Correlated network activity during development is not unique to sensory cortices or to rodents/mammals. Synchronous spontaneous activity has been observed in the thalamus, hippocampus, and the spinal cord. Correlated firing of developing thalamic neurons has been documented as early as E14.5 in vitro (Mire et al., 2012; Moreno-Juan et al., 2017). In one of the earliest calcium imaging studies in brain slices, ENOs were first reported in hippocampal CA1 neurons (Garaschuk et al., 1998), a finding that was expanded upon in subsequent studies (Allene et al., 2008; Crepel et al., 2007). Synchronous activity within CA1 in neonatal mice has also been documented in vivo (Dard et al., 2022; Leinekugel et al., 2002; Leprince et al., 2023; Valeeva et al., 2019). The dependence of ENOs on glutamate has also been demonstrated in entorhinal cortex in vitro (Unichenko et al., 2015). Bursts of neuronal activity have also been recorded in the neonatal rat spinal cord and were closely linked to spontaneous hindlimb myoclonic twitches (Inacio et al., 2016). Whether CNA also occurs in the neonatal amygdala, striatum, hypothalamus, or other brain regions remains to be determined, but there is no reason to believe that early patterns of activity in those regions would not be synchronous. Indeed, in vitro studies have revealed synchronous activity patterns in the embryonic and neonatal striatum (Dehorter et al., 2011; Ferrari et al., 2012), but this has yet to be confirmed in vivo.
Synchronous activity has been found across the kingdom, from invertebrates to mammals. In Drosophila larvae, activity is synchronous, referred to as patterned stimulus-independent neuronal activity (PSINA) (Akin et al., 2019; Bajar et al., 2022). In newborn ferrets and fetal cats, spontaneous bursts of synchronized activity sweep across the retina (Meister et al., 1991). In the dorsal lateral geniculate nucleus and surrounding thalamic regions of anesthetized ferrets, spontaneous rhythmic activity can be detected via in vivo extracellular recordings around 3 weeks of age; by age three months, spindle waves are robust and regular, occurring about every 5-7 s (McCormick et al., 1995). Synchronous bursts of activity were recorded between P22 and P28 in V1 of awake behaving young ferrets in which both thalamically mediated (LGN) and locally driven cortical CNA coexist (Chiu and Weliky, 2001). More recently, CNA was reported during cortical development in marsupial dunnarts as early as Stage 23 (equivalent of E15 in mice). Owing to its unique maturation in the pouch, these non-placental mammals may be well suited to uncover mechanisms of different activity patterns, such as patches in developing S1 vs. waves in V1 (Suarez et al., 2023).