The role of input and output modality pairings in dual-task performance: Evidence for content-dependent central interference
Introduction
Psychology has long sought an understanding of how humans perform concurrent tasks. Given the massive parallelism of the human brain, one might expect people to be able to perform multiple tasks concurrently with little interference. However, this expectation is not generally borne out in experimental settings, where people routinely show severe limitations when attempting to perform two distinct tasks simultaneously. Thousands of experiments have examined these dual-task costs, providing the basis for numerous influential theories of human cognitive architecture.
The extensive literature on dual-task performance describes many forms of interference between concurrent tasks (e.g., Allport et al., 1972, Diedrichsen et al., 2001, Greenwald, 1972, Hirst and Kalmar, 1987, Jolicoeur, 1999, Navon and Gopher, 1979a, Pashler, 1990, Pashler and Johnston, 1989, Schumacher et al., 2001, Spijkers and Heuer, 1995, Tombu and Jolicoeur, 2002, Welford, 1952, Wickens et al., 1983). Thus, dual-task costs likely emerge from manifold sources, a subset of which may be relevant for any particular pairing of tasks. For example, the simultaneous production of motor responses with the two hands leads to bimanual interference (Franz et al., 1991, Heuer, 1995, Spijkers et al., 1997, Swinnen and Wenderoth, 2004). Bimanual interference is not present when a task requiring a manual response is paired with a task requiring a vocal response. Similarly, conceptual overlap between tasks (e.g., Hommel, 1998, Logan and Schulkind, 2000, Navon and Miller, 1987), may lead to inter-task interactions that would not have occurred with non-overlapping task pairs.
Despite evidence for multiple sources of task-specific interference, contemporary theories of dual-task performance often assume that these conditions represent special cases and that, in the absence of conceptual overlap or response overlap, meaningful generalizations about dual-task operations can be drawn across a range of task pairings. That is, it is widely accepted that the coordination of task operations is governed by a common set of general principles or control processes (e.g., a central bottleneck) assumed to apply regardless of the particular tasks currently being performed. This research strategy is supported empirically by literally hundreds of studies showing seemingly similar patterns of dual-task costs despite impressive variation in the tasks. Nevertheless, there is still considerable debate regarding the best account for these phenomena.
In the present study, we focused on three outstanding questions emerging from current theorizing about dual-task limitations. First, is the concurrent performance of two distinct tasks subject to limitations in central processing? Second, do central limitations (if any) lead to serial processing of the two tasks? Third, does central interference depend only on the duration of competition for central capacity (as often assumed), or does it also depend critically on the specific content of the two tasks? As will be seen, the answers to these questions are somewhat unexpected given the dominant models of dual-task performance, compelling us to reexamine several fundamental assumptions of those models. We first discuss each of these questions and then describe our experimental approach towards addressing them.
The dominant theories of dual-task performance differ on whether limitations on concurrent task performance reflect structural properties inherent in the cognitive architecture or a strategic adaptation to a particular set of circumstances. Structural accounts generally attribute dual-task interference to contention for limited-capacity central resources. Strategic accounts generally posit no central resource limitations, instead attributing costs to the scheduling of task operations (which may be dictated by contention for peripheral sensory or motor resources).
One popular structural account, originally proposed by Welford (1952), is that the cognitive architecture of the human brain includes a single-channel central mechanism (see also, Pashler, 1984, Pashler, 1994b, Pashler and Johnston, 1989, Pashler and Johnston, 1998, Smith, 1967, Welford, 1967). According to this “central bottleneck” theory, dual-task costs occur because central processing for one of the tasks must be delayed while the central mechanism is occupied by another task.1 Thus, dual-task costs should be observed whenever two tasks are temporally aligned so as to simultaneously require central processing. This prediction and several others relating to the central bottleneck have been borne out in literally hundreds of studies, using a myriad of task combinations, suggesting that a central processing mechanism is required during the performance of a wide range of tasks (for a review, see Lien and Proctor, 2002, Pashler and Johnston, 1998).
