Cultural Variation in Behavioral Correlates of Cortical Thickness among 9-10-Year-Old Children

Original Article

Austin Addict Sci. 2023; 5(1): 1017.

Cultural Variation in Behavioral Correlates of Cortical Thickness among 9-10-Year-Old Children

Shervin Assari1-4*; Babak Najand4

1Department of Family Medicine, Charles R. Drew University of Medicine and Science, Los Angeles, CA, USA

2Department of Public Health, Charles R. Drew University of Medicine and Science, Los Angeles, CA, USA

3School of Nursing, Charles R. Drew University of Medicine and Science, Los Angeles, CA, USA

4Minorities’ Diminished Returns (MDRs), Los Angeles, CA, USA

*Corresponding author: Shervin Assari Department of Family Medicine, Charles R. Drew University of Medicine and Science, Los Angeles, CA, USA. Tel.: +1-734-858-8333 Email: assari@umich.edu

Received: April 19, 2023 Accepted: May 20, 2023 Published: May 27, 2023

Abstract

Background: One of the main characteristics that differentiate Asian and European cultures is self-construal, the former has a higher tendency to have interdependent, and the latter tends to have an independent self. Recent structural brain imaging studies have shown that the Prefrontal Cortex (PFC) is thinner in Asian than European individuals, attributed to a need for reduction of reward relevant to self to maximize the reward relevant to the group. There is more to find out about differential cortical thickness and behavioral correlates across these cultural groups

Aim: This study was performed to investigate the associations between cultural group membership, age, cortical thickness, and two sets of reward-related behaviors (e.g., reward responsiveness and prosocial behaviors) in a national sample of 9/10-year-old children in the U.S.

Materials and Methods: For this cross-sectional study, we used demographic, socioeconomic, structural, and behavioral data from the Adolescent Brain Cognitive Development (ABCD) study. Our analytical sample included 5942 American children between the ages of 9 and 10 who were either European (n=5741) or Asian (n=201). The cortical thickness for various Regions Of Interest (ROIs) was measured using Structural Magnetic Resonance Imaging (sMRI). Two aspects of the behavioral profile, reward sensitivity, and prosocial behaviors, were also measured using self-report data. As a proxy of self-construal, culture was the independent variable or the moderator variable, depending on the model. Mixed-effects regression models were used for data analysis to adjust for nested data across families and study sites.

Results: In the overall sample, asian children had a smaller thickness of cortical regions across all ROIs than European children. Age did not interact with culture on cortical thickness, suggesting a similar rate of pruning across cultures. Culture showed statistically significant interactions with cortical thickness across ROIs within and beyond PFC on children’s reward responsiveness and prosocial behaviors, indicating stronger associations for Asian than European children.

Conclusion: Compared to European children, Asian children show lower cortical thickness across ROIs, a phenomenon that is not limited to PFC and is not due to a differential rate of age-related pruning. There are stronger associations between the thickness of several cortical areas with prosocial behaviors and reward responsiveness in Asian children relative to European children, extending the existing literature on culture, cerebral cortex, and reward salience. Our findings support the hypothesis that Asian children’s cortical volume changes are a cultural adaptation to maximize conformity and harmony with the group in Asian culture by reducing the relevance of reward salience of self and maximizing it for the community. More research is needed on cultural differences in behavioral correlates of structural and functional measures of cortical regions among European and Asian children.

Keywords: Population groups; Prefrontal cortex; Cerebral cortex; Morphometry; cortical thickness; Culture; Ethnic groups; Asian; European

Background

Cultural psychologists such as Markus and Kitayama [1] have introduced self-construals of interdependence versus independence as a primary distinction of Asian from European culture [2]. Also described as one of the main East-West differences [3], a considerable body of research has shown that Asian individuals differ in their definition and view of self, relative to others [4]. One of the most robust findings on the contrasts between these two cultural patterns are seen in the higher social orientation of Asian than European culture, a finding repeatedly documented by multiple surveys comparing Asian and European individuals inside and outside the United States [5]. Asians place higher relative importance on others relative to self, compared to Europeans [1]. Although some questions have been raised regarding the validity and reliability of these findings and constructs [3,5,6], the consensus is that this cultural difference between Asians and Europeans is both valid and significant [7].

The systematically higher significance of social orientation in Asian culture than European culture has implications for behavioral profiles of Asian versus European cultures. Some of the behavioral implications of social orientation might be differences in down-regulation of urges and emotion regulation to maximize conformity, social harmony, and pursuing rewards for the group rather than self [8]. However, we are not aware of any studies that have compared Asian and European cultures for the associations between brain structure and reward-related behaviors.

