Toward a gaze-independent matrix speller brain-computer interface.

Many people affected by debilitating neuromuscular disorders such as amyotrophic lateral sclerosis (ALS), brainstem stroke, or spinal cord injury are impaired in their ability to communicate. Conventional assistive devices (e.g., letter boards, cheek or tongue switches, or eye trackers) that aim to restore communication all require muscular control, which is often lost in the progress of neuromuscular disorders. A brain–computer interface (BCI) uses brain signals directly, rather than muscles, to re-establish communication with the outside world. One well-known BCI approach is the so-called ‘‘P300 matrix speller’’ that was first described by Farwell and Donchin (1988). In this system, the user pays attention to a character in a matrix while each row and column is intensified rapidly and randomly. The brain produces a response to the row or column that contains the intended character (i.e., the oddball); this response is not present for the other rows or columns. The BCI typically averages several responses, detects the row and column with the strongest responses, and thereby identifies the character the user wants to select. The individual parameters of the matrix speller have each been studied and optimized extensively. This includes the matrix size (Allison and Pineda, 2003), stimulation frequency (Sellers et al., 2006a), stimulation intensity (Takano et al., 2009), classification algorithm (Krusienski et al., 2006), and electrode locations (Krusienski et al., 2008). The matrix speller approach has been used in several application contexts, such as web browser navigation (Mugler et al., 2008), control of ambient environment (Edlinger et al., 2009), wheelchair navigation (Rebsamen et al., 2007), and mouse movement (Citi et al., 2008). Moreover, it has recently been shown that more than 80% of the healthy population can use such a BCI (Guger et al., 2009), which demonstrates the broad applicability of this approach in people without disabilities. Most important to the eventual goal of BCI research, there is mounting evidence that the matrix speller can also restore function in severely disabled individuals (Piccione et al., 2006; Sellers et al., 2006b, 2010; Vaughan et al., 2006; Nijboer et al., 2008; Kübler and Birbaumer, 2008; Silvoni et al., 2009; Hoffmann et al., 2008; see Donchin and Arbel, 2009 or Mak and Wolpaw, 2009 for comprehensive reviews). However, these studies typically report spelling performance that is lower than that reported in similar studies using healthy individuals. This decreased spelling performance in patients compared to healthy individuals may be related in part to cognitive impairment (Phukan et al., 2007) or to the inability to maintain gaze. For example, eye gaze is often impaired or lost in subjects affected by ALS. Although some people with ALS maintain residual eye movement