Clarifying the Simple Assumption of the Secondary Task Technique

ABSTRACT - The secondary task technique has been used in the detection and measure of constructs described variously as attention, effort, elaboration, cognitive capacity, processing intensity, and such, which in turn may provide an indication of learning and automatism. The assumption that the secondary task provides evidence for the detection of such a variety of constructs may not always be valid. This paper attempts to clarify our current understanding of how the secondary task technique works.

Citation:

Robert S. Owen (1991) ,"Clarifying the Simple Assumption of the Secondary Task Technique", in NA - Advances in Consumer Research Volume 18, eds. Rebecca H. Holman and Michael R. Solomon, Provo, UT : Association for Consumer Research, Pages: 552-557.

Advances in Consumer Research Volume 18, 1991      Pages 552-557

CLARIFYING THE SIMPLE ASSUMPTION OF THE SECONDARY TASK TECHNIQUE

Robert S. Owen, Ohio State University

ABSTRACT -

The secondary task technique has been used in the detection and measure of constructs described variously as attention, effort, elaboration, cognitive capacity, processing intensity, and such, which in turn may provide an indication of learning and automatism. The assumption that the secondary task provides evidence for the detection of such a variety of constructs may not always be valid. This paper attempts to clarify our current understanding of how the secondary task technique works.

INTRODUCTION

Our inability to perform some combination of tasks without a decrement in performance of one or all tasks is the basis of the secondary task technique. Until the skill to concurrently perform two tasks has been acquired, focusing one's attention on the performance of one task can often cause a measurable degradation in the performance of the other. The observation of performance changes in one task can often be taken as a measure of changes in processing resources being devoted to the performance of a concurrent task, presumably providing an operational indication of attention, of the expenditure of effort, or of various other constructs or processes, which in turn can provide evidence of learning and skill acquisition.

The secondary task technique has been used in a number of contexts that are of interest to both marketing academicians and practitioners. For example, the secondary task technique has been used by Thorson, Reeves, and Schleuder (1985, 1987) to investigate the amount of viewer "attention" allocated to the processing of television messages; by Lord and his colleagues (Lord and Burnkrant 1988; Lord, Burnkrant, and Owen 1989) to measure changes in television viewer "involvement" and "elaboration"; and by Moore, Hausknecht, and Thamodaran (1986) in the investigation Of the amount of "attentional capacity" allocated to the processing of audio commercials. The secondary task technique has also been used by several researchers in psychology as a measure of the amount of "cognitive capacity" dedicated to the reading of text (e.g., Inhoff and Fleming 1989; Britton and Tesser 1982). Secondary task performance has even been associated with personality (Huddleston and Wilson 1974) and demographic variables (Stapleford 1973).

Although the secondary task technique, particularly the "RT-probe", does appear to be useful in the detection and measure of a variety of constructs and processes in a variety of situations, several questions can be raised regarding such uses. How is it that the same technique can be used in different studies to measure "attention", "effort", "elaboration", and so on? Do identical operationalizations in the detection and measure of these imply that these are identical constructs or processes? What are the implications when a secondary task probe fails to detect these constructs or processes? Does this failure necessarily imply that the construct or process was non-existent at the time of the measure?

The focus of this paper is on clarifying our current understanding of what it is that the secondary task technique, particularly the "RT-probe", actually measures. The generally accepted assumption, that the existence of some limited pool of resources is a phenomenal given and that the secondary task probe is the dipstick by which this pool is measured, may not always be valid justification for claims that may be made as a result of the use of the secondary task technique. A theoretical problem with this assumption as the basis- for using the secondary task probe as a measurement instrument is its circularity as a theoretical claim (cf., Navon 1984, 1985). An operational problem with this assumption in its simple form is that lt does not account for evidence of apparently "resource free" processing, including apparent parallel processing and automatism. What we do know is that changes in secondary task performance do seem to provide evidence of changes in the use of processing resources, whatever these "resources" really are, and that these changes may in turn provide evidence for the detection of one (and only one) dimension of a variety of constructs and processes.

