Baseline Pupil Diameter Is Not a Reliable Biomarker of Subjective Sleepiness

The Impact of Pupil Constriction on the Relationship Between Melanopic EDI and Melatonin Suppression in Young Adult Males

The pupil modulates the amount of light that reaches the retina. Not only luminance but also the spectral distribution defines the pupil size. Previous research has identified steady-state pupil size and melatonin attenuation to be predominantly driven by melanopsin, which is expressed by a unique subgroup of intrinsically photosensitive retinal ganglion cells (ipRGCs) that are sensitive to short-wavelength light (~480 nm). Here, we aimed to selectively target the melanopsin system during the evening, while measuring steady-state pupil size and melatonin concentrations under commonly experienced evening light levels (<90 lx). Therefore, we used a five-primary display prototype to generate light conditions that were matched in terms of L-, M-, and S-cone-opic irradiances, but with high and low melanopic irradiances (~3-fold difference). Seventy-two healthy, male participants completed a 2-week study protocol. The volunteers were assigned to one of the four groups that differed in luminance levels (27-285 cd/m2). Within the four groups, each volunteer was exposed to a low melanopic (LM) and a high melanopic (HM) condition. The two 17-h study protocols comprised 3.5 h of light exposure starting 4 h before habitual bedtime. Median pupil size was significantly smaller during HM than LM in all four light intensity groups. In addition, we observed a significant correlation between melanopic weighted corneal illuminance (melanopic equivalent daylight illuminance [mEDI]) and pupil size, such that higher mEDI values were associated with smaller pupil size. Using pupil size to estimate retinal irradiance showed a qualitatively similar goodness of fit as mEDI for predicting melatonin suppression. Based on our results here, it remains appropriate to use melanopic irradiance measured at eye level when comparing light-dependent effects on evening melatonin concentrations in healthy young people at rather low light levels.

Quantifying Workload and Stress in Intensive Care Unit Nurses: Preliminary Evaluation Using Continuous Eye-Tracking

OBJECTIVE

(1) To assess mental workloads of intensive care unit (ICU) nurses in 12-hour working shifts (days and nights) using eye movement data; (2) to explore the impact of stress on the ocular metrics of nurses performing patient care in the ICU.

BACKGROUND

Prior studies have employed workload scoring systems or accelerometer data to assess ICU nurses' workload. This is the first naturalistic attempt to explore nurses' mental workload using eye movement data.

METHODS

Tobii Pro Glasses 2 eye-tracking and Empatica E4 devices were used to collect eye movement and physiological data from 15 nurses during 12-hour shifts (252 observation hours). We used mixed-effect models and an ordinal regression model with a random effect to analyze the changes in eye movement metrics during high stress episodes.

RESULTS

While the cadence and characteristics of nurse workload can vary between day shift and night shift, no significant difference in eye movement values was detected. However, eye movement metrics showed that the initial handoff period of nursing shifts has a higher mental workload compared with other times. Analysis of ocular metrics showed that stress is positively associated with an increase in number of eye fixations and gaze entropy, but negatively correlated with the duration of saccades and pupil diameter.

CONCLUSION

Eye-tracking technology can be used to assess the temporal variation of stress and associated changes with mental workload in the ICU environment. A real-time system could be developed for monitoring stress and workload for intervention development.

Diurnal Variation in Accommodation, Binocular Vergence, and Pupil Size

SIGNIFICANCE

Our results show significant diurnal variations in accommodative function and the magnitude of the phoria. Therefore, when comparing visual measures in clinical or laboratory settings, performing the visual examination at the same time of day (±1 hour) is encouraged.

PURPOSE

The aim of this study was to evaluate the accommodation, binocular vergence, and pupil behavior on three different times during the day.

METHODS

Twenty collegiate students (22.8 ± 2.1 years) participated in this study. Participants visited the laboratory on three different days at 2-hourly intervals (morning, 9:00 to 11:00 am ; afternoon, 2:00 to 4:00 pm ; evening, 7:00 to 9:00 pm ). The binocular vergence and accommodative function were measured using clinical optometric procedures, and the accommodative response and pupil function were evaluated in binocular conditions using the WAM-5500 autorefractometer.

RESULTS

The accommodative amplitude for the right and left eyes showed statistically significant differences for the time interval ( P = .001 and P = .02, respectively), revealing higher accommodative amplitude in the morning and afternoon in comparison with the evening. Participants were more esophoric when assessed in the morning in comparison with the evening at far and near ( P = .02 and P = .01, respectively) and when assessed in the afternoon in comparison with the evening at far distance ( P = .02). The magnitude of accommodative response was higher in the morning, and it decreased throughout the day at 500 ( P < .001), 40 ( P = .05), and 20 cm ( P < .001). No statistically significant differences were obtained for any other variable.

