There is no such thing as a ‘phantom limb’. On the face of it, that’s obvious – a phantom limb is no longer present and, therefore, does not exist. That is not what I mean. I mean that the ‘phantom’ in the phrase is itself the phantom. To understand this, look at it the other way around: we don’t say that a person with a limb over which she has no control has a ‘phantom brain map’ for that inoperative limb.
This particular type of body centrism goes something like this: a person with a missing limb is incomplete; by ‘filling it out’ with a phantom part, the brain is, somehow, completing the body. The pioneering work of behavioural neurologist V.S. Ramachandran on phantom limbs, as entertainingly recounted in his book Phantoms in the Brain, has helped us to understand just how pervasive such sensations can be in amputees. He describes the way in which some of these patients feel the limb to be not only present but also ‘locked’, as in a case where an amputee felt that the fist of his missing arm was tightly clenched, fingernails digging painfully into his palm.
What such patient studies, and a great deal of other evidence from experiments with sensory substitution, tell us is that the brain expects input. Depending on the precise circumstances of the way the original input source is removed, the brain may ‘give up’ on that source, ‘remap’ it, or insist that it is still present. The Penfield homunculus is one way of illustrating which parts of the cortex control which parts of the body. These kinds of ‘maps’ can be useful, but they can also lead to the mistaken idea that there is a proper, normal, correct one-to-one mapping of motor and somatosensory cortex areas to body parts. Of course in an intact human body there is a ‘standard’ type of mapping (in body identity integrity disorder this goes awry creating the sensation that some standard body part should not be present); however, we cannot reasonably claim that the brain somehow ‘seeks out’ this usual mapping and tries to complete it. For such a claim to be true, there would have to exist somewhere in the brain a map of the map, and so on to infinite regress.
A better way to look at the brain’s apparent expectation of input – certainly better from a transhumanist point of view – is to see it as suggestive of extensibility as native function. Body plans have evolved in myriad diverse ways through natural selection, so perhaps we should not be surprised that the cognitive flexibility to accept novel sources of input seems to be part of our biological makeup. There is, of course, a vast difference between adaptation over aeons and adaptation over months/years, but I am not claiming that we have retained the cognitive flexibility to map to, for example, a trilobite body plan; only that we are capable of quite radical sensory extension even without artificial cognitive enhancements.
Nervous systems of Alalcomenaeus fossil (left), a larval horseshoe crab (middle) and a scorpion (right)
Human Nervous System
When Paul Bach-y-Rita created a chair that could give blind persons a novel (though very limited) form of ‘sight’, he was tapping into this extensible neuroplasticity. A camera mounted in front of the chair was connected to a network of vibrating plates in the back of the chair, effectively turning the visual image captured by the camera into vibrating ‘pixels’. A blind person sitting in the chair could feel these via their sense of touch, and therefore, ‘sense’ objects brought towards the camera.
Bach-y-Rita’s success in his early experiments in sensory substitution later led him to develop more-sophisticated devices of this kind. The BrainPort sends electrical pulses converted from its camera input to the tongue of the blind user, allowing that user to navigate his/her environment with quite a high degree of accuracy.
Note that I have avoided using the word ‘interpret’ in describing how the brain deals with such signals. For some reason, we tend to use this word when describing novel sources of brain input but not so much when describing ‘natural’ ones. In either case, ‘interpretation’ is misleading. This is what brains do – they deal with input as best they can, whether it comes from eyes, ears, noses, or BrainPorts. You may wish to call that ‘interpretation’; I prefer to call it sensing.
We are too rigid about senses. Most people still stick to the silly ‘common sense’ claim that we have only five. Even a brief reading of a neuroscience textbook will introduce a host of others: proprioception (the sense of the position of one’s limbs in space), nociception (the perception of pain signals), and thermoception (the perception of temperature) among them. And our senses overlap. Chopping up our total sensory experience and putting it into categories may help with classification, but it does not help when it comes to understanding perception as a spectrum of staggering breadth, beauty, and diversity. A lack of understanding of this spectrum can lead to inflexible, even prejudicial, attitudes; a clear understanding of it leads towards the conclusion that the human brain is more than ready for sensory extension.
The rubber hand trick usually raises a laugh. Put one of the participant’s hands out of view, put a rubber one in view alongside the visible hand, then stroke it and tap it. It’s not long before the participant begins to ‘feel’ hand-like sensations in the rubber hand; an unexpected hammer-blow to it elicits a pain-type reaction. It’s a crude but effective demonstration of sensory flexibility in action.
Some people find the seamlessness of such cognitive transitions alarming. Instinctive dualistic interpretations of the –mind-body problem will tend to cause this kind of dissonance. Curiously, some dualists find it easier to accept the idea of sophisticated robotic arms than of crude rubber hands becoming part of a body plan. Perhaps this is because some ‘external interference’ is required to integrate the robotic arm, whereas the integration of the rubber hand just happens, and in a way that makes it appear that the brain has been fooled; the rubber hand becomes the pesky phantom.
In time, we will learn how to repair all broken sensory abilities. Just as cochlear implants have created a revolution in the restoration of hearing, so will robotic and bioengineered visual systems restore sight to the blind, and so on. The implementation of many of those sensory technologies will require invasive surgery, but our native flexibility will serve us well when it comes to accepting those ‘plug-in’ technologies as part of our very being. And we will take on new sensory abilities; the perceptive capabilities of other animals (e.g. vibration-sensing antennae and ultraviolet-range vision) will inspire us and will encourage us to think ever more radically about how we can weave ourselves more intimately into the fabric of our cosmos.
Because that’s what senses do. They allow us to flex, and to be flexed by, the Universe. Brains do this. We – this extensible species – do this. Reality is about to become much more real. Go sense it.
D.J. is a futurist thinker and writer, and is signed up with Alcor for cryonic preservation. He lives in, and works from, a modern house overlooking the sea on the coast of the Isle of Skye, in the Highlands of Scotland.