Dana DeMaster

Developmental Differences in Medial Temporal Lobe Function during Memory Encoding

The ability to recollect details about past events improves during childhood. Most researchers favor the view that this improvement depends largely on the development of the prefrontal cortex, which is thought to have a protracted course of development relative to the medial temporal lobes (MTL). The primary goal of the present study was to test the hypothesis that the development of detail recollection is also associated with changes in MTL function. We collected functional magnetic resonance imaging data during an incidental encoding task in 80 participants, divided equally across four age groups: 8-year-olds, 10- to 11-year-olds, 14-year-olds, and young adults. Developmental differences in MTL activation profiles were observed. Fourteen-year-olds and adults engaged regions of the hippocampus and posterior parahippocampal gyrus selectively for subsequent detail recollection, whereas 8- and 10- to 11-year-olds did not. In 8-year-olds, these regions were recruited indiscriminately for detail recollection and item recognition; in 10- to 11-year-olds, activation in these regions did not consistently predict subsequent memory. These results suggest there are changes in the functional organization of the MTL, such that the hippocampus and posterior parahippocampal gyrus become increasingly specialized for recollection; these changes may be in part responsible for long-term memory improvements during childhood.

Developmental differences in hippocampal and cortical contributions to episodic retrieval

Episodic memory, or the ability to form and retrieve conscious memories about specific past events, improves during childhood. Previous adult neuroimaging results indicate a central role of the hippocampus in episodic retrieval, but it is not clear whether the contribution of the hippocampus changes during development. Traditionally, developmental improvements in episodic retrieval have been thought to depend on strategic processes mediated by the prefrontal cortex (PFC), a region that is considered to have a protracted course of development relative to the hippocampus. The primary goal of the present study was to test the hypothesis that the development of episodic retrieval is also associated with changes in hippocampal function. Children ages 8- to 11-years-old and adults ages 18-25 (N = 41) encoded black and white line drawings surrounded by either a green or red border. Functional magnetic resonance imaging (fMRI) data were acquired while participants attempted to recall which colour was originally paired with each drawing. Correct recall of item-colour pairings indicated successful episodic retrieval. Activity in the anterior hippocampus, but not in the posterior hippocampus, was associated with episodic retrieval in adults, whereas activity in the posterior, but not in the anterior hippocampus, was associated with episodic retrieval in children. Developmental differences were also found in regions in anterior lateral PFC and posterior parietal cortex. Overall, these results support the view that the development of episodic memory is supported by functional changes in the hippocampus as well as in other critical cortical regions.

Structural Development of the Hippocampus and Episodic Memory: Developmental Differences Along the Anterior/Posterior Axis

The hippocampus is critically involved in episodic memory, yet relatively little is known about how the development of this structure contributes to the development of episodic memory during middle to late childhood. Previous research has inconsistently reported associations between hippocampal volume and episodic memory performance during this period. We argue that this inconsistency may be due to assessing the hippocampus as a whole, and propose to examine associations separately for subregions along the longitudinal axis of the hippocampus. In the present study, we examined age-related differences in volumes of the hippocampal head, body, and tail, and collected episodic memory measures in children ages 8-11 years and young adults (N = 62). We found that adults had a smaller right hippocampal head, larger hippocampal body bilaterally, and smaller right hippocampal tail compared with children. In adults, but not in children, better episodic memory performance was associated with smaller right hippocampal head and larger hippocampal body. In children, but not in adults, better episodic memory was associated with larger left hippocampal tail. Overall, the results suggest that protracted development of hippocampal subregions contribute to age-related differences in episodic memory.

Development of memory for spatial context: Hippocampal and cortical contributions

The goal of the present study was to examine age-related differences in hippocampal and cortical contribution to episodic retrieval of spatial context in 3 age groups. Children ages 8-9 and 10-11 years old, and adults ages 18-25 (N=48) encoded black and white line drawings appearing either on the right side or the left side of a screen. Functional magnetic resonance imaging (fMRI) data were acquired while participants attempted to recall where each studied drawing had originally appeared. Correct recall of spatial source indicated successful episodic retrieval of spatial context. Activity in head and body of the hippocampus was associated with episodic retrieval in adults, but not in children. In children, individual differences in hippocampal activation for recognition predicted rates of correct spatial recall. Developmental differences were also found in regions in posterior parietal cortex, anterior prefrontal cortex, and insula. Overall, these results support the view that the development of episodic memory is supported by functional changes in the hippocampus as well as cortical regions.

Retrieval flexibility and reinstatement in the developing hippocampus.

