George Trumbull Ladd39 suggested in 1892 that the visual elements of dreams were derivations of the "psychical synthesis" of night-time retinal sense data. This hypothesis led him to speculate that the eyeballs move during dreaming. "As we look down the street of a strange city, for example, in a dream we probably focus our eyes somewhat as we should do in making the same observation when awake. . . ."
Ladd's ingenious surmise went unverified until 1955 when Aserinsky and Kleitman2 reported actual observations of ocular movements during sleep. They described 2 types which appear at separate times during the night in a predictable pattern: (a) slow eye movements (SEM's)—,slow, often asynchronous, gliding excursions of the eyeballs at sleep onset and after every body movement occurring during the night's sleep; (b) rapid eye movements (REM's)—bursts of quick, binocularly synchronous, single and grouped ocular deviations often in clusters