Subitizing
Subitizing is the rapid, accurate, and effortless ability to perceive small quantities of items in a set, typically when there are four or fewer items, without relying on linguistic or arithmetic processes. The term refers to the sensation of instantly knowing how many objects are in the visual scene when their number falls within the subitizing range.
Sets larger than about four to five items cannot be subitized unless the items appear in a pattern with which the person is familiar (such as the six dots on one face of a die). Large, familiar sets might be counted one-by-one (or the person might calculate the number through a rapid calculation if they can mentally group the elements into a few small sets). A person could also estimate the number of a large set—a skill similar to, but different from, subitizing. The term subitizing was coined in 1949 by E. L. Kaufman et al., and is derived from the Latin adjective subitus (meaning "sudden").
The accuracy, speed, and confidence with which observers make judgments of the number of items are critically dependent on the number of elements to be enumerated.
While the increase in response time for each additional element within a display is 250–350 ms per item outside the subitizing range, there is still a significant, albeit smaller, increase of 40–100 ms per item within the subitizing range. A similar pattern of reaction times is found in young children, although with steeper slopes for both the subitizing range and the enumeration range.
A 2006 study demonstrated that subitizing and counting are not restricted to visual perception, but also extend to tactile perception, when observers had to name the number of stimulated fingertips. Even though the existence of subitizing in tactile perception has been questioned, this effect has been replicated many times and can be therefore considered as robust. The subitizing effect has also been obtained in tactile perception with congenitally blind adults.