Photic memory for executive brain responses

Acute effects of light during daytime on central aspects of attention and affect: A systematic review

Light regulates both image- and various non-image forming responses in humans, including acute effects on attention and affect. To advance the understanding of light's immediate effects, this systematic review describes the acute effects of monochromatic/narrow bandwidth and polychromatic white light during daytime on distinct aspects of attention (alertness, sustained attention, working memory, attentional control and flexibility), and measures of affect (self-report measures, performance-based tests, psychophysiological measures) in healthy, adult human subjects. Original, peer-reviewed (quasi-) experimental studies published between 2000 and May 2024 were included according to predefined inclusion and exclusion criteria. Study quality was assessed, and results were synthesized across aspects of attention and affect and grouped according to light interventions; monochromatic/narrowband-width or polychromatic white light (regular white, bright white, and white with high correlated color temperature (CCT)). Results from included studies (n = 62) showed that alertness and working memory were most affected by light. Electroencephalographic markers of alertness improved the most with exposure to narrow bandwidth long-wavelength light, regular white, and white light with high CCT. Self-reported alertness and measures of working memory improved the most with bright white light. Results from studies testing the acute effects on sustained attention and attentional control and flexibility were inconclusive. Performance-based and psychophysiological measures of affect were only influenced by narrow bandwidth long-wavelength light. Polychromatic white light exerted mixed effects on self-reported affect. Studies were strongly heterogeneous in terms of light stimuli characteristics and reporting of light stimuli and control of variables influencing light's acute effects.

Prefrontal cortex neurons encode ambient light intensity differentially across regions and layers

While light can affect emotional and cognitive processes of the medial prefrontal cortex (mPFC), no light-encoding was hitherto identified in this region. Here, extracellular recordings in awake mice revealed that over half of studied mPFC neurons showed photosensitivity, that was diminished by inhibition of intrinsically photosensitive retinal ganglion cells (ipRGCs), or of the upstream thalamic perihabenular nucleus (PHb). In 15% of mPFC photosensitive neurons, firing rate changed monotonically along light-intensity steps and gradients. These light-intensity-encoding neurons comprised four types, two enhancing and two suppressing their firing rate with increased light intensity. Similar types were identified in the PHb, where they exhibited shorter latency and increased sensitivity. Light suppressed prelimbic activity but boosted infralimbic activity, mirroring the regions' contrasting roles in fear-conditioning, drug-seeking, and anxiety. We posit that prefrontal photosensitivity represents a substrate of light-susceptible, mPFC-mediated functions, which could be ultimately studied as a therapeutical target in psychiatric and addiction disorders.

Seasonality in regional brain glucose metabolism

BACKGROUND

Daylength and the rates of changes in daylength have been associated with seasonal fluctuations in psychiatric symptoms and in cognition and mood in healthy adults. However, variations in human brain glucose metabolism in concordance with seasonal changes remain under explored.

METHODS

In this cross-sectional study, we examined seasonal effects on brain glucose metabolism, which we measured using 18F-fluorodeoxyglucose-PET in 97 healthy participants. To maximize the sensitivity of regional effects, we computed relative metabolic measures by normalizing the regional measures to white matter metabolism. Additionally, we explored the role of rest-activity rhythms/sleep-wake activity measured with actigraphy in the seasonal variations of regional brain metabolic activity.

RESULTS

We found that seasonal variations of cerebral glucose metabolism differed across brain regions. Glucose metabolism in prefrontal regions increased with longer daylength and with greater day-to-day increases in daylength. The cuneus and olfactory bulb had the maximum and minimum metabolic values around the summer and winter solstice respectively (positively associated with daylength), whereas the temporal lobe, brainstem, and postcentral cortex showed maximum and minimum metabolic values around the spring and autumn equinoxes, respectively (positively associated with faster daylength gain). Longer daylength was associated with greater amplitude and robustness of diurnal activity rhythms suggesting circadian involvement.

CONCLUSIONS

The current findings advance our knowledge of seasonal patterns in a key indicator of brain function relevant for mood and cognition. These data could inform treatment interventions for psychiatric symptoms that peak at specific times of the year.

The cognitive impact of light: illuminating ipRGC circuit mechanisms

Ever-present in our environments, light entrains circadian rhythms over long timescales, influencing daily activity patterns, health and performance. Increasing evidence indicates that light also acts independently of the circadian system to directly impact physiology and behaviour, including cognition. Exposure to light stimulates brain areas involved in cognition and appears to improve a broad range of cognitive functions. However, the extent of these effects and their mechanisms are unknown. Intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged as the primary conduit through which light impacts non-image-forming behaviours and are a prime candidate for mediating the direct effects of light on cognition. Here, we review the current state of understanding of these effects in humans and mice, and the tools available to uncover circuit-level and photoreceptor-specific mechanisms. We also address current barriers to progress in this area. Current and future efforts to unravel the circuits through which light influences cognitive functions may inform the tailoring of lighting landscapes to optimize health and cognitive function.

Predicting ischemic stroke risk from atrial fibrillation based on multi-spectral fundus images using deep learning

Background

Ischemic stroke (IS) is one of the most common serious secondary diseases of atrial fibrillation (AF) within 1 year after its occurrence, both of which have manifestations of ischemia and hypoxia of the small vessels in the early phase of the condition. The fundus is a collection of capillaries, while the retina responds differently to light of different wavelengths. Predicting the risk of IS occurring secondary to AF, based on subtle differences in fundus images of different wavelengths, is yet to be explored. This study was conducted to predict the risk of IS occurring secondary to AF based on multi-spectrum fundus images using deep learning.

Methods

A total of 150 AF participants without suffering from IS within 1 year after discharge and 100 IS participants with persistent arrhythmia symptoms or a history of AF diagnosis in the last year (defined as patients who would develop IS within 1 year after AF, based on fundus pathological manifestations generally prior to symptoms of the brain) were recruited. Fundus images at 548, 605, and 810 nm wavelengths were collected. Three classical deep neural network (DNN) models (Inception V3, ResNet50, SE50) were trained. Sociodemographic and selected routine clinical data were obtained.

Results

The accuracy of all DNNs with the single-spectral or multi-spectral combination images at the three wavelengths as input reached above 78%. The IS detection performance of DNNs with 605 nm spectral images as input was relatively more stable than with the other wavelengths. The multi-spectral combination models acquired a higher area under the curve (AUC) scores than the single-spectral models.

Conclusions

The probability of IS secondary to AF could be predicted based on multi-spectrum fundus images using deep learning, and combinations of multi-spectrum images improved the performance of DNNs. Acquiring different spectral fundus images is advantageous for the early prevention of cardiovascular and cerebrovascular diseases. The method in this study is a beneficial preliminary and initiative exploration for diseases that are difficult to predict the onset time such as IS.

Outdoor artificial light at night and risk of early-onset dementia: A case-control study in the Modena population, Northern Italy

Background

Dementia is a neurological syndrome characterized by severe cognitive impairment with functional impact on everyday life. It can be classified as young onset dementia (EOD) in case of symptom onset before 65, and late onset dementia (LOD). The purpose of this study is to assess the risk of dementia due to light pollution, and specifically outdoor artificial light at night (LAN).

Methods

Using a case-control design, we enrolled dementia patients newly-diagnosed in the province of Modena in the period 2017-2019 and a referent population from their caregivers. We geo-referenced the address of residence on the date of recruitment, provided it was stable for the previous five years. We assessed LAN exposure through 2015 nighttime luminance satellite images from the Visible Infrared Imaging Radiometer Suite (VIIRS). Using a logistic regression model adjusted for age, sex, and education, we calculated the risk of dementia associated with increasing LAN exposure, namely using <10 nW/cm²/sr as reference and considering ≥10-<40 nW/cm²/sr intermediate and ≥40 nW/cm²/sr high exposure, respectively We also implemented non-linear assessment using a spline regression model.

