Dual-task interference as a function of cognitive processing load

Ada Psychologica 43 (1979) 71-84 8 North-Holland publishing Company

DUAL-TASK INTERFERENCE NITIVE PROCESSING LOAD

AS A FUNCTION

OF COG-

Leslie A. WHITAKER* Naval Submarine MedicalResearch Revised version received January

Laboratory, Groton, Connecticut. U.S..4. 1973

Sixteen su’>jects from the Naval Submarine Medical Research Laboratory participated in a dual-task study designed to measure processing requirements of a choice reaction time (RT) task. Twolevels of choice RT stimulus-response (S-R)compatibiiity were tested with each of two tracking tasks to provide different levels of dual-task loading. In one tracking task, the target’s temporal-spatial pattern was fiied; in the other, the target’s path was a function of the subject’s performance. In the choice RT task, compatibility was treated as a between-subjects factor, while the number of alternatives (set size) within a sequence was a within-subjects variable. Choice RT results indicated that earnpatibility and set size interacted; the increase in response latency as a function of set size was much greater when compatibility was low. An increase in choice RT response latency occurred when the secondary tracking task was added. Within a given compatibility level, this dual-task decrement was constant for ah levels of set sire; however, the magnitude of the dual-task decrement varied as a function of S-R compatibility. being greater when compatibility was low than when it was high. For these data, a model like Sternberg’s (1969) stagesmodel isseen to have more explanatory value than a pooled processing capacity model (e.g., Norman and Bobrow 1975).

Current models of human information processing may be classified according to their mechanisms for resource or attention allocation. In one ‘classof models, attention is described as essentially a unitary resource and tasks which require attention will show decrements in performance when the task demands exceed the available capacity of this resource. This group of models is illustrated by Moray (1967), Kahneman ( 1973), *

This research was supported by a National Council Post-doctoral Associateship and sponsored by the Naval Medical Research and Development Command. The opinions and assertions contained in thii article are the private ones of the author and should not be construed as official or as reflecting the views of the Navy Department. The author wishes to thank George Moeiler for his comments on an earlier draft of this paper. Requests for reprints should be sent to Leslie Whitaker, i’sychob;V, UniversitY of Missouri, St. Louis, MO 63121, U.S.A.

72

t. A. Whitaker ,‘DuaEtask interference

and Norman and Bobrow (1975) and models of this type may be called ‘pooled capacity’ models of information processing. In the second class of models, attention is presented as a resource which is available independently for each of several processing requirements. When task demaads overload the system, th.?y do so at one or several specific stages. Decrements in performance will be sensitive indicators of which stages are affected by which task requirements. Models which exemplify this group are Sternberg (1969) and Kaniowitz and Knight (1976). These models are known as ‘stages’ models of information processing. A test of the predictions from each of these c!assc~ ~8; ‘;z GXC!X~P~ by the use of concurrent tasks. Experiments employing dual-task paradigms compare the performance of each task by itself with its performance while a concurrent task is being executed. Such designs are likely to provide sensitive measures of processing requirements because a subject is required to employ all available processing capacity in the dual-task condition in an attempt to equal his or her perforflagce level in the comparable single task condition (Trumbo 1973). The pooled capacity models predict that processing capacity mati, available for one task or one requirement of a task will not be available for other processing requirements. Kantowitz and Knight (1976), in developing their hybrid processing model (a member of the class of ‘stages’ models), have very clearly described the necessary prediction of pooled capacity models; that is, varying the processing requirements of one task while holding the requirements of a concurrent task constant will result in an interaction. The decrement resulting from dual-task requirements will be greater when the first task is already requiring greater amounts of the limited attention resource. It should be noted that it is possible to combine two tasks which require SO little attention that their joint demands do not exceed the capacity available. In this case, either no dual-task decrement would be found or the decrement would he equal for all levels of task requirements. [A uniform (or additive) decrement could also be attributed to peripheral (input-output) interfercnca rather than to competing demands upon the attentional capaLy.1 If the dual-task decrement is found to be a function of processing requirements of one task, then Pooled capacity models predict that the available capacity is being exceeded and the situation should be sensitive to all variations in processi.ng demands. That is, dual-task decrement should be an increasing function of processing demands, regardless of the task component

