Using Flickering Light to Enhance Nonimage-Forming Visual Stimulation in Humans

Kinetic pupillary size using Pentacam in myopia

Purpose

To compare if the kinetic pupillary changes differs between high myopia (HM) and low/moderate myopia by Pentacam.

Setting

Chinese People's Liberation Army (PLA) General Hospital, Beijing, China.

Design

Comparative study.

Methods

In this cross-sectional retrospective study, 44 eyes of 44 patients were recruited in the Refractive Surgery Center of Chinese PLA General Hospital. Eyes were divided into two groups according to the refractive error: low/moderate myopia (22 eyes; -2.99 ± 1.09 D) and HM (22 eyes, -12.93 ± 3.44 D). At the beginning of the experiment, we made trials of scanning one false pupil by Pentacam. All patients underwent the Pentacam examination three times. Pupillary diameters (PD) during the scan process and other parameters were measured using the Pentacam. Coefficient variations of PD (CV) during the different scanning periods were analyzed comparatively between the two groups.

Results

Pentacam once time output 25 Scheimpflug images, with 13 ones during the period from 1st to 1.5th s and 12 ones during the period from 2.5th to 3rd s after the scanning onset. For the spatial order on all the 25 meridians, 13 Scheimpflug images came out when the Pentacam rotated from 60° to 153°meridians and the remaining 12 Scheimpflug images from 161° to 245° meridians. Among pupillary parameters, no statistical significance existed in PD25, PD13, and PD12 (pupil diameter's mean from all, former 13 and remaining 12 of 25 Scheimpflug images, respectively) (P > 0.05) between the two groups. However, there were statistically significant differences in CV25 and CV13 (coefficient variation of the pupil diameters from all and former 13 of 25 images, respectively) (P < 0.001), with no statistical significance in CV12 (coefficient variation of the pupil diameters from remaining 12 of 25 images) between both groups.

Conclusion

Twenty-five Scheimpflug images on Pentacam had the temporal and the spatial orders. CV in eyes with HM was lower than that in eyes with low/moderate myopia in a certain period of the Pentacam scan. Kinetic pupillary size in HM changed more slowly than that in low/moderate myopia during some scanning period analogous to the phasic response of the pupil reflex.

Optimum display luminance under a wide range of ambient light for cockpit displays

The self-luminous cockpit displays need to be adaptive to a wide range of ambient light levels, which changes from very low illuminance to very high levels. Yet, current studies on evaluation and luminance setting of displays in bright surroundings are still limited. In this study, a three-dimensional visual ergonomic experiment was carried out to investigate how bright a cockpit display should be to meet aircrew operational requirements under different illuminance. A lab study with a within-subjects (N = 12) design was conducted in a simulated cockpit. According to the Weber-Fechner's Law, human observers evaluated five display luminance conditions (10¹, 10^1.5, 10², 10^2.5, 10³ cd/m²) under five ambient illuminance conditions (10°, 10¹, 10², 10³, 10⁴ lx). Visual performance, visual fatigue and visual comfort were used as evaluation bases, which were measured by d2 task, subjective fatigue questionnaire and visual perception semantic scales. Nonlinear function fitting was used to calculate the optimal luminance under a certain illuminance. Finally, curvilinear regression was used to analyze the illuminance and its corresponding optimal luminance. Based on Silverstein luminance power function, a luminance adjustment model with the form of power function was obtained. The proposed three-dimensional model fits the experimental data well and is consistent with the existing studies. It can be regarded as a supplement and optimization of the previous model under high ambient illuminance. This study can contribute not only to the pleasing luminance setting of panel displays in aircraft cockpits but also to other self-luminous devices, such as tablet devices, outdoor monitoring equipment and advertising screens.

Circadian Responses to Light-Flash Exposure: Conceptualization and New Data Guiding Future Directions

A growing number of studies document circadian phase-shifting after exposure to millisecond light flashes. When strung together by intervening periods of darkness, these stimuli evoke pacemaker responses rivaling or outmatching those created by steady luminance, suggesting that the circadian system's relationship to light can be contextualized outside the principle of simple dose-dependence. In the current review, we present a brief chronology of this work. We then develop a conceptual model around it that attempts to relate the circadian effects of flashes to a natural integrative process the pacemaker uses to intermittently sample the photic information available at dawn and dusk. Presumably, these snapshots are employed as building blocks in the construction of a coherent representation of twilight the pacemaker consults to orient the next day's physiology (in that way, flash-resetting of pacemaker rhythms might be less an example of a circadian visual illusion and more an example of the kinds of gestalt inferences that the image-forming system routinely makes when identifying objects within the visual field; i.e., closure). We conclude our review with a discussion on the role of cones in the pacemaker's twilight predictions, providing new electrophysiological data suggesting that classical photoreceptors—but not melanopsin—are necessary for millisecond, intermediate-intensity flash responses in ipRGCs (intrinsically photosensitive retinal ganglion cells). Future investigations are necessary to confirm this “Cone Sentinel Model” of circadian flash-integration and twilight-prediction, and to further define the contribution of cones vs. rods in transducing pacemaker flash signals.

