Looking ahead from age 6 to 13: a deeper insight into the development of planning ability.

Planning ability gradually increases throughout childhood. However, it remains unknown whether this is attributable to global factors such as an increased ability and willingness to inhibit premature, impulsive responding, or due to the availability of specific planning operations, such as being able to mentally plan ahead more steps ('search depth') or to derive a clear temporal order of goals by the task layout ('goal hierarchy'). Here, we studied the development of planning ability with respect to these global and problem-specific aspects (search depth and goal hierarchy) of performance in 178 children from 6 to 13 years using the Tower of London task. As expected, global performance gradually developed with age. In accordance, planning durations increasingly reflected global problem demands with longer pre-planning in harder problems. Furthermore, specific planning parameters revealed that children were increasingly capable of mentally searching ahead more steps. In contrast, the ability to derive a goal hierarchy did not show age-related changes. While the global development of planning performance and adaptive planning durations were proposed to primarily reflect enhanced self-monitoring, the specific increase in search depth across childhood that most likely proceeds until young adult age represents more directly planning-related processes. Thus, development of planning ability is supported by multiple contributions.

Transcranial direct current stimulation over left and right DLPFC: Lateralized effects on planning performance and related eye movements.

Left and right dorsolateral prefrontal cortex (dlPFC) were recently found to be differentially affected by unilateral continuous theta-burst stimulation, reflected in an oppositional alteration of initial thinking time (ITT) in the Tower of London planning task. Here, we further explored this finding using bilateral transcranial direct current stimulation (tDCS) and simultaneous tracking of eye movements. Results revealed a decrease in ITT during concurrent cathodal tDCS of left dlPFC and anodal tDCS of right dlPFC. Eye-movement analyses showed that this facilitating tDCS effect was associated with the actual planning phase, thus reflecting a planning-specific impact of stimulation. For the reverse stimulation pattern of cathodal tDCS of right dlPFC and anodal tDCS of left dlPFC, an increase in gaze shifts was observed, without a significant impact on ITT. Taken together, these findings corroborate that enhanced planning performance can be obtained by boosting right dlPFC and dismantling the inhibitory impact of left dlPFC.

Planning steps forward in development: in girls earlier than in boys.

The development of planning ability in children initially aged four and five was examined longitudinally with a retest-interval of 12 months using the Tower of London task. As expected, problems to solve straightforward without mental look-ahead were mastered by most, even the youngest children. Problems demanding look-ahead were more difficult and accuracy improved significantly with age and over time. This development was strongly moderated by sex: In contrast to coeval boys, four year old girls showed an impressive performance enhancement at age five, reaching the performance of six year olds, whereas four year old boys lagged behind and caught up with girls at the age of six, the typical age of school enrollment. This sex-specific development of planning was clearly separated from overall intelligence: young boys showed a steeper increase in raw intelligence scores than girls, whereas in the older groups scores developed similarly. The observed sex differences in planning development are evident even within a narrow time window of twelve months and may relate to differences in maturational trajectories for girls and boys in dorsolateral prefrontal cortex.

Dissociable stages of problem solving (I): temporal characteristics revealed by eye-movement analyses.

Understanding the functional neuroanatomy of planning and problem solving may substantially benefit from better insight into the chronology of the cognitive processes involved. Based on the assumption that regularities in cognitive processing are reflected in overtly observable eye-movement patterns, here we recorded eye movements while participants worked on Tower of London (TOL) problems that comprised an experimental manipulation of different task demands. Single-trial saccade-locked analyses revealed that higher demands on forming an internal problem representation were associated with an increased number of gaze alternations between start state and goal state, but did not show any effect on the durations of these inspections of the states. In contrast, higher demands on actual planning in terms of mental manipulations of working memory contents coincided with a prolonged duration of the very last inspection of the start state (i.e., immediately preceding movement execution) but did not show any effect on the number of gaze alterations. Differential task demands on internalization and planning processes during problem solving hence selectively affect different eye-movement parameters. Moreover, these findings complement previous neuroimaging data on dissociable contributions of left and right dorsolateral prefrontal cortex in problem solving with novel evidence for a corresponding dissociation in the eye-movement patterns reflecting the associated cognitive processes.

Dissociable stages of problem solving (II): first evidence for process-contingent temporal order of activation in dorsolateral prefrontal cortex.

In a companion study, eye-movement analyses in the Tower of London task (TOL) revealed independent indicators of functionally separable cognitive processes during problem solving, with processes of building up an internal representation of the problem preceding actual planning processes. These results imply that processes of internalization and planning should also be distinguishable in time and space with respect to concomitant brain activation patterns. To investigate this possibility, here we conducted analyses of fMRI data for left and right dorsolateral prefrontal cortex (dlPFC) during problem solving in the TOL task by accounting for the trial-by-trial variability of onsets and durations of the different cognitive processing stages. Comparisons between stimulus-locked and response-locked modeling approaches affirmed that activation in left dlPFC was elicited particularly during early processes of internalization, comprising the extraction of goal information and the generation of an internal problem representation, whereas activation in right dlPFC was predominantly attributable to later processes of mental transformations on this representation, that is planning proper. Thus, present data corroborate the proposal that often observed bilateral dlPFC activation patterns during complex cognitive tasks such as problem solving may reflect functionally and, to some extent, even temporally separable processes with opposing lateralizations.