Hand Amputation *Will* Result in Altered Perception Around the Hands

Tamara Makin et. al report [link, link]: The space within reach of our hands is the medium for reaching, grasping, and avoidance movements. Accordingly, visual information in this action space is organized in hand?centered coordinates, generating a common frame of reference for perception and action. (...) We report here that amputation of a hand is indeed associated with a mild visual neglect of the amputated side: Participants with an amputated hand favored their intact side when comparing distances in a landmark?position judgment task. Importantly, this bias was absent when the targets were placed in far space. Our results thus demonstrate that the possibility for action within near space shapes the actor’s spatial perception. -- The findings suggest that losing a hand may shrink the action space on the amputated side, leading to permanent distortions in spatial perception. According to the researchers, This shows that the possibility for action in near space shapes our perception — the space near our hands is really special, and our ability to move in that space affects how we perceive it.

So I wondered what would influence this warped perception, this distortion, more: a static (cosmetic) or a functional prosthesis? From my own experience I prefer a static (cosmetic) prosthesis for walking or going to the movies or also for Tai Chi - when my shoulders, back and neck are important. But for feeling at least halfways symmetrical I also feel that wearing a body powered prosthesis makes me more aware of my disabled side. Also, I just recently started to increasingly use the disabled arm and the prosthesis - not just because I started to use appealing and powerful terminal devices but also, because this seemed to take time.

Intriguingly, I just got this statement from someone happily wearing a somewhat static but halfways functional arm that still allows to grasp and hold objects using minimized cable control: This "passive" arm helps me.  However, I built this arm like a tank and it weighs about 3 KG and it's protected.  My body is balanced, symmetrical.  My movement and power is spot on. Like me, he optimized between prosthetic parts he did not want to put up with and other parts that proved useful.

So I went to the supplementary data of this article [link, link] and, tada.

The PSE (point of subjective quality) is 0.50 for perfectly symmetric people. Arm amputees tend have their PSE warped so it is either e.g. 0.51 or 0.49, depending on what side the amputation is.

Based on the supplementary table data of the aforementioned paper, the difference of the PSE to the norm (0.50) is plotted below. I plotted it according to two types of prostheses: c (cosmetic) and f(functional). While amount of data is rather moderate (twelve participants), there is a slight trend: people with functional (f) prostheses err less to one side than people with a cosmetic (c) prosthesis. People with functional prostheses seem to have less distortion in their action space. Don't be fooled - you'll get a distorted perception no matter what.

So obviously, asymmetry is there to stay.

So what happens when you get older? As the next graph shows, that I also pulled from the supplementary data table of that article, the PSE difference to 0.5 gets bigger the longer the time since limb loss. In other words there is no way that is just stopping an invisible process (at first - later we'll all warp up).

Or is there?

Maybe there is.

The following plot shows the same data as above but separated into two groups: The red line approximates the fast increase of distortion of the action space as measured by PSE (and its difference to a normal 0.5) for users of cosmetic arms, the blue line approximates the considerably slower distortion increase for people with functional prostheses.

In other words: years of using a functional prosthesis seem to make a difference.

Anecdotically and just because it seems fit to mention here: when I was suffering from some overuse of the left arm last year, an old and experienced physiotherapist told me that a cable controlled and very comfortable prosthetic arm was the best choice I could have made and he recommended that I stick with that system for the next couple of years rather than just switching around. It felt like it was the right decision then, and that feeling keeps getting stronger now.

Obviously these are trends and other variables play a role too.

Then I wondered about the combination of experience and usage frequency as these must play a role too. So I calculated the normed PSE differences, normed usage frequency and normed years since amputation to get an overall score that combines 'little deviation of warp space' with 'experience':

pse diff -> N(pse diff) = [pse diff - mean(pse diff)] / std dev (pse diff)

years-> N(years) = [years - mean(years)] / std dev (years)

usage freq ->  N(usage freq) = [usage freq - mean(usage freq)] / std dev (usage freq)

To make the data scale positively in a range above zero, I flipped them in real number space:

N(pse diff)-> Nf pse = 6-N(pse diff)

N(years) -> Nf years= N(years)+6

N(usage freq) -> Nf usage freq= N(usage freq)+6

These then were multiplied to reach a score:

score = Nf pse * Nf years * Nf usage freq

The trend that was apparent above is now more accentuated.

It therefore appears as if frequent usage of a functional rather than cosmetic prosthesis reduces the altered perception of space.

This is relevant for all kinds of conclusions.

It means that a combination of more usage, longer usage and less distortion of perceptive field seem to be a bit more prevalent in users of functional rather than cosmetic arms. While it is just a statistical trend it confirms my own perception even though that is just anecdotical.

What it really could mean is this: for a prosthesis to really blend into a minimally warped body image, it must be worn and used. It must be worn and used day in and day out, all the time.

