Cortical morphometry and structural connectivity relate to executive function and estradiol level in healthy adolescents

Abstract Introduction Emotional and behavioral control is necessary self‐regulatory processes to maintain stable goal‐driven behavior. Studies indicate that variance in these executive function (EF) processes is related to morphological features of the brain and white matter (WM) differences. Furthermore, sex hormone level may modulate circuits in the brain important for cognitive function. Methods We aimed to investigate the structural neural correlates of EF behavior in gray matter (GM) and WM while taking into account estradiol level, in an adolescent population. The present study obtained neuroimaging behavioral and physiological data from the National Institute of Health's Pediatric Database (NIHPD). We analyzed the relationship between cortical morphometry and structural connectivity (N = 55), using a parent‐administered behavioral monitoring instrument (Behavior Rating Inventory of Executive Function—BRIEF), estradiol level, as well as their interaction. Results Executive function behavior and estradiol level related to bidirectional associations with cortical morphometry in the right posterior dorsolateral prefrontal cortex (pDLPFC) and primary motor cortex (PMC), as well as fractional anisotropy (FA) in the forceps major and minor. Lastly, the interaction of EF behavior and estradiol level related to decreased volume in the right PMC and was linked to altered FA in the right inferior fronto‐occipital fasciculus (iFOF). Conclusions The study provides evidence that the relationship between EF behavior and estradiol level related to bidirectional GM and WM differences, implying estradiol level has an influence on the putative structural regions underlying EF behavior. The findings represent a crucial link between EF behavior and hormonal influence on brain structure in adolescence.


| Relationship between executive function and cortical morphometry
Despite EF's critical role in guiding future-oriented behavior, inconsistencies exist regarding the morphological features that support it during adolescence. While some studies demonstrate that increases in total cortical and PFC GM volume (GMV) relate to higher scores on working memory and response inhibition tests (Kharitonova, Martin, Gabrieli, & Sheridan, 2013;Mahone, Martin, Kates, Hay, & Horska, 2009;Yurgelun-Todd, 2007); others show that GMV decreases in the PFC relate to increased ability to regulate emotion, better working memory capacity, and higher scores on verbal memory tests (Caballero et al., 2016;Yurgelun-Todd, 2007). Similarly, higher IQ during adolescence is associated with cortical thinning of left superior orbitofrontal cortex and superior motor area, and higher bilateral hemispheric surface area (SA) (Schnack et al., 2015). Thus, bidirectional morphological results in relation to EF need not be interpreted as contradictory, instead they could possibly reflect the fact that GM maturation follows an inverted-U shape over development, peaking at different ages depending on the region (Ducharme et al., 2015;Sowell, Thompson, Tessner, & Toga, 2001). Therefore, GM is considered closely related to maturation of a brain region (Crone, 2009;Giedd, 2004), suggesting that controlling for age is paramount when examining cortical GM in adolescent samples.

| Relationship between executive function and structural connectivity
In a more consistent pattern than GM maturation during adolescence, studies examining fractional anisotropy (FA; a WM integrity descriptor) during this period indicate relatively linear increases coinciding with improved EF performance. Specifically, increases in FA in the posterior corpus callosum during adolescence are associated with better working memory and IQ scores (Giedd, 2004;Giorgio et al., 2010;Nagy, Westerberg, & Klingberg, 2004). Similarly, research indicates increased FA of fronto-temporal-subcortical WM tracts (inferior fronto-occipital longitudinal fasciculus (iFOF), superior longitudinal fasciculus (SLF), arcuate fasciculus and the corticospinal tract) support enhanced communication between disparate regions of the cortex and reflect increases in top-down cognitive control of behavior (Asato et al., 2010;Peper, Heuvel, Mandl, Pol, & Honk, 2011). While previous studies show specific changes in GM and FA and indicate relationships with some facets of EF, a comprehensive cortical morphometry and structural connectivity investigation concerning the full range of EF constructs is lacking.

