Biomedical subjects

R E Kronauer

Publications and source records attributed to R E Kronauer.

At least 37 records · Page 2Linked to original sources

PubMed1999

Quantifying human circadian pacemaker response to brief, extended, and repeated light stimuli over the phototopic range.

The authors' previous models have been able to describe accurately the effects of extended (approximately 5 h) bright-light (>4000 lux) stimuli on the phase and amplitude of the human circadian pacemaker, but they are not sufficient to represent the surprising human sensitivity to both brief pulses of bright light and light of more moderate intensities. Therefore, the authors have devised a new model in which a dynamic stimulus processor (Process L) intervenes between the light stimuli and the traditional representation of the circadian pacemaker as a self-sustaining limit-cycle oscillator (Process P). The overall model incorporating Process L and Process P is intended to allow the prediction of phase shifts to photic stimuli of any temporal pattern (extended and brief light episodes) and any light intensity in the photopic range. Two time constants emerge in the Process L model: the characteristic duration for necessary bright-light pulses to achieve their full effect (5-10 min) and the characteristic stimulus-free (dark) interval that can be tolerated without incurring an excessive penalty in phase shifting (30-80 min). The effect of reducing light intensity is incorporated in Process L as an extension of the time necessary for the light pulse to be fully realized (a power-law relation between time and intensity). This new model generates a number of new testable hypotheses, including the surprising prediction that 24-h cycles consisting of 8 h of darkness and 16 h of only approximately 3.5 lux would be capable of entraining a large fraction of the adult population (approximately 45%). Experimental data on the response of the human circadian system to lower light intensities and briefer stimuli are needed to allow for further refinement and validation of the model proposed here.

Algorithms↗

PubMed1998

Refinement of a limit cycle oscillator model of the effects of light on the human circadian pacemaker.

In 1990, Kronauer proposed a mathematical model of the effects of light on the human circadian pacemaker. Although this model predicted many general features of the response of the human circadian pacemaker to light exposure, additional data now available enable us to refine the original model. We first refined the original model by incorporating the results of a dose response curve to light into the model's predicted relationship between light intensity and the strength of the drive onto the pacemaker. Data from three bright light phase resetting experiments were then used to refine the amplitude recovery characteristics of the model. Finally, the model was tested and further refined using data from an extensive phase resetting experiment in which a 3-cycle bright light stimulus was presented against a background of dim light. In order to describe the results of the four resetting experiments, the following major refinements to the original model were necessary: (i) the relationship between light intensity (I) and drive onto the pacemaker was reduced from I1/3 to I0.23 for light levels between 150 and 10,000 lux; (ii) the van der Pol oscillator from the original model was replaced with a higher-order limit cycle oscillator so that amplitude recovery is slower near the singularity and faster near the limit cycle; (iii) a direct effect of light on circadian period (tau x) was incorporated into the model such that as I increases, tau x decreases, which is in accordance with "Aschoff's rule". This refined model generates the following testable predictions: it should be difficult to enhance normal circadian amplitude via bright light; near the critical point of a type 0 phase response curve (PRC) the slope should be steeper than it is in a type 1 PRC; and circadian period measured during forced desynchrony should be directly affected by ambient light intensity.

Circadian Rhythm↗

PubMed1998

Motion detection on flashed, stationary pedestal gratings: evidence for an opponent-motion mechanism.

Contrast thresholds were measured for discriminating left vs right motion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion. This test was presented on a 1 c/deg stationary grating (pedestal) of twice-threshold, flashed for the duration of the test motion. Lu and Sperling [(1995). Vision Research, 35, 2697-2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the 'feature wobble' which it induces). On the contrary, we observe that the stationary pedestal has large effects on motion detection at 7 and 15 Hz, and smaller effects at 0.9-3.7 Hz, evidenced by a spatial phase dependency between the stationary pedestal and moving test. At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedestal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test motion reverses with the pedestal in the opposite phase. Phase dependency was also explored using a very brief (approximately 1 msec) static pedestal presented with the moving test. The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left and right moving components which interact with the moving test. The pedestal effects can be explained by the visual system's much higher sensitivity to the difference of the contrast of right vs left moving components than to either component alone.

Contrast Sensitivity↗

PubMed1998

Short-wave cone signal in the red-green detection mechanism.

Previous work shows that the red-green (RG) detection mechanism is highly sensitive, responding to equal and opposite long-wave (L) and middle-wave (M) cone contrast signals. This mechanism mediates red-green hue judgements under many conditions. We show that the RG detection mechanism also receives a weak input from the short-wave (S) cones that supports the L signal and equally opposes M. This was demonstrated with a pedestal paradigm, in which weak S cone flicker facilitates discrimination and detection of red-green flicker. Also, a near-threshold +S cone flash facilitates detection of red flashes and inhibits green flashes, and a near-threshold -S cone flash facilitates detection of green flashes and inhibits red flashes. The S contrast weight in RG is small relative to the L and M contrast weights. However, a comparison of our results with other studies suggests that the strength of the absolute S cone contrast contribution to the RG detection mechanism is 1/4 to 1/3 the strength of the S contribution to the blue-yellow (BY) detection mechanism. Thus, the S weight in RG is a significant fraction of the S weight in BY. This has important implications for the 'cardinal' color mechanisms, for it predicts that for detection or discrimination, the mechanisms limiting performance do not lie on orthogonal M-L and S axes within the equiluminant color plane.

Color Perception↗

PubMed1998

Nonphotic entrainment of the human circadian pacemaker.