And for that it must match some criteria:

  1. It must be stable, rock solid and reliable so it can function day after day, week after week, year after year. Such prostheses are mandatory, as unreliable prosthetic gadgets cost money first, are dropped later and so will usage frequency. And that may end up costing more money in the end through massages, physiotherapy, loss of work, et cetera, than meets the eye.
  2. A prosthesis must be comfortable to wear so it can be worn day after day, week after week. Uncomfortable prosthetic gadgets are dropped and so will usage frequency; I tried a myoelectric arm with a hard socket and, boy, was that painful; other people have repeated bouts of eczema and other skin problems due to the way their prostheses are made. Not everyone can deal with prosthetic sockets, some prefer or even require osseointegration - others need to find an optimal liner to go with their sockets.
  3. The prosthetic arm must be optimally functional in terms of ADL (activities of daily living). If it is not top functional - already if it is just cosmetic - the body of the wearer will asymmetrically warp space; typically, people get frustrated with myoelectric arms and then either wear cosmetic arms or no prosthetic arms any more.
  4. The arm needs to be affordable - so insurances can pay equipment, upkeep, repairs and supplies for decades rather than a one time equipment.
  5. Last but not the least, the arm must look and feel good to the wearer. And to others.
  6. The prosthetic action must be there - at the stump, at the residual limb. Not on the toes, not on the other hand, not on the scalp or anywhere else. After all, you want to maintain an activated state of the disabled body part. That may not be perfectly possible, after all parts have gone missing. But still, such as state seems to be effected, surprisingly, by activation of (now it comes) that disabled body part. So, motor control and sensory feedback either by wearing a hard socket, by getting osseointegration to transmit osseoperception - all that are steps in the right direction.

The more the better.  See what keeps you from wearing the prosthetic arm 24/7 and try to minimize that. For me, this meant to avoid uncomfortable and painful hard socket design of a myoelectric arm, to minimize wiggle and continuous frustrating part failure (avoid ill fashioned prosthetic parts, redesign better parts), to minimize skin problems, to start using terminal devices that allow for functional powerful and stress free grip performance, and to use cable control that feels integrated, graduated and organic and that blends in with body motion.

Conversely, the Otto Bock plastic sheath that shipped with my harness soaked up axilla sweat and started to really stink and that significantly reduced the prosthetic usage for me. The Otto Bock perlon cable and its minute screw connectors that permanently ripped significantly reduced prosthetic usage for me. The Otto Bock hooks and cable connectors that were ill fashioned really reduced the prosthetic usage for me. The Otto Bock wrist that ended up not interfacing well with the Otto Bock 'childrens' surprise' bolts really was problematic. The hook that slips from many smooth surfaces that ends up being a problem in public (shopping, cafeteria) or a nuisance at home (dropping tools or dishes) significantly reduced the prosthetic usage for me. And all of that which is stressful and annoying adds up, the EODF (end of day feeling) can be very bad. And it's not that wearing a prosthetic arm is easy to begin with, no, it starts as a hard and difficult situation in the best of all prosthetic worlds. So you have to understand that the cumulative stress of all the prosthetic crap sold by Otto Bock lead to me not wearing the prosthetic arm for my spare time every now and so often - which in turn does not make for a particularly friendly attitude towards the material at hand. Also cumulative stress of this type tends to make me angry and verbally aggressive - a reaction that the Otto Bock customer representative had no understanding for (which begs a number of extremely uncomfortable deep questions). As she mentioned ergotherapy to me, I did wonder whether she at all had an even rough concept of the domain she was talking about, and the specific problems that crap parts entail. And I concluded she was in all likelihood evil - as no company knows more about how to upset amputees than Otto Bock. In the end, care for my own symmetric and functional well being caused me to specifically aim for high quality parts for my arm.

A prosthetic arm must satisfy criteria 1, 2, 3, 4, 5 and 6 all at once. Only then will the EODF (End of Day Feeling) be good. Only if that EODF is good will there be another day of wearing that arm. And another day. And so on. That is what I am after. If someone takes a particular interest in disabled people and wants to build 'interesting' and 'fun' parts - I'm all for it. But then I want to see that in the way these parts are made. Then I expect prosthetic quality.

People that build these arms also read this blog and yet most of them don't listen.

I have been saying that for below elbow amputees such as me, body powered and functional warez may just be so much better than myoelectric attempts, since the very inception of this blog have I been saying that. Over years and decades, warped body shape may end up with neck problems, dizziness, chronic pain, muscular tenseness, cramps, rotatory cuff damage and more.

Yes, I can walk around with the bare stump showing - but perceptional space, then muscles and bones and posture, all may and eventually will maximally warp depending on what I do. And after a while, I may very well feel the consequence. Question is, do I want that?

That is why I feel as if the most robust, functional and comfortable setup wins way over the most impressive, modern or beautiful looking setup. Of course, the harness is a bit of a strain - but not wearing any prosthesis is even more of a strain as I have to do everything with my left hand.

So if you read this and you can still choose over the type of prosthesis: Whatever you will wear, choose a system that you can wear as long as possible over a whole day, as many days as possible in a sequence, that is maximally functional on a reliable level and that is as comfortable as ever possible. For high amputations this may be osseointegration and targeted reinnervation (that's probably what I'd get if I was in that situation), for below elbow amputations this is definitely body powered technology (unless I'd have a good reason not to), and for terminal devices it's what gets the stuff grasped (for me, current  favorites are the Becker hands and the V2P).

At the same time, massively expensive yet less useful and unreliable gadgetry is being developed by people who do not know what they are doing - consistently and since decades. How do we make them see? They cannot read obviously. How do we make them see? Art? Music? A speech? Posters? Word of mouth? Muppets? Jokes? Satire? Bribery? Martial arts? Black magic? Sleuth dogs? Screw drivers? Alcohol? Workshops? Call center marketing? Letters to the editor?

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Hand Amputation *Will* Result in Altered Perception Around the Hands; published January 10, 2010, 19:17; URL: https://www.swisswuff.ch/tech/?p=259.

BibTeX: @MISC{schweitzer_wolf_1581673269, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Hand Amputation *Will* Result in Altered Perception Around the Hands}}, month = {January},year = {2010}, url = {https://www.swisswuff.ch/tech/?p=259}}