Bilateral and two-item advantage in subitizing

- Subitizing is improved when items are presented bilaterally than unilaterally.
- Two items are enumerated faster than one item (two-item advantage).
- Redundant target effect correlates negatively with enumeration speed.

Subitizing refers to people's ability to enumerate small sets of items fast and accurately. The present study examined if the speed and scope of subitizing is improved when the items to be enumerated are presented bilaterally across hemifields rather than unilaterally in a single hemifield. Such an effect, known as the bilateral field advantage, has been observed in a number of other visual tasks. A second aim was to examine whether the speed of subitizing could be explained by the speed it takes to detect the items to be enumerated, as simple reaction times to multiple stimuli are known to be faster than responses to individual items (known as the redundant target effect, RTE). The results revealed a bilateral field advantage even for enumerating two items. Moreover, the two item condition was the optimal subitizing condition - even enumerating one single item took longer - but this effect was not due to the RTE. In fact, the RTE negatively correlated with the speed of enumerating the same stimuli.