Results

We recruited 58 EOD cases, 34 LOD cases and 54 controls. Average LAN exposure levels overlapped for EOD cases and controls, while LOD cases showed higher levels. Compared with the lowest exposure, the risk of EOD associated with LAN was higher in the intermediate exposure (OR = 1.36, 95% CI 0.54-3.39), but not in the high exposure category (OR = 1.04, 95% CI 0.32-3.34). In contrast, the risk of LOD was positively associated with LAN exposure, with ORs of 2.58 (95% CI 0.26-25.97) and 3.50 (95% CI 0.32-38.87) in the intermediate and high exposure categories, respectively. The spline regression analysis showed substantial lack of association between LAN and EOD, while almost linear although highly imprecise association emerged for LOD.

Conclusions

Although the precision of the estimates was affected by the limited sample size and the study design did not allow us to exclude the presence of residual confounding, these results suggest a possible role of LAN in the etiology of dementia, particularly of its late-onset form.

Customized designed notch filters and applied effects on glare and contrast sensitivity in patients with dry eye syndrome

PURPOSE

Glare visual acuity and contrast sensitivity are important indicators of visual quality. Studies have shown that the glare visual acuity and contrast sensitivity in dry eye patients tend to degenerate, further affecting their quality of life. The objective of this study was to investigate the effect of notch filters on glare VA and contrast sensitivity in patients with dry eye or with dry eye syndrome.

METHOD

36 subjects in the 20‒65 age group were diagnosed as having dry eye disease or perceived dry eye syndromes themselves who were included after the initial screening with the OSDI questionnaire, and one was subsequently excluded as they had undergone retinal detachment surgery. Finally, 35 subjects (14 male and 21 female) with a mean age of 40.66 ± 15.62 years participated in this study. All subjects wore their habitual prescriptions and four different filter lenses (namely 480, 620, dual 480 & 620 notch filter, and FL-41 tinted lens), and measured the parameters of glare visual acuity and contrast sensitivity using CSV-1000 and sine wave contrast test (SWCT), respectively. Student t-test and Repeated measurement analysis (R-ANOVA) were utilized by using SPSS 26.0 software.

RESULTS

A dual-wavelength 480 & 620 nm optical notch filter had a significant anti-glare effect decreasing glare disabilities or discomfort, and leading to better visual quality, the same effect was also shown on a 480 nm notch filter lens. All participants showed a significant difference among the baseline, three notch filters (480 nm, 620 nm, dual-wavelength 480 & 620 nm), and FL-41 tinted lens were used on SWCT_A (1.5 cpd, F = 3.054, p = 0.019) and SWCT_E (18 cpd, F = 2.840, p = 0.049); but did not show statistical different on SWCT_B (3 cpd, F = 0.333, p = 0.771), SWCT_C (6 cpd, F = 1.779, p = 0.159), and SWCT_D (12 cpd, F = 1.447, p = 0.228). The baseline showed the best visual performance on CS at a low spatial frequency (SWCT_A, 1.5 cpd), any filter might reduce the contrast sensitivity at low spatial frequencies in the clinical trial, whereas 480 nm notch filter showed the best effectiveness on CS at a high spatial frequency (SWCT_E, 18 cpd), the FL-41 lens that also filters out the 480 nm band does not achieve the same effect. Moreover, patients with dry eye or those older than 40 years old preferred optical multilayer notch filters to FL-41 tinted lenses.

CONCLUSION

The 480- & 620-nm dual-wavelength and 480-nm single-wavelength notch filters have the best effect on the glare visual acuity and contrast sensitivity (CS) at high spatial frequencies in dry eye patients. The 620-nm notch filter performs better in CS at low and mid-low spatial frequencies; the FL-41 tinted lens performs poorly for glare VA and CS spatial frequencies examination. Patients with glare disabilities or CS disturbance at high spatial frequencies may choose a 480-nm notch filter lens, and patients who have CS disturbance at low spatial frequencies may consider a 620-nm notch filter for the prescription.

Light as a Modulator of Non-Image-Forming Brain Functions—Positive and Negative Impacts of Increasing Light Availability

Light use is rising steeply, mainly because of the advent of light-emitting diode (LED) devices. LEDs are frequently blue-enriched light sources and may have different impacts on the non-image forming (NIF) system, which is maximally sensitive to blue-wavelength light. Most importantly, the timing of LED device use is widespread, leading to novel light exposure patterns on the NIF system. The goal of this narrative review is to discuss the multiple aspects that we think should be accounted for when attempting to predict how this situation will affect the NIF impact of light on brain functions. We first cover both the image-forming and NIF pathways of the brain. We then detail our current understanding of the impact of light on human cognition, sleep, alertness, and mood. Finally, we discuss questions concerning the adoption of LED lighting and screens, which offer new opportunities to improve well-being, but also raise concerns about increasing light exposure, which may be detrimental to health, particularly in the evening.

Light at night and cause-specific mortality risk in Mainland China: a nationwide observational study

BACKGROUND

While epidemiological studies have found correlations between light at night (LAN) and health effects, none has so far investigated the impacts of LAN on population mortality yet. We aimed to estimate the relative risk for mortality from exposure to LAN in Mainland China.

METHODS

This time-stratified case-crossover nationwide study used NPP-VIIRS to obtain daily LAN data of Mainland China between 2015 and 2019. The daily mortality data were obtained from the Disease Surveillance Point System in China. Conditional Poisson regression models were applied to examine the relative risk (RR) for mortality along daily LAN in each county, then meta-analysis was performed to combine the county-specific estimates at the national or regional level.

RESULTS

A total of 579 counties with an average daily LAN of 4.39 (range: 1.02-35.46) were included in the main analysis. The overall RRs per 100 nW/cm²/sr increases in daily LAN were 1.08 (95%CI: 1.05-1.11) for all-cause mortality and 1.08 (95%CI: 1.05-1.11) for natural-cause mortality. A positive association between LAN and all natural cause-specific mortality was observed, of which the strongest effect was observed on mortality caused by neuron system disease (RR = 1.32, 95%CI: 1.14-1.52). The results were robust in both younger and old, as well as in males and females. The more pronounced effect of LAN was observed in median LAN-level regions. Combined with an exposure-response curve, our study suggests a non-linear association between LAN and mortality in China.

CONCLUSIONS

Our study shows LAN is associated with mortality in China, particularly for neuron system disease-related mortality. These findings have important implications for public health policy establishment to minimize the health consequences of light pollution.

Examining the Efficacy of Bright Light Therapy on Cognitive Function in Hematopoietic Stem Cell Transplant Survivors

Patients who have undergone hematopoietic stem cell transplant (HSCT) may experience cognitive impairment that can persist after treatment. Several studies have shown that bright light therapy may improve cognition, potentially due to its effects on the circadian system via brain regions that respond preferentially to light. In this double-blind randomized controlled trial, the efficacy of bright light therapy on cognition was examined in HSCT survivors. Forty-seven HSCT survivors at an urban hospital in the United States were screened for mild cognitive impairment, randomized to either bright white light (BWL) or comparison dim red light (DRL) conditions using a block randomization approach, and instructed to use their assigned light box every morning upon awakening for 30 min for 4 weeks. Assessments occurred at baseline, the end of the second week of the intervention, the end of the intervention, and at follow-up (8 weeks later). The primary outcome was objective cognitive function as measured by a global composite score on neuropsychological tests. Secondary outcomes included cognitive performance in individual domains, self-reported cognitive function, fatigue, sleep and sleep quality, and circadian rhythm robustness. Repeated-measures linear mixed models for both objective and self-reported cognitive function indicated significant main effects for time (ps < 0.05) suggesting significant improvements in both conditions over time. Time by light condition interaction effects were not significant. Models focused on secondary outcomes yielded no significant effects. Both BWL and DRL groups demonstrated significant improvements in objective cognitive and self-reported cognitive function over time, but there was no hypothesized effect of BWL over DRL nor associations with circadian rhythm robustness. Therapeutic effects of both light conditions, practice effects, and/or placebo effects may account for the findings.Trial registration: ClinicalTrials.gov Identifier: NCT02677987 (9 February 2016).

