25.1. How our predictions concerning memory and inhibition fared
We predicted that inhibitory demands would account for a greater proportion of the variance in children’s performance than in adults, and the more so the younger the child. Consistent with our prediction, Dots-Incongruent (where inhibition of the spatially-compatible response was required on all trials) was more difficult than Dots-Congruent (where the spatially-compatible response was correct on all trials) and the more so the younger the children. Accuracy and impulsivity differences between those two conditions decreased over age. The RT difference on Dots-Incongruent versus Dots-Congruent (which differed only in their inhibitory demands) was greater for the youngest children than the RT difference on two- versus six-Abstract-Shapes (which differed only in their memory requirements). This finding surprises adults who have taken our task battery because for adults the inhibitory demand in Dots-Incongruent feels rather minimal while the memory demand in six-Abstract-Shapes feels quite substantial. However, this is fully consistent with greater costs being exacted by inhibitory demands even in steady-state than memory demands for the youngest children. Also consistent with this prediction, we found that the spatial incompatibility effect (the cost of inhibiting the pull to respond on the same side as the stimulus) was greater the younger the children. This suggests that the younger the children, the harder it was for them to muster inhibition at either (a) the level of attention to disregard an irrelevant aspect of the stimulus (its spatial location) and/or (b) the level of response to override the prepotent tendency to respond on the same side as the stimulus.
We looked at the spatial incompatibility effect in the context of lower- and higher-order rules, different memory loads, and in the context of task-switching. While in the Pictures test, the typical low-level rules pertaining to individual stimuli were used and memory demands were minimized by the use of icons over the response-sites, we also investigated the spatial-incompatibility effect in hybrid, conceptual tasks (Arrows and Dots) where the rules were more abstract, spatial location had to be integrated with stimulus identity, and no icons were provided to remind subjects of the stimulus–response mappings. The working memory requirements were greater for these later tasks because they required mental computation to determine the correct response. Instead of the rule being “for A press left” (the typical rule on Simon tasks, which requires attending only to the stimulus or a particular property of the stimulus), the rule for the Arrows and Dots tasks was “for A press on the side opposite A.” Knowledge of only which stimulus appeared or only where it appeared was insufficient on these tasks; those two pieces of information had to be integrated. The Arrows and Dots tasks differed from each other in that the memory demands were minimal on the Arrows task because the stimulus pointed to the correct response on each trial.
In the Pictures task spatial incompatibility effects were significant for both RT and accuracy for children of all ages, produced the greatest effect on RT, and decreased more in size as a function of age than on the Arrows or Dots tasks. Even at 13 years of age, children still showed a significantly greater Simon effect on the Pictures task than did young adults. The spatial incompatibility effect was weakest and showed the least change over age in the Dots task, even though in the Dots and Arrows tasks the spatial location of the stimuli had to be explicitly taken into account and on the Pictures task it did not. The Dots-Mixed condition was the only task in which the spatial incompatibility effect was not evident in both RT and accuracy. In Dots-Mixed, the spatial incompatibility effect was evident only in RT, its effect on RT was weaker for the youngest children (4–6 years old) and adults than in the Arrows or Pictures tests, and the size of its effect on RT did not change significantly over age. The Arrows condition produced a significant spatial incompatibility effect on both RT and accuracy, and a decrease in the size of the effect on accuracy (though not on RT) over age. These results – a stronger spatial incompatibility effect the easier the task – are consistent with results in the literature showing that this effect decreases as a function of task difficulty (Hommel, 1993
; Vu & Proctor, 2004
). However, those results have previously been interpreted to mean that anything that increases response time will decrease the Simon effect (giving the automatic activation of the irrelevant stimulus location information time to decay). In contrast, we found that although younger children took much longer to respond than older children, they showed a larger spatial incompatibility effect.