| Measuring executive function in children and adolescents
A tool often used to investigate multiple EF constructs and occasionally, their underlying neural substrates, is the Behavior Rating Inventory of Executive Function (BRIEF). This reliable and validated psychological battery is designed to measure EF behavior in children and adolescents (5-18 years) during everyday situations through behavioral observation (Clark, Pritchard, & Woodward, 2010). Initial factor analytic studies of the BRIEF support two robust indices: a Behavioral Regulation Index (BRI)-emphasizing inhibitory and emotional control (EC), and a Metacognition Index (MCI)-emphasizing working memory, planning, and strategic response preparation (Mahone et al., 2009). The sum of the two indices provides a Global Executive Composite (GEC), whereby elevated scores indicate more observed problems with EF behavior. Neurobiologically, research indicates that the BRIEF captures unique variance in predicting PFC development in children and adolescents (Mahone et al. (2009) and provides an economical port of entry to both behavioral regulation and cognitive issues that may in turn relate to cortical morphometry and structural connectivity measurements.

| Relationship between executive function, cortical morphometry and structural connectivity
However, findings from the few studies that have investigated cortical morphometry in healthy, typically developing adolescents in relation to EF are inconsistent, likely due to the magnitude of change during adolescence. While some studies demonstrate that increased frontal GMV relates to decreased working memory and emotional control (Faridi et al., 2015;Mahone et al., 2009), others show the inverse pattern: decreased temporal lobe GMV relates to decreased inhibition and emotional control (Faridi et al., 2015). Conversely, relationships between structural connectivity and the BRIEF during this variable period reflect an evident pattern: Reductions in FA relate to decreased EF behavior. A variety of pediatric clinical populations exhibit reduced FA in temporal, frontal, and corpus callosal regions in association with deficits on the GEC (Antshel, Conchelos, Lanzetta, Fremont, & Kates, 2005;Gautam et al., 2015;Wozniak et al., 2007).
The sole study linking EF behavior with FA in a healthy adolescent sample investigated the frontal aslant tract (FAT), a newly discovered white matter tract which connects posterior inferior frontal gyrus (IFG) with the pre-supplementary and supplementary motor areas (pre-SMA and SMA), regions proposed to underlie inhibition. The study indicates the FAT develops in a protracted manner into late adolescence/early adulthood and that right lateralization of this fiber pathway is significantly associated with decreased EF behavior as measured by the BRIEF (Garic, Broce, Graziano, Mattfeld, & Dick, 2018). Taken together, scant evidence indicates that EF behavior is associated with both GM and FA changes during childhood and adolescence, yet the results are conflicting. Therefore, a comprehensive cortical morphometry and structural connectivity study using the BRIEF to assess EF behavior within a healthy sample of adolescents can help clarify previous findings.