The effects of light colour on female rabbit reproductive performance and the expression of key genes in follicular development

The purpose of this study was to analyse the effects of light colour on rabbit reproductive performance and the expression of key follicular development genes. Rabbits (n = 1,068, 5 months old, 3.6–4.4 kg live body weight) were divided randomly into four groups, housed individually in wire mesh cages and exposed to red, green, blue, and white light-emitting diode (LED) light (control). The lighting schedule was 16 L : 8 D-15 d / 150 lx / 6:00 am–22:00 pm (3 d preartificial insemination to 12 d postartificial insemination). Red light and white light affected the conception rate and kindling rate and increased the total litter size at birth (p < 0.05). The effects of red light on litter size at weaning, litter weight at weaning, and individual weight at weaning increased compared with the green and blue groups. The effects of red light on live litter size at birth were increased compared with those in the blue group (p < 0.05). Compared to white light, green and blue light reduced the number of secondary follicles (p < 0.05). Compared to red light, green and blue light reduced the number of tertiary follicles (p < 0.05). Compared with white light, red LED light resulted in greater ovarian follicle stimulating hormone receptor and luteinizing hormone receptor mRNA expression (p < 0.05). Compared with green and blue LED light, red LED light resulted in greater B-cell lymphom-2 mRNA expression (p < 0.05). Compared with green LED light, red LED light inhibited FOXO1 mRNA expression in rabbit ovaries (p < 0.05). Red light can affect the reproductive performance of female rabbits and the expression of key genes for follicular development.

Impact of daytime spectral tuning on cognitive function

Light at night can improve alertness and cognition. Exposure to daytime light, however, has yielded less conclusive results. In addition to direct effects, daytime light may also mitigate the impact of nocturnal light exposure on alertness. To examine the impact of daytime lighting on daytime cognitive performance, and evening alertness, we studied nine healthy individuals using a within subject crossover design. On four visits, participants were exposed to one of four lighting conditions for 10 h (dim fluorescent, room fluorescent, broad-spectrum LED, standard white LED; the latter three conditions were matched for 100 lx) followed by an exposure to bright evening light. Cognitive performance, subjective and objective measures of alertness were regularly obtained. While daytime alertness was not impacted by light exposure, the broad-spectrum LED light improved several aspects of daytime cognition. The impact of evening light on alertness was not mitigated by the pre-exposure to different daytime lighting conditions. Results suggest that daytime exposure to white light with high melanopic efficacy has the potential to improve daytime cognitive function and that such improvements are likely to be direct rather than a consequence of light-induced changes in alertness.

Cross-sectional study of intraocular cataract lens replacement, circadian rest–activity rhythms, and sleep quality in older adults

Study Objectives

Age-related cataract decreases light transmission at the most sensitive spectrum for circadian photoentrainment, with negative ramifications for human health. Here, we assessed whether intraocular lens replacement (IOL) in older patients with previous cataract was associated with increased stability and amplitude of circadian rest–activity rhythms, and improved sleep quality.

Methods

Our cross-sectional study included sixteen healthy older individuals without ocular diseases (controls; 55–80 years; 63.6 ± 5.6y; 8 women) and 13 patients with previous cataract and bilateral IOL (eight with blue-blocking [BB] lens and five with ultraviolet-only [UV] blocking lens; 55–80 years; 69.9 ± 5.2y; 9 women). The study comprised three weeks of at home rest–activity assessments using wrist-worn actigraphs, and each week preceded a laboratory protocol. Primary outcomes were actigraphy-derived interdaily stability, intradaily variability, and relative amplitude of circadian rest–activity rhythms. Secondary outcomes were actigraphy-assessed sleep quality (i.e. time in bed, sleep duration, sleep efficiency, mean wake bout time and fragmentation index).

Results

Patients with IOL had significantly higher interdaily stability (“Group” effect: pFDR =.001), but not intradaily variability (“Group” effect: pFDR = n.s.), and significantly higher relative amplitude of rest–activity rhythms (“Group” effect: pFDR &lt; .001). Moreover, patients with IOL had significantly higher activity levels during the day and lower levels during the evening, as compared to healthy older controls (“Group” effect: pFDR = .03). Analyses of actigraphy-derived sleep parameters yielded no significant differences across groups (“Group” effect: all pFDR &gt; .1).

Conclusions

Our cross-sectional study suggests that enhancing spectral lens transmission in patients with cataract may benefit their circadian health.

The role of melanopsin photoreception on visual attention linked pupil responses

A decision during a visual task is marked by a task‐evoked pupil dilation (TEPD) that is linked to the global cortical arousal state. Melanopsin expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) form the afferent pathway for this pupil response. Melanopsin activation also influences mood and arousal and increases activity in decision‐making brain areas that receive direct ipRGC projections. Here, an optical photostimulation method controlled the excitations of all five photoreceptor classes in the human eye to isolate melanopsin‐mediated photoreception. We hypothesised that the TEPD can be driven by directing active visual covert attention through the ipRGC pathway. When observers are completely certain of the stimulus presence, melanopsin‐directed stimulation produces a TEPD of similar amplitude to a cone‐directed stimulation, with their combination producing larger amplitudes. This dilation is satisfactorily modelled by linear addition with a higher melanopsin weighting in ipRGCs. Visual reaction times were longest in response to melanopsin‐directed lights. Next, we asked whether the afferent photoreceptor input and decision certainty, controlled by priming the observer's a priori expectation, interact to drive the TEPD. Signal detection analysis showed that by fixing the predecision certainty (bias), the phasic arousal and TEPD amplitude vary with observer criterion (c′) and sensitivity (d′) but not with preferential activation of melanopsin. The signature feature of the melanopsin response during attention was a biphasic TEPD. We conclude that active covert attention can be modulated by visual information mediated via ipRGCs, but that phasic arousal responses marked using the TEPD are not increased by higher levels of melanopsin activation.

Current Insights into Optimal Lighting for Promoting Sleep and Circadian Health: Brighter Days and the Importance of Sunlight in the Built Environment

This perspective considers the possibility that daytime's intrusion into night made possible by electric lighting may not be as pernicious to sleep and circadian health as the encroachment of nighttime into day wrought by 20th century architectural practices that have left many people estranged from sunlight.

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 [...].

Exposure to Short Photoperiod Regime Restores Spatial Cognition in Ventral Subicular Lesioned Rats: Potential Role of Hippocampal Plasticity, Glucocorticoid Receptors, and Neurogenesis

Ambient light influences our mood, behavior, and cognition. Phototherapy has been considered as an effective non-pharmacological intervention strategy in the restoration of cognitive functions following central nervous system insults. However, the cellular and molecular underpinnings of phototherapy-mediated functional recovery are yet to be studied. The present study examines the effectiveness of short photoperiod regime (SPR; 6:18-h light:dark cycle) in restoring the cognitive functions in ventral subicular lesioned rats. Bilateral ventral subicular lesion (VSL) resulted in significant impairment of spatial navigational abilities when tested in the Morris water maze (MWM) task. Further, VSL resulted in reduced expression of glucocorticoid receptors (GRs) and activity-regulated cytoskeletal (Arc) protein and suppression of neurogenesis in the hippocampus. VSL also suppressed the magnitude of long-term potentiation (LTP) in the hippocampal Schaffer collateral-CA1 synapses. However, exposure to SPR for 21 days showed significant restoration of spatial performance in the MWM task as the ventral subicular lesioned rats could deploy higher cognitive allocentric navigational strategies to reach the hidden platform. Further, SPR resulted in enhanced expression of hippocampal GR and Arc protein and neurogenesis but not hippocampal LTP suggestive of appropriate need-based SPR intervention. In conclusion, the study demonstrates the effectiveness of SPR in establishing functional recovery as well as the possible molecular and cellular basis of cognitive recovery in a rat model of neurodegeneration. Such studies provide a framework in understanding the efficacy of non-pharmacological strategies in establishing functional recovery in neurodegenerative conditions.

