The goal of these experiments is to further our understanding of the mechanisms underlying task switching and, specifically, to ascertain whether distinct cognitive processes are engaged depending on the particular task being performed. Our results indicate that distinct forms of task switching are likely to exist and provide some insights into the nature of the cognitive process underlying them. A perceptual switch cost was induced simply by presenting the alternative feature set thereby requiring, in the case of a switch, that attention be reoriented from one set of locations and features to another. Consistent with the strong association between parietal cortex and selective attention (Corbetta & Shulman, 2002
, Kanwisher & Wojciulik, 2000
), the right superior parietal cortex was engaged when participants were required to perform a perceptual switch.
In contrast, a rule switch cost was present even when the alternative feature set was absent. Switching between sets of response rules requires the ability to retrieve the correct rule set and load it into working memory. One might argue that the crucial difference between the rule and perceptual conditions is that the former requires working memory while the latter does not. Often, rule-guided behavior relies on updating the contents of working memory and as such may represent a specific component of working memory (Kerns, et al., 2004
). According to this view selection and maintenance of context-appropriate responses are two reflections of the same underlying neural mechanism. In the present study rule switching was marked by greater activity of the DLPFC, a region strongly linked to rule-guided behavior in other studies and to the maintenance of context for responding across a range of higher cognitive tasks (see Miller & Cohen, 2001
, for a review).
While our findings are consistent with functional hypotheses for the DLPFC and superior parietal cortex proposed by others, it is possible that a region in a different part of the parietal cortex is also modulated by rule shifts, but did not reach threshold in our study. Thus, we are not trying to make strong claims that the DLPFC is only involved in rule switches or superior parietal cortex in perceptual switches. Our main claim is that we do not see one neural region that is engaged by both perceptual and rule shifts. As this claim rests on a null result, however, we must interpret this result with due caution.
We believe that the present results may bear on a current debate in the literature concerning the nature of task-switching with some arguing that the time taken to switch tasks reflects an active process of engaging the current task set (Rogers & Monsell, 1995
; Meiran, 1996
) while others argue that it reflects the time taken for the previous task set to decay (Altmann, 2003
; Ruge et al., 2005
). One could argue that a minimum requirement for a putative control structure would be a strong relationship between functional activity and successful performance. If task switching is an active process, then “good” shifters should engage neural regions involved in task switching to a greater degree than “poor” shifters. Indeed, in the rule switching condition, shifting speed was related to activity in the DLPFC. However, in the perceptual condition, parietal activity was not modulated by individual differences in switching behavior. It may be that both ideas are correct, and that switching difficulty reflects different processes depending on the type of switching being performed. Rule switching may require top-down control with the shift cost reflecting the time taken for the DLPFC to guide and coordinate other regions that undertake the processing for a particular task. In contrast, the shift cost in the perceptual condition may reflect passive interference from competing stimuli and activity of the parietal cortex reflects this greater interference. In any case, our results do not clearly support perceptual switching as reflecting an executive process important for successful performance.
The results of our fMRI study are broadly consistent with other imaging studies of task switching. In an informal review of task switching studies, we noticed that prefrontal activity was primarily reported in studies where rules switched. and summarize studies of task switching that performed whole-brain analyses and reported regions of interest in stereotactic coordinates. As reporting can vary from study to study, we located each peak coordinate in two ways. To determine whether activity was in the DLPFC, we classified Brodmann's area and frontal gyrus of peak activity according to the closest gray-matter region using the Talairach daemon (http://ric.uthscsa.edu/td_applet/
). We classified peak coordinates as being in the DLPFC if it fell within BA 9 or 46, although these locations should be considered approximate. Note, as well, that the Talairach daemon also has a fair bit of imprecision and the reader should interpret BA labels as approximate. In addition to the DLPFC, a region in the inferior frontal junction (IFJ) is observed consistently in studies of task switching (Derrfuss, et al., 2005
). We labeled regions as residing in the (IFJ) based on criteria detailed in a meta-analysis of this region in task switching studies (Derrfuss, et al., 2005
). Our goal was not to provide an exhaustive meta-analysis of the imaging literature on task switching, but simply to determine whether there were any meaningful patterns that could be observed across studies in regard to PFC activity.
Table 3 Areas of the frontal cortex reported in studies of viusospatial attention switching. Letters in parentheses refer to the type of switch performed according to Wager, et al., 2004. (L=Location, A = Attribute, T = task, R = rule, O=object). Note that we (more ...)
Table 4 Regions of DLPFC (BAs 9 & 46) and IFJ involved in switches of rule information. Letters in parentheses refer to the type of switch performed according to Wager, et al., 2004 (L=Location, A = Attribute, T = task, R = rule, O=object). Note that (more ...)
We have classified switch types into those where location, attributes, tasks, rules, or objects switches as in Wager et al.'s meta-analysis of task switching (see Wager, et al., 2004
for a description of these categories) reports studies of perceptual shifting where rule information did not change from trial-to-trial. In general, shifts of visuospatial attention to one or another perceptual aspect of a display did not evoke activity of the DLPFC or IFJ above threshold (see ), although these shifts did engage other frontal regions and the parietal cortex. In one study, participants responded whether a movement-defined or color-defined target was present or absent (Pollmann, 2000). Regions of the parietal cortex and inferior frontal cortex were more active when the target switched dimensions than when they repeated, but neither DLPFC nor IFJ activity was observed. The only study reporting activity in BA 9 was conducted by Weidner et al., (2002)
, and this activity was in a very medial part of the superior frontal gyrus.
