you listen. Many shapes of POC are therefore logically possible without any necessary theoretical interpretation. If there is a trade-off in some region of the POC, however, that suggests at least one common resource between the tasks, and that it is limited. Third, Navon and Gopher (1979) have extended the argument of Norman and Bobrow, using the analyses of micro-economics. That analogy brings out the fact that successful performance of a function may depend upon a number of resources of different kind, and that the combination used in one function may differ from that used in another. One job on a farm may be done by teams of four men, each with a tractor; while another job needs one man for each tractor. If the relative priorities of the two tasks are changed, transfer of men from the second to the first task will leave tractors spare. So the effect on the second task of a given shift of priorities will depend on the number of men per team required by the first task. As Gopher and Navon (1980) sum it up, they are suggesting that one should look at the interaction of priority and task difficulty before drawing conclusions about the presence of shared resources in two tasks. Again, many different shapes of POC are logically possible.

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The main contribution of these conceptual analyses is to expose the controversial character of some of the arguments in the area; should one look at the effects on task A of changing the difficulty of B, or the priority of B, or both combined? The first policy is the most common, but clearly the most suspect. At a modest level, POCs following the policy of Norman and Bobrow may at least set limits to the theories possible in the area. Relatively few of these curves have been compiled: when they have, the results quite often show a region near the maximum priority for each task, where performance does not get any better even with further emphasis on that task. Under such conditions the limit is not in some common resource between the two tasks. Many curves however show an intervening region of trade-off that is fairly linear, improvement in one task being balanced by deterioration in the other at a single steady rate. See for example Kinchla (1980). This single segment of trade-off looks like a common system that can be treated as a single resource, though it may of course consist of a very large number of separate ones with the linear trade-off being due to statistical averaging. No evidence seems to have been found for sharp jumps in POC of the kind expected if there are definite separate resources inside the person. Controlled and automatic processes While the approaches discussed in the last section are methodological, and discuss what one ought to conclude from particular patterns of data, there are also approaches that arrive at definite conclusions. One that is often quoted is the distinction made by Shiffrin and Schneider (1977) and Schneider and Shiffrin (1977) between controlled and automatic processes. One kind of process is seen as open to variation through temporary instructions or other changes; it is also seen as requiring some general resource that would also be needed by other processes in this group. The second kind of process, the automatic ones, are distinguished by the absence of these properties. They do not interfere with each other, and they cannot be stopped if the appropriate stimulus is present. Concretely, these concepts are based on the results of a particular experimental technique, visual search through a number of items in the field, with particular items representing targets. When the experiment starts, or if the target and other items are changed on every trial, it takes longer to search among presented items when there

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are more to search. It also takes longer if the set of targets is larger. If the target and non-target sets are constant, remaining the same from trial to trial, then after a considerable amount of practice the speed of search does not depend on the number of items in either set. (Remember Rabbitt 1964, 1967.) Thus, early in practice, one can interpret reaction to the target as suffering interference from the processing of the non-target items; and later in practice with a constant set one could argue that all items are being processed independently without interference. It is clear that early in practice, and with the varied set, detection is under the control of instructions; from what has been said, however, it may not be so clear that the practiced task, with the constant set, has ceased to be under control. A great deal of emphasis is placed in this case on one experimental result of Shiffrin and Schneider (1977). They told people to search for targets from a varied set on certain parts of the display, ignoring stimuli in other parts. If in a place to be ignored a target appeared from an earlier constant set it caused interference. The suggestion is that this stimulus was processed despite the instructions and that thus there was interference with the main ‘controlled’ processing. Two points should be noted at once about this experiment. First, the official target could appear in either of two locations placed diagonally on the display, and the irrelevant one appeared in one of the two locations on the other diagonal. Thus the irrelevant item was always in the same vertical position as one of the two target locations, and the same horizontal position as the other. It was separated from them by less than the 1” which we saw, from the work of the Eriksen group, as the minimum distance needed to achieve selection by filtering on a basis of spatial position. The careful reader may also wonder why, if automatic processing places no demands on the general resource need by controlled processing, there is nevertheless an interference when the ‘automatic’ stimulus arrives. This is clarified by Shiffrin et al. (1981); they point out that the interference appears when the practiced task requires action, whereas when the practiced task requires no action there is indeed no interference. The non-target items of Shiffrin and Schneider of course require no action; similarly, when Shiffrin et al. quote a combination of a practiced constant-set task with a varied-set task, finding no interference, targets in the two tasks never arrived at the same time. The