Light Promotes Neural Correlates of Fear Memory via Enhancing Brain-Derived Neurotrophic Factor (BDNF) Expression in the Prelimbic Cortex

Exposure to light has been shown to enhance vigilance and improve working memory, possibly due to changes in prefrontal function. Ample evidence supports the critical role of prefrontal cortex (PFC) in fear memory performance. However, the effects of light on memory processing and its potential mechanisms remain unclear. Here, through rats exposure conditioned to light at different memory phases, we sought evidence for the influences by employing behavioral tests, pharmacological infusions, immunoblotting, and electrophysiological recording. Exposure to light immediately following conditioning of 30 min or longer could effectively improve consolidation of fear memory without altering short-term memory or upgrading the original fear. The absence of significant freezing during baseline and intertrial interval periods ruled out the possibility of a general induction of freezing by light. Meanwhile, rats exposed to light in homecages or conditioning chambers exhibited a similar memory phenotype, indicating that light specifically enhanced the fear stimulus rather than the contextual environment. Furthermore, light exposure elevated the training-induced brain-derived neurotrophic factor (BDNF) expression in the prelimbic, but not infralimbic, subregion of the PFC. Moreover, the BDNF-TrkB pathway, but not the BDNF-p75^NTR pathway, was involved in light-mediated fear memory. The enhancement in BDNF activity effectively facilitated firing correlates of prelimbic pyramidal neurons but not fast-spiking interneurons. Blocking the training-induced BDNF by its antibody abolished the effects of light on neural function and fear memory. Therefore, our findings indicate that light enhances training-induced BDNF expression that promotes the neural correlate of memory function.

Color-dependent changes in humans during a verbal fluency task under colored light exposure assessed by SPA-fNIRS

Light evokes robust visual and nonvisual physiological and psychological effects in humans, such as emotional and behavioral responses, as well as changes in cognitive brain activity and performance. The aim of this study was to investigate how colored light exposure (CLE) and a verbal fluency task (VFT) interact and affect cerebral hemodynamics, oxygenation, and systemic physiology as determined by systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS). 32 healthy adults (17 female, 15 male, age: 25.5 ± 4.3 years) were exposed to blue and red light for 9 min while performing a VFT. Before and after the CLE, subjects were in darkness. We found that this long-term CLE-VFT paradigm elicited distinct changes in the prefrontal cortex and in most systemic physiological parameters. The subjects' performance depended significantly on the type of VFT and the sex of the subject. Compared to red light, blue evoked stronger responses in cerebral hemodynamics and oxygenation in the visual cortex. Color-dependent changes were evident in the recovery phase of several systemic physiological parameters. This study showed that the CLE has effects that endure at least 15 min after cessation of the CLE. This underlines the importance of considering the persistent influence of colored light on brain function, cognition, and systemic physiology in everyday life.

Age-related neuroendocrine and alerting responses to light

Aging is associated with sleep and circadian alterations, which can negatively affect quality of life and longevity. Importantly, the age-related reduction in light sensitivity, particularly in the short-wavelength range, may underlie sleep and circadian alterations in older people. While evidence suggests that non-image-forming (NIF) light responses may diminish in older individuals, most laboratory studies have low sample sizes, use non-ecological light settings (e.g., monochromatic light), and typically focus on melatonin suppression by light. Here, we investigated whether NIF light effects on endogenous melatonin levels and sleep frontal slow-wave activity (primary outcomes), and subjective sleepiness and sustained attention (secondary outcomes) attenuate with aging. We conducted a stringently controlled within-subject study with 3 laboratory protocols separated by ~ 1 week in 31 young (18-30 years; 15 women) and 16 older individuals (55-80 years; eight women). Each protocol included 2 h of evening exposure to commercially available blue-enriched polychromatic light (6500 K) or non-blue-enriched light (3000 K or 2500 K) at low levels (~ 40 lx, habitual in evening indoor settings). Aging significantly affected the influence of light on endogenous melatonin levels, subjective sleepiness, sustained attention, and frontal slow-wave activity (interaction: P < 0.001, P = 0.004, P = 0.007, P = 0.001, respectively). In young individuals, light exposure at 6500 K significantly attenuated the increase in endogenous melatonin levels, improved subjective sleepiness and sustained attention performance, and decreased frontal slow-wave activity in the beginning of sleep. Conversely, older individuals did not exhibit signficant differential light sensitivity effects. Our findings provide evidence for an association of aging and reduced light sensitivity, with ramifications to sleep, cognition, and circadian health in older people.

Individual differences in light sensitivity affect sleep and circadian rhythms

Artificial lighting is omnipresent in contemporary society with disruptive consequences for human sleep and circadian rhythms because of overexposure to light, particularly in the evening/night hours. Recent evidence shows large individual variations in circadian photosensitivity, such as melatonin suppression, due to artificial light exposure. Despite the emerging body of research indicating that the effects of light on sleep and circadian rhythms vary dramatically across individuals, recommendations for appropriate light exposure in real-life settings rarely consider such individual effects. This review addresses recently identified links among individual traits, for example, age, sex, chronotype, genetic haplotypes, and the effects of evening/night light on sleep and circadian hallmarks, based on human laboratory and field studies. Target biological mechanisms for individual differences in light sensitivity include differences occurring within the retina and downstream, such as the central circadian clock. This review also highlights that there are wide gaps of uncertainty, despite the growing awareness that individual differences shape the effects of evening/night light on sleep and circadian physiology. These include (1) why do certain individual traits differentially affect the influence of light on sleep and circadian rhythms; (2) what is the translational value of individual differences in light sensitivity in populations typically exposed to light at night, such as night shift workers; and (3) what is the magnitude of individual differences in light sensitivity in population-based studies? Collectively, the current findings provide strong support for considering individual differences when defining optimal lighting specifications, thus allowing for personalized lighting solutions that promote quality of life and health.

Modulation of recognition memory performance by light and its relationship with cortical EEG theta and gamma activities

Acute exposure to light exerts widespread effects on physiology, in addition to its key role in photoentrainment. Although the modulatory effect of light on physiological arousal is well demonstrated in mice, its effect on memory performance is inconclusive, as the direction of the effect depends on the nature of the behavioural task employed and/or the type of stimulus utilised. Moreover, in all rodent studies that reported significant effects of light on performance, brain activity was not assessed during the task and thus it is unclear how brain activity was modulated by light or the exact relationship between light-modulated brain activity and performance. Here we examine the modulatory effects of light of varying intensities on recognition memory performance and frontoparietal waking electroencephalography (EEG) in mice using the spontaneous recognition memory task. We report a light-intensity-dependent disruptive effect on recognition memory performance at the group level, but inspection of individual-level data indicates that light-intensity-dependent facilitation is observed in some cases. Using linear mixed-effects models, we then demonstrate that EEG fast theta (θ) activity at the time of encoding negatively predicts recognition memory performance, whereas slow gamma (γ) activity at the time of retrieval positively predicts performance. These relationships between θ/γ activity and performance are strengthened by increasing light intensity. Thus, light modulates θ and γ band activities involved in attentional and mnemonic processes, thereby affecting recognition memory performance. However, extraneous factors including the phase of the internal clock at which light is presented and homeostatic sleep pressure may determine how photic input is translated into behavioural performance.

