Chapter 5 Involuntary attention

Chapter 5 Involuntary Attention M. Eimer, D. Nattkemper, E. Schr6ger and W. Prinz Department of Psychology, University of Munich and Max-Planck-Institute ]:orPsychological Research, Munich, Germany

The concept of attention refers to at least three major functions. The first is the function of mobilizing aspecific mental energy to enable certain information processing activities (Berlyne, 1974). The second is concerned with integration of visual features into a localized and identifiable object (Treisman and Gelade, 1980). Finally, selection of information is considered to be a third function of attention. Selection is said to occur when the cognitive system chooses a small portion of currently present information for further processing while at the same time excluding the remaining information from further consideration. The act of selecting information is equivalent to directing attention to the events or stimuli containing the information. Directing attention, or attending, can come about in various ways. First, it can be intentional. This occurs, for instance, when a person searches for a certain object, which has the effect that all objects that are similar to the target receive more attention. Whenever attending is intentional, it is said to be voluntary. In contrast, attending is involuntary when it does not stem from intentions but is elicited from outside events. A simple example is a loud bang, which elicits attending in the absence of any intention to take notice of loud noises. While involuntary attention is elicited bottom-up, voluntary attention is top-down in the sense that attention is directed to outside events by inner intentions. This chapter is concerned with research on involuntary attention and in particular with the conditions in which it occurs. The distinction between voluntary and involuntary attention is already common in the early literature (D6rr, 1907; Ebbinghaus, 1911; Elsenhans, 1912; James, 1890, p. 304; Kreibig, 1897). 1 More recent dichotomies, such as the distinction between automatic and controlled processing, or between exogenous and endogenous control of attention (Neumann, 1984; Ohman, 1992; Shiffrin and Schneider, 1977; Theeuwes, 1991) are related to the distinction between involuntary and voluntary attention, but should still be distinguished for two major reasons. First, involuntary attending refers only to the selection of information, while 1As an exception, Wundt (1903, vol. III, p. 303) rejected the notion of involuntary attention, since he considered all attentional phenomena as an expression of the will.

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automaticity is concerned, in addition, with subsequent processing of information. 2 Second, as will be shown, involuntary attending is not merely a matter of the occurrence of outside events, but is also tied to certain internal conditions. Although the elicitation of involuntary attention is 'exogenous', its realization requires at least an interaction between endogenous and exogenous factors. This is easily overlooked if involuntary attention is said to be externally controlled. In the next section, a brief taxonomy will be developed of the conditions in which attending is involuntarily elicited. This will be done on the basis of observations from everyday life. It will be shown that this attempt towards a systematic classification of the phenomena requires theoretical considerations about the functional basis of the processes involved.

1

A TAXONOMY

OF THE PHENOMENA

What external events might elicit involuntary attending? It seems useful in this regard to distinguish between specific and aspecific involuntary selection (Prinz, 1983a). There is specific selection if features of an event or situation attract involuntary attention. Pictures of a pinup g i r l - or a handsome m a n - are usually apt to attract the gaze of m a l e - or f e m a l e - observers. The same can be said of a crying baby., with respect to its mother, or of a nicely displayed meal with respect to a hungry person. In such cases, attending results from specific events, which fit latent wishes, desires or interests of the observer. The structure of the antecedents of this type of selection m a y well be similar to those of voluntary selection in that attending is the consequence of a specific mental set. The critical difference with voluntary a t t e n t i o n - w h i c h leads to the qualification 'involuntary' in the abovedescribed e x a m p l e s - is the absence of an explicit intention to attend. Instead there is merely a latent disposition. Since in the case of specific selection the b o u n d a r y between an explicit intention and a latent disposition is unclear, there is a smooth transition between voluntary and involuntary attention. This is one of the main reasons to focus the argument on processes relating to aspecific selection, which refers to conditions in which directing attention does not depend on specific but on relational features. Relational features are characterized by deviations from what is common for a certain situation. A sudden noise in a silent environment attracts attention. Again, a single vertical bar, surrounded by many horizontal bars, pops out (Treisman, 1982). A false tone in a piece of music also elicits attending involuntarily. In such cases the specific content of the stimulus event is not responsible for attending: thus the vertical bar amidst many horizontal ones is not conspicuous because it is vertical but because it deviates from the spatial orientation of the other bars. If these were vertical, then a single horizontal bar would strike the eye. Hence, it is a matter of aspecific selection that in principle can be elicited by an unlimited variety of completely different stimuli. 2Involuntary selection and unlimited capacity of processing are combined in the concept of 'automatic processing'. But one might well think of a combination of involuntary selection with other kinds of processing (see, for example, the models discussed in section 1). Last but not least, for this reason it is necessary to make a difference between a selection and a processing component when describing attentional processes: the process of selection (of specific information for further processing) is not necessarily identical with the act of processing (of this selected information).

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In cases of aspecific selection it is a discrepant stimulus that involuntarily attracts attention. 3 The sudden noise stands out from the earlier relatively lower noise level. The pop-out stimulus does not fit the homogeneous pattern of the remaining situational context and the false tones do not tune in with the melody of the musical piece. The first and last of these examples are concerned with deviations from a structure developing in time, while the pop-out effect is a deviation from a spatial structure. Spatial deviations are usually dealt with in relation to problems of perceptual structuring and will therefore not be discussed in this chapter. Instead, this chapter will focus on the role of situational deviations occurring over time in eliciting involuntary attending. Two types of deviations will be distinguished: level shifts and rule deviations. Level shifts occur whenever there is an obtrusive change in the stimulus situation, e.g. the appearance of a new stimulus or a change in the value of a feature of a stimulus which is actually present. In contrast, rule deviations refer to changes in a rule-based sequence of events. This is the case, for instance, when a repetitive sequence is broken off. If, say, a dripping tap suddenly stops dripping, the absence of the sound of dripping constitutes a deviation of the formerly established repetitive sequence of events. Involuntary attending can also be elicited if the interrupted regularity relates to a more complex structure of events. This is the case in the example of the false tone. A false tone attracts attention because it represents a deviation from the rules according to which the sequence of tones is structured in that particular piece of music. H o w to explain that situational deviations elicit attention? It is a prerequisite that the situational deviation is registered, and in turn, this requires a comparison between a present and an earlier situational state. There could be different underlying mechanisms, depending on whether there is a level shift or a rule deviation. In the case of a level shift a simple sensor mechanism might be responsible, a mechanism that registers changes in elementary sensory features. In the case of a rule deviation, there must be a more complex mechanism, which enables registration of deviations from a prior existing event structure. Therefore, this mechanism must be capable of maintaining a representation of that prior event structure. Hence, these two mechanisms seem to differ in at least two distinct ways: first, with respect to encoding (elementary sensory features versus sequential structures), and second, with respect to the time w i n d o w over which information should be integrated so as to notice a deviation. (The time w i n d o w is clearly longer for rule deviations than for level shifts.) In the following two sections we consider involuntary attending elicited by situational deviations. Some of the experimental evidence will be discussed with respect to level shifts (following section) and rule deviations. In the final section the distinction between voluntary and involuntary attention will be considered again but this time with the emphasis on the possibility of distinguishing underlying processing mechanisms rather than categorizing empiric phenomena.

3The difference between specific and aspecific selection can also be formulated this way. In specific selection (voluntary or involuntary), events attain selection that indicate specific (e.g. intentionally defined) features. In aspecific selection (involuntary), stimuli can be selected that do not show specific (context defined) features.

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2

LEVEL SHIFTS AND

INVOLUNTARY

ATTENTION

In this section experiments are discussed in which the influence of elementary situational changes on directing attention is studied. The main concern will be with sudden changes of single physical parameters, e.g. a change in the distribution of brightness on a cathode ray tube, which is brought about by the sudden appearance of a new stimulus. Level shifts will first be described in the context of the orienting reflex. This will be followed by a review of the effects of level shifts on evoked potentials in which context a model of a mechanism will be presented with regard to attending to auditory level shifts. Finally, reaction time studies will be discussed on attending in the visual domain, aiming at the question of the functional distinction among processes involved in directing attention.