Acepromazine and Chlorpromazine as Pharmaceutical-grade Alternatives to Chlorprothixene for Pupillary Light Reflex Imaging in Mice

Studies of visual responses in isoflurane-anesthetized mice often use the sedative chlorprothixene to decrease the amount of isoflurane used because excessive isoflurane could adversely affect light-evoked responses. However, data are not available to justify the use of this nonpharmaceutical-grade chemical. The current study tested whether pharmaceutical-grade sedatives would be appropriate alternatives for imaging pupillary light reflexes. Male 15-wk-old mice were injected intraperitoneally with 1 mg/kg chlorprothixene, 5 mg/kg acepromazine, 10 mg/kg chlorpromazine, or saline. After anesthetic induction, anesthesia maintenance used 0.5% and 1% isoflurane for sedative- and saline-injected mice, respectively. A photostimulus (16.0 log photons cm^-2 s^-1; 470 nm) was presented to the right eye for 20 min, during which the left eye was imaged for consensual pupillary constriction and involuntary pupil drift. Time to immobilization, loss of righting reflex, physiologic parameters, gain of righting reflex, and degree of recovery were assessed also. The sedative groups were statistically indistinguishable for all measures. By contrast, pupillary drift occurred far more often in saline-treated mice than in the sedative groups. Furthermore, saline-treated mice took longer to reach maximal pupil constriction than all sedative groups and had lower heart rates compared with chlorpromazine- and chlorprothixene-sedated mice. Full recovery (as defined by purposeful movement, response to tactile stimuli, and full alertness) was not regularly achieved in any sedative group. In conclusion, at the doses tested, acepromazine and chlorpromazine are suitable pharmaceutical-grade alternatives to chlorprothixene for pupil imaging and conceivably other in vivo photoresponse measurements; however, given the lack of full recovery, lower dosages should be investigated further for use in survival procedures.

Binocular facilitation in light-mediated melatonin suppression?

In humans, the production of melatonin is suppressed by light exposure. This effect is mediated by a retinal pathway involving the melanopsin-containing intrinsincally photosensitive retinal ganglion cells (ipRGCs), which exhibit maximum sensitivity to short-wavelength light. Here, based on extant and published data, we examine how signals from the two eyes are integrated in driving the suppression of melatonin by light. We find that melatonin suppression by light exposure to two eyes corresponds to a sensitivity shift by about 1.2 log units (factor ~17.4).

Circadian phase-shifting by light: Beyond photons

Circadian entrainment to the solar light:dark schedule is thought to be maintained by a simple photon counting method. According to this hypothesis, the pacemaker adjusts the phase of the body's endogenous rhythms in accordance to the intensity and duration with which it encounters a perceived twilight signal. While previous data have generally supported the hypothesis, more recent analysis has codified other factors besides irradiance that influence the magnitude of resetting responses to light delivered within the same phase of the circadian cycle. In particular, the frequency with which light is alternated with darkness, or whether it's packaged in millisecond flashes versus continuous blocks, can significantly alter the dose-response relationship. Here, we used a drosophilid model to test whether circadian photon-counting trends can be broken with light administration protocols spanning just 15 minutes. In the early part of the delay zone, a 15-min continuous light pulse was fragmented until it could no longer produce a full-magnitude shift of the flies' locomotor activity rhythms. The remaining exposure was then reorganized along various fractionation schemes that employed pulses with different widths and interstimulus intervals. Our results suggest that the pacemaker integrates the phase-shifting effects of equiluminous light differently depending on the stimulus pattern with which light is made available. For example, despite having fewer photons, certain ratios of light and darkness could be optimized on a timescale of seconds and minutes so as to achieve pacemaker resetting close to par with steady luminance. These data provide further evidence that the circadian pacemaker's responses to light entail more than photon counting and motivate continued discussion on how phototherapy can be best optimized in clinical practice to improve conditions linked to circadian impairment.