| Relationship between executive function, cortical morphometry, structural connectivity and estradiol
During this developmental period of high flux, the hormone estradiol, the predominant estrogen, has been shown to have a significant impact on the structural reorganization of the prefrontal cortex (McCarthy, 2008;Nguyen et al., 2013), a crucial region underlying EF (Yuan & Raz, 2014). The hormone has complex effects in the two genders, however, because estrogen receptor distribution in the prefrontal cortex varies (Cooke, Nanjappa, Ko, Prins, & Hess, 2017;Gillies & McArthur, 2010).
Therefore, estradiol may have both similar and different (sometimes opposite effects) due to underlying brain dimorphisms. Nonetheless, this hormone influences cognitive function through complex interactions with dopaminergic and oxytocinergic systems that govern EF (Kuhn et al., 2010;Steinberg, 2005), the description of which is not within the scope of this paper. The complex, menstrual phase-dependent evidence from studies in adult women points to estradiol level playing both a facilitative and/or hindering role in cognitive function. Some studies report higher levels of circulating estradiol being associated with improved working memory performance (Hampson & Morley, 2013), while others show increased estradiol had a negative impact on general processing speed, working memory performance (Sommer et al., 2018), and slower response times and decreased accuracy on EF tasks that were instead related to progesterone level during the luteal phase (Hidalgo-Lopez & Pletzer, 2017). Alongside the adult literature, morphometric studies in young adults demonstrate increased circulating levels of estradiol are associated with cortical thinning of the IFG (Witte, Savli, Holik, Kasper, & Lanzenberger, 2010), a region linked to self-regulation (Aron & Poldrak, 2006;Depue, Curran, & Banich, 2007;Depue, Orr, Smolker, Naaz, & Banich, 2015). Structural connectivity evidence points to elevated estradiol level influencing decreases in FA, which is associated with reduced behavioral control during early pubertal development (Peper, Reus, Heuvel, & Schutter, 2015). Elevated estradiol level in adolescent girls shows a negative relationship with right angular gyrus (AG) and the superior longitudinal fasciculus (SLF) FA, a brain region and a WM tract involved in attention, spatial and social cognition (Herting, Maxwell, Irvine, & Nagel, 2012). However, scant evidence between the relationship between EF behavior, estradiol, and specific brain changes exists. Therefore, the present study aimed to comprehensively investigate the relationship between EF behavior (as measured by the BRIEF questionnaire) and estradiol level, individually and interactively on cortical morphometry and FA in a healthy adolescent sample. Specifically, the aims were to examine the relation between (a) the BRIEF and estradiol level, (b) the BRIEF, cortical morphometry, and FA, (c) estradiol level, cortical morphometry and FA, and (d) any interaction between the BRIEF and estradiol level with cortical morphometry and FA. In parallel, we hypothesized based on the limited literature findings that (a) EF behavior and estradiol level will be inversely related, (b) decreased EF behavior will relate to decreased GMV of the LPFC and decreased FA of WM tracts subserving EF (iFOF/SLF), (c) increased estradiol level will relate to decreased FA and cortical morphometry of the LPFC, and (d) increased estradiol level combined with decreased EF behavior would subsequently exacerbate these previous findings.
This comprehensive study, therefore, investigated how individual differences in EF behavior and estradiol level relate to variation in aspects of cortical morphometry and FA in a healthy, adolescent sample.

| Participants
Cross-sectional data were obtained from the Pediatric MRI Data Repository (Release 4.0) of the NIH MRI Study of Normal Brain Development, a project developed to characterize healthy brain maturation in relation to behavior in a large, multisite study (Evans & Brain Development Cooperative, 2006). This multi-center project conducted epidemiologically based recruitment of a large, demographically balanced sample across a wide age range, using strict exclusion factors and comprehensive clinical/behavioral measures. A mixed cross-sectional and longitudinal design was used to create an MRI/clinical/behavioral database from approximately 500 children, aged 7 days to 18 years, to be shared with researchers and the clinical medicine community. Using a uniform acquisition protocol, data were collected at six Pediatric Study Centers and consolidated at a Data Coordinating Center. Enrolled subjects underwent a standardized protocol to characterize neurobehavioral and pubertal status. The data were demographically representative of the U.S. population in terms of variables including gender, race, and socioeconomic status (Waber, Forbes, Almli, Blood, & Cooperative, 2012). Exclusion criteria included but were not limited to IQ < 70, history of medical illness with CNS implications, and any Axis I psychiatric disorder (other than simple or social phobia, adjustment disorder, oppositional defiant disorder, enuresis, encopresis, or nicotine dependency; see Waber et al. (2007) for a complete list of inclusion and exclusion criteria). Participants underwent brain MRIs and extensive neuropsychological testing on up to three occasions at two-year intervals. For the purposes of this report, a sample of 55 participants (age range 7-18) with cross-sectional data (1 time point) was selected with structural imaging data (T1), diffusion tensor imaging data (DTI), behavioral (BRIEF), and hormonal data (estradiol) ( Table 1).
Seven participants were missing estradiol data; therefore, they were not included in subsequent analyses involving estradiol. Collection site was treated as a nuisance factor in all subsequent analyses.