2.1

Orienting Reflex

An orienting reflex (OR) is defined as the complex of physiological and behavioral reactions arising in the case of a change in stimulus situation (Pribram and McGuinness, 1995).4 Among the responses are alpha blocking in the electroencephalogram (latency 150-250ms), decrease in skin resistance (latency 2-6s), vasodilatation of blood vessels in the head, changes in pupil diameter as well as in pulse and breathing frequency and, finally, eye and head movements in the direction of the location of the change. Directing attention to a new or changing stimulus is usually considered to be a main function of the OR (N/i/it/inen, 1992); in addition, the OR can also be viewed as an indicator that a change in situation has been noticed. The conditions on which an OR occurs, on which it habituates and, again, dishabituates, are of course of special interest for the present discussion. Level shifts typically elicit an OR as in cases in which a tactile, auditory or visual stimulus is presented for the first time. Again, regularly repeated or permanently presented stimuli lead to habituation of the OR, which may dishabituate in the case of a subsequent change in the stimulus situation (Sokolov, 1963, p. 39, 1975). It is interesting that an OR is elicited not only by the onset, e.g. a loud bang, but also by the offset, e.g. a sudden darkness, of a stimulus (Sokolov, 1963, p. 82, 1975). Any change in intensity, either increase or decrease, suffices to bring about the OR, the strength of which is correlated with the extent of the intensity change (Sokolov, 1963, p. 39). Sokolov (1963, 1975) explained the effect, both the habituation and the dishabituation of the OR, in terms of the 'neural model of the stimulus'. His basic reasoning is as follows: the properties of repetitive as well as of permanent stimuli are extracted by the organism and stored for a certain lapse of time. The stored neural representation of the stimuli contains simple parameters such as intensity or color, but also more complex aspects such as the sequence of successively presented stimuli and the duration of the inter-stimulus intervals. An OR arises whenever the parameters of an actual stimulus situation do not correspond to the parameters as 4The connections between the various measures will not be discussed, but see Barry (1984). Again the relations between the OR and evoked potentials are not covered, but see Donchin (1981), N/i/it/inen (1979), Schandry and Hofling (1979), and Rockstroh and Elbert (1990).

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represented in the neural model. Following Sokolov, the strength of an OR depends, at least within certain limits, on the degree to which a parameter of the actual stimulus deviates from its representation in the neural model, and also on how many parameters change simultaneously. Alternative attempts towards modeling habituation and dishabituation without invoking the concept of a neural model stem, among others, from Groves and Thompson (1970) and from Horn (1967). Thus, the dual-process theory of Groves and Thompson distinguishes between a habituation system and a state system. The habituation system is responsible for the decrease in response readiness of the effectors in the case of repeated stimulation. In turn, the state system is responsible for an initial increase in response readiness, which is also followed by a decrease in the case of repeated stimulation. Assuming that both systems contribute independently to the degree of response readiness, many phenomena relating to habituation and dishabituation can be reasonably explained. One may argue, though, as did Pribram and McGuinness (1975), that the alternative theories apply only to effects of simple stimulus changes. As pointed out in section 1, an OR can be elicited by either level shifts or rule deviations. To explain the latter type of phenomena it seems more plausible to postulate a storage system, the contents of which can be compared with the newly presented stimulus. The meaningfulness of a stimulus is often considered as a necessary condition for the OR to occur (Maltzman, 1979). One could, indeed, argue that level shifts elicit an OR owing to their potential meaning for the organism. It is not denied that stimulus changes or newly arriving stimuli are potentially meaningful. Yet, directing attention cannot be ascribed to the meaning of the stimulus, since the stimulus is not selected on the basis of particular features. Instead, as was already argued, the control of involuntary attention occurs bottom-up, merely on the basis of a change in situation (see first paragraphs and section 1). In support of this view, Gati and Ben-Shakar (1990) have demonstrated in two studies that an OR can be elicited by mere newness. This could occur, in fact, on a fully abstract level. In their featurematching model the authors postulated two comparison mechanisms to which newly arriving stimuli are subjected, one mechanism resulting in voluntary and the other in involuntary attending. On the one hand, a newly arriving stimulus is compared with the stimulus representation that is relevant in the context of the experiment, for instance the target stimulus as defined by the experimenter. On the other hand, the new stimulus is compared with the representation of the recently presented stimuli. In both cases incoming stimuli are compared with representations of stimuli. The result of the first comparison determines the relevance of the new stimulus, while the result of the second comparison concerns its newness. The degree of relevance and newness of the newly arriving stimulus shows, respectively, a positive and a negative correlation with the internal model of the situation. 5 According to Gati and Ben-Shakar, meaningfulness and newness have additive effects on the OR. The effect of newness on the OR, measured by the electrodermal response, was supported by the following experiment. In a variation on the so-called 'guilty knowledge technique' (Lykken, 1959) subjects were told to imagine SSimilar considerations suggest two different selection mechanisms, one based on a 'match' and the other on a 'mismatch' between a newly arriving stimulus and the internal model of the situation (Ohman, 1979;N~i~it~inen,Gaillard and M~intysalo, 1980; Prinz, 1983b, 1990a,b).

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that they were suspected of murder. They were either shown a picture or given a verbal description of the victim, and were further instructed to act in the experiment as if they were innocent. Before representing the relevant stimulus (the victim), several other persons were brought in, and made familiar in the same way with the victim but now they had a variable number (zero to three) of characteristics in common with the victim. As an example, a characteristic could be 'the victim is an architect'. The results showed that the OR was stronger in the case of a characteristic that was seldom found for the control persons. In a further similar study the factors 'newness' and 'relevance' were simultaneously manipulated with the result that both factors had a main effect but no interaction. These results are consistent with the author's model in which, first, features of the control stimuli are continuously analyzed and integrated into representations, and second, newly arriving stimuli are compared with these just-established representations.

2.2

Evoked Potentials

An evoked potential (EP) is an event-related sequence of changes in electrical brain potentials. The changes are relatively small in comparison with spontaneous background activity, but can be isolated by way of adequate averaging techniques. An EP consists of several relative minima and maxima, which are indicated both by their direction (negative, positive) and by their number (e.g. N2 for the second negative wave following the stimulus) or by their latency (e.g. P300 for a positive wave that occurs about 300 ms after presentation of the stimulus). 6'7 There is ample evidence that level shifts as well as rule deviations lead to characteristic changes in the EP. Following a sudden stimulus, either visual, auditory or tactile, an N1 followed by a P2 is commonly observed (Picton, 1980; R6sler, 1982, p. 18). This N 1 - P 2 complex is also found when attention is not explicitly directed to the eliciting stimulus, which suggests involuntary attention, s Thus, N/i/it/inen, Gaillard and M/intysalo (1980) presented frequent standard tones and infrequent deviations via earphones to one ear at a time. The task of the subjects was to count the deviant tones presented to either one of the ears, and to neglect those presented to both ears. The N1 occurred both at standard stimuli and at deviant stimuli irrespective of how they had been presented or, in other words, irrespective of whether or not attention had been directed to the deviant stimuli. The neural processes involved in the generation of the N1 wave have been related to processes of directing attention (N/i/it/inen, 1988; Verbaten et al., 1986). Various generator processes may contribute to the N1 as elicited by auditory stimuli (N/i/it/inen, 1988, 1990; N/i/it/inen and Picton, 1987; Scherg, Vajsar and Picton, 1989). N/i/it/inen (1988, 1990) has argued that processes in the supratemporal auditory cortices are in particular responsible for eliciting attention in the case of 6The various waves are labeled as 'components'; this term is also used to indicate the cortical processes are assumed to underlie the occurrence of the EP waves (N/i/it/inen and Picton, 1987). 7See Chapter 9 for a discussion of technical details. 8yet, the amplitudes of the N1 and P2 are usually somewhat smaller than when attention is directed in advance to the eliciting stimulus, which suggests that the N1-P2 complex is also affected by voluntary attention (Hillyard and Hansen, 1986; Hillyard et al., 1973; Picton and Hillyard, 1988).

