Electroencephalography (EEG) has been used to generate music for over 40 years,but the most recent developments in brain–computer interfaces (BCI) allowgreater control and more flexible expression for using new musical instrumentswith EEG. We developed a real-time musical performance system using BCItechnology and sonification techniques to generate imagined musical chords withorganically fluctuating timbre. We aim to emulate the expressivity oftraditional acoustic instruments. The BCI part of the system extracts patternsfrom the neural activity while a performer imagines a score of music. Thesonification part of the system captures non-stationary changes in the brainwaves and reflects them in the timbre by additive synthesis. In this paper, wediscuss the conceptual design, system development, and the performance of thisinstrument.