B. Bottom–Up Versus Top–Down Attention Attentional selection is a central part of perception and cognition. As such, it is influenced by many factors, both internal and external to the observer. What is attended depends, for instance, on the observer’s motivation and the specific task he or she is performing. In a set of classic experiments, Yarbus [22] showed that eye movements (overt attention) of the same observer viewing the same visual scene differ dramatically depending on what information the observer is looking for in the scene. Attentional selection that depends on the internal state of the observer is referred to as ‘‘top–down attention.’’ It is very difficult to develop biologically realistic detailed models of such mechanisms which may include influences such as the personal history of the observer.