An early antecedent to modern random dot stereograms --'the secret stereoscopic writing' of Ramón y Cajal.

The use of computerized random dot stimuli in modern neuroscience was introduced by Julesz in the 1960s. This method made it possible to study exclusively cortical processing of binocular information by disparity-sensitive neurons, and it has attained widespread use among neuroscientists and psychologists. It is now largely forgotten that in the last century, the famous neuroanatomist Ramón y Cajal had worked on random dot stereograms as a means of encoding written information. A brief note was finally published in a Spanish journal on photography in 1901. We present a translation of this text and summarize the early ideas on random dot stereograms, and we also supply a brief historical account on stereoscopic perception.