Cerebus system

We recorded force and neural signals using the Cerebus system (Blackrock Microsystems, Inc.). We did not attempt to identify spikes belonging to individual neurons, but instead used a simple multi-unit signal, registering a spike whenever an electrode signal exceeded -5 times the RMS amplitude for that electrode, as calculated at the beginning of every session. We computed firing rates as the number of spikes detected in each electrode during each time bin, divided by the 50ms bin duration. We excluded neural channels for which the firing rate, averaged over the entire training dataset, fell below 0.5 Hz. The mean number of remaining channels was 78±8 (mean±1SD).
We fitted an exponential sinusoidal tuning curve to the firing rate data, and identified a preferred direction (PD) for each of the multi-unit signals included in our analyses by assuming a lag of 150ms between neural activity and force. We used bootstrapping with 100 repetitions and a generalized linear model with a Poisson noise model [36 (link)] to compute the statistics leading to the identification of the PD.

Neurophysiological data was recorded via chronically implanted multi-microelectrode (“Utah”) arrays that were located in area V4 (monkeys B and F) or V1 (monkey K) (see [19 (link)] for details regarding surgery and implantation). Each electrode was spaced 400 μm from its neighboring electrodes, and 1.5 mm (0.6 and 1.5 mm for monkey K) long. Neural data from monkeys B and F was recorded at a sampling rate of 24414.1 Hz using a Tucker Davis Technology system and at 30 kHz for monkeys K and Br on a Blackrock Microsystems Cerebus System. Following 13 sessions in monkey B and 6 sessions in monkey F, permanent focal aspiration lesions of isohemispheric primary visual cortex (V1) were performed (see [23 (link)] for details). After the lesion, post-lesion data were recorded in 15 and 6 sessions for monkey B and monkey F, respectively. To confirm the visual deficit (scotoma) following the V1 lesion, monkeys performed a perimetry task covering the lower right quadrant (see [20 (link)] for details). Data from monkey K was collected in two sessions. Layer-resolved V1 data was recorded from monkey Br using a linear microelectrode array, consisting of 22–24 active microelectrodes, linearly spaced 0.1 mm apart, with impedances ranging 0.2–0.8 MΩ at 1kHz (UProbe, Plexon). Electrical reference for data from the UProbe was the probe shaft.

Benchtop recordings were taken in both acute and chronically implanted rats in a stereotaxic frame with the head fixed by ear bars and a bite bar. Recordings were referenced to a local reference wire on the implanted electrodes and the stereotaxic frame served as ground. Data were acquired at 30 kHz with a Cerebus system (Blackrock Microsystems) with 0.25 microvolt/bit resolution. Brain activity was modulated by altering the anesthetic depth during the experiment. This was accomplished by changing the concentration of isoflurane provided, or in several acute studies by transitioning from isoflurane to morphine. In one acutely implanted rat, the electrode was advanced in 50 micron increments over a 600-micron range to observe the spatial localization of multi-unit activity waveforms.

Neural signals were recorded in rats two days after surgery followed by weekly recordings for over two months. Each recording session lasted for 10 min. Data were acquired using a Cerebus system (Blackrock Microsystems, Salt Lake City, UT) and each channel was recorded at 30,000 samples per second. To remove any movement artifacts associated with recording from awake animals, a common average reference was subtracted from each channel^{21 (link)} using a custom-routine in MATLAB (Mathworks, Natick, MA). Spikes were detected by signal magnitude excursions of beyond a threshold. Each channel had a specific threshold based on six standard deviations of the baseline noise for that channel. The captured spikes were then sorted for each channel into well-resolved units in Offline Sorter V.3 (Plexon, Inc, Dallas, TX) and confirmed by visual inspections. Peak-to-peak amplitude (V_p-p) and spike rate were calculated for each unit. The signal-to-noise ratio (SNR) for each unit was defined as the mean V_p-p over root mean square (RMS) value of the noise for the corresponding unit.