L. A. Wihker

/Dud-task

interf‘erence

i3

which is varied to increase that processing requirement. In summary, the prediction of the pooled capacity models is that a significant interaction between the factor Single/Dual and one task variable implies that. an interaction will be found between the facttir Single/Duai and all task variables affecting processing requirements. Stages models can make the same prediction, but they can also explain a different set of results. If dual-task decrement interacts with one variable, it need not interact with all such variables. That is, dual-task loading may result in an mteraction with levels of one task variable, but this loading could result in a constant desrement at all levels of a second task variable. Both the interaction and the additive effects can be explained because stages models assume that capacity is allocated separately to indepzcdent stages. Increasing the processing demands in one stage may overlap wiih the demands from the concurrent task, hence an interaction. In the case of the second processing variable, there may be no overlap with the concurrent task and hence no evidence of an interaction resulting from competing demands. A specific test of these contrasting predictions was condlicted in the present experiment. Two types of tasks were executed: a choic; reaction time (choice RT) and a pursuit tracking task. In the choice RT task, set size and S-R compatibility were varied. In general, choice RT increases linearly as a function of the average information in the stimuli presented (e.g., Hick 1952). However, the compatibility of the stimulus-response (S-R) pairings can determine the slope of this function. An example of a highly compatible S-R pairing is the task of responding by saying the same word that is heard as the stimulus (Broadbent and Gregory 1962). Davis et al. (1961) found that, for this shadowing task, choice RT did not increase as a function of stimulus set size. The role of S--R compatibility has been discussed in models of human information processing (e.g., Broadbent 197 1; Teichner and Krebs 1974) and, despite disagreement about the specific locus of the change, various models propose that some’ central processing requirements are decreased, or possibly eliminated, when S-R pairings are highly compatible (Sanders 1967). Two levels of a pursuit tracking task were combined with the choice RT tasks in a dual-task program. One tracking pattern consisted of a predictable step function.’ In order for the subject to perform well on this task., he or she had to anticipate both the spatial and temporal location of the next step. In this way, it would be possible for the

74

L. A. Whitaiier /Dual-task interference

subject to decrease or (eliminate the reaction time lag, which results from waiting until the step is seen before responding. Poulton (1952) referred to this behavior as perceptual anticipation. However, in order to anticipate the step, knowledge of the pattern must be maintained in active memory. This processing stage has been proposed as the site of the processing capacity limitations (Norman 1968). In aoditbn, two general reviews of perceptual-motor performance measures (Fitts et al. 1959; Poulton 1974) have suggested that anticipation of the temporal and/or the spatid pa-tern of a tracking target requires considerable processing capacity. In contrast, the other tracking task was thought to require little or no anticipation. The target mored continuously in a quasirandom two, dimensional pattern. The diameter of this pattern was an increasing function of the subject’s tracking skill. That is, the pattern was adaptive (Kelley 1967). No instcntaneousjumps occurred in this task; instead the subject maintained a continuous motion along a convoluted cloverleaf pattern. This r‘ask was thought to require a smzller memory component for the pattern itself. The addition of either of these tmcking tasks would be expected to interfere with performance on the choice RT task to the extent that the two concurrent tasks tapped a common limited resource. A pooled capacity model predicts interference will increase whenever task demands are increased - regardless of the source; e.g., set size or compatibility level. On the other hand, a stage specific model predicts that task interference will be a function of the source of the cognitive requirements; e.g., compatibility, but not set size. In summary, choice RT was expected to be an increasing function of the number of stimulus-response alternatives, the slope being greater for less compatible S-R pairings. The dual-task loading was predicted to interfere with choice RT to a greater extent for secondary (tracking) tasks which require more memory and temporal-spatial anticipation. The efifect of dual-task loading on choice RT variables, set size and compatibility, would be used to analyze the nature of the dual-task interference. A stages model of cognitive processing predicts interactive effects if and only if two variables affect the same processing stage. On the. other hand, from pooled capacity models cognitive processing is better explained as an assessment against a general processing capability, wh;lch is not stage specific.

