Longitudinal indices of human cognition and brain structure

Changes in human cognition, brain structure, and function occur with development and throughout aging. This change occurs across timescales of years, and scientific inquiry into these changes has inspired the neuroscientific fields of development, life span, and aging. Likewise, interventions that remediate or enhance cognitive performance can thwart or truncate the temporal progression by which change might naturally occur, and are drawing increasing interest from basic science, clinical, and commercial audiences alike. Understanding the neural basis of cognitive change is a primary goal of human neuroscience. Although crosssectional studies inform questions related to life span and intervention differences, only longitudinal studies which compare the same participants across multiple time points can mechanistically explain change. This In Focus issue brings together longitudinal research from developmental neuroscience and cognitive training in effort to gain insights into how changes occur in cognition and brain structure over timescales of weeks to years. There are unique challenges associated with longitudinal research. Logistical challenges include planning, sometimes years in advance, for potential problems related to participant retention and research implementation. Furthermore, longitudinal studies are resource consuming in terms of finances and manpower, often preventing earlyand midcareer researchers from pursuing ambitious, potentially paradigmshifting lines of research. Here, research involving human participants was brought to a standstill in March 2020 due to the unforeseen global COVID19 pandemic. Scientific researchers had to adjust plans for data collection and dissemination, rework project aims, reconsider what is feasible, and strive to balance professional and personal while working from home. Perhaps the least devastating outcome of the pandemic was the 50% dropout of planned contributions to this In Focus issue. With that in mind, below we showcase the three articles of this issue. We then conclude with a call for future research that takes a longitudinal approach across multiple intersecting fields of inquiry to bring us closer to understanding the neural basis of cognitive change. A core challenge in longitudinal research is the development and use of measures with robust test– retest reliability (Ofen et al., 2019). If a measure does not show stable outcomes in the same participant across multiple points close in time, without intervention, then it cannot reliably measure stability or change. Homayouni et al. (2021) tackled this core challenge by validating a measure of brain structure, specifically, hippocampal subfield volumes using structural MRI. The hippocampus is critical to declarative memory, and crosssectional life span differences in hippocampal subfields have been linked to life span differences in declarative memory performance (Daugherty et al., 2017). In this study, hippocampal subfields were measured in the same pediatric participants, aged 7– 20 years, at baseline and both 1month and 2year delays (Figure 1a). The reliability of the measure was demonstrated by means of structural stability over 1 month and change over 2 years, replicated in two independent samples. These results further demonstrate the utility of the measure as sensitive enough to detect longitudinal changes in brain structure over an expected maturational timescale of 2 years. Interventions that aim to remediate or enhance cognitive performance, if successful, can thwart or truncate life span timescales of change, or simply enhance performance. The other studies in this issue tackled questions related to cognitive trainingbased intervention, a field that is plagued with unreliable outcomes (Boot & Kramer, 2014; Hampshire et al., 2019; Kable et al., 2017; Owen et al., 2010; Sala & Edited by Cristina Ghiani and Junie Warrington.