Connectedness effects in enumeration of small numbers.

The connectedness effect is the tendency to perceive connected items as fewer units, such as two connected dots seen as one. It commonly occurs with intermediate numerosity perception alongside the activity of the approximate number system (ANS), indicating that intermediate quantities are perceived as discrete units rather than continuous magnitudes. The present study explored how this effect influences the enumeration of small numerosities (fewer than 5), which are accurately assessed through a mechanism known as subitizing under normal conditions. In the single enumeration task, where participants enumerated 2-4 dots solely from the indicated target patch prior to stimulus presentation, connectivity did not induce underestimation, indicating that subitizing is impervious to the connectedness effect. Conversely, connectivity led to significant underestimation in the dual enumeration task, where participants had to simultaneously estimate dots in both patches and respond upon cueing of the target patch. Furthermore, the connectedness effect is more pronounced in the simultaneous comparison task compared with the sequential task. Weber fractions for small numerosities correlate with those for intermediate numerosities in the simultaneous comparison task, whereas no such correlation is observed in the sequential task. This suggests that subitizing prevails in single/sequential tasks, while estimation takes precedence in dual/simultaneous tasks under attentional load. The connectedness effect does not impact the subitizing mechanism in single tasks, but it occurs alongside estimation regardless of the number regime, leading to significant underestimation in dual tasks. Approximate estimation relies on segmented objects, rather than continuous magnitude, even for very small numerosities.