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Single Neuron Recording: Applications to Brain Computer Interfaces and Neural Prostheses Jonathan Scott and Elizabeth Lyman Design Electrodes are used to detect and record neural signals A key component in any single neuron recording device Microwires Arrays of soldered wire, typically 30-50 µm in diameter Standard materials for the conducting core are stainless steel, platinum or tungsten, and insulating materials such as Teflon Composed of eight or more wires, spaced about 250 µm apart Effectiveness of microwire is influenced by the cut of the tip of the wire: blunt cuts record a higher quality signal, angled cuts a lower quality signal Blunt cuts are more damaging to the brain upon implantation Silicon Arrays Composed of layers of conducting and insulating material on a silicon base Probe with receptors extends from the array, still connected to the dielectric body The probe array is about 15 µm thick, and can be up to 3000 µm long The probe contains a 1 cm long, 5 µm thick flexible, ribbon cable that allows the array to move up and down as the brain pulses Planar silicon arrays are one sided, three dimensional arrays are double sided Function Single neuron recording is based on action potentials Electric activity created by a depolarizing current The electrodes detect the action potentials as they leave the brain The spike waveforms are recorded, compiled and analyzed The ‘white noise’ is filtered out to allow for precise interpretation Signal to noise ratio (SNR) is computed as the ratio of the amplitude of the average waveform to the standard deviation of the waveform noise Higher SNR is preferred Interpreting filtered data: The intensity of the recorded activity is related to the direction of movement When plotted in three dimensions, the action potential recordings form a cosine wave Wave is ‘tuned’ to the movement direction The preferred direction can be singled out by the intensity of the action potential firing The intentions of the user can be decoded by representing each unit with a vector in the preferred direction, weighted by intensity, and vectorially summing these two contributions Site Placement Placement of the electrodes on the probe can affect the functionality of the array Three possible locations for electrode placement: Edge: has better accessibility to neurons, enabling them to pick up on more action spikes. Electrodes on edge sites also tend to remain functional for longer periods of time than electrodes on the center, because the electrodes on the edge are less affected by the brain’s ‘foreign body response’. Center: does not record signals as well as the edge and tip; fair amount of impedance on the receptors Tip: seems to be the most receptive area to place electrodes, but is not endorsed, because it is very difficult to measure to reliability of electrodes placed on the tip of the probe. The expectation is that the tip sites have the lowest impedance because of a reduction in foreign body response due to the depth of the probe in the cortex. Current Research: Intended Movement Basis of research: goal signals vs. trajectory signals Ex. goal signal: intention to reach for an apple Ex. trajectory signal: indicates intended direction of the hand movement during the reach Research team at Caltech see goal signals as the key to more accurate implementation of neural signals to neural prostheses Goal signal will be present when the subject plans to move, whether or not the subject decides to act on their plans In their research, Caltech found that when the subject feels more strongly about a certain object or concept, the neural firings are more intense because more action potentials are released The ability to interpret preference can be a huge benefit to patients who wish to be able to communicate and act on their own Conclusion: With a goal in mind, trajectory and movement can be calculated by an external device Unintended Movement University of Pittsburgh’s Motorlab researches drift Spikes of brain activity can manifest as a ‘drift’ in which the user’s prosthesis can cause undesired movement Drift typically occurs when the user is unfocused, or rather, not focused on their prosthetic device This study defined and characterized the distinct Idle and Active neural states to relate to individual units’ tuning functions Neural states are strongly defined by their population firing rate pattern Helped to advance state detection by detecting the states on an instantaneous basis Findings suggest that the process of predicting kinematics can be improved by identifying state changes in tuning functions Unintended movement is a qualitative issue Pitt research team improved the technique used to measure state changes in the neural signals from the motor cortex This knowledge can be used to prevent drift from occurring Sustainability Ethical Complications Ability to Consent Due to its’ invasive nature, single neuron recording technology can not be tested on healthy and willing volunteers Single neuron recording technology would be most beneficial to patients with ‘locked-in’ syndrome The issue with this option is that completely locked-in patients, though they have the full cognitive ability to comprehend the necessary information, cannot actually communicate their consent Potential to Alter Personality Manipulating the brain may cause unpredictable changes in individuals and their personality It becomes necessary to identify which changes of the brain and its functions would shift a person’s identity Preserving the episodic memory, memory of autobiographical events, is extremely important Doing this will maintain the patients overall personality, even if some slight, unintentional alteration are made Responsibility Challenge to our common conception of responsibility: the motor activity carried out by a neural prosthetic is determined by the user’s brain waves, and the computer’s interpretation of those signals It’s important to define who the responsible entity is in different situations Enhancement vs. Restoration Should neural prostheses be limited to restoring normal function, or be used to enhance human abilities? Translation of plotted data Direction determined by intensityMovement determined by vectors Extraction of data Action PotentialCompile dataSignal Noise Ratio Signal Acquisition EEGECoGMicrowiresSilicon arrays Implementation Goal signals Drift Electrodes are precise tools, but short lived Most implants begin to degenerate after a few months, resulting in a decline in the devices ability to record individual action potentials Two main causes for the rapid degradation of the recording devices: 1) biological or physical response and 2) mechanical failure Biological/Physical Response Implantation triggers foreign body response, and causes damage to areas surrounding the site of implantation Microelectrode can tear connective tissue, also known as glia, causing unwanted blood flow in the extracellular environment of the brain Glial response can create a scar around the affected area, and essentially block off the microelectrode from the neural cells it intends to record Continued presence of the recording device can cause over activation of glial response, leading to heavy scarring, and cell death at the site of implantation Mechanical Failure Differences in the structure of the array and the brain can cause mechanical stress on the recording device The surface of the array is more adhesive in nature than brain tissue – reducing adhesiveness would result in less stress placed on both structures Electrode is rigid, while the brain is flexible Increasing the flexibility of the array would reduce injury due to strain Increased flexibility makes arrays more difficult to handle, and insert into the brain An ideal level of elasticity would be lower than that of the brain, or at the very minimum would match it ; at this point in time, the level of flexibility of arrays is several orders higher than the brain