| Behavior rating inventory of executive function (BRIEF)
The BRIEF was completed on the same day as the scan by a parent or guardian that had contact with the child within the prior 6 months. The Subscales (Table 2) were used for further correlation and regression analyses with cortical, FA, and hormonal measurements, controlling for age and gender. Multiple comparison correction was carried out using the Benjamini & Hochberg, 1995procedure (Benjamini & Hochberg, 1995, controlling the false discovery rate (FDR) at p < .05.

| Estradiol
At each visit during the assessment day, all subjects provided two separate 1-3cc samples of saliva at two time points between 12 and 6 p.m. The maximum range for the collection of the two hormonal time points was 7 hr and 40 min. Saliva was collected while the subject was relaxed and not after potentially stressful procedures (e.g., MRI). Samples were collected, stored at -20 to −80°C, and shipped in batches from each site to UCLA. Samples were assayed by pub- We additionally performed independent samples t tests comparing pre-and postpubertal groups by BRIEF subscales, estradiol level, and structural connectivity.

| Diffusion tensor imaging (DTI)
Data were acquired at a subset of sites (Boston, Cincinnati,

| Surface-based morphometry (SBM)
Cortical reconstruction and volumetric segmentation was performed with the Freesurfer image analysis suite (v5.6.0), which is documented and freely available for download online (http://surfer.nmr.mgh.harva rd.edu/). The technical details of these procedures are described in prior publications . Briefly, this processing includes motion correction and averaging (Reuter, Rosas, & Fischl, 2010) of volumetric T1-weighted images, removal of nonbrain tissue using a hybrid watershed/surface deformation procedure (Ségonne et

Ability to control impulses (inhibitory control) and to stop engaging in a behavior
Shift Ability to move freely from one activity or situation to another; to tolerate change, to switch or alternate attention

Emotional control
Ability to regulate emotional responses appropriately

Initiate
Ability to begin an activity and to independently generate ideas or problemsolving strategies Working memory Ability to hold information when completing a task, when encoding information, or when generating goals/plans in a sequential manner Plan/Organize Ability to anticipate future events; to set goals; to develop steps; to grasp main ideas; to organize and understand the main points in written or verbal presentations

Organization of materials
Ability to put order in work, play, and storage spaces (e.g., desks, lockers, backpacks, and bedrooms)

Monitor
Ability to check work and to assess one's own performance; ability to keep track of the effect of one's own behavior on other people TA B L E 2 BRIEF subscale descriptions F I G U R E 1 Normalized estradiol values divided in by gender and pre-and postpuberty Groups al., 2004), automated Talairach transformation, intensity normalization (Sled, Zijdenbos, & Evans, 1998), tessellation of the gray matter white matter boundary, automated topology correction (Fischl, Liu, & Dale, 2001;Ségonne, Pacheco, & Fischl, 2007), and surface deformation following intensity gradients to optimally place the gray/white and gray/ cerebrospinal fluid borders at the location where the greatest shift in intensity defines the transition to the other tissue class Dale & Sereno, 1993;Fischl & Dale, 2000). Once the cortical models are complete, a number of deformable procedures were carried out for further data processing and analysis including surface inflation , registration to a spherical atlas which utilized individual cortical folding patterns to match cortical geometry across subjects (Fischl, Sereno, Tootell, & Dale, 1999), parcellation of the cerebral cortex into units based on gyral and sulcal structure (Desikan et al., 2006;Fischl et al., 2004), and creation of a variety of surface-based data including maps of cortical volume, surface area (SA), thickness, curvature, sulcal depth, and local gyrification index. The resulting probability maps were input into a general linear model (GLM) evaluating regressions between all vertices and BRIEF subscales, estradiol level, as well as the BRIEF subscale interaction with estradiol level (calculated by multiplying the raw BRIEF score with the estradiol level) controlling for age, gender, time of estradiol collection (when estradiol was present in the analysis), intracranial volume (ICV) and collection site. Vertex-wise threshold was set at p < .001 level. Cluster-wise threshold was corrected for at p < .05 level using nonparametric permutation testing with Monte Carlo simulation.

