The fact that the span of apprehension averages only four or five... probably results from the high rate of encoding. In a tachistoscopic experiment the subject must read the fading icon as rapidly as possible.
p. 42 ; As cited in: A.H.C. Van der Heijden, "Visual attention," in: Handbook of Perception and Action, Vol. 3. 1996 - Cognitive Psychology, 1967