For example, we rely on our sense of touch to apply just enough force on objects so as to not drop them when fingertips are numbed with a local anaesthetic, we exert much more force than is necessary 9. Touch and proprioception are also critical to our ability to dexterously manipulate objects, as evidenced by the deficits that result when we lose these sensory signals. Tactile signals allow us to maintain contact with an object without having to attend to it visually. Nerve fibres that innervate the skin convey information about the initiation and termination of contact with an object, about which parts of the hand make contact with the object, about the forces exerted at each location, and about the object itself (its size, shape and texture 7, 8). Nerve fibres that innervate the muscles, tendons, joints and skin convey information about the posture and movements of the limb and about the forces they exert. Instead, in non-disabled individuals, limb control relies heavily on somatosensory signals that track the state of the limb and its interactions with objects. However, the performance of unidirectional efferent neuroprosthetic systems is limited by the inadequate sensory information available to the user: movements are guided mainly using vision 5, even if proprioceptive signals stemming from the residual forearm muscles used for myoelectric control of the bionic hand, and sound from the robotic actuators (and other incidental cues), may also contribute 6. There are a variety of technologies for the extraction of control signals from the user’s muscles, nerves or brain, or from the user’s residual movements 4. The development of such prostheses involves methods that infer intended movements from signals acquired from the user (neural or otherwise), and the execution of these motor intentions by the prosthetic device. Bionic (robotic) limbs can restore independence to these individuals. Each year, thousands of people suffer the consequences of upper-limb paralysis 2 or of amputation 3 caused by a traumatic event or disease. Losing hand function can cause severe physical incapacity and even mental disabilities. Also important are sensory signals from the hand that convey information about the hand’s state (movements and posture) and about its interactions with objects (contact timing, location and pressure). Hand control also requires a sophisticated neural system to configure the digits in task-appropriate ways and to apply finely graded forces on objects. The hand’s versatility is partly enabled by its anatomical complexity (it comprises many joints and is driven by many muscles 1). For example, grasping involves a range of behaviours, from precision grip with the index finger and thumb, to power grasp with all of the fingers and the palm. The hand allows for a wide array of interactions with objects.
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