===Neural theories===
In the late 19th and early 20th Centuries, it was widely believed that déjà vu could be caused by the mis-timing of [[neuronal]] firing. This timing error was thought to lead the brain to believe that it was encountering a stimulus for the second time, when in fact, it was simply re-experiencing the same event from a slightly delayed source. A number of variations of these theories exist, with miscommunication of the two [[cerebral hemispheres]] and abnormally fast neuronal firing also given as explanations for the sensation.  Perhaps the most widely acknowledged neuronal theory is the optical pathway delay theory which explains déjà vu as being the product of a delayed optical input from one [[eye]]. Closely following the input from the first eye (when it should be simultaneous), this misleads conscious awareness and suggests a sensation of familiarity when there should not be one. Although intuitively plausible, this theory is untestable due to the minute times involved in neuronal firing, and inconsistent with reports that blind individuals experience déjà vu in the same way as sighted individuals (O'Connor & Moulin, 2006).