2. Intrinsic Arousal Dynamics Alternately Support Sensory Encoding and Hippocampal Ripples

COEX 

The variable nature of the brain's response to identical stimuli has long intrigued researchers. The state of arousal appears to be a critical factor given the enhancement of sensory responses during active behaviors, such as whisking or locomotion (1, 2), and through manipulation of the noradrenergic and cholinergic systems (3, 4). However, in absence of overt active behaviors or external arousal modulators, response variability persists, even over time frames of a few seconds (5–7). The nature and source of this seconds-scale fluctuation in the brain's response remains unclear. Recent studies revealed that ongoing spiking activity during rest is organized as a highly structured cascade dynamic that entrains ~70% of neurons across various regions into a temporal sequence of activations spanning multiple seconds (8). Importantly, this cascade dynamic is phase coupled to rapid, seconds-scale arousal modulations, which may in turn contribute to the seconds-scale variance of neural responses. By analyzing large-scale neuronal recordings from mice, we investigated whether the cascade dynamic persists during periods of continuous external sensory stimulation and, if so, whether the dynamic affects how the brain processes sensory inputs.