Abstract
Nowadays, everybody knows what a hybrid car is. A hybrid car normally has two engines to enhance energy efficiency and reduce CO2 output. Similarly, a hybrid brain-computer interface (BCI) is composed of two BCIs, or at least one BCI and another system. A hybrid BCI, like any BCI, must fulfill the following four criteria: (i) the device must rely on signals recorded directly from the brain; (ii) there must be at least one recordable brain signal that the user can intentionally modulate to effect goal-directed behaviour; (iii) real time processing; and (iv) the user must obtain feedback. This paper introduces hybrid BCIs that have already been published or are in development. We also introduce concepts for future work. We describe BCIs that classify two EEG patterns: one is the event-related (de)synchronisation (ERD, ERS) of sensorimotor rhythms, and the other is the steady-state visual evoked potential (SSVEP). Hybrid BCIs can either process their inputs simultaneously, or operate two systems sequentially, where the first system can act as a “brain switch”. For example, we describe a hybrid BCI that simultaneously combines ERD and SSVEP BCIs. We also describe a sequential hybrid BCI, in which subjects could use a brain switch to control an SSVEP-based hand orthosis. Subjects who used this hybrid BCI exhibited about half the false positives encountered while using the SSVEP BCI alone. A brain switch can also rely on hemodynamic changes measured through near-infrared spectroscopy (NIRS). Hybrid BCIs can also use one brain signal and a different type of input. This additional input can be an electrophysiological signal such as the heart rate, or a signal from an external device such as an eye tracking system.
Highlights
Brain–computer interface (BCI) research is advancing very rapidly
We show the rates of TPs, FPs of switch (FP) and PPVb of the ERD brain-computer interface (BCI) and PPVa and rate of errors/minute during resting periods (FPr) of the state visual evoked potential (SSVEP) BCI
Hybrid BCIs could involve a second type of input operating sequentially and/or simultaneously
Summary
Brain–computer interface (BCI) research is advancing very rapidly. Most BCI research still focuses on restoring communication and control in severely paralysed patients (Birbaumer et al, 1999; Wolpaw et al, 2002; Pfurtscheller et al, 2008a), but BCIs are quickly becoming useful to healthy people too (Allison et al, 2007; Nijholt et al, 2008). Non-invasive BCIs utilize changes in the dynamics of brain oscillations such as event-related (de)synchronization (ERD, ERS), steady-state evoked potentials (SSEPs), P300 evoked potentials and related components, real-time fMRI BOLD signals or near-infrared spectroscopy (NIRS)-measured oxyhemoglobin signals (Pfurtscheller et al, 2005a; Birbaumer and Cohen, 2007; Sitaram et al, 2007) Each of these BCIs has advantages and disadvantages. A simultaneous hybrid BCI can either use two different brain signals (e.g. electrical and hemodynamic signals), one brain signal (e.g. EEG) associated with two mental strategies (motor imagery and spatial visual attention; Figure 1C), or one brain signal and another input Such an additional input can be a physiological signal like the electrocardiogram (ECG, Figure 1B) or a signal from an external device such as an eye gaze control system (Zander et al, in press). Hybrid BCIs, like any BCI, must fulfil four criteria to function as BCI: 1. Direct: The system must rely on activity recorded directly from the brain
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