Early in development, spontaneous cortical activity in ferrets is organised into modular, low-dimensional patterns, and this organisation is highly similar across multiple cortical areas [1]. Currently, it is unclear how cortical activity changes in development, whether these changes reflect the different functional specialisations across cortical areas, and what circuit changes underlie these changes.Here, we investigated how the structure of spontaneous cortical activity changes over the course of development by recording it with two-photon calcium imaging before (P21-24), around (P27-32) and after (P39-43) eye opening in five cortical areas (V1, A1, S1, PPC, PFC). Surprisingly, activity patterns in all five areas followed a similar developmental trend: a pronounced decrease in the correlation of nearby neurons and a strong increase in dimensionality, indicating a transition, from a modular to a more fine-scaled organisation.To explore a possible circuit mechanism underlying these common developmental changes, we studied a linear recurrent neural network model. Assuming recurrent interactions (RI) follow a local excitation and lateral inhibition, the network reproduces the modular structure of spontaneous activity observed in the young cortex [2]. We then considered three possible changes of RI and their effect on dimensionality and correlation structure: 1) transition towards local inhibition and lateral excitation, 2) increase in their heterogeneity, 3) decrease in their effective strength. We found that scenario 3) agrees best with the experimentally observed changes in activity, suggesting that a mild effective weakening of RI during development could underlie the reorganisation from modular to fine-scaled activity observed in diverse cortical areas.References:[1] N Powell, B Hein, D Kong, J Elpelt, H Mulholland, M Kaschube, G Smith. Common modular architecture across diverse cortical areas in early development. Proceedings of the National Academy of Sciences. In press.[2 ] GB Smith, B Hein, DE Whitney, D Fitzpatrick, M Kaschube. Distributed network interactions and their emergence in developing neocortex. Nature neuroscience 21 (11), 1600-1608 (2018); https://doi.org/10.1038/s41593-018-0247-5