Episodic memory improves during childhood and this improvement has been associated with age differences in hippocampal function, but previous research has not manipulated the possible underlying mechanisms. We tested the hypothesis that age‐related differences in hippocampal activation may reflect changes in retrieval flexibility. We expected these activation differences to be observed most prominently in the anterior hippocampus. Functional magnetic resonance imaging (fMRI) data were collected from children ages 8 and 10, and adults (N = 63) during an associative recognition task that required participants to recognize pairs of pictures which either appeared in the same location as during encoding (Same location), or in a flipped location, such that each picture switched their location with the other member of the pair (Flipped location). Recognition of same‐location pairs placed lower demands on flexible retrieval compared to recognition of flipped‐location pairs. Behaviorally, 8‐year‐olds exhibited the strongest correct recognition gains for same‐location compared to flipped‐location pairs, and females unexpectedly outperformed males across all ages. When we examined correct recognition, adults recruited the hippocampal head more strongly for flipped‐ versus same‐location pairs compared to both groups of children; in contrast both adults and 10‐year‐olds recruited the hippocampal tail more strongly for flipped‐ versus same‐location pairs compared to 8‐year‐olds. This pattern was stronger in the left hippocampus and for females. Moreover hippocampal discrimination between recognized and forgotten items in the same‐location condition was stronger in 8‐year‐olds compared to adults, and was stronger in the flipped‐location condition in adults compared to 8‐year‐olds; this pattern was stronger in the left hippocampus. Individual differences in this discrimination contrast for flipped‐location trials in the head and body predicted performance on an index of creative thinking. Overall, these results lend new support to the idea that hippocampal development may reflect change in retrieval flexibility with implications for additional forms of flexible cognition.

Longitudinal diffusion tensor imaging after pediatric traumatic brain injury: Impact of age at injury and time since injury on pathway integrity.

Following pediatric traumatic brain injury (TBI), longitudinal diffusion tensor imaging may characterize alterations in initial recovery and subsequent trajectory of white matter development. Our primary aim examined effects of age at injury and time since injury on pathway microstructure in children ages 6–15 scanned 3 and 24 months after TBI. Microstructural values generated using tract‐based spatial statistics extracted from core association, limbic, and projection pathways were analyzed using general linear mixed models. Relative to children with orthopedic injury, the TBI group had lower fractional anisotropy (FA) bilaterally in all seven pathways. In left‐hemisphere association pathways, school‐aged children with TBI had the lowest initial pathway integrity and showed the greatest increase in FA over time suggesting continued development despite incomplete recovery. Adolescents showed limited change in FA and radial diffusivity and had the greatest residual deficit suggesting relatively arrested development. Radial diffusivity was persistently elevated in the TBI group, implicating dysmyelination as a core contributor to chronic post‐traumatic neurodegenerative changes. The secondary aim compared FA values over time in the total sample, including participants contributing either one or two scans to the analysis, to the longitudinal cases contributing two scans. For each pathway, FA values and effect sizes were very similar and indicated extremely small differences in measurement of change over time in the total and longitudinal samples. Statistical approaches incorporating missing data may reliably estimate the effects of TBI and provide increased power to identify whether pathways show neurodegeneration, arrested development, or continued growth following pediatric TBI. Hum Brain Mapp 37:3929–3945, 2016.

Children's head motion during fMRI tasks is heritable and stable over time

Head motion during fMRI scans negatively impacts data quality, and as post-acquisition techniques for addressing motion become increasingly stringent, data retention decreases. Studies conducted with adult participants suggest that movement acts as a relatively stable, heritable phenotype that serves as a marker for other genetically influenced phenotypes. Whether these patterns extend downward to childhood has critical implications for the interpretation and generalizability of fMRI data acquired from children. We examined factors affecting scanner motion in two samples: a population-based twin sample of 73 participants (ages 7–12 years) and a case-control sample of 32 non-struggling and 78 struggling readers (ages 8–11 years), 30 of whom were scanned multiple times. Age, but not ADHD symptoms, was significantly related to scanner movement. Movement also varied as a function of task type, run length, and session length. Twin pair concordance for head motion was high for monozygotic twins and moderate for dizygotic twins. Cross-session test-retest reliability was high. Together, these findings suggest that children’s head motion is a genetically influenced trait that has the potential to systematically affect individual differences in BOLD changes within and across groups. We discuss recommendations for future work and best practices for pediatric neuroimaging.

Nurturing the preterm infant brain: leveraging neuroplasticity to improve neurobehavioral outcomes

An intrinsic feature of the developing brain is high susceptibility to environmental influence—known as plasticity. Research indicates cascading disruption to neurological development following preterm (PT) birth; yet, the interactive effects of PT birth and plasticity remain unclear. It is possible that, with regard to neuropsychological outcomes in the PT population, plasticity is a double-edged sword. On one side, high plasticity of rapidly developing neural tissue makes the PT brain more vulnerable to injury resulting from events, including inflammation, hypoxia, and ischemia. On the other side, plasticity may be a mechanism through which positive experience can normalize neurological development for PT children. Much of the available literature on PT neurological development is clinically weighted and focused on diagnostic utility for predicting long-term outcomes. Although diagnostic utility is valuable, research establishing neuroprotective factors is equally beneficial. This review will: (1) detail specific mechanisms through which plasticity is adaptive or maladaptive depending on the experience; (2) integrate research from neuroimaging, intervention, and clinical science fields in a summary of findings suggesting inherent plasticity of the PT brain as a mechanism to improve child outcomes; and (3) summarize how responsive caregiving experiences situate parents as agents of change in normalizing PT infant brain development.

Memory and the hippocampal formation following pediatric traumatic brain injury

Previous research indicates disruption of learning and memory in children who have experienced traumatic brain injury (TBI).