Phantom Sensations a Compensation Mechanism

In the previous blog post the function of spinal sensory nerves was summarized, and the phantom symptoms that arise after spinal sensory nerve damage or direct brain injury were introduced.  As we previously explained, these phantom symptoms are a form of a compensation mechanism that is produced by the brain.  The most common phantom symptom that occurs after spinal sensory nerve damage is called Phantom Limb Syndrome, and the phantom symptoms that occur after injury to the brain are known as tactile hallucinations.  Phantom Limb Syndrome is a type of phantom symptom that is produced by the brain even when direct brain injury has not occurred.  This blog post will describe a case in which both a phantom symptom and a tactile hallucination are similar, but yet different in the ways that they develop.

Phantom Limb SyndromeThe way that this phenomena works in the brain: A limb, for example an arm or leg, sends tactile information such as touch to the somatosensory cortex of the brain via a spinal nerve.  This information is then processed by a group of cortical neuron connections.  When the arm is amputated, the spinal nerve is damaged, leading to no sensory input to the somatosensory cortex, and the cortical neuron connections become unresponsive (UR).  The unresponsive (UR) cortical neurons in the somatosensory cortex that once processed information from this limb initiate a compensation mechanism a few weeks after the injury occurred.  The compensation mechanism, involves the invasion  of neighboring and responsive (R) cortical neurons to the unresponsive (UR) area via shifting and remapping mechanisms, resulting in new connections with the UR neurons (Purves 2008).  Cortical neurons in the UR group have made new connections with cortical neurons in the R group.  These new connections process information from other areas in the body.  This results in two different kinds of tactile hallucinations:

1. Illusions of sensations on the "non-existent" limb are present when skin near the non- existent limb is stroked; for example the shoulder, back or neck. Most interestingly, some individuals actually feel tactile sensations in the "non-existent" limb when another part of their body is stroked, such as their face! Why? this is because the R cortical neurons that made new connections with the UR cortical neurons, are responsible for processing tactile information that is sent to the brain from the face (Gallance and Spence 2012).  
2. Both the illusion that the limb is still present and sensations such as pain on the "non-existent" limb, is another type of tactile hallucination.  This occurs because the brain was not directly damaged, therefore a conscious awareness including tactile memories of the limb that were once there, still exist.  The new connections between both R and UR cortical neuron groups in the somatosensory cortex did not take away these memories in the brain, but rather increased the tactile sensitivity of other body parts.

 

Phantom Limb Syndrome occurs after a limb has been amputated due to a direct injury to the limb.  It has been given this name because of the illusion that remains after the limb is no longer present. These illusions are composed of sensations that are felt on the limb that is no longer attached to the body; these sensations are known as tactile hallucinations.     

    

Citations:

Gallance, A. and Spence, C 2012. "Touch and the body: the role of the somatosensory cortex in tactile awareness". Psyche, vol. 16, no. 1, pp.30-60.

Krulwich, Robert. How Do You Amputate A Phantom Limb . N.d. Photograph. 2009. The Center for Brain and Cognition, UCSD Web. 21 Mar 2014. <http://www.npr.org/templates/story/story.php?storyId=101788221>.

Purves, D. and Augustine, G . Neuroscience. 4. Sunderland, MA: Sinauer Associates Inc., 2008. 857. Print.