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Resources and dual-task performance; resource allocation versus task integration
Acta Psychologica North-Holland
72 (1989) 221-232
221
RESOURCES AND DUAL-TASK PERFORMANCE; RESOURCE ALLOCATION VERSUS TASK INTEGRATION
*
Mieke DONK and Andries F. SANDERS R WTH Aachen, FRG
Accepted
February
1989
This study aims at contributing to the explanation of dual-task performance in terms of either resource allocation or of task interference and integration. Twenty-four subjects carried out, single and in combination, a motor interval production task and a perceptual target detection task on the basis of combined memory and display search. The demands of the target detection task were varied by increasing or decreasing the presentation rate of successive search displays. Furthermore, the presentation rate was either constant or variable. The dual-task condition had a negative effect on interval production, the extent of which was unaffected by either rate or variability of display presentation. This means that there was no evidence for synchronising interval production with display presentation, so that the major opportunity for task integration did not substantiate. It is suggested that the two tasks use different resource pools in addition to a common mechanism, the limited capacity of which causes a general interference in dual-task conditions.
Introduction This article is concerned with an empirical test of some predictions derived from the multiple resource theory of dual-task performance as proposed by Navon and Gopher (1979), Sanders (1979) and Wickens (1984). In contrast to a single capacity view IMoray 1967), multiple resource theory assumes more than one commodity with resource-like properties such as specific resource pools for perceptual, central and motor processes (Sanders 1979; Wickens 1984). Several proponents of multiple resource theory have put forward an additional general purpose mechanism (Posner 1985; Keele et al. 1985) which would be especially operational when more tasks are performed * Requests Jagerstrasse
for reprints should be sent to M. Donk, 17/19, 5100 Aachen, FRG.
OOOl-6918/89/$3.50
0 1989, Elsevier Science Publishers
Dept.
of Psychology,
B.V. (North-Holland)
RWTH
Aachen,
222
M. Dank, A.F. Sanders / Resources and dual-task performance
together. Among others, this mechanism is thought to be responsible for coordination and for allocation of task-specific resources (Navon and Gopher 1979; Hunt and Lansman 1981; McLeod 1977) in multiple task conditions. The limited capacity of the general purpose mechanism is shown in costs of concurrence which are quite common when tasks are carried out in combination. One function of the general purpose mechanism might be concerned with timing activities (Pokomy 1985; Noble et al. 1981; Keele et al. 1985; Michon 1967). Pokorny has shown that the occurrence of auditory events affects timing in an interval production task in which the auditory events were embedded (Pokorny 1985). Various interindividual studies (Smith 1957; Keele et al. 1985) have also shown correlations between perceptual discrimination of intervals and the accuracy of producing intervals. These and other results suggest that the mechanism used for encoding perceptual time relations has some commonality with that used for timing interval production. A major issue in any type of resource theory concerns the validity of one of its basic axioms, i.e. task invariance. Task invariance refers to the assumption that the way a task is carried out does not depend on whether it is performed single or in combination with another task. The basic operations in performing each of two tasks together should also not change when their relative priority is manipulated. Different priority rules in dual-task performance should only reflect changes in the amounts of allocated resources to either task (Gopher and Sanders 1984). Alternatively, the operations involved in either task might not be invariant but change in order to achieve minimal interference. The work of Hirst et al. (1980) and Spelke et al. (1976) is a case in point. Their ‘attention-is-a-skill’ hypothesis holds that in dual-task performance some form of task integration develops over time. It follows that extended training in dual tasks should lead to elimination of most dual-task interference. Indeed, studies on extended practice in processing a combination of a visual and an auditory message have shown impressive practice effects (Hirst et al. 1980; Spelke et al. 1976). The more recent structural approaches of Neumann (1987), Navon (1985) and Navon and Miller (1986) also assume that task integration has either the effect of eliminating cross-talk between processing the contents of two simultaneous tasks (Navon and Miller 1986) or of reducing the number of their action goals (Neumann 1987). It is
M. Donk, A. F. Sanders / Resources and dual-task performance
223
important to in this that Neumann well as and Miller the role integration in behaviour. Applied dual-task performance would imply towards integration the tasks. this fails would no attempt performing tasks in and proceed performing through shifting between (Broadbent 1982). this study will be whether timing can be in a of a target detection and a interval production or whether timing aspects both tasks independent. Thus, basic goal a test the task assumption which so essential resource theories. target detection and an production task used both and in The interval task required to generate pre-learned tapping of 550 which has shown to a comfortable rate (Michon In the detection task were shown number of ‘frames’ (Schneider Shiffrin 1977) were presented rapid succession. frame consisted a fixation surrounded by randomly chosen Subjects had task to counts of number of each of predefined target had been The demands the target task were by means the presentation of the the onset-to-onset terval (IFI) 550 msec 750 msec or 1100 (slow); the presentation time a frame always 350 In addition, frames were presented at constant or a variable A single theory predicts under dual-task performance in interval production depends on demands of target detection Michon (1967) assumed that requires information as any kind of The duration intervals has be stored, retrieved, and with other intervals or running clock The effect additional demands from task on production in is supposed be on ‘noise’ generated the subject producing regular (Michon 1967). tapping is to be demanding which that the the presentation of the in the detection task, more irregular should be, of course subjects fully performance in target detection In order emphasise that
224
M. Donk, A.F. Sanders / Resources and dual-task performance
in that Multiple resource usually assume there is resource trade-off tasks which their resources different pools. that tapping target detection entirely different - i.e. vs. perceptual central (Wickens - a resource theory obviously predict tapping perforshould be by an target detection There is the possibility multiple resources an additional consuming mechanism for coordinating in multiple conditions. In case a dual-task decrement expected a decrement is insensitive variations in culty or of either Finally, there the possibility task integration, particular with to synchronising to frame As mentioned, would violate invariance assumption resource theories suggest an of dual-task along the of Navon and Neumann Task integration that the ity of should heavily on the and size IFI. Synchronisation be much to accomplish the constantconditions at speed corresponding the tapping
Method Subjects Nine female and fifteen male subjects between eighteen and thirty years participated in one two-hour session. All subjects had normal or corrected to normal visual acuity and reported to have no earlier experience with any experiment of this kind. All subjects were paid for participation (DM15.00 to DM21.00 depending on performance on the target detection task). Experimental
tasks: The target detection task
A modified version of the task used by Logan (1978) served as the perceptual experimental task. Subjects were seated at a distance of approximately 70 cm from a computer screen on which the stimuli were centrally displayed. A series of successive frames (Schneider and Shiffrin 1977) was presented for one minute. Each frame consisted of a fixation point surrounded by four capital consonants (each letter was 0.6 X 1.0 cm, visual angle = 0.49O; each frame was 1.8 X 3.5 cm, visual angle = 1.47 “).
