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Gadi Geiger and Jerome Lettvin, cognitive scientists at the Massachusetts Institute of Technology, used a mechanical shutter, called a tachistoscope, to briefly flash a row of letters extending from the center of a subject’s field of vision out to its perimeter.

W.J. came into the Sperry lab from his home in Southern California to find Gazzaniga waiting with a tachistoscope, a device that could present visual stimuli for specific periods of time—and, crucially, could present a stimulus to the right side or the left side of each eye separately.