Developmental maturation of millimeter-scale functional networks across brain areas

Abstract

Interacting with the environment to process sensory information, generate perceptions, and shape behavior engages neural networks in brain areas with highly varied representations, ranging from unimodal sensory cortices to higher-order association areas. Recent work suggests a much greater degree of commonality across areas, with distributed and modular networks present in both sensory and non-sensory areas during early development. However, it is currently unknown whether this initially common modular structure undergoes an equally common developmental trajectory, or whether such a modular functional organization persists in some areas-such as primary visual cortex-but not others. Here we examine the development of network organization across diverse cortical regions in ferrets of both sexes using in vivo widefield calcium imaging of spontaneous activity. We find that all regions examined, including both primary sensory cortices (visual, auditory, and somatosensory-V1, A1, and S1, respectively) and higher order association areas (prefrontal and posterior parietal cortices) exhibit a largely similar pattern of changes over an approximately 3 week developmental period spanning eye opening and the transition to predominantly externally-driven sensory activity. We find that both a modular functional organization and millimeter-scale correlated networks remain present across all cortical areas examined. These networks weakened over development in most cortical areas, but strengthened in V1. Overall, the conserved maintenance of modular organization across different cortical areas suggests a common pathway of network refinement, and suggests that a modular organization-known to encode functional representations in visual areas-may be similarly engaged in highly diverse brain areas.

Figure 1:. Imaging distinct cortical regions in ferrets at different developmental ages.

a. Target cortical locations. PFC – prefrontal cortex, S1 – somatosensory cortex, A1 – auditory cortex, PPC – posterior parietal cortex, V1 – visual cortex. b. Experimental timeline. Animals were injected with AAV expressing GCaMP6s 10–14 days prior to imaging. Imaging was performed at P27–32 or P39–43. P21–24 data from ref (Powell et al., 2024) (indicated by *) is presented for comparison with other ages. c. Imaged locations for P27–32 animals reconstructed from histology, colored based on assigned cortical area. d. Same as (c) for P39–43 animals.

Figure 2:. Spontaneous events exhibit widespread and distributed modular activity in diverse brain areas at P27–32.

a. Example timecourse of spontaneous activity in different cortical areas. Numbers indicate events shown in b and c. b. Spontaneous events show modular patterns of activation in PFC, PPC, A1, S1, and V1 at P27–32. Left: raw event pattern showing clear modular patterns of activity in all areas at time (1) in (a). Right: Same event after applying a highpass spatial filter. c. Second representative event from same experiments as (b), at time (2) in (a). Scale bars (a): 0.1 ΔF/F, 20 sec; (b-c): 1 mm.

Figure 3:. Spontaneous events continue to exhibit widespread and distributed modular activity at P39–43.

a. Example timecourse of spontaneous activity in different cortical areas. Numbers indicate events shown in b and c. b. Spontaneous events show modular patterns of activation in all areas at P39–43. Left: raw event pattern showing clear modular patterns of activity in all areas at time (1) in (a). Right: Same event after applying a highpass spatial filter. c. Second representative event from same experiments as (b), at time (2) in (a). Scale bars (a): 0.1 ΔF/F, 20 sec; (b-c): 1 mm.

Figure 4:. Developmental maturation of modular spontaneous activity across cortical areas.

a. Modularity of spontaneous events declines with age, but remains significant in all cases versus shuffle controls. b. The wavelength of active modules is relatively consistent across areas and shows a slight but significant decline with age in A1. c. The amplitude of active modules declines in a similar manner with age in all cortical areas examined. P21–24 data in (a-c) replotted for comparison from (Powell et al., 2024).

Figure 5:. Millimeter-scale modular correlations across diverse brain areas after eye-opening.

a. Correlations across spontaneous events for representative experiments at P27–32. Pixelwise correlations are shown relative to two different seed points for each area. Note that the spatial patterns of correlations vary between seed points, reflecting multiple distinct correlated networks. b. Modular correlations are still present at P39–43 in all cortical areas examined. Scale bar: 1 mm.

Figure 6:. Maturation of correlation strength and dimensionality across cortical areas.

a. Long-range correlations (1.8–2.2 mm) remain statistically significant versus control across age in all areas, while showing a significant decline with age in all areas except V1. Closed circles indicate experiments with significant correlations relative to shuffled controls. Open circles are non-significant at p<0.05. b. Dimensionality increases significantly with age in most cortical areas. Data from P21–24 animals in (a-c) replotted for comparison from (Powell et al., 2024).