Roles of the Hippocampus and Prefrontal Cortex in Young Children

• In Event: 1-163 - Memory and the Developing Brain: Sampling Behavior and Defining Constructs

Integrative Statement

Episodic memory undergoes robust development during childhood and depends on the hippocampus (HC) and prefrontal cortex (PFC), with each supporting different memory aspects. The HC, specifically the subfields Cornu Ammonis (CA)-3 and dentate gyrus (DG), is important for memory that requires representations for specific details to distinguish between similar stimuli (Reagh et al., 2014; Yassa et al., 2011). The PFC may be important when memory is evaluated using tasks that can be completed with familiarity-based and decision processes (Yonelinas et al., 2005). Protracted developmental patterns have been observed in both HC (CA3 and DG in specific, Daugherty et al., 2017; Keresztes et al., 2017) and PFC (lateral PFC in specific, Gogtay et al., 2004). With this protracted development, the roles of the HC and PFC in memory performance in young children remain unclear.

To assess the differential contributions of the HC and PFC in memory, we designed a memory task with two different recognition tests: Old/New recognition test and two-alternative forced-choice (2AFC) test. During encoding, participants studied 64 pictures of objects presented one at a time, with an additional six pictures used for an n-Back test to ensure children maintain attention to the studied pictures. During retrieval, memory for half of the previously studied pictures was tested through Old/New recognition test, and memory for the other half was tested through 2AFC test. Half of the studied pictures depicted objects from only four categories (‘within’ category: eight exemplar objects per category), whereas the other 32 pictures depicted objects from 32 unique categories (‘across’ category: one exemplar object per category). Episodic memory was tested using this memory paradigm in thirty-five 5-6-year-olds (mean age=6.20, N=35, 40% females). Lateral PFC regional volumes were calculated with FreeSurfer. HC subfields (CA3-DG, CA1-2, and Subiculum) volumes were calculated manually with high inter-rater reliability (ICC2>0.85).

We found that recognition memory for ‘within’ category objects, when tested by Old/New test, was related to individual differences in volumes of CA3-DG (r = -0.36, p=0.04), but not to that of other hippocampal subfields or any PFC volumes. In contrast, recognition memory, when tested with 2AFC, was uniquely related to volumes of the lateral PFC, for both ‘within’ (r = -0.43, p=0.01) and ‘across’ (r = -0.42, p=0.02) category objects. Interestingly, recognition memory tested through Old/New for ‘across’ category stimuli was related to all hippocampal subfields and lateral PFC volumes, particularly the caudal middle frontal lobe.

These findings suggest that in young children, unique roles of the HC and PFC already exist, such that the variability in CA3-DG volume is related to memory that requires representation of fine details, whereas variability in PFC volumes is related to memory that relies on decision processes that may incorporate familiarity judgements. The roles of these neural substrates for memory function may continue to increase over development. Our findings suggest that the two memory tests of similar information (‘within’ category) is advantageous in differentiating the relationships between memory performance and structural differences in the PFC versus HC.

Authors

• Qijing Yu, Wayne State University
  Presenting Author

• Sruthi Ramesh, Wayne State University
  Non-Presenting Author

• Bryn Thompson, Wayne State University
  Non-Presenting Author

• Roya Homayouni, Wayne State University
  Non-Presenting Author

• Zhijian Chen, Wayne State University
  Non-Presenting Author

• Mayu Nishimura, The Hospital for Sick Children
  Non-Presenting Author

• Noa Ofen, Wayne State University
  Non-Presenting Author