THE RT-PROBE SECONDARY TASK

Although most recent users of the secondary task technique have used the RT-probe, a variety of secondary tasks could be used (Ogden, Levine, and Eisner 1979), including the maintenance of hand pressure (Welch 1898) and finger tapping tasks (Friedman, Polson, and Dafoe 1988; Jastrow 1892). Of greatest interest to most current users of the secondary task technique is the measurement of response times to visual or auditory stimuli while performing a concurrent primary task. A brief audible-beep, to which the subject must respond by pressing a button switch, can presumably function as a probe into the processing of a concurrent primary task. Changes in response times to the secondary or probe task are taken as an indicator of changes in the use of processing resources devoted to the performance of the primary task. There are variations on the secondary stimuli and response tasks that can be used, such as the use of a flash of light and a verbal response, but all are based on the same basic underlying assumption: that there appear to be limits in our abilities to utilize processing resources and that greater expenditure of resources toward the performance of the primary task can measurably degrade the performance of the concurrent secondary task. There is yet, however, some amount of controversy regarding just what these "resources" might be and how they function.

ATTENTION, EFFORT, ELABORATION, ETC.: WHAT'S THE DIFFERENCE?

Although the secondary task technique is most certainly a valuable research tool, it really cannot detect anything other than apparent interference between concurrently performed tasks. Under the assumption that concurrently performed tasks will interfere with each other, the secondary task technique has been used to measure constructs and processes described variously as attention, effort, elaboration, processing intensity, and such. Is it really possible that the response time to an audible beep can become a common operational definition of these various constructs which may not have common theoretical meanings? If these constructs are theoretically different, how is it that they can all be measured in exactly the same manner? That is, if all are defined in the amount of secondary task interference that they cause, how can they be any different? A yardstick only provides a measure of length and nothing more: can the secondary task probe provide a direct measure of attention and of effort and of elaboration?

Although such constructs may be related, they are theoretically somewhat different. What we need is to know what specific dimensional aspect the secondary task describes about the construct under investigation. The RT-probe secondary task generally does not provide a direct measure of attention or of elaboration or of most other constructs we would usually want to investigate, but provides a measure of only a single dimensional quantity which is common to these constructs. Evidence that the secondary task measured something is not alone sufficient evidence of whatever construct the investigator might claim to measure. There must be some a priori theoretical reason for considering that the secondary task probe provides evidence of some dimension of a construct.

Consider, for example, two tasks with which we might use the RT-probe secondary task. The first primary task involves mental arithmetic and the second primary task involves reading an interesting story. Both primary tasks can be expected to produce measurable decreases in RT-probe secondary task performance. It might be reasonable to assert that we have measured "effort" in the arithmetic task and "elaboration" in the interesting reading task, even though we might no think that "elaboration" was involved in the mental arithmetic task or that subjects would regard reading an interesting story as especially effortful. This is because we do have some feel for other dimensions such as how and when processing resources might be utilized in performing these tasks and for the outcome of the performance of these tasks. In this example, we might consider recall as another dimension associated with the elaboration construct but not with the effort construct. We might consider certain skill improvements to be associated with the effort construct, but not with the elaboration construct.

As long as the theoretical context of the construct was considered and more than a single dimension of the construct under test was investigated, we might have reasonable strength in our claim of evidence for the construct under investigation. The cautionary note, then, is that there must first be some theoretical justification for using this methodology and that it should not be the sole measurement instrument of a multidimensional construct. One possible multidimensional framework, which attempts to integrate measures of attention with measures of attitude, has been suggested by Owen (1990).

So what exactly is this single common dimension that is detected or measured by the secondary task probe? Why and how do concurrently performed-tasks interfere with each other? The basic assumption in using the secondary task probe is that it provides an index of primary task consumption of "processing resources". Precisely what are these "resources" upon which the use of the secondary task technique is grounded? If a change in the use of these resources as indicated by the secondary task probe has some sort of meaning, what is the meaning of a failure of the secondary task probe to detect resource usage?

WHAT IS A "LIMITED RESOURCE"?

The theoretical basis for using the secondary task probe in the measure or detection of a particular construct is a little more problematic than one might initially assume; it is still not clear just what it is that the secondary task probe measures and does not measure beyond concurrent task interference. The theoretical justification for the secondary task technique is based on the assumption that the construct or process under investigation consumes from some asymptotically limited pool of processing resources; changes in secondary task performance are presumed to provide a measure of changes in the use of those resources, providing evidence as to the investigator's claims regarding the construct under investigation. Acceptance of this assumption requires that one adopt a circular form of reasoning. Although the relationship between secondary task performance and "attention", "effort", "capacity", or "processing resources" seems obvious enough to some researchers, others have found the conceptual, theoretical, and operational definition of these constructs and processes to be more elusive (cf., Hirst and Kalmar 1987; Kinchla 1980; Navon 1984; Stelmach and Hughes 1983; Wickens 1984). Secondary task performance can be explained in terms of structural limitations, processing channel limitations, central processor limitations, resource sharing, and so on, and none of these explanations seems to have completely obsolesced the others.