Central bottleneck theories have recently been challenged on the grounds that dual-task interference might represent a strategic adaptation to the demands of the experiment, not an inherent limitation of a fixed cognitive architecture, at least after sufficient practice. This interpretation is supported by recent studies showing that participants can learn to perform two tasks simultaneously as quickly and accurately as they perform each task individually. For example, Schumacher et al. (2001) required participants to make a manual keypress based on the location of a visual stimulus and a vocal response based on the pitch of a tone. Participants performed single-task blocks using only one of the two tasks, and mixed blocks consisting of single-task trials randomly intermixed with dual-task trials in which the visual and auditory stimuli were presented simultaneously. After several sessions, performance on dual-task trials was nearly identical to performance on single-task trials. These researchers concluded that dual-task limitations can be overcome by eliminating conflict for specific peripheral resources, and by providing practice and sufficient incentives for good performance. Hazeltine, Teague, and Ivry (2002) replicated Schumacher et al.’s key findings and demonstrated that the elimination of dual-tasks interference was not due to the integration of the two tasks into a single super-task.
Despite these findings, the debate over the source of dual-task costs remains. Levy and Pashler (2001) pointed out that the visual–manual task used by Schumacher et al. was so easy that it might not have required any central processing. Furthermore, they noted that a lack of interference could be accounted for by a “latent” bottleneck, in which the central bottleneck operations on the two tasks do not overlap in time. The probability of minimal temporal overlap in central operations is increased when the when the durations of the central operations of the two tasks are short (see also, Byrne and Anderson, 2001, Ruthruff, Johnston et al., 2003, Tombu and Jolicoeur, 2004), which is likely to be the case after substantial practice.
The central bottleneck model and some strategic accounts of dual-task performance (e.g., EPIC, Meyer & Kieras, 1997b) assume that costs result from serial performance of certain critical operations on the two tasks. However, competition for central resources need not result in a strict processing bottleneck. An alternative theory is that two tasks can simultaneously share the central resource, so that neither task is performed as quickly as it would be performed alone (e.g., Navon and Miller, 2002, Tombu and Jolicoeur, 2002, Tombu and Jolicoeur, 2003). In many cases, it might be more efficient to allocate all of the central capacity to one of the tasks until it is completed, in which case this model makes similar predictions as the single-channel bottleneck model. However, the assumption that central capacity can be shared allows the model to account for some findings that are difficult for the single-channel bottleneck model to accommodate.
The question of whether response selection can proceed in parallel for two tasks has received a flurry of recent attention, although no consensus has yet emerged (e.g., Byrne and Anderson, 2001, Hazeltine et al., 2002, Hommel, 1998, Logan and Schulkind, 2000, Navon and Miller, 2002, Ruthruff, Pashler et al., 2003, Ruthruff, Pashler et al., 2001, Schumacher et al., 1998, Schumacher et al., 2001, Tombu and Jolicoeur, 2003, Van Selst et al., 1999). This unresolved issue is of critical importance for understanding the cognitive architecture underlying response selection.
Thus far, all of the accounts we have described share the fundamental assumption (explicit or implicit) that central interference is due to generic limitations, either structural features of the architecture or strategic considerations, and that these limitations apply regardless of the pairings of stimuli and responses. Additional forms of peripheral interference may arise if both tasks use, for example, visual stimuli or manual responses. However, in the absence of conflict for peripheral resources, selecting a manual response to a visual stimulus should not result in different patterns of dual-task costs than, for instance, selecting a vocal response to a visual stimulus.
