The work at hand has argued that the present data do not offer support for task-set decay during the RCI, whereas the data seem to be well accounted for by assuming that episodic task-set retrieval is modulated by temporal distinctiveness, which is varied in changing RCIs. This novel account is largely consistent with existing data in the literature (e.g., Altmann, 2005). However, it appears to be inconsistent with findings by Koch (2001). As already described, RCI was manipulated between-subjects in that study, and the data showed that switch costs were smaller with long RCI than with short RCI, which was taken
120 GENERAL DISCUSSION
as support of an assumed process of task-set decay. Note that the RCIs were constant for each individual subject in Koch’s (2001) study, so that the RCI manipulation did not affect temporal distinctiveness as defined in the present paper. To reconcile these data with the present account, one can speculate that decay of cue-repetition priming might have contributed to the observed RCI effects in the present experiments. In Koch’s (2001) study, subjects switched among three tasks, rather than between two tasks which might have influenced the priming level. It would certainly be interesting to examine the influence of number of tasks on task-set repetition priming in future studies.
Another issue concerns the observation of generally decreased RTs with increased RCI in the blocked RCI condition of Experiment 3, which replicated Altmann’s (2005) findings observed in a between-subjects design. This finding certainly contradicts any prediction of a short-term decay account. However, while the constant size of the repetition priming effect in these conditions is fully consistent with the temporal-distinctiveness account, this in itself does not offer any basis for predicting this effect either. To explain this effect one can assume that performance is affected by temporal preparation when the RCI is predictably constant over a large number of trials, as is the case with blocked RCIs. Temporal preparation might thus become strategically involved with blocked or constant RCIs, whereas it plays only little role if the RCI remains unpredictably unchanged (as in the mixed RCI conditions in Experiments 3-4) or when predictable RCI changes alternate with unchanged RCI (as in the predictable RCI condition in Experiment 4). It is possible that a predictable long
GENERAL DISCUSSION 121
RCI encourages preparation for the temporal onset of the upcoming stimulus (see, e.g., Los, in press, for a review). Note that this stimulus is the task cue in the present context, whereas studies on temporal preparation in single-task contexts typically vary the intervals between the response and the onset of the target stimulus (e.g., Los & van der Heuvel, 2001) or between cue and target in task switching studies (e.g., Meiran & Chorev, 2005), so that it is not easy to compare the effects of these different manipulations. Therefore, one can only speculate that predictable long RCIs resulted in generally facilitated cue processing, for both task switches and task repetitions alike (Meiran, Chorev, et al., 2000). Hence, while the temporal-distinctiveness account does not specifically predict this effect, it is also not damaged by its observation.
It should also be noted that the present account, which refers to priming in cue-triggered episodic task retrieval that is modulated by temporal distinctiveness, appears to fit the present data much better than the short-term decay account, but our account is not yet cast in terms of a formal model. For example, temporal distinctiveness was defined simply based on the relation of the current RCI to the preceding RCI, but it is possible that a mathematically more complex definition might be required if more quantitative predictions need to be derived.
Given that the present task-switching paradigm differs substantially from experimental paradigms typically used in research on temporal distinctiveness in serial memory (see, e.g., Brown, Neath, et al., 2007; Neath & Surprenant, 2003), a simple transfer of a formal definition is not trivial. Also, currently the present work focused on explaining the task-repetition benefit. To this end the
122 GENERAL DISCUSSION
definition of temporal distinctiveness was restricted to the current and the immediately preceding RCI, as these seemed to be the most relevant in the present context. Nevertheless, it would certainly be important to see whether temporal distinctiveness is important over longer runs of tasks, as well.
Likewise, it might be interesting to examine whether temporal distinctiveness is a variable that can affect cue-triggered task retrieval in task switches, even though the present data do not suggest a strong influence of RCI on task switches. From a theoretical perspective, it is conceivable that episodic retrieval might occur just as much on switch trials. Presumably, the cue would retrieve prior episodes in which that cue occurred, and this in turn should help performance in the same way as on repeat trials. However, the present manipulations are of the relative recency of the n-1 and n-2 trial (and hence the temporal distinctiveness of trial n-1), and the relevant prior episode for a switch trial is never trial n-1. Thus, one would not expect to get an effect of RCI on switch trials. However, there might be an effect of the temporal distinctiveness of the most recent trial on which the same task cue was presented. For example, if the episode is surrounded by two long RCIs, it should be more distinctive than if surrounded by two short RCIs. Empirically, at this point the design of the present experiments does not allow a reasonable analysis of the influence of temporal distinctiveness on task switches due to long lags (i.e., distances) of a repetition of the same episode. It might therefore be interesting for future designs to examine whether there are still some diminishing benefits of temporal distinctiveness as the most recent episode retreats into the past (e.g. for ABA trials). This would be in line with the famous “telephone poles
GENERAL DISCUSSION 123
analogy” by Crowder (1976), which compares memories to telephones poles when one is on a moving train. In analogy to telephone poles in the receding distance becoming less and less distinctive from their neighbors, each item in the memory list is assumed to become less distinctive from the other items (in a list) as the episode recedes into the past (Crowder, 1976). In any case, for future research it seems desirable to develop our account into a more quantitative format that also allows computational modeling.