To prioritize relationship with others (one's social orientation toward a community) above one's self-interest [1,9,10], Asians may have a higher need for suppressing their emotions and motivations, which may be in contrast to the benefit of their group [10]. Asian’s interdependence culture, viewing the self as connected to others, emphasizing harmonious relations with others, and favoring the community's good all may require minimizing the rewards that are merely relevant to self [10]. The reward system should be consistently down-regulated, and emotion regulation should be empowered in Asians [10]. In contrast to Asian culture, European’s independent culture emphasizes uniqueness, views self as separate from others, and favors pursuing own reward independent of group [1]. As such, Europeans require far less down-regulation and regulation of emotions [11,12]. These differences may have implications for reward-related behaviors between and within Asian and European cultures [1].

Some recent research has shown that Prefrontal Cortex (PFC) differences may exist between Asian and European individuals, supporting the above hypothesis of Asian culture adaptation to these cultural differences [11,12]. In one study, Kitayama et al. studied 135 Japanese young adults and collected data on structural magnetic resonance imaging and self-construal [11]. They measured (a) independent and interdependent self-construal, (b) the degree to which individuals form vivid images of external objects (object imagery), and (c) the PFC volume, particularly for the Orbitofrontal Cortex (OFC) [11]. The authors investigated OFC because of its role in value-based decision-making. The authors hypothesized that OFC thickness, which has a role in personal goals and desires, would be inversely linked to interdependent self-construal scores. The highest level of interdependent self-construal was associated with lower OFC volume and high object imagery in that study. The authors argued that their findings are consistent with previous evidence that interdependence, as realized via obligation and duty, requires reduced self-interest and maximizes cognitive attunement to environmental context [11]. In another study [12], Kitayama et al. analyzed data of 132 young adults (both European Americans and Asian-born East Asians) for self-construal, structural MRI, and genetic data [12]. Authors found that gray matter volume of the medial prefrontal cortex and the orbitofrontal cortex were smaller among Asian than European individuals. Moreover, the difference in gray matter volume was significantly more pronounced among carriers of the 7/2-R allele of the dopamine (DRD4) gene than among non-carriers. This pattern was robust in an alternative measure assessing cortical thickness. The authors also found that among Asian carriers, the number of years spent in the U.S. was predictive of increased gray matter volume in the OFC cortex. Both these studies have provided evidence consistent with a view that culture shapes the cortical thickness [11,12].

For at least five reasons, there is a need for additional studies in this field. First, most of this literature is on adults, and less is known about the relevance of brain structure for culture in children. Second, given the replication crisis in psychological studies, there is a need to replicate a study that has recently emerged. Third, most of these studies have a small sample size, and there is a need for investigations that have larger statistical power. Fourth, past research is mainly on PFC. However, it is likely that this is a general pattern that holds for PFC and other cortical regions. As such, there is a need to explore these cultural variations in cortical thickness across Regions of Interest (ROIs). Finally, these studies have not tested differential correlations between brain structures and reward-related behaviors such as reward responsiveness and prosocial behaviors between Asian and European children.

Aims

In a national sample of 9/10-year-old American children (general population), the current study was performed with three aims in mind. First, to compare Asian and European children for cortical thickness across Regions of Interest (ROIs), particularly PFC thickness. Second, to investigate cultural differences in the associations between cortical thickness with age. Third, to investigate cultural differences in the associations between cortical thickness with reward responsiveness and prosocial behaviors. Cortical thickness across Regions of Interest (ROIs), including but not limited to PFC was expected to be smaller in Asian than European culture (Hypothesis 1). Smaller cortical thickness and stronger associations between cortical thickness and reward responsiveness and prosocial behaviors in Asian than European children are based on the existing hypothesis on differences between Asians and Europeans in group orientation, sympathy, conformity, and reward dependence, as described by Kitayama and others [1]. We also expect cortical thickness to show a stronger inverse association with rage for Asian than European children, suggesting faster pruning in Asian than European children, which would contribute to the thinner cortex in Asians than Europeans (Hypothesis 2). We also expect cortical thickness to show stronger associations with reward responsiveness and prosocial behaviors for Asian than European children (Hypothesis 2).

Materials and Methods

Design and Setting

With a cross-sectional design, this study applied a secondary analysis of data from the Adolescent Brain Cognitive Development (ABCD) study [13-17]. The ABCD is a national brain development study of American children [13,18].

Sample and Sampling

The ABCD participants were sampled from 21 sites in multiple cities across different states in the United States. The ABCD sample is mainly enrolled through the U.S. school system. The ABCD sampling strategy applied a careful design of pre-adolescents sampling across various sites [13,14,16,18-33]. To ensure that the ABCD sample is representative, the ABCD has used a weight (propensity score). Using weights (propensity scores), the final ABCD results are generalizable to the U.S., and the weighted participants are a close approximation of national sociodemographic, sex, culture, race, and ethnicity. A full description of the ABCD sample and sampling is published here [34].

