Jacqueline Humbert, Brainwave Etch a Sketch, 1974. "One person's Alpha waves control the movement of a dot along the oscilloscope's X axis, while the other person's are responsible for movement along the Y axis. The resulting form of the drawing depends on the degree of cooperation, sensitivity and agility in consciously controlling brain signals that the two participants are able to achieve with each other." (Quote and photos from Biofeedback and the Arts, 1976, ed. by David Rosenboom)
