GoKawiilExperimental procedures and approval
All procedures were performed under protocols approved by the Northwestern University Institutional Animal Care and Use Committee. Three adult rhesus macaque monkeys were trained in the experimental procedures described below.
Surgery
In each monkey, we surgically implanted a 96-electrode silicon electrode array (Blackrock Microsystems) into M1 contralateral to the arm used to control the cursor. During surgery, monkeys were anaesthetized with isoflurane (2–3%). Anaesthetic depth was assessed at all times by monitoring jaw muscle tone and vital signs. A craniotomy was performed above M1, and the dura was incised and reflected. Electrode arrays were implanted in the proximal arm area, as determined by referencing cortical landmarks and by intraoperative electrical stimulation with a silver ball electrode (2–5 mA, 200-μs pulses at 60 Hz). During stimulation, a reduced level of isoflurane (minimum alveolar concentration lower than 0.5) was supplemented with intravenous remifentanil (0.15–0.30 μg per kg per min) to reduce suppression of cortical excitability. The electrode array was positioned on the crown of the right precentral gyrus, approximately in line with the superior ramus (medial edge) of the arcuate sulcus. The electrode shank length was 1.5 mm. The preamplifier was grounded to the CerePort pedestal and referenced to two platinum wires with 3-mm exposed wire length placed under the dura. A piece of artificial pericardium (Preclude, Gore Medical) was applied above the array, and the dura was closed using 4.0 sutures (Nurolon, Ethicon). Another piece of pericardium was applied over the dura, and the craniotomy partially filled with two-part silicone (Kwik-Cast, World Precision Instruments). The craniotomy was then closed, and the skin was closed. All monkeys were given postoperative analgesics buprenorphine and meloxicam for two and four days, respectively.
Neural signal recording
All neural and behavioural data were recorded using a 96-channel Multichannel Acquisition Processor (Plexon). LFP signals were sampled at 1 kHz and band-pass-filtered from 200 Hz to 300 Hz. We deliberately chose this band to be as conservative as possible, because previous studies suggested that components of LFPs above 150 Hz were most contaminated by spike leakage25,28,29. We computed the spectral power of each electrode from the band-passed LFP by applying a 256-point Hanning window (overlapped by 206 points) and calculating the squared amplitude (power) of the windowed signal’s discrete Fourier transform, yielding 50-ms-binned HGA. Multiunit spikes (threshold crossings) were high-pass-filtered at 300 Hz and sampled at 40 kHz. The signals were further downsampled to 1 kHz. The threshold on each electrode was set at 4.5 s.d. from the root-mean-square baseline activity on the electrode. Note that we also ran a subset of experiments using a lower threshold of 3 s.d. (in the ‘hash’, to reduce the possibility that we were missing some smaller-amplitude spikes near the electrode) and obtained similar results—monkeys could still decorrelate the signals. We defined spikes as unsorted threshold crossings on each electrode. To identify shunted electrodes, we calculated the pairwise R of the spike rates, binned at 1 ms, between all electrode pairs, and removed any electrode that had a pairwise R higher than 0.3 with any other electrodes. For all the analyses except the spike-triggered HGA analysis, we used 50-ms bins for both HGA and spike rates. The above procedures were performed for each file separately. In each file, we also removed electrodes on which the spike rates averaged less than five spikes per second. The available electrodes were used for further analysis after these shunted and low-spike-rate electrodes were removed.
Hand-control task
Three adult rhesus macaques, two male and one female (monkeys C, J and M), used a two-link manipulandum to move a cursor (1-cm-diameter circle) within a rectangular planar workspace (20 cm × 20 cm). The task was a four-target centre–out task, with 2-cm2 outer targets spaced at 90° intervals around a circle of radius 10 cm. Each trial began with a target at the centre of the circle. After a random hold time of 0.5–0.6 s within the centre target, a randomly selected outer target was illuminated and the centre target disappeared, signalling the monkey to start a reach. The monkey needed to reach the outer target within 1.5 s and hold for a random amount of time between 0.2 s and 0.4 s to obtain a liquid reward. The task was performed in 10-min files.
ONF task
To maximize monkeys’ ability to control the cursor, we first selected electrodes on which spiking correlated highly with movement during the reach task. First, we computed the absolute value of the Spearman correlation coefficient (|R|) between binned spikes and HGA on each electrode and the velocity in the X and Y directions, respectively, for a total of four correlations (spikes–X, spikes–Y, high gamma–X and high gamma–Y) over a 10-min period. Next, we added the |R| corresponding to X and Y directions for each signal (that is, spikes–X + high gamma–Y; spikes–Y + high gamma–X), giving us two |R| values corresponding to the two possible arrangements of the signals for control. We then selected the electrodes with highest cumulative |R| as inputs (CEs) to use for the ONF task. Later, after monkeys learned to perform ONF control, we selected electrodes that had higher spike–HGA correlations (see below) during hand control.
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