Earlier studies of the mechanism of information processing viewed the processing system as something like a single channel transmission line (see Welford 1967 for review). Miller (1956), for instance, viewed the human as "a kind of communication system" with a finite limit in its channel capacity. Many of the earlier studies of attention and the mechanism of the information processing system were attempts to locate where this capacity limitation, or "bottleneck", occurred. Broadbent (1954) concluded that there seemed to be a many-to-one selection switch in this channel and that there was a limit as to how fast this switch could operate in selecting parallel input signals for sequential passage through a single channel processing system. The idea that this processing bottleneck occurred near the input of the processing channel was to result in Broadbent's "Filler Theory" (Broadbent 1957, 1958).

Although Broadbent's theory is one of the most well known of the earlier theories of attention, it very quickly came under attack regarding the location of the channel bottleneck and the mechanism of this bottleneck. Two well known attempts to refine the Filler Theory in these regards were those of Deutsch and Deutsch (1963) and Treisman (1966). It became evident, however, that there could be situations in which the processing system did not behave as if it was a single channel transmission line.

If the processing system operates in a serial manner as proposed by the single channel hypotheses, i.e., being able to sequentially process only one task at a time, then the time taken to perform two tasks concurrently should be linearly additive. That is, if the system must function by completing one task and then "switching" to the other, then the amount of time taken to complete both tasks should be predicted by adding the times to complete each task individually. Empirical evidence has shown otherwise; the time taken to perform the tasks concurrently is sometimes much less than the sum of the times taken to perform the tasks individually (Keele 1967). This seems to provide evidence against a single-channel serial system.

Further evidence against the single channel hypothesis was provided by experiments that allowed for compatible input and output tasks. In a manner similar to Broadbent's (1954) dichotic listening task, Moray and Jordan (1966) presented subjects with different but simultaneous messages in each ear. Subjects were asked to type these messages, the numbers zero through nine, on keyboards with ten keys labeled zero through nine; the subject was to type the left ear messages on left hand switches, and right ear messages on right hand switches. This was assumed to provide subjects with a means of parallel output matched to parallel input. The results seemed to indicate that something more than single channel transmission was possible.

This led Moray (1967) to propose that apparent limits in the ability to process information may be better conceptualized in terms of a limited capacity central processor rather than a limited capacity communication channel. Under this central processor conceptualization, the overall size of the processor is limited but the processor is very flexible. This limited capacity central processor conceptualization was expanded and refined by Kahneman (1973). Kahneman did not view this model as a replacement for the earlier models. The earlier models were viewed as explanations of structural limitations in processing (e.g., eyes cannot be focused on two objects simultaneously, regardless of the processing abilities of the rest of the system) and the limited capacity processor model as an explanation of how some processing activities can be carried out together; neither view was considered as adequate alone. The model that was proposed by Moray and Kahneman has been labeled the undifferentiated capacity hypothesis by Kerr (1973).

Kahneman seemed deliberately vague in labeling this limited processing resource, noting that it "may be variously labeled 'effort', 'capacity', or 'attention"' (p. 9). -Kahneman's conceptualization viewed the processing system as possessing a very general pool of "capacity" or "effort" or "attention" which may be allocated to the performance of various tasks.

A problem with this undifferentiated resource capacity model is that the processing of some tasks does appear to be differentiated. For example, it is easier to attend to auditory and visual messages concurrently than to two auditory messages (Rollins and Hendricks 1980; Treisman and Davies 1973). This could be due to structural limitations or it could be possible that there are separate processing channels for auditory and visual information prior to input to the central processing mechanism. According to the multiple resource theory, it could also be possible that there are different kinds of resource pools, and that dual task performance interference occurs when two processes attempt to use the same pool (Navon and Gopher 1979; Wickins 1980, 1984). Friedman, Polson, and Dafoe (1988), for instance, found differences between each cerebral hemisphere and task interference. This explanation implies that not all primary tasks will interfere in the same way with secondary task performance; the detection of differences in secondary task performance between two tasks may be not necessarily be due to differences in "attention" or in the expenditure of "effort".