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auditory level shifts. 9 The basic rationale of N/i/it/inen's hypothesis is that certain stimulus properties are extracted on a subcortical level. Level shifts activate subcortical systems for detecting transients, which, in turn, send interrupt signals to central processing mechanisms. This has the effect that the processes involved in generating the N1 are activated. If a certain threshold is exceeded, attention is directed to the sensory proceesses at work and to the results of those earlier sensory processes that are still available in the iconic storage system. According to N/i/it~inen (1988, 1990), a variety of results supports the notion that attending is elicited by the N1 generators. The N1 is correlated with the presentation of a stimulus; its amplitude depends on the detection threshold of auditory stimuli. Again, the N1 amplitude depends on the presence rather than on the content of the stimulus. Thus, a dissociation is commonly observed between the amplitude of the N1 and the loudness of the stimulus. In addition, the N1 is quite similar for different stimuli, such as clicks, tones and speech-like noises. Attendingas well as N1 amplitude are reduced during sleep, and more so as sleep is at a deeper stage. These results are consistent not only with the hypothesis that attending and N1 are related, but also that N1 is primarily related to stimulus detection. Results arguing against these assumptions include the association between N1 amplitude and subjective distractibility, and the dissociation between the N1 amplitude and the detection of suprathreshold signals: the N1 has a smaller amplitude as the inter-stimulus interval between stimuli is smaller (Davis et al., 1966). Again, the N1 is at the first than at later stimuli of a sequence (Verbaten et al. 1986; Woods and Elmasian, 1986). The subjective phenomenal appeal of the stimuli decreases in both cases, while the detectability of the stimuli remains the same. A combination of N1 and attending appears not only after changes in stimulus energy (increases or decreases in the case of continuous signals) but also after changes in qualitative aspects of the stimulus, such as frequency. In a study by Speer, Zimmer and Odenthal (1969), a frequency change of a 1000 Hz tone led to differences between the N 1 - P 2 amplitude in the order of 6-12 mV, the difference increasing with the amount of modulation. In the same vein, the N1 latency decreased as the frequency changes were larger. This mechanism of eliciting attention may be characterized as a detector of level shifts, either quantitatively or qualitatively, which simply reacts to an incoming stimulus. As long as the signal, which is triggered by the stimulus, is sufficienctly strong, the earlier extracted and stored physical properties of the stimulus come into the focus of attention (N~i/it/inen, 1988, p. 133). At that point, it is decided whether or not the stimulus will be subjected to further processing. The same mechanism of attending to auditory stimuli may actually apply to visual and tactile stimuli. It is mentioned in passing that, when processing visual stimuli, differential effects have been observed on the EP, depending on whether there had been a peripheral visual onset, either as a cue or as distractor, in advance of the target stimulus (Hillyard, M6nte a n d Neville, 1985; Luck et al., 1990). The next section is concerned with behavioral studies on effects of level shifts on reaction time in the visual domain. However, to our knowledge, there is no systematic research on the 9It is still an open question whether precisely this component is responsible for directing attention. According to Verbaten (1990), eliciting attention is more a matter of an aspecific component, which, in contrast to the modality-specificsupratemporal component, also occurs in the case of visual stimuli, and which is correlated with the occurrence of an OR.

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relation between level shifts in the visual modality on the one hand, and N1 generators and eliciting attention on the other hand.

2.3

Reaction Time Studies

In recent years there has been much research on the relation between level shifts and attending in reaction time experiments, and in particular on processes involved in covert orienting (Posner, 1980). These processes cannot be directly observed, but are restricted to internal events. Covert orienting can occur voluntarily as well as involuntarily. The following discussion focuses on conditions in which covert attending is involuntarily elicited, and will be limited to visual situations only. 1~ The existence of phenomena of covert orienting presupposes that subjects are capable of dissociating the direction of attention from the direction of eye-fixation. In the experiments there are various techniques persuading subjects to concentrate on a particular region of the visual field, while at the same time maintaining their original eye-fixation. By measuring eye movements, through electro-oculography or otherwise, it is checked whether subjects really obey the instruction to keep their eyes fixated, and only those trials are subjected to further analysis. An example of a study on covert orienting stems from Posner, Nissen and Ogden (1978). At their eye-fixation point, subjects view an arrow that points either to the left or to the right, indicating the direction to which attention should be oriented. Following a variable interval a target stimulus is presented, either to the left or to the right of the fixation point, requiring a simple key-pressing response. In 80% of the trials, the target corresponds to the direction of the arrow. In comparison with the control condition, in which there is no advance information about the location of the target, the results of the experimental condition show significantly faster reaction times when the location of the target corresponds to the direction of the arrow, while reaction times increase if arrow and target do not correspond. Posner and colleagues interpreted these results as evidence for the hypothesis that the central cue had elicited covert orienting. Subjects start attending the cued direction prior to the presentation of the target, which has the effect that processing is facilitated if the target occurs at the indicated spot. The opposite effect occurs when cue and target do not correspond. Yet, involuntary attention seems not to be involved in this study, since it was the subjects' task to decode a symbolic stimulus (the centrally presented arrow), and to direct attention accordingly. Hence, the direct reason for attending concerns the subject's intentions. In contrast, involuntary covert orienting should be elicited irrespective of, and even contrary to, the intentions of a subject. To evoke involuntary processes, Jonides (1981) investigated a variation on the earlier-described Posner paradigm. During a trial eight letters were presented on an imaginary circle, the midpoint of which was fixated by the subject. The task was to detect a target letter among the eight letters. Briefly (50-100 ms) before the letters, subjects saw a cue stimulus. This 1~ Bernstein, Clark and Edelstein (1969a) and Bernstein and Edelstein (1971), with respect to the effect of auditory stimulus onsets on the direction of visual attention.

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could be an arrow presented in the middle of the display, indicating the direction of the target with a certain predefined probability. Alternatively the cue could be presented in the periphery, pointing towards the position of the target with the same probability. Jonides (1981, experiment 2) found that, in the case of the peripheral cue, directing attention was hard to suppress voluntarily. In contrast to the case of the central cue, subjects were incapable of ignoring the peripheral cue even w h e n they were explicitly instructed to do so, and w h e n the validity of the peripheral cue was only 12.5% and, hence, at chance level. The few targets that were present at the indicated location were processed faster, while processing at all other target positions was slower. Jonides concluded from these results that peripheral cues attract attention involuntarily, while the attentional response to central cues is actually voluntary and d e p e n d e n t on instruction. 1 Which properties of the peripheral cue are responsible for involuntary attending? Yantis and Jonides (1984) suspected that a peripheral cue elicits attention in the case of an abrupt onset, and they p r o p o s e d a mechanism that could be responsible for eliciting attention to abruptly appearing stimuli. ~2 Yantis and Jonides tested their proposition in an experiment in which the effect on attending of an abrupt stimulus onset was c o m p a r e d with that of a gradually appearing stiumulus. Subjects were asked again to react to a target letter amidst a configuration of other letters, which were all presented on a screen. Besides the target there could be one or three context letters. One of the presented letters appeared abruptly, while the remaining one(s) arose gradually. For the gradually arising letters, Yantis and Jonides applied a technique that was first used by Todd and van Gelder (1979). A rectangular digit '8' was presented, separate segments of which were gradually extinguished within 80 ms. In this w a y the letters E, H, P, S and U (so-called offset stimuli) could be established, which were then used as either target or context letters. If the target letter was presented abruptly, reaction time was largely i n d e p e n d e n t of the n u m b e r of gradually arising context letters. However, if the target was gradually established, reaction time increased as a function of the total n u m b e r of presented letters. Yantis and Jonides interpreted the effect by the hypothesis that a abruptly presented stimulus attracts attention involuntarily and, therefore, is processed with priority. Given a serial self-terminating search and a gradually presented target, subjects w o u l d first process the abruptly presented context letter, followed by a serial search until detecting the target. This would, indeed, have the effect that reaction time increased with the n u m b e r of context letters. This did not occur if the target letter was the abrupt stimulus, since in that case the target was processed first, w h e r e u p o n the search could be abandoned. On the basis of these 11Peripherally elicited attention fulfills Neumann's (1984) second criterion for automatic processing, namely that the processes are not subject to voluntary control. Jonides (1981, experiment 1) also found that attending following a peripheral cue obeys Neumann's first criterion for automaticity. Performance in the case of a peripheral cue appeared less affected by a secondary memory task than in the case of a central cue. As expected from automatic processes, this suggests that attending on the basis of the peripheral cue does not consume capacity and, hence, does not interfere with a secondary task. 12The authors refer to the functional difference between two types of retinal ganglion cells as indication for a biological basis of such a mechanism (1984, p. 602). X cells are sensitive to continuous stimulation, while Y cells reach a maximum activation when the intensity of a stimulus suddenly changes. Again, their receptive fields are larger than those of X cells. In addition Y cells are regularly distributed over the whole retina while X cells are mainly found in the fovea.