concept of ‘automatic processing’ is therefore that non-tcrrger stimuli become equivalent to the blank screen that would otherwise exist: while target stimuli do still produce some further demand on resources, and (it is suggested) cannot be stopped from doing so. Controlled processing on the other hand, in which the person is instructed to respond to a fresh stimulus in a fresh background, interferes with other tasks even when no action is required; and is therefore seen as requiring some further resource (‘attention’?). These concepts have been very widely influential: they are beginning to appear in quite elementary texts, and have formed a framework for a large number of experiments. Some doubts will be expressed about them later. Conscious and unconscious processes Another dichotomy having some connections with that of controlled and automatic is the set of distinctions made by Posner (e.g., 1978). Indeed, the same terms are sometimes used for both distinctions (e.g., Keele and Neil1 1978) but the concepts are slightly different so the labels ‘conscious and unconscious’ are used here to avoid confusion. Posner himself is less reluctant to use the term ‘conscious’ than many authors, so the terms can perhaps be excused. A wider variety of tasks are used in this case than the previous one, and exposition is correspondingly more difficult, but the concepts can be illustrated from experiments on priming. If a pair of letters is presented and the subject asked to decide whether they are different or identical, they can be preceded by a prime letter. This letter is in some cases the same as one or both of the decision letters, and if it is the same, reaction is faster. This benefit is exaggerated if the prime letter is reliably informative about the following letters, that is, if on most trials it is one of them. A smaller benefit does however appear even if the prime is only rarely followed by a pair containing the same letter. In addition to the benefit of a valid prime, there is in some cases a cost for having an invalid prime, as compared to having no prime. That is, if you see a letter and it turns out not to be in the decision pair, you may be slower than if you had no prime. This happens only if the prime is reliable; you have been led to expect that the prime really will be in the pair, so you start some process that has to be stopped or overturned

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if it turns out to be wrong. The cost also differs from the benefit in taking longer to develop; the benefit starts immediately after the prime and is maximum at about 150 msec, whereas the cost is only then beginning. A similar pattern of costs and benefits occurs with the priming of words (Neely 1977). If a word is presented shortly after another word with which it has been long associated in experience, it will give a faster reaction and this will apply even with a very short interval between prime and decision stimulus. If subjects are told as a conscious instruction that a prime of one type means that a word from a certain category will occur, this gives a benefit even though the prime is not, in everyday life, associated with the decision word. But now the benefit does not start until a longer interval between stimuli. The prime has to start a process that takes some time to run off and gives benefit only when it has finished. Costs in either case apply only if the interval is long. So, there is a fast process that gives benefit without cost, and appears with unreliable primes or with associations derived simply from experience. This is the process labelled ‘unconscious’ in this section; the ‘conscious’ process takes longer, needs reliable primes or specific instructions, and gives costs as well as benefits. These processes can be seen (e.g., by Keele and Neil1 1978) as descendants of the late-selection view, with the unconscious representing widespread and uncontrolled activation of memory codes corresponding to any stimulus that arrives. The conscious processes then become the late selective operation, necessary to avoid interference between overt actions, and so carrying cost. For completeness, it may be worth recalling the study of Broadbent and Broadbent (1980), in which words were primed reliably or unreliably and accuracy rather than speed determined. Again, two processes were found because the effect of reliable primes was greater if the stimulus preserved details of the word rather than global outline. But, in accuracy, there was no extra cost in false alarms for reliable primes. The Broadbents obtained rough latency data, confirming that invalid reliable primes did give a cost on that measure; and suggest that the stopping of the ‘conscious’ process when it is invalid merely starts everything back at zero without reducing final accuracy, The ‘unconscious’ process, on the other hand, does give false alarms. Again, we shall come back to Posner’s distinction.

