There is considerable controversy as to how the brain extracts numerosity information
from a visual scene and as to how much attention is needed for this process. Traditionally, it has
been assumed that visual enumeration is subserved by two functionally distinct mechanisms:
the fast and accurate apprehension of 1 to about 4 items, a process termed "subitizing", and the
slow and error-prone enumeration of larger numerosities referred to as "counting". Further to
a functional dichotomy between these two mechanisms, an attentional dichotomy has been proposed.
Subitizing has been thought of as a pre-attentive and parallel process, whereas counting
is supposed to require serial attention.
In this work, the hypothesis of a parallel and pre-attentive subitizing mechanism was
tested. To this aim, the amount of attention that could be allocated to an enumeration task was
experimentally manipulated. In Experiment 1, attentional set was manipulated such that attention
could either be drawn to the relevant of two subsets to enumerate or had to be distributed
to both subsets. Furthermore, the relationship of enumeration to perceptual grouping and item
discrimination was explored. In Experiment 2, a dual-task approach was employed in which
the amount of attentional resources available to enumeration was systematically modulated by
imposing an additional task and by varying its attentional load. Experiment 3 investigated
the neural correlates of visual enumeration under attentional load using functional magnetic
resonance imaging (fMRI).
Results indicated that (1) enumeration, particularly subitizing, was clearly compromised
under conditions of distributed or reduced attention. (2) Both the enumeration of small
and large numerosities was a�ffected by such attentional manipulations. (3) Subitizing selectively
activated brain areas associated with stimulus-driven attention. (4) Enumeration is contingent
on other potentially attention-demanding visual processes such as perceptual grouping. The
evidence presented here seriously challenges the traditionally held claim of a parallel and preattentive
subitizing mechanism and suggests instead that small numerosity judgement requires
visual attention. This weakens the argument of an attentional as well as a functional dichotomy
and strengthens the idea that enumeration may be subserved by a single, continuous mechanism