Plasticity is a necessary and powerful force in the developing brain. By adulthood, however, things have mostly settled down. But every now and then, there are events that can upturn the relative stability of the adult brain, forcing a rearrangement of its otherwise consistent connections.
Stroke is one such event. The cutoff of blood to brain tissues leads to the death of neurons, disrupting the neural network in and around the affected region. In a frantic and somewhat messy attempt to salvage what remains, the brain responds by releasing a molecular cocktail to encourage the growth of new cells and of new axons on existing cells, trying to reestablish the lost connections. But this attempt to reconnect is largely random and time sensitive; it rarely is able to restore the brain to the way it was before the stroke. Scientists are studying how to best intervene to enhance the brain’s natural response to injury and improve stroke outcomes.
But the brain’s natural response to trauma is not always helpful. Take, for example, the problem of “referred phantom sensation.” When a person loses a limb, the cortical area that normally processes sensory signals from that limb no longer receives its normal input. Yet a large fraction of amputees report feeling sensation in their phantom limb on a regular basis. What’s more, some patients actually feel sensation in their phantom limbs in response to being touched in another part of the body. For example, a case study from Oxford University describes how an amputee felt sensation in her amputated right arm when certain parts of the right side of her face were touched. The responses were somatotopically mapped –that is, nearby areas of the face corresponded roughly to nearby areas of the arm (see figure below). It appears that, because it’s lacking its own input, the region assigned to the now-missing limb is overtaken by inputs from another region. In most people, the area of cortex representing the head is located directly next to that representing the arm, so perhaps it is unsurprising that the arm area would be overtaken by head inputs. It’s assumed that plasticity, either in the cortex or the subcortical structures which feed into it (or both), is responsible for this phenomenon. But exactly how–either through the strengthening of existing synapses or the sprouting of new ones into the affected area–is not clear.
While this ability of the cortex to rearrange itself can be damaging, it can also be utilized. Prosthetic devices such as cochlear implants rely on it. Over time and use, the brain learns how to properly process inputs it gets from the implant, allowing a person to improve in important areas such as speech perception. The double-edged nature of the brain’s dramatic response to dramatic change can be fully appreciated.
– Grace Lindsay
Grace Lindsay is a first-year Ph.D. student in the Neurobiology and Behavior Program at Columbia University. She got her BS in neuroscience from the University of Pittsburgh in 2011 and then spent a year doing research at the Bernstein Center in Freiburg, Germany. She blogs about all things neuroscience at neurdiness.wordpress.com.