Decoding Location-specific and Location-invariant Stages of Numerosity Processing in Subitizing

Extracting the number of objects in perceived scenes is a fundamental cognitive ability. Number processing is proposed to rely on two consecutive stages: an early object location map that captures individuated objects in a location-specific way and a subsequent location-invariant representation that captures numerosity at an abstract level. However, it is unclear whether this framework applies to small numerosities that can be individuated at once ("subitized"). Here we used EEG-based multivariate pattern decoding to test for location-specific and location-invariant stages of numerosity processing in the subitizing range. In two experiments, 1-3 targets were presented in the left or right hemifield, which allowed for decoding target numerosity within each hemifield separately (location-specific) or across hemifields (location-invariant). Experiment 1 indicated the presence of a location-specific stage (180-200 ms post-stimulus), followed by a location-invariant stage (300 ms post-stimulus). Experiment 2 showed that both location-specific and invariant components are engaged only during tasks that explicitly require numerosity processing, ruling out automatic, passive recording of numerosity. Overall, the results suggest that numerosity coding in subitizing is strongly grounded on an attention-based, location-specific stage. This stage remains active in parallel with the subsequent activation of a location-invariant stage, where a full representation of numerosity is finalized.