L. A. Whitaker /Dual-task interference

‘15

The experiment Method Subjects

Sixteen staff members of the Naval Submarine Medical Research Laboratory participated as Ss. Their ages ranged from 22 to 56 years. Four of these participants were women. Tasks

When a dual-task paradigm is employed to assess cognitive processing requirements, the tasks must be selected in a way which avoids confounding central with peripheral (input, outpu;) interference (Kerr I973). Therefore, in each condition, a tracking trsk, which used visual input and manual output, was paired with a concurrent choice RT task, which presented auditory stimuli and required a vocal response. Tracking. Pursuit tracking (in which both the target and the cursor are displayed) was used. The adaptive trackinb locus ,vas a continuous two-dimensional path which was modified as a function of the expertise of the S. The S’s task was to keep the target and cursor superimposed throughout the two-minute trial. Failures to discriminate target and cursor is a major source of error in this task. The step-tracking target locus was B series of six jumps among unique horizontal positions. The dwell time at each position was tiixed at one second. The pattern was repeated 20 times during each two-minute trial. Within a trial, there was no distinguishing mark between the end of one sequence and the beginning of the next. Ss were not informed of the number of steps but they were told that the pattern was fixed and repetitive. They were instructed to learn the pattern and were shown how the ability to anticipate the next step accurately would improve their performance. The S’s task WJS to keep the target and the cursor on the same (imaginary) vertical line. Target and cursor were separated in the Y-axis in this task. Choice R T

The digits one to four were the set of possible stimulus items. The S’s task was to respond as quickly as possible by saying the response digit which was paired with the stimulus digit presented. In the compatible S-R condition, each digit was paired with itself as a response. That is, Ss were to say the digit that they beard. In the incompatible condition, Ss responded to each stimulus digit with another digit from the set. EachS’s instructions specified the exact S-R pairings. Throughout the experiment, an S used a constant S-R mapping. Ttie S was always informed of the exact digit set which would be presented in an;’ given . ial. A trial consisted of 2C stimulus items, selected from that set and presented in ri ndom order. A new list was presented on each trial. The designated digits were drawn from a random number table and no further restrictions were placed on this sampling. The interstimulus interval (I&i) varied from three to seven seconds. Since there was no event uncertainty for the one-alternative lists, the variable ISI was used to provide temporal uncertainty for ail stimulus lists.

76

L. A. Whitaker JL’bal-task interference

The choice RT trials required 80 to 100 seconds, depending uprjn the distributions of t1.e ISI for stimuli in that trial. In the dual-task trials, the digits always began 10 to 15 seconds after the tracking task began. Design Each group of four Ss was tested in one cell of a factorial 2 X 2 design. The level of choice RT S-R compatibility determined *.he two levels of the first factor, while the type of tracking task determined the two levels of the second. Groups were labeled by task conditions.. There were two within-subjects variables: (1) size of the choke RT atimuh~ set on any triai (one, two, or four digits) and (2) levels of tcsk combinuiion -appropriate single task controls and the dual-task combinations. This last variable will be designated as Load to indicate that dual-tasks are loaded processing conditions. .dpparaFus

Choice KT. A Roberts four track stereophonic tape recorder (Model 330) was used to present the auditory stimuli. Each response le.tency was recorded in the following manner: The stimulus item triggered a voice operated relay (Scientific Prototype, Model 761G). The impulse from this relay started the clock counter of a Data General NOVA 1220 mini-computer. The S’s vocal response operated a second voice operated relay (Grason-Stadler, Model E7300A-1). This pulse stopped the counter. The response latency automatically was recorded on a teletype and the image displayed on a television screen, which the experimenter monitored. The experimenter also monitored both the stimuli and the responses through a headphone. Hence, she was able to record any response errors which ockurred. Tracking. In each tracking task, the target and the cursor were displayed on a Tektronix four-trace oscilloscope Model RM-56lA. The S controlred the cursor via a pressure-sensitive two-dimensional joystick (Measurement Systems, Model 435 MS lS1) A pressure of five pounds moved the display element 4 cm on the scope face. The joystick provided simple gain (zero-order) control of the cursor. The forcing function for the adaptive tracking task was generated by an Electronics Associates Analog computer (EAI TR-20). These functions were X = sine (0.3 14) t + rim? (1.974) t and Y = EOS(0.3 14) t + cos (! .974) f. Be.fore reaching the target display element, these functions were modified by an e:.ror-factor, C = J (K (J%+.@)) dt + c. where Ex and & are squared momentary errors between the target and the cursor positions. Both X and Y were multiplied by C. The net result was a target display which appeared as a slowly rolling spot of light tumbling in a convoluted circular pattern. The diameter of this pattern was initially 4 cm. If the S tracked well, the diameter expanded; if (s)he tracked badly, it contracted. The dependent measure was the value of C. Since this quantity is an additive inverse function of the error, increased scores indicate better tracking performance. This value, C, was sampled at 0.6 set intervals and printed out by a Hewlett Packard Data Acquisition System Model 20 12-C. The step function stimulus was generated by a D?ta Trak Programmer (Model 5110). FOP this equipment, the 6-step tracking pattern was etched on a conductive Plastic form. An electronic sensor arm locked onto the etched pattern. As the drum

