It is a common observation that when we notice, think about, or observe some stimulus, it may be difficult to notice, think about, or observe other stimuli. In other words, perception and other cognitive processes are often selective. One explanation for these observations is that in order to notice some stimulus, we must attend to it and we can only attend to a limited number of stimuli at a time. Often, other explanations are available. For example, it may be difficult to notice two things because they are spatially separated, and we can only look in one direction at a time. Such peripheral explanations are usually suggested as alternatives to an attentional explanation. Attention can, then, be viewed as a central adaptation to information overload. There is more information present in the stimulus array than is possible to process so it is necessary to selectively attend to some stimuli at the expense of others.