Melanopsin- and L-cone-induced pupil constriction is inhibited by S- and M-cones in humans

The human retina contains five photoreceptor types: rods; short (S)-, mid (M)-, and long (L)-wavelength-sensitive cones; and melanopsin-expressing ganglion cells. Recently, it has been shown that selective increments in M-cone activation are paradoxically perceived as brightness decrements, as opposed to L-cone increments. Here we show that similar effects are also observed in the pupillary light response, whereby M-cone or S-cone increments lead to pupil dilation whereas L-cone or melanopic illuminance increments resulted in pupil constriction. Additionally, intermittent photoreceptor activation increased pupil constriction over a 30-min interval. Modulation of L-cone or melanopic illuminance within the 0.25-4-Hz frequency range resulted in more sustained pupillary constriction than light of constant intensity. Opposite results were found for S-cone and M-cone modulations (2 Hz), mirroring the dichotomy observed in the transient responses. The transient and sustained pupillary light responses therefore suggest that S- and M-cones provide inhibitory input to the pupillary control system when selectively activated, whereas L-cones and melanopsin response fulfill an excitatory role. These findings provide insight into functional networks in the human retina and the effect of color-coding in nonvisual responses to light, and imply that nonvisual and visual brightness discrimination may share a common pathway that starts in the retina.

Subadditive responses to extremely short blue and green pulsed light on visual evoked potentials, pupillary constriction and electroretinograms

Background

The simultaneous exposure to blue and green light was reported to result in less melatonin suppression than monochromatic exposure to blue or green light. Here, we conducted an experiment using extremely short blue- and green-pulsed light to examine their visual and nonvisual effects on visual evoked potentials (VEPs), pupillary constriction, electroretinograms (ERGs), and subjective evaluations.

Methods

Twelve adult male subjects were exposed to three light conditions: blue-pulsed light (2.5-ms pulse width), green-pulsed light (2.5-ms pulse width), and simultaneous blue- and green-pulsed light with white background light. We measured the subject’s pupil diameter three times in each condition. Then, after 10 min of rest, the subject was exposed to the same three light conditions. We measured the averaged ERG and VEP during 210 pulsed-light exposures in each condition. We also determined subjective evaluations using a visual analog scale (VAS) method.

Results

The pupillary constriction during the simultaneous exposure to blue- and green-pulsed light was significantly lower than that during the blue-pulsed light exposure despite the double irradiance intensity of the combination. We also found that the b/|a| wave of the ERGs during the simultaneous exposure to blue- and green-pulsed light was lower than that during the blue-pulsed light exposure. We confirmed the subadditive response to pulsed light on pupillary constriction and ERG. However, the P100 of the VEPs during the blue-pulsed light were smaller than those during the simultaneous blue- and green-pulsed light and green-pulsed light, indicating that the P100 amplitude might depend on the luminance of light.

Conclusions

Our findings demonstrated the effect of the subadditive response to extremely short pulsed light on pupillary constriction and ERG responses. The effects on ipRGCs by the blue-pulsed light exposure are apparently reduced by the simultaneous irradiation of green light. The blue versus yellow (b/y) bipolar cells in the retina might be responsible for this phenomenon.

Human pupillary light reflex during successive irradiation with 1-ms blue- and green-pulsed light

Background

In the human retina, the contribution of intrinsically photosensitive retinal ganglion cells (ipRGCs) to the regulation of the pupillary response remains poorly understood. The objective of the current study was to determine the response dynamics of the pupillary light reflex to short, successive pulses of light. In order to better assess the roles of ipRGCs and cones, we used pulses of blue and green light.

Methods

Each participant was exposed to 1-ms blue (466 nm) and/or green (527 nm) light pulses simultaneously or separately, with inter-stimulus intervals (ISIs) of 0, 250, 500, 750, or 1000 ms. Pupil diameter was measured using an infrared camera system.

Results

We found that human pupillary light responses during simultaneous irradiation or successive irradiation with ISIs ≤ 250 ms were equivalent, though successive irradiation of blue- and green-pulsed light with ISIs ≥ 500 ms induced markedly increased pupillary constriction.

Conclusions

We propose that this result may be related to cell hyperpolarization that occurs in the retina just after the first light stimulus is turned off, with the threshold for this effect being between 250 and 500 ms in the human retina.

