Introduction

One of the fundamental mysteries in neuroscience is how the brain develops and adapts to its experiences regardless of genetics. It has become a hot topic in media from documentaries like Three Identical Strangers, following the real story of three triplets separated at birth and reunited, to classic movies like Parent Trap, which plays out a similar situation in a more humorous setting. However, neurobiological differences from these contrasting experiences stem much deeper than Lindsay Lohan’s stellar performance of a stark British and American accent between twins. Recent studies have highlighted an intriguing view of how many factors in our lifestyles, such as education, diet, or social connections affect our neural development. Underlying most, if not all of these factors is socioeconomic status as a key determinant for a person’s lifestyle. In other words, different experiences resulting from socioeconomic inequality could significantly influence the physiological development and lifetime cognitive functioning of the human brain.

A Biological Basis

Neural plasticity, the ability of the nervous system to change, is crucial to understanding how environmental and social factors influence brain development and function over time [1]. The brain can be thought of as an extensive, complex web of electrical connections facilitated by neurons. After a stimulus is received, a neuron sends an electrical impulse called an action potential down its axon. As the action potential reaches the end of the neuron, chemical messengers called neurotransmitters are passed between neurons through junctions known as synapses. This triggers action potentials on the adjacent neuron if stimulated enough. This process repeats, creating an electrical signal that propagates across many neurons. When two nearby neurons are stimulated at the same time, connections are created between them and relevant existing connections are strengthened. In other words, “neurons that fire together, wire together.” Sometimes, multiple neurons compete for a single synaptic connection. Most notably in early development, there are periods of time in which a specific connection must be made before it is overridden by other connections fighting for the same synapse. These events are known as critical periods, and they explain the “use it or lose it” nature of some abilities [1]. This explains why some people cannot wiggle their ears and find it impossible to learn how to when they are older. Such synaptic connections must be formed at a young age. Similarly, this concept could also explain why a lack of exposure to critical stimuli in early life, such as emotional connections, language acquisition, or basic sensory functions could hinder proper neural development. For example, research has shown that sensory deprivation during critical periods hinders later development of neural networks required for face recognition [2]. These insights have led to interesting implications for the neural basis of socioeconomic inequality as a newly emerging field of study.

Poverty and Brain Development

Socioeconomic status (SES) has been strongly linked to significant changes in brain activity during infancy, which is when critical periods are most prevalent for learning tasks such as language development and attention [3]. In a study led by Dr. Przemyslaw Tomalski at the University of East London, researchers investigated if there were any neural determinants linked to SES. To do this, 45 infants from a wide variety of income and education levels were split into groups. Infants were then placed in groups of lower gross family income (LI) and higher income (HI) as well as into three family-occupational groups ranging from (1), meaning a higher managerial or professional job, and (3), being unemployed for over 6 months. The infants then watched a series of simple short videos, such as a colorful toy moving, while connected to an electroencephalogram (EEG). This machine utilizes a group of sensors over the scalp to record the electrical signals within the brain. Among these signals recorded is gamma activity, which is linked to memory and attention. Reduced gamma activity is even associated with neurological disorders including schizophrenia, Alzheimer’s, and epilepsy. They found that gamma activity was significantly higher in HI and higher ranking occupational groups. In other words, children who come from families in a higher socioeconomic standing are likely to have increased brain stimulation, which could further promote neural growth during important critical periods in infancy. Conversely, children in low-income families had lower gamma stimulation, leading to decreased neural growth [3].

Other studies have also shown connections between poverty and physiological changes such as brain volume [4]. Researchers at the University of Michigan found, in a diverse group of participants between the ages of 4 and 22, that lower family income was linked to reduced grey matter in the frontal and temporal cortex [5]. These areas are responsible for some aspects of planning and perception, and contain the hippocampus, which plays a significant role in learning and memory. There was a notable 8-9% reduction in volume amongst children living at or under the poverty line compared to expected sizes among the general population [5]. Having observed correlations from poverty to neurological and physiological changes, we must now also consider how environmental factors play in brain development.

