Pupil Size Coupling to Cortical States Protects the Stability of Deep Sleep via Parasympathetic Modulation

Abstract

During wakefulness, pupil diameter can reflect changes in attention, vigilance, and cortical states. How pupil size relates to cortical activity during sleep, however, remains unknown. Pupillometry during natural sleep is inherently challenging since the eyelids are usually closed. Here, we present a novel head-fixed sleep paradigm in combination with infrared back-illumination pupillometry (iBip) allowing robust tracking of pupil diameter in sleeping mice. We found that pupil size can be used as a reliable indicator of sleep states and that cortical activity becomes tightly coupled to pupil size fluctuations during non-rapid eye movement (NREM) sleep. Pharmacological blocking experiments indicate that the observed pupil size changes during sleep are mediated via the parasympathetic system. We furthermore found that constrictions of the pupil during NREM episodes might play a protective role for stability of sleep depth. These findings reveal a fundamental relationship between cortical activity and pupil size, which has so far been hidden behind closed eyelids.

Keywords: EEG; head fixed; infrared back-illumination pupillometry; mouse; parasympathetic; pupil tracking; sleep; sleep spindles; ultra-slow oscillations.

Figure 1

Head Fixed Sleep and iBip Pupil Tracking (A) X-ray images of a naturally sleeping mouse in a curled-up position. (B) Position of a sitting mouse during natural sleep. (C) Head-fixed mouse with a head angle of 30 degrees. (D) Front-view schematics of the infrared back-illumination pupillometry (iBip) pupil-tracking system. The 940 nm infrared light-emitting diode (LED) is placed above the skull, which allows the bright LED light to penetrate the head and back-illuminate the pupils. (E) Top row: power spectrogram of M1 ECoG signal during awake, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep states. Second row: a close-up of the ECoG and electromyography (EMG) signals during awake, NREM, and REM states. Third row: ECoG signal and pupil diameter during different cortical states. Data are from a habituated head-fixed mouse. The pupil diameter data are missing due to eye blinks at the beginning or end of REM periods. (F) Images of pupil with infrared back illumination in awake, NREM and REM states. See also Figures S1 and S2 and Movie S1.

Figure 2

Pupil Size Fluctuations in Different Sleep States and Its Coupling to Brain Oscillations (A) Left: median distribution (shaded regions are quartiles) of pupil diameter during REM, NREM, and awake states (n = 16 sessions). The pupil diameter has been normalized to its maximum recorded value in each experimental session. Insets show the location probability of the pupil’s contour in one representative session for each state separately. Right: median pupil diameter versus its distribution width (the middle range containing 95% of the data points) of the 16 sessions. (B) Top: ECoG recording of an example session with the identified sleep states based on delta, theta, and EMG power classification criteria (see STAR Methods for details). Bottom: predicted sleep states based on the pupil diameter recording (red trace) of the same session using a trained neural network algorithm (see STAR Methods for details). Note that the episodes marked in white include mixed NREM and awake bouts shorter than 100 s and were therefore excluded. (C) Example session illustrating co-fluctuations of pupil size (red) and the Hilbert amplitude of ECoG oscillations in the alpha frequency band depicted in gray and its low-passed trace (denoted as alpha amplitude) in black. (D) Mean (±SEM) Pearson’s correlation between ECoG oscillatory magnitude of each frequency band and pupil diameter (n = 16 sessions) and the corresponding correlation lags, where negative values indicate that changes in oscillation size lead changes in pupil size. ^∗p < 0.01, ^∗∗p < 0.001 (Student’s t test, Bonferroni corrected). (E) Left: mean (±SEM) variance accounted for between measured and predicted pupil size based on ECoG oscillations of each frequency band. Right: example session comparing the measured pupil size (red) to the fitted (top) and predicted (bottom) pupil size with a general linear model using alpha amplitude as a regressor. See also Figure S2 and STAR Methods for details.

Figure 3

Mechanisms Underlying Coupling of Pupil Diameter and Brain Activity (A) Schematics of the regulation of pupil size in relation to cortex through sympathetic and parasympathetic pathways (adapted from [28]). (B) Pupillary oscillations in both eyes in baseline condition in NREM sleep. (C) Pupillary oscillations in intact and dapiprazole-instilled eyes in NREM sleep. (D) Pupillary oscillations in intact and tropicamide-instilled eyes in NREM sleep. (E) Pupil size comparison of the eyes in baseline, dapiprazole-instilled, and tropicamide-instilled conditions in NREM sleep. Colored dots correspond to individual NREM bouts, and black dots are means (±SEM) of binned data (ten equally sized bins between the minimum and maximum size of the control pupil). (F) Cross-correlation of the pupil diameters in opposing eyes in NREM sleep in baseline, dapiprazole-instilled, and tropicamide-instilled conditions. (G) Cross-correlation of pupil diameter and the alpha power in the contralateral M1 in control, dapiprazole-instilled, and tropicamide-instilled conditions. See also Figure S3 and Movie S1.

Figure 4

Potential Role of Pupillary Constrictions during NREM Sleep (A) Schematics of the light-stimulation experiment. (B) Pupil size traces of the drugged and control eyes with EMG and bandlimited ECoG power signals in a typical stimulation experiment. 1-s-long light stimulations are marked with green vertical bars on the pupil diameter traces. (C) EMG activity at the time of light stimulation (two animals, 11 sessions, 196 control and 207 drugged-eye stimulation trials, mean [±SEM] in C–F). (D–F) Delta (D), alpha (E), and gamma (F) power change at the time of light stimulation.