Imagine you are on your way to the grocery store, but you forget to take the shopping list. Now, you are trying to recall the items from that list. You may recall a vegetable, a tomato, which cues your memory of celery, broccoli, and onions, which belong to the same category. You may then recall items that are protein, snacks, etc. In contrast, a preschooler might recall tomato, beef, goldfish, celery, and so on, without organizing them based on different categories.
Laboratory work has supported the improvement of the ability to spontaneously use an elaborative mnemonic strategy from childhood to adulthood, such as organizing grocery items based on semantic categories. Increased spontaneous use of elaborative mnemonic strategy benefits memory performance and is an important contributing factor for memory development. As the brain is critical for memory functioning and undergoes robust changes during development, we examined the contribution of the brain to the increasing use of mnemonic strategy as one develops.
In our study titled “Age-associated increase in mnemonic strategy use is linked to prefrontal cortex development,” published in the journal NeuroImage, we have identified brain regions that may play critical roles in the development of the spontaneous use of elaborative mnemonic strategies.
The prefrontal cortex (PFC) is a brain region essential for high-order cognitive abilities, such as decision making, future planning, emotional regulation, and cognitive control, which are critical for using mnemonic strategy spontaneously. The PFC in humans occupies a much larger proportion of the brain than any other animals. Over individual human development, this region also shows the most protracted maturation, particularly in the right hemisphere, which may underlie the protracted development in the spontaneous use of mnemonic strategies.
Evidence from neural imaging and neuropsychological studies supports the role of the PFC in spontaneous use of elaborative mnemonic strategies. Patients with PFC lesions have difficulties initiating mnemonic strategy use during memory tasks, but when prompted by an experimenter to use a mnemonic strategy, their memory performance becomes comparable to that of normal participants. In a healthy adult lifespan sample, Kirchhoff and colleagues found that age differences in PFC volume contributes to an age-associated decrease in spontaneous use of semantic strategies. However, little evidence exists in studies of child development about the relationship between protracted structural maturation of the PFC and facilitation of spontaneous elaborative mnemonic strategy use.
We studied memory performance in 120 participants aged 5-25 years using a standardized task, the California Verbal Learning Test (CVLT). During the task, we asked participants to memorize a list of 15 words from three semantic categories (i.e., fruit, things to wear, and things to play with) and asked them to immediately recall as many words as they could in any order. As we did not require sequential recall, participants might use a semantic mnemonic strategy to recall words from the same category together, regardless of the presentation order. Consistent with the literature, we found the use of semantic strategy benefits memory performance and increases significantly from children to young adults.
From the same 120 participants, we also collected structural brain images to examine the role of PFC maturation in the development of spontaneous use of mnemonic strategy. We calculated PFC volumes using a semi-automatic brain structural segmentation tool called FreeSurfer. Consistent with previous findings, we observed what may be considered as protracted maturation of the PFC: younger participants had larger PFC compared to older participants. It is not possible to pin down those age differences in volume on specific neuronal processes, but those differences may reflect synaptic or neuronal pruning.
Regardless of the specific neuronal processes underlying observed differences in volume, it is likely that this volumetric reduction from 5-25 years reflects the typical process that unfolds during development. Moreover, reduced volume resulted from the typical process likely relates to observed gains in performance over age, perhaps reflecting a higher efficiency of cognitive processes. We also found that across the whole sample, smaller volumes of some PFC regions, including the left superior frontal, left inferior frontal, and right middle frontal regions, are associated with more spontaneous use of semantic strategies.
Last, we examined the possible relationships between PFC structural maturation and the development of spontaneous use of semantic strategy. To test this question, we utilized mediation analysis, a statistical analysis that allows us to examine the possible mechanistic role of the PFC underlying development of semantic strategy use during the memory process. We found volumes of only one PFC region, the right dorsolateral frontal (DLPFC), has a significant indirect effect on the relationship between age and semantic strategy use, such that older age is associated with smaller right DLPFC, which is further associated with more spontaneous use of semantic strategy.
The specific role of the right DLPFC in the development of semantic strategy use may relate to the particularly late structural maturation of the DLPFC and its role in cognitive control. The differences in DLPFC structure between adults and children, which suggests protracted DLPFC maturation, may underlie the differences we observe between adults and children in using elaborative mnemonic strategies spontaneously to facilitate memory.