Brain-machine interfaces: an overview <s> In <s> The activity of single neurons in precentral cortex of unanesthetized monkeys (Macaca mulatta) was conditioned by reinforcing high rates of neuronal discharge with delivery of a food pellet. Auditory or visual feedback of unit firing rates was usually provided in addition to food reinforcement. After several training sessions, monkeys could increase the activity of newly isolated cells by 50 to 500 percent above rates before reinforcement. <s> BIB001 </s> Brain-machine interfaces: an overview <s> In <s> Recordings have been obtained simultaneously from several, individually selected neurons in the motor cortex of unanesthetized monkey as the animal performed simple arm movements. With the use of comparatively simple quantitative procedures, the activity of small sets of cells was found to be adequate for rather accurate real-time prediction of the time course of various response measurements. In addition, the results suggest that hypotheses concerning the response variables "controlled" by cortical motor systems may well depend upon whether or not the temporal relations between simultaneously active neurons are taken into account. <s> BIB002 </s> Brain-machine interfaces: an overview <s> In <s> The concept of a separate premotor cortical field involved in the cerebral control of movement went out of favor among neurophysiologists during the quarter century from 1952 to 1977, but recent studies have led to its rehabilitation. The premotor cortex appears to be one of at least three fields within the motor cortex, two others being the primary motor cortex and the supplementary motor cortex. Several proposals have been presented concerning the functional specializations of the premotor cortex. Although no specific hypotheses have very strong support at present, the best evidence favors a role for premotor cortex in the preparation for and the sensory guidance of movement. <s> BIB003 </s> Brain-machine interfaces: an overview <s> In <s> For the severely handicapped patient, such as a quadriplegic, a large number of independent signals would be desirable to control neuromuscular stimulators that could impart movement to the paralyzed limbs. We have investigated the possibility of making long-term connections to the central nervous system with microelectrodes. Monkeys have been implanted with arrays of intracortical electrodes for periods of up to 37 months, indicating that long-term connections to the nervous system are possible. A second question investigated was whether the implanted monkeys could learn to modify the firing patterns of recorded neurons to control a device outside of their bodies. Through the use of an 8 target tracking task a monkey was able to produce an information transfer rate of 2.45 bits/sec when cortical cell signals were the monkey's output. The same task was performed having the monkey move a handle by wrist flexion and extension (i.e., using the intact motor system as the output). The information transfer rate increased to 4.48 bits/sec, or less than a twofold improvement. Thus, the direct output of cortical cells can provide information transfer only moderately less precise than the intact motor system. Our preliminary studies have been encouraging on obtaining connections to the nervous system to control external devices. However, numerous improvements are required in electrode design, fabrication, implantation, and signal processing techniques before this method of obtaining control signals would be feasible for human applications. <s> BIB004 </s> Brain-machine interfaces: an overview <s> In <s> The aims of this paper are to (i) provide a brief history of cochlear implants; (ii) present a status report on the current state of implant engineering and the levels of speech understanding enabled by that engineering; (iii) describe limitations of current signal processing strategies and (iv) suggest new directions for research. With current technology the “average” implant patient, when listening to predictable conversations in quiet, is able to communicate with relative ease. However, in an environment typical of a workplace the average patient has a great deal of difficulty. Patients who are “above average” in terms of speech understanding, can achieve 100% correct scores on the most difficult tests of speech understanding in quiet but also have significant difficulty when signals are presented in noise. The major factors in these outcomes appear to be (i) a loss of low-frequency, fine structure information possibly due to the envelope extraction algorithms common to cochlear implant signal processing; (ii) a limitation in the number of effective channels of stimulation due to overlap in electric fields from electrodes, and (iii) central processing deficits, especially for patients with poor speech understanding. Two recent developments, bilateral implants and combined electric and acoustic stimulation, have promise to remediate some of the difficulties experienced by patients in noise and to reinstate low-frequency fine structure information. If other possibilities are realized, e.g., electrodes that emit drugs to inhibit cell death following trauma and to induce the growth of neurites toward electrodes, then the future is very bright indeed. <s> BIB005
1963, Walter conducted an experiment that can be considered the first demonstrations of a BMI. He recorded readiness potentials in the motor cortex of patients undergoing neural surgery  . The patients were instructed to push a button in order to advance a slide projector. Readiness potentials occurred shortly before the button presses and were prominent enough to serve as a trigger for the projector. Accordingly, Walter connected the motor cortex to the projector directly. The subjects still continued to press the button, but the Figure 1 . Brain-machine interface (BMI) for reaching and grasping by a robotic arm. Extracellular activity of cortical neurons was sampled with multielectrode arrays implanted in multiple cortical areas of a rhesus monkey. These neuronal signals were decoded and redirected to the controller of a robotic arm equipped with a gripper. The position of the robotic arm was displayed to the monkey as a computer cursor. The cursor size conveyed the information about the gripping force. The behavioral task required reaching and grasping virtual targets. In a manual version of the task, the monkey operated the robot by moving a two-dimensional joystick. The gripping force was produced when the monkeys squeezed the joystick handle. To enact direct brain control, the joystick was disconnected from the robot, and the robot was operated by the motor commands extracted from the cortical activity. Adapted from [6] . readiness potentials often completed the job before the subjects moved. A few years later, scientists at the National Institutes of Health (NIH) announced that the development of neurally controlled devices was their research goal. Group leader Frank wrote, "We will be engaged in the development of principles and techniques by which information from the nervous system can be used to control external devices such as prosthetic devices, communications equipment, teleoperators... and ultimately perhaps even computers"  . To accomplish this goal, the scientists implanted several electrodes in monkey motor cortex and recorded from a small population of neurons BIB002 . Their monkeys performed a wrist movement task. In an offline analysis of these recordings, multiple linear regression analysis was applied to reconstruct the movement traces from the neuronal activity with good accuracy. Next, the reconstruction algorithm was implemented online, and NIH monkeys were able to move a cursor on a LED display with their cortical modulations BIB004 . At about the same time, Fetz and his colleagues conducted pioneering studies on volitional control of single cortical neurons. Monkeys were operantly conditioned to voluntarily modulate firing rates of single cortical neurons BIB001 . Such control was possible even in the absence of overt limb movements. In parallel to these studies on motor BMIs, research has started on sensory BMIs. Cochlear implant was the most notable development BIB005 . The methodology of single-unit recordings was employed in many subsequent studies. Wise and his colleagues observed that neurons in premotor cortex were attuned to movement direction several seconds before movements started BIB003 . In these experiments monkeys were instructed of movement direction, but had to withhold movements until a trigger stimulus 