L. A. Whitaker /Dual-tad: interference

71

containing the form rotated, the position of this arm determined the target input voltage and, hence, the horizontal position of the target stimulus. The voltage was also used as input to the comparator in the analog computer. Mean square error between the target and the cursor voltage was integrated over time. The score was sampled at intervals of 0.6 set and, in addition, the total score for the two min trials ‘was accumulated. The step function was a repetitive one-dimensional (hnrizonta!.) pattern, which was not modified by the S’s performance. The tlrget’s path bisected the oscilloscope display screen. The cursor’s path was 0.25 cm to 0.50 cm below that of the target. This separation allows-d the subject to discriminate between target and cursor. Procedure

All training and testing were conducted in a double-compartment hyperbaric chamber but participants were not subjected to hyperbaric pressures. This fal:ility wasused because it contained the equipment necessary for administering bath t.&,:;. Ss were assigned to conditions by order of their appearance in the laborator), On the first day, each S was trained for four one-hour sessions on the choice RT and the tracking task alone. Ss were trained in pairs; one S rested for approximately two min while the other S executed a training trial. Each type of task was performed by the same person for half of a session, then Ss traded tasks for the balance df the hour. The second session Cay was scheduled two days after the first. Ss reported individually and were trained on the dual-task combinations during twc. tinehalf hour training sessions in the morning. Ss were instructed to treat the choice; RT task as the primary one. These instructions were intended to prevent some Ss from choosing tracking as primary, while other S: selected the choice RT. Instructions were worded in a way to encourage Ss to believe that the tasks might be successfully combined without interference, and ro mo.\etary payoff was provided for performance on-either task. Testing was always conducted in the afternoon. Each S participated in a single one-hour test session in which tasks were presented both singly and in the dual-task corn binations. 18 trials were scheduled: six tracking single, two at each set size for choi,se RT single, and two for each set size for dual-task. The order of these trials was counterbalanced across Ss.

Results F:or each S, a mean latency for correct response’s (choice RTj arid/c???J mean trsckit?g score was obtained for each experimental condition. Thus, there were six A.*_1+n& choice RT scores (a smgie srrd 3 &by. . . . . . mndition at each of three set sizes) and four tracking scores (single control and three levels of dual-tasf. set size! for each S. The mean scores for the four independent groups are preset ted in table 1. Performance of indiiiduals was examined for systematic differences related to S’s sex. None was found in either task.

l/3 33 236b) 224 244 232

213/2X 2411268 2821305

o/4 36. 224@ 210 229 213

2321274 351/390 503/569

Adaptive trackind Incompatible choice RT

3/l 33 126c) ?65 173 140

2531282 2961299 2911310

Step tracking/ Compatible choice RT

O/4 30 86C) 118 138 120

3041436 510/651 1181818

Step trackme/ Incompatible choice RT -

b) Adaptive tracking scores arc mce;i?;rd m a9 adapt&e t_m_ck+ e*r-**t+ _b WV .a&- ., pC, *.*h;-\ ~fiti.tifih,creases as *Wackiug performance improves. c, SteR tracking scores are measured in integrated mean square error, which decreases as tracking performance improves.

a) Subscript indicates the number of stimulus-response alternatives (set size) in the concurrent choice RT task.

dual,

. . z

female/male (N) age Wn)

Mean tracking score

single, /dual, a) sir&, /dual, single, /dual.

for each treatment.