The larger Simon effect found for the younger children might indicate that their ability to exercise inhibition of the pull to respond on the same side as the stimulus was weaker than that of older children. It may have also been affected by the greater likelihood of younger children to use verbal mediation. Though the Pictures task could be solved by simple perceptual matching, some younger children named the stimulus out loud on some trials. Similarly, on the Dots task, younger children often called out the rule (“same” or “different”) on trials in the Mixed condition. In adults, the Simon effect is stronger, and does not diminish with consecutive incompatible trials, when the stimulus or response has a verbal property (Proctor & Vu, 2002
The lack of an accuracy cost on spatially incompatible (Incongruent) trials in the Mixed block of the Dots task is in sharp contrast to the results when comparing separate blocks of Congruent and Incongruent trials on the task, which showed a significant spatial incompatibility effect for children of all ages in both speed and accuracy, though not for adults. The lack of a spatial incompatibility effect for accuracy within the Dots-Mixed condition, in conjunction with the increase in RT and reduction in accuracy for all trials in that condition relative to the other two Dots conditions, may indicate that participants exercised inhibition on both Congruent and Incongruent trials when those trials were intermixed in a difficult task-switching context. It may also be due to an order effect (see below).
The results of Crone et al. (in press)
, with a task similar to our Dots task and with subjects ages 7–8, 10–12, and 20–25 years, are similar to ours. Like us, they found a stronger Simon effect comparing across the single-task blocks than in the Mixed block, but unlike us they found that the Simon effect disappeared altogether in the Mixed block. Like us, they found the single-task block Simon effect to be significant for both accuracy and RT, but unlike us they found no change in the size of the accuracy or RT Simon effects over age.
Our study of spatial compatibility effects with conceptual rules and switching between those rules, as well as stimulus-level S–R associations, deserves additional follow-up. Our Dots and Arrows tests required integrating stimulus-appearance information with spatial location information. Since spatial location was relevant to the correct response they were not pure Simon tasks, but hybrid spatial incompatibility tasks. There is evidence that the neural bases for working memory of object-appearance and spatial-location information are somewhat different (Haxby, Petit, Ungerleider, & Courtney, 2000
; Levy & Goldman-Rakic, 2000
; Mecklinger & Mueller, 1996
) and spatial location is exactly the stimulus property that feeds the spatial-compatibility bias. In standard Simon tasks, subjects would perform better if they could (theoretically) screen out the location of the stimulus, but on our Dots and Arrows tasks information about the location of the stimulus is critical for determining the correct response. Will results be similar to those on our Dots test, if the conceptual rules are, for example, “If the stimulus is an animal, press right; if the stimulus is a vehicle, press left”? Here, the same number of mental steps ( Is the stimulus an animal or a vehicle?  Where do I press for that?) would be required as in our Dots test ( Which rule [same side or opposite side] pertains to this stimulus?  Which side is the stimulus on?). That would be a conceptual task with higher-order rules (like our Dots task), but unlike our Dots task it would be a pure Simon task, not a hybrid.
Similarly, Wascher and colleagues (Wascher, Schatz, Kuder, & Verleger, 2001
; Wascher & Wolber, 2004
; Wiegand & Wascher, 2005
) provide evidence for the involvement of both visuomotor and cognitive mechanisms in Simon task performance. Modifying the task slightly (e.g., presenting vertical rather than horizontal stimuli) changed the distributions of RT scores, and was taken as evidence for involvement of a cognitive component. An interesting developmental question is, “Does the effect function across Simon task variants change over age and if so in what ways and why?” Our manipulations (increasing or decreasing the working memory load) might also change the distribution of RT scores but, unfortunately, the vincentization procedure used by Wascher and colleagues involves splitting the RT distributions into quantiles (i.e., quartiles or even deciles) and would result in relatively few datapoints per bin in the current study. Future research with these tasks with more data per subject could allow more thorough investigation of the RT distributions across development.