The Absence of Environmental Stimuli

A clear connection between a loss of stimuli and neural development could explain why brain growth is stunted. Recall that “neurons that fire together wire together,” meaning that neural plasticity is influenced by connections made from repeated stimuli that activate multiple neurons simultaneously. Therefore, a lack of certain environmental stimuli may have a profound impact on neurogenesis, or the creation of new neurons. This was demonstrated in a 2015 study from UC Irvine where researchers compared mice in an environment with very few stimulants to mice that spent as little as 40 days in an enriched environment filled with toys, tunnels, and wheels, along with other mice for social interaction [6]. The researchers observed a significant increase in neurogenesis in the mice who were placed in an enriched environment. This effect was compounded by marked improvements by these mice in the Morris water maze test. In this test, a mouse is forced to swim through a partially submerged platform to measure its spatial learning ability and memory. It is important to note that these effects were more prominent in younger mice than older mice, consistent with the understanding that nearly all neural growth occurs in earlier years when environmental stimuli are most critical for development [6] [7]. While these findings alone are intriguing, they are merely indicative and cannot yet be extrapolated to humans without further study.

To address this connection, a fascinating study conducted in India by Dr. Dhanya Parameshwaran aimed to connect certain lifestyles interpreted as different levels of stimuli to cognitive development [8]. In this study, a group of 402 people from a wide range of incomes had their brain activity measured using an EEG. The results unveiled a significant difference in resting-state EEG signals between low-income individuals and high-income individuals. Thus, brain activity may be correlated with either the amount of money someone makes or the resulting lifestyle. As this finding suggests, low-income conditions, such as minimal education, lack of phone usage, and absence of travel were associated with lower resting-state EEG signals. One primary explanation for this pattern is that these low-income conditions contribute to a narrow range of stimuli and a lack of enriching stimuli compared to the high-income conditions [8]. Using the long-established link between stimulus input and neural plasticity, it makes sense that a decreased level of enriching stimuli would diminish the effects of neural plasticity and hinder proper brain development in the critical early years [9]. In the same way that the neurogenesis in mice was greater while in an enriched environment, neural plasticity is reduced in a low-income and less-enriched environment.

The Presence of Abrasive Stimuli

Another possible explanation for these neural deficits is the presence of more daily life stressors accompanying those with less financial stability compared to those with more. Children who grow up in low-income environments are more likely to experience challenges in living conditions and conflict among family members which can be significantly detrimental to their mental health [10]. In 2015, clinicians at Boston College explored these connections by following a group of 495 children from economically poor neighborhoods from infancy to age 6. They assessed the child’s developmental status, level of environmental chaos, and parental interaction through a series of interviews with both the parents and children at various time intervals as well as interviewer observations of households. At the end of the study, they found that the low-income surroundings of these children were strongly associated with detrimental physical and mental health [10].

These results could be explained by an above-average basal level of cortisol hormone, a chemical signal released in periods of stress [11]. A study led by Dr. Clancy Blair from New York University examined groups of infants across Eastern North Carolina and Central Pennsylvania, evaluating their household lifestyle through questionnaires given to the parents and observations of interactions between the parent and child when given a puzzle to complete together. They then took samples of the child’s salivary cortisol and compared it to the previously collected data. Their results found similar trends to the study at Boston College, highlighting higher overall levels of cortisol associated with poorer housing quality and lack of positive caregiving during puzzle completion tasks. Correlations between higher cortisol levels and African American ethnicity were also found, which may indicate an enduring intergenerational effect of social injustice on stress in African Americans from intractable disparities in birth and health outcomes [11]. There is strong evidence that these increased levels can interfere with the development of higher order processing as well as long-term attention and memory, which the same researchers describe as the building block of early cognitive and social competence [4]. As such, it is worthwhile to acknowledge the possibility that high amounts of stress-inducing stimuli, unlike the high amounts of positive stimuli in the aforementioned “enriched environment,” could adversely affect critical brain development in early childhood.

Exploring Other Dimensions

While these aforementioned studies may demonstrate that poverty is directly linked to hindered brain development, there are also indirect effects from inadequate access to other resources that stems from poverty. For example, educational accessibility is often affected by an individual’s socioeconomic status whether due to location or costs of higher education. Studies done at the University of Montreal have shown that years of education protect against loss of cortical volume [12]. Researchers determined this through magnetic resonance imaging (MRI), in which a machine which records the structure and size of participants' brains, and compared the data to their years of education. Functional MRIs (fMRIs), which record brain activity in various regions, revealed increased activity for higher educational levels in brain regions associated with working memory including the right medial frontal gyri, middle cingulate gyri, and right inferior parietal lobule [12]. Conversely, this could suggest that having a lower income poses a threat to healthy brain development due to the lack of access to education. Furthermore, inadequate nutrition as a result of low access to typically more expensive healthy diets disproportionately affects people experiencing poverty. Barriers to healthy food have been shown to negatively impact IQ and other academic-related skills, especially when occurring in the early years, suggesting that poverty indirectly harms cognitive development [13]. These are only a few possible connections that demonstrate the importance of holistic analysis of external factors when investigating as broad a topic as poverty and brain development.

