How similar are the brains of London taxi drivers, United States Navy SEALs, and elite athletes? The answer: more similar than they seem at first glance. While they all perform drastically different tasks – from driving a car in a maze of a city, to combat in extreme circumstances, to cycling exceptional distances – their brains have metamorphized to be uniquely suited to the specific task which they perform at an elite level. The brains of elite performers optimize themselves for specific tasks through structural and functional changes brought about by consistent training over an extended period of time.
Neuroplasticity is the process of updating the brain structurally and functionally in response to specific experiences or needs. The study of this process began in the late 1700s and has experienced many changes in perception since. Modern instrumentation including MRI, functional MRI (fMRI), and electroencephalography (EEG) has opened new insights into the constant changes in the human brain.
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To effectively review the topic of neuroplasticity in elite performers, a brief historical overview will be given. Then, the basic science behind neuroplasticity will be addressed. Last, three case studies of different groups will be evaluated with an integrated discussion of the neuroimaging methods which engendered these discoveries. The first case study of London taxi drivers will elucidate major structural plasticity. The second case study of Navy SEALs will examine functional plasticity to optimize performance. The third case study of elite athletes will demonstrate the symbiotic interplay of structural and functional plasticity. Sixty years ago, the scientific consensus was that the brain is a fixed and stable structure once an individual had reached adulthood. People believed that the brain contained a fixed number of cells, which represented the entire set of brain cells a person would ever have. In a sense, this is largely true. Most cells in the brain do not proliferate, but some do. The idea that the brain is a stable and fixed structure, however, is patently false. The brain is a dynamic and adaptive organ which updates itself to operate optimally under the set of conditions to which it is exposed. The idea that neuroplasticity occurs – that the brain changes both in terms of its size and its connections – is not new, but it has gone in and out of fashion.
The idea of neuroplasticity began in the 1780s with Malacarne’s experiments on the response of a dog’s brain to mental exercises. In the first half of the 1800s, Jean-Baptiste Lamarck proposed that the specific regions of the brain were specialized by performing certain tasks. In 1890, William James made the first true mention of plasticity in The Principles of Psychology (Costandi, 2016). Also in the 1890s, Spanish scientist Santiago Ramon y Cajal published papers stating that neurons compose the brain and he later hypothesized that changes could occur at the synapses (Costandi, 2016). About ten years later, however, he softened his position that the brain could be remodeled. In his famous textbook, Degeneration and Regeneration of the Nervous System, he stated that: “In the adult centers, the nerve paths are something fixed, ended and immutable. Everything may die, nothing may be regenerated” (J.G.G., 1929, & Costandi, 2016). This idea of stasis became the standard vein of thinking until the 1960s, when Hubel and Wiesel began their work on the developing brain (Costandi, 2016). Neuroplasticity is the brain’s ability to change or update itself to support different functions throughout the course of one’s life in response to changing demands and experiences. There are two broad subsets of neuroplasticity: functional plasticity and structural plasticity. Functional plasticity is any change in the physiological aspects of neurons (Costandi, 2016). A change in the basal firing rate of a neuron is an example of this. Structural plasticity encompasses any microscopic or macroscopic physical change to the brain (Costandi, 2016). Microscopic changes include the formation of new synapses, new dendrites, or increased myelination of axons. Macroscopic updates include a change in the size of certain lobes of the brain, for example.
It is now accepted that microscopic and macroscopic updates can occur at any stage of life, albeit to varying extents. The brain is most plastic during childhood, but adults can experience constructive or destructive changes. These changes can foster increased efficiency at a task; or, degradation can lead to diseases like dementia (Duong et. al., 2017). However, the focus of this review is neuroplasticity in elite performers, how these changes manifest themselves, and what advantages and disadvantages they yield.
The timescale on which macroscopic and microscopic changes vary. The formation or destruction of a synapse can occur in milliseconds. However, it can take hours to create or destroy dendrites. A new, functional synapse can be constructed in a few hours (Squire, 2014). The longevity of these changes depends on the duration and intensity of the stimulus. Stimuli include learning, stress, and training. The three following case studies delve into the manifestations of neuroplasticity induced by these stimuli. London taxi drivers have to memorize massive amounts of information and synthesize it on a regular basis to determine the fastest route between locations. Since this learning typically occurs after the age of eighteen, they provide an interesting subset of the population for studying neurological changes in an adult population. It takes anywhere from two to four years to learn the 25,000 streets and thousands of landmarks of greater London necessary to become a licensed cabbie (Transport for London, 2020). After this, prospective cabbies have to pass stringent tests to show they know the city and surrounding area of London and the fastest routes between landmarks. The failure rate is seventy percent. Those who pass boast a dramatic increase in the volume of grey matter in the hippocampal region of their brains.
A 2006 study compared the brains in a group of licensed London taxi-cab drivers to ordinary London bus drivers. The two groups were matched in age, handedness, and IQ. A whole-brain structural MRI scan was taken of each subject of the study using a 1.5 T magnetic field (Maguire et. al., 2006).