In young adults, in whom inhibitory control is more mature, we had predicted that memory demands would exact a greater cost than inhibitory demands. Beginning at 10 years, increased memory demands (six versus two rules) took a greater toll on accuracy than did consistently inhibiting the tendency to respond on the same side as the stimulus (Dots-Incongruent versus Dots-Congruent)—the opposite of the pattern observed in the youngest children. Also in line with the greater role of memory in what is demanding for adults, taxing cognitive flexibility and inhibition in a switching context was not that hard for adults if memory demands were minimal. Unlike the case for children of all ages in our study, the Arrows test was easy for adults. However, the Dots-Mixed condition, which presented the same demands on inhibition and cognitive flexibility as did the Arrows test but in addition taxed working memory more, was difficult even for adults. The difference in accuracy on Dots-Mixed and Arrows, which differed only in the greater working memory demands in the Dots condition, was greater for young adults than for children at any age from 7 to 13 years. The greater memory demands in the Dots condition made a big difference for adults and the youngest children, but not for the majority of children (aged 7–13 years).
Since we are discussing performance on Dots-Incongruent and Dots-Congruent here, it is appropriate to note that one of the striking differences between the results for children and adults was that effects elicited only in Mixed blocks (e.g., in Dots-Mixed) with adults were found in children even in single-task blocks (e.g., Dots-Incongruent versus Dots-Congruent Blocks). For example, even though switching between rules that require inhibiting a prepotent response or making it is what was most difficult at all ages, even inhibition in steady state (Dots-Incongruent) was more difficult for children than going with their prepotent response on every trial (Dots-Congruent). At every age, without exception, children were slower and less accurate in the Dots-Incongruent block than the Dots-Congruent Block. Thus, inhibition, even in steady state, was sufficiently difficult for children to elicit a cost in their performance. This was not true for adults. Adults performed comparably in both speed and accuracy in the Dots-Congruent and Dots-Incongruent conditions. They required having to switch between the two conditions for a significant effect on their performance to be evident.
These results for adults are consistent with those reported in other studies of the spatial incompatibility effect in adults. As we found here, adults tend not to show the spatial incompatibility effect if Congruent and Incongruent trials are administered in separate, single-task blocks (Stürmer et al., 2002
; Valle-Inclán et al., 2002
; Verbruggen et al., 2005
; Wühr, 2004
). Adults evidently re-set their default response if several trials in a row are Incongruent and so are no slower on those Incongruent trials than on Congruent ones. Indeed, adults can show a reverse spatial incompatibility effect when switching back to responding on the same side as a stimulus after several trials of responding on the side opposite the stimulus (e.g., Logan & Zbrodoff, 1979
). Children from 4 to 13 years of age, on the other hand, evidently did not re-set their default response. They showed the spatial incompatibility effect with Congruent and Incongruent trials administered in separate, single-task blocks. Inhibition in steady state took a toll on the performance of children even as old as 13 years, but not on that of adults.
Note that participants (including adults) in our study showed a spatial compatibility effect in Dots-Mixed although all subjects were tested on Dots-Incongruent immediately before that. Just as adults typically show little cost in making spatially-incompatible responses if they are tested on a block where they need to do that on every trial or most trials, Tagliabue, Zorzi, Umiltà, and Bassignani (2000)
found no Simon effect in Mixed blocks when adults were given a run of Incongruent trials beforehand. Given those results it is possible that we might have found a much stronger spatial incompatibility effect in Dots-Mixed if it had been preceded by a block of Congruent trials. In order to test participants at all ages under the same conditions we had not varied task order, so further study would be needed to address that interesting possibility. It is also possible that other effects observed here might appear different if the tasks were administered in a different order. For example, Dots-Incongruent was always tested after Dots-Congruent. We do not think that caused a difference in the memory demands between the two conditions because the rule for Dots-Incongruent was taught and practiced immediately before that block just as the rule for Dots-Congruent was taught and practiced immediately before that block, but given that we did not vary task order we cannot prove that that is the case.
