Correlated color temperature and light intensity: Complementary features in non-visual light field.

An appropriate exposure to the light-dark cycle, with high irradiances during the day and darkness during the night is essential to keep our physiology on time. However, considering the increasing exposure to artificial light at night and its potential harmful effects on health (i.e. chronodisruption and associated health conditions), it is essential to understand the non-visual effects of light in humans. Melatonin suppression is considered the gold standard for nocturnal light effects, and the activation of intrinsically photosensitive retinal ganglion cells (ipRGCs) through the assessment of pupillary light reflex (PLR) has been recently gaining attention. Also, some theoretical models for melatonin suppression and retinal photoreceptors activation have been proposed. Our aim in this study was to determine the influence of correlated color temperature (CCT) on melatonin suppression and PLR, considering two commercial light sources, as well as to explore the possible correlation between both processes. Also, the contribution of irradiance (associated to CCT) was explored through mathematical modelling on a wider range of light sources. For that, melatonin suppression and PLR were experimentally assessed on 16 healthy and young volunteers under two light conditions (warmer, CCT 3000 K; and cooler, CCT 5700 K, at ~5·1018 photons/cm2/sec). Our experimental results yielded greater post-stimulus constriction under the cooler (5700 K, 13.3 ± 1.9%) than under the warmer light (3000 K, 8.7 ± 1.2%) (p < 0.01), although no significant differences were found between both conditions in terms of melatonin suppression. Interestingly, we failed to demonstrate correlation between PLR and melatonin suppression. Although methodological limitations cannot be discarded, this could be due to the existence of different subpopulations of Type 1 ipRGCs differentially contributing to PLR and melatonin suppression, which opens the way for further research on ipRGCs projection in humans. The application of theoretical modelling suggested that CCT should not be considered separately from irradiance when designing nocturnal/diurnal illumination systems. Further experimental studies on wider ranges of CCTs and light intensities are needed to confirm these conclusions.

The effects of transcutaneous auricular vagal nerve stimulation on pupil size.

OBJECTIVE: Mechanisms of action and optimal stimulation parameters of transcutaneous auricular vagus nerve stimulation (taVNS) are currently unknown. Pupil size has gained attention as a promising biomarker of vagal activation in different studies on animal models. The aim of this study is to investigate the effects of taVNS on pupil diameter in healthy subjects. METHODS: All subjects received taVNS at the left external acoustic meatus and control stimulation at the left earlobe during the same experimental session. Different intensities (0.5 mA; 1.0 mA; 2.0 mA; 3.0 mA) for both conditions were tested. Tonic pupil size was recorded in both eyes at baseline and during each stimulation using an infrared-automated pupillometer in three different illuminance conditions (scotopic, mesopic, photopic). RESULTS: In scotopic illuminance condition, a significant interaction between intensity and condition (real vs control) was found for the left eye. Post-Hoc analysis showed that during real taVNS at 2 mA, pupil size was significantly larger in comparison to baseline and 2 mA control stimulation. CONCLUSIONS: Our study demonstrates that taVNS induces pupil dilation under specific illuminance conditions and at specific stimulation intensity. SIGNIFICANCE: The effects of taVNS are strictly dependent on technical aspects, such as stimulation parameters and experimental set-up.

Infants relax in response to unfamiliar foreign lullabies.

Music is characterized by acoustic forms that are predictive of its behavioural functions. For example, adult listeners accurately identify unfamiliar lullabies as infant-directed on the basis of their musical features alone. This property could reflect a function of listeners' experiences, the basic design of the human mind, or both. Here, we show that US infants (N = 144) relax in response to eight unfamiliar foreign lullabies, relative to matched non-lullaby songs from other foreign societies, as indexed by heart rate, pupillometry and electrodermal activity. They do so consistently throughout the first year of life, suggesting that the response is not a function of their musical experiences, which are limited relative to those of adults. The infants' parents overwhelmingly chose lullabies as the songs that they would use to calm their fussy infant, despite their unfamiliarity. Together, these findings suggest that infants may be predisposed to respond to common features of lullabies found in different cultures.

Transcutaneous Vagus Nerve Stimulation in Humans Induces Pupil Dilation and Attenuates Alpha Oscillations.

