If the eyes are exposed to an external visual stimulus, then this is quickly extinguished and the eyes are exposed to a similar stimulus from a different place, the two stimuli may be perceived as a single stimulus which has moved across the intervening space. Such apparent motion was first investigated by Exner1 in 1875 and given the designation "Phi phenomenon" by Wertheimer2 in 1912. Since the advent of advertising signs which utilize this phenomenon, it has become generally familiar. It has been extensively investigated,3 and much is now known as to the conditions which are most apt to elicit it, including the time and space intervals. Experimental conditions have been produced under which the observer cannot distinguish between the apparent movement in the Phi phenomenon and actual movement when the conditions for one are substituted for those of the other. It is known that the phenomenon
VERHOEFF FH. PHI PHENOMENON AND ANOMALOUS PROJECTION. Arch Ophthalmol. 1940;24(2):247–251. doi:10.1001/archopht.1940.00870020039004
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