Based on our hypothesis that inhibitory control is extremely problematic for very young children, we predicted they would perform poorly on all trials requiring inhibition (Incongruent trials and switch trials) and that those effects would be additive. That is, we predicted they would perform worst on switching to the Incongruent (more difficult) condition; opposite to the pattern typically reported in adults. We predicted that after that early period, we would see greater switch costs at all ages for switching to the easier (Congruent) condition than to the harder (Incongruent) condition (consistent with the asymmetric switch costs previously reported in adults [Allport & Wylie, 2000
; Allport et al., 1994
; De Jong, 1995
; Kleinsorge & Heuer, 1999
; Los, 1996
and Stoffels, 1996
; Wylie & Allport, 2000
]). Further, for intermediate-age children, who are beginning to exercise better inhibitory control, we reasoned that doing so should require greater effort than in older participants and so predicted that undoing that inhibition (switching back to making the dominant response) should exact a greater cost in those children than in adults. Thus, we predicted that beginning after 6 or 7 years of age, asymmetric switch costs would be larger in younger than older participants. These predictions were confirmed for RT. On both Arrows and Dots-Mixed, children of 4–6 years showed a greater RT cost for switching to the Incongruent rule than the Congruent one. In both of those conditions, adults and older children showed a greater RT switch cost for switching to the Congruent than the Incongruent rule (consistent with previous reports of asymmetric switch costs). In both Arrows and Dots-Mixed, the differential RT cost of switching to the Congruent rather than the Incongruent rule was largest at 7–10 years of age. For accuracy, on the other hand, across the age spectrum on both Arrows and Dots-Mixed, people were more accurate when both the rule and the response changed than when just the rule changed. Thus the asymmetric switch costs reported in the literature primarily for RT, were found here for accuracy at all ages (even among the youngest children).
Crone et al. (in press)
found results for RT that resemble ours for accuracy and results for accuracy that resemble ours for RT. Across their age spectrum (7–23 years), they found faster responses when both the rule and the response changed than when just the rule switched. Across our age spectrum (4–45 years), we found a greater percentage of correct responses when both the rule and the response changed than when just the rule switched. We found this effect on accuracy to be largest at 8–10 years; they found this effect on RT to be largest at 7–8 years. Mirroring in reverse these greater effects in young children versus older children and adults, Mayr (2001)
found the RT effect (faster rule switching when the response also changed) to be greater in older versus younger adults. Crone et al. found that accuracy costs did not show the pattern they expected; more errors occurred on switch trials when the response-site also changed than when it remained the same. We similarly found slower responding on rule-switch, response-switch trials until age 13 on our Dots-Mixed task which resembles Crone et al.’s (this encompasses two of Crone et al.’s three age groups), but unlike Crone et al.’s results for accuracy, we found faster responding when both rule and response switched for adults.
We had predicted that even the youngest children would find it easy to hold two rules in mind and that although it would be harder for everyone to hold more rules in mind than fewer, the relative difficulty of this would not change over age. Indeed, as predicted, holding two arbitrary rules in mind was easy even for our youngest participants. At all ages performance was excellent on the two-Abstracts-Shapes and Dots-Congruent conditions (the latter requiring holding a superordinate rule in mind and mentally translating that into the appropriate embedded rule on each trial). This is consistent with other evidence that children can hold two conditional rules in mind by 4(½)–5 years of age (Campione & Brown, 1974
; Doan & Cooper, 1971
; Gollin, 1964
; Gollin & Liss, 1962
; Osier & Kofsky, 1965
; Shepard, 1957
). Although everyone found the increased memory load (six versus two Abstract Shapes) more difficult, the size of the effect changed little over age in either speed or accuracy when all subjects were included in the analyses. More fine-grained analyses, however, showed that how the difference in difficulty was handled differed over age. The speed-accuracy tradeoff changed over age. Accuracy on six-shapes more closely approximated that on two-shapes the older the subjects. RT in the two conditions, however, diverged more the older the subjects.