Yet another explanation is the dual process theory, which recognizes a continuum of information processing, anchored by slow, effortful, serial controlled processing at one extreme and by fast, resource-free, parallel automatic processing at the other (e.g., Schneider and Shiffrin 1977; Shiffrin and Schneider 1977). Related to this latter notion is the "processing is a skill" explanation (Hirst 1986; Hirst, Spelke, Reaves, Caharack, and Neisser 1980; Spelke, Hirst, and Neisser 1976). These explanations attempt to explain how processing can sometimes appear to be resource free; processing can take place yet remain undetected by a secondary task probe.

A more recent explanation is that concurrent task interference may not be due to limitations in "capacity" in the sense of some entity being divided in the processing of concurrent tasks, but is due instead to attention sharing. David Navon, who was once an advocate of the capacity explanations of concurrent task interference, has become somewhat more hesitant toward the capacity assumption in recent years. A possible explanation for differences in the interference of various dual tasks is in the way they share a relatively limitless resource; there may be no limitations in a capacity sense, but interference may be a result of some sort of confusion due to what Navon calls outcome conflict (Navon 1985; Navon and Miller 1987). One possible source of outcome conflict discussed by Navon is "cross talk" among parallel, independent processes similar to electrical cross talk in parallel wires.

Given that there can be so many viable explanations for dual task interference, it might seem that the secondary task technique is not a useful technique beyond the investigation of its own theoretical basis. Navon (1984) questions the validity of tests which rely on a resource explanation and suggests that an uncritical acceptance of such resource assumptions can result in experimental results that might only be methodological artifacts (Navon 1985). On the other hand, the secondary task technique does appear to detect and measure something related to the interference between concurrently performed tasks. As long as this observation allows us to make inferences regarding constructs and processes such as attention, effort, or elaboration, then the secondary task technique would appear to be useful and valid regardless of our inability to completely understand why or how it works (cf., Navon 1984).

A cautionary note is again, then, that there should be some theoretical justification behind using this methodology. One report in the marketing literature, for instance, justifies the use of the secondary task technique on the grounds that "psychologists have used his method for many years" and by discussing what sorts of findings are "typical" of such studies. It is important to be explicit about assumptions regarding the resource requirements of specific pairs of tasks (cf., Friedman, Polson, and Dafoe 1988); careful consideration needs to be given as to why a particular pair of tasks may or may not be expected to interfere in a predictable, detectable, and measurable way.

CONCLUDING REMARKS

The secondary task technique does appear to be a very useful tool, but our current understanding of its underlying assumptions with regard to a variety of constructs is not so strong as one might initially believe. What has been emphasized in this paper is that there must be a stronger theoretical basis when using the secondary task technique than a casual, vague reference to "limited processing resources". Not all pairs of tasks will always cause detectable interference, yet some form of processing can still take place. Most importantly, the construct that is detected is generally not the construct of interest, but only a single dimension of that construct, evidenced only by concurrent task performance interference.

One must fully consider, then, what dimensional aspect of a construct or process the secondary task probe is expected to measure. One must be cautious in claims that a change (or no change) in secondary task performance alone provides conclusive evidence regarding the construct under investigation. The studies of Britton suggest, for instance, that the reading of difficult text can at times consume more "processing resources" than simple text (Britton and Tesser 1982), but also suggest that simple text can at times consume more "processing resources" than difficult text (Britton et al. 1978, 1979, 1980). We cannot really say what is "typical" of such studies, nor can we expect in both cases that the construct that was detected was, say, "elaboration" or "effort". The detection of decreased secondary task performance during, say, the processing of a marketing communication is also not universally an indication of any particular construct or process.

The secondary task probe merely detects apparent changes in the use of "processing resources". Whether this indicates a change in the quantity of usage of a particular resource, a change in the way a resource is shared, or a change to a different resource is currently unclear. Although it would seem reasonable to assume that changes in secondary task performance are an indication of something, regardless of our current ability to fully understand whatever that "something" really is, it is currently not clear how such changes relate to a variety of constructs such as attention, effort, elaboration, and so on. Perhaps we need to better define the various dimensions which might be used to conceptually, theoretically, and operationally describe these constructs and processes.

It seems appropriate to conclude with two quotes that are so often used to introduce papers on the subject of attention:

"Everyone knows what attention is."