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starting points, Yantis and Jonides developed a quantitative model, the predictions of which were in accord with their experimental data. In a further study, Jonides and Yantis (1988, experiment 2) varied the number of context letters (2, 4 or 6) and obtained similar results. In addition, they studied to what extent stimulus properties other than direct onset might elicit involuntary attending (1988, experiment 1). They did find an increase of reaction time as a function of the number of context letters, if either color or brightness was the distinctive feature between the target letter and the context letters, suggesting that these properties seem incapable of eliciting involuntary attending. There remains, of course, the question to what extent this result depends on the extent to which attention is focused during a trial. Is abrupt stimulus onset really the only feature that elicits involuntary attending? Krumhansl (1982) found that abrupt changes in the form of a stimulus also facilitate its localization and identification. 13 Miller (1989) investigated whether it is the abrupt presentation of a stimulus situation. In the experiments of Yantis and Jonides, there was clearly a more general change in stimulus situation in the case of abrupt than of gradual onset in which some segments gradually disappeared. Miller attempted to deconfound both factors by applying a variation on the technique of Todd and van Gelder. The orginally presented pattern from which the offset stimuli were handed out little by little- the '8' in the experiments of Yantis and Jonides- had several additional segments. In this way the total change in the visual situation was at least equal and probably more in the case of gradual than of abrupt presentation. In this condition, there were no indications for involuntary attending to the abrupt stimulus, since reaction time increased with the number of context stimuli. Miller concluded therefore that not only onset, but also offset (although probably less so), can elicit involuntary covert attending. The decisive factor seems to be the net sum of the total changes in visual stimulation. The more the appearance of a stimulus is accompanied by a general situational change, the more it attracts involuntary attention. The described experiments all studied elementary situational conditions for involuntary attending, which indeed appears to be elicited by simple level shifts. Elementary changes in situation occur frequently and simultaneously outside the laboratory, and it seems plausible that they do not all lead to involuntary attending. Abrupt onset cannot be a sufficient condition for involuntary attention, therefore. Thus, Yantis and Jonides (1984) proposed that involuntary attending occurs only when there is one single abrupt change. 14 What are the more specific conditions on which abrupt stimulus onsets can be operative? Recently this question has been elaborated into either of two directions. One is concerned with the temporal dimension: is involuntary attending merely a transient phenomenon, which in subsequent processing is replaced or completed by voluntary attention? The other direction wonders about the relation between involuntary attending and the direction of attention prior to the moment that involuntary attending was elicited. What is the effect of an abrupt event when attention has already been directed to a certain area? 13This result has been interpreted in the framework of the model of Yantis and Jonides (1984). 14'Apparently only when the visual field contains but one such event (as when a relatively static scene is viewed and a moving object appears in the visual periphery) can attention be engaged in the manner illustrated by these experiments.' (p. 617).

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The temporal course of involuntary attending as elicited by abrupt onset has been studied by M611er and Rabbitt (1989). In one of their studies (experiment 1), four peripherally localized squares were initially presented in which targets or context letters could be positioned later. Subjects were asked to direct attention to a certain square that was indicated by a cue. In one condition the cue was an arrow presented at the fixation point (central cue). In another condition, the sides of one of the squares lit u p d u r i n g 50 ms (peripheral cue). The variables were the validity of the cue and the temporal interval between presentation of the cue and of the target (between 100 and 725 ms), while the probability of correct localization and identification of the target was measured. M611er and Rabbitt observed that the temporal course of facilitation in the case of a valid cue, or of inhibition in the case of an invalid cue, strongly d e p e n d e d on the type of the cue (with valid cues) or erroneous responses (with invalid cues) occurred at a temporal interval between cue and target of 175 ms. In contrast, valid central cues had their maximal effect only after 400ms. M611er and Rabbitt suggested, therefore, two i n d e p e n d e n t mechanisms for attending. A fast mechanism, elicited by peripheral cues reaching its maximal effect after 100-200 ms, causes the above-described early effect with peripheral cues. A second, slower mechanism is elicited by central cues and has its maximal effect only after 275-400 ms. Does the fast mechanism represent involuntary attending? In a further study, Miiller and Rabbitt (1989, experiment 2) investigated to what extent the presence of a peripheral cue affects the reaction to a simultaneous central cue. What happens if both cues deviate with respect to the indicated target location? The results s h o w e d that, if a valid central cue was presented first, followed after a 500 ms interval by (the lighting u p of) an invalid square, the proportion of correct reactions was significantly less than in the opposite case of an invalid central and a valid peripheral cue. In the latter case, the target a p p e a r e d in the square that was lit up but not indicated by the central cue. It should be noted that this pattern of results was found only w h e n the interval between peripheral cue and target was 100 ms at the most. If this interval was longer, say 400-700 ms, the effects changed in that n o w the proportion of correct responses was larger if the central cue had been valid. Mfiller and Rabbitt interpreted these data as suggesting that the slower attending as a result of the central cue is interrupted in the case of an abruptly presented peripheral cue. This causes fast attending to its location, which occurs against the subject's intention and despite the process of v o l u n t a r y attending, as controlled by the central cue. This suggests indeed that the fast process is at the basis of involuntary attending. If there is a longer interval between the cue and the target, the effect of the fast mechanism gets lost. In this case attention can be voluntarily redirected to the position as indicated by the central cue. 15

15In a further experiment, M611erand Rabbitt (1989, experiment 3) investigated whether the occurrence of a (for the target position noninformative) lightflash in temporal proximity of a valid peripheral cue leads to a reduced detection accuracy. The most interesting results were found in the condition in which target and context stimuli were presented 100ms after the lightflash. In that case, the proportion of correct detections was smaller as the temporal interval between peripheral cue and flash was longer. If the flash occurred as early as 100 ms after the peripheral cue, i.e. within the temporal span in which the operation of the fast orienting mechanism is presumed, the effect of the flash was smaller than at an interval of 300-500 ms at which involuntary attending has made way for voluntary attending.

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The results of experiments by Nakayama and Mackeben (1989) also support the assumption of two different mechanisms for directing attention. Here, the task of the subjects was to detect a square-like target amidst 64 other figures. The target figure differed from the context elements by a specific combination of two features (e.g. the only horizontally located white rectangle). Subjects received first a cue, e.g. a square, marking the future position of the target stimulus. Following a variable temporal interval, the stimulus figures were briefly presented (33-117 ms). The question concerned the effect of the interval between cue and target on its correct identification. Nakayama and Mackeben established that identification gradually improved when the interval between cue and target was prolonged to 200 ms, but clearly declined at longer intervals. This effect was again interpreted as suggesting that the cue elicits fast involuntary attending, which accounts for the improvement in performance, but which reaches its optimal activity between 100 and 200 ms. This fast mechanism can therefore contribute little to directing attention, when the target arrives at a longer interval. Like M/iller and Rabbitt, Nakayama and Mackeben suspect that at longer intervals only the second slower and voluntarily controlled mechanism remains operative. Hence single stimuli, appearing in an otherwise stable context, attract involuntary attention with a specific temporal course, irrespective of, or even contrary to, existing intentions. Is involuntary attention independent of the momentary state of attending in which a subject is actually engaged? This question has been addressed in a recent series of studies, the results of which appear to argue against this assumption. Instead, involuntary attending seems to be elicited only if observers have not yet focused their attention elsewhere. Yantis and Jonides (1990) investigated to what extent the validity of a central cue might affect the occurrence of involuntary attending. In their first experiment they presented an 80% valid central cue. In one half of the trials, the target letter appeared abruptly at a position on the screen which had been signaled by a dotted pattern, while the context letter was presented gradually by way of the earlierdescribed technique. This was reversed in the other half of the trials. Yantis and Jonides found that after presentation of a valid central cue the mode of target presentation (abruptly or gradually) did not play a role. Identification was about equally fast in both cases. However, at the 20% invalid central cues the advantage of abrupt presentation on reaction time reappeared. Apparently abruptly appearing stimuli do not necessarily elicit involuntary attending. In order to investigate further the limits of involuntary attending, Yantis and Jonides (1990, experiments 2 and 3) varied the temporal interval between cue and target as well as the cue validity. In experiment 2 a target letter (appearing abruptly in half of the trials) was presented together with three context letters. The central cue was 100% valid and was presented either 200ms before, 200ms after or simultaneously with the letters. In the first case, there was again no significant difference between reaction time to abrupt targets or gradual target letters. However, in other conditions the abrupt targets were detected significantly faster than the gradual ones. This was interpreted by Yantis and Jonides as follows. If a valid central cue can be used for voluntary attending, i.e. the condition in which the cue preceded the target, then there is no effect of abruptly appearing context letters on the speed of detecting a gradually emerging target letter. The sudden appearance of context letters does not elicit involuntary attending when attention is already focused; it only does as long as attention is not yet focused on a prospective target.

