Wearable EEG and beyond

Abstract

The electroencephalogram (EEG) is a widely used non-invasive method for monitoring the brain. It is based upon placing conductive electrodes on the scalp which measure the small electrical potentials that arise outside of the head due to neuronal action within the brain. Historically this has been a large and bulky technology, restricted to the monitoring of subjects in a lab or clinic while they are stationary. Over the last decade much research effort has been put into the creation of "wearable EEG" which overcomes these limitations and allows the long term non-invasive recording of brain signals while people are out of the lab and moving about. This paper reviews the recent progress in this field, with particular emphasis on the electrodes used to make connections to the head and the physical EEG hardware. The emergence of conformal "tattoo" type EEG electrodes is highlighted as a key next step for giving very small and socially discrete units. In addition, new recommendations for the performance validation of novel electrode technologies are given, with standards in this area seen as the current main bottleneck to the wider take up of wearable EEG. The paper concludes by considering the next steps in the creation of next generation wearable EEG units, showing that a wide range of research avenues are present.

Keywords: Electrodes; Electroencephalography; Wearable.

Fig. 1

A conventional lab based EEG set up with metal electrodes on the scalp held in place by a cap with wires to connect to a recording instrumentation box

Fig. 2

a An example traditional Silver/Silver-Chloride EEG electrode, typically 1 cm in diameter. b An example placed in a cap with a conductive gel present to ensure a good electrical contact is made between the electrode metal and the scalp

Fig. 3

Examples of current dry fingered EEG electrodes. a Wearable sensing [40]. b Cognionics [41]. c Neuroelectrics [42]. d IMEC [5]. e Mindo [43]. f g.tec g.SAHARA [44] Figure originally taken from [45]

Fig. 4

Personalisation parameters in fingered EEG electrodes for making a better connection to the scalp

Fig. 5

Modern wearable electrode design is a very multi-disciplinary problem requiring the different aspects to be tackled holistically

Fig. 6

Comparison of two different electrode configurations shows how the presence/positioning of a reference electrode can distort the signals collected from a test electrode. a Test set up using a two electrode electro-physiological recorder. Electrodes in the blue test case (1 and 2) are placed on the wrist. Electrodes in the red test case (A and B) are on the wrist and chest. No third reference electrode is present when using the camNtech actiwave. b Wrist only recordings are very low in amplitude with no clear QRS complex. c With the chest electrode present a QRS complex apears at the wrist

Fig. 7

Effect of artificially added random noise on the reported correlation coefficient between two otherwise identical EEG traces. a Noise source S(f) is used to artificially corrupt the recorded EEG with white Gaussian noise. b The noise amplitude and analysis length have a substantial effect on the reported correlation coefficient

Fig. 8

An example EEG head phantom made of conductive gelatine [67]. This has electrodes embedded on the inside to allow pre-recorded EEG to be re-played and measured on the surface

Fig. 9

Power trade-off in wearable EEG units, taken from [17]. Some power is used for real-time onboard data compression/reduction, but this leads to large power savings in the wireless transmission stage, allowing the overall device operating lifetime to be improved

Fig. 10

An overview of the evolution of EEG modalities, with reducing size and increased portability. Ear EEG picture originally from [84]. Some wearable units present only on the head are now available, and the research challenge is in creating beyond wearable devices which are flexible and socially discrete for long term use

Fig. 11

Examples of conformal temporary tattoo electrodes for head electrophysiological monitoring. a Forehead monitoring from [89]. b On the ear monitoring from [90]. c Behind the ear monitoring from [91]

Fig. 12

Concept of future closed loop EEG systems which both sense and actuate. Here EEG is used to guide a.c. transcranial current stimulation (tACS), adjusting the settings (e.g. phase, frequency) based upon the underlying EEG. This necessitates real-time signal processing, and real-time removal of the tACS stimulation artefact