Usability of Transient VEPs in BCIs

Abstract

Among non-invasive Brain Computer Interfaces (BCIs), electroencephalogram (EEG) has been the most commonly used for them because EEG is advantageous in terms of its simplicity and ease of use, which meets BCI specifications when considering practical use. In general, EEG signals (EEGs) can be classified into two categories, spontaneous EEGs and stimulus evoked EEGs. Focusing on stimulus evoked EEGs, signals called P300 and Visual Evoked Potentials (VEPs) are often utilized for BCIs. Both types of BCIs extract the intention of users by detecting which target on the PC monitor users are gazing at (Sellers & Donchin, 2006; Sellers, et al., 2006). While P300 signals are thought to be derived from the thoughts of users, VEPs are simply derived from physical reaction to visual stimulation. In that sense, VEP-based BCIs are thus known as the simplest BCIs. Most VEP-based BCIs utilize so-called “steady-state VEPs” (SSVEPs) which are generated in reaction to high-speed blinking light (Allison, et al., 2008; Cheng, et al., 2002). Because SSVEPs are characterized by sinusoidal-like waveforms whose frequencies are synchronized with the frequency of blinking light, the gazing target of users can be identified by using frequency analysis of SSVEPs from among several visual targets with different blinking frequencies. On the other hand, there is another type of VEP called a “transient VEP.” Transient VEPs are generated in reaction to low-speed blinking light (i.e., blinking frequency of less than 3.5 Hz), and they can be characterized with a negative peak of around 75 ms and a positive peak of around 100 ms (N75 and P100 in Fig. 1). Unlike SSVEPs, transient VEPs are rarely used for BCIs because it is considered that they need longer detection time than SSVEPs. However, there are several issues which need to be addressed regarding the use of SSVEPbased BCIs. The first issue is discomfort caused by blinking light. When gazing at highspeed blinking light, some people exhibit symptoms similar to optically stimulated epileptic seizure such as annoyance, headache, or nausea (Graf, et al., 1994; Guerrini & Genton, 2004). Most of the subjects in the authors’ study group actually felt discomfort caused by the blinking stimuli. The second issue is that SSVEPs are not detected in all people. One of the reasons for this is considered to be that some people unconsciously refuse to gaze at discomfort targets, and the authors’ group included some users in which SSVEPs were not detected. SSVEP-based BCIs cannot be practically used for such kind of users. Considering these issues, the authors have proposed a transient VEP-based BCI which reduces discomfort caused by gazing at high-speed blinking light (Yoshimura & Itakura,