Analytical Sample

This study included 5938 9/10-year-old children who had data on our study variables, including negative urgency. Children from European or Asian cultures were included. Participants from other cultural and ethnic groups such as Black, Native American, Hispanic/Latino, or other/mixed were excluded. No additional eligibility criteria were considered.

Measures and Measurements

Cerebral cortex thickness. For various Regions of Interest (ROIs), the cerebral cortex thickness was measured using harmonized sMRI across 21 study sites. Harmonization and standardization of ABCD imaging modalities are well described here [35]. The ABCD centers conducted high-resolution T1-weighted structural MRI scans (1-mm isotropic voxels) with one of the following scanners: Philips Healthcare (Andover, Massachusetts), GE Healthcare (Waukesha, Wisconsin), or Siemens Healthcare (Erlangen, Germany) [14]. All the structural MRI data were processed using FreeSurfer version 5.3.0 [36,37], in line with the standard processing pipelines [14]. The process included the removal of nonbrain tissue, the segmentation of gray and European matter [38] and the parcellation of the cerebral cortex [39]. Every scan session underwent a radiological review. Anextended quality control protocol was implemented, which included a visual inspection of T1 images and Free Surfer outputs for an acceptable quality [40]. MRI images that did not pass the quality control were excluded. The cortical parcellation in this study was based on the Desikan-Killiany atlas of ROIs [40]. Although the primary ROIs in this study were PFC and OFC, we reported values for the volumetric data provided by the ABCD data for all ROIs in cerebral cortex. One of the confounders was intra-cranial volume to adjust for the differences between skull and whole-brain size across cultures.

Culture: Culture, identified by parents, was a categorical variable with the following levels: Asian and European (reference group). This variable was the independent variable for aim 1 and the effect modifier for the other aims.

Parental educational attainment: Parental educational attainment was a five-level categorical variable. Responses included 1= less than high school diploma; 2 = high school diploma or GED; 3 = some college; 4 = college degree; and 5 = some graduate education.

Parental marital status: The household's marital status was a dichotomous variable: married = 1 and non-married = 0.

Family income: Family income was a three-level categorical variable. The item used to measure parental educational attainment was: "What is your total combined parental educational attainment for the past 12 months? This should include income (before taxes and deductions) from all sources, wages, rent from properties, social security, disability and veteran's benefits, unemployment benefits, workman". Levels were 1= less than $50,000; 2 = $50,000 to $99,000; 3 = $100,000 or more.

Age: Age was measured in months and was a continuous measure.

Sex: Sex, 1 = males and 0 = females, was a dichotomous variable.

Data Analysis

We used the Data Analysis and Exploration Portal (DEAP) for data analysis. Developed as a part of the National Data Archive (NDA), National Institutes of Health (NIH), DEAP is a data analysis platform that uses R software to perform analysis of the ABCD data. As ABCD participants are nested within families, who are themselves sampled across 21 sites, DEAP uses mixed (random) effect models for analysis of data. Such an approach adjusts for the ABCD data's nested nature. Standard Errors (SEs) are estimated for various levels of analysis (individual, family, site). To describe our sample, we reported mean Standard Deviation (SD) for continuous variables, frequencies, and percentages for categorical variables in the pooled sample and by culture. We used Chi-square or independent sample t-test for bivariate analysis. Mixed-effects multivariable models were performed. In these models, cortical thickness for ROIs was the outcome, culture was the moderator, and sex, age, household income, parental education, and family structure were covariates. All these models controlled for study site and family ID as well. For models in the pooled sample, we also controlled for whole-brain size (intracranial volume). First, we ran models with culture as the main independent variable and ROI cortical thickness as the outcomes. Then we ran models within each culture (culture as strata), with cortical thickness across ROIs as the predictor and reward responsiveness or prosocial behavior as the outcomes. We also ran models with and without interaction for regions that showed culture-specific correlations with reward responsiveness or prosocial behavior. Regression coefficient (b), and p-values were reported for each model parameter. Appendix 1 shows the formula used in the DEAP system. Appendix 2 is a fit of our models. Appendix 3 reports the results of our regression to test the effect of culture on ROIs without whole-brain volume as a covariate.

Ethical Aspect

For this study, we used a fully de-identified data set and therefore the study was exempted from a full review Institutional Review Board (IRB). However, the main study protocol, the ABCD, was approved by the IRB at the University of California, San Diego (UCSD), and several other institutions. Participants signed consent or assent depending on their age [18].

Results

Descriptive Data

Table 1 depicts the summary statistics of the pooled sample and by culture. The current analysis was performed on 5942, 9/10-year-old children, from which 5741 were European and 201 were Asian.