Theories that attribute dual-task costs to a structural component of the cognitive architecture (e.g., a single-channel mechanism Byrne and Anderson, 2001, McCann and Johnston, 1992, Pashler, 1994b), including capacity-sharing theories (e.g., Kahneman, 1973, Navon and Miller, 2002, Tombu and Jolicoeur, 2003), hold that central mechanisms use generic (content-independent) resources. Provided that there is no physical or conceptual overlap between the stimuli and/or responses for the two tasks (e.g., the tasks do not involve overlapping stimulus classifications), dual-task costs should be determined by the duration of central operations, not by the specific relationships between the two tasks. Hence, if participants perform an auditory–vocal task concurrently with a visual–manual task, current theories tell us that costs should arise only from competition for generic central resources, because the input and output modalities have each been separated to avoid conflict. There is no reason to suspect that we should observe any different pattern if we ask subjects to perform the same central operations but change the input-output pairings by asking them to perform the same tasks with auditory–manual and visual–vocal mappings. That is, according to content-independent theory, central operations reflect abstract conceptual choices, not influenced by the specific input or output channels. Although bottleneck and limited capacity models can be modified so that operations subject to the central limitations would depend on the composition of the tasks, such changes dramatically alter the fundamental assumption of these models that the central operations are generic.
Theories that attribute dual-task costs to voluntary sequential central processing (e.g., Meyer and Kieras, 1997b, Meyer et al., 1995) also assume that the contents of the central operations—the specific stimulus–response (S–R) mappings—do not directly affect dual-task costs. That is, although the duration of the central operations may have a powerful effect on the duration of the voluntarily imposed bottleneck delay (and hence the magnitude of dual-task costs), the S–R translation processes for the two tasks do not interact. It would be possible to expand these models to allow for direct interactions between central operations (e.g., delays might be imposed differently depending on the contents of the tasks). However, such a modification would be significant since it is not justified by the proposed cognitive architecture (which explicitly asserts that resource competition occurs only between input processors and between output processors). In essence, such changes to the models would make the claim that there are no restrictions on central processing contingent on the particular tasks being performed.
All of these accounts allow for task-specific interactions on peripheral mechanisms, such as when there is overlap between the input modalities or between the output modalities. For example, it is widely postulated that two tasks involving manual responses call upon common mechanisms that control the two hands (Heuer, 1995, Meyer and Kieras, 1997a, Pashler, 1990, Van Selst et al., 1999). Therefore, requiring two manual responses, even if made by different hands, causes competition for the manual processor and results in dual-task costs above and beyond those caused by limited central resources. Critically, the locus of the interference is assumed to be peripheral rather than central.
To summarize, each of the models, listed in the columns of Table 1, offer distinct sets of answers to our questions, listed in rows. Whereas the theories diverge regarding whether performance is constrained by central limitations and whether central operations must performed serially, there is consensus among the accounts that central limitations are not content-dependent. We now consider a class of theories that offer a different answer to this question.
An alternative to the prevailing view, often neglected in recent work, is that central mechanisms are subject to content-dependent interference. By content-dependent interference, we refer to interactions between the specific central codes associated with the concurrently performed task. Unlike the abstract S–R codes postulated for generic central operations, content-dependent central operations reflect aspects of the underlying cognitive architecture. That is, the central operations are determined by the specific linkage between input and output modalities. Thus, the selection of a manual response to a particular stimulus will engage a set of central operations that differ from those engaged by the selection of a vocal response, and the nature of the dual-task interference will differ accordingly. We term this type of account a content-dependent theory, because the specific codes invoked by the S–R translation (the contents of the particular action required by the task on a given trial) determine the pattern of dual-task costs.
Although the content-dependence question and the serial/parallel question are logically distinct, they are not independent. Serial processing of central operations would seem to minimize the opportunity for content-dependent interference between tasks. Parallel central processing (which is supported by the data presented below), in contrast, suggests active processing of central codes on both tasks at the same time, using the same set of buffers/memory stores. Thus, the opportunity for mutual interference is greatly increased. For instance, interference might arise if both tasks rely upon similar central codes (e.g., if both use acoustic codes). As an analogy, it is much easier to simultaneously listen to a male and female voice rather than to two male voices. Note that although capacity-sharing accounts also assume interference between parallel central processes, they attribute interference to a generic resource limitation (a sort of mental “energy”) rather than to specific interactions.
