We first used EEG measurements to fine-tune the frequency-tagging approach to separately measure responses to numerosity as well as to continuous magnitudes. Our findings demonstrated that numerosity can be independently processed at an early stage in the visual cortex, even when completely isolated from other magnitude changes. The similar implicit discrimination for numerosity as for some continuous magnitudes, which correspond to basic visual percepts, shows that both can be extracted independently, hence substantiating the nature of numerosity as a primary feature of the visual scene. Further MEG measurements replicated the same discrimination results but in another modality (MEG vs. EEG). We further showed that early visual regions would be able to discriminate numerosity and some of the continuous magnitudes (total area and convex hull) and the parietal regions may support the persistence of the information over short timescales. The frequency-tagged neuromagnetic responses provide evidence in favor of an automatic feature-based attention spontaneously directed towards numerosity and some continuous magnitude properties related to the whole visual scene. Finally, our results highlighted the role of parietal regions in the early steps of magnitude processing (both numerosity and total area) even in absence of any explicit task and decision making mechanisms. Taken together, our innovative frequency-based approach allowed us to isolate responses to numerosity and to continuous magnitudes bringing new insights to a longstanding debate in the field of numerical cognition. Moreover, implementation of the frequency-tagging approach to MEG revealed the relative contribution of different regions of the brain to automatic magnitude discrimination and to the attentional sustain of these representations over short time scales.
The results of the Freq4Num project have been presented at international conferences (Organisation for Human Brain Mapping, Cognitive Neuroscience Society, the Third Jean Piaget Conference) and national meetings (Belgian FNRS group of contact « Numbers and the Brain », Belgian Association for Psychological Sciences, Belgian Socity for Neurosciences). The work done during the Freq4Num project allowed to bring crucial new insights in the field of numerical cognition, as attested by the publication of the seminal results of the project in an interdisciplinary journal (Proceedings of the National Academy of Sciences). Other important publications were associated to the project consisted in a meta-analysis and in the development of an open methodological tool related to non-symbolic numerical cognition. Further outcomes should be accepted for publication in the upcoming months.