(William James 1890, p. 403)

"The discovery of attention didn't result in any immediate triumph of experimental method. It was something like a hornet's nest: The first touch brought out a whole swarm of instant problems. . . The discovery of a reliable measure of attention would appear to be one of the most important problems that await solution by experimental psychology in the future." (Edward Titchener 1908, ch. 5)

REFERENCES

Britton, Bruce K., Timothy S. Holdredge, Cheryl Curry, and Robert D. Westbrook (1979), "Use of Cognitive Capacity in Reading Identical Texts with Different Amounts of Discourse Level Meaning," Journal of Experimental Psychology: Human Learning and Memory, 5, 262-270.

Britton, Bruce K. and Abraham Tesser (1982), "Effects of Prior Knowledge on Use of Cognitive Capacity in Three Complex Cognitive Tasks," Journal of Verbal Learning and Verbal Behavior, 21, 421-436.

Britton, Bruce K., Robert D. Westbrook, and Timothy S. Holdredge (1978), "Reading and Cognitive Capacity Usage: Effects of Text Difficulty," Journal of Experimental Psychology: Human Learning and Memory, 4, 582-591.

Britton, Bruce K., Robbie Zeigler, and Robert D. Westbrook (1980), "Use of Cognitive Capacity in Reacting Easy and Difficult Text: Two Tests of an Allocation of Attention Hypothesis," Journal of Reading Behavior, 12, 23-30.

Broadbent, D.E. (1954), "The Role of Localization in Attention and Memory Span," Journal of Experimental Psychology, 47, 191-196.

Broadbent, D.E. (1957), "A Mechanical Model for Human Attention and Immediate Memory," Psychological Review, 64, 205-215.

Broadbent, D.E. (1958), Perception and Communication. London: Pergamon Press Ltd.

Deutsch, J. and D. Deutsch (1963), "Attention: Some Theoretical Considerations,'' Psychological Review, 70, 80-90.

Friedman, Alinda, Martha Campbell Polson, and Cameron- G. Dafoe (1988), "Dividing Attention Between the Hands and the Head: Performance Trade-offs Between Rapid Finger Tapping and Verbal Memory," Journal of Experimental Psychology: Human Perception and Performance, 14, 60-68.

Hirst, William (1986), "The Psychology of Attention," in Mind and Brain, eds. Joseph E. LeDoux and William Hurst, New York: Cambridge University Press. 105-141.

Hirst, William and David Kalmar (1987), "Characterizing Attentional Resources," Journal of Experimental Psychology: General, 116, 68-81.

Hirst, William, Elizabeth E. Spelke, Celia Reaves, George Caharack, and Ulric Neisser (1980), "Dividing Attention Without-Alternation or Automaticity," Journal of Experimental Psychology: General, 109, 98-117.

Huddleston, E. F., and R. V. Wilson (197]), "An Evaluation of the Usefulness of Four Secondary Tasks in Assessing the Effort of a Lag in Simulated Aircraft Dynamics," Ergonomics, 14, 371 -380.

Inhoff, Albrecht Werner, and Kevin Fleming (1989), "Probe-Detection Times During the Reading of Easy and Difficult Text," Journal of Experimental Psychology: Learning, Memory, and Cognition, 15(2), 339-351.

James, William (1890), The Principles of Psychology. New York: Holt.

Jastrow, J. (1892), "The Interference of Mental Processes," American Journal of Psychology, 4, 219-223.

Kahneman, Daniel (1973), Attention and Effort, Englewood Cliffs: Prentice Hall, Inc.

Keele, S.W. (1967), "Compatibility and Time-Sharing in Serial Reaction Time," Journal of Experimental Psychology, 75, 529-539.

Kerr, Beth (1973), "Processing Demands During Mental Operations," Memory and Cognition, 1, 401-412.

Kinchla, R.A. (1980), "The Measurement of Attention," in Attention and Performance VIII, ed. Rayond S. Nickerson, Hillsdale: Lawrence Erlbaum Associates, Publishers. 213-238.

Lord, Kenneth R. and Robert E. Burnkrant (1988), "Television Program Elaboration Effects on Commercial Processing," in Advances in Consumer Research, vol. 15, ed. Michael J. Houston, 213-218.

Lord, Kenneth R., Robert E. Burnkrant, and Robert S. Owen (1989), "An Experimental Comparison of Self-Report and Response Time Measures of Consumer Information Processing," in Proceedings of the American Marketing -Association 1989 Summer Educator's Conference.