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To what extent was suppression of involuntary attention due to the fact that the central cue was fully valid? This question was addressed in experiment 3 of the Yantis and Jonides (1990) study, in which cue validity was varied between 25% and 100%. The results s h o w e d that, at a low cue validity (25%), abrupt targets were actually detected faster than gradually emerging targets. 16 Thus, the general tenet of these results is that involuntary attending caused by s u d d e n changes in the situation is not a matter of mere external control, independent of internal factors. Focused attention directed by a valid cue eliminated the effectiveness of abrupt stimuli. A b r u p t stimuli appear to exert an effect only w h e n attention is not yet focused, as in the conditions in which the central cue appeared together with or after the stimulus pattern, and in which the validity of the cue was at chance level. The question whether a s u d d e n event m a y elicit involuntary attending if attention has already been focused on a different spatial location was also studied by Theeuwes (1991). He varied the temporal interval between a 100% valid central cue and the presentation of target and context letters ( - 6 0 0 m s , - 3 0 0 m s , + 2 0 0 m s ) . In contrast to Yantis and Jonides, Theeuwes presented all letters gradually but, in addition, a bar abruptly a p p e a r e d beside one of the letters between 160 ms before and 80 ms after presentation of the letters. If there was a sufficiently large interval between the central cue and the presentation of the letters (600 or 300 ms), then there was no inhibitory effect of the abrupt appearance of the peripheral bar at a non-target location on reaction time to the target. Hence, the abruptly appearing event did not cause interfering involuntary attending because attention was already directed to another location on the basis of the valid central cue. However, Theeuwes did find that a peripheral bar beside the target letter had the effect of slowing d o w n reaction time. 1v If the central cue was not presented until 200 ms after the stimulus l e t t e r s - and, consequently, could not lead to voluntarily controlled a t t e n d i n g - then the abrupt appearance of the peripheral bar beside the target letter had the effect of a significant decrease in reaction time. In this case, attention had not yet focused and therefore involuntary attending could be elicited, which is in line with the results of Yantis and Jonides. The question then arises as to whether abrupt appearance and abrupt disappearance of stimuli have the same kind of effect. Theeuwes (1991, experiment 2) investigated this by presenting four peripheral bars at the start of a trial, one of which (either close to the target or a context letter) abruptly disappeared at a certain interval before or after presentation of stimulus letters. No significant effects on reaction time were found in comparison with a control condition without abruptly disappearing peripheral bars. Yet there was one exception to this result: if the central cue was presented 2 0 0 m s after the stimulus letters, i.e. attention was unfocused at the time the letters were presented, then reaction time was less in the 16Ifthe central cue was valid at 75% the reaction time data were less consistent. Yantis and Jonides (1990, p. 130) suggested that the subjects used different strategies for the employment of the central target. But the abrupt onset of a target letter does not lead involuntarily to attentional shifts; the effects of such an onset are more dependent on the strategy that is followed by the subjects. If the central cue is valid at 100%, the onset modus of the targets plays no role, as already mentioned. ~7It should be noted, though, that this occurred only when presentation of the peripheral bar and the letters were separated by an interval of less than 160 ms. Theeuwes suggested that the slowing down might be ascribed to lateral masking of the letter by the bar or to lateral interference.

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case of abrupt disappearance of the peripheral bar beside the target letter. Thus Theeuwes and Yantis and Yonides arrived at the same conclusion: a sudden appearance, and less pronounced a disappeance, of a stimulus evokes involuntary attending to the corresponding location if attention is not focused. However, if attention is focused because of a valid central cue, the sudden stimuli do not elicit involuntary attending, at least not as long as they are outside the focus of attention. The results of the discussed experiments are partly contradictory. On the one hand, M~iller and Rabbitt (1989) found that voluntary attending can be affected or even interrupted by a peripheral change in situation, while Yantis and Jonides (1990) and Theeuwes (1991) arrived at the opposite conclusion: if attention is focused, sudden stimuli outside the focus of attention have no effect. How can these divergent results be reconciled? Yantis and Jonides (1990, p. 123) presume that a decisive difference between their experiments and those of M/filer and Rabbit (1989) relates to the relatively long interval between the presentation of the central cue and the letters in the Miiller and Rabbitt experiments. In addition, Mfller and Rabbitt's 50% cue validity was not particularly high. According to Yantis and Jonides, both factors might have added to incomplete focusing of attention. The more attention is divided, the larger is the probability that an abrupt event elicits involuntary attending. Plausible as this explanation may be, it still cannot satisfactorily account for the differences between the earlier experiments- suggesting early involuntary attendi n g - and the more recent studies, which cast at least some doubt on this interpretation. The relations between single situational variables and the extent of attentional focusing remain to be investigated. The temporal interval between central cues, distractors and targets, and the validity of either peripheral or central cues, are certainly among these variables. It should be scrutinized to what extent abrupt presentation of targets, distractors and cues leads to different consequences for involuntary attending. The dependence of involuntary attending on current orienting of attention should also be studied in greater detail. On an still more basic level, the question should also be raised to what extent the described phenomena of voluntary and involuntary attention are indeed based upon separate mechanisms. An alternative view could be to conceptualize visual attending as a matter of a single mechanism, in which case the difference between voluntary and involuntary attending is not related to different mechanisms but solely a matter of the conditions of eliciting either one. Studies by Warner, Juola and Koshino (1990) have s h o w n - contrary to, say, M611er and Rabbitt (1989)- no systematic differences, either in temporal course or in cost-benefit pattern, between involuntarily and voluntarily elicited attending. They might be considered, therefore, as support for the existence of a single mechanism (see also Cheal and Lyon, 1991).

3

RULE DEVIATIONS

AND INVOLUNTARY

ATTENDING

In this section the discussion will center around the question to what extent rule deviations may elicit involuntary attending. First, it will be shown on the basis of studies about the orienting reflex that, indeed, deviations from a sequence of regular events may lead to attending. The data favor a mechanism that models

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rules occurring in sequences of stimuli, which registers deviations and which attends if a deviation is noticed. However, the problem with experiments in the tradition of the orienting reflex is that they tell little about the nature of the mechanisms involved. Differential effects of rule deviations on evoked potentials- as discussed b e l o w - a p p e a r to support more detailed hypotheses about involuntary attending. The final section deals with behavioral data from studies on visual search, which show that relatively complex rules can also be adequately modeled, and that violating these rules may lead to involuntary attending.

3.1

Orienting Reflex

An orienting reflex (OR) can be elicited by level shifts as well as by rule deviations. For example, there is a quite simple rule deviation, when a fixed order in which a combination of two stimuli always occurs is reversed. The rule deviation could also consist of omitting one of the stimuli of the combination. Thus, an initial OR habituates in the case of a recurring combination of a soft and a loud noise, but it dishabituates when the loud noise is presented without the soft noise (Sokolov, 1975). There is also an OR when the stimuli belong to different sensory modalities, as in the case of a tone and a light (Badia and Delfran, 1970; Siddle and Packer, 1987). In one of the conditions of the Badia and Delfran study, 15 tones were presented, each time followed by a light. The 16th presentation had only the tone, which led to an OR. In another condition there was only the light at the 16th presentation which again elicited an OR. Other simple rule deviations, which evoke an OR, are changes in stimulus duration or in inter-stimulus interval in the case of a sequence of repetitive stimuli (Sokolov, 1975). Thus, the results imply a mechanism that can detect deviations in the context of regularly occurring events. This mechanism should be capable of anticipating events on the basis of earlier processed information, and registering deviations due to a model of the sequential dependencies of the various events. In this way the mechanism can distinguish between appropriate and inappropriate events in sequences of stimuli. In the above-mentioned studies the rule deviations were simple changes in a repetitive sequence. This raises the further question as to whether deviations from more complex regularities will also be recognized. A more complex sequence might be, for instance, A-B-C-D-E-F-G-I-J-K-L-M-N-B, in which the letter 'B' appears twice, first at an appropriate and then at an inappropriate position, at least according to the alphabetical order, which would require an 'O'. Will such more complex rule interruptions be detected, and will they elicit an OR? In a study by Unger (1964), this has indeed been observed. Subjects received a sequence of numerically ascending numbers until the O R - operationalized in terms of vasoconstriction of the blood vessels of the f i n g e r - h a d habituated to the presentation. TM Following successful habituation, a number that did not fit in the sequence, e.g. 15 after 16, elicited a clear OR. A further experimental result in favor of a mechanism for modeling sequential dependencies was described by Zimny, Pawlick and Saur (1969). They investigated the effect of a test stimulus, i.e. the number 600, as a function of the structure of standard stimuli, i.e. the numbers 21 18Subjects who did not habituate were excluded from the experiment.

