![]() Other properties of local inhibitory circuits, such as the spatial decay of synaptic connectivity, may also be tailored to specific regions’ roles in sensory processing, decision-making, or the control of behavioral output. ![]() Theoretical work suggests that this difference underlies flexible coding properties of association cortex ( Wang and Yang, 2018). Though many aspects of inhibitory microcircuitry are conserved across cortical areas, the relative density of SOM to PV neurons increases from sensory to association cortex ( Dienel et al., 2020 Kim et al., 2017). SOM neurons can powerfully influence correlated variability in local networks ( Chen et al., 2015b Veit et al., 2017), due to their broad lateral pooling of excitatory inputs ( Adesnik et al., 2012) and dense innervation of local neurons ( Fino and Yuste, 2011), and disinhibit excitatory neurons through PV neurons ( Pfeffer et al., 2013). Cortical inhibitory neurons can largely be divided into three nonoverlapping subtypes, which express parvalbumin (PV), somatostatin (SOM), or vasoactive intestinal peptide (VIP), and participate in distinct local circuit motifs ( Tremblay et al., 2016). The spatial and temporal properties of population activity depend on the interactions among a rich diversity of excitatory and inhibitory cell types, which are not yet well understood, especially outside of sensory cortex. The spatial and temporal scales of cortical microcircuits may thus be tailored to the computational demands of each region, such as transforming topographically organized feedforward inputs in sensory cortex or integrating multimodal information over time in association cortex. Although the spatial scale of noise correlations has not been well-characterized outside of sensory cortex, in primate prefrontal cortex, noise correlations do not seem to decay with distance ( Safavi et al., 2018), suggesting that the spatial scale of functional connectivity may also vary across the cortical hierarchy. Pairwise noise correlations in sensory cortex, which tend to be highest among synaptically connected neurons ( Ko et al., 2011), decrease sharply over intersomatic distances ( Chelaru and Dragoi, 2016 Rosenbaum et al., 2017 Rothschild et al., 2010 Schulz et al., 2015 Smith and Kohn, 2008 Smith and Sommer, 2013). The spatial scale of shared variability may also increase systematically across cortex. As a result, information coding has a shorter timescale in AC than in PPC ( Runyan et al., 2017). For example, shared variability across neurons is lower in magnitude and decays more rapidly across time in auditory cortex (AC), a sensory region, than in posterior parietal cortex (PPC), an association level region. The timescale of information processing increases systematically along the cortical hierarchy, which allows sensory and association regions to specialize in cortical computations with distinct temporal scales ( Chaudhuri et al., 2015 Chen et al., 2015a Murray et al., 2014). A well-studied feature of population activity, the shared variability of activity between neurons that is unrelated to stimulus-evoked responses, can limit the information neural populations encode ( Averbeck et al., 2006 Bartolo et al., 2020 Moreno-Bote et al., 2014), but also improves the stability and consistency of representations over time ( Runyan et al., 2017 Valente et al., 2021). Incoming signals interact with rich, ongoing population activity dynamics throughout the brain. Sensory cortices process incoming signals from a single modality, while downstream association areas integrate inputs from multiple modalities to guide behavior. The mammalian neocortex is a hierarchically organized and highly interconnected network that can be subdivided into regions with functional specializations. Our results imply both generalization and specialization in the functional structure of inhibitory subnetworks across cortex. However, in both regions, activity of SOM neurons is more highly correlated than Non-SOM neurons’ activity. The spatial structure of shared variability among SOM and Non-SOM neurons differs across regions: correlations decay rapidly with distance in AC, but not PPC. Here, in mice, we compare the functional interactions among somatostatin-expressing (SOM) inhibitory interneurons and the rest of the neural population in auditory cortex (AC), a sensory region of cortex, and posterior parietal cortex (PPC), an association region. It is unclear whether specializations in the patterns of interactions among excitatory and inhibitory neurons underlie systematic differences in activity dynamics across cortex. These intrinsic dynamics are the consequence of interactions among local excitatory and inhibitory neurons and affect inter-region communication and information coding. Incoming signals interact with rich, ongoing population activity dynamics in cortical circuits.
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