Speier W, Fried I, Pouratian N. Improved P300 speller performance using electrocorticography, spectral features, and natural language processing. Clin Neurophysiol. 2013 Jul;124(7):1321-8. DOI: 10.1016/j.clinph.2013.02.002. Epub 2013 Mar 5. PMID: 23465430; PMCID: PMC3679217.