Recent advances and open challenges in hybrid brain-computer interfacing: a technological review of non-invasive human research

Context. In recent years, hybrid brain-computer interfaces (hBCIs) have proven to be a promising path towards practical brain-computer interfacing. These hybrid interfaces capitalize on the concurrent recording of various physiological signals, or of the elicitation of more than one mental process, to increase the number of possible input commands and achieve more flexible and robust systems. Although hBCIs have previously been reviewed in some articles, a more recent, and complete survey of the literature is missing to lay the foundations for further research. Objective. This work aims at systematically reviewing recent articles on the topic of non-invasive hBCIs, to comprehensively identify the current trends, limitations and challenges that these studies report. Methods. Three major databases covering the fields of science and engineering were queried. From these and others sources, 55 journal articles from 2008 to November 2014 were selected and analyzed. Results. Twenty-two items were investigated to offer a complete perspective on the current state of non-invasive hBCI research in humans, covering the study rationale, experimental protocol, signal-processing methodology, and system evaluation. Based on this analysis, recommendations were formulated to direct further work in the field. Significance. We hope this review will constitute the groundwork for future hBCI studies.