Effect of photoperiod and light intensity on learning ability and memory formation of the pond snail Lymnaea stagnalis

Natural light is regarded as a key regulator of biological systems and typically serves as a Zeitgeber for biological rhythms. As a natural abiotic factor, it is recognized to regulate multiple behavioral and physiological processes in animals. Disruption of the natural light regime due to light pollution may result in significant effects on animal learning and memory development. Here, we investigated whether sensitivity to various photoperiods or light intensities had an impact on intermediate-term memory (ITM) and long-term memory (LTM) formation in the pond snail Lymnaea stagnalis. We also investigated the change in the gene expression level of molluscan insulin-related peptide II (MIP II) is response to the given light treatments. The results show that the best light condition for proper LTM formation is exposure to a short day (8 h light) and low light intensity (1 and 10 lx). Moreover, the more extreme light conditions (16 h and 24 h light) prevent the formation of both ITM and LTM. We found no change in MIP II expression in any of the light treatments, which may indicate that MIP II is not directly involved in the operant conditioning used here, even though it is known to be involved in learning. The finding that snails did not learn in complete darkness indicates that light is a necessary factor for proper learning and memory formation. Furthermore, dim light enhances both ITM and LTM formation, which suggests that there is an optimum since both no light and too bright light prevented learning and memory. Our findings suggest that the upsurge of artificial day length and/or night light intensity may also negatively impact memory consolidation in the wild.

Objective and Subjective Characteristics of Vigilance under Different Narrow-Bandwidth Light Conditions: Do Shorter Wavelengths Have an Alertness-Enhancing Effect?

The aim of this study was to explore the effects of 20 min of narrow-bandwidth light exposure of different wavelengths (455, 508, and 629 nm, with irradiance of 14 µW/cm2) on various neuropsychological and neurophysiological parameters of vigilance in healthy volunteers and to provide further evidence of the behavioral (subjective sleepiness, reaction time) and electrophysiological (P300 and spectral characteristics) responses to light. The results show that the short-wavelength light condition (455 nm) was found to be most effective in terms of its alerting effect for the following variables: subjective sleepiness, latency of P300 response, and absolute EEG power in higher beta (24-34 Hz) and gamma (35-50 Hz) range at each of the 19 recording electrodes. However, no differences in current power density were observed at the level of cortical EEG sources estimated by exact low-resolution electromagnetic tomography. Our results are in line with other research that shows significant alerting effects of blue (short-wavelength) light in comparison to lights of longer wavelengths. Our results confirm earlier findings that exposure to short-wavelength light during the day may enhance cognitive performance in task-specific scenarios.

Intraocular cataract lens replacement and light exposure potentially impact procedural learning in older adults

Procedural learning declines with age and appropriately timed light exposure can improve cognitive performance in older individuals. Because cataract reduces light transmission and is associated with cognitive decline in older adults, we explored whether lens replacement (intraocular blue-blocking [BB] or UV-only blocking) in older patients with cataracts enhances the beneficial effects of light on procedural learning. Healthy older participants (n = 16) and older patients with post-cataract surgery (n = 13 with BB or UV lens replacement) underwent a randomized within-subject crossover laboratory design with three protocols. In each protocol, 3.5 hr dim-dark adaptation was followed by 2 hr evening blue-enriched (6,500K) or non-blue-enriched light exposure (3,000K or 2,500K), 30 min dim post-light, ~8 hr sleep and 2 hr morning dim light. Procedural learning was assessed by the alternating serial reaction time task (ASRT), as part of a larger test battery. Here, ASRT performance was indexed by type of trial (random or sequence) and sequence-specific (high or low probability) measures. During evening light exposure, we observed a significant effect of the interaction of "group" versus "light condition" on the type of trial (p = .04; p = .16; unadjusted and adjusted p-values, respectively) and sequence-specific learning (p = .04; p = .16; unadjusted and adjusted p-values, respectively), whereby patients with UV lens replacement performed better than patients with BB lens or non-cataract controls, during blue-enriched light exposure. Lens replacement in patients with cataracts may potentially be associated with beneficial effects of blue light on procedural learning. Thus, optimizing spectral lens transmission in patients with cataracts may help improve specific aspects of cognitive function, such as procedural learning.

Environmental Light Is Required for Maintenance of Long-Term Memory in Drosophila

Long-term memory (LTM) is stored as functional modifications of relevant neural circuits in the brain. A large body of evidence indicates that the initial establishment of such modifications through the process known as memory consolidation requires learning-dependent transcriptional activation and de novo protein synthesis. However, it remains poorly understood how the consolidated memory is maintained for a long period in the brain, despite constant turnover of molecular substrates. Using the Drosophila courtship conditioning assay of adult males as a memory paradigm, here, we show that in Drosophila, environmental light plays a critical role in LTM maintenance. LTM is impaired when flies are kept in constant darkness (DD) during the memory maintenance phase. Because light activates the brain neurons expressing the neuropeptide pigment-dispersing factor (Pdf), we examined the possible involvement of Pdf neurons in LTM maintenance. Temporal activation of Pdf neurons compensated for the DD-dependent LTM impairment, whereas temporal knockdown of Pdf during the memory maintenance phase impaired LTM in light/dark cycles. Furthermore, we demonstrated that the transcription factor cAMP response element-binding protein (CREB) is required in the memory center, namely, the mushroom bodies (MBs), for LTM maintenance, and Pdf signaling regulates light-dependent transcription via CREB. Our results demonstrate for the first time that universally available environmental light plays a critical role in LTM maintenance by activating the evolutionarily conserved memory modulator CREB in MBs via the Pdf signaling pathway.SIGNIFICANCE STATEMENT Temporary memory can be consolidated into long-term memory (LTM) through de novo protein synthesis and functional modifications of neuronal circuits in the brain. Once established, LTM requires continual maintenance so that it is kept for an extended period against molecular turnover and cellular reorganization that may disrupt memory traces. How is LTM maintained mechanistically? Despite the critical importance of LTM maintenance, its molecular and cellular underpinnings remain elusive. This study using Drosophila is significant because it revealed for the first time in any organism that universally available environmental light plays an essential role in LTM maintenance. Interestingly, light does so by activating the evolutionarily conserved transcription factor cAMP response element-binding protein via peptidergic signaling.

Bright light exposure advances consolidation of motor skill accuracy in humans

Light has attracted increasing attention as a critical determinant of memory processing. While sleep selectively consolidates newly encoded memories according to their future relevance, the role of light in human memory consolidation is largely unknown. Here, we report how bright light (BL), provided during encoding, influences online and offline consolidation of motor skill learning. We sought to determine whether relatively slower and faster key-press transitions within individuals were differentially consolidated by BL. Healthy human subjects were briefly exposed to either BL (>8000 lx) or control light (CL; <500 lx) during memory encoding at 13:00 h, when light minimally affects circadian phase-shifting, and were retested 24 h later. The effects of BL on online and offline performance gains were determined by accuracy and speed. BL-exposed subjects showed better overall performance accuracy during training and lower overnight accuracy gains after a subsequent night of sleep than did CL-exposed subjects. BL preferentially improved the initially most difficult individual key-press transitions during practice; these were only improved overnight under CL. By contrast, accuracy during what had been the easiest key-press transitions at the beginning of the experiment was unaffected by light conditions or online/offline learning processes. BL effects were not observed for performance speed, mood, or sleep-wake patterns. Brief BL exposure during training may advance motor memory selection and consolidation that optimally meet individual requirements for potential gains, which would otherwise depend on post-training sleep. This suggests a new way of enhancing brain plasticity to compensate for impaired sleep-dependent memory consolidation in neuropsychiatric conditions.

Association of Intraocular Cataract Lens Replacement With Circadian Rhythms, Cognitive Function, and Sleep in Older Adults

Importance

Cataract is associated with a progressive decline in light transmission due to the clouding and yellowing of the natural crystalline lens. While the downstream effects of aging lenses include long-term disruption of circadian rhythms, cognitive function, and sleep regulation, it remains unknown whether there is an association of intraocular cataract lens (IOLs) replacement with circadian rhythms, cognition, and sleep.