CONCLUSIONS

This study shows small diurnal variations in some accommodative and binocular vergence parameters, but no effects were observed for the pupil response. These outcomes are of special relevance for eye care specialists when performing repeated accommodative or binocular vergence measures. However, the diurnal variations were modest and may not influence a routine orthoptic examination.

Recommendation of using both initial pupil size and constriction latency in pupillary light reflex as objective indicators reflecting subacute pain-related mental fatigue

A previous study examining clinical subacute pain models under different methodological conditions showed that pain-induced mental fatigue can be associated with decreased initial pupil size (INIT)/shortened constriction latency (LAT) in the pupillary light reflex (PLR). We aimed to investigate the potential of INIT/LAT as objective indicators reflecting mental fatigue under the same methodological conditions. We recruited 118 patients planning to undergo three types of representative otolaryngological head and neck surgery procedures. We used the numerical rating scale (NRS) to assess subjective pain intensity and two mental fatigue-related mood categories of the Profile of Mood States, as well as INIT and LAT measurements (1) in the afternoon one day before surgery (pre1-surgery), (2) in the morning of the day of surgery (pre2-surgery), and (3) in the morning of the day following surgery (post-surgery). We assessed time point-dependent changes using one- or two-way analysis of variance, as well as responses of PLR parameters to mental fatigue using linear mixed-effects models (LMMs). As a result, NRS scores, the two mood categories, as well as LAT and INIT, showed significant time point-dependent changes. In post-hoc analyses, only INIT showed significant changes between the two pre-surgery time points. Thus, INIT values fluctuated even under pain-free conditions due to differences in the time of the day. LMMs demonstrated decreased INIT/shortened LAT related to mental fatigue. All surgical groups showed similar associations between mental fatigue and INIT/LAT findings. As each parameter has advantages and disadvantages, it is recommended to use both INIT and LAT as the indicators.

Investigating the Reliability of Pupillometry as a Measure of Individualized Listening Effort

Recordings of the pupillary response have been used in numerous studies to assess listening effort during a speech-in-noise task. Most studies focused on averaged responses across listeners, whereas less is known about pupil dilation as an indicator of the individuals’ listening effort. The present study investigated the reliability of several pupil features as potential indicators of individual listening effort and the impact of different normalization procedures on the reliability. The pupil diameters of 31 normal-hearing listeners were recorded during multiple visits while performing a speech-in-noise task. The signal-to-noise ratios (SNRs) of the stimuli ranged from −12 dB to +4 dB. All listeners were measured twice at separate visits, and 11 were re-tested at a third visit. To examine the reliability of the pupil responses across visits, the intraclass correlation coefficient was applied to the peak and mean pupil dilation and to the temporal features of the pupil response, extracted using growth curve analysis. The reliability of the pupillary response was assessed in relation to SNR and different normalization procedures over multiple visits. The most reliable pupil features were the traditional mean and peak pupil dilation. The highest reliability results were obtained when the data were baseline-corrected and normalized to the individual pupil response range across all visits. Moreover, the present study results showed only a minor impact of the SNR and the number of visits on the reliability of the pupil response. Overall, the results may provide an important basis for developing a standardized test for pupillometry in the clinic.

Importance of the locus coeruleus-norepinephrine system in sleep-wake regulation: implications for aging and Alzheimer’s disease

Five decades ago, seminal studies positioned the brainstem locus coeruleus (LC) norepinephrine (NE) system as a key substrate for the regulation of wakefulness and sleep, and this picture has recently been elaborated thanks to methodological advances in the precise investigation and experimental modulation of LC structure and functions. This review presents and discusses findings that support the major role of the LC-NE system at different levels of sleep-wake organization, ranging from its involvement in the overall architecture of the sleep-wake cycle to its associations with sleep microstructure, while accounting for the intricate neuroanatomy surrounding the LC. Given the particular position held by the LC-NE system by being at the intersection of sleep-wake dysregulation and initial pathophysiological processes of Alzheimer's disease (AD), we conclude by examining emerging opportunities to investigate LC-NE mediated relationships between sleep-wake alteration and AD in human aging. We further propose several research perspectives that could support the LC-NE system as a promising target for the identification of at-risk individuals in the preclinical stages of AD, and for the development of novel preventive interventions.

Reply to Bracke et al. Comment on "Prayag et al. Light Modulation of Human Clocks, Wake, and Sleep. Clocks&Sleep 2019, 1, 193-208"

We thank Bracke and colleagues [...].

Oxycodone Effect on Pupil Constriction in Recreational Opioid Users: A Pharmacokinetic/Pharmacodynamic Meta-Analysis Approach

INTRODUCTION

Understanding the effect of oxycodone pharmacokinetics (PK) on µ-opioid receptor binding benefits from an integrated approach to compiling the results of multiple studies. The current pharmacokinetic/pharmacodynamic (PK/PD) model analysis brings together various studies to support the interpretation of newly collected PK/PD data, putting the new results into the perspective of the full concentration-effect curve.