M. Donk, A.F. Sanders / Resources and dual-task performance
225
Prior to each one-minute trial, two target letters were defined. The task was to detect the occurrence of targets and to count their number. Maximally one target could occur in each frame. During the presentation of the frames, no overt reponses were required. After one minute the screen turned black and the number of detected targets was reported followed by feedback about the actual number. In all conditions the frame-time (Schneider and Shiffrin 1977) was 350 msec. Three onset-to-onset Inter-Frame-Intervals (IFI) were used to create three different presentation rates: (a) fast, IF1 = 550 msec; (b) middle, IF1 = 750 msec; and (c) slow, IF1 = 1100 msec. In addition, IF1 was either variable or constant in one sequence of trials. In the variable case IF1 had a rectangular distribution with a range of f 120 msec and a mean value corresponding to the condition fast, middle or slow. The number of targets in one trial varied randomly between 19 and 31 but the mean number of targets was the same in all conditions. The interval production task The interval production task (Michon 1967) required the subjects to produce a sequence of tapping intervals of 550 msec. Tapping was self-generated with the right index finger on a small metal plate. All tapping intervals were registered and subjects received feedback about their mean and variability. During the training period, subjects synchronised tapping with a series of tones which was presented over headphones. After a number of practice trials, the tones were omitted so that subsequent intervals were self-generated. Prior to each new experimental tapping trial, tapping was always synchronised with a series of tones during ten seconds.
Design A complete within-subjects design was used. The factors ‘variability of IFI’ (constant vs. variable) and ‘presentation rate’ (fast vs. middle vs. slow) were counterbalanced over subjects. All subjects did first single tapping followed by single target detection. Then both tasks were performed together. This sequence of trials was repeated six times for each subject. The fixed sequence of ‘single tapping-single detection-combined tapping and detection’ was preferred so as to guarantee practice in both specific tasks before exposing subjects to the corresponding dual task trial. The disadvantage of this procedure is of course that practice and dual task effects are confounded so that dual task effects might be underestimated. Twenty-four subjects were randomly assigned to six groups of four subjects each. Three groups, e.g. groups I, II, III, started always with a constant-IF1 condition followed by a variable-IF1 condition. The other three groups, e.g. groups IV, V, VI, started with a variable-IF1 condition. For all groups the variable vs. constant conditions alternated. The order of the presentation rates was fast-middle-slow for groups I and IV, middle-slow-fast for groups II and V, slow-fast-middle for groups III and VI.
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Procedure
Two subjects were always tested in alternation. While testing one subject, the other subject had a break. Subjects started with an 18-minute practice session in which they were familiarised with both tasks by written instruction and by performing both tasks single and together. The practice session was followed by a break upon which the actual experiment started. Three sessions were run which also lasted about 18 minutes each and which were separated by 18-minute breaks. In a session, two of the six trial sequences were run. A session always consisted of a variable and a constant condition at one level of the factor presentation rate. Before each sequence the experimenter loaded the program which generated the stimuli and registered the tapping intervals. When the program was loaded subjects received the tone interval of 550 msec during 10 seconds over the headphone. They synchronised their tapping rate with this tone. After ten seconds the tone was stopped while the subject continued tapping: After another ten seconds had passed the tapping intervals were registered during 60 seconds. At the end of this trial subjects received feedback about their performance. After the single tapping trial, subjects performed the target detection task. First, two randomly determined target letters were presented. Following a signal of the experimenter successive frames were presented during 60 seconds at a rate as specified by one of the six conditions. After the last frame had appeared the screen turned black and the subject told the experimenter how many targets had been detected. Finally subjects performed the tapping task and the target detection task together during another 60-second trial. A dual task period started with ten seconds tapping in synchronisation with tones and by a further ten seconds single tapping. Then the presentation of the frames started as well as the registration of the tapping intervals during one minute. At the end, subjects reported how many targets had been detected. This dual-task trial was followed by feedback about the preceding single-task and dual-task trial. A break followed two sequences. Withon one sequence, the presentation rate and the variability of the IFI’s were the same in the single- and the dual-task conditions. With regard to priority, performance on the target detection task was emphasised. Subjects could earn additional money as the numbers of actual and reported targets were more similar. This bonus applied equally to target detection in the single- and in the dual-task trials.
