Open Ephys electroencephalography (Open Ephys + EEG): a modular, low-cost, open-source solution to human neural recording

Abstract

Objective: Electroencephalography (EEG) offers a unique opportunity to study human neural activity non-invasively with millisecond resolution using minimal equipment in or outside of a lab setting. EEG can be combined with a number of techniques for closed-loop experiments, where external devices are driven by specific neural signals. However, reliable, commercially available EEG systems are expensive, often making them impractical for individual use and research development. Moreover, by design, a majority of these systems cannot be easily altered to the specification needed by the end user. We focused on mitigating these issues by implementing open-source tools to develop a new EEG platform to drive down research costs and promote collaboration and innovation.

Approach: Here, we present methods to expand the open-source electrophysiology system, Open Ephys (www.openephys.org), to include human EEG recordings. We describe the equipment and protocol necessary to interface various EEG caps with the Open Ephys acquisition board, and detail methods for processing data. We present applications of Open Ephys + EEG as a research tool and discuss how this innovative EEG technology lays a framework for improved closed-loop paradigms and novel brain-computer interface experiments.

Main results: The Open Ephys + EEG system can record reliable human EEG data, as well as human EMG data. A side-by-side comparison of eyes closed 8-14 Hz activity between the Open Ephys + EEG system and the Brainvision ActiCHamp EEG system showed similar average power and signal to noise.

Significance: Open Ephys + EEG enables users to acquire high-quality human EEG data comparable to that of commercially available systems, while maintaining the price point and extensibility inherent to open-source systems.

Figure 1.

Schematic for Open Ephys with EEG connectivity. a) Scalp potentials are registered by an electrode cap, which are then sent to b) the EEG breakout board that interfaces with c) Intan Amplifier based headstages. Amplified signals are sent via an SPI cable to the d) Acquisition Board and are then sent via USB to e) a PC for visualization and data storage.

Figure 2.

Open Ephys EEG breakout board. a) Unpopulated board showing Pak-50 connector positions for connecting to electrode cap (green), Omnetics positions for connecting to Amplifier, headstage (blue), header pins for re-referencing, and sourcing additional electrophysiological signals such as EMG or EOG (orange). b) Default connection; [top figure] jumpers placed in the red positions route four single EEG cap connections (Pak-50) to the Omnetics connector to record all EEG channels, jumpers placed in the orange positions route EEG reference position to Omnetics connector; [bottom figure] the board layout corresponding to the default schematic shows jumpers colored red and orange for correct placement for the positions detailed in the schematic. c) Re-routing connections; [top figure] jumpers placed in the blue positions re-route up to four connections from the Omnetics connector on the far right to allow recording other biopotential measurements such as EMG or EOG, jumpers placed in the green positions re-route the reference connections for the corresponding Omnetics connectors, allowing you to specify the reference of the signal; [bottom figure] the board layout corresponding to the re-routing schematic shows jumpers colored in blue and green for correct placement for the positions detailed in the schematic, as well as two connector pins of the same colors to show the corresponding connections for external electrodes (out of figure). For top figures in b) and c) solid black lines indicate recorded sources, dotted black lines indicate nonrecorded sources.

Figure 3.

Input and output connections of Open Ephys Acquisition Board. a) +/−5V analog output, b) +/−5V analog input, c) 0/5V digital output, d) 0/5V digital input, e) SPI terminal for connecting to Intan based headstages. Photoadapted from www.open-ephys.org.

Figure 4.

Open Ephys GUI; example recording setup. a) List of processors that can be dragged and dropped into the b) signal chain. c) The Rhythm FPGA initializes the Acquisition Board and opens up communication for data to be sent along the signal chain. d) A bandpass filter filters the data within any prescribed range. e) The LFP Viewer allows for visualization of incoming data. f) Data is easily saved by clicking on the drop down button and entering in the relevant information.

Figure 5.

Schematic of standard 10–20 electrode cap layout. Recording electrodes Pz and C3 (light green) used for analysis in sections 4.1 and 4.2, reference electrode (light blue), and ground electrode (light grey).

Figure 6.

Occipital alpha band activity from three different subjects, a), b), and c) taken from the Pz electrode during an alert (eyes open) and a rest (eyes closed) state. The top left plot for a, b, and c are the raw, unfiltered voltage traces of the transition state for eyes closed to eyes open (denoted by the dotted black line). The bottom left plot for a, b, and c are the filtered voltage traces (0.1–100 Hz band-pass, 60 Hz notch) that show a slight reduction in power, but still strong presence of alpha band activity. Finally, the right plot for a, b, and c show the power spectral density plots of eyes open (dotted grey line) and eyes closed (solid black line) states. There is a prevalent alpha peak (around 10 Hz) for each individual in the eyes closed state, and a strong reduction in alpha power in the eyes open state.

Figure 7.

Signal comparison between Brainvision actiCHamp system and Open Ephys + EEG system using the same Brainvision actiCap electrode cap in recording eyes closed alpha activity. Transition from eyes closed to eyes open in the [top figure] Brainvision system and the [bottom figure] Open Ephys + EEG are very similar. b) Average PSD of Brainvision (dotted black line) and Open Ephys + EEG (solid blue line) for eyes closed epochs.

Figure 8.

Grand average sensory evoked potential at electrode C3 (solid black) of perceived stimuli from tactile detection task. Stimulus delivery occurs at 0 msec. Standard deviation for average of three subjects (dotted black), 78 trials each. P300 deflection shown at ~ 400 ms in light green (p < 0.05). A 40 Hz low-pass filter was used for visualization.

Figure 9.

Example of acquired EMG data from forearm flexion using Open Ephys with the EEG adaptor (Fig 2.). EMG electrodes were placed on the subject’s forearm with a reference on the elbow; the forearm was at rest for the first 2 seconds, and then the forearm was flexed from 2–3 seconds, before resting once more.