All our experience of the world, and ability to act on it, are channelled through our body. The pioneering computer scientist, Alan Turing, correctly realised the human mind is special not particularly because of its computing power, but because the body provides it with a unique interface to the world. Current research in psychology and neuroscience is probing how the brain represents the body. Recent advances have revealed that body representation is fundamentally multisensory, arising from the combination of many different sensory signals. These include classical “senses,” such as touch and vision, and also much more specific signals, such as the flexion or extension of each muscle, which define the body’s posture in space. This information is integrated to construct a multisensory representation of the current state of the body. Intriguingly, multisensory signals also affect what we perceive our body to be like, for example by making us feel like a rubber hand really is our hand! Our thoughts about what our body is are highly flexible, and track the multisensory inputs that the brain receives.
A common illustration of just how flexible the sense of our body is comes from changes in the brain’s representation of the body due to tool use. Humans, and some other animals, are able to use tools as additions to the body. When we use a long pole to retrieve an object we couldn’t otherwise reach, the pole becomes, in some sense, an extension of our body. Is this merely a poetic way of speaking, or does the brain actually incorporate the tool into its representation of the body? Studies of monkeys learning to use a rake to obtain distant objects show that this may be more than a mere metaphor. Multisensory brain cells respond both to touch on the hand or visual objects appearing near the hand. When the monkeys used the rake, these cells began to respond to objects appearing anywhere along the length of the tool, suggesting the brain represented the rake as actually being part of the hand.
A recent paper in Psychological Science elegantly illustrates the plasticity of body representation, and provides further evidence that representations of the body really do expand to include ‘external’ objects we hold. Thomas Carlson of the University of Maryland and colleagues at Harvard University and Utrecht University in the Netherlands used an unusual subjective experience of the body first reported by Franklin Taylor of Princeton University in 1941. If you look towards your hand in a darkened room and see it illuminated by a bright flash, an afterimage of your hand remains after the flash. If you then move your hand, the afterimage changes, though no actual visual signal is present. The precise effect, like so much of the richness of human sensation, is difficult to catch in words, but is like a fading, or loss of clarity of the hand. This fading is normally explained by the multisensory nature of body representation: when the hand moves, but the afterimage does not, visual information and ‘proprioceptive’ information from muscles no longer agree about where the hand is. The visual impression of the hand fading may be a by-product of this inability to integrate different sensations due to conflict about where they are in space.
Carlson et al used this fading to investigate the limits of the brain’s representation of the body. When participants held an object and then moved their hand after the flash, the object’s afterimage faded as well. Further, if participants reached for the object after the flash, the object still faded. The brain may detect a conflict between the location of the object in the afterimage (on the table) and the location where the object is actually felt (in their hand). Alternatively, the object may be rapidly assimilated into the representation of the body and therefore subject to the same perceptual conflict.
The authors also ran additional tests in which volunteers dropped the object, providing new evidence for a spatial bound on plasticity of body representation. Objects released from a mechanical gripper held in the hand did not fade, while those released directly from the hand’s grasp did fade. These conditions suggest that direct contact with the skin may be an important cue to plastic bodily extension. A critical, though elaborate, test might involve forming an initial afterimage of the hand, gripper and object placed separately on the table, then picking up the gripper, and using this to pick up the object. The authors would predict a fading of the afterimage of the gripper, due to its incorporation in the body representation. However, the object should not be incorporated because it would not be touched directly.
These results elegantly confirm that the human brain maintains a highly flexible representation of the body, despite the tendency in everyday life to think of ourselves as having a fixed personal identity, linked to our body. Two distinctive features of mental body representation emerge. First, from the brain’s perspective, the body is by far the most familiar object in the world: the body, as William James elegantly put it, is “always there.” In these experiments, the mechanical gripper could be treated as part of one’s own body. However, this did not extend to an object held in the gripper, perhaps because this situation wasn’t familiar enough. We speculate that skilled prosthesis users might experience objects grasped in the prosthetic hand as fading, even though first-time users in the present experiment did not. Given the ability to move an object through voluntary action, and sufficient sensory experience of doing so, the capacity to extend the self may be virtually unlimited.
Second, these studies suggest a view of the body as an interface between the brain and the external world. This view has important implications for human psychology generally. The sensorimotor mechanisms of the body are effectively a tool for our voluntary actions to respond to the environment and to change it. While we think of our body as a fixed feature of our lives, the brain displays a surprising ability to accept as part of ‘me’ whatever I happen to be touching and using at any given time.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters co-editor Gareth Cook, a Pulitzer prize–winning journalist at the Boston Globe, where he edits the Sunday Ideas section. He can be reached at garethideas AT gmail.com