Guest Editor's Introduction

Brain–Computer Interface (BCI) systems are quickly moving out of the laboratory and becoming practical communication and control systems. BCIs have been validated in homes, hospitals, expositions, and other noisy environments. BCIs may provide accurate control through different brain signals, and can allow people to spell, select items, browse the internet, manage smart home systems, and control robotic devices including wheelchairs, orthoses, and prostheses. Today the world of BCI applications is expanding and new fields are opening. One new direction involves BCIs to control virtual reality (VR), including BCIs for games, or using VR as a powerful feedback medium to reduce the need for BCI training. Virtual environments (VE) can provide an excellent testing ground for procedures that could be adapted to real world scenarios, especially for patients with disabilities. If people can learn to control their movements or perform specific tasks in a VE, this could justify the much greater expense of building physical devices such as a wheelchair or robot arm that is controlled by a BCI. Recent work has clearly shown that BCIs can allow control in VEs, including control of a virtual character (avatar). Such a control can be realized either by a motor imagery-based BCI or by spatial attention to flickering or flashing stimuli (SSVEP-based and P300-based BCIs). Motor imagery BCIs do not require external stimulation to elicit the necessary brain activity, but often require extensive training. BCIs based on visual evoked potentials require external stimuli to elicit these potentials, but require little or no training. Subjects who use immersive VEs make fewer errors, report that BCIs are easier to learn and use, and state that they enjoy BCI use more. One reason for this might be that virtual environments enhance vividness and mental effort, which leads to more discriminable EEG patterns. This improves BCI classification accuracy. Therefore, there is an unmet opportunity to further enhance BCI usability by developing and testing rich, engaging virtual environments for BCIs. An interesting aspect is that mental simulation of movement (motor imagery) results in cardiovascular changes. The heart rate (HR) generally decreases during motor imagery in laboratory conditions without VR feedback, but can be increased during effortful imagery and/or enhanced mental effort, as in the case of VR feedback in BCI experiments. This underlines the importance of VR feedback in modifying emotional experiences and enhances autonomic and visceral responses. The HR changes can be in the order of several beats per minute, and can increase the classification accuracy of a motor imagery-based BCI when both the EEG and the HR are analyzed simultaneously. This special issue presents new work about BCI applications in virtual environments. There are contributions about the use of P300-based immersive VR (Groenegress et al., and Donnerer and Steed) and SSVEP-based avatar control (Faller et al.), a new flexible open-source software platform to combine BCI and VR systems (Renard et al.), and motor imagery-based BCI applications in VR (Lotte et al., and Velasco-Álvarez et al.).