A review of hybrid brain-computer interface systems <s> Conclusion <s> This work introduces a hybrid brain-computer interface (BCI) composed of an imagery-based brain switch and a steady-state visual evoked potential (SSVEP)-based BCI. The brain switch (event related synchronization (ERS)-based BCI) was used to activate the four-step SSVEP-based orthosis (via gazing at a 8 Hz LED to open and gazing at a 13 Hz LED to close) only when needed for control, and to deactivate the LEDs during resting periods. Only two EEG channels were required, one over the motor cortex and one over the visual cortex. As a basis for comparison, the orthosis was also operated without using the brain switch. Six subjects participated in this study. This combination of two BCIs operated with different mental strategies is one example of a “hybrid” BCI and revealed a much lower rate of FPs per minute during resting periods or breaks compared to the SSVEP BCI alone ( FP = 1.46 ± 1.18 versus 5.40 ± 0.90). Four out of the six subjects succeeded in operating the self-paced hybrid BCI with a good performance (positive prediction value PPVb > 0.70). <s> BIB001 </s> A review of hybrid brain-computer interface systems <s> Conclusion <s> Brain-computer interfaces (BCI) provide a new communication channel between the human brain and a computer without using any muscle activities. Applications of BCI systems comprise communication, restoration of movements or environmental control. Within this study we propose a combined P300 and steady-state visually evoked potential (SSVEP) based BCI system for controlling finally a smart home environment. Firstly a P300 based BCI system was developed and tested in a virtual smart home environment implementation to work with a high accuracy and a high degree of freedom. Secondly, in order to initiate and stop the operation of the P300 BCI a SSVEP based toggle switch was implemented. Results indicate that a P300 based system is very well suitable for applications with several controllable devices and where a discrete control command is desired. A SSVEP based system is more suitable if a continuous control signal is needed and the number of commands is rather limited. The combination of a SSVEP based BCI as a toggle switch to initiate and stop the P300 selection yielded in all subjects very high reliability and accuracy. <s> BIB002
To overcome limitations and disadvantages of conventional BCIs, different BCI systems or BCI and non-BCI systems can be combined to form a "hybrid BCI". In this paper, different methods of establishing a hybrid BCI system were discussed Advances in Human-Computer Interaction 7 and compared. Hybrid BCIs have been used for different applications such as 2D control of a cursor, target selection, and virtual environment. There are several advantages of sequential combination when one of the BCIs is used as a switch or different BCIs are used for different tasks sequentially. When combined sequentially, complicated tasks can be distributed to several stages in series. For each stage, a specific BCI can be used. An example of this approach is a virtual environment application BIB002 . Based on the required type of control commands, different BCI systems can be implemented. In BIB001 , one BCI (ERD) was used as a switch for another BCI (SSVEP) and the false positive rate was decreased for this sequential hybrid BCI. However, the main advantage of the simultaneous combination is that in general the accuracy can be improved if the BCIs are combined appropriately for all subjects. With adaptive pattern recognition algorithms, a hybrid system can adapt to subjects based on their performance. In addition, classification methods can use more BCI outputs. Hybrid BCIs combining different systems simultaneously may be more complicated than a single BCI and more difficult to be accepted by all users. Therefore, the paradigm design of a hybrid BCI plays a very important role in the overall performance of the system. Similarly, when a BCI system is combined with a non-BCI, which is not based on EEG signals, the system performance can be improved. In general, in a hybrid BCI, the complexity of the system paradigm is increased compared to a non-hybrid BCI. Therefore, the use of hybrid systems might be more complicated from the user's point of view. Thus, in designing a hybrid system paradigm, the complexity and user acceptability are important performance criteria to be considered carefully. Another consideration for the user acceptability is the number of channels used in a hybrid BCI system. In conclusion, although hybrid BCIs have shown great improvements in several performance criteria such as accuracy and information transfer rate, complexity of the system, and user acceptability should be reported as important performance criteria of hybrid BCI systems. With the current trend in introducing hybrid BCIs, we will soon see more than two BCI systems combined sequentially or simultaneously. It is also possible to combined BCIs in a combined sequentially/simultaneously approach. This will create a network of BCIs which cannot be distinguished as sequential or simultaneous any more. For example, for tasks with heavy object selections, SSVEP can be used for one stage and P300 for another stage. After the object selection, ERD/ERS BCI would be presented for continuous control tasks for other commands.