| Behavioral/hormonal results
No significant results were found when investigating the relationship between EF behavior and estradiol level (Table 3A). Given the notable effects of age/gender in the sample, a separate analysis investigating the unique role of age/gender on the relationship between BRIEF subscales and estradiol level was explored (i.e., not controlling for age and gender, Table 3B). This resulted in no significant findings, suggesting age and gender do not have an impact on executive function (EF) behavior and estradiol level in this sample (see Supplementary Material for more detail about these null findings).

| Neuroimaging results
Next, we interrogated the BRIEF subscales, estradiol level, and their interaction, with cortical morphometry and FA. Of note, pubertal status showed no relationship with cortical morphometry or FA. The focus of this paper, therefore, presents only estradiol level in relationship to EF, cortical morphometry, and structural connectivity, as well as the impact of BRIEF subscale-by-estradiol interaction on brain measurements. The interaction between EF behavior and estradiol level is of particular interest, due to the fact previous studies suggest both variables have a close relationship to cortical morphometry and structural connectivity.
Given the possible effects of age/gender in the sample, an analysis investigating the unique role of age and/or gender on cortical morphometry was explored.  Figure 4). To be consistent throughout the results and discussion, we refer to the positive relationship between EF behavior and estradiol level from the standpoint of higher BRIEF subscale scores (i.e., decreased EF behavior) and elevated estradiol level, to explain their effects on cortical morphometry and FA.

Cortical morphometry
To examine relationships between cortical morphometry and EF (as  (Table 4, Figure 2).

White matter integrity
Having examined gray matter relationships with EF behavior, we next investigated FA. The Plan/Organize subscale showed a negative relationship with FA of the forceps minor (R 2 = .07, p = .01).
Conversely, the Inhibit subscale showed a positive relationship with FA in the forceps major (R 2 = .04, p = .02). These results indicated decreased ability putting order into play was associated with lower FA in the forceps minor, while decreased control over impulses was associated with higher FA in the forceps major (Table 4, Figure 2).

Cortical morphometry
No significant results were observed between cortical morphometry and estradiol level.

White matter integrity
Next, we examined the relationship between estradiol level and FA.
A negative relationship between estradiol level and FA was observed in the right inferior fronto-occipital fasciculus (iFOF) (R 2 = .09, p = .01), indicating higher estradiol level related to lower FA in the right iFOF (Table 5, Figure 3).

Cortical morphometry
Because we hypothesized that estradiol level may interact with EF behavior, we investigated the BRIEF subscale-by-estradiol interaction and its effect on cortical morphometry. A negative relationship was observed between the Inhibit-by-estradiol interaction and GMV in the right PMC and between the Working Memory-by-estradiol and GMV in the right PMC [−log(p) = −4.00, p = .0001; −log(p) = −2.47, p = .003, respectively]. These results indicated that increased difficulty inhibiting one's actions and increased levels of estradiol related to less GMV in the right PMC. Additionally, increased difficulty holding information online and increased levels of estradiol related to less GMV in the right PMC (Table 6, Figure 4).

White matter integrity
Finally, we examined the relationship between BRIEF subscales-byestradiol and FA interaction. The results showed negative relationships between the Initiate-by-estradiol interaction and FA in the right iFOF (R 2 = .15, p = .01), and between the Working Memoryby-estradiol interaction and FA in the right iFOF (R 2 = .16, p = .008).
The results suggested that increased estradiol level and decreased motivation of task initiation related to lower FA values in the right iFOF (Table 6, Figure 4). Lastly, increased magnitude of the interaction between EF behavior and estradiol level related to decreased cortical gray matter morphometry and white matter tract integrity. Below we discuss each finding and its relative implications.

