A Tale of Plasticities

You may have heard it repeated in the media and pop culture that humans’ brains don’t change at all after birth. However, the past several decades of neuroscience research have shown that this is far from the truth.

The human brain is remarkably plastic, meaning that it has the ability to change and react to your environment and experiences. Starting at the cellular level, human neurons adapt to new inputs. Through a process called synaptogenesis, new connections between existing neurons can be formed. These connections are formed when the axon of one neuron makes contact with the dendrite of another, typically at a protrusion on the dendrite called the dendritic spine. The stability of dendritic spines, in particular, has been shown to represent the storage of lifelong memories. 

Synaptogenesis is experience-dependent and occurs throughout one’s lifetime. This process is incredibly dynamic, as increased co-activation between two neurons strengthens their connections, and infrequent co-activation leads to a gradual decrease in synaptic strength. If a connection weakens enough, synaptic pruning occurs and the connection formed between two neurons is eliminated. This isn’t all bad news, however. Synaptic pruning occurs throughout the lifespan and is necessary for normal brain development.

But what does it take to change the brain? Memorizing pi to a hundred thousand digits? Becoming a world-renowned chess player? It turns out, even the complexity of your surroundings has a huge influence on the brain.

Back in the 1960s, Marian Diamond and her colleagues at the University of California, Berkeley performed a series of simple experiments to determine whether an enriched environment affects the brains of rats in the lab. Typically, rodents used in biomedical research were housed alone in cages with sparse opportunities for interaction. Diamond compared the brains of rodents kept in “enriched” environments, which included a second rat, toys, and a small wooden maze, to those kept alone in toy-less “impoverished” environments. Her results indicated that the enriched environment increased the thickness of the rat cortex, particularly in the visual cortex. Although these findings are not at all controversial today, they upended the prevailing assumption at the time that the brain’s development was genetically predetermined.

Today, using magnetic resonance imaging (MRI), we are able to see the effects of environment and experience on both brain structure and function. These experiments have given us a better understanding of plasticity on the whole-brain level in humans. 

One famous early example of this research comes from the United Kingdom, where researchers scanned the brains of 16 London taxi drivers to examine the effects of “extensive navigation experience” on brain structure. In order to be a certified taxi driver in London, one must pass “The Knowledge,” a demanding test requiring in-depth understanding of hundreds of routes through and locations within the historic city. Comparing the brains of these taxi drivers to non-taxi driver controls, the researchers found that the taxi drivers had more gray matter in the bilateral posterior hippocampus, an area involved in storing a spatial representation of one’s surroundings. Additionally, long-term drivers tended to have more gray matter in this region relative to newer drivers. This was one of the first studies to demonstrate that expertise, spatial navigation in this case, can be attributed to specific and localized differences in human brain anatomy.

Another interesting study took a more interventional approach. Rather than looking at the changes following expertise, another group in Germany performed a study where participants were scanned before and after acquiring a new skill. These scientists scanned the brains of volunteers before and after 3 months of juggling training. Becoming a successful juggler requires, among other skills, good hand-eye coordination and concentration, but this study found that participants showed significant gray matter gains in the bilateral hMT/V5, an area of the brain associated with complex visual motion processing. Furthermore, a follow-up study found that changes in white matter, the connections between regional gray matter, also occur near similar regions following juggling training. These scientists demonstrated a causal relationship between experience and neuroanatomical plasticity by using an intervention approach, which is especially important in clinical settings to show the effectiveness of a drug or behavioral treatment in generating positive outcomes in patients. Thus, the accumulation of evidence for neuroplasticity from basic science studies has laid the groundwork for neural markers or brain imaging to be used as outcome measures in clinical trials.

Despite the novelty of these findings from MRI research, the exact mechanism for these changes in adult human brain structure remains unknown, in part because the methods to examine this are too invasive to perform on living humans. However, studies using post-mortem human tissue have shown that neurogenesis, the production of new neurons, occurs in the hippocampus throughout adulthood. Although a more recent high-profile study has failed to find evidence for the neurogenesis in adults, there are several other processes that are believed to give rise to observed changes in gray matter. Among these processes supporting neuroplasticity are angiogenesis, which is the development of new blood vessels, and synaptogenesis. To better understand how these, and other processes, result in neuroanatomical changes, researchers are working to develop more sophisticated neuroimaging methods and increase collaboration between neuroscientists studying the brain at the cognitive and molecular levels.

As you can see, diverse factors ranging from memory to expertise can affect the brain. Despite the many recent discoveries in this field, there are still countless questions remaining for researchers who study neuroplasticity. In particular, one line of research has used neuroplasticity as an approach to understand and promote functional recovery after injury due to stroke or traumatic brain injury (TBI). But one thing is abundantly clear: your brain has changed, at least a little bit, since you started reading this!

How does plasticity change how we think about the adult brain? Tell us your thoughts in the comments!

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References:

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