The visual attention mechanism may have at least the following basic components [Tsotsos, et. al. 1995]:
(1) the selection of a region of interest in the visual field;
(2) the selection of feature dimensions and values of interest;
(3) the control of information flow through the network of neurons that constitutes the visual system; and
(4) the shifting from one selected region to the next in time .
from:http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/SUN1/attn.htm