1 INTRODUCTION
1.1 Natural User Interfaces
1.2 Contributions
2 BIOLOGICAL FOUNDATIONS
2.1 Brain-Computer Interfaces: Neurological Phenomena
2.2 Brain-Computer Interfaces: Signal Acquisition Modalities
2.3 Electrooculography
2.4 Muscular Myoelectric Activity
3 NUI HARDWARE
3.1 Electrodes
3.2 Brain-Computer Interfaces
3.3 Functional Near-Infrared Spectroscopy
3.4 Surface Electromyography
3.5 Electrooculography
3.6 Silent Speech Interfaces
3.7 NUI Hardware Challenges
3.8 Hardware Challenges of Combining Mobile Output and NUI-related Hardware
4 NUI-ENABLED APPLICATIONS
5 CHALLENGES AND OPEN PROBLEMS
5.1 Future Directions
6 CONCLUSION