The impact of temporal modulations in irradiance under light adapted conditions on the mouse suprachiasmatic nuclei (SCN)

Electrophysiological responses of SCN neurons to light steps are well established, but responses to more natural modulations in irradiance have been much less studied. We address this deficit first by showing that variations in irradiance for human subjects are biased towards low temporal frequencies and small magnitudes. Using extracellular recordings we show that neurons in the mouse SCN are responsive to stimuli with these characteristics, tracking sinusoidal modulations in irradiance best at lower temporal frequencies and responding to abrupt changes in irradiance over a range of commonly encountered contrasts. The spectral sensitivity of these light adapted responses indicates that they are driven primarily by cones, but with melanopsin (and/or rods) contributing under more gradual changes. Higher frequency modulations in irradiance increased time averaged firing of SCN neurons (typically considered to encode background light intensity) modestly over that encountered during steady exposure, but did not have a detectable effect on the circadian phase resetting efficiency of light. Our findings highlight the SCN’s ability to encode naturalistic temporal modulations in irradiance, while revealing that the circadian system can effectively integrate such signals over time such that phase-resetting responses remain proportional to the mean light exposure.

LED Lights With Hidden Intensity-Modulated Blue Channels Aiming for Enhanced Subconscious Visual Responses

A new form of light-emitting diode (LED) light suitable for general illumination is proposed to enhance subconscious, nonimage-forming visual responses, which are essential to our well-being. Pulsing light has been shown to reduce photoreceptor adaptation and elicit stronger subconscious visual responses at an indoor illumination level. Using the silent substitution technique, a melanopsin-selective flicker can be added into white light. A linear optimization algorithm was developed to suppress any perceivable fluctuation of light intensity and colors of illuminated objects. Two examples of lights are given to illustrate the potential applications of the proposed multi-LED light for general illumination and therapeutic purposes.

Responses to Spatial Contrast in the Mouse Suprachiasmatic Nuclei

A direct retinal projection targets the suprachiasmatic nucleus (SCN) (an important hypothalamic control center). The accepted function of this projection is to convey information about ambient light (irradiance) to synchronize the SCN’s endogenous circadian clock with local time and drive the diurnal variations in physiology and behavior [1, 2, 3, 4]. Here, we report that it also renders the SCN responsive to visual images. We map spatial receptive fields (RFs) for SCN neurons and find that only a minority are excited (or inhibited) by light from across the scene as expected for irradiance detectors. The most commonly encountered units have RFs with small excitatory centers, combined with very extensive inhibitory surrounds that reduce their sensitivity to global changes in light in favor of responses to spatial patterns. Other units have larger excitatory RF centers, but these always cover a coherent region of visual space, implying visuotopic order at the single-unit level. Approximately 75% of light-responsive SCN units modulate their firing according to simple spatial patterns (drifting or inverting gratings) without changes in irradiance. The time-averaged firing rate of the SCN is modestly increased under these conditions, but including spatial contrast did not significantly alter the circadian phase resetting efficiency of light. Our data indicate that the SCN contains information about irradiance and spatial patterns. This newly appreciated sensory capacity provides a mechanism by which behavioral and physiological systems downstream of the SCN could respond to visual images [5].

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Many SCN units have receptive fields optimized for spatial discrimination

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A large majority of SCN neurons track changes in spatial patterns

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Spatial patterns can enhance SCN-maintained firing, but not circadian phase resetting

Characterizing and modeling the intrinsic light response of rat ganglion-cell photoreceptors

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate both image-forming vision and non-image-forming visual responses such as pupillary constriction and circadian photoentrainment. Five types of ipRGCs, named M1-M5, have been discovered in rodents. To further investigate their photoresponse properties, we made multielectrode array spike recordings from rat ipRGCs, classified them into M1, M2/M4, and M3/M5 clusters, and measured their intrinsic, melanopsin-based responses to single and flickering light pulses. Results showed that ipRGC spiking can track flickers up to ∼0.2 Hz in frequency and that flicker intervals between 5 and 14 s evoke the most spikes. We also learned that melanopsin's integration time is intensity and cluster dependent. Using these data, we constructed a mathematical model for each cluster's intrinsic photoresponse. We found that the data for the M1 cluster are best fit by a model that assumes a large photoresponse, causing the cell to enter depolarization block. Our models also led us to hypothesize that the M2/M4 and M3/M5 clusters experience comparable photoexcitation but that the M3/M5 cascade decays significantly faster than the M2/M4 cascade, resulting in different response waveforms between these clusters. These mathematical models will help predict how each ipRGC cluster might respond to stimuli of any waveform and could inform the invention of lighting technologies that promote health through melanopsin stimulation.