Results Tapping
The results on tapping are presented in table 1. They suggest that the mean tapping interval is somewhat smaller in the dual-task than in the single-task baseline conditions. Furthermore, the variances of the tapping intervals seem to be considerably
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Table 1 Means and mean variances of tapping intervals as a function of single- and dual-task conditions (the single task figures refer to the results of the single runs carried out within the task sequence
concerned). Target detection task
Means of tapping intervals (sd)
Mean variances of tapping intervals (sd)
Presentation rate
IFIvariability
Single
Dual
Single
Dual
Fast
Constant Variable
543 (24) 543 (21)
533 (20) 527 (20)
596 (316) 540 (189)
911 (386) 932 (503)
Middle
Constant Variable
542 (21) 544 (18)
522 (20) 523 (18)
565 (213) 516 (219)
924 (408) 941(477)
Slow
Constant Variable
544 (22) 540 (17)
528 (19) 526 (18)
628 (330) 528 (169)
953 (457) 965 (406)
larger when the interval production task is performed in combination with the target detection task. These impressions were confirmed by an ANOVA. A three factor repeated measurements analysis of variance was performed with the individual mean tapping intervals as cells with load (single- vs. dual-task performance), presentation rate (fast, middle, slow) and IFI-variability (constant vs. variable) as main factors. Mean tapping time interval for the dual-task condition was shorter (526.50 msec) than for the single-task condition (542.67 msec: F(l, 23) = 31.81, p < 0.001). This minor effect on tapping speed might be due to commonly observed effects of additional information processing requirements on interval estimation. Increasing tapping speed could represent an attempt to minimize the variance of tapping under dual-task conditions (Newell 1980). The single-task trials served as baselines for tapping in the dual-task trials. It is not surprising that the results on the single-tapping trials were not significantly different from each other. In order to investigate the effects of presentation rate and IFI-variability on the mean tapping intervals, a two factor analysis of variance was performed on the dual-task data only. Both factors appeared to be marginally significant (F(2, 46) = 3.54, p < 0.05 for the factor presentation rate; and F(1, 23) = 5.05, p < 0.05 for the factor IFI-variability). The Least Significant Difference for the comparison between the presentation rates was 8.30 ((Y = 0.05); The mean tapping interval was significantly larger in the dual-fast-constant condition than in the dual-middle-constant condition (11.13 msec). Other pairwise comparisons with respect to presentation rates were not significant (see table 1). Also, mean tapping interval for the dual-fast-constant condition was 6.64 msec larger than for the dual-fast-variable condition (LSD( LX= 0.01) = 5.48 msec). No other pairwise comparison with respect to IFI-variability produced significant differences. Another three factor repeated measurements analysis of variance was performed on the individual variances in tapping with load (single- vs. dual-task performance),
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presentation rate (fast, middle, slow) and IFI-variability (constant vs. variable) as variables. The only significant effect was a larger mean variance in the dual-task than in the single-task condition; P(1, 23) = 33.31, p < 0.001. A two factor analysis of variance on the dual-task data only did not reveal any significance at all; F(1, 23) = 0.22 for the variable IFI-variability, and F(2, 46) = 0.36 for the variable presentation rate. This implies that in the target detection task neither presentation rate nor IFI-variability affected the variability of the tapping intervals to any significant extent. The target
detection
task
The results on the target detection task are presented in table 2. The dependent variable was the absolute proportion of incorrectly detected and missed targets ( 1actual number of targets - reported number of targets l/actual number of targets). In the single detection trials the overall proportion amounted to 0.12 and in the dual trials to 0.10. A Sign test on these data revealed that the overall error proportion in the single-task trials was significantly larger than in the dual-task trials (p = 0.032, N = 24, x = 7). Pairwise comparisons for each condition showed only a significantly larger proportion in the single-middle-variable condition (p = 0.021, N = 20, x = 5; see table 2). The better performance in the dual conditions could be due to the fixed sequence of single and dual trials so that each dual trial could profit from further practice gained at the preceding single trial. The effect may mainly reflect practicing the target set for that sequence. It seems clear, though, that subjects obeyed the instructions to give high priority to target detection. As anticipated, performance in the target detection task depends on presentation rate; the fast conditions produced an overall error rate of 0.19, the middle conditions 0.10, and the slow conditions 0.06. All pairwise comparisons were highly significant
Table 2 Proportion of errors in the target detection task for all conditions. Presentation rate
IFIvariability
Single
Dual
Significance level
Fast
Constant Variable
0.21 0.21
0.18 0.16
0.202 0.076
Middle
Constant Variable
0.14 0.11
0.07 0.07
0.067 0.021=
Slow
Constant Variable
0.05 0.06
0.06 0.05
0.332 0.412
a Significant at 0.05 level.
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(fast-middle: p ~0.001, N= 24, x = 1; middle-slow; p =O.OOl, fast-slow: p < 0.001, N = 24, x = 0). Additional Sign tests revealed no significant differences in error tween the constant and variable conditions neither under single- nor requirements. Thus, the variability of IF1 did not affect the processing target detection task.