MRI is useful for examining the structural features of the brain owing to its high spatial resolution. In this study, 1 mm spatial resolution was achieved in the images. Protons cause a small magnetic field, and MRI harnesses this property to give a clear image of the brain. The subject is placed in a high external magnetic field in order to orient the spins of the species of interest in the same way. This orientation is called the bulk magnetization of the sample. The bulk magnetization is defined as “up,” so if a spin is opposite to this direction it is classified as “down.” To obtain an image of the sample, a radio frequency (RF) pulse is applied to excite a particular species in the sample. The RF pulse forces the nuclear spin axis from its equilibrium position. The relaxation of the sample is then measured and can be manipulated to provide an image of the sample. The relaxation of the sample is commonly characterized by T1 and T2 relaxation times in MRI. T1 relaxation, or spin lattice relaxation, is given by measuring the protons’ return to equilibrium by dissipating energy through elastic collisions or rotational energy loss. T2 relaxation, or spin-spin relaxation, is the relaxation of particles through the dephasing of spins of nearby protons. Taking the Fourier Transform of the data acquired gives the recognizable image of the brain.
The images collected by Maguire et. al. provided clear evidence of structural changes due to the intellectual demands of being a London cabbie. Compared to London bus drivers, taxi drivers showed significantly more gray matter volume in the mid-posterior hippocampus and less volume in the anterior hippocampal region (Maguire et. al., 2006). The hippocampus has been linked to many functions, particularly learning, spatial memory, and navigation. Interestingly, this trend of increased volume in the mid-posterior hippocampus and less volume in the anterior hippocampus followed a linear trend with the number of years driving a cab. There were no significant differences in the brains of bus drivers no matter the number of years driving (Maguire et. al., 2006). While it could be argued that cabbies had more brain volume than bus drivers before they started driving, the exacerbation of these affects as time went on argues against this theory.
This change in hippocampal volume is clear evidence of macroscopic structural plasticity in the brain. The taxi drivers in London are the elite performers in this situation. They ingest and synthesize incredible amounts of information on a daily basis. This study points to the fact that the amount and usage of this information is what contributes to the structural changes observed between the two groups. The taxi and bus drivers experienced similar amounts of stress while driving on busy London streets and both performed the physical task of driving all day. The main difference between the two groups is that the bus drivers drove a constrained route, while the cabbies constantly had to integrate their memorized information with the demands of their customers’ desired trips. This strongly indicates that the learning and spatial task of designing unique routes through London was the reason for the observed changes (Maguire et. al., 2006).
Although the cab drivers experienced these changes to their brain that made them well suited for their job, cabbies performed worse than the bus drivers when tasked with acquiring new visuospatial information (Maguire et. al., 2006). This suggests two things. First, there is a mental cost of altering the brain while mastering specific tasks. Second, neuroplasticity updates the brain to allow an individual to perform a specific task, but not every task, more efficiently.
Navy Sea, Air, and Land forces (SEALs) are a group of elite fighters who are highly trained and primed for combat missions in the harshest environments. Their training intentionally confuses them, deprives them of sleep, and subjects them to exceptional amounts of stress. These conditions, stretched over a training period of twenty-four weeks resets the activity levels in SEALs’ brains in response to stimuli like fear.
In 2012, Simmons et. al. took ten Navy SEALs and 11 age-matched healthy male volunteers and performed an anxiety activation task in an MRI scanner. The subjects of the study were shown either an image to relax them (positive image) or a combat image to induce stress (negative image). They were then instructed to press a button corresponding to whether a circle or a square was shown to them. The brain activity in response to the task was measured by fMRI. While the basics of structural MRI have previously been explained, fMRI can measure time dependent activation changes in a subject. Oxygen usage can be measured in the brain by measuring how much deoxy-hemoglobin is present in specific regions. Since only deoxy-hemoglobin, and not oxygenated hemoglobin, is paramagnetic, it is measurable by fMRI. The uptick in the signal from deoxy-hemoglobin means that oxygen is being used in that region of the brain. This measurement can yield the cerebral metabolic rate of oxygen consumption. If the cerebral metabolic rate of oxygen consumption is taken in conjunction with the change in cerebral blood flow and cerebral blood volume, the blood-oxygenation level dependent (BOLD) response can be measured. This gives a measure of real time activation in the brain. The T2* measurement pattern in fMRI detects small inhomogeneities in the relaxation of the sample after excitation, which makes it ideally suited for the measurements necessary to record the BOLD response.
Simmons et. al. found that SEALs modulated their brain activity depending on the stimuli provided. If a SEAL was shown a positive image, their brain activity in the insula region was higher than the control group. However, if the SEALs were shown a negative image, their brain showed significantly lower levels of activation than the control group in the insula region of the brain which is responsible for the reaction to the situation.
