D2.1 Requirements and Specification - CORBYS

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D2.1 Requirements and Specification due to a reduction in skin/electrode interface effects for dry electrodes over the length of the trial, RMS artefact values for dry electrodes were 8.2 dB lower than wet electrodes. This outcome may be attributed to the geometry of the dry electrodes housing (Searle & Kirkup, 2010). 4. Usability: wet electrodes with its lengthy application time, subject discomfort and irritation, the need for extensive caregiver support and training, have made the technology impractical. Instead the dry electrode with its easy and reliable application in a home environment and its use without any skin preparation or gel application, may help the widespread use of BCI technology (Mason, 2005). 5. Stability: the wet electrode signal tends to become unstable over time due to the use of conductive gel, it dries during prolonged measurements reducing signal quality and signal performance. On the other hand dry electrodes are like wet types at the beginning but become more stable with ongoing time (Matteucci et al, 2007). 6. Portability: typical EEG equipments using wet electrodes are not mobile, they have high power consumption and a mass of wires connecting the cap to a PC that process, store, display and analyse the signals. However, the requirements for power, control, and read-out are reduced in dry electrodes, making a more feasible a wireless solution (Sullivan et al, 2008). Some of these features have been corroborated by research teams that have developed working prototypes of dry EEG sensors, and demonstrated that the signal obtained can be largely comparable to wet electrodes (Popescu, 2007). They have also been tested in BCI paradigms such as alpha/mu rhythms and flash visual evoked potential (FVEP). They showed a high correlation with signals recorded in parallel with wet standard electrodes in the alpha/mu rhythms experiment and a negligible difference in the FVEP experiment (Gargiulo et al, 2010). In addition to the previous results, the N100 auditory evoked potential, the auditory evoked P300 event related potentials and the sensory-motor rhythm (SMR) for 2-class motor imagery were also tested. Dry electrodes showed a signal close to the one recorded with gel-based electrodes between at least 7 Hz and 44 Hz Bristle-sensors (Grozea et al, 2011). 150
D2.1 Requirements and Specification Figure 40: EEG Dry Devices On the other hand, clinical acquisition systems for dry devices are not available on the market, instead, several commercial offerings have been made that could fulfil BCI requirements. An important key feature is that these systems are focussed on every day use of EEG equipment over a long-term period. For that reason, particular attention is paid to the headset design which has to be lightweight, comfortable and at the same time ensures coverage in the area of interest. One of the main key points of these systems is that they are portable since they incorporate wireless connection link with a computer. For instance, the Enobio system (Figure 40(b)) developed by Starlab, is a device with 4 dry active digital electrodes placed on the forehead, it shows similar responses compared to Active Two system from Biosemi (gel based device) (Cester et al, 2008; Riera et al, 2008;, Starlab, n.d). A Neural Impulse Actuator (NIA) (Figure 40(e)) is also available, a OCZ Technology product, it uses three carbon-fibre sensors placed in the forehead (OCZ Technology, n.d); instead, NeuroSky provides with MindSet (Figure 40(d)) a sensor system with just one single dry sensor at EEG position FP1 (International 10-20 system) (NeuroSky, n.d). These are wearable wireless devices, however all of them have constrained in EEG research possibilities since the sensor location do not cover the areas of interest in the BCI community (like the sensory-motor cortex) and they sense the active in the prefrontal areas focalising in mental activity such as attention and meditation. So far, Epoch (Figure 40(a)), commercialised by Emotiv, is the most complete in fulfilling BCI requirements: it collects data from 14 saline sensors (they eliminate the electrolytic gel of wet electrodes but a saline solution has to be used) placed in a wearable wireless neuroheadset, located at AF3, AF4, F3, F4, F7, F8 FC5, FC6, T7, T8, P7, P8, O1 and O2. P300 experiments were already conducted (Campbell et al, 2010). Unfortunately the Epoch neuroheadset does not support experiments involving motor cortex (brain area widely used in BCI) (Emotiv, n.d). 151
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D2.1 Requirements and Specification - CORBYS

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