Sleep helps us remember the details of past events more clearly. When we sleep, neural mechanisms facilitate the consolidation of memories formed during the waking day. Specifically, memories are temporarily stored in a brain structure called the hippocampus. During the consolidation process, memories are replayed and integrated into long-term storage centers in the neocortex of the brain. Poor sleep impairs sleep-based memory consolidation and memory retrieval. In other words, when our sleep is fragmented, our memory is less clear.
One way to assess the clarity of a memory is to measure neural similarity, or the overlap between patterns of neural activity. My colleagues and I presented participants with a series of word pairs to remember while we recorded their neural activity using electroencephalography. We used this task to measure neural activity when participants studied (i.e., encoded) and were tested on (i.e., retrieved) the word pairs. The overlap between their neural patterns for a given word pair at study and test is an index of neural similarity.
Interestingly, we found that sleep quality was associated with neural activity for word pairs that were paired differently. When people had more consistent sleep quality from night-to-night (measured with wrist-worn monitors), they had greater neural similarity when they correctly rejected word pairs that were paired differently. For example, if they saw the pair “wing – clock” during the study period and correctly identified “fork – clock” as a different pairing at test, they demonstrated higher neural similarity.
There were several strengths of the study. We used an objective measure of sleep quality — wrist-worn monitors. We also measured sleep quality for seven nights, which allows for assessing night-to-night sleep variations. Our participants were racially and ethnically diverse people across the adult lifespan. However, our study was limited by its small, convenience-based sample of participants (74 people) and cross-sectional design. We cannot determine if poorer sleep causes lower neural similarity with this data.
Taken together, our study suggests that memory integrity, or the ability to clearly remember the details of past events, may be linked with consistent sleep patterns. Thus, in addition to sleeping for enough time, sleep consistency also contributes to better memory retrieval.
As humans, we all experience stress. It is a normal, and sometimes even beneficial, part of life. A small amount of stress can help motivate someone to prepare for a job interview or study for an important exam. There are times, however, when stressors become too overwhelming and even detrimental to health. Scientists, from medical researchers to psychologists, have studied stress for decades and documented some of these negative impacts on the brain. When thinking about the importance of the foundational, early years of a person, the presence or lack of stress can play a crucial role in development. For instance, extensive research shows that living in poverty is extraordinarily stressful for families and can negatively influence children’s brain development. The impacts of stress resulting from situations such as growing up in poverty warrant further investigation, especially considering that in 2020, one in six children in the U.S. was living in poverty.
Researchers can use various methods to assess how factors like stress impact the brain of growing children. Developmental scientists can use a tool called EEG, short for electroencephalography, to study the brain. EEG measures electrical activity in the brain by recording the communication between brain cells. It is an ideal neuroimaging method for understanding infant brain development since it allows for infants to be awake and moving, and even sitting on their caregiver’s lap during recording. Besides being infant-friendly, EEG is a useful tool for looking at brain development, given that there is a known pattern of how brain activity changes across the first few years of life.
Specifically, when using EEG to look at brain development, scientists typically see two different patterns. Broadly, infants have a mix of different types of brain activity that we call low-frequency and high-frequency power. Low-frequency power (e.g., theta) tends to be higher when the brain is at rest, while high-frequency power (e.g., alpha, beta, and gamma) tends to be used for more complex thinking like reasoning or language. As infants grow, scientists see that low-frequency power decreases and high-frequency power increases. Importantly, we can use EEG to assess how factors like stress impact the tradeoff of low-frequency and high-frequency power in the developing brain.
Research shows that children growing up in chronically stressful environments often show alterations in the typical pattern of brain activity development. To further understand the mechanisms underlying this pattern of development, scientists have begun to study which biological and environmental factors may be at play. For instance, researchers can examine the role of caregiver stress, socioeconomic status, home environment, and neighborhood factors, just to name a few.
A recent paper by Dr. Sonya V. Troller-Renfree and colleagues examined maternal stress by looking at the amount of stress hormone (cortisol) found in hair. This measure assesses chronic stress and provides researchers with the average cortisol level of the mother from the preceding 3 months. Dr. Troller-Renfree’s research group hypothesized that infants who have mothers with higher stress hormone, compared to mothers with lower levels of stress, would show differences in their brain activity. Specifically, the researchers predicted that infants of more chronically stressed mothers would exhibit proportionally more low-frequency power and proportionally less high-frequency power compared to infants with physiologically less-stressed mothers.
Indeed, their results showed that infants of mothers who had higher levels of hair cortisol demonstrated higher levels of low-frequency (theta) activity and lower levels of high-frequency (alpha and gamma) brain activity. This finding is consistent with previous research showing that stress and adversity impacts early neural development. Importantly, Dr. Troller-Renfree’s team sampled a diverse pool of participants (both in terms of socioeconomic status and race), therefore bolstering the generalizability of their findings.
So what are the implications of these alterations? Research suggests that similar patterns of neural activity are associated with negative outcomes later in a child’s life, including language development and psychiatric problems. Nevertheless, this does not mean that a child will undoubtedly experience these issues. Additionally, it may be possible that these patterns, while associated with negative outcomes in some areas, may also be adaptive in other circumstances. Furthermore, the issue of the mechanisms by which a mother’s stress impacts the developing child still remains unclear. How exactly does a mother’s stress level impact the brain of her child?
Based on previous research by other scientists, Dr. Troller-Renfree posits a few mechanisms that must be further explored. For example, it is possible that stress impacts crucial mother-child interactions. It could be that stress hormones are passed from mother to baby in utero or through breastmilk. Moreover, it is also possible that environmental factors impact stress and brain development.
It is crucial that developmental scientists continue studying these mechanisms so that targeted intervention programs can be formed for families facing stress. Indeed, the esteemed pediatrician and researcher Dr. Jack Shonkoff of the Center on the Developing Child said in an episode of The Brain Architects Podcast: “In fact, one of the cardinal principles of the science of early childhood development is that if we want to create the best kind of environment for learning and healthy development for young children, we have to make sure that the adults who care for them are having their needs met as well.” As a society, we must recognize how detrimental stress can be to the developing child and invest in finding effective ways to alleviate caregiver stress.
Dr. Sonya V. Troller-Renfree is a Goldberg Postdoctoral Fellow in the Neurocognition, Early Experience and Development Lab at Teachers College, Columbia University. Her research focuses on the effects of early adversity and poverty on cognitive and neural development. She intends to continue examining these questions as part of her new, federally-funded Pathway to Independence Award (K99/00). You can stay up-to-date on her research findings on Twitter at @STRscience or on her website: www.sonyatrollerrenfree.com.