Abstract
BackgroundBrain computer interfaces (BCI) based on electro-encephalography (EEG) have been shown to detect mental states accurately and non-invasively, but the equipment required so far is cumbersome and the resulting signal is difficult to analyze. BCI requires accurate classification of small amplitude brain signal components in single trials from recordings which can be compromised by currents induced by muscle activity.Methodology/Principal FindingsA novel EEG cap based on dry electrodes was developed which does not need time-consuming gel application and uses far fewer electrodes than on a standard EEG cap set-up. After optimizing the placement of the 6 dry electrodes through off-line analysis of standard cap experiments, dry cap performance was tested in the context of a well established BCI cursor control paradigm in 5 healthy subjects using analysis methods which do not necessitate user training. The resulting information transfer rate was on average about 30% slower than the standard cap. The potential contribution of involuntary muscle activity artifact to the BCI control signal was found to be inconsequential, while the detected signal was consistent with brain activity originating near the motor cortex.Conclusions/SignificanceOur study shows that a surprisingly simple and convenient method of brain activity imaging is possible, and that simple and robust analysis techniques exist which discriminate among mental states in single trials. Within 15 minutes the dry BCI device is set-up, calibrated and ready to use. Peak performance matched reported EEG BCI state of the art in one subject. The results promise a practical non-invasive BCI solution for severely paralyzed patients, without the bottleneck of setup effort and limited recording duration that hampers current EEG recording technique. The presented recording method itself, BCI not considered, could significantly widen the use of EEG for emerging applications requiring long-term brain activity and mental state monitoring.
Highlights
Electro-encephalography (EEG) is the oldest brain imaging technology, and among non-invasive methods it still offers the highest temporal resolution
We introduce a new EEG cap design with few electrodes and show that the much sought-after ‘dry electrode’ technology is surprisingly frugal and accurate enough for excellent online discrimination
The main object of the study was to compare the Information Transfer Rate (ITR) obtainable with the dry cap with that previously established for the full cap for an existing paradigm using the same subjects
Summary
Electro-encephalography (EEG) is the oldest brain imaging technology, and among non-invasive methods it still offers the highest temporal resolution. EEG can monitor brain events very responsively in time, it suffers from high inter-trial variability and spatial mixing: numerous electrical sources active at any given time in the brain are superimposed onto the scalp across distances of over 5 cm [4] These limitations have led to the assumption that many electrodes are necessary, and that one needs to average signal features across time or repeated trials to accurately discriminate mental states. Brain computer interfaces (BCI) based on electro-encephalography (EEG) have been shown to detect mental states accurately and non-invasively, but the equipment required so far is cumbersome and the resulting signal is difficult to analyze. A novel EEG cap based on dry electrodes was developed which does not need time-consuming gel application and uses far fewer electrodes than on a standard EEG cap set-up. The presented recording method itself, BCI not considered, could significantly widen the use of EEG for emerging applications requiring longterm brain activity and mental state monitoring
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