Objective

To test whether IOL replacement (blue blocking [BB] or ultraviolet [UV] only blocking) in older patients with previous cataract is associated with the beneficial light effects on the circadian system, cognition, and sleep regulation.

Design, Setting, and Participants

Cross-sectional study at the Centre for Chronobiology, University of Basel in Switzerland from February 2012 to April 2014, analyzed between June 2012 and September 2018. Sixteen healthy older controls and 13 patients with previous cataract and IOL replacement participated without medication and no medical and sleep comorbidities.

Exposures

Three and a half hours of prior light control (dim-dark adaptation), followed by 2 hours of evening blue-enriched (6500 K) or non-blue-enriched light exposure (3000 K and 2500 K), 30 minutes in dim post-light exposure, 8 hours of sleep opportunity, and 2 hours of morning dim light following sleep.

Main Outcomes and Measures

Salivary melatonin, cognitive tests, and sleep structure and electroencephalographic activity to test the association of IOLs with markers of circadian rhythmicity, cognitive performance, and sleep regulation, respectively.

Results

The participants included 16 healthy older controls with a mean (standard error of the mean [SEM]) of 63.6 (5.6) years; 8 women and 13 patients with previous cataract (mean [SEM] age, 69.9 [5.2] years; 10 women); 5 patients had UV IOLs and 8 had BB IOLs. Patients with previous cataract and IOLs had an attenuated increase in melatonin levels during light exposure (mean [SEM] increase in the BB group: 23.3% [2.6%] and in the UV lens group: 19.1% [2.1%]) than controls (mean [SEM] increase, 48.8% [5.2%]) (difference between means, 27.7; 95% CI, 15.4%-41.7%; P < .001). Cognitive function, indexed by sustained attention performance, was improved in patients with UV lens (mean [SEM], 276.9 [11.1] milliseconds) compared with patients with BB lens (mean [SEM], 348.3 [17.8] milliseconds) (difference between means, 71.4; 95% CI, 29.5%-113.1%; P = .002) during light exposure and in the morning after sleep. Patients with UV lens had increased slow-wave sleep (mean [SEM] increase, 13% [3.4%]) compared with controls (mean [SEM] increase, 5.2% [0.8%]) (percentage of total sleep time; difference between means, 7.9; 95% CI, 2.4%-13.4%; P = .02) and frontal non-rapid eye movement slow-wave activity (0.75-4.5 Hz) during the first sleep cycle (mean [SEM], 79.9 [13.6] μV2/Hz) compared with patients with BB lens (mean [SEM], 53.2 [10.7] μV2/Hz) (difference between means, 26.7; 95% CI, 9.2-48.9; P = .03).

Conclusions and Relevance

These in-laboratory empirical findings suggest that optimizing the spectral lens transmission in patients with previous cataract may minimize the adverse age-related effects on circadian rhythms, cognition, and sleep.

Light Modulation of Human Clocks, Wake, and Sleep

Light, through its non-imaging forming effects, plays a dominant role on a myriad of physiological functions, including the human sleep–wake cycle. The non-image forming effects of light heavily rely on specific properties such as intensity, duration, timing, pattern, and wavelengths. Here, we address how specific properties of light influence sleep and wakefulness in humans through acute effects, e.g., on alertness, and/or effects on the circadian timing system. Of critical relevance, we discuss how different characteristics of light exposure across the 24-h day can lead to changes in sleep–wake timing, sleep propensity, sleep architecture, and sleep and wake electroencephalogram (EEG) power spectra. Ultimately, knowledge on how light affects sleep and wakefulness can improve light settings at home and at the workplace to improve health and well-being and optimize treatments of chronobiological disorders.

Light modulates oscillatory alpha activity in the occipital cortex of totally visually blind individuals with intact non-image-forming photoreception

The discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs) marked a major shift in our understanding of how light information is processed by the mammalian brain. These ipRGCs influence multiple functions not directly related to image formation such as circadian resetting and entrainment, pupil constriction, enhancement of alertness, as well as the modulation of cognition. More recently, it was demonstrated that ipRGCs may also contribute to basic visual functions. The impact of ipRGCs on visual function, independently of image forming photoreceptors, remains difficult to isolate, however, particularly in humans. We previously showed that exposure to intense monochromatic blue light (465 nm) induced non-conscious light perception in a forced choice task in three rare totally visually blind individuals without detectable rod and cone function, but who retained non-image-forming responses to light, very likely via ipRGCs. The neural foundation of such light perception in the absence of conscious vision is unknown, however. In this study, we characterized the brain activity of these three participants using electroencephalography (EEG), and demonstrate that unconsciously perceived light triggers an early and reliable transient desynchronization (i.e. decreased power) of the alpha EEG rhythm (8–14 Hz) over the occipital cortex. These results provide compelling insight into how ipRGC may contribute to transient changes in ongoing brain activity. They suggest that occipital alpha rhythm synchrony, which is typically linked to the visual system, is modulated by ipRGCs photoreception; a process that may contribute to the non-conscious light perception in those blind individuals.

Plasticity in the Sensitivity to Light in Aging: Decreased Non-visual Impact of Light on Cognitive Brain Activity in Older Individuals but No Impact of Lens Replacement

Beyond its essential visual role, light, and particularly blue light, has numerous non-visual effects, including stimulating cognitive functions and alertness. Non-visual effects of light may decrease with aging and contribute to cognitive and sleepiness complaints in aging. However, both the brain and the eye profoundly change in aging. Whether the stimulating effects light on cognitive brain functions varies in aging and how ocular changes may be involved is not established. We compared the impact of blue and orange lights on non-visual cognitive brain activity in younger (23.6 ± 2.5 years), and older individuals with their natural lenses (NL; 66.7 ± 5.1 years) or with intraocular lens (IOL) replacement following cataract surgery (69.6 ± 4.9 years). Analyses reveal that blue light modulates executive brain responses in both young and older individuals. Light effects were, however, stronger in young individuals including in the hippocampus and frontal and cingular cortices. Light effects did not significantly differ between older-IOL and older-NL while regression analyses indicated that differential brain engagement was not underlying age-related differences in light effects. These findings show that, although its impact decreases, light can stimulate cognitive brain activity in aging. Since lens replacement did not affect light impact, the brain seems to adapt to the progressive decrease in retinal light exposure in aging.

Sustained effects of prior red light on pupil diameter and vigilance during subsequent darkness

Environmental light can exert potent effects on physiology and behaviour, including pupil size, vigilance and sleep. Previous work showed that these non-image forming effects can last long beyond discontinuation of short-wavelength light exposure. The possible functional effects after switching off long-wavelength light, however, have been insufficiently characterized. In a series of controlled experiments in healthy adult volunteers, we evaluated the effects of five minutes of intense red light on physiology and performance during subsequent darkness. As compared to prior darkness, prior red light induced a subsequent sustained pupil dilation. Prior red light also increased subsequent heart rate and heart rate variability when subjects were asked to perform a sustained vigilance task during the dark exposure. While these changes suggest an increase in the mental effort required for the task, it could not prevent a post-red slowing of response speed. The suggestion that exposure to intense red light affects vigilance during subsequent darkness, was confirmed in a controlled polysomnographic study that indeed showed a post-red facilitation of sleep onset. Our findings suggest the possibility of using red light as a nightcap.

Examinations of the methods used to power supply of different light sources and their effect on bioelectrical brain activity

OBJECTIVE

The article represents the preliminary study, with the aim of the experiment being to examine whether different types of light sources used commonly in building interiors combined with various color temperature have an effect on EEG activity. The effect of frequency pulsation and color temperature on brain activity in EEG examinations in the beta 2 band was assumed.

MATERIAL/PARTICIPANTS

Twenty healthy men aged 19-25 years participated in the experiment.

