Summary: Copyright 2008 Psychonomic Society, Inc. 390
The human visual system is constantly confronted with
an overwhelming amount of information, only a sub-
set of which is selected for detailed processing. Unlike
overt selection by eye movement, covert attention can
sample multiple objects at once (Awh & Pashler, 2000).
The maintenance of multiple attentional foci, however, is
limited in capacity. For instance, when observers track a
subset of moving objects with attention, accuracy declines
as the number of tracked targets increases (Pylyshyn &
Storm, 1988). Previous studies have interpreted this de-
cline in terms of a limit in the number of attentional foci.
However, when objects move in a restricted visual field,
increasing the number of targets often results in a decrease
in the spatial separation among targets. If attention is lim-
ited in maintaining close spacing among multiple foci,
tracking accuracy should decline as tracked targets are
spaced more tightly.
Indeed, visual attention is limited in its resolution to
distinguish targets from nontargets. Attentive tracking is