Socioeconomic Status and the Brain

June 26, 2018

Written by: Katerina Placek

 

Nearly every society features differences in access to resources and/or material goods, such that some individuals are ‘better off’ than others. These differences are modernly captured by socioeconomic status (SES), a combined measure of economic and social position that is often based on income, education, and occupation. A person with high SES commonly has higher annual income, greater years of education, and/or a more prestigious occupation when compared to a person with low SES. Importantly, a person’s SES is predictive of a remarkable range of life outcomes including physical health, mental health, academic and occupational performance, and IQ. This makes understanding how SES impacts human development an essential area of study from broader social and public health standpoints. But why should neuroscientists be concerned with SES?

Over the past two decades or so, neuroscientists have begun to study how differences in SES may be reflected in brain biology and function. This is because of discoveries demonstrating that 1) the development of the brain is influenced by environmental factors associated with SES, and 2) SES impacts human cognitive ability and mental health. Indeed, environmental risk exposure to toxins including lead and air pollutants is more common in persons from low SES backgrounds¹, and is causally associated with delayed cognitive and behavioral development and dis-regulation of a protein called brain-derived neurotrophic factor (BDNF)² which is essential to brain growth. Furthermore, persons from higher SES backgrounds perform better on standardized tests of intellectual achievement (e.g. kindergarten readiness³, intelligence quotient (IQ) ⁴, and have lower rates of depression and psychosis relative to persons from lower SES backgrounds.

Given this motivation, have neuroscientists actually found brain differences associated with SES? The answer is yes – in fact, neuroscientists find differences between persons from high and low SES in both brain anatomy and brain function. A few of the best-replicated differences are highlighted below.

Neuroscientists can measure brain anatomy and brain function in living people using magnetic resonance imaging (MRI). You have likely heard of MRI in the context of medical care – MRI is a technique used to form pictures of the anatomy and physiology of the body by taking advantage of the differences in magnetization between different tissues (e.g. bone, blood, muscle, fat). When MRI is used to measure brain anatomy, it is referred to as structural MRI. If we were to compare the human brain to the engine of a car, structural MRI allows us to look ‘under the hood’ and examine the components of the engine. However, measures of brain function allow us to evaluate how well the engine performs – is the engine fuel efficient? Does it enable maximum power output? MRI can also be used to measure changes in brain function associated with mental processes such as cognition in a technique called functional MRI. Functional MRI allows neuroscientists to determine which regions of the brain are more active than other regions during different tasks by measuring the magnetization of oxygen in blood, with the understanding that blood flow increases to brain areas that are more active.

Using structural MRI, neuroscientists’ most frequent finding is that children from low SES families have smaller measured volume of the hippocampus compared to children from higher SES families⁵. The hippocampus is located in the temporal lobe (see Figure 1) and is essential for the consolidation of information into long-term memory (e.g. remembering what you had for dinner last night) and for spatial navigation (e.g. knowing how to get from your house to your school/workplace).  Importantly, this finding agrees with studies of human cognitive ability. Memory ability is consistently found to be worse in persons from low SES backgrounds⁶. Smaller hippocampal volume has also been observed in low SES adults, but this finding is not always common across multiple studies– future work is necessary to determine whether the hippocampal differences in childhood associated with SES actually persist into adulthood.

 

Using measures of brain function including functional MRI, neuroscientists have observed SES-related differences in activity of the brain. One study used functional MRI to measure brain activity while children completed a complex task. This task required children to learn previously unknown associations between buttons on a keypad and groups of pictures ‘on the fly’ as they completed the task. The results of this study indicated that children from higher SES families not only showed better task performance relative to children from low SES families, but also showed different patterns of brain activity in the prefrontal cortex⁷. The prefrontal cortex consists of the outer layers of the frontal lobe of the brain essential for higher-order cognitive processes including planning, decision-making, and attention (see Figure 2). Specifically, higher-SES children showed greater activity in one region of the prefrontal cortex when correctly learning novel rules, and less activity in different regions associated with distracted attention.

Differences in activity of the prefrontal cortex have also been demonstrated using another method of measuring brain function called electroencephalography (EEG). A researcher using EEG measures brain activity by placing electrodes on the scalp and measuring the amplified signal (see Figure 3). Results from research using EEG shows that low SES children exhibit larger responses to distracting stimuli⁸ and smaller responses in prefrontal cortical regions important for focusing attention on relevant stimuli⁹.

The neuroscience of SES including the differences in brain anatomy and function outlined above hold important implications for both neuroscience and for society. No human brain exists outside of a socioeconomic context, so the SES of human participants in neuroscience experiments should be considered in the design, analysis and interpretation of research findings. This can include prioritizing socioeconomic diversity of research participants, and/or discussing the generalizability of research findings to people living in different socioeconomic circumstances. Importantly, findings from neuroscience research impact society – they are used to inform public policies that include how we educate members of society and how we maintain a healthy environment healthy. With this in mind, the neuroscience of SES is important for understanding the human brain and for shaping the society we live in.

 

References:

1. Evans, G. W. & Kantrowitz, E. Socioeconomic Status and Health: The Potential Role of Environmental Risk Exposure. Annual Review of Public Health 23, 303–331 (2002).
2. Stansfield, K. H., Pilsner, J. R., Lu, Q., Wright, R. O. & Guilarte, T. R. Dysregulation of BDNF-TrkB signaling in developing hippocampal neurons by Pb(2+): implications for an environmental basis of neurodevelopmental disorders. Toxicol. Sci. 127, 277–295 (2012).
3. Noble, K. G., Norman, M. F. & Farah, M. J. Neurocognitive correlates of socioeconomic status in kindergarten children. Developmental Science 8, 74–87 (2005).
4. Turkheimer, E., Haley, A., Waldron, M., D’Onofrio, B. & Gottesman, I. I. Socioeconomic Status Modifies Heritability of IQ in Young Children. Psychological Science 14, 623–628 (2016).
5. Noble, K. G. et al. Socioeconomic disparities in neurocognitive development in the first two years of life. Developmental Psychobiology 57, 535–551 (2015).
6. Farah, M. J. The Neuroscience of Socioeconomic Status: Correlates, Causes, and Consequences. Neuron 96, 56–71 (2017).
7. Sheridan, M. A., Sarsour, K., Jutte, D., D’Esposito, M. & Boyce, W. T. The Impact of Social Disparity on Prefrontal Function in Childhood. PLoS ONE 7, e35744 (2012).
8. Stevens, C., Lauinger, B. & Neville, H. Differences in the neural mechanisms of selective attention in children from different socioeconomic backgrounds: an event-related brain potential study. Developmental Science 12, 634–646 (2009).
9. Kishiyama, M. M., Boyce, W. T., Jimenez, A. M., Perry, L. M. & Knight, R. T. Socioeconomic disparities affect prefrontal function in children. Journal of Cognitive Neuroscience 21, 1106–1115 (2009).

 

 

Images:

Figure 1: From Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body, Bartleby.com: Gray’s Anatomy, Plate 739, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3907047

Figure 2: Polygon data were generated by Database Center for Life Science(DBCLS). Polygon data are from BodyParts3D. CC BY-SA 2.1, https://commons.wikimedia.org/w/index.php?curid=32490636

Figure 3: By Antoine Lutz – Barry Kerzin, Public Domain, https://commons.wikimedia.org/w/index.php?curid=56906405