OpBox: Open Source Tools for Simultaneous EEG and EMG Acquisition from Multiple Subjects

Overall architecture of the OpBox system. On the leftmost are the subjects and devices that may be placed within a behavioral chamber. Arrows in the top part of the figure indicate the flow of electrophysiology (EEG and EMG) data to the amplifier, data acquisition card, and software scripts. Additional arrows indicate the ability to add behavioral and video modules as well. Pictures of modules described in this paper in detail (OpBox amplifier and MATLAB GUI) are shown above their respective nodes.

OpBox amplifier designs. A, Circuit schematic for one channel of the electrophysiology amplifier. For clarity, power connections and decoupling capacitors are not shown. B, Photograph of a four-channel amplifier PCB with all components. All channels in this four-channel amplifier share the same reference and there is an additional ground connection. C, Photograph of a three-channel amplifier PCB with all components. In this three-channel amplifier, channels 1 and 2 share a reference, while channel 3 is a separate bipolar channel. Details for B, C are as follows: Signal inputs via screw terminals for unamplified electrophysiology input signals (EEG and EMG). Single channel amplifier components. Amplifier chips are highlighted with dashed yellow outlines. Leftmost is an instrumentation amplifier (INA114, Texas Instruments) with low offset voltage (50 μV), drift (0.25 μV/°C), and high common-mode rejection (115 dB at G = 1000), set for a fixed voltage gain of 10×. The middle and rightmost chips are operational amplifiers (TLC277, Texas Instruments) with gain of 10× each, for a total system gain of 1000×. Each also implements one of two successive two pole active high-pass (0.28 Hz, Q ≈ 0.71) or low-pass (200 Hz, Q ≈ 0.71) filters in Sallen–Key configurations (6 dB/octave). Output connections for amplified signals. Four-channel amps use a single RJ45 (Ethernet) connector for all channels, three-channel amps use BNC connectors for each channel. LED indicators to check power connections and battery level for the positive and negative rails (±3V). Each power rail uses two AA batteries in series (+3 V and −3 V). Photographs in B, C were adjusted to improve contrast and remove lab logo for blind review.

Simultaneous EEG/EMG recordings from rat subjects using OpBox scripts. A, Data stream from a single subject. The largest plot shows overlaid amplified voltages from all channels (−1 to +1 V) over the past 5 s. The overlapping blue and green traces represent EEG, while the red trace is EMG from nuchal muscles. Rectangular traces in the EEG plot represent digital behavioral event markers (each color of yellow, magenta, and cyan represents a different type of behavioral event). Box number and subject ID are shown to the left of the plot. The top right of this plot shows elapsed time since the start of the recording. Below each EEG plot, a real-time FFT based power spectrum (0–100 Hz, plotted with log power on the y-axis) is shown to the left. The mean accumulated evoked potential (1-s total window length, auditory stimulus onsets are at time = 0 ms) is shown in the bottom right; y-axis is in amplified volts and the number of stimuli is indicated in the top left corner. This GUI is running in MATLAB on a Windows desktop computer. B, Full GUI window showing simultaneous acquisition from twelve subjects. Eight rats are performing an auditory Go/No-go discrimination, each with their own mean evoked potentials displayed. The last four subjects are recordings of spontaneous behavior, i.e., while not performing a task, and thus have no evoked potentials. This multisubject GUI allows the experimenter to monitor all subjects simultaneously with a consistent configuration.

Amplification of a 10-Hz test signal. A, Comparison of power spectrum density over a 5-min period, where a 10-Hz, 100-μV peak-to-peak sine wave was sent to one channel of each amplifier using a function generator and voltage divider circuit. Spectrograms were taken of the amplifier outputs using 4 s, non-overlapping windows (n = 75 windows per amplifier). Spectra are shown with shaded 95% confidence intervals. No line filters were used. Signals were also sent via an inexpensive commutator used by our lab to verify their performance. Commutators were spun by hand. B, Approximate rotational position in radians of our commutator where 0 is equivalent to the initial position. C, Raw recording of the test signal. D, Spectrogram of the signal.

High sample rate tests. OpBox and Grass amplifiers were sampled at 100 kHz via OpBox scripts in MATLAB, and the Intan amplifier was sampled at their max rate of 30 kHz using the Open Ephys GUI. A, Phase distortion from Grass and OpBox amplifiers at 5 different frequencies; 1-mV peak-to-peak sine wave signals at varying frequencies were split from a function generator and sent to one channel of the amplifier via a voltage divider as well as directly to the DAQ. Data from both inputs were sampled simultaneously at 100 kHz. Recordings were Hilbert transformed and the difference in phase angle was calculated for each frequency tested. B, Measured gain for Grass and OpBox amplifiers at each frequency. Using the same recordings, RMS values over the entire 10 s were calculated and the ratio of amplified signals versus direct to DAQ signals divided by 1000 (to match the voltage divider output) are reported for each frequency. C, Amplifier responses to a step response, a 10-s, 500-μV unipolar rectangular wave. A simulated version of the amplifier input is shown in the top plot. D, Amplifier responses to an approximate impulse response, a 1-ms, 500-μV unipolar pulse. A simulated version of the amplifier input is shown in the top plot.