N=
24,
x= 4;
proportions beunder dual-task demands of the
Discussion The experiment has three main outcomes. First, there is a decrease of tapping regularity under all dual-task conditions, irrespective of presentation rate and IFI-variability of the target detection task. Second, target detection task performance was even somewhat better under dual- than under single-task conditions suggesting that it had indeed high priority. Finally, as anticipated, performance in the target detection task depends on the presentation rate of the frames. Thus tapping regularity was unaffected by the demands of the target detection task. This result is clearly at odds with the suggestion that tapping regularity can serve as a general measure of perceptual-motorload (Michon 1967). It also follows that a single resource explanation in the sense of Moray (1967) is inappropriate. There was also a lack of support for the notion of task integration. Neumann (1987) has proposed that the main problem in time sharing is not to combine stimuli or actions, but rather to deal with them independently. Yet, the present study shows that the tapping variances were unrelated to IFI-variability, which suggests independent processing of the perceptual time relations and the production of intervals. Admittedly, the frames of the target detection task were not synchronised at the beginning of each dual trial with the subject-generated taps. There were, however, ample opportunities to reach synchronisation by means of minor adaptations. In fact, subjects were even not aware of the possibility of synchronising. Thus, task integration seems less self-evident than anticipated. It is possible that different results would have been obtained if subjects had been explicitly instructed to synchronise their taps with frame presentation. It is also possible that the present timing demands are too simple. Task integration may be mandatory when more complex dual tasks are involved or when subjects have received more extensive training. A major question that
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remains to be answered is whether and under what circumstances task integration occurs. Yet, at the simple conditions of this study it seems no necessary element of dual-task performance. As pointed out in the introduction, the absence of a performance exchange relation between tapping and target detection supports an interpretation of multiple resources (Navon and Gopher 1979; Sanders 1979; Wickens 1984). The target detection task seems to tap its resources from a perceptual and probably a central pool while the tapping task seems to tap a motor resource. The combination of target detection and tapping was chosen so as to avoid problems of structural interference (Kahneman 1973) at either the input or the output level. It follows that the observed costs of concurrence are determined by resource limitations, presumably due to an additional general purpose mechanism which is only relevant in dual-task conditions (Wickens 1984). This resource might be invoked wherever activities must be coordinated or when allocation of different resource types is needed. Yet, the suggestion that it also is concerned with timing activities of various nature seems not confirmed. In a study in which two motor activities were time-shared, performance was less as the temporal periods of left- and right-hand responses were not harmonically .related (Klapp 1979). Interference between different effecters following different temporal courses, suggests a common time-keeping problem. Yet, this might be fully due to motor resources and does not require the assumption of a general purpose mechanism. A different conclusion might follow from the work of Pokorny (1985) who found that the occurrence of perceptual events affects timing in motor production. Presenting tones during intertap intervals affects size and regularity of the tapping intervals. Pokorny concluded that the mechanism used for encoding the temporal properties of tones has some commonality with that used for timing interval production. The absence of a similar effect in this study suggests that encoding time properties of perceptual events is not necessarily coupled to timing motor activity. The divergence between our results and those of Pokorny might be due to a stronger predominance of temporal properties in auditory than in visual presentation (Posner 1978). Auditory stimulation might impose stronger constraints for simultaneous motoractivity which implies stronger demands on the coordinating mechanism. Direct comparisons between auditory and visual signal sequences on
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231
the one hand and interval production on the other hand is surely required. Summarizing, the results from this study favour a multiple resource view in the sense of Wickens (1984) in which the target detection task taps its resources from a perceptual and a central pool and the interval production task from a motor capacity. Furthermore, both tasks have a dual-task-specific mechanism in common. The limited capacity of this mechanism is probably causing the costs of concurrence. Finally, it does not seem that this mechanism is concerned with timing activities. The question with respect to the nature of this mechanism needs further investigation.
References Broadbent, D., 1982. Task combination and the selective intake of information. Acta Psychologica 50, 253-290. Oh stages! Oh resources!’ In: W. Prinz and A.F. Gopher, D. and A.F. Sanders, 1984. “‘S-Oh-R”: Sanders (eds.), Cognition and motor processes. Berlin: Springer. pp. 231-253. Hirst, W., E.S. Spelke, CC. Reaves, G. Coharack and U. Neisser, 1980. Dividing attention without alternation or automaticity. Journal of Experimental Psychology: General 109,98-117. Hunt, E. and M. Lansman, 1981. ‘Individual differences in attention’. In: R. Stemberg (ed.), Advances in the psychology of intelligence, Vol. 1. Hillsdale, NJ: Erlbaum. Kahneman, D., 1973. Attention and effort. Englewood Cliffs, NJ: Prentice Hall. Keele, S.W., R.A. Pokomy, D.M. Corcos and R. Ivry, 1985. Do perception and motor production share common timing mechanisms: A correlational analysis. Acta Psychologica 60, 173-191. Klapp, ST., 1979. Doing two things at once: The role of temporal compatibility. Memory and Cognition 7, 375-381. Logan, G.D., 1978. Attention in character-classification tasks: Evidence for the automaticity of component stages. Journal of Experimental Psychology: General 107, 32-63. McLeod, P., 1977. A dual task response modality effect: Support for multi-processor models of attention. Quarterly Journal of Experimental Psychology 29, 651-667. Michon, J.A., 1967. Timing in temporal tracking. Soesterberg: Institute for Perception-TNO. Moray, N., 1967. Where is attention limited. A survey and a model. Acta Psychologica 27, 84-92. Navon, D., 1985. ‘Attention division or attention sharing’. In: M.I. Posner and O.S.M. Matin (eds.), Attention and performance XI. Hillsdale, NJ: Erlbaum. Navon, D. and D. Gopher, 1979. On the economy of the human processing system. Psychological Review 86, 214-255. Navon, D. and J. Miller, 1986. The role of outcome conflict in dual-task interference (ICS Report 8601). San Diego, CA: Institute for Cognitive Science. Neumann, O., 1987. ‘Beyond capacity: A functional view of attention’. In: H. Heuer and A.F. Sanders (eds.), Perspectives on perception and action. Hillsdale, NJ: Erlbaum. Newell, K.M., 1980. ‘The speed-accuracy paradox in movement control: Errors of time and space’. In: G.E. Stelmach and J. Requin (eds.), Tutorials in motor behavior. Amsterdam: North-Holland.
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Noble, M.E., A.F. Sanders and D.A. Trumbo, 1981. Concurrence costs in double stimulation tasks. Acta Psychologica 49, 141-158. Pokomy, R.A., 1985. Searching for interaction between timing of motor tasks and timing of perceptual tasks. Doctoral dissertation, University of Oregon, Eugene, OR. Posner, MI., 1978. Chronometric explorations of mind. Hillsdale, NJ: Erlbaum. Posner, MI., 1985. Hierarchical distributed networks in the neuropsychology of selective attention. Technical Report No. 85-1, Cognitive Science Program, University of Oregon, Eugene, OR. Sanders, A.F., 1979. ‘Some remarks on mental load’. In: N. Moray (ed.), Mental workload: Its theory and measurement. New York: Plenum Press. Schneider, W. and R.M. Shiffrin, 1977. Controlled and automatic human information processing: I. Detection, search, and attention. Psychological Review 84, l-66. Smith, O.W., 1957. Relationship of rhythm discrimination to motor rhythm performance. Journal of Applied Psychology 41, 365-369. Spelke, E., W. Hirst and U. Neisser, 1976. Skills of divided attention. Cognition 4, 215-230. Wickens, C.D., 1984. ‘Processing resources in attention’. In: R. Parasuraman and D.R. Davies (eds.), Varieties of attention. New York: Academic Press. pp. 63-102.