We are not the first to argue that central interference depends critically on the composition of the particular tasks. Indeed, content-dependent theories have a long history (e.g., Greenwald, 1972, Hirst and Kalmar, 1987, McLeod and Posner, 1984, Navon and Miller, 1987, Pashler, 1990, Shaffer, 1975). A well-known class of content-dependent theories includes multiple-resource theory (e.g., Allport et al., 1972, Navon and Gopher, 1979a, Navon and Gopher, 1979b, Wickens, 1980, Wickens, 1984) in which interference is maximized when two tasks require similar pools of resources. One prominent example of multiple resource theory is the code compatibility account of Wickens et al. (1983). These authors proposed that dual-task interference depends critically on the interaction between the type of central code (spatial versus verbal) and the type of input modality (auditory versus visual) and output modality (manual versus speech).
A second class of content-dependent theories supposes that the costs arise not from competition for a limited set of task-specific resources but instead from crosstalk between conflicting representations. Along these lines, Navon and Miller (1987) proposed that dual-task interference might result because each task produces “outputs, throughputs, or side effects that are harmful to the processing” of the other task (see also Allport, 1987, Hirst and Kalmar, 1987). If, for instance, both tasks involve the same stimulus categories, then the near-simultaneous activation of both stimulus categories can create crosstalk or a binding problem (see Logan & Gordon, 2001) and lead to decrements in performance.
These examples are typical in that it is assumed that content-dependent interference stems from overlap between salient features of the stimuli or between features of the responses for the two tasks. That is, theorizing about content-dependent interference has largely been restricted to interactions between tasks that possess overlapping features, such as when the stimuli share a semantic (e.g., Hirst and Kalmar, 1987, Logan and Schulkind, 2000, Navon and Miller, 1987) or spatial (e.g., Wickens, 1984) component, or the responses are produced in the same modality (e.g., Pashler, 1990). The experiments discussed below lead us to an alternative theory of content-dependent processing in which interference can occur even when there is no explicit overlap between the specific stimuli or responses for the two tasks, but because the stimuli activate existing linkages between input and output modalities.
Although investigations of content-dependent interactions have largely been restricted to cases when the two tasks involve conceptually overlapping S–R mappings, there is some evidence that this type of overlap is not a requirement. Levy and Pashler (2001) assessed dual-task performance after modest training (two 1-h training sessions) with two different pairings of input and output modalities. In their Experiment 1, participants performed the Standard modality-pairings used in the Schumacher et al. (2001) study. We call these the Standard pairings, because participants responded manually to visual stimuli and vocally to auditory stimuli, which is standard in the dual-task literature. In Experiment 2, a new group of participants responded to the visual stimulus with a vocal response and to the auditory stimulus with a manual response. We call these the Non-standard pairings. Levy and Pashler (2001) reasoned that if the use of proper instructions and moderate practice are sufficient to produce perfect time-sharing, then it should occur in both modality-pairing conditions. Contrary to this prediction, both single-task and dual-task RTs were longer with the Non-standard pairings compared to the Standard pairings. Levy and Pashler (2001) interpreted their findings as evidence that there are structural limitations to performance. They assumed that with the Standard modality-pairings either the visual–manual task did not require central operations or that the central bottleneck was latent.
Another explanation suggested by these findings, not explicitly considered by Levy and Pashler, is that modality-pairings determine the linkage between stimulus representations and responses, and that the specifics of these linkages have a critical influence on dual-task performance. With this point in mind, we note that a very limited set of tasks has been used in previous studies showing near-perfect dual-task performance (e.g., Hazeltine et al., 2002, Schumacher et al., 2001). In particular, these studies paired a visual–manual task with an auditory–vocal task, which, according to content-dependent theories, may differ fundamentally from other pairings of input and output modalities.
