EEG signature of grouping strategies in numerosity perception

Abstract

The moment we see a group of objects, we can appreciate its numerosity. Our numerical estimates can be imprecise for large sets (>4 items), but they become much faster and more accurate if items are clustered into groups compared to when they are randomly displaced. This phenomenon, termed groupitizing, is thought to leverage on the capacity to quickly identify groups from 1 to 4 items (subitizing) within larger sets, however evidence in support for this hypothesis is scarce. The present study searched for an electrophysiological signature of subitizing while participants estimated grouped numerosities exceeding this range by measuring event-related potential (ERP) responses to visual arrays of different numerosities and spatial configurations. The EEG signal was recorded while 22 participants performed a numerosity estimation task on arrays with numerosities in the subitizing (3 or 4) or estimation (6 or 8) ranges. In the latter case, items could be spatially arranged into subgroups (3 or 4) or randomly scattered. In both ranges, we observed a decrease in N1 peak latency as the number of items increased. Importantly, when items were arranged to form subgroups, we showed that the N1 peak latency reflected both changes in total numerosity and changes in the number of subgroups. However, this result was mainly driven by the number of subgroups to suggest that clustered elements might trigger the recruitment of the subitizing system at a relatively early stage. At a later stage, we found that P2p was mostly modulated by the total numerosity in the set, with much less sensitivity for the number of subgroups these might be segregated in. Overall, this experiment suggests that the N1 component is sensitive to both local and global parcelling of elements in a scene suggesting that it could be crucially involved in the emergence of the groupitizing advantage. On the other hand, the later P2p component seems to be much more bounded to the global aspects of the scene coding the total number of elements while being mostly blind to the number of subgroups in which elements are parsed.

Keywords: EEG; ERPs; groupitizing; numerosity perception; subitizing.

Figure 1

Stimuli configuration and overview of the experimental design. (A) Examples of stimulus configurations in estimation and subitizing ranges. (B) Participants performed eight sessions estimating quantities in the estimation range, each comprising four blocks with grouped (G) and ungrouped (U) stimuli alternated and one session estimating quantities within the subitizing range. (C) Example of the time course of the stimuli.

Figure 2

Behavioral results. Average Weber fraction for 6 and 8 items, for both grouped and ungrouped conditions.

Figure 3

ERPs responses to stimuli in the subitizing range. ERPs response averaged across electrodes PO3, PO4, PO7, PO8, POz, O1, O2 (A) and across electrodes PO3, PO4, POz, P3, P4 and Pz (B) for 3 (red curves) and 4 items (blue curves). In panel (A), image insert shows a detail of the N1 component; in panel (B), a detail of the P2p component. N1 latency (C), N1 amplitude (D) and P2p amplitude (E) in response to low and high numerosities averaged across participants. Error bars represent standard error of the mean corrected for a within-subject design (Cousineau and O’Brien, 2014).

Figure 4

N1 latency (A), N1 amplitude (B) and P2p amplitude (C) in response to 6 and 8 items separately for the ungrouped (red circles) and grouped condition (purple squares) averaged across participants. Error bars represent standard error of the mean corrected for a within-subject design (Cousineau and O’Brien, 2014).

Figure 5

ERPs responses to grouped stimuli in the estimation range. ERPs response averaged across electrodes PO3, PO4, PO7, PO8, POz, O1, O2 for 6 (orange) and 8 (blue) items in (A) the congruent condition (6 items divided in 4 subgroups and 8 items divided in 4 subgroups) and in (B) the incongruent condition (6 items divided in 4 subgroups and 8 items divided in 3 subgroups). Image inserts in panels (A,B) show the peak latency of the N1 component. N1 latency (C), N1 amplitude (D) and P2p amplitude (E) in response to 6 and 8 items separately for the ungrouped (red circles), grouped- Congruent (N6 grouped into 3 clusters and N8 grouped into 4 clusters; blue squares) and grouped- Incongruent (N6 grouped into 4 clusters and N8 grouped into 3 clusters; green squares) condition averaged across participants. Error bars represent standard error of the mean corrected for a within-subject design (Cousineau and O’Brien, 2014).

Figure 6

N1 latency differences. The bar graph shows the N1 latency differences (high – lower numerosity, in ms) averaged across participants in response to stimuli in the subitizing (grey), and in the estimation range for ungrouped (red) and congruent grouped (green) stimuli (* = p < 0.05; ** = p < 0.01). Error bars represent standard error of the mean corrected for a within-subject design (Cousineau and O’Brien, 2014).