72 (1989) 221-232
221
RESOURCES AND DUAL-TASK PERFORMANCE; RESOURCE ALLOCATION VERSUS TASK INTEGRATION
*
Mieke DONK and Andries F. SANDERS R WTH Aachen, FRG
Accepted
February
1989
This study aims at contributing to the explanation of dual-task performance in terms of either resource allocation or of task interference and integration. Twenty-four subjects carried out, single and in combination, a motor interval production task and a perceptual target detection task on the basis of combined memory and display search. The demands of the target detection task were varied by increasing or decreasing the presentation rate of successive search displays. Furthermore, the presentation rate was either constant or variable. The dual-task condition had a negative effect on interval production, the extent of which was unaffected by either rate or variability of display presentation. This means that there was no evidence for synchronising interval production with display presentation, so that the major opportunity for task integration did not substantiate. It is suggested that the two tasks use different resource pools in addition to a common mechanism, the limited capacity of which causes a general interference in dual-task conditions.
Introduction This article is concerned with an empirical test of some predictions derived from the multiple resource theory of dual-task performance as proposed by Navon and Gopher (1979), Sanders (1979) and Wickens (1984). In contrast to a single capacity view IMoray 1967), multiple resource theory assumes more than one commodity with resource-like properties such as specific resource pools for perceptual, central and motor processes (Sanders 1979; Wickens 1984). Several proponents of multiple resource theory have put forward an additional general purpose mechanism (Posner 1985; Keele et al. 1985) which would be especially operational when more tasks are performed * Requests Jagerstrasse
for reprints should be sent to M. Donk, 17/19, 5100 Aachen, FRG.
OOOl-6918/89/$3.50
0 1989, Elsevier Science Publishers
Dept.
of Psychology,
B.V. (North-Holland)
RWTH
Aachen,
222
M. Dank, A.F. Sanders / Resources and dual-task performance
together. Among others, this mechanism is thought to be responsible for coordination and for allocation of task-specific resources (Navon and Gopher 1979; Hunt and Lansman 1981; McLeod 1977) in multiple task conditions. The limited capacity of the general purpose mechanism is shown in costs of concurrence which are quite common when tasks are carried out in combination. One function of the general purpose mechanism might be concerned with timing activities (Pokomy 1985; Noble et al. 1981; Keele et al. 1985; Michon 1967). Pokorny has shown that the occurrence of auditory events affects timing in an interval production task in which the auditory events were embedded (Pokorny 1985). Various interindividual studies (Smith 1957; Keele et al. 1985) have also shown correlations between perceptual discrimination of intervals and the accuracy of producing intervals. These and other results suggest that the mechanism used for encoding perceptual time relations has some commonality with that used for timing interval production. A major issue in any type of resource theory concerns the validity of one of its basic axioms, i.e. task invariance. Task invariance refers to the assumption that the way a task is carried out does not depend on whether it is performed single or in combination with another task. The basic operations in performing each of two tasks together should also not change when their relative priority is manipulated. Different priority rules in dual-task performance should only reflect changes in the amounts of allocated resources to either task (Gopher and Sanders 1984). Alternatively, the operations involved in either task might not be invariant but change in order to achieve minimal interference. The work of Hirst et al. (1980) and Spelke et al. (1976) is a case in point. Their ‘attention-is-a-skill’ hypothesis holds that in dual-task performance some form of task integration develops over time. It follows that extended training in dual tasks should lead to elimination of most dual-task interference. Indeed, studies on extended practice in processing a combination of a visual and an auditory message have shown impressive practice effects (Hirst et al. 1980; Spelke et al. 1976). The more recent structural approaches of Neumann (1987), Navon (1985) and Navon and Miller (1986) also assume that task integration has either the effect of eliminating cross-talk between processing the contents of two simultaneous tasks (Navon and Miller 1986) or of reducing the number of their action goals (Neumann 1987). It is
M. Donk, A. F. Sanders / Resources and dual-task performance
223
important to in this that Neumann well as and Miller the role integration in behaviour. Applied dual-task performance would imply towards integration the tasks. this fails would no attempt performing tasks in and proceed performing through shifting between (Broadbent 1982). this study will be whether timing can be in a of a target detection and a interval production or whether timing aspects both tasks independent. Thus, basic goal a test the task assumption which so essential resource theories. target detection and an production task used both and in The interval task required to generate pre-learned tapping of 550 which has shown to a comfortable rate (Michon In the detection task were shown number of ‘frames’ (Schneider Shiffrin 1977) were presented rapid succession. frame consisted a fixation surrounded by randomly chosen Subjects had task to counts of number of each of predefined target had been The demands the target task were by means the presentation of the the onset-to-onset terval (IFI) 550 msec 750 msec or 1100 (slow); the presentation time a frame always 350 In addition, frames were presented at constant or a variable A single theory predicts under dual-task performance in interval production depends on demands of target detection Michon (1967) assumed that requires information as any kind of The duration intervals has be stored, retrieved, and with other intervals or running clock The effect additional demands from task on production in is supposed be on ‘noise’ generated the subject producing regular (Michon 1967). tapping is to be demanding which that the the presentation of the in the detection task, more irregular should be, of course subjects fully performance in target detection In order emphasise that
224
M. Donk, A.F. Sanders / Resources and dual-task performance
in that Multiple resource usually assume there is resource trade-off tasks which their resources different pools. that tapping target detection entirely different - i.e. vs. perceptual central (Wickens - a resource theory obviously predict tapping perforshould be by an target detection There is the possibility multiple resources an additional consuming mechanism for coordinating in multiple conditions. In case a dual-task decrement expected a decrement is insensitive variations in culty or of either Finally, there the possibility task integration, particular with to synchronising to frame As mentioned, would violate invariance assumption resource theories suggest an of dual-task along the of Navon and Neumann Task integration that the ity of should heavily on the and size IFI. Synchronisation be much to accomplish the constantconditions at speed corresponding the tapping
Method Subjects Nine female and fifteen male subjects between eighteen and thirty years participated in one two-hour session. All subjects had normal or corrected to normal visual acuity and reported to have no earlier experience with any experiment of this kind. All subjects were paid for participation (DM15.00 to DM21.00 depending on performance on the target detection task). Experimental
tasks: The target detection task
A modified version of the task used by Logan (1978) served as the perceptual experimental task. Subjects were seated at a distance of approximately 70 cm from a computer screen on which the stimuli were centrally displayed. A series of successive frames (Schneider and Shiffrin 1977) was presented for one minute. Each frame consisted of a fixation point surrounded by four capital consonants (each letter was 0.6 X 1.0 cm, visual angle = 0.49O; each frame was 1.8 X 3.5 cm, visual angle = 1.47 “).
