proprioception

Phantom Limbs and Proprioception

Like an additional clue in the complex mystery of how the human brain operates, the concept of phantom limbs and phantom pain provides an anomaly that neuroscientists must account for when trying to explain our quotidian experience with how we perceive and control our bodies in space.

Phantom Limbs and Proprioception

A young man sits blindfolded on an exam table as a neuroscientist prods him with a Q-tip. Doctors had already confirmed that he has nothing wrong with him neurologically. Yet, when the scientist touches certain parts of his face, the young man reports sensations in his left hand, surprising the researcher and startling the man himself. Not only is it strange that he is feeling dual sensations on his face and in his hand, but the hand itself doesn’t exist. Three weeks prior, after a car crash, the young man’s left arm was amputated from the forearm down. Professor V.S. Ramachandran uses this story in both his research on phantom limbs and in his book “The Tell-Tale Brain: A Neuroscientist’s Quest for What Makes us Human”:

“For most parts of the body he reported the location of the sensation accurately. But when we touched his ipsilateral face, he reported with considerable surprise that he felt the touch not only on his face—as expected—but also on his missing phantom hand. Touching different parts of the face elicited precisely localized sensations on different parts of the phantom arm. The margins of different fingers were clearly delineated and there was a crudely topographic organization. Stroking the cheek was felt as stroking on the phantom and tapping was felt as tapping.” (V.S. Ramachandran and Eric L. Alschuler, “The use of Visual Feedback, in Particular Mirror Visual Feedback, in Restoring Brain Function,” Brain 2009: 132; 1693–1710.)

Like an additional clue in the complex mystery of how the human brain operates, the concept of phantom limbs and phantom pain provides an anomaly that neuroscientists must account for when trying to explain our quotidian experience with how we perceive and control our bodies in space. 

Proprioception, the self-generated sense of the position and movement of the body’s limbs and trunk—and also of effort, force, and heaviness—is sometimes thought of as a sixth sense, not quite fitting into Aristotle’s nicely defined five senses: touch, taste, hearing, smell, and sight. Undoubtedly, humans have this ingrained sense of position and movement. It is why, for example, you can manipulate objects that are out of your sight, why you know the location of your limbs in space even when blindfolded, why it makes sense that you can learn to move a barbell through space in a straight vertical line without concurrent visual feedback. Experiences like the young man’s phantom limb have required that scientists look outside of the five traditional senses to help explain the ability to sense your body’s position in space and, perhaps, to learn how to manipulate that ability.

Proprioception seems to involve multiple feedback loops, being much more than a back-and-forth, sense-interpretation communication between the body’s sensory inputs and central processing. Consider that your body has the expected traditional sensory inputs, receptors in the skin, muscles, and bones of your body. These receptors feed real-time information to your brain. If that were the end of the story, however, we would expect that when a person loses a limb the sensory input would just go quiet, dark. Where does phantom pain or even phantom movement (the feeling that a missing limb is moving through space or trapped and unable to move) come from?

Your brain contains a sensory map of the positioning and layout of your body. In general, particular areas of your sensory cortex correspond to the positions of receptor cells in your skin, muscles, and joints. Visually, you may have encountered this sensory map as a cartoonish homunculus of mixed up parts. The proximity of the hand and face might help explain the phantom sensations of the young man who felt the q-tip on his face and missing limb simultaneously.  

“Even though the map is accurate for the most part, some portions of it are scrambled with respect to the body’s actual layout.” Professor V.S. Ramachandran explains in his book, “The Tell-Tale Brain,” probable reasons for the mislaid map in the brain of amputee patients, and he explains how he has used the mapping tendencies of the brain to help alleviate phantom pain or even “amputate” phantom limbs. Dr. Ramachandran explains, “There is no longer an arm, but there is still a map of the arm in the brain. The job of this map, its raison d’etre, is to represent its arm. The arm may be gone but the brain map, having nothing better to do soldiers on. It keeps representing the arm, second by second, day after day.” Where the brain is looking for input, it might substitute sensory input on your face, for example, for that of a missing arm. (Or give one some small consolations for losing a foot.[1])

 

OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]

“The subject of proprioception lies at the boundary between neurophysiology and neuropsychology.” (Uwe Proske, and Simon C. Gandevia, “The Proprioceptive Senses: Their Roles in Signaling Body Shape, Body Position and Movement, and Muscle Force,” https://doi.org/10.1152/physrev.00048.2011.) An amputee experiencing a phantom limb, or researchers observing the faulty perception of a person’s movement or limbs exposes the complex, though often unnoticed, nature of our sense and control of movement and being. Ideally, your map is accurate with no dark, unexplored areas. If you want to move your arms, open a door, or push the pedals of your car without visual feedback, you should be able to do so. Similarly, we tap into this sense map when we engage in barbell training. One of the first things you will do when you first start lifting is to place a bar on your back, out of sight. Your eyes stay focused on a point on the floor, and you learn to squat, moving an object through space, using muscles you cannot see in a mirror, maintaining your balance, and trying to maintain control of many moving parts. 

If you have never squatted before, the first rep (or first dozen) was probably a little tricky. Most new lifters will say that there’s “a lot to think about” when you are trying to lift. That is because your body image, your central body map, is being refined in a new way. With good feedback and deliberate practice, you can learn quickly how a squat is supposed to feel. Then, there is less to think about.

Still, without feedback often as a lifter, you don’t know what you don’t know. One of the areas we tend to see deficient control over the body is in a lifter’s inability to contract the spinal erectors voluntarily, manipulating the joints of the spine, which is necessary to extend and hold rigid the spine while lifting, particularly during the low bar back squat and deadlift. Some understanding of how proprioception works and how we can use knowledge of the senses to better learn movement can help troubleshoot issues like the ability to set your back and others that might be less about being “good” at lifting and more about the perception of your central body map.