Vagus nerve stimulation (VNS) is widely used to treat drug-resistant epilepsy and depression. While the precise mechanisms mediating its long-term therapeutic effects are not fully resolved, they likely involve locus coeruleus (LC) stimulation via the nucleus of the solitary tract, which receives afferent vagal inputs. In rats, VNS elevates LC firing and forebrain noradrenaline levels, whereas LC lesions suppress VNS therapeutic efficacy. Noninvasive transcutaneous VNS (tVNS) uses electrical stimulation that targets the auricular branch of the vagus nerve at the cymba conchae of the ear. However, the extent to which tVNS mimics VNS remains unclear. Here, we investigated the short-term effects of tVNS in healthy human male volunteers (n = 24), using high-density EEG and pupillometry during visual fixation at rest. We compared short (3.4 s) trials of tVNS to sham electrical stimulation at the earlobe (far from the vagus nerve branch) to control for somatosensory stimulation. Although tVNS and sham stimulation did not differ in subjective intensity ratings, tVNS led to robust pupil dilation (peaking 4-5 s after trial onset) that was significantly higher than following sham stimulation. We further quantified, using parallel factor analysis, how tVNS modulates idle occipital alpha (8-13Hz) activity identified in each participant. We found greater attenuation of alpha oscillations by tVNS than by sham stimulation. This demonstrates that tVNS reliably induces pupillary and EEG markers of arousal beyond the effects of somatosensory stimulation, thus supporting the hypothesis that tVNS elevates noradrenaline and other arousal-promoting neuromodulatory signaling, and mimics invasive VNS.SIGNIFICANCE STATEMENT Current noninvasive brain stimulation techniques are mostly confined to modulating cortical activity, as is typical with transcranial magnetic or transcranial direct/alternating current electrical stimulation. Transcutaneous vagus nerve stimulation (tVNS) has been proposed to stimulate subcortical arousal-promoting nuclei, though previous studies yielded inconsistent results. Here we show that short (3.4 s) tVNS pulses in naive healthy male volunteers induced transient pupil dilation and attenuation of occipital alpha oscillations. These markers of brain arousal are in line with the established effects of invasive VNS on locus coeruleus-noradrenaline signaling, and support that tVNS mimics VNS. Therefore, tVNS can be used as a tool for studying how endogenous subcortical neuromodulatory signaling affects human cognition, including perception, attention, memory, and decision-making; and also for developing novel clinical applications.

Background luminance effects on pupil size associated with emotion and saccade preparation.

Pupil dilation is consistently evoked by affective and cognitive processing, and this dilation can result from sympathetic activation or parasympathetic inhibition. The relative contributions of the sympathetic and parasympathetic systems on the pupillary response induced by emotion and cognition may be different. Sympathetic and parasympathetic activity is regulated by global luminance level. Higher luminance levels lead to greater activation of the parasympathetic system while lower luminance levels lead to greater activation of the sympathetic system. To understand the contributions of the sympathetic and parasympathetic nervous systems to pupillary responses associated with emotion and saccade preparation, emotional auditory stimuli were presented following the fixation cue whose color indicated instruction to perform a pro- or anti-saccade while varying the background luminance level. Pupil dilation was evoked by emotional auditory stimuli and modulated by arousal level. More importantly, greater pupil dilation was observed with a dark background, compared to a bright background. In contrast, pupil dilation responses associated with saccade preparation were larger with the bright background than the dark background. Together, these results suggest that arousal-induced pupil dilation was mainly mediated by sympathetic activation, but pupil dilation related to saccade preparation was primarily mediated by parasympathetic inhibition.

Effective combination of pupil light reflex and heart rate variability to assess foot bath effects on autonomic function in healthy adults.

OBJECTIVES: To evaluate the effects of a foot bath on autonomic nervous system (ANS) activities as measured by pupil light reflex (PLR) and heart rate variability (HRV) in healthy adults. METHODS: PLR and HRV were assessed before and during a foot bath in healthy subjects (n = 110). The subjects dipped their feet in 40 °C water at a level below their knees for 15 min The light reflex in PLR was elicited at four ascending stimulus intensities. Linear-mixed effect models were fit with PLR parameters, and a Wilcoxon signed-rank test with HRV parameters. In PLR analysis, an interaction term between the foot bath intervention and the stimulus intensity was included as a fixed effect. RESULTS: The PLR analysis showed significant interactions in five out of seven parameters. The altered patterns indicated enhanced activity of the parasympathetic nervous system (PNS) due to the intervention. In contrast, the HRV analysis showed few alterations of PNS activities due to the intervention. CONCLUSIONS: The study results demonstrated differences in altered PNS activity during a foot bath, when assessed by PLR and HRV. In PLR, the stimulus intensity was a crucial factor for the evaluation of foot bath effects. Taken together, the combination of PLR and HRV effectively evaluates ANS activities during a foot bath.

Speech Planning at Turn Transitions in Dialog Is Associated With Increased Processing Load.