Group Adaptive tracking/ Compatible choice RT

scores

__. __..,_.- --- PMean choice RT (ii msec)

Table 1 Mean pcrf~rmance

B .

z

F g

P

P

L. .A. Whitaker /Dual-task interference

79

Choice R T

Response latency increased as a function of set size. As predicted, this effect was much greater when S-R compatibility was lower. In contrast, the dual-task decrement did not increase with the number of alternatives; however, this decrement was greater for incompatible than for compatible S-R pairings (see fig 1). A considerntion of single VS. dual task conditions in table 1 indicates that loading with either tracking task resulted in comparable decrements in choice RT performance.

Fig. 1. RespoWZ latency on the choice RT tzsk as 3 function of number of alternatives in the stimulus sequence. Tne parameters are level of S-R compatibility and task loading (single vs. dual task conditions).

These resuits were shown to be statistically reliable by a mixed factors analysis of variance for allgroups. Response latency increased as a function of the number of alternatives (F(2.24) = 56.87, p < 0.001); however, consideration of the significant Compatibility X Set Size interaction (F(2,24) = 31.622, p < 0.001) indicated that the effect of set size differed significantly as a function of the level of S-R compatibility. The slope was much greater when S-R coritpatibihty was lower (see fig. 1). In addition, compatible pairings resulted m significantly faster response latencies than did incompatibie pairings (F(I,12) = 15.270, p < 0.01). The average choice RT latency for Adaptive tracking grou!ps was faster than that for the Step tracking groups (322 and 436 msec, respectively); however, this between groups difference only approached significance (F( 1 ,12) = 4.407, p < 0.058). The main effect oc loading was significa.nt (F( 1 ,12) = 16.272, p < 0.01). The Load X Compatibility interaction was also signifi’cant (F(1 ,12) = 6.023, p < 0.05) with secondary task.

80

i. A. @‘hitaker/ Dudtask interference

loading yielding more interference! when the S-R compatibility wa.s low (77 msec) than when it was high (18 msec) (see fig. 1). Increasing the set size did not alter the extent of the dual-task loading effect (F(2,24) <: l), nor did the nature of the tracking task (r( 1,I 2) - 1.226, p > 0.25). Error rate was less than 1% for all compatible conditions. However, for the incompatible conditions, the mean percentage of incorrect responses increased as a function of set size. For both single and dual task trials, the mean error rate increased from 9% to 4% for the Step/Incompatible group and from 0% to 11% for the Adaptive/Incompatible group. Individual r-tests were conducted to determine the statistical reliability of the error differences for treatment conditions in both incompatible groups. None of ihese tests (both between and within groups) resultad in a significant difference. Examination of the data revealed that these re: Jtively large differences in per cent error resulted from one or two Ss. Several other Ss had no increase in error rate as a function of either set sizn or secondary-task loading, thus explaining this failure to rench statistical significar Although the choice KT W&S;he principal task being considlared, results of the tracking task are of interest for any disc:&ion of total processing requirements. Tracking The step tracking performance showed 2 decrement for all dual-task conditions when compared to the single-task tracking controls. However: adaptive tracking scores showed a decrement only for dual-task conditions using one- or four-alternatives. The two-alternative set size resulted in performance at least equal to that in the single-task control (see table if. Separate mired factors analyses of variance were conducted for Adaptive and Step tracking scores. Each showed ,: significant -ain effect of loading (F(3,18) = 8.026 and 11.274, respectively, p c.. 0 Wl). Tests of the simple effects within this factor (Tukey WSD) indicated that the Adaptive groups tracked significantly better in the single tark condition than in the dualtask condition with one choice RT stimulus alternative. In addition , fz-r Adaptive: tracking groups the two-alternatlve choice RT dual-task was executed significantly better than either of the other dualtask conditions. The simple effects teat showed that Step tracking groups were significantly better in the single task than in any dual-task condition and that the duai-l:ask conditions did not differ alnongst themselves. Choice RT compatibility level had no significant main effect en tracking performance nor did compatibility interact with task load.