Across conditions, older participants slowed down to preserve their accuracy on more difficult trials. Thus they showed sizeable RT differences and small accuracy differences. An elegant analysis of this tendency of adults to alter their response times to preserve a constant level of accuracy in the face of variations in task difficulty is provided by Usher and McClelland (2001)
and Usher, Olami, and McClelland (2002)
. In our study, younger children often showed less of a change in speed and hence showed very large differences in accuracy across trials of differing difficulty. For example, older participants were better able to modulate their speed and slowed down in the difficult Dots-Mixed condition relative to the easy Dots-Congruent condition to minimize a reduction in accuracy. Younger participants (even those given a very long time to respond) kept their response speed more constant across conditions, perhaps because they were too impulsive to take more time when they needed it, at the cost of accuracy in the difficult conditions. Hence, for example, the accuracy difference between the Dots-Mixed and Dots-Congruent conditions decreased with age but RT differences between those two conditions increased over age (see ). Similarly, the mixing cost of Congruent and Incongruent trials being mixed together declined over age for accuracy but increased over age for RT (see ). Likewise over age, differences in performance on the six- and two-Abstract-Shapes conditions declined in accuracy but increased in RT.
The very youngest children (4–5 years of age) were given a long time to respond (3000 ms) so it is unlikely that they lacked sufficient time to modulate their response speed. It is more likely that they had difficulty inhibiting impulsive responding, i.e., difficulty withholding their response long enough to take the time they really needed. For instance, the RTs for children of 4–5 years on nonswitch Incongruent trials in the Dots-Mixed condition differed little from their RTs in the easier single-task Dots-Incongruent block. This was true even though their RTs in both conditions were on average less than half of the time allotted, so they had time to compute their responses but did not make use of that extra time. Their inhibitory problems can also be seen in their greater likelihood to respond impulsively before a stimulus appeared and to fail to promptly stop pressing a response button after responding.
These findings concerning not taking the time they needed are fully consistent with results reported by Gerstadt, Diamond, and Hong (1994)
and Diamond, Kirkham, and Amso (2002)
on a different task, the Day–Night Stroop-like task, where children had to say “night” to a daytime image and “day” to a nighttime image. Gerstadt et al. (1994)
found that: (a) those children of 3(½)–4(½) years who took more time to compute their answers were able to answer correctly on more trials than children who answered more quickly and (b) within child, on those trials where a child of 3(½)–4(½) years took longer to respond, the child was more likely to be correct. Diamond et al. (2002)
manipulated time to view the stimulus and compute the response by chanting a ditty to the child either after the stimulus was presented but before the child could respond or between trials before the stimulus was presented. Diamond et al. found that 4- and 4(½)-year-old children were correct on significantly more trials in the manipulation that gave them more stimulus-viewing and more response-computation time (ditty chanted while stimulus was visible) but performed no better than in the basic condition when the extra time could be used to remind themselves of the rules but not to instantiate the correct rule for the current trial (ditty chanted between trials, before stimulus was visible).
On the other hand, we have evidence here that if a task is sufficiently easy that 4–5-years-olds can compute the answer in roughly a second, they will modulate their speed to preserve their accuracy. On the Pictures test, for example, children of 4–5 years slowed way down on Incongruent trials relative to Congruent ones, thereby preserving their accuracy so that the difference in their accuracy on Incongruent versus Congruent trials was smaller than that seen by older children of 6–8 years given less time to compute their responses (see ). Similarly, on the Arrows test, children of 4–5 years used the ample time allowed them to maintain an accuracy level of over 80%, a level of accuracy not seen when given less time to respond until children were 10–11 years old. Children of 4–6 years also showed smaller local switch costs on the Arrows test than did the older children; they achieved this by using their allotted time to slow down on the switch trials; their RT switch costs were over twice those of participants at any other age.
Certainly there is considerable evidence that 6-year-olds benefited from having a longer time to respond (3000 ms versus 1250 ms). The results clearly show that by 6 years, if allowed more time to respond, children will take advantage of that to reduce their errors.
The relative lack of response speed modulation in children of 6–8 years tested in the adult condition probably had a different cause than that for children of 4–5. In the case of the 6–8-year-olds, the response window (1250 ms ISI; 750 ms stimulus presentation) was likely too brief to allow them the time they needed to slow down sufficiently in the more difficult conditions to preserve their accuracy. Thus, even on the easy Pictures task, they could not slow down sufficiently on incompatible trials to preserve their accuracy, and so although their RTs were longer on incompatible trials, their accuracy suffered on those trials more than was found at any other age.