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Prior to each one-minute trial, two target letters were defined. The task was to detect the occurrence of targets and to count their number. Maximally one target could occur in each frame. During the presentation of the frames, no overt reponses were required. After one minute the screen turned black and the number of detected targets was reported followed by feedback about the actual number. In all conditions the frame-time (Schneider and Shiffrin 1977) was 350 msec. Three onset-to-onset Inter-Frame-Intervals (IFI) were used to create three different presentation rates: (a) fast, IF1 = 550 msec; (b) middle, IF1 = 750 msec; and (c) slow, IF1 = 1100 msec. In addition, IF1 was either variable or constant in one sequence of trials. In the variable case IF1 had a rectangular distribution with a range of f 120 msec and a mean value corresponding to the condition fast, middle or slow. The number of targets in one trial varied randomly between 19 and 31 but the mean number of targets was the same in all conditions. The interval production task The interval production task (Michon 1967) required the subjects to produce a sequence of tapping intervals of 550 msec. Tapping was self-generated with the right index finger on a small metal plate. All tapping intervals were registered and subjects received feedback about their mean and variability. During the training period, subjects synchronised tapping with a series of tones which was presented over headphones. After a number of practice trials, the tones were omitted so that subsequent intervals were self-generated. Prior to each new experimental tapping trial, tapping was always synchronised with a series of tones during ten seconds.
Design A complete within-subjects design was used. The factors ‘variability of IFI’ (constant vs. variable) and ‘presentation rate’ (fast vs. middle vs. slow) were counterbalanced over subjects. All subjects did first single tapping followed by single target detection. Then both tasks were performed together. This sequence of trials was repeated six times for each subject. The fixed sequence of ‘single tapping-single detection-combined tapping and detection’ was preferred so as to guarantee practice in both specific tasks before exposing subjects to the corresponding dual task trial. The disadvantage of this procedure is of course that practice and dual task effects are confounded so that dual task effects might be underestimated. Twenty-four subjects were randomly assigned to six groups of four subjects each. Three groups, e.g. groups I, II, III, started always with a constant-IF1 condition followed by a variable-IF1 condition. The other three groups, e.g. groups IV, V, VI, started with a variable-IF1 condition. For all groups the variable vs. constant conditions alternated. The order of the presentation rates was fast-middle-slow for groups I and IV, middle-slow-fast for groups II and V, slow-fast-middle for groups III and VI.
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Procedure
Two subjects were always tested in alternation. While testing one subject, the other subject had a break. Subjects started with an 18-minute practice session in which they were familiarised with both tasks by written instruction and by performing both tasks single and together. The practice session was followed by a break upon which the actual experiment started. Three sessions were run which also lasted about 18 minutes each and which were separated by 18-minute breaks. In a session, two of the six trial sequences were run. A session always consisted of a variable and a constant condition at one level of the factor presentation rate. Before each sequence the experimenter loaded the program which generated the stimuli and registered the tapping intervals. When the program was loaded subjects received the tone interval of 550 msec during 10 seconds over the headphone. They synchronised their tapping rate with this tone. After ten seconds the tone was stopped while the subject continued tapping: After another ten seconds had passed the tapping intervals were registered during 60 seconds. At the end of this trial subjects received feedback about their performance. After the single tapping trial, subjects performed the target detection task. First, two randomly determined target letters were presented. Following a signal of the experimenter successive frames were presented during 60 seconds at a rate as specified by one of the six conditions. After the last frame had appeared the screen turned black and the subject told the experimenter how many targets had been detected. Finally subjects performed the tapping task and the target detection task together during another 60-second trial. A dual task period started with ten seconds tapping in synchronisation with tones and by a further ten seconds single tapping. Then the presentation of the frames started as well as the registration of the tapping intervals during one minute. At the end, subjects reported how many targets had been detected. This dual-task trial was followed by feedback about the preceding single-task and dual-task trial. A break followed two sequences. Withon one sequence, the presentation rate and the variability of the IFI’s were the same in the single- and the dual-task conditions. With regard to priority, performance on the target detection task was emphasised. Subjects could earn additional money as the numbers of actual and reported targets were more similar. This bonus applied equally to target detection in the single- and in the dual-task trials.
