by Ariel Hairston
figures by MacKenzie Mauger
Our lives are stuffed with opportunities for excitement and stimulation. You might wake up in the morning and find yourself staring at your modest collection of succulents, or that small oil painting you bought at a flea market. The first moments of your day might be spent taking a walk around the block, the smell of tree bark hanging in the air. Yet, there are many who don’t have the resources to build such sensorially stimulating environments. Moreover, natural disasters and war can strip people of the homes they’ve carefully curated, replacing them with only the bare necessities. Given the natural inclination to “decorate” one can’t help but wonder how an individual’s physical space can impact their mental space. Environmental enrichments, which include visually stimulating objects such as art work, plants or toys, social interaction, and exercise have been widely studied for their potential effects on improving cognition and mood.
Enriched environments enhance memory and mood in mice
The scientist Donald Hebb first considered the possible effects of enriched environments in the late 1940s when he compared his pet rats to rats he kept in the lab. Research has shown that rodents exposed to enriched environments (for example, toys, huts, and exercise wheels) perform better than mice in control environments on tasks designed to test working and long-term memory (Figure 1). These studies also show that mice prefer enriched environments over a control environment, known as standard housing, which only includes food, water, and bedding.
Researchers who study memory formation and recall have also shown low levels of stress as a contributor to improved memory. It has been shown to enhance alertness, thereby improving memory and other intellectual activities. Research in this area indicates that environmental enrichment can promote activation of the stress response – a state in which the central nervous system triggers the release of stress hormones that enable one to quickly deal with a threat in the environment. Indeed, both positive and negative life experiences can cause the release of stress hormones in the body at comparable levels. In addition to the improvements in memory, mice reared in enriched environments show greater motivation for completing tasks, as well as increased resilience to anxiety and depression. This indicates that they are better able to cope with novel, negative stressors.
Enriched environments increase neuron production in adults
A large body of work has shown that enriched environments impact the hippocampus, a region of the brain implicated in the formation and recall of memories, enhancing neuronal excitability and promoting long-term-potentiation, a process that strengthens the connections between neurons in the brain, and has been widely thought to lay the foundation for long-term memory.
The hippocampus is also one of the only sites in the adult brain where new neurons are generated. This process, known as neurogenesis, occurs in a subregion of the hippocampus termed the dentate gyrus. Neurogenesis supports cognitive function and forming and updating contextual memories as one experiences both novel and familiar environments. However, high levels of stress can decrease neurogenesis. It has been found that enriched environments can enhance neurogenesis and can prevent the stress-induced decline of neurogenesis. Further, enriched environments can prevent damage and death of mature neurons.
Neurons in the hippocampus regulate stress and anxiety
The dentate gyrus of the hippocampus can be further subdivided into dorsal and ventral regions that are responsible for different brain functions (Figure 2). Dorsal regions of the dentate gyrus support our ability to connect related facts and memories, as well as distinguish between similar but distinct experiences. Thus, neurogenesis in this region is believed to bolster cognitive function and memory formation. Alternatively, damage in the ventral dentate gyrus alters the ability to form and recall fearful experiences and causes decreased sociability. Experimentally activating neurons in the ventral dentate gyrus of mice can reduce innate anxiety behaviors. Indeed, researchers have found that ventral dentate neurons are connected to neurons in the prefrontal cortex, another region in the brain implicated in functions related to cognition and mood. Thus, this connection may explain how the dentate gyrus could be involved in regulating anxiety-driven behaviors.
The ventral dentate gyrus is also connected to the amygdala, a brain region important for assigning contextual memories as pleasant or unpleasant, and the ventral tegmental area, which contains neurons that produce and release dopamine, a signaling molecule involved in reward prediction processes in the brain. Finally, it has been found that projections from the ventral dentate gyrus also connect to neurons that control the production and release of stress hormones in the hypothalamus. Thus, neurons in the dentate gyrus have an important role in controlling how the brain processes stressful and emotional events through connections with these other areas.
Environmental enrichment can impact neural activity in the hippocampus
Intriguingly, researchers have found that neurogenesis can inhibit dentate gyrus functions, such as regulating anxiety-driven behaviors. To this end, neurogenesis generates immature excitatory neurons, cells that activate other neurons. These immature neurons form connections with and activate another cell type in the dentate gyrus known as inhibitory neurons, which lower the activity of the neurons that they are connected to. By enhancing the production of adult-born neurons, and thus increasing inhibitory neuron connections and lowering activity, enriched environments can indirectly suppress the dentate gyrus from promoting anxiety-like behaviors (Figure 3).
Environmental deprivation can harm brain development and functioning
In addition to the extensive research that has been done investigating the effects of enriched environments in enhancing cognition, there has been some exploration into the effects of environmental deprivation on cognitive function. As far back as 1976, research by Elizabeth Gardner and colleagues found that environmental deprivation can produce deficits in memory retention. More recent human studies demonstrated that not having access to green spaces in urban areas can negatively affect mood and cognition. For example, a recent study compared brain volumes of Romanian refugee orphans who had experienced environmental deprivation with brain volumes of orphans in the United Kingdom who had not. They showed that refugees had a decrease in brain volume specifically in the temporal and prefrontal lobes. This difference was also associated with lower intelligence quotient (IQ) scores, as well as a higher prevalence of symptoms associated with attention-deficit/hyperactivity disorder. These structural differences in the brain are consistent with the plethora of work describing the negative effects of environmental deprivation on cognitive functioning.
A large body of work over many decades has been devoted to determining how environmental enrichments can improve the cognitive function of research subjects. However, seeking environmental enrichment is an inherent behavior. There is, in fact, a biological basis to our drives to decorate our apartments and homes or to seek out colorful experiences. Our systems are built to use experiential variety as a training ground for unexpected stresses. That flea market sketch hanging in your kitchen might seem like a frivolous addition to your living space, but it is those small things that not only improve your quality of life, but enhance your cognitive strength and resilience as well.
Ariel Hairston is a PhD student at Harvard’s Program in Neuroscience.
MacKenzie Mauger is a fourth-year Ph.D. student in the Biological and Biomedical Sciences program at Harvard Medical School, where she is studying cell type-specific gene repression. You can find her on Twitter as @MacKenzieMauger.