Spatiotemporal dynamics of processing nonsymbolic number: an event-related potential source localization study

Abstract

Coordinated studies with adults, infants, and nonhuman animals provide evidence for two distinct systems of nonverbal number representation. The "parallel individuation" (PI) system selects and retains information about one to three individual entities and the "numerical magnitude" system establishes representations of the approximate cardinal value of a group. Recent event-related potential (ERP) work has demonstrated that these systems reliably evoke functionally and temporally distinct patterns of brain response that correspond to established behavioral signatures. However, relatively little is known about the neural generators of these ERP signatures. To address this question, we targeted known ERP signatures of these systems, by contrasting processing of small versus large nonsymbolic numbers, and used a source localization algorithm (LORETA) to identify their cortical origins. Early processing of small numbers, showing the signature effects of PI on the N1 (∼150 ms), was localized primarily to extrastriate visual regions. In contrast, qualitatively and temporally distinct processing of large numbers, showing the signatures of approximate number representation on the mid-latency P2p (∼200-250 ms), was localized primarily to right intraparietal regions. In comparison, mid-latency small number processing was localized to the right temporal-parietal junction and left-lateralized intraparietal regions. These results add spatial information to the emerging ERP literature documenting the process by which we represent number. Furthermore, these results substantiate recent claims that early attentional processes determine whether a collection of objects will be represented through PI or as an approximate numerical magnitude by providing evidence that downstream processing diverges to distinct cortical regions.

Figure 1

Grand average ERP response to small and large numbers. A: Grand average waveform for small and large numbers over the posterior scalp. B: Average scalp topography for large and small numbers at time points characterizing the N1, P2p, and P3.

Figure 2

Effects of cardinal value on N1. A: Average ERP waveforms over the posterior scalp for each cardinal value presented from −200 to 600 msec after stimulus onset. B: Source localization of N1 cardinal value effect for small numbers. C: Mean N1 amplitudes for cardinal values presented in the small and large number range. The “*” indicates a significant (P < 0.05) difference or interaction.

Figure 3

Effects of number change on P2p for large numbers and P3 for small numbers. A: Average waveform over the posterior scalp for each of the experimental conditions within the large number range. B: Graph comparing mean P2p amplitude across large number experimental conditions. C: Average waveform over the posterior scalp for each of the experimental conditions within the small number range. D: Graph comparing mean P3 amplitude across small number experimental conditions.

Figure 4

Source localization of number change effects. Blue represents localization of the large number P2p effect (number change > no change). Red represents the localization of the small number P3 effect (no change > number change).

Figure 5

Source localization for P2p number‐specificity effects. Blue highlights represent ERP localization of areas that responded more to large number‐shape changes compared to large number‐number changes. Red areas represent the ERP localization of areas that responded more to large‐number number changes compared to large‐number shape changes.

Figure 6

Comparison of our ERP source localization to peak voxels activated in previous fMRI studies of nonsymbolic large number processing. Red and blue areas represent LORETA voxels showing significant P2p effects for large numbers and P3 effects for small numbers. Other colored areas represent closest LORETA voxel to reported peak voxels in several prominent fMRI studies of nonsymbolic large number processing. Please note that the spatial resolution and actual voxel size varies substantially between fMRI and ERP source localization using LORETA. Furthermore, only reported peaks of fMRI activity transformed to LORETA space are mapped. Therefore, this figure is meant as an approximation and conclusions regarding exact overlap or distinctness should be made with caution.