Results Tapping
The results on tapping are presented in table 1. They suggest that the mean tapping interval is somewhat smaller in the dual-task than in the single-task baseline conditions. Furthermore, the variances of the tapping intervals seem to be considerably
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Table 1 Means and mean variances of tapping intervals as a function of single- and dual-task conditions (the single task figures refer to the results of the single runs carried out within the task sequence
concerned). Target detection task
Means of tapping intervals (sd)
Mean variances of tapping intervals (sd)
Presentation rate
IFIvariability
Single
Dual
Single
Dual
Fast
Constant Variable
543 (24) 543 (21)
533 (20) 527 (20)
596 (316) 540 (189)
911 (386) 932 (503)
Middle
Constant Variable
542 (21) 544 (18)
522 (20) 523 (18)
565 (213) 516 (219)
924 (408) 941(477)
Slow
Constant Variable
544 (22) 540 (17)
528 (19) 526 (18)
628 (330) 528 (169)
953 (457) 965 (406)
larger when the interval production task is performed in combination with the target detection task. These impressions were confirmed by an ANOVA. A three factor repeated measurements analysis of variance was performed with the individual mean tapping intervals as cells with load (single- vs. dual-task performance), presentation rate (fast, middle, slow) and IFI-variability (constant vs. variable) as main factors. Mean tapping time interval for the dual-task condition was shorter (526.50 msec) than for the single-task condition (542.67 msec: F(l, 23) = 31.81, p < 0.001). This minor effect on tapping speed might be due to commonly observed effects of additional information processing requirements on interval estimation. Increasing tapping speed could represent an attempt to minimize the variance of tapping under dual-task conditions (Newell 1980). The single-task trials served as baselines for tapping in the dual-task trials. It is not surprising that the results on the single-tapping trials were not significantly different from each other. In order to investigate the effects of presentation rate and IFI-variability on the mean tapping intervals, a two factor analysis of variance was performed on the dual-task data only. Both factors appeared to be marginally significant (F(2, 46) = 3.54, p < 0.05 for the factor presentation rate; and F(1, 23) = 5.05, p < 0.05 for the factor IFI-variability). The Least Significant Difference for the comparison between the presentation rates was 8.30 ((Y = 0.05); The mean tapping interval was significantly larger in the dual-fast-constant condition than in the dual-middle-constant condition (11.13 msec). Other pairwise comparisons with respect to presentation rates were not significant (see table 1). Also, mean tapping interval for the dual-fast-constant condition was 6.64 msec larger than for the dual-fast-variable condition (LSD( LX= 0.01) = 5.48 msec). No other pairwise comparison with respect to IFI-variability produced significant differences. Another three factor repeated measurements analysis of variance was performed on the individual variances in tapping with load (single- vs. dual-task performance),
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presentation rate (fast, middle, slow) and IFI-variability (constant vs. variable) as variables. The only significant effect was a larger mean variance in the dual-task than in the single-task condition; P(1, 23) = 33.31, p < 0.001. A two factor analysis of variance on the dual-task data only did not reveal any significance at all; F(1, 23) = 0.22 for the variable IFI-variability, and F(2, 46) = 0.36 for the variable presentation rate. This implies that in the target detection task neither presentation rate nor IFI-variability affected the variability of the tapping intervals to any significant extent. The target
detection
task
The results on the target detection task are presented in table 2. The dependent variable was the absolute proportion of incorrectly detected and missed targets ( 1actual number of targets - reported number of targets l/actual number of targets). In the single detection trials the overall proportion amounted to 0.12 and in the dual trials to 0.10. A Sign test on these data revealed that the overall error proportion in the single-task trials was significantly larger than in the dual-task trials (p = 0.032, N = 24, x = 7). Pairwise comparisons for each condition showed only a significantly larger proportion in the single-middle-variable condition (p = 0.021, N = 20, x = 5; see table 2). The better performance in the dual conditions could be due to the fixed sequence of single and dual trials so that each dual trial could profit from further practice gained at the preceding single trial. The effect may mainly reflect practicing the target set for that sequence. It seems clear, though, that subjects obeyed the instructions to give high priority to target detection. As anticipated, performance in the target detection task depends on presentation rate; the fast conditions produced an overall error rate of 0.19, the middle conditions 0.10, and the slow conditions 0.06. All pairwise comparisons were highly significant
Table 2 Proportion of errors in the target detection task for all conditions. Presentation rate
IFIvariability
Single
Dual
Significance level
Fast
Constant Variable
0.21 0.21
0.18 0.16
0.202 0.076
Middle
Constant Variable
0.14 0.11
0.07 0.07
0.067 0.021=
Slow
Constant Variable
0.05 0.06
0.06 0.05
0.332 0.412
a Significant at 0.05 level.
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(fast-middle: p ~0.001, N= 24, x = 1; middle-slow; p =O.OOl, fast-slow: p < 0.001, N = 24, x = 0). Additional Sign tests revealed no significant differences in error tween the constant and variable conditions neither under single- nor requirements. Thus, the variability of IF1 did not affect the processing target detection task.
N=
24,
x= 4;
proportions beunder dual-task demands of the
Discussion The experiment has three main outcomes. First, there is a decrease of tapping regularity under all dual-task conditions, irrespective of presentation rate and IFI-variability of the target detection task. Second, target detection task performance was even somewhat better under dual- than under single-task conditions suggesting that it had indeed high priority. Finally, as anticipated, performance in the target detection task depends on the presentation rate of the frames. Thus tapping regularity was unaffected by the demands of the target detection task. This result is clearly at odds with the suggestion that tapping regularity can serve as a general measure of perceptual-motorload (Michon 1967). It also follows that a single resource explanation in the sense of Moray (1967) is inappropriate. There was also a lack of support for the notion of task integration. Neumann (1987) has proposed that the main problem in time sharing is not to combine stimuli or actions, but rather to deal with them independently. Yet, the present study shows that the tapping variances were unrelated to IFI-variability, which suggests independent processing of the perceptual time relations and the production of intervals. Admittedly, the frames of the target detection task were not synchronised at the beginning of each dual trial with the subject-generated taps. There were, however, ample opportunities to reach synchronisation by means of minor adaptations. In fact, subjects were even not aware of the possibility of synchronising. Thus, task integration seems less self-evident than anticipated. It is possible that different results would have been obtained if subjects had been explicitly instructed to synchronise their taps with frame presentation. It is also possible that the present timing demands are too simple. Task integration may be mandatory when more complex dual tasks are involved or when subjects have received more extensive training. A major question that