Miller, George A. (1956), "The Magical Number Seven, Plus or Minus Two: Some Limits on our Capacity for Processing Information," Psychological Review, 63(2), 81-97.

Moore, Danny L., Douglas Hausknecht, and Kanchana Thamodaran (1986), "Time Compression Response Opportunity in Persuasion," Journal of Consumer Research, 13 (June), 85-99.

Moray, N. (1967), "Where is Capacity Limited? A Survey and a Model," in Attention and Performance, ed. A.F. Sanders, Amsterdam: North-Holland Publishing Co., 84-92.

Moray, N. and A. Jordan (1966), "Practice and Compatibility in Two-Channel Short-term Memory," Psychonomic Science, 4, 427.

Navon, David (1984), "Resources - A Theoretical Soupstone?" Psychological Review, 91, 216234.

Navon, David (1985), "Attention Division or Attention Sharing?" in Attention and Performance XI, eds. Michael I. Posner and Oscar S.M. Marin. Hillsdale: Lawrence Erlbaum Associates, Publishers, 133-146.

Navon, David and Jeff Miller (1987), "Role of Outcome Conflict in Dual-Task Interference," Journal of Experimental Psychology: Human Perception and Performance, 13(3), 435448.

Navon, David and D. Gopher (1979), "On the Economy of the Human Processing System," Psychological Review, 86, 214-255.

Ogden, George D., Jerrold M. Levine, and Ellen J. Eisner (1979), "Measurement of Workload by Secondary Tasks," Human Factors, 21(5), 529548.

Owen, Robert S. (1990), "Integrating Attitude and Attention Theories," in Developments in Marketing Science, Vol. 13, ed. B. J. Dunlap, The Academy of Marketing Science, 52-55.

Rollins, H.A. and R. Hendricks (1980), "Processing of Words Presented Simultaneously to Eye and Ear," Journal of Experimental Psychology: Human Perception and Performance, 6, 99-109.

Schneider, Walter and Richard M. Shiffrin (1977), "Controlled and Automatic Human Information Processing: I. Detection, Search, and Attention," Psychological Review, 84, 1-66.

Shiffrin, Richard M. and Waller Schneider (1977), "Controlled and Automatic Human Information Processing: II. Perceptual Learning, Automatic Attending, and a General Theory," Psychological Review, 84, 127-190.

Spelke, Elizabeth, William Hirst, and Ulric Neisser (1976), "Skills of Divided Attention," Cognition, 4, 215-230.

Stapleford, R. L. (1973), "Comparisons of Population Subgroups' Performance on a Keyword Psychomotor Task, " Proceedings of the Ninth Annual Conference on Manual Control, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Stelmach, George E. and Barry Hughes (1983), "Does Motor Skill Require a Theory of Attention?" in Memory and Control of Action, ed. Richard A. Magill, New York: North-Holland Publishing Company, 67-92.

Thorson, Esther, Byron Reeves, and Joan Schleuder (1985), "Message Complexity and Attention to Television," Communication Research, 12(4), 427 -454.

Thorson, Esther, Byron Reeves, and Joan Schleuder (1987), "Attention to Local and Global Complexity in Television Messages," in Communication Yearbook 10, ed. Margaret McLaughlin, Newbury Park, CA: Sage Publications, 366-383.

Titchener, Edward B. (1908), Lectures on the Elementary Psychology of Feeling and Attention, New York: McMillan.

Treisman, Anne M. (1966), "Our Limited Attention," Advances in Science, 22, 600611.

Treisman, Anne M. and Alison A. Davies (1973), "Divided Attention to Ear and Eye," in Attention and Performance IV, ed. Sylvan Kornblum, New York: Academic Press, 101-125.

Welch, Jeannette C. (1989), "On the Measurement of Mental Activity through Muscular Activity and the Determination of a Constant Attention," American Journal of Physiology, 1, 283-306.

Welford, A.T. (1967), "Single-Channel Operation in the Brain," in Attention and Performance, ed. A.F. Sanders, Amsterdam: North-Holland Publishing Co., 5-22.

Wickens, Christopher D. (1980), "The Structure of Attentional Resources," in Attention and Performance VIII, ed. Raymond S. Nickerson, Hillsdale: Lawrence Erlbaum Associates, Publishers, 239-257.

Wickens, Christopher D. (1984), "Processing Resources and Attention," in Varieties of Attention, eds. Raja Parasuraman and D.K. Davies, Orlando: Academic Press, Inc., 63- 102.

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