Inferring the Components of Residual Switch Costs

There are many theories that have attempted to explain the underlying mechanisms of task switching. While these theories have furthered the understanding of switch costs, there are inconsistencies that prevent an all-encompassing explanation. Dykstra et al. (in prep) provided evidence which suggests that switch costs may vary depending on the type of task performed. While this finding is interesting, the probability of switches were not equal between tasks. This may have altered switch costs. The present study utilized the inferred switch task from Dykstra et al. (in prep) and adjusted the switch probabilities to match the comparison task. We found that even after corrections were made, the inferred switch cost was still greater than the comparison switch cost. This finding may help clarify current theories of task switching. COMPONENTS OF RESIDUAL SWITCH COSTS 3 Inferring the Components of Residual Switch Costs Being able to switch between multiple tasks is usually perceived as an effortless endeavor. When studying for an upcoming test, one is easily able to switch from reading a textbook, to checking their phone. However, switching between tasks is not without consequence. Having to go from one thing to another introduces cognitive delays which makes the overall process slower. This delay is known as a switch cost. Jerslid (1927) was one of the first to look at how individuals switch between tasks. In this experiment, participants were required to complete two different lists. One list had participants continuously adding three to a number. The other list had participants switch between adding and subtracting three. Performance on the alternating list was much slower than on the repetitive list. The delay present when alternating tasks became known as a switch cost. Since Jerslid, many have developed theories that attempt to encapsulate the framework of switch costs. One such theory is task-set inertia. The theory of task-set inertia proposes that switch costs are a result of proactive interference from the preceding trial (Allport et al., 1994; Allport and Wylie, 2000). Whenever one is switching between tasks, information in relation to the previous task set conflicts with information required for the next trial. This is supported by the fact that when switching from a more dominant task, a smaller switch cost is present in comparison to the opposite scenario. It is theorized that when performing a weaker task, greater inhibition of the dominant task is required. When a switch from the weaker task to the dominant task is engaged, the greater inhibition of the dominant task takes longer to overcome. This interference translates to switch costs. However, not all theories agree with this account. An alternative framework for switch costs is the two stage model of task-set reconfiguration. COMPONENTS OF RESIDUAL SWITCH COSTS 4 In most two stage models of task switching, there is a portion of the switch cost that occurs prior to stimulus onset and another that occurs once a stimulus has been presented. Rogers and Monsell (1995) developed a task switching paradigm that allowed participants to know of a switch before stimulus presentation. They adjusted the interval between the response on each trial and the stimulus onset of the subsequent trial. They found that if the interval continued to increase, the switch cost would be significantly reduced. However, once the response-stimulus interval (RSI) was lengthened to a second, there was no further reduction in switch trial reaction times (RT). This suggests that a portion of the switch cost reflects time needed to prepare for the next task, as well as an additional process that cannot be prepared prior to stimulus presentation. Specifically, Rogers and Monsell propose that the RSI allows for endogenous reconfiguration of the task set before the next trial. The switch cost that remains reflects an exogenous component of reconfiguration. This refers to stimuli eliciting a habitual response from individuals even if it is in opposition with the intended action on a given trial. This component is known as the residual switch cost. However, the exogenous account is not the only explanation for residual switch costs. Meiran (2000) developed a model of task switching which suggests that residual switch costs are a result of conflicting responses. When a response is bivalent, this can cause interference between the two possible outcomes. An example of this would be when one key on the keyboard can mean either ‘odd’ or ‘greater than five’. This model suggests that even after an RSI long enough to reconfigure the appropriate response set, all possible responses are still present in working memory. While one response may be more readily available, there is still conflict between the two possible outcomes. Conflict reflects the endogenous process of COMPONENTS OF RESIDUAL SWITCH COSTS 5 selecting the appropriate response. This is in opposition to the exogenous theory presented by Rogers and Monsell (1995), demonstrating that it may be unclear as to what comprises the residual switch cost. Another factor that influences switch costs is the probability of switches. Monsell and Mizon (2006) manipulated the probability of a switch trial in a cued task switching paradigm. They found that as the probability of switch trials increased, the switch cost decreased. Monsell and Mizon propose that participants may have noticed when the probability of a switch was high. If this is the case, it would be more beneficial for participants to always expect a switch. This results in preparation for the alternative task before a switch indicating cue was presented, reducing the switch trial RT. It also leads to an incorrect reconfiguration during repeat trials. This would then require an additional reconfiguration after stimulus presentation, thus increasing repeat RT. These results would lower the overall switch costs. An alternative explanation suggests that participants may have been ‘neutral’ between the two tasks, meaning that neither task was prepared for prior to cue presentation. This would result in preparation being required for both repeat and switch trials. Using this strategy, mappings are updated on every trial, resulting in lower switch costs. Overall, this study demonstrates that the probability of switch trials influences switch costs. It also suggests that there may be multiple strategies for switching between tasks. While it was originally suggested that switch costs were primarily based on interference from previous trials or task-set reconfiguration (Allport and Wylie, 2000; Rogers and Monsell, 1995), the two theories may not be mutually exclusive. Monsell (2003) proposes that switch costs may be accounted for by both. Reasoning for this stems from the residual switch cost. Monsell states that interference from the previous task set is likely a contributing factor in the COMPONENTS OF RESIDUAL SWITCH COSTS 6 residual switch cost. But even that does not provide a complete explanation. The cause of the residual switch cost has been difficult to definitively define. Rogers and Monsell (1995), Meiran (2000), and several others (Allport et al., 1994; Koch and Allport, 2006) all propose different ideas. It could be the case that one of these accounts is correct, but it seems more likely that several of these theories play a role. Recently, Dykstra et al. (in prep) provided interesting results that may provide insight into the composition of switch costs. In most task switching experiments, participants are required to switch between the dimensions of a bivalent stimulus in which they need to attend. Dykstra et al. (in prep) instead had participants switch between two response mappings. In this study, participants were presented with colored shapes. Only one feature of those colored shapes would be relevant on any given task. Participants were cued at the end of each trial on which response mappings would be needed for the next trial. There were two versions of this response switching task that are of particular interest to the current study. The directed switch task and the inferred switch task. In the directed switch task, participants were taught two mappings, 'A' and 'B'. Participants were then instructed after each response on which mapping to use for the next trial. This was indicated by an audio cue of either 'A' or 'B'. In the inferred switch task, participants would be given an audio cue that corresponded to the response that was made according to the current mapping that was in use. For example, one mapping had green associated with 'J' and red associated with 'K'. After pressing the 'J' key, if participants heard ‘red’ and there was a red shape present, they would continue using the same mapping. If participants heard ‘green’ after pressing 'J', since the audio cue did not match the stimuli on the screen, this would have been an indicator to switch mappings. COMPONENTS OF RESIDUAL SWITCH COSTS 7 Even though this was not a traditional task switching experiment, Dykstra et al. (in prep) still found switch costs to be present when participants were required to switch between response mappings. A finding of particular interest was that switch costs were greater in the inferred task than in the directed task. While it is tempting to determine that there is something unique about the inferred switch task, there is one difference between the two tasks that may have led to an increase in switch costs. This was the probability of switches. Dykstra et al. (in prep) had the proportion of switches in the directed task set at 50%. In the inferred task, the proportion of switches was set at 15%. Previous research has found that the lower the probability of a switch in a cued task switching paradigm, the greater the switch cost (Monsell and Mizon, 2006). The fact that the inferred switch task had such a low probability of switches may have been cause for an increase in switch costs. The