NEWS FEATURE NATURE Vol 442 13 July 2006 IN SEARCH OF THE SIXTH SENSE hut your eyes Now touch your nose Chances are you can do this without even thinking about it For this you can thank your sense of proprioception which is so much a part of us that most of us are unaware that it exists This sixth sense lets our brain know the relative positions in space of different parts of our bodies Without it our brains are lost Ian Waterman knows how that loss feels More than 30 years ago he lost this sense almost overnight when a flu like virus damaged the required sensory nerves His muscles worked perfectly but he could not control them I lost ownership of my body he says He could no longer stand or even sit up by himself and doctors said he would never be able to do so again Waterman s condition arose from a disease called acute sensory neuropathy and is so rare that only a dozen or so similar cases are known to the medical literature Some neuroscientists are taking a cue from Waterman s experiences and starting to investigate whether robotic devices controlled by thought alone could be integrated with an artificial sense of proprioception If so they reason these neuroprosthetics could be made S to work in a much more life like way What s more they hope to gain a deeper understanding of how proprioception works and how they might be able to manipulate it Some months after his virus attacked Waterman only 19 years old was lying in bed applying all his mental energy to the fight for control of his body He tensed his stomach muscles lifted his head and stared down at the limbs that seemed no longer to belong to him He willed himself to sit up Concentrated effort Later he realized that it was the visual feedback that allowed his body to unexpectedly obey the mental instruction But the euphoria of the moment made me lose concentration and I nearly fell out of bed he remembers From then on he learnt to compensate for his deficit in proprioception with other forms of sensory feedback to help him understand where his limbs are and thus control them It requires constant intense concentration but now despite his profound impairments he can manage fairly normal movements Most of the input that he relies on is visual standing up with his eyes closed is still nearly impossible 2006 Nature Publishing Group but he can also tune in to the tug of a jacket sleeve to work out the direction his arm is moving Or to the cool air on his armpit when he raises his arm in a loose shirt Neuroprosthetic engineers are realizing that many sensory feedback signals could be similarly harnessed A neuroprosthetic is more accurately called a brain machine interface Hundreds of electrodes fixed into tiny arrays are placed in or on the surface of the cortex the thin folded outer surface of the brain that controls complex functions including the organization of movement The electrodes record the electrical signals from the cortex s neurons and these are translated by a computer algorithm and used to drive specific actions the movement of a cursor on a computer screen for example or of an artificial limb In this issue of Nature two papers1 3 demonstrate dramatic progress in the area A team consisting of John Donoghue s group based at Brown University in Rhode Island and Cybernetics Neurotechnology Systems in Foxborough Massachusetts implanted 96 electrodes into Matt Nagle s motor cortex the brain region that processes information about move125 R MASSEY GETTY Implants in the brain could one day help paralysed people move robotic arms and legs But first scientists need to work out how our brains know where our limbs are says Alison Abbott NEWS FEATURE R FRIEDMAN NATURE Vol 442 13 July 2006 and redirected to the appropriate part of the cortex or the sensory cortex itself The how refers to the design of the electrical signals to be fed into the cortex These could mimic the sensory system s natural nerve impulses based on parameters such as frequency and amplitude Or they could involve creating artificial signals that the sensory cortex is able to distinguish in the hope that the brain can be trained to associate particular signals with particular parameters Once scientists have worked out how best to encode the signals the idea would be to place sensors on artificial limbs to generate signals representing proprioceptive information such as angle of joint vibration force of grip and other sensory information that Waterman has found helpful such as temperature Mind control Matt Nagle s neuroprosthetic lets him move a cursor using thought alone A handful of researchers is starting to try to work out where and how to stimulate the sensory nervous system to reproduce the sorts of information that a limb might send to the sensory cortex It is early days none of their work is published And as so little is known about the system there is no obvious place to start Theoretically the where could be the nerves running from the limb into the spinal cord or the spinal cord itself see graphic Or it could be higher in the brain s thalamus where incoming sensory signals are integrated HOW SOME OF THE BODY S SENSORY INPUTS WIRE INTO THE BRAIN Thalamus Closing the loop The two papers show how closely neuroprosthetics are approaching medical reality But although moving a computer cursor by thought alone may be dazzling scientists have long term ambitions to make neuroprosthetics reproduce more complex functions Could patients direct a robotic arm to pick up a coffee cup for example For this the devices need to deliver feedback to the brain we need to close the loop says Daofen Chen director of the neural prosthesis programme at the US National Institute of Neurological Disorders and Stroke in Bethesda Maryland The brain s sensory cortex receives signals proprioception touch pain and so on from the body see graphic and in response constantly modifies its movement related commands The current generation of output only neuroprosthetics are open loop systems with more limitations than Ian Waterman who can at least use visual temperature and tactile feedback Brain machine interfaces will have to become interactive says Chen But now that we would like to exploit it we realize we know next to nothing about sensory input 126 Face Arm Trunk Flow of information ment Nagle is a quadriplegic patient and the first human volunteer to reach this advanced stage of testing see picture above Hooked up to computers and attended by a team of technicians Nagle could