The integration

of features

The last major concepts we shall mention are the recent views of Treisman (Treisman and Gelade 1980). She distinguishes actions based on detection of stimulus features, and on the integration of features. As a concrete example, consider visual search through a display containing Ps and Bs, looking for a target R. This takes a time dependent initially on the number of non-target stimuli present, but with practice that number becomes much less important, as in the results of Shiffrin and Schneider. The target can of course be detected by the presence of a diagonal line that is missing from any of the irrelevant items. If the background is modified to P and Q, with the small diagonal of (2 very similar to that of the R, the target can only be identified by the combination of two features each of which is present in isolation in every one of the irrelevant ones. This task now becomes much more dependent on the number of irrelevant items, and some effect is still detectable up to quite high levels of practice: as is agreed by Shiffrin et al. (1981). Treisman is therefore suggesting that the detection of single features is taking place by a parallel process; but that their integration into a single combined unity is something that can only be undertaken serially, for one item at a time. This view is of course a recognisable descendant of her earlier ones, since the isolated features could trigger the processes primed by context. Without such priming. integration might be needed to launch one rather than another of the possible words in a listening task. Thus her results on breakthrough in dichotic listening, as well as the results on ‘automatic processing’, will fit this new formulation.

Conclusions Certain broad literature.

feelings force themselves

upon one, after looking

at this

The end of late selection The field has been dominated during the 1970s by a version of late selection theory as simplistic as the very first filter formulation. This

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view is held in different forms by different authors but generally suggests that all inputs are analysed and that interference occurs only at response. Yet in the same decade the evidence has piled up to show such a view is wrong. If we had to choose, Treisman’s theory of twenty years ago is much more nearly correct. On the one hand, the difficulties raised by her in the 1960s for late selection have never been answered. On the other hand, the studies by Eriksen and by Posner are even more convincing than Treisman’s original ones. If no opportunity is given for selection, interference depends on individual features of the stimulus and not only on the responses made. If the person knows what part of the stimulus field to select, the interference disappears. The interference must therefore be before the choice of response, and the selection must be earlier still. From the side of late-selection theory, only three kinds of fresh evidence have been produced. First, fresh instances of breakthrough. which Treisman’s view has always handled. Second, evidence that stimuli requiring no action produce no interference, which is consistent with any theory. Third, evidence that long strings of statistically dependent stimuli and motor responses can be made simultaneously, which was already known. The popularity of late selection does not stem from any empirical evidence. Rather it seems to be one of separation between different academic communities. One can distinguish traditions drawn from psychophysics (Eriksen), from skilled performance (Posner), from learning and mathematical psychology (Shiffrin), and from higher mental processes or artificial intelligence (Neisser), with communication across traditions being inferior to that within each. Problems of the concept of automatic processing The widespread popularity of a notion of automatic processing illustrates some of the difficulties. As we have seen, there are two strands to the concept; processing without interference and processing that cannot be stopped. Consider the first strand; when a task is practiced there is undoubtedly a reduction in the interference. The 1950s explanation was that processing was reduced to the minimum operations necessary to perform the task. Then as now, it was known that non-target events, requiring no action, do not interfere. Target events do, and the recent evidence shows that they do so to an extent heavily dependent upon the number and nature of the features needing to be

