A P300-based brain–computer interface for people with amyotrophic lateral sclerosis
Introduction
Brain–computer interfaces (BCIs) circumvent motor output and convey messages directly from the brain to a computer (Kübler et al., 2001, Wolpaw et al., 2002). Thus, BCIs may be able to provide a new communication channel to individuals with severe neurological or muscular diseases. This includes patients with locked-in syndrome (LIS). LIS is characterized by complete motor paralysis, except for eye movements, with intact cognition and sensation (Laureys et al., 2004). Amyotrophic lateral sclerosis (ALS) is a progressive neurological disease that often leads to LIS (Karitzky and Ludolph, 2001). A communication tool that is independent of muscle control would allow individuals with LIS to regain a level of autonomy, and to be less dependent upon others for communication, particularly after they have lost reliable eye-movement.
The P300 event-related potential (ERP) is one possible BCI control signal. The P300 is a positive deflection in the electroencephalogram (EEG) that occurs 200–700 ms after stimulus onset and is typically recorded over central–parietal scalp locations (Fabiani et al., 1987). The response is evoked by attention to rare stimuli in a random series of stimulus events (i.e., the oddball paradigm) (Fabiani et al., 1987).
Farwell and Donchin showed that the P300 can be used to select items displayed on a computer monitor (Farwell and Donchin, 1988, Donchin et al., 2000). The authors presented study participants with a 6 × 6 matrix where each of the 36 cells contained one character (Fig. 1). The participants were instructed to attend to one of the 36 cells (the target) while the matrix rows and columns flashed in a random order. This design represents an oddball paradigm. In one trial of 12 flashes (6 rows and 6 columns), the target cell flashes only twice: once in a column and once in a row. These two rare events in the context of the other 10 flashes typically elicit a P300 response.
While recent studies have successfully demonstrated several alternatives for P300-based communication and control, none have explored the use of a matrix speller for spontaneous communication. Sellers and Donchin (2006) tested a four-choice paradigm with three healthy volunteers and three volunteers with ALS. The words ‘yes’, ‘no’, ‘pass,’ and ‘end’ were presented one at a time as auditory, visual, or auditory and visual stimuli. The participant’s task was to count the number of times the designated target (either ‘yes’ or ‘no’) was presented in a random sequence of the four choices. The authors showed that a target probability of 25% dependably elicited a P300 response, and that the response remained stable over a period of 10 sessions in people with and without ALS.
Piccione et al. (2006) also tested a four-choice P300 paradigm with seven healthy volunteers and five individuals with tetraplegia, including one individual with ALS. In this paradigm, the participants attended to one of four flashing arrows to indicate which direction they wished to move a cursor: up, down, left, or right. Hoffmann et al. (2008) tested a six-choice visual paradigm that allowed four healthy volunteers and five people severely disabled by ALS to select among icons representing four devices, a door, and a window.
Recently, Sellers et al. (2006b) presented preliminary data from subjects A and B in this study to illustrate their P300 response during successful use of the 6 × 6 matrix speller. The present study was designed to extend those encouraging initial findings by studying a larger group of individuals with ALS, evaluating the stability of their BCI performance in repeated sessions over a prolonged period of time, and by determining if an ALS population could use a P300-based matrix speller to communicate spontaneous words and phrases.
Section snippets
Participants
Eight individuals with amyotrophic lateral sclerosis (ALS), who were severely paralyzed (ALS-FRS score of 12 + 8), provided informed consent for the study. The study was approved by the Ethical Review Board of the Medical Faculty of the University of Tübingen in Germany. All participants were tested in their home environment. Six individuals completed Phase I of the study in which they copy-spelled 51 characters in each of 12 experimental sessions (described below in Section 2.5). A seventh
Phase I: copy spelling
Our first hypothesis was that the response ERPs to the desired matrix item can be discriminated from the responses to the other matrix items accurately enough to be used for communication. Thus, we examined classification accuracy both online and offline. Our second hypothesis was that classification accuracy does not change over time. To test both hypotheses, we conducted a two-way analysis of variance (ANOVA) including the factors of analysis mode using online vs. offline conditions on
Discussion
Previously, Sellers and Donchin (2006) found that both healthy volunteers and volunteers with advanced ALS could use a P300-based four-choice matrix with auditory and/or visual stimuli; and Sellers et al. (2006b)) reported that several ALS patients could use the P300-based BCI and that two patients could use a 6 × 6 matrix to copy text. The present study substantially extends this initial work by demonstrating that individuals with severe paralysis caused by ALS can use a P300-based BCI that
Conclusions
The data presented in this study support the hypothesis that individuals severely disabled by ALS can use a P300-based BCI for writing text and that performance was stable for many months in terms of the ERP response, and in terms of classification accuracy. We expect that BCI devices are poised to make significant and meaningful contributions in the daily lives of those who are severely disabled by ALS or other devastating neuromuscular disorders.
Acknowledgements
We thank Tilman Gaber, Slavica von Hartlieb, Jeroen Lakerveld, Boris Kleber, Seung-Soo Lee, and Barbara Wilhelm from the Institute of Medical Psychology and Behavioral Neurobiology, University of Tübingen, Germany, for their support in training participants and Gerwin Schalk and Dennis McFarland from the Wadsworth Center, Albany, USA, for providing software support. We acknowledge the support and the patience of our participants. We particularly acknowledge the contribution of our dear and
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