Mammalian pupils respond to light1,2 and dilate with arousal, attention, cognitive workload, and emotions,3 thus reflecting the state of the brain. Pupil size also varies during sleep, constricting during deep non-REM sleep4-7 and dilating slightly during REM sleep.4-6 Anecdotal reports suggest that, unlike mammals, birds constrict their pupils during aroused states, such as courtship and aggression,8-10 raising the possibility that pupillary behavior also differs between mammals and birds during sleep. Here, we measured pupil size in awake pigeons and used their translucent eyelid to investigate sleep-state-dependent changes in pupil size. Male pigeons constricted their pupils during courtship and other male-female interactions but not while engaging in other waking behaviors. Unlike mouse pupils, the pigeons' pupils were dilated during non-REM sleep, while over 1,000 bursts of constriction and relaxation, which we call rapid iris movements (RIMs), occurred primarily during REM sleep. Consistent with the avian iris being composed largely of striated muscles,11-15 rather than smooth muscles, as in mammals, pharmacological experiments revealed that RIMs are mediated by nicotinic cholinergic receptors in the iris muscles. Despite receiving input from a parasympathetic nucleus, but consistent with its striated nature, the avian iris sphincter muscle behaves like skeletal muscles controlled by the somatic nervous system, constricting during courtship displays, relaxing during non-REM sleep, and twitching during REM sleep. We speculate that during wakefulness, pupillary constrictions are involved in social communication, whereas RIMs occurring during REM sleep might maintain the efficacy of this motor system and/or reflect the processing of associated memories.
Keywords: REM; avian; cuckoo; iris; memory; pigeon; pupil; pupillometry; sleep; twitches.
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