Given our hypothesis that working memory and inhibition are independent, we had predicted that performance on tasks that tax primarily memory or primarily inhibition would not be highly correlated. Instead, when the tasks were matched for difficulty, speed on working memory and inhibition tasks was highly correlated. Individuals who were fast at exercising inhibition also tended to be fast on working memory measures, even after accounting for age effects. Accuracy across working memory and inhibition measures was also correlated, though not as strongly.
Finally, we had predicted that the most difficult condition at all ages would be the one that taxed inhibition and memory in a switching context (Dots-Mixed) and that that would even be more difficult than having to hold much more information in mind but with no inhibition or switching component (six-Abstract-Shapes). Indeed, as predicted, we found that at every age, including for young adults, holding two superordinate rules in mind and switching randomly between the rule for making a prepotent response and the rule for inhibiting that to make the opposite response (Dots-Mixed) was the most difficult condition, harder even than holding six arbitrary rules in mind for stimuli that did not easily lend themselves to verbal labels (six-Abstract-Shapes).
25.2. How our predictions concerning cognitive flexibility and task switching fared
Cognitive flexibility (switching, overcoming inertial tendencies) was far harder than consistent inhibition in steady state or than holding and manipulating a couple of items in mind, and showed a much longer developmental progression. The cost, and longer developmental progression, of cognitive flexibility can be seen most clearly on the Arrows test, where little or no memory was required as the arrow pointed to the correct response on every trial. Since we hypothesized that switching is so difficult, we had predicted that having to switch between tasks even when memory demands were minimized (as in the Arrows test) would show a long developmental progression. This was confirmed. Even by age 10, the percentage of correct responses did not exceed 80% on the Arrows test, and even by the age of 13, children were not yet performing at adult levels on the Arrows task.
At all ages, the RT costs for having to exercise inhibition in a switching context on the Arrows task versus not having to exercise inhibition or switch (Dots-Congruent condition) took a much greater toll on response speed than did increasing the memory load from two to six arbitrary rules However, consistent with inhibition in a switching context being disproportionately difficult for young children and memory being disproportionately difficult for adults, even young adults, the larger accuracy difference between Arrows versus Dots-Congruent than between two- versus six-Abstract-Shapes was found only for the younger two-thirds of the subjects (children ≤9 years).
Consistent with cognitive flexibility improving with age, performance differences on Dots-Incongruent and Dots-Mixed decreased over age. If cognitive flexibility is improving, however, one would also expect the difference in performance on Dots-Incongruent and Arrows to decrease over age. The markedly faster speed and better accuracy in the Dots-Incongruent condition compared with the Arrows test, however, remained strong throughout the age range for children, though the accuracy difference disappeared among young adults. This may suggest that much of the age-related reduction in the cost of exercising cognitive flexibility comes after 13 years of age.
Consistent with the “all or none” principle (Diamond, in preparation), it should be easier to inhibit a dominant response all the time than only some of the time. We thus predicted that performance at all ages would be better in Dots-Incongruent (where inhibition was consistently required on all trials) than in Mixed blocks of Dots or Arrows (where inhibition is only required on the 50% of trials that are Incongruent), and that this difference would be greater the younger the children. Indeed, we found that inhibiting the spatially-compatible response some of the time in a switching context despite minimal memory requirements (the Arrows task) took a greater toll on speed and accuracy at every age than did inhibiting the spatially-compatible response consistently on all trials (Dots-Incongruent). Not surprisingly, the differences were even larger between Dots-Mixed and Dots-Incongruent. Our prediction that the difference in performance between Dots-Mixed and Dots-Incongruent would decrease over age as cognitive flexibility improved fared less well as these differences remained large at all ages, though the difference in accuracy was larger the younger the children. Similarly, the accuracy difference between Dots-Incongruent and Arrows was smaller in adults than in children, but otherwise the markedly better performance on Dots-Incongruent than on Arrows was equally true across all ages.
