Cognitive processes surely exist, so it can hardly be unscientific to study them. - Ulric Neisser

To deal with the whole visual input at once, and make discriminations based on any combination of features in the field, would require too large a brain, or too much "previous experience" to be plausible.

The attentive synthesis of any particular letter or figure takes an appreciable time, of the order of 100ms...If a whole row of letters is to be identified, they must be synthesized one at a time... To "identify" generally means to name, and hence to synthesize not only a visual object but a linguistic-auditory one... Hence the span of apprehension is limited to what can be synthesized, and then verbally stored.

Attention is not a mysterious concentration of psychic energy; it is simply an allotment of analyzing mechanisms to a limited region of the field. To pay attention to a figure is to make certain analyses of, or certain constructions in, the corresponding part of the icon.