Wiussion Tracking

,

In the step tracking task, all dual-task conditions resulted in a com-

L. A. Whitaker / Dual-task inttrference

81

parable decrement in tracking performance. No evidence of the increased processing load as a function of set size and/or S-R compatibility of the choice RT rask was found in the tracking scores. One hypothesis to explain these resuits was a change in subject performance strategy. The subjects might have stopped anticipating the steps and begun to lag in the dual-task conditions, thereby freeing active memory to be utilized in the concurrent choice RT task. This strategy could have made the tracking task insensitive to changes in the memory requirements rcslllting from changing information load (e.g., set size). However, inspection of the tracking records (visicorder printout) did not indicate any such change in performance between single and dual-task trials. In subsequent research, the author again obtained a constant step tracking dlecrement at all levels of concurrent choice RT set size (Whitaker and Findley 1977). Therefore, this appears to be a robust, though perplexing, phenomenon. Adaptive tracking performance on the dual-task conditions is difficult to exp1ai.n with even a posf hoc rationale. Subsequent rese;trch in which tracking itask parameters are varied may be necessary to understand the saw-toothed function obtained in this experiment. Track ‘scores cannot be interpreted in the same way i.hat response latencies are; response latencies presumably measure cognitive processing time directly. Tracking scores are a function of sp:.iial :md temporal accuracy. It can only be said that, on the basis of these tracking results, the assumption of memory and/or antic;pation differences between the two tracking tasks was not supported. Consideration of the effects of dual-task requirements on choice RT is consistent with this idea. These effects are discussed in the following section. Choice

RT

The effect of adding a secondary task to the choice RT task was different for the variables, set size and compatibility. The dual-task decrement was less for the Compatible .conditions (18 mriec) than for the Incompatible conditions (77 msec). This diiferelice resulted in a significant Loading X Compatibility interaction. On the other hand, the dual-task loading effect was constant across all levels of set size (55, 5 1, and 37 msec for set sizes 1,2, and 4, respectively). The prediction of the pooled capacity models is that the signi!icant interaction between the t,ask variables, loading and compatibility,

82

L. A. Whitaker / Dual-tag;; interference

requires an interaction between loading and all other task variables -which affect processing capacity. The results of the present experiment indicate #at set size does require processing capacity (latency is an increasing function of set size); however, dual-task loading does not lteract with set size. This combination of results can be explained by assuming that loading and compatibility require at least one common stage, while loading and set size probably require indetaendent stages. The: last piece of evidence is that set size end compatil-AiV{ interact and, therefore, seem to require a common stage also. This pattern of results leads to the following model (see fig. 2). The stage affected by set size and compatibility is named response selection on the basis of earlier models (e.g., Teichner and Krebs 1974). It remains only to find a functional label for Stage S. Without this labeling requirement, stages could proliferate without providing a cue to their function or Fuidance for manipulating other task variables in order to explore that new stage further.

Fig. 2. Processing stages and the task variables which affect them.

This additional stage may become importarn o!ily when the subject is required to share processing capacity between two tasks. Subjects in this study were told to treat the choice RT task as their primary task. In order to combine these tasks while trying to prevent a decrement in the choice F.T performance, some processing must be expended to execute an allocator, monitor, or executive (sharing) function. This function would be more difficult (more costly) under conditions of low compatibility because both tasks reqtiire extensive monitoring. In both the tracking task and the incompatible choice RT task, subjects must monitor their own responses in order to convince them!selves that the executed responses were correct. This feedback monitoring is a necessav component for the maintenance of perceptual-motor performance (Welford 1976) and may be selectively more important for the less compatible choice RT tasks because the response does not represent a

