It is well known that building a setup for recording neural spikes is not trivial. Many older electrophysiological systems are bulky, expensive and difficult to use. Their system components require elaborate shielding and grounding to reduce the electromagnetic interference. Recently, the technology advances have been quite rapid. It has become possible to build the millimeter-scale electrophysiology amplifiers from commercial off-the-shelf components. Here, I would like to share an amplifier design that is capable of intracellular and extracellular recording, as well as LFP, EMG, and EEG. It is small, easy to build and extremely cheap. The system uses differential mode of recording, thus eliminating the need of extensive shielding from environmental noise. The circuit had been tested on freely-moving animal exposed to their regular environment and was able to record neural spikes with a good signal-to-noise ratio (SNR).
The amplifier is divided into two stages. The first stage is an Instrumentation Amplifier (IA) with the differential input. The input signals have DC removed (with a high-pass filter) with highly tuned RC components. It is important to have very closely matched components to have high Common Mode Rejection Ratio (CMRR). The capacitors Cpf and Cnf are for power supply regulation with typical value of 0.1µF. The gain of the instrumentation amplifier is typically set in the range from 100 to 1000. The second stage is a second order low-pass filter (sellen-key). The gain of this filter should be maintained about 10; resulting in a total gain of the amplifier of ~10,000.
Following are the example values for the filter amplifier:
Chp1 = CHR = 100nF, RH1 = RHR = 4.68MΩ; Low Cut-off frequency = 0.34Hz;
R1= 10kΩ, R2= 150kΩ, C1= 1nF, C2= 1nF, R3= 10kΩ, R4= 100kΩ; High Cut-off frequency = 4,109Hz;
NOTE: For multiple channels build the same circuit and use them in parallel. For single reference electrode just short all the inverting terminals (-) of all instrumentation amplifiers and use only one reference high-pass filter (CHR, RHR) instead.
Central-nervous-system based neuromotor prosthesis (NMP) holds a great deal of promise for complete spinal cord injury (SCI) yet is still far from the clinical use. Cortical-level NMP uses direct cortical recording and requires craniotomy for implanting a microelectrode array in the motor cortical area of an injured person. First successful human trial of the cortical NMP in a quadriplegic person, the BrainGate, was done by Dr. Donoghue and colleagues back in 2006. Last year, Dr. Edgerton and colleagues have applied a spinal NMP to train a paraplegic person to stand and walk on a treadmill. As these cortical and spinal NMPs are reaching maturity, the question emerges, whether all people with SCI can benefit from this technology. In this post, I will try provide my perspective about the potential technology users.
SCI has different severity, motor complete or incomplete, and occurs at different spinal levels, from cervical to thoracic and lumbosacral, resulting in quadriplegia or paraplegia. NMP is potentially most viable for motor-complete SCI since people with incomplete SCI can benefit from extensive rehabilitation training. Quadriplegics with motor-complete SCI would likely benefit the most from this technology. One of major challenges for implementation of cortical NMP for quadriplegics is the availability of real-time adaptive decoding algorithms for controlling the body balance, needed to enable standing and locomotion. As a quadriplegic person completely loses his/her posture control, it is unlikely that they could use existing decoding algorithms for cortical NMP for standing and stepping. Still, such a person can use the cortical NMP for controlling an external device or an upper limb (through stimulation of peripheral nerves or muscles). Volitional control of an individual hand muscle by this kind of cortical CNMP has already been demonstrated in non-human primates by Dr. Fetz and colleagues.
It is not clear whether paraplegics can benefit from the NMP technology to the same degree as quadriplegics. As paraplegics have useful hand and arm functions, cortical NMP might be too risky and invasive of a procedure to justify the potential benefits. Perhaps, a spinal NMP controlled by a hand or a processor that interprets the person’s movement intent can be more beneficial for standing and walking. In a spinally-intact person, the leg movements and locomotion require no visual feedback and are adjusted in time and space through a local feedback circuitry in lumbo-sacral region of spinal cord. Provided that this feedback loop is intact in the paraplegic person, would be extremely beneficial to use this loop along with the spinal Central Pattern Generator (CPG) for enabling the locomotion. Recent human studies by Edgerton and others indicate simply turning these spinal neural circuits ON and OFF might not be enough for standing and/or stepping. Hopefully, with more robust decoding and encoding algorithms, the spinal NMP might become a viable clinical solution for paraplegics.
Considering these arguments, I would like to suggest that an ideal candidate for cortical NMP would be a quadriplegic, while an ideal candidate for spinal NMP would be a paraplegic.