Conclusion

Through a combination of absent enriching stimuli and added abrasive stimuli, lower socioeconomic status clearly has the potential to have profound impacts on both the physiological and cognitive development of the brain. While neuroscience and public health are normally viewed as separate disciplines, this is a dangerous precedent to set. As a relatively new topic with many of the aforementioned studies being based on subjective interview data, more research is required to form definitive conclusions. Further study would also allow additional insight for potential impacts not heavily explored in this article, such as resulting risk factors for neurological diseases or racial determinants. An estimated 34 million Americans live in poverty today, highlighting the urgent need for solutions in order to avoid lifelong neural consequences [14]. How this can be done is a discussion for itself, but it certainly involves policy change, resource distribution, and acknowledging the many aspects of inequality from race, gender, identity, and many more. Rather than ignoring the issues that plague impoverished communities across the nation, we need to bring more attention and applied action to address inequalities still present in society.

References

1. Cech, D., & Martin, S. (2012). Functional movement development across the life span. doi:10.1016/c2009-0-60730-3
2. Mondloch, C. J., Segalowitz, S. J., Lewis, T. L., Dywan, J., Le Grand, R., & Maurer, D. (2013). The effect of early visual deprivation on the development of face detection. Developmental Science, 16(5), 728-742. doi:10.1111/desc.12065
3. Tomalski, P., Moore, D. G., Ribeiro, H., Axelsson, E. L., Murphy, E., Karmiloff-Smith, A., . . . Kushnerenko, E. (2013). Socioeconomic status and functional brain development - associations in early infancy. Developmental Science, 16(5), 676-687. doi:10.1111/desc.12079
4. Blair, C., & Raver, C. C. (2016). Poverty, stress, and brain development: New directions for prevention and intervention. Academic Pediatrics, 16(3). doi:10.1016/j.acap.2016.01.010
5. Hair, N. L., Hanson, J. L., Wolfe, B. L., & Pollak, S. D. (2015). Association of child POVERTY, brain development, and academic achievement. JAMA Pediatrics, 169(9), 822. doi:10.1001/jamapediatrics.2015.1475
6. Clemenson, G. D., Deng, W., & Gage, F. H. (2015). Environmental enrichment and neurogenesis: From mice to humans. Current Opinion in Behavioral Sciences, 4, 56-62. doi:10.1016/j.cobeha.2015.02.005
7. Tierney, A. L., & Nelson, C. A., 3rd (2009). Brain Development and the Role of Experience in the Early Years. Zero to three, 30(2), 9–13.
8. Parameshwaran, D., Sathishkumar, S., & Thiagarajan, T. C. (2021). The impact of socioeconomic and stimulus inequality on human brain physiology. Scientific Reports, 11(1). doi:10.1038/s41598-021-85236-z
9. Oakes, L. M. (2017). Plasticity may change inputs as well as processes, structures, and responses. Cognitive Development, 42, 4-14. doi:10.1016/j.cogdev.2017.02.012
10. Coley, R. L., Lynch, A. D., & Kull, M. (2015). Early exposure to environmental chaos and children's physical and mental health. Early Childhood Research Quarterly, 32, 94-104. doi:10.1016/j.ecresq.2015.03.001
11. Blair, C., Raver, C. C., Granger, D., Mills-Koonce, R., & Hibel, L. (2011). Allostasis and allostatic load in the context of poverty in early childhood. Development and Psychopathology, 23(3), 845-857. doi:10.1017/s0954579411000344
12. Boller, B., Mellah, S., Ducharme-Laliberté, G., & Belleville, S. (2016). Relationships between years of education, regional grey matter volumes, and working memory-related brain activity in healthy older adults. Brain Imaging and Behavior, 11(2), 304-317. doi:10.1007/s11682-016-9621-7
13. Waber, D. P., Bryce, C. P., Girard, J. M., Zichlin, M., Fitzmaurice, G. M., & Galler, J. R. (2013). Impaired IQ and academic skills in adults who experienced moderate to severe infantile malnutrition: A 40-year study. Nutritional Neuroscience, 17(2), 58-64. doi:10.1179/1476830513y.0000000061
14. Bureau, U. (2020, September 15). Income and poverty in the United States: 2019. Retrieved May 15, 2021, from https://www.census.gov/library/publications/2020/demo/p60-270.html#:~:text=In%202019%2C%20there%20were%2034.0,and%20Table%20B%2D1).