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to 60. Subjects received once a numerically ascending sequence of stimuli (21, 22, 23, 24 .... 59, 60). The structure of the sequence was violated, however, after the standards 28, 34, 44, 53 and 58. The test stimulus 600 was presented, whereupon the sequence continued in the normal way. The first and second presentation of the 600, after either 28 or 34, did not elicit an OR. After the third presentation of the test stimulus (following 44) the electrodermal activity, as an indicator of OR, increased. The last two presentations of the 600 did not elicit an OR. The authors suspect that a sufficiently specific neural model had been established after 24 presentations of the standard, i.e. after the number 44, so as to bring about dishabituation of the OR. At still later presentations the test stimulus was included in the neural model with the result that the OR was no longer evoked. It should be noted that in the experiments of Unger and of Zimny et al., the OR was not elicited by a novel stimulus but by a stimulus that had already been presented during the sequence. Instead, the standard stimuli were 'new' in a formal sense. Thus, the decisive condition for the OR to occur was not the novelty of the stimulus, but the fact that a rule was violated. Hence, the data favor a mechanism that recognizes sequential dependencies in stimulus materials, anticipates future situations, registers discrepant events and, finally, elicits attending.

3.2

Evoked Potentials

Studies on attending through evoked potentials frequently use the so-called 'oddball' paradigm. The subject receives a sequence of stimuli, many of which (the standards) have a high probability of occurrence while other stimuli (deviants) are relatively rare (Fabiani et al., 1987). The passive form of the oddball paradigm is particularly suitable for studying involuntary attending since it diverts attention from the stimuli in which the experimenter is really interested. This is realized by having subjects perform a task that occupies another channel or sensory modality, e.g. when investigating evoked potentials to auditory stimuli. Alternatively they may be instructed to process information presented at the right ear and to ignore information presented at the left ear, while the experimentally interesting sequence of standard and deviant stimuli actually occurs at the left ear. According to N~i~it~inen, there is, in addition to the earlier-discussed mechanism (section 2), a second mechanism that elicits involuntary attending in the case of.auditory stimuli (N~i~it~inen, 1988, 1990; N~i~it~inen, Simpson and Loveless, 1982; Ohman, 1992), in which 'mismatch negativity' (MMN) fulfills a key role. The MMN is part of the N2 component and shows up in the time function of the potential as an additional negativity in the case of the deviant as compared with the standard. The maximum of the MMN is at about 100-200 ms after stimulus onset, and, in contrast to the effect on the N1, MMN is usually elicited in the passive oddball paradigm by deviants only and not by standards. MMN also clearly differentiates better between stimulus repetition and stimulus change than does the N1 (Sams et al., 1985). The amplitude of the MMN is largest in the frontal area and shows a slight dominance in the right hemisphere irrespective of whether the stimuli are presented contralaterally or ipsilaterally. Various generator processes appear to be involved in establishing MMN and there seems to be at least one supratemporal generator in either one of the auditory cortices (Hari et al., 1984; Scherg et al., 1989) and a further

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generator at a frontal location (N/i/it/inen and Michie, 1979, p.115; Giard et al., 1990). A deviant stimulus which elicits M M N may differ from a standard in intensity (N/i/it/inen et al., 1987; 1993), in duration (N/i/it/inen et al., 1989; Paavilainen et al., 1991), in the location of the stimulus source (Paavilainen et al., 1989), in interstimulus interval (ISI) (Ford and Hillyard, 1981; Nordby, Roth and Pfefferbaum, 1988) or in frequency (Nordby et al., 1988; Sams et al., 1985). For example, in the Sams et al. (1985) study the deviation was frequency. In 80% of the trials, subjects were given 1000 Hz tones, while in the remaining 20% of the trials the frequency was either 1002, 1004, 1008, 1016 or 1032 Hz (stimulus duration: 50 ms; ISI: I s; intensity: 80 dB (SPL)). Both 1016 and 1032 Hz stimuli showed a clear M M N in comparison with the standards. M M N occurs not only when parameters of simple stimuli change but also with more complex auditory stimuli. Thus, in various experiments by Schr6ger, N/i/it/inen and Paavilainen (1992) the standard was a combination of successive tones comprising eight components of different frequencies. The deviant differed from the standard only in the sixth component and was identical otherwise. Yet, an M M N appeared after sufficient presentations of the stimuli. Following N/i/it/inen (1988, 1990), the M M N results may be explained as follows: the physical parameters of the standard stimulus are encoded as neural representation of that stimulus. Each incoming auditory stimulus is compared with this m e m o r y representation. If the comparison process results in a mismatch between actual stimulus information and m e m o r y representation, M M N occurs. Thus, MMN represents the extent to which the representation of a given stimulus deviates from the neural representation of the standard stimulus. 19 According to N/i/it/inen and Michie (1979, p. 115), a deviation first causes activity in the M M N generator in the auditory cortex which, in turn, initiates activity in the frontal generator. Provided a variable threshold value is exceeded, this last generator might elicit attending the discrepant stimulus which results, as indicated by an N 2 b - P 3 a complex (N/i/it/inen and Gaillard, 1983), in an OR. The biological function of the mechanism is temporarily to store repetitive properties of auditory stimuli and to elicit attending in the case of sufficiently large changes. In a study by Winkler et al. (1990), M M N was also observed in a more ecologically valid situation, which supports the notion of the biological function of the above-mentioned mechanism. The authors showed that a preceding constant repetitive stimulus is no prerequisite for eliciting M M N to a deviant stimulus, but that the standard stimulus might just as well be variable. In their experiment subjects were instructed to read in a book and to ignore auditory stimuli. The standard was a 600 Hz tone with an average intensity of 80 dB (SPL). The intensity of the standard was varied in such a way that the intensity difference between two successive standards amounted to 0, 0.2, 0.4, 0.8 or 1.6 dB while the range of intensity variation within a block was 0, 0.8, 1.6, 3.2 or 6.4 dB. The standard had a probability of occurrence of 0.9. The deviant could differ either in intensity (600 Hz; 19The MMN generator process might also be viewed as the neural basis of the mechanism for detecting deviations, as proposed by Sokolov (1975) (see also Sams et al., 1985). There are, however, several differences between MMN and an OR, which show that an OR can be elicited by processes that do not elicit MMN. Thus, an OR, but not MMN, is commonly observed following the first of a sequence of stimuli, and also when the inter-stimulus interval between stimuli exceeds 10s. In addition it is contested whether MMN also occurs after other than auditory stimuli (e.g. N/i/it/inen, 1990; Ciesielski, 1990; Cammann, 1990).

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70 dB) or in frequency (650 Hz; 80 dB). MMN occurred both in the case of a deviant frequency and a deviant intensity. Separate analyses for each individual block of trials showed that MMN decreased as the range of variation of the standards increased. Again, the conditions leading to MMN in the passive oddball paradigm often, although not always, elicit a positivity after 300 ms (P3) (Ritter, Vaughan and Costa, 1968; Sams et al., 1985; Snyder and Hillyard, 1976; Squires, Squires and Hillyard, 1975; Squires et al., 1977). The subjects in the Ritter et al. study (1968, experiment 5) were reading during which activity a click was presented every 2 s at one ear. In about 4% of the trials the click appeared at the contralateral ear which caused a P3 component. 2~ In one condition of the Snyder and Hillyard experiment, subjects were instructed to read and to ignore the presented clicks while deviant clicks were counted in another condition. The intensity of the clicks was 65 or 75 dB (SPL). Varied between blocks, 90% of the clicks were either loud or soft. In the 'reading' condition a frontal positive shift was observed with a maximum after 258 ms, i.e. a P3 component. In the counting condition a parietal positive shift was found after 378 ms, i.e. a P3b component. The authors interpreted the results as evidence for the notion that the N2-P3a complex indicates the activity of a detector for background events. Evidence for bimodal interference was obtained in an experiment of Squires et al. (1977), in which combined light-tone stimuli were presented (blue lightflash/ 1000 Hz tone; orange lightflash/1100 Hz tone; blue/1100 Hz and orange/1000 Hz). Visual as well as auditory deviants could occur. A deviant stimulus in one modality was independent of a deviant stimulus in the other modality, so that, besides light-tone pairs as defined above, there were light-tone pairs with a deviant in either or both modalities. In one condition, the subjects' task was to count visual deviants and to ignore auditory stimuli; this instruction was reversed in another condition. In separate blocks of trials, EPs were not measured but, instead, subjects reacted to standard and to deviant signals with respect to one of the two modalities by pressing an appropriate response key, while ignoring the stimuli from the other modality. According to expectation, the deviants of the attended modality elicited a P3, but in addition the deviants of the ignored modality also elicited a P3. Again, reaction times for the frequent (standard) stimuli of the attended modality were faster when the stimulus of the ignored modality was also standard than when it was deviant. This interference effect may be explained by assuming that the irrelevant deviant had still attracted attention; in turn the P3 component, connected to the irrelevant deviants, might indicate the extent to which attention was distracted from the primary task. Compatible with this interpretation are the results of the earlier outlined study of Sams et al. (1985, p. 445), in which only a P3 component was found if subjects became aware of a deviant stimulus. P3 effects to deviants, which also suggests involuntary attending, occur in the 'active' oddball paradigm in which subjects react to predefined target stimuli. Here, the typical effect is a pronounced P3 complex in the case of task-relevant targets, in comparison with the case of irrelevant standard stimuli. However, this effect can be readily related to voluntary attending (Picton and Hillyard, 1988) and is, therefore,