| Relationship between executive function and estradiol level
Firstly, we wanted to determine the relationship between estradiol level and EF behavior. We did not find any significant relationships between estradiol level and EF behavior, regardless of correcting or not correcting for age/gender. Previous evidence suggests that estradiol level is indeed related to EF (Hampson & Morley, 2013;Hidalgo-Lopez & Pletzer, 2017), albeit the results differ depending on the age range of the sample. It is possible our age range (7-18) did not have enough variability to produce statistically significant results.

| Relationship between executive function and cortical morphometry
We next aimed to determine how EF behavior, aspects of cortical TA B L E 6 Relationships of BRIEFby-estradiol subscales interaction with cortical GM and FA demonstrates the forceps minor is a fiber bundle which connects the lateral and medial surfaces of the frontal lobes and crosses the midline via the genu of the corpus callosum (Genova, DeLuca, Chiaravalloti, & Wylie, 2013). When damaged by disease, the forceps minor is linked to robustly diminished processing speed and cognitive impairment, indicating its interhemispheric connections between the PFC contribute to EF (Biesbroek et al., 2016;Genova et al., 2013). The association between damage to these tracts and reduced performance in the trail-making task has been reported in schizophrenia (Perez-Iglesias et al., 2010) and traumatic brain injury (Kraus et al., 2007). Previous studies in multiple sclerosis (MS) have also noted a correlation between reduced FA in the forceps minor and Paced Auditory Serial Addition Test (PASAT) performance (Van Hecke et al., 2010). Our results therefore echo previous findings: Decreased EF behavior is related with decreased FA of the forceps minor.
Conversely, decreased EF behavior related to controlling impulses (Inhibit subscale) was associated with increased FA in another WM tract, the forceps major. The forceps major is a fiber bundle which connects the occipital lobes and crosses the midline via the splenium of the corpus callosum (Prasad, Upton, Nimgaonkar, & Keshavan, 2015) and is thought to aid visuo-spatial function (Tamura et al., 2007). Lesions of the forceps major are associated with deficits in multitasking (Burgess, Veitch, de Lacy Costello, & Shallice, 2000), allocation of attentional resources, and other information processing requiring integrated hemispheric function (Rossi et al., 2012). An increase of FA in the forceps major suggests efficient and speedy processing of incoming visuo-spatial material and thus may result in difficulties inhibiting behavior. Indeed, patients with conditions posited to arise from axonal overconnectivity such as autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), and schizophrenia exhibit reduced inhibitory control (Solso et al., 2016;Tamm, Barnea-Goraly, & Reiss, 2012;Taylor, Theberge, Williamson, Densmore, & Neufeld, 2016). Thus, our findings suggest that FA changes in the forceps major affect attention-based cognitive functions such as impulse control and highlight the complex relationship between white matter structure and EF behavior.

| Relationship between executive function, cortical morphometry and structural connectivity
We next investigated whether estradiol level had any relationship to cortical morphometry and FA. We next hypothesized decreased cortical morphometry in the LPFC and reduced FA, as studies indicate that decreased cortical morphometry and FA may both be related to increases in estradiol level in adolescent individuals (Herting et al., 2014(Herting et al., , 2012Peper et al., 2009;Witte et al., 2010). Contrary to our hypothesis, we found no such relationship with cortical morphometry. However, our results indicated that increased estradiol level related to decreased FA of the right iFOF.
The iFOF, a long association WM bundle connects the inferior and lateral regions of the PFC through the inferior temporal lobes, terminating in lateral occipital regions (Ashtari, 2012). Research indicates the iFOF plays a critical role in attention and visual processing (Catani & Thiebaut de Schotten, 2008;Wu, Sun, Wang, & Wang, 2016