In contrast, the DLPFC or IFJ is more consistently reported when task-relevant information changes from trial-to-trial (see ). The types of rule information that may switch in these studies include stimulus-response rules (e.g., respond in the direction of the arrow or the opposite; respond to the location of the target or the opposite), the relevance of conceptual properties of objects (e.g., whether it is man-made or. larger than a breadbox), or which set of items in working memory should be updated (e.g., add one to either the circle shape counter or the triangle shape counter). While most studies listed in report activity in lateral PFC, the location of peak activation in these studies varies widely. For example, regions in the inferior frontal gyrus, middle frontal gyrus, and the IFJ all seem to be associated with rule switching and it is not apparent from the table that there is a clear association between different kinds of rules and PFC regions. For the most part, the peak coordinate appears to reside primarily in the IFJ or left BA 9, either in the inferior or middle frontal gyrus rather than BA 46 (although see Konishi, et al., 2001, Sylvester, et al., 2003
and Parris, et al., 2007
). In contrast, we found a more anterior region in BA 46 to be engaged by rule shifts. While it is unclear how our more anterior region is functionally different from the majority of more posterior regions reported in rule switching studies in , it is clearly distinct from the site found in the parietal cortex for perceptual switching.
One potential exception to the hypothesis that the lateral PFC is important for rule shifts involves imaging studies examining neural activity during a shifting task created by Rogers and Monsell (1995)
. In this task, letters and numbers are simultaneously presented (e.g., “G3”, “A4”) and participants must either say whether the letter is a consonant or vowel or a number is odd or even. On the surface, the switch required in this paradigm would appear to be rule-based because participants have to determine which task should be undertaken (consonant/vowel or odd/even). However, it could be argued that successful performance in this task relies more on visuospatial switching. Once participants in this study know where
to attend – to the left or right of the compound stimulus – they know what to do because a letter cannot be classified as odd or even nor a number classified as a consonant or vowel. Accordingly, imaging experiments using this design have not reported significant DLPFC or IFJ activity (see ). Thus, one reason for the lack of consistent activity in the DLPFC in neuroimaging studies of task switching (Wager, et al., 2004
) may be due to the variety of shifting paradigms employed across studies; some entail switches of rule information whereas others do not involve this type of switching.
In these experiments, our goal was to isolate the effects of rule- and perceptually-based switches on performance and neural engagement. However, most studies have not specifically examined the cognitive processes entailed by their particular task switching paradigm. A good example in both the imaging and neuropsychological literatures is the Wisconsin Card Sorting Task which we would classify as both a rule (sorting rules can change from trial-to-trial) and a perceptual switch (number, color, and shape are features that are always present). Interestingly, while this task is thought to be a test of frontal dysfunction, those with posterior lesions can also be impaired on this task (Anderson, et al., 1991
). A consideration of the underlying cognition for task switches may help to clarify some of the inconsistent finding in both the behavioral, neuropsychological, and neuroimaging research. Indeed, we were able to demonstrate the clinical utility of our design by demonstrating that patients with frontal lobe abnormalities associated with schizophrenia were impaired on rule switching but showed intact performance with perceptual switching (Ravizza, et al., submitted). While our broad categorization of shift types does not explain every inconsistency, we believe it is a promising method of understanding the essence of a task switch.
The design of our experiment is very similar to that used by Rushworth et al (2001
who distinguished between switches of attentional (visual) and intentional (response) set. In the attentional condition, participants switched between attending to colors and shapes while detecting rare targets. In the intentional condition, participants were required to switch the S-R mappings associated with two shapes. While formally similar to our experimental paradigm, Rushworth's studies differ from ours in several aspects including the design, results, and interpretation. First, our study was designed to assess shifting differences across the entire brain (excluding the inferior cerebellum) whereas Rushworth and colleagues used a region of interest analysis focusing on either the parietal or medial frontal cortex. Second, Rushworth reported the opposite results for activity in the superior parietal cortex; that is, this region was more active in the visuomotor switching task than the attentional task. While speculative, this may be due to the fact that the motor task required spatial reversals (i.e., square=left hand, triangle=right hand vs. square=right hand, triangle = left hand) that may call upon visuospatial transformation processes computed by the superior parietal cortex. In contrast, our response rules were not reversals but separate sets of S-R mappings for letters and shapes that remained constant. Third, Rushworth distinguishes between switches of attention vs. switches of action. While we do manipulate sets of response rules, we propose that rule switching is not limited to rules of action. Thus, we predict our findings in the PFC to generalize to other kinds of situations where people must switch between sets of rules - motor or otherwise. However, we tested only one type of shift in rule information (i.e., stimulus-response mappings), and it is possible that the behavioral and neural effects we observed in the rule shifting condition will be relevant only for switching between response rules rather than to all type of rule shifts.
Similarly, we are unable in this design to determine whether a perceptual switch involves reorienting attention to the relevant feature or to the relevant location (to the center or the periphery of the compound letter/shape object). There might be important distinctions behaviorally and neurally between shifts of feature and shifts of spatial location that we are unable to observe in this study. This leaves open the question as to whether the right superior parietal cortex is involved in location or feature switches or both, and it is quite possible that separable regions of the parietal cortex could be found for each type of shift.
These results emphasize that a consideration of the task is crucial to understanding the cognitive and neural mechanisms that allow for this form of flexible behavior. In sum, an appreciation of both the “task” and the “switch” in necessary to any study of task switching.