Valerie Parente
March 21, 2017
Analyzing Phantom Limb Syndrome
Phantom limb syndrome refers to the sensations a person perceives on a limb that no longer exists. Phantom sensations can have the quality of tingling, hot or cold, itching, pressure, vibration, pain, or any other aspect of touch. These feelings teeter anywhere from mild to severe. This kind of phenomena is not uncommon among amputees. Phantom limbs occur in 95 percent of amputees who lose an arm or leg (Choi, 2013). It might come as a surprise that there have been reports of phantom illusions in more than just limbs, including phantom appendix pains, phantom menstrual cramps after hysterectomies, and phantom nipples (Choi, 2013). Even more surprisingly, phantom sensations can occur in more than just those with an amputated limb. Children born without a limb due to congenital aplasia have reported feeling a vivid phantom of the missing body part (Melzack, 1989). Through research and experimentation performed with a focus on cortical mapping and neuroscientific approaches, both amputees and non-amputees have contributed substantial evidence indicating that phantom sensations might be a result of the brain’s miraculous top-down processing and plasticity.
Before delving into the details and implications of phantom limb phenomena it is important to understand how deceitful the mind can be. A study conducted by Arvid Guterstam of the Karolinska Institute, called the “Invisible Hand Experiment” demonstrates how an unattached limb can still be “felt” as part of the one’s body. 234 healthy adult volunteers sat with their right hand hidden and a researcher used a paintbrush to brush the thin air in front of the volunteer as well as their visibly hidden real hand. To Guterstam’s shock, it took less than a minute for the majority of participants to claim they “felt” as if their stroked hand was located in the empty region of air in which the visible paintbrush had conspicuously been brushing thin air (Choi, 2013). The participants indeed perceived what Guterstam denoted an “invisible hand”, reminiscent of the phantom limb. Furthermore, when fourteen of these participants, whom perceived an invisible hand, had their brains scanned using functional magnetic resonance imaging (fMRI) the scans showed increased activity in the same parts of the brain that are active when individuals see their real hand being touched (Choi, 2013). The powerful conviction of the invisible hand illusion was further emphasized when researchers took a knife and made a stabbing motion toward the empty space “occupied” by said invisible hand and measured the sweat response in participants. The sweat response in a handful of volunteers acted as an indication of stress towards a make-believe threat (Choi, 2013). The threat of being stabbed, even when flagrantly irrational to the naked eye, was still perceived as reasonable enough to incite the top-down processing of the brain to evoke stress. Clearly the brain has some sort of mental ability to convince individuals that phantom body parts are being stimulated in ways that the rest of their physical body cannot account for.
A reoccurring theme amongst phantom limb syndrome research and experimentation seems to be the prevalence of top-down processing. A phenomenon such as phantom limb pain, much like the perception of an invisible hand, is a touch-based sensory illusion derived from cortical top-down processing. Top-down processing, the brain’s way of organizing immense amounts of information (Berezin, 2014) acts so that we as humans, can function adequately in everyday life without becoming overwhelmed by the constant sensory and motor input confronting our moment to moment.
Examination of phantom limb syndrome has been led researchers to multiple conclusions. One explanation of this phenomena proposes that the same brain processes activated when a body is intact can also be activated when a body part is amputated (Melzack, 1989). Another explanation of phantom limb syndrome attributes the syndrome to neural pathways. Neural networks are responsible for dictating how and what humans experience as sensations which feel as if they exclusively originate in the body. It is the brain and its magnificent web of neurons that give an individual a sense of being in their own body, not the literal body in itself. By this logic, an input does not need to be present to generate an output and convince the mind of sensations like pain, tingling, or even complacent stillness. This is understandable, for even non-amputees, with insensate arms or hands, might feel that their arm is in a certain position when their eyes are closed, but, upon opening their eyes they might become surprised to see the arm bent in a way they had not felt (Melzack, 1989). Data such as this suggests that neural activity can exist in the brain when there is no input from the body. This being said, inputs from the body can still shape the output of these neural networks, but they are not a requirement to generate an experience of one’s body (Melzack, 1989).
Top-down processing proves to be a key ingredient when theorizing the origin of phantom limb pain, leading to many theories based on cortical mapping. The human brain in its infantile days (prior to amputation), developed so that any direct sensation of touch and motor experience merged together in a cortical map (Berezin, 2014). Sensory and motor neurons worked together, imprinting the brain with a map of, for example, an arm, a leg, a finger, etc (Berezin, 2014). Many pieces of evidence supporting cortical maps include amputees’ reports of having idiosyncratic characteristics upon phantom limbs that match with its limb prior to amputation. Examples include bunions on a phantom foot and rings on a phantom finger (Melzack, 1989). Even patients with Parkinson’s disease have reported feeling a tremor in a phantom limb (Melzack, 1989). Apparently cortical maps of certain body parts can outlive the body part. Peripheral nerves in an amputated leg might be absent, but the top-down process of cortical mapping still exists and can still be triggered. Such a trigger seems to be a consistent concept amongst many phantom limb researchers, including Ronald Melzack. Melzack, in 1989 of McGill University described the brain’s trigger as sensory input in his theory of the “neuromatrix”. The neuromatrix, Melzack proposed, is the self’s genetically built-in experience of one unified body which can be influenced- but not exclusively controlled- by the input of sensory information. Such an omniscient neural network would be constantly producing outputs that relay information about body position, motion, sensation, and therefore the overall physical “self”. It is only when sensory input interferes with the ongoing neural network by the growth, survival, or death of synapses between neurons that a particular body experience, like pain or pleasure, merges with the output pattern awaiting later activation (Melzack, 1989). Melzack (1989) goes on to call the assortment of patterns “neurosignatures” that are inherent in each brain but do not act out until the overarching neuromatrix is triggered by output of hyperactive cells. The neuromatrix explanation of phantom limb sensation asserts that any experience of touch and motion in the body is predetermined in the brain, reinforcing the prevalence of top-down processing and therefore a map of the body stamped upon the human brain.