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remains to be answered is whether and under what circumstances task integration occurs. Yet, at the simple conditions of this study it seems no necessary element of dual-task performance. As pointed out in the introduction, the absence of a performance exchange relation between tapping and target detection supports an interpretation of multiple resources (Navon and Gopher 1979; Sanders 1979; Wickens 1984). The target detection task seems to tap its resources from a perceptual and probably a central pool while the tapping task seems to tap a motor resource. The combination of target detection and tapping was chosen so as to avoid problems of structural interference (Kahneman 1973) at either the input or the output level. It follows that the observed costs of concurrence are determined by resource limitations, presumably due to an additional general purpose mechanism which is only relevant in dual-task conditions (Wickens 1984). This resource might be invoked wherever activities must be coordinated or when allocation of different resource types is needed. Yet, the suggestion that it also is concerned with timing activities of various nature seems not confirmed. In a study in which two motor activities were time-shared, performance was less as the temporal periods of left- and right-hand responses were not harmonically .related (Klapp 1979). Interference between different effecters following different temporal courses, suggests a common time-keeping problem. Yet, this might be fully due to motor resources and does not require the assumption of a general purpose mechanism. A different conclusion might follow from the work of Pokorny (1985) who found that the occurrence of perceptual events affects timing in motor production. Presenting tones during intertap intervals affects size and regularity of the tapping intervals. Pokorny concluded that the mechanism used for encoding the temporal properties of tones has some commonality with that used for timing interval production. The absence of a similar effect in this study suggests that encoding time properties of perceptual events is not necessarily coupled to timing motor activity. The divergence between our results and those of Pokorny might be due to a stronger predominance of temporal properties in auditory than in visual presentation (Posner 1978). Auditory stimulation might impose stronger constraints for simultaneous motoractivity which implies stronger demands on the coordinating mechanism. Direct comparisons between auditory and visual signal sequences on
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the one hand and interval production on the other hand is surely required. Summarizing, the results from this study favour a multiple resource view in the sense of Wickens (1984) in which the target detection task taps its resources from a perceptual and a central pool and the interval production task from a motor capacity. Furthermore, both tasks have a dual-task-specific mechanism in common. The limited capacity of this mechanism is probably causing the costs of concurrence. Finally, it does not seem that this mechanism is concerned with timing activities. The question with respect to the nature of this mechanism needs further investigation.
References Broadbent, D., 1982. Task combination and the selective intake of information. Acta Psychologica 50, 253-290. Oh stages! Oh resources!’ In: W. Prinz and A.F. Gopher, D. and A.F. Sanders, 1984. “‘S-Oh-R”: Sanders (eds.), Cognition and motor processes. Berlin: Springer. pp. 231-253. Hirst, W., E.S. Spelke, CC. Reaves, G. Coharack and U. Neisser, 1980. Dividing attention without alternation or automaticity. Journal of Experimental Psychology: General 109,98-117. Hunt, E. and M. Lansman, 1981. ‘Individual differences in attention’. In: R. Stemberg (ed.), Advances in the psychology of intelligence, Vol. 1. Hillsdale, NJ: Erlbaum. Kahneman, D., 1973. Attention and effort. Englewood Cliffs, NJ: Prentice Hall. Keele, S.W., R.A. Pokomy, D.M. Corcos and R. Ivry, 1985. Do perception and motor production share common timing mechanisms: A correlational analysis. Acta Psychologica 60, 173-191. Klapp, ST., 1979. Doing two things at once: The role of temporal compatibility. Memory and Cognition 7, 375-381. Logan, G.D., 1978. Attention in character-classification tasks: Evidence for the automaticity of component stages. Journal of Experimental Psychology: General 107, 32-63. McLeod, P., 1977. A dual task response modality effect: Support for multi-processor models of attention. Quarterly Journal of Experimental Psychology 29, 651-667. Michon, J.A., 1967. Timing in temporal tracking. Soesterberg: Institute for Perception-TNO. Moray, N., 1967. Where is attention limited. A survey and a model. Acta Psychologica 27, 84-92. Navon, D., 1985. ‘Attention division or attention sharing’. In: M.I. Posner and O.S.M. Matin (eds.), Attention and performance XI. Hillsdale, NJ: Erlbaum. Navon, D. and D. Gopher, 1979. On the economy of the human processing system. Psychological Review 86, 214-255. Navon, D. and J. Miller, 1986. The role of outcome conflict in dual-task interference (ICS Report 8601). San Diego, CA: Institute for Cognitive Science. Neumann, O., 1987. ‘Beyond capacity: A functional view of attention’. In: H. Heuer and A.F. Sanders (eds.), Perspectives on perception and action. Hillsdale, NJ: Erlbaum. Newell, K.M., 1980. ‘The speed-accuracy paradox in movement control: Errors of time and space’. In: G.E. Stelmach and J. Requin (eds.), Tutorials in motor behavior. Amsterdam: North-Holland.
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Noble, M.E., A.F. Sanders and D.A. Trumbo, 1981. Concurrence costs in double stimulation tasks. Acta Psychologica 49, 141-158. Pokomy, R.A., 1985. Searching for interaction between timing of motor tasks and timing of perceptual tasks. Doctoral dissertation, University of Oregon, Eugene, OR. Posner, MI., 1978. Chronometric explorations of mind. Hillsdale, NJ: Erlbaum. Posner, MI., 1985. Hierarchical distributed networks in the neuropsychology of selective attention. Technical Report No. 85-1, Cognitive Science Program, University of Oregon, Eugene, OR. Sanders, A.F., 1979. ‘Some remarks on mental load’. In: N. Moray (ed.), Mental workload: Its theory and measurement. New York: Plenum Press. Schneider, W. and R.M. Shiffrin, 1977. Controlled and automatic human information processing: I. Detection, search, and attention. Psychological Review 84, l-66. Smith, O.W., 1957. Relationship of rhythm discrimination to motor rhythm performance. Journal of Applied Psychology 41, 365-369. Spelke, E., W. Hirst and U. Neisser, 1976. Skills of divided attention. Cognition 4, 215-230. Wickens, C.D., 1984. ‘Processing resources in attention’. In: R. Parasuraman and D.R. Davies (eds.), Varieties of attention. New York: Academic Press. pp. 63-102.