2~

it is unclear from their data whether there was also an MMN.

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irrelevant to the present argument. Yet, a P3 component is evoked not only by target stimuli but also by task-irrelevant deviants (novel stimuli, non-targets) which clearly differ from the standard (Kemner and Verbaten, 1990, for the auditory, visual and somatosensory modalities). 22 Admittedly, attending to such stimuli is not involuntary, because the instruction implies that each stimulus must be inspected and assessed whether there is a relevant target stimulus. I n v o l u n t a r y - in the sense of bottom-up control- is only the reaction to the standard stimuli. Hence, there is no involuntary attending to the stimulus, but only to certain aspects of the stimulus. It is impossible to ascribe the 'registration' of deviants to intentional selection, because the specific features of the novel stimuli could not be anticipated. Therefore, attending could not be top-down controlled. The effect on the novel stimuli differs, among others, from that on the targets in that it clearly habituates in the course of an experiment, which does not occur with target stimuli (Courchesne, Courchesne and Hillyard, 1978; R6sler, Hasselmann and Sojka, 1987). This is precisely what should be expected from an OR-based mechanism, because novel stimuli lose their deviant characteristics with an increasing number of repetitions. It would be dysfunctional not to include them in the standard model of the situation. Donchin (1979, 1981) and Donchin and Coles (1988) consider the P3 component in the case of rare events as reflecting elaboration of the context (but see criticism of Verleger, 1988). So far the rule deviations that led to the described EP effects all had a physical origin. What is the situation in cases of semantic rule interruptions? Nasman and Rosenfeld (1990) showed that stimuli from a deviant category had a larger P3 amplitude than nondeviant stimuli. Again, this did not only apply to targets but also to non-targets. In five different tasks, stimuli were presented to subjects from a set of nine possible alternatives. Eight stimuli consisted of two-digit numbers, while the ninth was a double letter (e.g. BB). The P3 amplitudes were larger for a non-target of the deviant category than for other non-targets. The categorically deviant non-targets evoked an almost equally large P3 amplitude as categorically nondeviant targets. The relative effect of physical and semantic deviants on EP components has been repeatedly studied. In the three experiments by Kutas and Hillyard (1980), physical deviants showed a P3 (with P210, P360 and P560 as local maxima), while semantic deviants showed an N400 component. Seven-word sentences were presented visually and word for word (stimulus duration 1000 ms, ISI 900 ms). A total of 160 different sentences was used, so that no sentence was ever repeated. In the Kutas and Hillyard study, which aimed at studying the effect of physical deviants, 25% of the sentences ended with a word in bold type (e.g. 'She put on her high-heeled shoes'). In the two studies in which the effect of semantic deviants was investigated, 25% of the sentences were incongruent in that the last word did not fit the context (e.g. 'He took a sip from the transmitter'), the incongruence being more pronounced in the one than in the other of the two studies. The negativity with a maximum at 400 ms was stronger as semantic incongruence was more pronounced. In these studies by Kutas and Hillyard the variations related not only to the dimension 'physical-semantic', but also to the nature of the deviation. In the

21Effects of N1 and MMN in the 'active' oddball paradigm are not discussed here (but see Chapter 9).

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example the physical deviation implied a deviation from a repetitive event in that the lettertype of the final word did not correspond to that of the other six. In the case of the semantic deviation, there was a deviation from a more complex regularity of events. The semantic deviation did not simply consist of a new event added to a sequence of repetitive events since the seventh word was also new in the congruent sentences. The issue was rather that it was a word that did not belong to the set of words that could be expected on the basis of the first six words. Thus, the dimension 'deviation from a repetitive sequence' versus 'deviation from a complex rule' was orthogonal to the dimension 'physical versus semantic deviation'. Hence both dimensions were confounded and, consequently, the experiments of Kutas and Hillyard do not allow conclusions about the dimension to which the differential effect of the change in situation on the EP should be ascribed. Both dimensions were not confounded in an experiment by Besson and Macar (1987), the sole aim of which was to study effects of deviations from more complex rules. Stimulus sequences of seven items each (duration 750 ms, ISI 1000 ms) were presented in four different conditions. The stimuli were: (1) six- or seven-word sentences presented visually and word for word; (2) geometric patterns the size of which either increased or decreased in the course of a stimulus sequence; (3) tones with either an increasing or decreasing frequency; and (4) tunes from well-known music. In each of the conditions, 25% of the stimulus sequences ended with an incongruent stimulus. In condition 1 it was a semantically incongruent word; in condition 2 it was a pattern that violated the implicit rule about the change in size; in condition 3 it was a wrong frequency with respect to the stimulus sequence; and in condition 4 it was a tone that did not belong to the melody. In conditions 1, 2 and 4 a more pronounced N1 component showed up after incongruent stimuli as compared with congruent ones. In conditions 2, 3 and 4, incongruent stimuli evoked a P3 complex after 350-450 ms. Finally, incongruent stimuli had the effect that the positivity shifted into the negative direction in condition 1, i.e. in the case of sentences (maximum about 350ms). Hence the P3 complex appeared to be superseded by the N400. It is of course essential that the more complex nonlinguistic deviants (wrong size, wrong frequency, wrong tone) did not evoke an N400. The authors suggest that the first words of each sentence may semantically prime later words, and therefore preactivated words are processed more easily. An N400 component is evoked if the later word does not belong to the primed semantic category. In summary, it can be said that various investigators, studying evoked potentials, have developed functional notions about internal registration of rule interruptions in which attending is elicited by a deviation of an actual event from an internal model.

3.3

Visual Search

Evidence for the effect of deviations from rules also stems from research on continuous selection processes, which, for example, take place in continuous search tasks (see also Chapter 2). In experiments of this type, which have been developed by Neisser (1963, 1967), subjects are instructed to scan, line by line, a list of letters