METHODS

The research stand was lit by: LED diodes with color temperatures of 3000K, 4200K, 6500K, with the power supplied using the pulse width modulation (PWM) method with the current frequency of 122Hz, linear fluorescent tubes (3000K, 6500K), with the power supplied with the frequency of 50Hz and 52kHz from the electromagnetic and electronic ballasts, and the conventional light bulb, with the power supplied directly from the mains electricity, used as a reference light. System Flex 30 apparatus with TrueScan software was used to record the EEG signal. The examination used two factors (speed and accuracy) of the Kraepelin's work curve to describe changes in work performance for various types of lighting.

RESULTS

The results demonstrate that the use of different types of emission of light and color temperature of the light have an effect on bioelectrical brain activity and work performance.

CONCLUSIONS

The highest activity of brain waves concerns the beta band in the frequency range of 21-22Hz, regardless of the type of the light source (LED, fluorescent tube). The methods used to supply power and color temperature of fluorescent tubes do not significantly affect bioelectrical brain activity during "work", but previous lighting with fluorescent tubes during work has an essential effect on bioelectrical brain activity during rest. Regardless of the color temperature, LED lighting with PWM power supply leads to the highest bioelectrical activity (mainly in the range of 21-22Hz) in the brain during work and rest, which might suggests the usefulness of this method of supplying power for everyday work. Incandescent light does not affect the bioelectrical brain activity during work and rest.

Light and Cognition: Roles for Circadian Rhythms, Sleep, and Arousal

Light exerts a wide range of effects on mammalian physiology and behavior. As well as synchronizing circadian rhythms to the external environment, light has been shown to modulate autonomic and neuroendocrine responses as well as regulating sleep and influencing cognitive processes such as attention, arousal, and performance. The last two decades have seen major advances in our understanding of the retinal photoreceptors that mediate these non-image forming responses to light, as well as the neural pathways and molecular mechanisms by which circadian rhythms are generated and entrained to the external light/dark (LD) cycle. By contrast, our understanding of the mechanisms by which lighting influences cognitive processes is more equivocal. The effects of light on different cognitive processes are complex. As well as the direct effects of light on alertness, indirect effects may also occur due to disrupted circadian entrainment. Despite the widespread use of disrupted LD cycles to study the role circadian rhythms on cognition, the different experimental protocols used have subtly different effects on circadian function which are not always comparable. Moreover, these protocols will also disrupt sleep and alter physiological arousal, both of which are known to modulate cognition. Studies have used different assays that are dependent on different cognitive and sensory processes, which may also contribute to their variable findings. Here, we propose that studies addressing the effects of different lighting conditions on cognitive processes must also account for their effects on circadian rhythms, sleep, and arousal if we are to fully understand the physiological basis of these responses.

Sex differences in light sensitivity impact on brightness perception, vigilant attention and sleep in humans

Artificial light endows a "round-the-clock", 24-h/7-d society. Chronic exposure to light at night contributes to health hazards for humans, including disorders of sleep. Yet the influence of inter-individual traits, such as sex-differences, on light sensitivity remains to be established. Here we investigated potential sex-differences to evening light exposure of 40 lx at 6500 K (blue-enriched) or at 2500 K (non-blue-enriched), and their impact on brightness perception, vigilant attention and sleep physiology. In contrast to women, men had higher brightness perception and faster reaction times in a sustained attention task during blue-enriched light than non-blue-enriched. After blue-enriched light exposure, men had significantly higher all-night frontal NREM sleep slow-wave activity (SWA: 2-4 Hz), than women, particularly during the beginning of the sleep episode. Furthermore, brightness perception during blue-enriched light significantly predicted men's improved sustained attention performance and increased frontal NREM SWA. Our data indicate that, in contrast to women, men show a stronger response to blue-enriched light in the late evening even at very low light levels (40lux), as indexed by increased vigilant attention and sleep EEG hallmarks. Collectively, the data indicate that sex differences in light sensitivity might play a key role for ensuring the success of individually-targeted light interventions.

In a Heartbeat: Light and Cardiovascular Physiology

Light impinging on the retina fulfils a dual function: it serves for vision and it is required for proper entrainment of the endogenous circadian timing system to the 24-h day, thus influencing behaviors that promote health and optimal quality of life but are independent of image formation. The circadian pacemaker located in the suprachiasmatic nuclei modulates the cardiovascular system with an intrinsic ability to anticipate morning solar time and with a circadian nature of adverse cardiovascular events. Here, we infer that light exposure might affect cardiovascular function and provide evidence from existing research. Findings show a time-of-day dependent increase in relative sympathetic tone associated with bright light in the morning but not in the evening hours. Furthermore, dynamic light in the early morning hours can reduce the deleterious sleep-to-wake evoked transition on cardiac modulation. On the contrary, effects of numerous light parameters, such as illuminance level and wavelength of monochromatic light, on cardiac function are mixed. Therefore, in future research studies, light modalities, such as timing, duration, and its wavelength composition, should be taken in to account when testing the potential of light as a non-invasive countermeasure for adverse cardiovascular events.

Acute exposure to blue wavelength light during memory consolidation improves verbal memory performance

Acute exposure to light within the blue wavelengths has been shown to enhance alertness and vigilance, and lead to improved speed on reaction time tasks, possibly due to activation of the noradrenergic system. It remains unclear, however, whether the effects of blue light extend beyond simple alertness processes to also enhance other aspects of cognition, such as memory performance. The aim of this study was to investigate the effects of a thirty minute pulse of blue light versus placebo (amber light) exposure in healthy normally rested individuals in the morning during verbal memory consolidation (i.e., 1.5 hours after memory acquisition) using an abbreviated version of the California Verbal Learning Test (CVLT-II). At delayed recall, individuals who received blue light (n = 12) during the consolidation period showed significantly better long-delay verbal recall than individuals who received amber light exposure (n = 18), while controlling for the effects of general intelligence, depressive symptoms and habitual wake time. These findings extend previous work demonstrating the effect of blue light on brain activation and alertness to further demonstrate its effectiveness at facilitating better memory consolidation and subsequent retention of verbal material. Although preliminary, these findings point to a potential application of blue wavelength light to optimize memory performance in healthy populations. It remains to be determined whether blue light exposure may also enhance performance in clinical populations with memory deficits.

Modulation of recognition memory performance by light requires both melanopsin and classical photoreceptors

Acute light exposure exerts various effects on physiology and behaviour. Although the effects of light on brain network activity in humans are well demonstrated, the effects of light on cognitive performance are inconclusive, with the size, as well as direction, of the effect depending on the nature of the task. Similarly, in nocturnal rodents, bright light can either facilitate or disrupt performance depending on the type of task employed. Crucially, it is unclear whether the effects of light on behavioural performance are mediated via the classical image-forming rods and cones or the melanopsin-expressing photosensitive retinal ganglion cells. Here, we investigate the modulatory effects of light on memory performance in mice using the spontaneous object recognition task. Importantly, we examine which photoreceptors are required to mediate the effects of light on memory performance. By using a cross-over design, we show that object recognition memory is disrupted when the test phase is conducted under a bright light (350 lux), regardless of the light level in the sample phase (10 or 350 lux), demonstrating that exposure to a bright light at the time of test, rather than at the time of encoding, impairs performance. Strikingly, the modulatory effect of light on memory performance is completely abolished in both melanopsin-deficient and rodless-coneless mice. Our findings provide direct evidence that melanopsin-driven and rod/cone-driven photoresponses are integrated in order to mediate the effect of light on memory performance.