Thus, the findings of Levy and Pashler (2001) provide preliminary evidence that modality pairings affect dual-task performance even when the stimulus and response modalities are distinct (although they did not interpret their results in this way). That is, when considered in tandem with other dual-task studies using similar tasks (e.g., Hazeltine et al., 2002, Schumacher et al., 2001), their findings suggest that the input–output pairings within a task play a crucial role in executive control processes. This conclusion is consistent with content-dependent accounts of response selection; reductions in dual-task costs depend on the specific S–R pairings associated with the two tasks. Such a form of content-dependent interference differs critically from previous reports of content-dependent peripheral interference (between inputs or outputs), because on the dual-task trials of Levy and Pashler the set of stimuli and responses were the same for the Standard and Non-standard groups. Thus, the difference in dual-task costs cannot be attributed to increased competition for peripheral resources. In sum, the Levy and Pashler result is consistent with a content-dependent account if it is assumed that the Non-standard modality-pairings (auditory–manual and visual–vocal) produce more code interference than the Standard modality pairings (auditory–vocal and visual–manual), and hence greater dual-task costs.
One factor complicating the interpretation of the Levy and Pashler (2001) findings, however, is that Non-standard pairings happened to create less compatible tasks. Arguably, it is much easier to respond to the location of a visual stimulus by pressing a button in the corresponding position than to say the number assigned to that position. This ease of the S–R mapping is not an inherent consequence of the standard modality pairings, but rather an accidental consequence of the particular tasks being studied. Consistent with this point, the Non-standard pairings produced longer single-task RTs in addition to the larger dual-task costs.
The pairing of particular stimuli with particular responses can shorten central operations by increasing either set-level compatibility or element-level compatibility (Kornblum, Hasbroucq, & Osman, 1990). Set-level compatibility refers to the correspondence between the relationship among the stimuli and the relationship among the responses. For example, if the leftmost stimulus location is mapped to the leftmost keypress and the rightmost stimulus location is mapped to the rightmost keypress, there is a high-degree of set-level compatibility. Note that each stimulus, considered by itself, is not inherently compatible with its response; the compatibility arises only when the sets of stimuli and responses are considered. This contrasts with element-level compatibility, which relates to the compatibility of a response with a stimulus in the isolation of the other S–R pairs. For example, saying the word “red” in response to the printed word “RED” has a high-degree of element-level compatibility. Any change in the S–R pairings may affect both set-level and element-level compatibility.
An important characteristic of content-dependent interactions, as we define them, is that they are distinct from the effects of S–R compatibility. Both capacity-sharing and bottleneck models can accommodate findings in which set-level or element-level compatibility effects increase dual-task costs (Kahneman, 1973, McCann and Johnston, 1992, Navon and Miller, 2002, Tombu and Jolicoeur, 2003). According to the bottleneck model, reduced compatibility increases the duration of response selection for one of the tasks, which can in turn increase the bottleneck delay in response selection for the other task. According to capacity-sharing accounts, S–R mappings with less element-level compatibility require a greater allocation of central resources, leaving fewer resources available for any other task and increasing dual-task costs (see Navon and Gopher, 1979a, Navon and Miller, 2002). The bottom line is that if a particular manipulation of input–output pairings alters set- or element-level compatibility, it will be unclear whether any differences in dual-task costs are due to modality pairings in general or to the change in compatibility.2 Thus, the challenge for content-dependent theories of central interference is to differentiate modality-pairing effects from S–R compatibility effects.
In addition to Levy and Pashler, a few dual-task studies have also examined the effect of modality pairings (Shaffer, 1975, Wickens, 1980, Wickens, 1984, Wickens et al., 1983) on dual-task performance. Shaffer (1975), for example, had a single skilled typist type one set of words and shadow another set simultaneously. Performance was much worse when the participant had to type words presented in the auditory modality and read aloud words presented visually than when the reversed arrangement was required. Although suggestive, these studies fall far short of demonstrating a direct influence of modality-pairings on central operations. The continuous nature of the tasks and the coarse level of analysis make it possible that interference was occurring at a peripheral level. Moreover, as in the Levy and Pashler (2001) study, modality-pairings were confounded with element-level S–R compatibility. At present, we see no compelling evidence for or against the hypothesis that modality-pairings have a significant influence on dual-task performance.