This decreased level of activation, when prepared for a negative stimulus and increased activation in the insula region of the brain when experiencing a positive stimulus, is evidence of neural tuning in Navy SEALs (Simmons et. al., 2012). Neural tuning is the idea that elite performers have adapted their brain to be prepared for a situation by responding in the most energy efficient manner. In this study, the SEALs were uniquely ready to respond quickly to a threat, and conserve energy when there was none. Since the insula is proximal to the vagus nerve which has parasympathetic effects if activated, the decreased activation would lead to an increased heart rate, more energy usage, and better reaction to a threat (Simmons et. al., 2012 & Breit et. al., 2018). This tuning of the neural response is a form of functional plasticity because there was not necessarily evidence of structural changes in the brain, but there is evidence of physiological tuning of firing rates due to training. Elite athletes regularly undertake mentally and physically strenuous tasks. Distance athletes are subject to physical and mental stresses for extended periods of time on a regular basis due to their necessary training schedule. A 2015 study by Ludyga et. al. examined eleven female and 18 male bicyclists using EEG. They found that the elite performers – those who had the highest maximal oxygen consumption (VO2MAX) – had structural and functional changes to their brain which worked in harmony to produce higher performance. It is important to note that VO2MAX refers to their ability to consume more oxygen if the situation requires it, not that they consume more oxygen when performing the same task as a non-elite athlete.
EEG is a noninvasive method to image the firing of action potentials, which are the basis for information transmission in the brain and to the body. An array of electrodes is placed on the surface of the scalp to measure the electrical potential generated by the ion currents caused by the firing of action potentials. While the temporal resolution is high in EEG, the spatial resolution of this method is lacking because potentials from distant sources can also be recorded by the electrodes. Another potential pitfall of EEG is a motion artifact, generated by rubbing the electrode against something else. Both of these are sources of noise in the measurement. The former is a limitation of the imaging technique. The latter was reduced in this study by recruiting experienced cyclists, who kept their upper body stable while pedaling (Ludyga et. al., 2016). This study found that the elite cyclists, those with a higher VO2MAX, had a lower level of arousal in the brain than an ordinary cyclist when generating the same power output (Ludyga et. al., 2016). This was noticed particularly in the central region of the brain which is the location of sensorimotor information integration for the lower extremities of the body. The precise location in the central region of the brain, however, cannot be determined by EEG due to its low spatial resolution. This represents a gap in the current literature. Further studies integrating methods like fMRI to measure the BOLD response are necessary to answer this question.
The EEG recording can be broken down into frequency components using the Fourier Transform. This yields useful parameters like alpha and beta power which correlate to neurons in non-operative modes during activation of other neurons in the brain and neurons at rest respectively. Alpha power is an inverse indicator of mental arousal and higher cognitive performance (Klimesch, 1999). Ludyga et. al. found that the elite athletes had similar changes in the ratio of alpha to beta powers, but that their baselines were different. The elite athletes used less total energy by selectively activating only the neurons needed for the task. This is because elite athletes’ brains are better at switching off neurons that are not being used for a specific task (Ludyga et. al., 2016). This results in higher neural efficiency and this finding, in conjunction with other studies, suggests this is caused by both functional and structural neuroplasticity.
Animal studies suggest that improved processing results from higher levels of integration of the cerebellar-thalamic-cortical circuit (Holschneider et. al., 2007). The increased levels of VO2MAX in the elite performers also suggest that there is an increase in vasculature in specific regions of the brain to supply more blood to only the necessary neurons. Neuroplasticity is influenced by the training, but also by the simple physical demands of needing more oxygen during extreme and extended exertion. This is evidence of how structural and functional plasticity work symbiotically to increase neural efficiency in elite performers.
This phenomenon is not unique to cyclists. Elite pistol shooters also showed decreased total levels of activation, but more efficient activation of neurons necessary for the task. Interestingly, there was even a significant difference in activation between good shots and bad shots in the elite pistol shooter group (Percio et. al., 2011). Elite performers, whether it be in driving a taxi-cab, combat, or athletics, experience modifications to brought on by training to perform their desired task at the highest level. These changes may be structural, like the increased size of the posterior hippocampus in London cabbies. They can also be functional, like the altered neural firing rates of Navy SEALs. The changes can be a combination of the two, exemplified by cyclist changing the level of blood flow reaching regions of the brain important to a task, and the specificity of activated neurons.
Each of these discoveries was made possible by using different neuroimaging techniques. MRI gives structural images of the brain. Functional MRI can be used to analyze changes in brain activation. EEG can measure the neural firing rates directly.
These discoveries contribute to the existing and rapidly evolving body of knowledge about the causes, manifestations, and effects of neuroplasticity. The brain, which was once thought to be a fixed and immutable structure has been shown to be a constantly changing organ which adapts to an individual’s circumstances. While these changes may prime an individual for a certain task, it may come at a cost to other functions the brain may one day have to perform. So, while London cabbies, Navy SEALs, and elite athletes have very different jobs, their brains are all uniquely suited to optimally perform their respective tasks.
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