| Relationship between executive function, cortical morphometry, structural connectivity and estradiol
The final aim of the study was to determine the relationship between the interaction between BRIEF subscales and estradiol (i.e., BRIEF subscale-by-estradiol) with cortical GM and FA. At last, we hypothesized that decreased EF behavior would be related to increased levels of estradiol, which may consequently relate to reductions in aspects of cortical morphometry and structural connectivity. Our cortical morphometry findings indicated that decreased EF behavior related to controlling impulses (Inhibit subscale) and holding information online (Working Memory Subscale) coupled with increased estradiol level was associated with less GMV in the PMC. Studies indicate extensive connections exist from the anterior PFC to PMC (Fregni et al., 2005), which are thought to coordinate the integration of higher level EF processes and motor planning in service of goal attainment. Moreover, research demonstrates the LPFC has an increased number of estradiol receptors (Almey, Milner, & Brake, 2015) which may result in increased sensitivity of estradiol in this region posited to underlie EF processes. Indeed, estradiol's impact on working memory is well documented, with high levels of estradiol impairing LPFC-dependent working memory, while low-level estradiol weakly facilitating it (Bimonte & Denenberg, 1999;Holmes, Wide, & Galea, 2002;Wide, Hanratty, Ting, & Galea, 2004). Therefore, our results append to existing findings, suggesting that changes in cortical morphometry may reflect more complex interactions between EF behavior and estradiol level affecting the LPFC.
Finally, our study's WM analyses suggested more difficulties with EF behavior related to beginning an activity (Initiate subscale), holding information online when completing a task (Working Memory subscale), and elevated estradiol level were associated with lower FA in the right iFOF. Previous research indicates that elevated estradiol level is related to decreases in EF behavior in adolescents (Lenroot & Giedd, 2010;Peper et al., 2009Peper et al., , 2011 and that FA in the iFOF may be an important neural correlate of EF (Santiago et al., 2015). The study had several limitations. First, the study design was cross-sectional and not longitudinal, which prevented depiction of individual trajectories, differences in change, and direct estimation of relationships between change across different morphometric measurements. The conclusions from the present study should be replicated in longitudinal studies. Although a longitudinal approach has many merits, because multimodal imaging and hormonal data were only available for a large enough sample during one visit per participant, our study's aims were only possible with a cross-sectional approach. Second, although pubertal status was taken into account (using the Tanner Stage), menstrual cycle data were not recorded for the female participants, which could further result in fluctuations in estradiol level across the cycle and affect EF behavior. Thirdly, since the BRIEF subscales are highly intercorrelated, discerning their individual impact on brain morphometry is difficult, but speak to their contribution to EF as a whole. Fourthly, no other hormones related to the menstrual cycle were collected. Literature suggests that during the menstrual cycle, both estradiol and progesterone levels fluctuate rapidly and a difference of a few hours can matter dramatically for estradiol levels. Rapidly changing effects of this hormone, coupled with age differences, suggest that these are important factors to keep in mind when researching the effects of cycling in females. Lastly, the measurement of estradiol level from saliva has drawbacks, especially in an adolescent population. Although great care was taken to understand the relationship between estradiol level, EF behavior and aspects of cortical morphometry and structural connectivity, saliva measurements are greatly affected by the use of exogenous hormones such as birth control or transdermal creams (Lewis, McGill, Patton, Patton, & Elder, 2002). Furthermore, results should be interpreted cautiously due to lack of contraceptive and menstrual cycle data.
Future studies should continue to combine EF behavior (such as the BRIEF subscales), estradiol level, and multimodal neuroimaging methods in order to disentangle the function-estradiol-structure relationship in this critical neurodevelopmental period in cortical morphometry and structural connectivity thought to underlie EF processes. Specifically, the roles of peptide hormones like oxytocin and vasopressin should be investigated in the neural development of the adolescent brain and its relationship to EF processes.  mni.mcgill.ca/nihpd/ info/parti cipat ing_cente rs.html. This manuscript reflects the views of the authors and may not reflect the opinions or views of the NIH.

CO N FLI C T O F I NTE R E S T
None declared.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.