analysed to discriminate them from irrelevant background. On the face of it these results are highly predictable from the older views. If a concept of automatic processing supposes that the same operations take place in practiced and unpracticed tasks, but that the latter case requires some extra and undefined resource, then it seems to need ad hoc postulates to explain why some practiced processes do not interfere and others do. If ‘automatic processing’ means fewer analytic operations, then it does not differ from the older theory. and the term is misleading. As regards the second strand, practiced processes certainly can be stopped. We have seen that the evidence for interference from a previously practiced target stimulus depends on the use of a visual arrangement in which there was no opportunity for filtering on a spatial basis. It has been confirmed by Francolini and Egeth (1980) for the visual case and by Johnston and Wilson ( 1980) for the acoustic case, that interference is reduced by allowing filtering. Duncan (1979) has shown that Treisman’s effect of conjunctive features in target detection can similarly be reduced by spatial filtering. Stroop interference, which is also sometimes attributed to automatic processes, depends similarly on the removal of opportunity to select by filtering or by pigeon-holing: when the words are in a known different place, or from a different semantic class, we have seen that interference is reduced. The occurrence of high interference from a few letters of a semantically related word, in particular, argues strongly against the idea that every feature is being fully and uncontrollably processed beyond the initial stage of temporary memory. There are also the cases in which the interference in the Stroop is abolished by changing the task set to the subject with the same stimuli (Killion 1982; Martin 1978). To put it baldly, interference from a familiar word seems to occur only if you are looking at it rather than somewhere else, and if you are either doing nothing or doing something that is likely to prime that particular word. The process is clearly very much under strategic control, and can be stopped by the selection strategies established in the 1960s. Neither criterion of ‘automaticity’ seems terribly helpful. Yet there are real distinctions to be drawn between different perceptual operations and the interference suffered by each. Remember the distinction of costs and benefits, and the different time courses of each. Perhaps it may be helpful to recall that the theory of Treisman (1960), or her more ponderous lieutenant (Broadbent 1971), also distinguished two stages.

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One was the stage of accumulating the evidence for the presence of each possible word, and the other stage was the decision in favour of one of the possibilities. If a degraded stimulus “bxoxd” is delivered to the senses, it might be “broad” “ brood”, or just conceivably “blood” or “blond”. All the possibilities have to be considered before the correct one is selected; and context and probability enter into the final decision. Once the decision is made however only the chosen word will affect further events. Thus at the earlier stage context may benefit one word but not eliminate another. At the late stage the others have been eliminated, and costs will be serious. It is worth reflecting however that it was the early stage at which Treisman and Broadbent located interference effects of the ‘breakthrough’ type; it was there that a few features could trigger a primed word. Once triggered, it could pass into various memory systems. and survive interference. Thus if we were to map the considerable evidence from Broadbent (1971) onto this version of the ‘automatic/controlled’ distinction, let alone add the data from the Eriksen school, it would be the automatic process that suffered interference and the controlled that did not. The advances in sensory analysis In fact, the mechanisms of the ‘breakthrough of the unattended’ seem to have been very substantially clarified in the last decade. It used to be thought that unattended events had managed to overcome the mechanism of selection. In the metaphor of the searchlight, one region was thought to be lit up and another dark; and somehow a person standing in the dark had managed to be recognised. The data of the Eriksen school show rather that selectivity takes in one part of the sensory field, but that this part is not infinitely small. Even when the beam is sharply focussed, it is large enough to include a fair region around the point of greatest concentration. and it is not surprising that an event within that area is identified. In the 1960s most work was done with hearing, and experimenters tended to assume that selectivity could confine itself to one ear alone; but this is really quite unlikely. The fact that combining hearing and vision is different from combining the two ears, as noted by Broadbent (1956b) suggests that many cases of breakthrough in the literature are due to the irrelevant stimuli being within the selected region of the sensory field. They need not have to overcome the lack of selection; they are close enough to the