Digging deeper into the brain’s role in these mysterious phantom experiences theories can be found venturing beyond cortical mapping and into neuroplasticity territory. Experimentation and research on an amputee, whom had lost his index finger and had a surgically reconstructed thumb, carried out at the University of Verona provides evidence attributing phantom limb sensations to the reorganization and plasticity within the somatosensory cortex following amputation (Aglioti, Smania, Atzei, & Berlucchi, 1997). In a three-year-long clinical case study of a 51 year old male having had his left index finger amputated, researchers stimulated certain areas on the patient’s skin both near and far from the amputation line, including areas along his left cheekbone (Aglioti, Smania, Atzei, & Berlucchi, 1997). It is important to note that portions of the somatosensory cortex, when deprived of their own sensory inputs, respond to sensory inputs that would normally be activated in other cortical zones (Aglioti, Smania, Atzei, & Berlucchi, 1997). With this knowledge, researchers applied pressure to the left portion of the patient’s face as well as his fifth, forth, and third fingers. These sites elicited sensation in his phantom index finger. Three years later, when this test was repeated (three years after the amputation), the patient’s phantom sensations in his nonexistent index finger arose when the right side of the face was stimulated instead of the left side of the face in the prior two sessions. This kind of changing mislocalized pain emphasizes the reorganization of the somatosensory system an adult primate (Aglioti, Smania, Atzei, & Berlucchi, 1997). Such a finding can be backed up by experimentation on a squirrel monkey in 1995 by C.E. Schroeder, S. Seto, J.C. Arexxo, and P.E. Garraghty. Research on this squirrel monkey revealed that the somatosensory cortex contained dominant and latent input, both capable of intermingling (Aglioti, Smania, Atzei, & Berlucchi, 1997). Further study on a cortical locus exposed that the usually subliminal latent inputs morphed into full expression when dominant inputs were removed (Aglioti, Smania, Atzei, & Berlucchi, 1997). Through these studies it can be deduced that the inputs which were once subliminally processed in the somatosensory cortex acted as a substitute for the former dominant inputs, thus graduating from subconscious awareness to conscious awareness. Plasticity within the sensory processing cortex of the brain, the very command center responsible for decoding anything tactile, could be responsible for illusory feeling of a phantom limb.
The implications posed by research and experimentation on phantom limbs, including research on non-amputees such as those involved in the “invisible hand experiment”, seep into the philosophical topic of selfhood- the concept which questions where the self begins and ends. The invisible hand experiment theoretically suggests that, if an individual can experience an invisible hand as their own, then why not an entire invisible body as their own? An essential “invisible man illusion” according to cognitive neuroscientist of Karolinska Institute, Henrik Ehrsson, is plausible (Choi, 2013). More ideas of “being” in one’s “self” stemming from Melzack’s neuromatrix theory of the phantom phenomena. If there really is a pattern built upon all inputs from the body, then all experiences of the individual’s body are permeated with a quality of self.
A close relative to the notion of “self” that can be weeded out of phantom limb data is the notion of consciousness. The malleability of consciousness is a subsequent discovery stemming from the reorganization of inputs described by Aglioti, Smania, Atzei, and Berlucchi, in 1997. If what were once subconscious sensations are able to graduate into conscious sensations, what does this mean for the potential of the mind? Can the fundamental consciousness of a human being be manipulated? Consciousness might be the one of the grandest ontological enigmas of humankind, arising in subjects ranging from quantum physics, philosophy, and now psychology. This psychological study of phantom limb phenomena brings us a significant step closer to uncovering the mystery of consciousness.
All of psychology studies the mind through premises of “self” and “consciousness”, yet since the spawn of this institution nobody has been able to clearly define what these entities are. Phantom limb syndrome research has offered great insight into where the self and consciousness begins and ends. Consequential data to research on this subject suggests a very hopeful framework for the future of psychology, a framework that has been long coveted to advance into a neuropsychological era based upon a scientifically proven model of “self” and “consciousness”.
References
Aglioti, S., Smania, N., Atzei, A., & Berlucchi, G. (1997). Spatio-Temporal Properties of the Pattern of Evoked Phantom Sensations in a Left Index Amputee Patient. Behavioral Neuroscience, 111(5), 867-872. http://dx.doi.org.libproxy.uml.edu/10.1037/0735-7044.111.5.867
Berezin, R. (2014, January 13). The Cortical Top-Down Processing of Life. Psychology Today. Retrieved March 18, 2017, from https://www.psychologytoday.com/blog/the-theater-the-brain/201401/the-cortical-top-down-processing-life
Choi, C. Q. (2013, April 12). Even Non-Amputees Can Feel a Phantom Limb. Live Science. Retrieved March 18, 2017, from http://www.livescience.com/28694-non-amputees-feel-phantom-limb.html
Melzack, R. (1989). Phantom limbs, the self and the brain (the D. O. Hebb Memorial Lecture). Canadian Psychology, 30(1), 1-16. http://dx.doi.org.libproxy.uml.edu/10.1037/h0079793
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