According to Rensink (2002) in Riegler & Riegler, coherence, the quality of being logically consistent, is very important to the notion of visual attention. When visual attention is absent, features is difficult to perceive. There are two aspects of visual attention: divided attention and selective attention.
1-Divided attention: Divided attention is the ability to pay attention on several objects at the same time. In Riegler & Riegler, British psychologists Donals Broadbent (1958) and Colin Cherry (1953) found good examples in case of pilots who flied planes. In front of a pilot, there was a control panel consisted of numerous buttons, switches, lights, and instructions. The pilot had to pay attention on everything, from left to right, from top to bottom. Nothing on the control panel could be omitted. The tasks that the pilot had to performed seem overload. However, the pilot had to control it anyway. In the regular life, workers in many fields have to apply divided attentions, too. An receptionist sitting at the front desk of a company, an operator of a manufactor, an engineer who works at a control room, etc., all work with divided attentions. Their eyes glance from this buttion to another, their mouths talk through a headphone, their right hands dial (or type) a number while their left hand pick up or put in some plugs. Practically, this task is not an easy one. Only the high functioned workers can do that kind of job.
2-Selective attention: On the opposite site of divided attention, selective attention is the careful look at something special. Riegler & Riegler used the example on the watcher who watched a basket ball game. He or she only payid attention on the bouncing back of the ball. Another watcher only watch the players of the team that the watchers support. Similarly, a careful driver will not notice on anything else beside the lane that the driver is driving on.