Spontaneous activation of cortical somatosensory networks depresses their excitability in preterm human neonates

Introduction The activity of the developing cortex is characteristically discontinuous where sudden high amplitude bursts interrupt periods of quiescent background. While the functional importance of this activity is clear, its aetiology is not known. Here, we hypothesise that this alternating pattern arises because of “refractoriness” of cortical networks following spontaneous activation. Methods To test this hypothesis, we assessed whether spontaneous activity in sensory networks depressed their excitability by measuring the impact of ongoing activity on the response to an external sensory stimulus. We recorded cortical activity before and after mechanical tactile stimulation of hands and feet in 35 preterm infants of median 32 weeks post-menstrual age. Results Mechanical stimulation evoked wideband energy increases with two distinct peaks within the delta and alpha-beta band. The delta activity engaged extended cortical areas, while the faster activity engaged local somatotopically specific areas. By then characterising the spectro-spatial properties of the spontaneous activity preceding stimulation, we showed that baseline energy with a distribution and spectral profile similar to that of somatosensory-evoked activity dampened the energy changes elicited by touching the body. Discussion Sensory-evoked activity in preterm human neonates likely represents the coordinated activation of extended (tangential) and local (e.g. columnar) cortical aggregates. The occurrence of spontaneous cortical events in the same cortical regions depresses their excitability preventing their immediate re-engagement. This “refractoriness” offers the first etiological explanation to the cyclical burst-quiescence pattern typical of preterm cortical activity.