However when "pop out" does 
not occur and it takes subjects 
considerable time to find the 
separate texture region or the 
unique element, it has been 
assumed that the targets are 
such that parallel processing 
doesn’t allow "finding" the 
region or element easily. In 
these circumstances it would 
seem that one must make use of 
attention, and if in fact one 
must attend to each element in 
time, serial rather than 
parallel processing must occur 
during search. When "pop out" 
does occur, it has been 
demonstrated repeatedly that 
the number of background 
elements, called distractors (as 
distinct from the target), does 
not influence the time required 
to find the target--further room 
for parallel processing. When 
"pop out" fails to occur, 
however, the more distractors 
there are, the longer the 
search task takes--further 
proof for serial attention to 
each item. One condition in 
which "pop out" fails to occur 
is when the target combines 
two properties, say a triangle 
that is red, and the distractors 
are, say, red circles and black 
triangles. The target is still 
unique but only by virtue of its 
combination of the two 
properties, redness and 
triangularity. What is the role 
of attention in this condition? 
Anne Treisman, who uncovered 
this effect, has suggested that 
attention serves to bind the two 
properties together into one 
perceptual object in a 
particular location. This 
analysis is based on the 
presumption that the 
perception of each property 
occurs through very separate 
processing in the brain and 
that unified perception of 
multifaceted objects requires 
an additional operation to 
integrate them. Treisman’s 
theory is therefore aptly called 
"feature integration theory."