Functional MRI of sleeping pigeons

Mammalian sleep plays a crucial role in maintaining optimal brain health by facilitating the clearance of toxic proteins that accumulate during wakefulness. The glymphatic system, responsible for flushing cerebral spinal fluid (CSF) through the brain, is particularly active during non-rapid eye movement (NREM) sleep in mice. In humans, functional magnetic resonance imaging (fMRI) has revealed an increase in ventricular CSF flow during NREM sleep. However, the relationship between sleep and CSF flow remains unexplored in avian species.Our study breaks new ground by employing fMRI to observe naturally sleeping pigeons, shedding light on the correlation between sleep states and CSF dynamics. Surprisingly, we found that rapid eye movement (REM) sleep in pigeons, characterized by wake-like brain activity, activates regions associated with visual information processing, especially those involved in interpreting optic flow during flight.Notably, our findings indicate that ventricular CSF flow in pigeons increases during NREM sleep compared to wakefulness, aligning with observations in mammals. However, during REM sleep, there is a sharp decline in CSF flow. This suggests a potential trade-off between the functions associated with brain activation during REM sleep, such as visual processing, and the essential waste clearance mechanisms predominantly occurring during NREM sleep. These insights into avian sleep patterns contribute to our understanding of the intricate relationship between different sleep states and their impact on cerebral fluid dynamics across species.