Section snippets
The present study
The goal of the present study is to empirically address the three questions posed above and to lay the groundwork for an integrative theory of dual-task interference. We describe how the current approach bears upon each of these questions in turn.
Experiment 1a
Experiment 1a addressed the central limitation question by evaluating dual-task performance across practice for two tasks with Non-standard pairings in a paradigm similar to that of Schumacher et al., 2001, Hazeltine et al., 2002. Participants completed 8 sessions in which they responded manually according to the pitch of tones and vocally according to the category of the referents of visually presented words. In other words, participants practiced two tasks with the Non-standard pairings. As
Experiment 1b
In Experiment 1a, dual-task costs with Non-standard modality pairings were still robust after 8 sessions of practice. These results suggest that the pattern of dual-task costs reported by Schumacher et al., 2001, Hazeltine et al., 2002 occurs only under restricted conditions, for instance, when one of the tasks involves spatial correspondence or when the tasks use the Standard modality-pairings. Given that the inputs and outputs of the pairings used in Experiment 1a are similar to those used in
Experiment 2a
It is conceivable that the differences in dual-task costs relate to the greater exposure to this dual-task method and the stimulus classifications by the participants when they performed Experiment 1b (Standard modality pairings) than when they performed Experiment 1a (Non-standard modality pairings). For instance, if the more complicated visual task required more practice before the categories could be accessed without the bottleneck, then the lack of cost in Experiment 1b might simply reflect
Experiment 2b
The results of Experiment 2a suggest that content-dependent interference plays a major role in dual-task performance after moderate practice, as evidenced by the larger dual-task costs associated with the Non-standard group. However, it remains an open question as to the nature of these costs as they persist throughout practice. Although the IRI distributions in Experiment 2a suggest that the Standard and Non-standard groups were not limited by a single-channel bottleneck, we sought converging
Experiment 3
The results from Experiments 1 and 2 suggest that the near-elimination of dual-task costs depends on the modality pairings of the two tasks that must be performed together. Throughout 16 sessions of training, participants in the Non-standard group showed significantly larger dual-task costs than participants in the Standard group, even though both groups used essentially the same stimuli and responses.
Along with the differences in dual-task costs, the modality pairings also caused differences
General discussion
We identified three outstanding questions regarding the nature of dual-task costs: Are dual-task costs due to limitations in central capacity? Must central operations be performed in a serial fashion? Is central interference content-dependent? We consider each of these questions and how the present data answer them in turn.
Summary
The present results answer three important questions about the nature of dual-task costs. First, the findings extend previous demonstrations of near-perfect time-sharing to more difficult tasks, demonstrating that central processing is not always limited in capacity. Second, the data indicate that the ability to select two responses in parallel may be achieved across a variety of conditions. However, although the responses appeared to be selected in parallel, dual-task costs remained robust
Acknowledgments
This work was funded by the Airspace Operations Systems Project of NASA’s Airspace Systems Program. Harold Pashler, Bernard Hommel, Sylvan Kornblum, and Howie Zelaznik provided many helpful comments on earlier drafts of this manuscript. The authors are also grateful to Elizabeth Kelly, Jennifer Kaiser, Tri Li, and Uyen Tran for their assistance in collecting data.
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2022, Acta PsychologicaCitation Excerpt :One important factor seems to be time or duration (Helton & Russell, 2015; Ross et al., 2014) between the sessions to allow offline processing mechanisms in the brain (Tambini et al., 2010; Wamsley, 2019). Thus, practice interventions usually extend over several weeks (Dux et al., 2009; Hazeltine et al., 2006; Strobach et al., 2014), while fatigue interventions are typically completed in one day, with almost no rest occurring between sessions (Hopstaken et al., 2015; van der Linden, Frese, & Sonnentag, 2003). Additionally, some studies suggested that it is crucial to eliminate any motivation and engagement such as feedback, sense of time, and reward, to establish the state of fatigue (Hopstaken et al., 2015; Katzir et al., 2020; Nakagawa et al., 2013).