main stimulus to get selected with it. Can u/l the instances of breakthrough be explained by this mechanism? It is uncertain. Stroop-like effects do occur between the eye and ear (Greenwald 1970; Lewis 1972; Navon 1977), and it is hard to imagine that selectivity cannot be reduced to one sense excluding the other. In the case of Navon. however, subjects were told to react to vision as well as discriminating the auditory stimulus; this was deliberately done to secure attention to both senses. In the case of Lewis, subjects were only told to watch the screen. but a word occurred suddenly against a previously blank background. In the case of Greenwald, a spoken word broke in on a background of silence. It seems plausible that a sudden stimulus onset acts to increase intake from that sensory region and to decrease it from elsewhere; that indeed was supported by data in Broadbent (1958). We do not know therefore whether to stick to the ideas of Treisman (1960). and think of the searchlight as having a penumbra of partial illumination around the full beam, sufficient to allow recognition of expected events: or whether the penumbra merely picks up enough to change the sharpness or direction of the beam. Further work is needed. The new data certainly emphasise, however, the importance of interactive verification. When a cue warns that an event is about to happen in a certain place, that draws the selective system towards that place. Similarly, broad characteristics of the event itself can indicate that further selectivity should be directed to that event. Remember the results of Kinchla showing that detectability of small letters depended on the large letter in which they were embedded. Some cases of breakthrough may therefore be due to the capture of the selective system by a few features of the unattended event. We know much more now, therefore, of the nature of early selection. Some cases of failure of response to events are undoubtedly because these events have not been selected. But what of the rather different failures that occur when two events have both been selected? Why do visual features occurring within 1” of the target item cause deterioration; or why do a word and a patch of colour interfere when the person does not know which location is to contain the colour. so that both are selected? Where, to borrow Duncan’s phrase, is capacity limited? If we leave aside cases of obvious motor interference, the difficulty must lie in the selection of an action given the events present in the sensory buffer. There are three answers that might be given to Duncan’s

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question. Two of them would suggest that the limit is not one of capacity in the strict informational sense. First, it could be suggested that each task such as speech comprehension on the one hand, or manual key-pressing to lights on the other, makes use of a processing resource specific to that task. In that case interference would arise only when two stimuli reach this same processor but point to opposite actions. Such a notion of separate processing appears informally in various textbooks; it fits well with a late selection interpretation of some kinds of interference. It would also explain, for instance the Eriksen results. The problem is however the massive evidence for interference between tasks of apparently very different content. Remember the interference between tracking and verbal learning, between memory and reaction time, between piano playing and repeating heard speech. Wickens (1980) lists large numbers of experiments showing interference between visual tracking and auditory tasks with no apparent similarity. And why have no Performance Operating Characteristics been found with more than one segment of trade-off? Until such difficulties have been explained, it is hard to take such a theory seriously. A second view is that isolated sensory features may influence action in parallel, but that any analysis of integrated groups of features must be performed serially. Each event made up of a number of features requires a single successive analysis. This is in outline the position of Treisman and Gelade (1980); certainly their results are a major step forward. The conjunctive use of features is more likely to interfere with other activities; but this does not mean that each conjunction is analysed serially, and that no simultaneous analysis of conjunction can occur. The linear increase in time taken for detection when the irrelevant items increase can be fitted as well - indeed, better ~ by models of parallel analysis based on statistical decision theory (Ratcliff 1978). At a less formal level, the Treisman view is formulated primarily for visual search; it is hard to see how to develop it to handle the numerous studies of task combination. For instance, Miller ( 1981a) shows identification of a target being assisted by the existence of two versions of it, in a manner that cannot possibly be serial. The notion that a single information channel must be a single processor, or deal with events serially, is a common confusion in this area. It is not so; the origins of information theory lay in the problem that ten separate radio frequencies each with a narrow bandwidth,

could transmit ten Morse messages, or five and a rough speech message. or one reasonable speech message. They are a ‘single channel’ mathematically and functionally, not physically. The third stance one can take at the present time is the notion of limited capacity; that trade-offs exist between the number of activities in progress at one time and the detail of the analysis possible in each. The processes occurring within the person are numerous and widespread physically: but the empirical data on task combination suggest that each feature extracted from the sensory buffer affects those processes very widely. As a result, the transfer of information from the sensory buffer, into a decision to act, may interfere with activities having no apparent similarity to the task in hand. Limited capacity of the central system is therefore the alternative most plausible to some of us; neither strict seriality. nor unlimited parallel processing. The data are very hard to fit with either of the latter views. From this basic standpoint, the most promising developments for the future will come from the work of, say, Treisman, Posner. or Eriksen, rather than some of the other authors mentioned. But we are all engaged in a common enterprise, and whatever the future, we can be sure that Actu Psychologicu will play a large part in it.

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