The Mirror Box  

Suppose now we have to troubleshoot the existing “map” of your limbs. While the ability to control a barbell in space, perform unfamiliar movements, and do seemingly simple things, like set and hold your back in rigid extension, are much less dramatic, we can perhaps learn something from the treatment and experience of phantom limbs. Dr. Ramachandran tapped into the -physiology/-psychology explanation in his work with amputees and his knowledge of how the brain develops a “central body map.” And he did so using a relatively low-tech method. 

In “The Tell-Tale Brain,” Dr. Ramachandran describes his approach to treating phantom limbs as first understanding the source of feeling from the limb. “Motor output signals to the muscles are (in effect) cc’d to the parietal lobes, where they are compared to sensory feedback signals from the muscles, skin, joints and eyes. If the parietal lobes detect any mismatches between the intended movements and the hand’s actual movements, they make corrective adjustments to the next round of motor signals.” When a limb is amputated, these command centers are on “autopilot,” and, somewhat paradoxically, this part of the brain does not access your rational knowledge that the limb is missing. The command centers continue to send motor signals to the limb and cc’d signals to the mapping part of your brain where they are misinterpreted as actual movements of the phantom limb.

To grossly simplify the problem, the brain doesn’t know what it doesn’t know; and in these extreme cases, it lacks reliable sensory input to help it learn. Dr. Ramachandran addressed this problem by offering a different kind of sensory input to help the brain learn and adjust. He created what is known as a “mirror box.” He writes,

“I placed an upright mirror in the center of a cardboard box whose top and front had been removed. If you stood in front of the box, held your hands on either side of the mirror and looked down at them from an angle, you would see the reflection of one hand precisely superimposed on the felt location of your other hand. In other words, you would get the vivid but false impression that you were looking at both of your hands; in fact, you would only be looking at one actual hand and one reflection of a hand.” 

By having patients visually experience their phantom limb, some were able to move painfully “frozen” limbs and experience relief. Others, with some prolonged use of the mirror box, removed the phantom limbs from their perception altogether, in effect amputating the phantom limb. Dr. Ramachandran and others have demonstrated that the brain is susceptible to additional sensory input, which, when used strategically, can either trick the brain into a false reality such as moving phantom limbs or update its central body map.

Combining Sensations

There is some parallel between phantom limbs and athletic movements such as those we use when barbell training or in any movement-based skill. The goal in any movement skill is the correct orientation of your body in space to either accomplish a particular movement or manipulate an object. Consider the model of the squat. A bar is placed on your back, and you are asked to move it in a straight vertical line while maintaining your balance and while controlling specific muscle groups, like your spinal erectors, to ensure the safe and effective transmission of force that will allow you to move the unseen (but definitely felt) barbell.

The successful execution of a physical skill comes with some combination of experience and learned movement. Experience informs the inside out, or top-down, way that your brain organizes and interprets sensory input. For someone experiencing a phantom limb, whether that person’s phantom moves or is frozen in place often depends on whether the limb was paralyzed before amputation. The brain’s experience sets the expectations for the movement. Experienced lifters have less to “think about” during a heavy squat than novices, just as experienced athletes seem to have improved reaction times based on less information than novice athletes. 

Where experience is lacking, one must learn. While you should eschew mirrors when lifting—they take your focus away from the movement important aspects of correct execution—we can use some lessons from Dr. Ramachandran’s mirror box experiments. Additional sensory feedback to establish the correct movement is the equivalent of training your brain with the sensory experience of what the correct movement should feel like. 

Collate sensory input into your training to build your body map of various skills and improve your athleticism. In general, the additional sensory inputs that will help you develop physical skills are the proprioceptive senses, tactile sense, and visual confirmation of the movement.

To develop your proprioceptive senses requires feedback about how the lift or movement feels during or shortly after its execution. If you think a lift feels correct and a coach confirms that sensation, you have an internal model that you can attempt to replicate. Or, if a movement feels correct to you, but a knowledgeable coach or friend tells you otherwise, you know that you must update the coordinated sensation of the movement through deliberate practice.  

Similarly, the tactile sense of learning often relates to how you control individual body parts. Learning how to extend their lumbar or thoracic spine and maintain that extension often is a challenging concept. One can learn this movement through tactile feedback—a hand placed in the correct spot—along with a verbal or visual cue. The sense of touch is a powerful one when it comes to creating and updating your central body map.

Visual confirmation of a movement can come from many sources. We suggest videoing all your work sets (and ideally sharing that video with a coach). The video can help you visually confirm the markers of correct movement: depth, positioning, bar path, balance. That confirmation either reinforces the feel of the movement or leads to changes for correction. Visual confirmation requires that you or your coach have some knowledge of what the movement should look like and, if there are errors, how to fix it.

These lessons apply to most physical skills and are why the more skill-dependant an activity is the more likely you are to find coaches in the field to help learn those skills. Running coaches, for example, are much less prevalent than are Brazilian Jiu-Jitsu coaches or tennis coaches or golf coaches. On a spectrum of skilled to unskilled movement, lifting lies somewhere in the middle. The lifts we perform can feel awkward and clumsy, like trying to manipulate a body part you didn’t know existed before you attempted to squat for the first time. If you approach the lifts as a collection of sensory inputs and experience, you will help direct your efforts toward better learning and better movement in no time.

 

NOTES

[1] From the Tell-Tale Brain:

“Finally, after amputation of the foot of another patient, sensations from the penis were felt in the phantom foot. (Indeed, the patient claimed that his orgasm spread into his foot and was therefore “much bigger than it used to be.”) This occurs because of another of these odd discontinuities in the brain’s map of the body: The map of the genitals is right next to the map of the foot.”

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