Also consistent with the “all or none” principle is that performance should be better on not-switching anything (repeat-rule, repeat-response trials) and on switching everything (switch trials where the response-site also switches) than on trials where either the rule or response-site changes but not the other. We had predicted that these effects, heretofore documented only in adults and older children (Kleinsorge, 1999
; Meiran, 2000a
; Rogers & Monsell, 1995
; Schuch & Koch, 2004
), would also be found in young children. We predicted that throughout our age span, participants would do better at switching tasks if the response-site also changed and would be slower and less accurate on switch trials when the response-site remained the same as on the previous trial. In both Dots-Mixed and Arrows, older children and adults were indeed better at switching tasks if the response-site also changed than if the response-site remained the same as on the preceding trial. However, contrary to our prediction, the youngest children (children of 4–8 years) performed better on switch trials where the response-site remained the same. They were faster on switch than nonswitch trials and on response-switch rather than response-stay trials and those effects tended to be additive. These results raise the possibility that perhaps the hypothesized “all or none” default of cognitive systems is an efficient characteristic of the mature cognitive system. It appears, in this particular context anyway, that piecemeal, additive effects are more characteristic of young children’s performance.
Research in adults has shown that performance on nonswitch trials (where the rule remains the same as on the previous trial) is worse when these trials are presented in the context of periodically having to switch than in the context of a block of all nonswitch trials (e.g., Fagot, 1994
; Mayr, 2000a
). We found that indeed performance was worse-slower, less accurate, and characterized by more anticipatory errors-on nonswitch trials within the Mixed block of the Dots task than within either single-task block of the task. Such global switch costs were among the strongest effects found in this study. It is not that participants forgot the rules when they had to hold both the Congruent and Incongruent rules in mind for the Mixed block. Indeed, children often called out the correct higher-order rule on trials in the Mixed condition (e.g., “same,” “opposite,” “opposite,” “same”) even as they were making errors. The problem seemed to be in quickly translating that rule into the correct response. The presence of global switch costs at all ages in our study is consistent with task-switching studies in children, young adults, and older adults; all studies consistently find global switch costs throughout the age spectrum.
We had predicted that global switch costs would decrease over age. That prediction was only partially confirmed. The global switch cost in accuracy declined from 9 to 13 years of age, as predicted, but the global switch cost in RT increased from 6 years through early adulthood (see ). Adults adjusted their speed to preserve their accuracy; younger children did that much less hence the difference in the speed-accuracy trade-off with age. This mirrors exactly what was found by Cohen et al. (2001)
using a very different task-switching paradigm. They used Meiran’s (1996)
task-switching paradigm presented as a computer game. A smiley face appeared at one of four quadrants of a square, preceded by a cue indicating the relevant dimension (horizontal [“is the cue in the left or right half?”] or vertical [“is the cue in the top or bottom half?”]). This was administered to 150 children (ages 5–11 years) and 16 young adults. They found that global switch costs in accuracy decreased from 5 to 11 years, and even 11-year-olds were not as accurate in mixed blocks as young adults, but global switch costs in speed of responding increased over age (just as we found here).
Contrary to our findings, however, though with only a few overlapping ages, Reimers and Maylor (2005)
found that global RT switch costs decreased linearly from 10 to 18 years. Like us, Crone et al. (in press)
found significant global switch costs in both speed and accuracy. However, unlike us, Cohen et al. (2001)
, or Reimers and Maylor (2005)
, Crone and colleagues found no change in the size of global switch costs with age in either speed or accuracy from 7 to 8 years to 23 years. Most studies in older adults report greater global RT switch costs in elders than in young adults (Kray et al., 2004
; Kray & Lindenberger, 2000
; Mayr, 2000
; van Asselen & Ridderinkhof, 2000
), though Kray et al. (2002)
report that no difference in global RT switch costs is found between younger and older adults when switches between tasks are unpredictable. The difference between predictable and unpredictable switches would also explain the differences across studies in whether global RT switch costs differ between young children and adults. The only study to find smaller global RT switch costs with increasing age from young children to adults was the one study that also predictably switched between tasks (Reimers & Maylor, 2005
), where a predictable double-alternation switching pattern was used in the Mixed block.