Speech planning is a sophisticated process. In dialog, it regularly starts in overlap with an incoming turn by a conversation partner. We show that planning spoken responses in overlap with incoming turns is associated with higher processing load than planning in silence. In a dialogic experiment, participants took turns with a confederate describing lists of objects. The confederate's utterances (to which participants responded) were pre-recorded and varied in whether they ended in a verb or an object noun and whether this ending was predictable or not. We found that response planning in overlap with sentence-final verbs evokes larger task-evoked pupillary responses, while end predictability had no effect. This finding indicates that planning in overlap leads to higher processing load for next speakers in dialog and that next speakers do not proactively modulate the time course of their response planning based on their predictions of turn endings. The turn-taking system exerts pressure on the language processing system by pushing speakers to plan in overlap despite the ensuing increase in processing load.

An oculometrics-based biofeedback system to impede fatigue development during computer work: A proof-of-concept study.

A biofeedback system may objectively identify fatigue and provide an individualized timing plan for micro-breaks. We developed and implemented a biofeedback system based on oculometrics using continuous recordings of eye movements and pupil dilations to moderate fatigue development in its early stages. Twenty healthy young participants (10 males and 10 females) performed a cyclic computer task for 31-35 min over two sessions: 1) self-triggered micro-breaks (manual sessions), and 2) biofeedback-triggered micro-breaks (automatic sessions). The sessions were held with one-week inter-session interval and in a counterbalanced order across participants. Each session involved 180 cycles of the computer task and after each 20 cycles (a segment), the task paused for 5-s to acquire perceived fatigue using Karolinska Sleepiness Scale (KSS). Following the pause, a 25-s micro-break involving seated exercises was carried out whether it was triggered by the biofeedback system following the detection of fatigue (KSS≥5) in the automatic sessions or by the participants in the manual sessions. National Aeronautics and Space Administration Task Load Index (NASA-TLX) was administered after sessions. The functioning core of the biofeedback system was based on a Decision Tree Ensemble model for fatigue classification, which was developed using an oculometrics dataset previously collected during the same computer task. The biofeedback system identified fatigue with a mean accuracy of approx. 70%. Perceived workload obtained from NASA-TLX was significantly lower in the automatic sessions compared with the manual sessions, p = 0.01 Cohen's dz = 0.89. The results give support to the effectiveness of integrating oculometrics-based biofeedback in timing plan of micro-breaks to impede fatigue development during computer work.

Are Fast Complex Movements Unimaginable? Pupillometric Studies of Motor Imagery in Expert Piano Playing.

Motor imagery (MI; mental simulation of actions) shares certain mental representations and processes with executed movement (ME). This neurocognitive overlap between MI and ME may explain why the systematic use of MI improves skilled performance in numerous domains. Unfortunately, the attentional mechanisms underlying MI remain unresolved. Therefore, the present studies investigated the role of attentional effort (as measured by pupil dilation) in MI. We evaluated the effects of movement complexity and speed on expert pianists' pupil dilation as they physically executed and used MI to perform easy/complex and slow/fast music phrases. Results revealed that easy movements required similar levels of attentional effort during MI and ME. However, during complex movements performed at a fast speed, the correspondence between execution and imagery of movement was disrupted.

Ocular Topical Anesthesia Does Not Attenuate Light-Induced Discomfort Using Blue and Red Light Stimuli.

Purpose: To investigate whether melanopsin-containing ophthalmic trigeminal ganglion cells provide significant input to mediate light-induced discomfort. This is done by studying the effect of ocular topical anesthesia on light-induced discomfort threshold to blue light and red light stimuli using a psychophysical approach. Method: Ten visually normal participants completed the experiment consisting of two trials: an anesthesia trial in which light stimuli were presented to both eyes following 0.5% proparacaine eye drops administration, and a placebo trial in which normal saline drops were used. In each trial, a randomized series of 280 blue and red light flashes were presented over seven intensity steps with 20 repetitions for each color and light intensity. Participants were instructed to report whether they perceived each stimulus as either "uncomfortably bright" or "not uncomfortably bright" by pressing a button. The proportion of "uncomfortable" responses was pooled to generate individual psychometric functions, from which 50% discomfort thresholds (defined as the light intensity at which the individuals perceived the stimulus to be uncomfortably bright/unpleasant 50% of the time) were calculated. Results: When blue light was presented, there was no significant difference in the light-induced discomfort thresholds between anesthesia and placebo trials (P = 0.44). Similarly, when red light was used, no significant difference in threshold values was found between the anesthesia and placebo trials (P = 0.28). Conclusions: Ocular topical anesthesia does not alter the light-induced discomfort thresholds to either blue or red light, suggesting that the melanopsin-containing ophthalmic trigeminal ganglion cells provide little or no significant input in mediating light-induced discomfort under normal physiologic conditions.