Volumetric and microstructural regional changes of the hippocampus underlying development of extended delay long-term memory

Episodic memory function improves through childhood and adolescence, in part due to structural maturation of the medial temporal cortex. Although partly different processes support long-term memory over shorter vs. longer intervals, memory is usually assessed after less than an hour. The aim of the present study was to test whether there are unique developmental changes in extended memory, and whether these are related to structural maturation of sub-regions of the hippocampus. 650 children and adolescents from 4.1 to 24.8 years were assessed in total 962 times (mean interval ≈ 1.8 years). Memory was assessed by the California Verbal Learning Test (CVLT) and the Rey Complex Figure Test (CFT). In addition to 30 min recall, an extended delay recall condition was administered ≈ 10 days after encoding. We found unique developmental effects on extended delay memory independently of 30 min recall performance. For visuo-constructive memory, this could be accounted for by visuo-constructive ability levels. Performance was modestly related to anterior and posterior hippocampal volume and mean diffusion. The relationships did not show an anterior-posterior hippocampal axis difference. In conclusion, extended delay memory shows unique development, likely due to changes in encoding depth or efficacy, or improvements of long-term consolidation processes.