L. A. Whitaker /Dual-task interference

83

compelling relationship to the stimulus. Green.wald (1972) has suggested that S-R pairings in #which the stimulus and the response share important characteristics be called ‘ideomotor compatib!e’. This name describes highly compatible S-R pairings for which the response selection stage may be virtually bypassed because the cognitive representation of the stimulus wJ1 be so similar to that of its response that the response can be executed on the b&s of the stimulus representation alone. For the auditory-vocal choice RT task ea$loyed in the present research, in the compatible S-R conditions the stimulus and the response are the same stimuli except for differences in voice qualip. Under these conditions, the subject’s monitoring task for the choice RT task is reduced to ascertaining whether the response produced was the same as the stimulus heard. That is, the check of the appropriateness of the S-R pairing is a much less demanding task in the compatible than in the incompatib!e condition. This would relieve the executive stage of some of the processing requirements from an entire task (the compatible choice RT task) and would reduce the total decrement of the dual-task loading for the compatible choice RT conditions. Kantowitz and Knight (1974) have proposed a sharing mechanism for the successful execution of two demanding concurrent tasks. In their mixed hybrid model, this function i.s satisfied by a general draw on the total system capacity (much like the pooled capacity models), while stages having their own dedicated capacity stores could handle specific processing requirements (e.g., encoding, response selection) independently. The present study was not designed as a test of the Kantowitz and Knight model and does not meet the requirements of a test of that model; therefore, while the Stage S may also serve as an executive or sharing function, its relation to the Kantowitz and Knight model car.. only be suggested. As an alternative explanati9r, pooled capacity models, those without assumptions of stage-specific processing limitations, seem to be less satisfactory. The du&task decrement in response latency is evidence that the combined capacity requirements of the two tasks exceeded the total capacity available. The present results indicate that, when available capacity was decreased by loading with a second task, varying compatibility produced different amounts of decrement (a Loading X Compatibility interaction). In contrast, varying set size did not alter the amount of interference which resulted from Load (no Loading X Set Size interaction).

References Broadbent, D. E.. 1971. Decision and stress, New York: Academic Press. Broadbent, D. E. and Y. Gregory, 1962. DonB- and C-reactions and S-R compatibility. Journal of Experimental Psychology 63,575-578. Davis, R., N. Moray and A. T&mat?, 1961. Imitative responses and the rate of gain of information. Quarterly Journal of Exp unentaI Psychology 13,7_ 91. Fitts, P. M., H. P. B&rick, M. Nobk and G. E. Briegs, 1959. Skilled performance: Part 1 and 11. USAF Wright Air Development Center. Final Report, No. AF 41(657)-70(1959). GreenwaId, A. G., 1972. Gn doing two things at once: time sharing as a function of ideomotor compatibility. Journal of Experimental Psychology 94,52-57. Hick, W- E., 1952. On the rate of gaiu of information. Quarterly Journal of Experimental Psychology 4,l l-26. K&nemaq D., 1973. Attention and effort. New Jersey: Prentice-Hall. Kantowitz, B. H. and J. L. Knight, Jr., 1974. Testing tapping timesharing. Journal of Experimental Psychology 103,331-336. Kantowl@ B. H. and 1. L. K&&t, Jr., 1976. Testing tapping timesharing. II: Auditory secondary task. Acta Psychologica 40.343-362. Kelly, C. R., 1967. Further research with adaptive tasks. NONR4986iOO), Dunlap and AssocIates, Inc., Wash&ton, D.C. Kerr, B., 1973., Processing demands during mental operations. Memory and Cognition 1, 401-412. Moray, N., 1%7. Where Is capacity limited? A survey and a model. In: A. F. Sanders, ed., Attention and Performance. Amsterdam: North-Holland. Norman, D. A., 1968. Toward a theory of memory and attention. Psychological Review 75, 522-537. Norman, D. A. and D. G. Bobrow, 1975. Gn data-limited and resource-limited processes. Cognitive Psychology 7,44-64. Poulton, E. C., 1952. Perceptual anticipation In tracking with two-pointer and one-pointer displays. British Journal of Psychology 43,222-229. Poulton, E. C., 1974. Trackiig skii and manual control. New York: Academic Press. Sanders, A. F., 1967. Some aspects of reaction processes. Acta Psychologica 27,115-130. Stemberg, S., 1%9. The discovery of processing stages: extensions of Dondcrs’ method. In: W. G. Koster, ed., Attention and performance II. Amsterdam: North-Holland Publishii company. pp. 276-315. Tejchner, W. It. and M. J. Krebs, 1974. Laws of visual choice reaction time. Psychological Review 81,75-98. Trumbo, D. A.,, 1973. Some laboratory tasks for the assessment of stressor effects. Psychiatria, Neurologka, Neurochiirgia 76,199-207. Welford, A. T., 1976. Skilled performance: perceptual and motor skiIlr Brighton, England: Scott, Foresman and Co. Whitaker, L. A. and M. S. Findley, 1977. Nitrogen narcosis measured by dual-task performance. Journal of Applkd Psychology 62,735-746.