Additionally, it should be noted that the present study was focused on temporal factors that modulate the task-repetition benefit. It can be argued that in the explicit cuing version of task switching, RCI manipulations affect the size of the task-repetition benefit due to variations of temporal distinctiveness in episodic retrieval rather than by an underlying unconditional decay process. However, this argument does not exclude other potent influences on the task-repetition benefit. As already discussed, task preparation is one of the major factors that influence task-switching performance (e.g., Altmann, 2004; Meiran, 1996;
Rogers & Monsell, 1995), but the present study was not designed to address this factor. Also, it has been shown that task-repetition priming is affected by a number of other factors, such as persisting stimulus-task associations (e.g., Koch & Allport, 2006; Waszak et al., 2003), response repetitions (e.g., Kleinsorge & Heuer, 1999), or general contextual factors (e.g., Lien & Ruthruff, 2004). It will be an important task for future research to examine whether and how the influence of these factors relate to temporal distinctiveness in episodic task retrieval.
124 GENERAL DISCUSSION
Finally, returning to the general context of cognitive control in the Introduction, one needs to stress the importance of using different task-switching paradigms and/ or different tasks. Cognitive control is a “general concept that applies to a wide variety of tasks in many different ways” (Logan, 2003, p. 48). Thus, since different task-switching paradigms might tap different abilities (Meiran, in press), more research is needed in order to disentangle task specific processes from cognitive control.
125
9 Conclusion
The present study aimed at contributing to the knowledge about underlying mechanisms of action control in changing task context. The task-switching paradigm allows to compare performance between switching tasks and repeating tasks. The resulting “switch costs” are typically interpreted in terms of cognitive control processes (Monsell, 2003), which are thought to enable flexible adaptations to environmental changes. The study at hand used a variant of the task-switching paradigm, namely the task-cuing paradigm, and focused on effects of manipulating the interval between a response and a subsequent cue (i.e., the RCI). Specifically, the present series of experiments re-examined the idea of passive decay of task-representations (task sets) in memory, as commonly assumed in many models of task switching. Task-set decay has been thought to be indexed by manipulating the RCI in a task-cuing paradigm.
The major support for decay of task set is traditionally assumed to come from studies which found that switch costs decreased as RCI increased, suggesting an underlying process of decay of set activation. The assumption of task-set decay plays a theoretical role in current literature on task switching (e.g., Meiran, Chorev, et al., 2000). However, the empirical evidence for task-set decay is much less consistent than is theoretically desirable. Therefore, empirical evidence for RCI effects on switch costs can serve as a basis for deriving assumptions about corresponding underlying processes in task
126 CONCLUSION
switching. This in turn can be helpful for a better understanding of the mechanism(s) of task switching in general and was the aim of the study at hand.
In contrast to previous works (see, e.g., Koch, 2001; Koch & Allport, 2006;
Meiran, Chorev, et al., 2000; Sohn & Anderson, 2001), the present study suggests that short-term decay of task set is not a major factor in explaining switch costs in task switching. The data observed in the present seven experiments refute this suggestion by presenting empirical evidence from RCI manipulations that critically damage the decay account of RCI effects.
Specifically, the data show that RCI effects on switch costs are mainly due to an influence on task repetitions, whereas task switches are hardly affected.
Further, RCI influenced the task-repetition benefit only when RCI changed from the previous to the current trial. As an alternative to short-term task-set decay, the present work proposes that episodic task retrieval in task repetitions is modulated by temporal distinctiveness, which can be affected by RCI manipulations. Further, the data imply that the effects of RCI in task switching are primarily linked to the stimulus-related components and probably not to response-related components. Thus, based on the novel account for RCI effects it is suggested that the Input-Set (or Stimulus-Set) is the component of the tasks set which is affected by temporal distinctiveness during episodic retrieval.
CONCLUSION 127
Numerous processes are involved in cognitive control. The task-cuing paradigm serves as a fruitful method to study different components of cognitive control.
An important contribution of the present study is to demonstrate the relevance of the role of episodic task retrieval influenced by temporal distinctiveness in task switching. An important goal for future research will be to further investigate the variable of temporal distinctiveness in cognitive control.
129
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