Neurological trauma, dysfunction or disease can leave persons with severe and life threatening motor or sensory disabilities that can compromise the ability to control basic vital functions. Persons with neurological impairments often rely on personal assistants, adaptive equipment and environmental modifications to facilitate their daily activities. Neural prostheses are highly effective methods for restoring function to individuals with neurological deficits by electrically manipulating the peripheral or central nervous systems. By passing small electrical currents through a nerve or directly to the motor units of a muscle via intramuscular, epimysial, and surface electrodes, neural prostheses can initiate action potentials which in turn trigger the release of chemical neurotransmitters to affect an end organ, such as a muscle. Techniques exist to selectively activate axons of any size or location within a nerve or fascicle, making it possible to preferentially target small sensory fibers or duplicate neutral motor unit recruitment in order to minimize fatigue and grade the strength of a stimulated muscular contraction. In addition to exciting the nervous system, the proper current waveform and configuration of electrodes can block nerve conduction and inhibit action potential transmission. Thus, in principle any end organ normally under neural control is a candidate for neural prosthetic control.
Neural prostheses may consist of wholly external components with only limited surface or percutaneous electrical contacts, combinations of both external and implanted components or in some cases fully implanted systems with limited or no interface to components external to the body. In some cases, recording components of neural prostheses may interface with external systems that impact the user such as external mechanical orthotics or other devices. In further advances in the field of neural prosthesis, networked systems are developed whereby combinations of sensors and actuators are implanted within the user and networked into a common system. Some exemplary systems are described for example in U.S. Pat. No. 5,167,229 to Peckham, et. al., and U.S. Pat. No. 7,260,436 to Kilgore et. al.
Neural prostheses commonly operate in one of two separate control modes, open loop and closed loop. In the case of open loop control, the neural prostheses applies electrical signals to the body based on a pre-defined simulation pattern that does not change after initiation based en the movement or change of body state. The pre-defined stimulation pattern is triggered using a variety of inputs, including joysticks, voice commands, or feedback from other sensors that provide information about the current state or orientation of the body and the user's desired state. Closed loop control modes use information about the state of the user's body during stimulation to further resolve and tune the stimulation to achieve more accurate and precise motion. In order to implement these various control modes, it is common for neural prostheses to use sensors to allow the user to issue command signals to the system and for estimating the body state, i.e. its position, orientation, force etc.
Existing neural prostheses utilize two different types of command signals, a logical or trigger command signal and a continuous or graduated control signal. The logical control signals are used to turn external devices on or off, initiate a predefined motion or stimulation pattern, or cycle through a set of different patterns such as different grasp patterns such as lateral and palmer grasp in the case of an upper extremity neural prosthesis, and lock or unlock a device at various force levels. Examples of logical command signals include push button switches, reaching a specific threshold value with a command signal recorded from a part of the body, or holding a command signal at a certain threshold level for a predetermined length of time. A continuous command signal is required to control degree of motion or position and force applied by the neural prosthesis. Some examples of command signal sources include joint positions or potentiometer readings obtained from joints where the user retains volitional control, myoelectric signals obtained from muscles above the lesion where the user maintains functional control such as voluntary control over wrist extensor muscles by a tetraplegic user with a C6 level injury. The goal in the design of neural prostheses is to create command inputs that are a natural extension of the user's intact motor system.
It is clear that neural prosthetic approaches can provide both therapeutic and functional benefits to individuals with impairments due to neurological injury or disorder. However, a significant disadvantage of prior neural prostheses is due to a lack natural command signals that are easy for a user to internalize and use to command the neural prosthesis.
There is increasing evidence that even in cases of severe spinal trauma that result in clinically complete Spinal Cord Injuries (SCI) some axons remain intact across the lesion. Traditional techniques for assessing SCI involve manual muscle testing and sensory testing. These traditional techniques have a functional basis for evaluating whether or not volitional control exists; meaning volitional control below the lesion is measured by evaluating the force manifest by the muscle under volitional control of the injured subject. Thus an SCI is determined to be functionally or clinically complete based on the presence or absence of visible or palpable movement in the muscles below the lesion. In recent studies seeking to develop new diagnostic techniques to analyze lesions in clinically complete SCI it has been found that there exists sufficient numbers of axons that cross the lesion that allow volitional electrical signals that cross the lesion and be manifest below the lesion, even though the signals are not strong enough to cause visible muscle contraction. For example, the volitional electrical signal may incomplete innervate a muscle and thus not have the ability to trigger enough motor units to cause a physically manifest contraction of the muscle. However, this sub-functional activation of motor units within the muscle does result in a measurable electromyography (EMG) signals. Someone with an SCI injury or other injury or trauma to the nervous system that is functionally or clinically complete, with no clinically manifest movement of muscles below the lesion, but who upon closer analysis is found to still generate volitional electrical potentials in muscles and nerves below the lesion is referred to herein using the term discomplete neural lesion.