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following each other either randomly or according to certain rules, and to look for one (or several) target letter(s). 22 Each list consists of a large number of context symbols (typically 300-1000 letters or digits), but contains only one single target. Scanning ends as soon as the target is detected. Neisser's original description of the performance of the subjects was in terms of search speed, defined by the average search time per line or symbol. More differentiated descriptions are possible by recording eye movements during scanning (Jacobs, 1986; Nattkemper, 1990; Nattkemper and Prinz, 1984; Prinz and Nattkemper, 1986, 1987; Rayner and Fisher, 1987). A variety of experiments, summarized by Prinz (1977, 1986), have led to the assumption that search activity is largely controlled by representational structures, which involve involuntary attending based on internal models of the environment. Three kinds of experiments are essential with respect to this view. The first is concerned with the phenomenon of detecting pseudo-targets (Prinz, Tweer and Feige, 1974). Subjects stop searching or hesitate when detecting certain symbols which they are not actually looking for. This occurs when, following some practice, lists are presented containing a new item that belongs neither to the target nor to the non-target list used so far. (The logic of these experiments is similar to those discussed in the previous section, in that rarely occurring deviants are inserted in a sequence of events.) It is surprising that detecting pseudo-targets does not require extensive experience with the usual context events, but occurs at an early stage of practice (Prinz, 1979, experiment 1). This phenomenon means first of all that, while searching, subjects notice deviations from a regular context, which may potentially lead to an interruption of the search. This cannot be explained by a theory that assumes mechanisms of searching and finding are ultimately a matter of voluntary attending (Neisser, 1967). Under this view the instruction to search for a distinct target would have the effect of selectively activating a representation of that target. With this representation 'in mind', the list is scanned until stimulus information is met that corresponds with the memory representation of that target. According to this theory, detection would be the result of voluntary attending, in the sense that a distinct intention, directed to the to-be-detected set of stimuli, constitutes the functional basis that stimuli of that specific category are processed as a first priority. The very fact of pseudo-target detection means that interruption of search does not necessarily imply a match between stimulus information and an internal representation of the to-be-searched target object. The phenomenon may be explained by reversing the logic of the Neisser theory: search may not be primarily controlled by representations of the targets but rather by an internal model of potential context events (Prinz, 1979; Prinz and Ataian, 1973). This internal model contains stored representations of context elements, and is established by the first context elements that occur in the list. All further repetitions of the context elements refresh and actualize the stored representations. To explain the genesis and maintenance of the internal model, if suffices to postulate that a memory representation which has been activated by given stimulus information primes itself. The 22The mechanisms used for searching and finding in such tasks are largely a matter of voluntary attention. In the present discussion only effects that are unrelated to voluntary attention will be considered.

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prime decays over time unless it is refreshed by repeated activation. Thus, the activation of a memory representation alters its functional state in a way that facilitates its subsequent activation. Search is continued as long as the stimuli fit the internal model of potential context events. Any stimulus that does not fit the model, either a target or any deviant from the regular context elements, is subjected to a further analysis. This mode of control becomes plausible when considering the essential aspects of the search task. Targets are rare events, while context elements are continuously and repeatedly met. Under these circumstances the generation of an internal model about potential context symbols is an economic way of controlling search. This is even more valid if the search task is considered in terms of decisions-to-act. While scanning the list, a decision is made at each fixation either to stop (in case of a deviation from the regular context) or to continue search with a saccade to another position of the list (in case of only regular context events). The decision to continue search is the common one and, hence, it is relevant to specify the criterion for making these decisions in terms of the properties of the more frequent and not in terms of the less frequent alternative (see also Heuer and Prinz, 1987). In two additional types of experiment it was found that registration of deviants is not limited to novel stimuli, such as pseudo-targets, but also occurs in the case of a rule interruption. What happens when there is a deviation from an earlier established rule structure? The first line of evidence emerged from studies in which the complexity of the context was varied within search lists (Nattkemper, Ullmann and Prinz, 1991). The complexity of the context was defined in terms of the number of different context letters in a certain area of horizontally adjacent elements. Complexity was varied across two successive lines within search lists. The critical issue was whether direct effects of such a variation are observed on scanning behavior, e.g. on saccadic eye movements. The results of these studies show two relevant features. The first fixation after the change in complexity exceeded the duration of the corresponding first fixation in lines where complexity had remained unchanged. This effect appeared to be aspecific, since it was observed at both increasing and decreasing complexity. A specific effect was found on the next saccade, the amplitude of which was smaller as context complexity increased and larger as it decreased. Thus, although the first fixation after the change in context complexity did not show a specific effect, the specific effect of the next saccade suggests that the direction of the change did not remain unnoticed. This saccade must be prog r a m m e d - or adjusted to the new structure of the list- on the basis of information processed during its preceding fixation. Presumably, therefore, the direction of the change is noticed during the fixation and saccade amplitude is immediately adjusted to the new list structure. Evidence concerning effects of a rule deviation on search is also obtained from search experiments in which the sequence of context elements is not at random but has a certain regularity that is suddenly violated. One may, for instance, establish such regularities by introducing a certain nonrandom algorithm generating specific letter bigrams or trigrams (Nattkemper and Prinz, 1991). In a critical block of trials, a violation may be realized by combining the same elements in a novel and thus unknown way. These experiments have aspects in common with research on implicit learning of regularities in event sequences (see Reber, 1989, for a review), which have demonstrated that subjects are capable of taking into account structural properties

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of stimulus information in shaping their behavior, while incapable of describing these properties. Consistent results stem from different paradigms. First, it has been shown that serial reproduction as well as prediction of a stimulus sequence with some experimental grammar is better than when the same stimulus sequence has a random order (Dulany, Carlson and Dewey, 1984; Reber, 1967; Reber and Millward, 1971). Second, a continually improving detection performance is observed when a certain rule determines the sequence of positions at which targets appear (Lewicki, Czyzewska and Hoffmann, 1987; Lewicki, Hill and Bizot, 1988). In search studies subjects do not usually notice the distinct regularities in the sequence of context elements, nor do they consciously recognize violations of the sequence. Yet, the critical session, in which the regularity is violated, revealed a clear increase of regressive eye movements (Nattkemper and Prinz, 1991). Regressive eye movements may be viewed as an indication for difficulties in processing information (Prinz, Nattkemper and Ullmann, 1991). Hence, this result suggests that violations of thus far existing event regularities lead to specific problems in processing that often require a retesting of previously analyzed areas of the context. The observation that deviations from the rule cause modifications in the pattern of eye movements suggests also that the regularities have become part of the representational structure that controls the search activities. As a result of this excursion into the area of visual search, it may be concluded that aspecific selection processes, based on internal models, are also operative in continuous tasks. However, it should be noted that, so far, the data from search experiments merely demonstrate that continuous search is mainly controlled by internal models. The mechanisms determining the generation, maintenance and ongoing actualization of the internal models are largely unknown.

4

CONCLUDING REMARKS

Involuntary attention in this contribution refers to processes of attending that are not elicited by intentions but by certain outside events. The discussion has been fully limited to aspecific processes of attending, elicited by the deviation of stimulus events from a given context. A distinction has been drawn between more simple and more complex deviations, i.e. level shifts and rule interruptions, although these cannot be considered as clearly separated categories but rather reflect a continuum. Suddenly appearing novel events (the onset of section 2) are at the one end, while more complex rule interruptions, as met in the discussion on visual search (section 3.3), are at the opposite end of the continuum. This proposed differentiation notwithstanding, the phenomena of involuntary attending have been incompletely covered in several respects. First, there are probably various types of other outside events that can elicit processes of involuntary, i.e. unintentional, attending. Such processes, which might be lumped together under the label of 'specific selection' (compare section 1), have not been dealt with. A second restriction may be even more essential: when referring to 'involuntary attention' or 'aspecific selection' the implicit classification (voluntary/involuntary) has always been in terms of elicitors of attending. The relevant processes are involuntary to the extent to which they are determined by outside events, and aspecific in the sense that the events are defined in relation to the situational context, namely as deviants from that context. The obvious essential question is

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whether distinct elictors of attending are actually accompanied by distinct differences in the nature of the processes. Do involuntary and intentional attending merely differ with respect to their actual determinants, or are the processes involved related to essentially different mechanisms? More generally formulated, the issue is to what extent the various elicitors reflect functional differences. When considering the processes outlined in this chapter under this perspective, there are reasons to doubt functional differences. As mentioned at the end of section 2, the recent research on attending, elicited by either central or peripheral cues, does not unequivocally suggest the operation of two mutually independent attention systems. The observed differences in the effects of central symbolic cues and peripheral onsets may, in fact, be explained by the assumption that a single common attentional mechanism is activated by these types of stimuli, albeit in a different way. In the context of the discussion about the relations between rule interruptions and involuntary attention (section 3), the central question of the research on visual search as well as that on event-correlated potentials concerned the type of mechanism through which rule interruptions can be identified. The question whether, following successful detection of a deviant, attending is qualitatively different from voluntary determined attention has not been answered, nor has it become clear how one could experimentally prove a possible difference. One of the reasons for this state of affairs is that, thus far at least, electrophysiological indicators of attentional processes and their effects have been almost exclusively concerned with voluntary attending (see also Chapter 9). Thus, voluntary and involuntary attending are distinguished only by the way in which they are elicited. The question of whether involuntary attention exists in the sense of an independent attention system remains fully open.

ACKNOWLEDGEMENT D. Nattkemper was supported by grant Pr 118/9 from the Deutsche Forschungsgemeinschaft (DFG). The final version of this manuscript was completed in November 1991. Therefore the review of the literature only takes into account publications available up to 1991.

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