Alerting or Somnogenic Light: Pick Your Color

In mammals, light exerts pervasive effects on physiology and behavior in two ways: indirectly through clock synchronization and the phase adjustment of circadian rhythms, and directly through the promotion of alertness and sleep, respectively, in diurnal and nocturnal species. A recent report by Pilorz and colleagues describes an even more complex role for the acute effects of light. In mice, blue light acutely causes behavioral arousal, whereas green wavelengths promote sleep. These opposing effects are mediated by melanopsin-based phototransduction through different neural pathways. These findings reconcile nocturnal and diurnal species through a common alerting response to blue light. One can hypothesize that the opposite responses to natural polychromatic light in night- or day-active animals may reflect higher sensitivity of nocturnal species to green, and diurnals to blue wavelengths, resulting in hypnogenic and alerting effects, respectively. Additional questions remain to be clarified. How do different light wavelengths affect other behaviors such as mood and cognition? How do those results apply to humans? How does light pose either a risk or benefit, depending on whether one needs to be asleep or alert? Indeed, in addition to timing, luminance levels, and light exposure duration, these findings stress the need to understand how best to adapt the color spectrum of light to our needs and to take this into account for the design of daily lighting concepts—a key challenge for today’s society, especially with the emergence of LED light technology.

Thin-film optical notch filter spectacle coatings for the treatment of migraine and photophobia

Previous evidence suggests optical treatments hold promise for treating migraine and photophobia. We designed an optical notch filter, centered at 480nm to reduce direct stimulation of intrinsically photosensitive retinal ganglion cells. We used thin-film technology to integrate the filter into spectacle lenses. Our objective was to determine if an optical notch filter, designed to attenuate activity of intrinsically photosensitive retinal ganglion cells, could reduce headache impact in chronic migraine subjects. For this randomized, double-masked study, our primary endpoint was the Headache Impact Test (HIT-6; GlaxoSmithKline, Brentford, Middlesex, UK). We developed two filters: the therapeutic filter blocked visible light at 480nm; a 620nm filter was designed as a sham. Participants were asked to wear lenses with one of the filters for 2weeks; after 2weeks when no lenses were worn, they wore lenses with the other filter for 2weeks. Of 48 subjects, 37 completed the study. Wearing either the 480 or 620nm lenses resulted in clinically and statistically significant HIT-6 reductions. However, there was no significant difference when comparing overall effect of the 480 and 620nm lenses. Although the 620nm filter was designed as a sham intervention, research published following the trial indicated that melanopsin, the photopigment in intrinsically photosensitive retinal ganglion cells, is bi-stable. This molecular property may explain the unexpected efficacy of the 620nm filter. These preliminary findings indicate that lenses outfitted with a thin-film optical notch filter may be useful in treating chronic migraine.

Light-sensitive brain pathways and aging

Notwithstanding its effects on the classical visual system allowing image formation, light acts upon several non-image-forming (NIF) functions including body temperature, hormonal secretions, sleep-wake cycle, alertness, and cognitive performance. Studies have shown that NIF functions are maximally sensitive to blue wavelengths (460–480 nm), in comparison to longer light wavelengths. Higher blue light sensitivity has been reported for melatonin suppression, pupillary constriction, vigilance, and performance improvement but also for modulation of cognitive brain functions. Studies investigating acute stimulating effects of light on brain activity during the execution of cognitive tasks have suggested that brain activations progress from subcortical regions involved in alertness, such as the thalamus, the hypothalamus, and the brainstem, before reaching cortical regions associated with the ongoing task. In the course of aging, lower blue light sensitivity of some NIF functions has been reported. Here, we first describe neural pathways underlying effects of light on NIF functions and we discuss eye and cerebral mechanisms associated with aging which may affect NIF light sensitivity. Thereafter, we report results of investigations on pupillary constriction and cognitive brain sensitivity to light in the course of aging. Whereas the impact of light on cognitive brain responses appears to decrease substantially, pupillary constriction seems to remain more intact over the lifespan. Altogether, these results demonstrate that aging research should take into account the diversity of the pathways underlying the effects of light on specific NIF functions which may explain their differences in light sensitivity.

Seasonality in human cognitive brain responses

Daily variations in the environment have shaped life on Earth, with circadian cycles identified in most living organisms. Likewise, seasons correspond to annual environmental fluctuations to which organisms have adapted. However, little is known about seasonal variations in human brain physiology. We investigated annual rhythms of brain activity in a cross-sectional study of healthy young participants. They were maintained in an environment free of seasonal cues for 4.5 d, after which brain responses were assessed using functional magnetic resonance imaging (fMRI) while they performed two different cognitive tasks. Brain responses to both tasks varied significantly across seasons, but the phase of these annual rhythms was strikingly different, speaking for a complex impact of season on human brain function. For the sustained attention task, the maximum and minimum responses were located around summer and winter solstices, respectively, whereas for the working memory task, maximum and minimum responses were observed around autumn and spring equinoxes. These findings reveal previously unappreciated process-specific seasonality in human cognitive brain function that could contribute to intraindividual cognitive changes at specific times of year and changes in affective control in vulnerable populations.

Post-illumination pupil response after blue light: Reliability of optimized melanopsin-based phototransduction assessment

Melanopsin-containing retinal ganglion cells have recently been shown highly relevant to the non-image forming effects of light, through their direct projections on brain circuits that regulate alertness, mood and circadian rhythms. A quantitative assessment of functionality of the melanopsin-signaling pathway could be highly relevant in order to mechanistically understand individual differences in the effects of light on these regulatory systems. We here propose and validate a reliable quantification of the melanopsin-dependent Post-Illumination Pupil Response (PIPR) after blue light, and evaluated its sensitivity to dark adaptation, time of day, body posture, and light exposure history. Pupil diameter of the left eye was continuously measured during a series of light exposures to the right eye, of which the pupil was dilated using tropicamide 0.5%. The light exposure paradigm consisted of the following five consecutive blocks of five minutes: baseline dark; monochromatic red light (peak wavelength: 630 nm, luminance: 375 cd/m(2)) to maximize the effect of subsequent blue light; dark; monochromatic blue light (peak wavelength: 470 nm, luminance: 375 cd/m(2)); and post-blue dark. PIPR was quantified as the difference between baseline dark pupil diameter and post-blue dark pupil diameter (PIPR-mm). In addition, a relative PIPR was calculated by dividing PIPR by baseline pupil diameter (PIPR-%). In total 54 PIPR assessments were obtained in 25 healthy young adults (10 males, mean age ± SD: 26.9 ± 4.0 yr). From repeated measurements on two consecutive days in 15 of the 25 participants (6 males, mean age ± SD: 27.8 ± 4.3 yrs) test-retest reliability of both PIPR outcome parameters was calculated. In the presence of considerable between-subject differences, both outcome parameters had very high test-retest reliability: Cronbach's α > 0.90 and Intraclass Correlation Coefficient > 0.85. In 12 of the 25 participants (6 males, mean age ± SD: 26.5 ± 3.6 yr) we examined the potential confounding effects of dark adaptation, time of the day (morning vs. afternoon), body posture (upright vs. supine position), and 24-h environmental light history on the PIPR assessment. Mixed effect regression models were used to analyze these possible confounders. A supine position caused larger PIPR-mm (β = 0.29 mm, SE = 0.10, p = 0.01) and PIPR-% (β = 4.34%, SE = 1.69, p = 0.02), which was due to an increase in baseline dark pupil diameter; this finding is of relevance for studies requiring a supine posture, as in functional Magnetic Resonance Imaging, constant routine protocols, and bed-ridden patients. There were no effects of dark adaptation, time of day, and light history. In conclusion, the presented method provides a reliable and robust assessment of the PIPR to allow for studies on individual differences in melanopsin-based phototransduction and effects of interventions.

The psychiatry of light

Bright light therapy and the broader realm of chronotherapy remain underappreciated and underutilized, despite their empirical support. Efficacy extends beyond seasonal affective disorder and includes nonseasonal depression and sleep disorders, with emerging evidence for a role in treating attention-deficit/hyperactivity disorder, delirium, and dementia. A practical overview is offered, including key aspects of underlying biology, indications for treatment, parameters of treatment, adverse effects, and transformation of our relationship to light and darkness in contemporary life.