Cepeda et al. (2001)
calculated global switch costs differently from the studies above. They compared performance on all trials in Mixed blocks (not just the nonswitch trials) to performance in single-task blocks. Just as we found that the difference in performance on Dots-Mixed versus Dots-Congruent or Dots-Incongruent decreased over age from 6 years to young adulthood, so too Cepeda and colleagues found that the difference in performance in their Mixed blocks versus their single-task blocks decreased from their youngest age (7 years) to young adulthood. Cohen et al. (2001)
similarly report a linear decline in the difference in performance in Mixed blocks versus single-task blocks for both speed and accuracy, with their oldest children (age 11 years) still showing a larger difference in both dependent measures than young adults.
This illustrates an important point. Inhibiting a dominant response requires effort, but it is not nearly as difficult if that inhibition needs to be consistently maintained (as in Dots-Incongruent). What is far more demanding is switching back and forth between sometimes inhibiting a dominant response and sometimes making it. What is truly difficult is overcoming one’s inertial tendency to continue in the same mindset, switching between one mental set and another. Even now many investigators still administer the Stroop task in single-task blocks (blocks of always reading the word and blocks of always naming the ink color). While it requires effort to focus on the ink color (and one can see that toll in slowed responding) one can get in the mode of always focusing on the ink color and the task is quite manageable. It is far harder not to be able to rely on always ignoring the word; to have to switch back and forth between sometimes reading the word and sometimes naming the ink color.
Because of floor effects (people are already slower and more error-prone in the Incongruent-only block), we predicted that the effect of context (the Mixed block versus single-task block) would be greater on Congruent than Incongruent trials. We further predicted that this should be more evident the younger the child. That is, we predicted that the younger the child, the closer performance on “easy” (Congruent, nonswitch) trials would fall to the level of “harder” trials in the context of sometimes having to switch back and forth. Consistent with this prediction, we found that the cost of mixing nonswitch trials in with switch trials and mixing Congruent trials in with Incongruent ones, versus having single-task blocks, was greater for the Congruent (easier) nonswitch trials than the Incongruent nonswitch trials. When these trials were administered in separate single-task blocks, fewer errors occurred on Congruent trials (except for adults where accuracy did not differ in the Congruent and Incongruent single-task blocks) but when they were intermixed within the same block comparable numbers of errors occurred on Congruent and Incongruent trials. Participants were able to respond much faster on nonswitch Congruent trials when all trials in the block were Congruent than when some were Incongruent, though only Congruent trials following a Congruent trial were included in these analyses. The same was true for Incongruent trials but to a lesser extent. The effect of context (Mixed block versus single-task block) appears to have been larger for the faster, more automatic response (responding on the same side as the stimulus) than for the slower, more demanding response (inhibiting the dominant response and responding opposite to it). However, contrary to the portion of our prediction concerning development, the size of the greater effect of context on Congruent versus Incongruent trials did not change significantly over age.
The difficulty of the harder condition is underestimated in single-task blocks (always having to respond opposite to the side of the stimulus tends to reduce its difficulty because you get in the mode of doing that) and the ease of the easier condition is underestimated in Mixed blocks (because people tend to slow down across the board on such blocks). Comparing non-switch Incongruent trials in the Dots-Mixed block to nonswitch Congruent trials in the Dots-Congruent block may come closest to approximating the full difficulty of inhibiting the tendency to make the spatially compatible response.
A striking difference in our findings for children and adults was that while RT was unquestionably a more sensitive measure than percentage of correct responses for adults, the latter was often the more sensitive measure for children, especially younger children. For example, for our youngest children (4–6 years of age), age-related improvements in each of the three conditions of the Dots task and in each of the two conditions of our Abstract Shapes task were far more evident in accuracy than in speed. Age-related improvements in the ability to inhibit spatially compatible responses were far more evident in reduced accuracy differences between Congruent and Incongruent trials over age than in reduced RT differences in each of the tasks that tested this (Pictures, Arrows, and Dots; see ). Similarly, age-related improvements in the ability to hold multiple items in mind were far more evident in the reduced change in accuracy over age for holding six rather than two arbitrary rules in mind than it was in reduced change in RT (see ).