“To be human is to 'be', that is we are born with the special ability to become what we want and that is why our species is aptly called "human beings".”
The human brain evolves tremendously, simply put, it is plastic, prone to change when exposed to new stimuli. Until the mid-1960s, the adult brain was thought to be "hard-wired," which could not be mutated to reorganize its structure and function. However, this fundamental notion typically underwent a paradigm shift when studies on rhesus monkeys proved that the mammalian brain could be conditioned to respond to a particular stimulus naturally. When a new map develops in the cerebral cortex out of an existing map, which leads to cortical re-mapping, the cognitive process of cortical re-mapping can be interpreted efficiently by studying phantom limbs which is a phenomenon experienced in patients who have lost a limb. Interestingly these patients can feel a limb that no longer exists! How is a map changing? How is it not rigid like the conventional ones?
We all use Google maps to move between towns and cities; every state is in its spot, and every country is on its continent. However, if someone said that the maps alter each day one wakes up from a nap? One would think the idea is absurd, but this is accurate when it comes to one's brain; every activity carried out, and every decision made changes the brain a little. What does that mean? Is the brain plastic? Every region in the brain is associated with carrying out a particular function. The cerebrum is responsible for hearing, vision, speech, reasoning, and other higher reasoning tasks, whereas the cerebellum coordinates muscle movements, posture, and balance. However "rigid" the brain might seem, it is not really. Until the early 1980s, the brain was considered a "hard-wired black box." However, this paradigm shifted as more research began in the areas of neural plasticity.
When we wake up in the morning and brush our teeth, we do not use our brains consciously; this action is classically conditioned in our brain. When a person learns an automatic conditioned response paired with a specific stimulus, behavior is created. Ivan Pavlov, who is considered the father of classical conditioning, was the first to study this. He realized that over time dogs would salivate not only at the sight of food but also when a bell was rung right before giving the dogs' food. They were quickly able to associate the ringing of the bell with food. What is interesting here is that this conditioning can be changed. Cortical remapping or reorganization is a process through which an existing map in the cortex mutates due to a stimulus leading to the formation of a new cortical map. A very famous example of this is the study conducted by E. T. Rolls at Oxford University in the 1970s. The study was performed on rhesus monkeys who were conditioned to licking a blue light bulb in the cage to receive black currant juice as a reward. After the monkeys were conditioned, the researchers switched black currant with brine, which the monkeys despised. When the monkeys now licked the blue bulb, which led to them receiving brine, within a matter of days, they learned that they were not receiving the expected reward and hence stopped licking the bulb. What does this experiment indicate? Remapping. The monkeys were able to undo a learned behavior, which means the brain is prone to change. Micheal M. Merzenich, a neuroscientist at the University of California, claims that "the brain was constructed to change." 
In 1973, Penfield and Boldrey studied the effects of stimulation of the cerebral cortex, which consisted of delineating brain regions with motor and sensory phenomena affecting a particular part of the body. They could precisely confirm the topography of cortical localization and pictorial represent it as the "Penfield Homonculus," a visual representation of the map of body space in the cortex, with the body size representing the size of the area in the cortex devoted to it. Two very distinct regions might share neighboring neural representation in the homunculus of the brain. An exciting study by Aglioti suggested that women who underwent mastectomy found that about one-fourth of the patients experienced phantom breast when the pinna region of the ear lobe was stimulated, suggesting that the cortical representation of the two body parts was adjacent to each other. Studying phantoms is crucial because it supports the functional remapping hypothesis.
However, what causes these neural maps to strengthen or weaken over time? This happens due to two processes, potentiation and depression. An axon terminal sends neurotransmitters to the receptors of the target cell located on its membrane. A neuron with a high action potential releases abundant neurotransmitters into the synaptic cleft, leading to a higher number of synapses between the two neurons and a more dense network between the dendrites and axon terminals, a process called synaptic sprouting. On the other hand, when the action potential is low, very little neurotransmitter binds to the target receptor; very few synapses form with a very loose connection between the interacting neurons, called synaptic pruning, which leads to the elimination of these unused pathways. Interestingly, newborns are born with way more synapses than adults; over time, the less used synapses get eliminated by pruning.
Simply put, pathways that are accessed frequently get hard-wired into the brain, and others get eliminated. Hebb’s law states that “Neurons that fire together, wire together.” These neurons can wire together only when the areas involved stay activated, and this only happens when an individual’s attention is focused. As it is rightly said, “The power is in focus!”
LoLordo V.M. (1979) Classical Conditioning: The Pavlovian Perspective. In: Animal Learning. NATO Advanced Study Institutes Series, vol 19. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-3387-6_2
Holloway M. The mutable brain. Sci Am. 2003 Sep;289(3):78-85. doi: 10.1038/scientificamerican0903-78. PMID: 12951831.
Halligan, P. W; Zeman, A.; Berger, A. (1999). Phantoms in the brain. BMJ, 319(7210), 587–588. doi:10.1136/bmj.319.7210.587
Schott, G D (1993). Penfield's homunculus: a note on cerebral cartography. Journal of Neurology, Neurosurgery & Psychiatry, 56(4), 329–333. doi:10.1136/jnnp.56.4.329
Aglioti S, Cortese F, Franchini C. Rapid sensory remapping in the adult brain as inferred from phantom breast perception. Neuroreport 1994;5:473-6.
Annette Sterr, Matthias M. Müller, Thomas Elbert, Brigitte Rockstroh, Christo Pantev, Edward Taub, Journal of Neuroscience 1 June 1998, 18 (11) 4417-4423; DOI: 10.1523/JNEUROSCI.18-11-04417.1998
Ramachandran VS, HirsteinW. The perception of phantom limbs. Brain 1998;121:1603-30.
Low, L. K., & Cheng, H. J. (2006). Axon pruning: an essential step underlying the developmental plasticity of neuronal connections. Philosophical transactions of the Royal Society of London. Series B, Biological Sciences, 361(1473), 1531–1544. https://doi.org/10.1098/rstb.2006.1883
Keysers, C., & Gazzola, V. (2014). Hebbian learning and predictive mirror neurons for actions, sensations, and emotions. Philosophical transactions of the Royal Society of London. Series B, Biological Sciences, 369(1644), 20130175. https://doi.org/10.1098/rstb.2013.0175
Schwartz, J., & Gladding, R. (2012). You are not your brain: the 4-step solution for changing bad habits, ending unhealthy thinking, and taking control of your life. Avery.
About the author
Name: Aarushi Chitkara
Credentials: Second-year BSc - Life Sciences student at St Xavier’s College, Mumbai
Bio: Anything to do with the brain and the mind piques my interest, I enjoy learning new things, reading books, and clicking pictures of the clouds. The constant debate between the tangible brain and an abstract mind is engrossing and enrapturing, The fact that we have simply scratched the surface and there is so much more than the brain needs to learn about the brain is fascinating.
Social Media Handles:
Instagram - @aarushhy
LinkedIn - www.linkedin.com/in/aarushi-chitkara