METHODS

A two-step modeling approach was applied to characterize the PK of oxycodone and its PK/PD relationship for the pupil diameter as a biomarker for µ-opioid receptor binding in recreational opioid users. First, a model-based meta-analysis (MBMA) was used to quantify the state-of-the-art knowledge from seven published studies, each of which contained part of the data needed for full characterization. Subsequently, the estimated parameters with uncertainty from the MBMA were used as prior information for a model developed on newly collected clinical data after intranasal administration in a clinical abuse potential trial.

RESULTS

The inclusion of intravenous data in the MBMA showed that the PK of oxycodone can be described by a two-compartmental model, and allowed for the estimation of absolute bioavailability after intranasal and oral administration. A hysteresis loop was observed when plotting plasma concentrations and pupil constriction, which was approximated using an effect compartment. The totality of literature data enabled the identification of a Hill equation for the drug effect. The model with prior information fitted successfully to the newly collected data, where most parameter estimates had their confidence intervals overlapping with the prior distribution. The new data led to a slightly lower intranasal absorption rate constant, explaining the longer apparent half-life of oxycodone in the newly collected data. The PK/PD model parameters were confirmed by the new data, leading to the following estimates: half maximal inhibitory concentration (IC₅₀) of 26.5 ng/mL, maximum pupil restriction of 66.0% from baseline, and a Hill factor of 1.05.

CONCLUSIONS

The new data confirmed the PK profile and the PK/PD relationship identified using the MBMA, resulting in similar parameter estimates except for the intranasal absorption rate constant. The latter was lower than in the MBMA and explained the slightly longer apparent half-life of oxycodone in the newly collected data.

The flicker Pupil Light Response (fPLR)

PURPOSE

The photoreceptor classes driving the flicker pupil light response (fPLR) to monochromatic sinusoidal temporal modulation are largely unknown. Here, we determine the photoreceptor inputs to the fPLR.

METHODS

The 0.5-Hz fPLR was measured in healthy observers using a Maxwellian view (41° diameter) pupillometer at five narrowband wavelengths (short: 409 nm; intermediate: 462, 507, 530 nm; and long: 592 nm) over ∼10 log units of irradiance spanning scotopic to photopic levels (5.6 to 15.6 log quanta·cm-2·s-1; -6.9 to 3.6 log cd·m-2). The relative photoreceptor contributions to the fPLR were then derived from these amplitude-irradiance functions using a criterion fPLR.

RESULTS

The fPLR amplitude is small (≤ 3.9 ± 3.1%; mean ± SEM) below 8.0 log quanta·cm-2·s-1 then increases with retinal irradiance in accordance with a Hill function that asymptotes between 13.0 to 15.0 log quanta·cm-2·s-1 (wavelength dependent). The Hill slope is steepest for the intermediate wavelengths. Further increases in irradiance (>15.0 log quanta·cm-2·s-1) produce a distinct suppression of the fPLR for the intermediate wavelengths. The fPLR phase delay shows a linear decrease with increasing irradiance. The spectral sensitivity of the fPLR is dominated by inner retinal melanopsin ganglion cell and outer retinal rod photoreceptor inputs to the afferent pupil control pathway; the relative melanopsin : rhodopsin weighting decreases with the transition from photopic to scotopic lighting.

CONCLUSIONS

The fPLR can be used as a marker of melanopsin and rod interactions during the flicker stimulation and to quantify their contributions to the post-illumination pupil response (PIPR).

TRANSLATIONAL RELEVANCE

These irradiance and wavelength responses will be useful in standardizing the measurements of the fPLR using chromatic pupillometry.

Steady-State Pupil Size Varies with Circadian Phase and Sleep Homeostasis in Healthy Young Men

Pupil size informs about sympathovagal balance as well as cognitive and affective processes, and perception. It is also directly linked to phasic activity of the brainstem locus coeruleus, so that pupil measures have gained recent attention. Steady-state pupil size and its variability have been directly linked to sleep homeostasis and circadian phase, but results have been inconsistent. Here, we report robust changes in steady-state pupil size during 29 h of continuous wakefulness in healthy young men (N = 20; 18–30 years old) maintained in dim-light in strictly controlled constant routine conditions. These variations were associated with variations in motivation and sustained attention performance. Pupil size variability did not significantly change during the protocol. Yet, pupil size variability was linearly associated with subjective fatigue, sociability, and anguish. No associations were found between neither steady-state pupil size nor pupil size variability, and objective EEG measure of alertness and subjective sleepiness. Our data support therefore the notion that, compared with its variability, steady-state pupil size is strongly influenced by the concomitant changes in sleep need and circadian phase. In addition, steady-state pupil size appears to be related to motivation and attention, while its variability may be related to separate affective dimensions and subjective fatigue.