In this investigation, we tested the hypothesis that interference resolution is mediated by a separable executive process that is shared by tasks in different cognitive domains. Our goal was to provide converging behavioral evidence to complement the existing neuroimaging data suggesting that these tasks share a common neural substrate. The process-specific fatigue effects demonstrated between pairs of tasks, provides behavioral support for their reliance on a common mechanism. In particular, the results showed that performing a working memory task with high interference trials resulted in reduced ability to resolve interference in both a semantic task and an episodic task. In contrast, participants who trained on a working memory task without interference had no reduction in the ability to resolve interference on the transfer tasks. When two tasks that did not involve the same executive demands (response inhibition vs. interference resolution) were used, neither version of the training task had any effect on the transfer task. This pattern of results was predicted based on neuroimaging evidence indicating anatomical overlap, or lack thereof, between the neural circuitry recruited by the executive demands these tasks. The current observations suggest that tasks used to investigate performance in several different cognitive domains (episodic memory, working memory, and semantic memory) share a common executive component. Furthermore, this component can also be separated from other specific executive functions, such as response inhibition.
These conclusions are generally consistent with studies using latent variable techniques to investigate whether the central executive is unitary or can be separated into several partially independent mechanisms. For example, using a large sample of subjects, Miyake et al. (2000)
investigated several tasks that required executive processes in order to determine the separability of three putative executive functions (shifting, updating and inhibition). Using confirmatory factor analysis they found that these three executive components are correlated with each other, but are clearly separable, indicating both unity and diversity of executive functions. Another common finding, however, is that intercorrelations among widely used executive tasks can be low and lack statistical significance (e.g., Lehto, 1996
), as one might expect if the tasks draw on separable executive functions.
Although the exact properties of the mechanism(s) recruited in the present high interference conditions remain to be specified, our findings provide some constraints on the nature of the shared executive process and have implications for the functional organization of IFG. First, as mentioned above, the lack of an interaction between the verb generate task and the stop signal task in experiment two dissociates interference resolution demands from response conflict or inhibition, an outcome that is consistent with prior work from our lab (Nelson, Reuter-Lorenz, Sylvester, Jonides, & Smith, 2003
) and others (see e.g. Aron, Robbins, & Poldrack, 2004
). Second, while there may be greater demands on context retrieval operations in the high PI conditions of the working memory and episodic memory tasks (e.g. Badre & Wagner, 2004
), there is no apparent need for context retrieval in the high interference condition of verb generation. Therefore context retrieval does not provide a satisfactory description of a control process that might be shared by all three tasks. Another process attributed to subregions of IFG is controlled memory retrieval, which refers to the engagement of top-down retrieval operations when more automatic processes (i.e. familiarity, associative strength) are insufficient to guide responses (e.g. Badre & Wagner, 2004
). Although it could be argued that the high interference conditions of all three tasks involve controlled retrieval, the source from which information is retrieved clearly differs between the tasks in that verb generation draws from semantic memory, the paired-associates task from episodic memory, and the letter identification task presumably draws from working memory. Moreover, controlled semantic retrieval has been linked to left BA 47 which is anterior (e.g. Gold, Balota, Kirchhoff, & Buckner, 2005
; Poldrack et al., 1999
) to the 44/45 region that is commonly activated by the three interacting tasks in the present study.
We believe the most satisfying account of the present results is the possibility of a general selection process that is engaged when goal-relevant representations must be selected from among competitors (Badre, Poldrack, Pare-Blagoev, Insler, & Wagner, 2005
). A general selection process that is recruited to meet the demands imposed by competing representations would provide a unitary account of the interactions we observe among the three high interference conditions in the present set of studies. Importantly, a selection process can be involved in multiple memory and task domains if this mechanism operates on active representations that are maintained in working memory. Thus, the general selection processes implicated here could potentially be involved in a range of tasks that extends beyond those used in the current study. One potential operation that could implement selection is an attentional template that consists of properties relevant for the current task (see Jonides & Nee, 2006
for a discussion). The particular properties of the template, and the representations on which it acts would be determined by the dynamic interplay between the shared neuroanatomical resource and task specific circuitry that could presumably be identified via connectivity analyses applied to neuroimaging data.
The results from these experiments support a resource model of executive processes. Some evidence for resource depletion of executive functions (i.e. negative transfer) more generally has been found in studies of suppression of stereotypes and negative attitudes and self-regulation (e.g. Richeson et al., 2003
; Vohs & Heatherton, 2000
). Neural correlates of resource depletion related fatigue effects have also been observed in lower-level systems. For example, reduced functional activation of primary motor cortex has been found following fatiguing exercise of the hand (Benwell et al., 2006
). Furthermore, prolonged exposure to a visual pattern disturbs visual perception, and single cell recording show have shown that prolonged stimulation of about 30 sec fatigues visual neurons in the primary visual cortex to give a weaker response that they otherwise would (Carandini, 2000
). However, the current study is, to our knowledge, the first attempt to use a resource depletion framework to examine a specific executive control process. Whether practice or experience involving executive function is transferred as reduced (i.e., fatigue) or enhanced (i.e., training) performance on a subsequent executive task is a question requiring further investigation. The answer appears to be related to a number of factors, including the extent of practice, and whether time is allowed for recovery.
It might be argued that selection processes, of the type the present experiments were designed to tap, may be active at some level in many everyday activities and therefore very resilient to fatigue effects. It should be noted, however, that the effects of fatigue on selection processes are rather small and do not result in the inability for an individual to perform the task. A slight increase in RT to perform a certain cognitive operation in everyday life activities due to cognitive fatigue may not be recognizable if not specifically tested in an experimental setting. Also, even if an individual is engaged in a task that involves a specific executive component, everyday activities (e.g. a conversation) often allow for recovery, unlike the continuous engagement of selection processes in the present experiments.
The reliance of the stop-signal task to test to inhibition effects on subsequent selection processes may have some limitations. For example, longer RTs in the HIGH inhibition group compared to the LOW inhibition group could reflect a tendency to wait for the stop-signal in spite of instructions, instead of inhibitory processes. With the current design, the interpretation that participants in the HIGH inhibition group change their decision criteria in response to the increased number of stop-trials cannot be completely ruled out.
In conclusion, the current data extend prior neuroimaging evidence for executive processes that are shared across cognitive domains by demonstrating behavioral interactions between tasks hypothesized to recruit the same process. These observations support a nonunitary central executive composed of separable subcomponents, one of which may be a general selection mechanism that resolves interference. The results also agree with the view that cognitive control processes are resource limited and can be temporarily depleted.