Abstract
Toward addressing many neuroprosthetic applications, the Neurochip3 (NC3) is a multichannel bidirectional brain-computer interface that operates autonomously and can support closed-loop activity-dependent stimulation. It consists of four circuit boards populated with off-the-shelf components and is sufficiently compact to be carried on the head of a non-human primate (NHP). NC3 has six main components: (1) an analog front-end with an Intan biophysical signal amplifier (16 differential or 32 single-ended channels) and a 3-axis accelerometer, (2) a digital control system comprised of a Cyclone V FPGA and Atmel SAM4 MCU, (3) a micro SD Card for 128 GB or more storage, (4) a 6-channel differential stimulator with ±60 V compliance, (5) a rechargeable battery pack supporting autonomous operation for up to 24 h and, (6) infrared transceiver and serial ports for communication. The NC3 and earlier versions have been successfully deployed in many closed-loop operations to induce synaptic plasticity and bridge lost biological connections, as well as deliver activity-dependent intracranial reinforcement. These paradigms to strengthen or replace impaired connections have many applications in neuroprosthetics and neurorehabilitation.
Highlights
Bidirectional brain-computer interfaces (BBCI) can simultaneously record from and stimulate brain sites
The results of this study suggested that over 17 successive nights of epidural stimulation phaselocked to oscillatory slow-wave sleep (SWS) activity facilitated learning of a neuroprosthetic task
Up to 700 of these pulses can be output per second if the voltage is limited to ±50 V. We have found this range adequate for reaching the 0.5–5 mA threshold for evoking motor output with relatively low impedance electrodes (∼10 kOhm at 1 kHz) placed at the cortical surface, and for delivering 10– 500 μA stimuli through higher impedance microelectrodes as found in the Utah electrode array (Utah Array)
Summary
Bidirectional brain-computer interfaces (BBCI) can simultaneously record from and stimulate brain sites. Motor unit action potentials from triceps are discriminated by a dual time-amplitude window running on the NC3 MCU; discriminator events trigger stimulation at a spinal site from which elbow extension is evoked with larger stimulation currents. A useful feature of the NC3 is its capacity for autonomous operation for long periods of time, providing continuous functionality during different behavioral states such as sleep and waking states This capability was exploited in a study to examine state-dependence of vagally evoked cortical potentials (VEPs) in response to stimulation of the vagus nerve (Rembado et al, 2021). NC3 was used to evoke action potentials from stimuli delivered at a neighboring site (as in section “Cortical Stimulation” above) throughout wake and sleep, and the strength of stimulus-evoked responses varied between states in a manner that depends on the specific neuron (Figure 10B) (Yun, unpublished). In the experiments performed to date, the weights and delays of the SNN were arbitrarily chosen to demonstrate its closed-loop operation; future implementations could vary these parameters in user-defined ways (Mishler, unpublished)
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