In organisms as diverse as single-celled algae and humans, light is the primary stimulus mediating entrainment of the circadian biological clock. Reports that some totally blind individuals appear entrained to the 24-h day have suggested that nonphotic stimuli may also be effective circadian synchronizers in humans, although the nonphotic stimuli are probably comparatively weak synchronizers, because the circadian rhythms of many totally blind individuals "free run" even when they maintain a 24-h activity-rest schedule. To investigate entrainment by nonphotic synchronizers, we studied the endogenous circadian melatonin and core body temperature rhythms of 15 totally blind subjects who lacked conscious light perception and exhibited no suppression of plasma melatonin in response to ocular bright-light exposure. Nine of these fifteen blind individuals were able to maintain synchronization to the 24-h day, albeit often at an atypical phase angle of entrainment. Nonphotic stimuli also synchronized the endogenous circadian rhythms of a totally blind individual to a non-24-h schedule while living in constant near darkness. We conclude that nonphotic stimuli can entrain the human circadian pacemaker in some individuals lacking ocular circadian photoreception.

Adult↗

PubMed1997

Photopic transduction implicated in human circadian entrainment.

Despite the preeminence of light as the synchronizer of the circadian timing system, the phototransductive machinery in mammals which transmits photic information from the retina to the hypothalamic circadian pacemaker remains largely undefined. To determine the class of photopigments which this phototransductive system uses, we exposed a group (n = 7) of human subjects to red light below the sensitivity threshold of a scotopic (i.e. rhodopsin/rod-based) system, yet of sufficient strength to activate a photopic (i.e. cone-based) system. Exposure to this light stimulus was sufficient to reset significantly the human circadian pacemaker, indicating that the cone pigments which mediate color vision can also mediate circadian vision.

Adolescent↗

PubMed1997

Colour adaptation modifies the long-wave versus middle-wave cone weights and temporal phases in human luminance (but not red-green) mechanism.

1. The human luminance (LUM) mechanism detects rapid flicker and motion, responding to a linear sum of contrast signals, L' and M', from the long-wave (L) and middle-wave (M) cones. The red-green mechanism detects hue variations, responding to a linear difference of L' and M' contrast signals. 2. The two detection mechanisms were isolated to assess how chromatic adaptation affects summation of L' and M' signals in each mechanism. On coloured background (from blue to red), we measured, as a function of temporal frequency, both the relative temporal phase of the L' and M' signals producing optimal summation and the relative L' and M' contrast weights of the signals (at the optimal phase for summation). 3. Within the red-green mechanism at 6 Hz, the phase shift between the L' and M' signals was negligible on each coloured field, and the L' and M' contrast weights were equal and of opposite sign. 4. Relative phase shifts between the L' and M' signals in the LUM mechanism were markedly affected by adapting field colour. For stimuli of 1 cycle deg-1 and 9 Hz, the temporal phase shift was zero on a green-yellow field (approximately 570 nm). On an orange field, the L' signal lagged M' by as much as 70 deg phase while on a green field M' lagged L' by as much as 70 deg. The asymmetric phase shift about yellow adaptation reveals a spectrally opponent process which controls the phase shift. The phase shift occurs at an early site, for colour adaptation of the other eye had no effect, and the phase shift measured monocularly was identical for flicker and motion, thus occurring before the motion signal is extracted (this requires an extra delay). 5. The L' versus M' phase shift in the LUM mechanism was generally greatest at intermediate temporal frequencies (4-12 Hz) and was small at high frequencies (20-25 Hz). The phase shift was greatest at low spatial frequencies and strongly reduced at high spatial frequencies (5 cycle deg-1), indicating that the receptive field surround of neurones is important for the phase shift. 6. These temporal phase shifts were confirmed by measuring motion contrast thresholds for drifting L cone and M cone gratings summed in different spatial phases. Owing to the large phase shifts on green or orange fields, the L and M components were detected about equally well by the LUM mechanism (at 1 cycle deg-1 and 9 Hz) when summed spatially in phase or in antiphase. Antiphase summation is typically thought to produce an equiluminant red-green grating. 7. At low spatial frequency, the relative L' and M' contrast weights in the LUM mechanism (assessed at the optimal phase for summation) changed strongly with field colour and temporal frequency. 8. The phase shifts and changing contrast weights were modelled with phasic retinal ganglion cells, with chromatic adaptation strongly modifying the receptive field surround. The cells summate L' and M' in their centre, while the surround L' and M' signals are both antagonistic to the centre for approximately 570 nm yellow adaptation. Green or orange adaptation is assumed to modify the L and M surround inputs, causing them to be opponent with respect to each other, but with reversed polarity on the green versus orange field (to explain the chromatic reversal of the phase shift). Large changes in the relative L' and M' weights on green versus orange fields indicate the clear presence of the spectrally opponent surround even at 20 Hz. The spectrally opponent surround appears sluggish, with a long delay (approximately 20 ms) relative to the centre.

Adult↗

PubMed1997

Human circadian pacemaker is sensitive to light throughout subjective day without evidence of transients.

Fifty-six resetting trials were conducted across the subjective day in 43 young men using a three-cycle bright-light (approximately 10,000 lx). The phase-response curve (PRC) to these trials was assessed for the presence of a "dead zone" of photic insensitivity and was compared with another three-cycle PRC that had used a background of approximately 150 lx. To assess possible transients after the light stimulus, the trials were divided into 43 steady-state trials, which occurred after several baseline days, and 13 consecutive trials, which occurred immediately after a previous resetting trial. We found that 1) bright light induces phase shifts throughout subjective day with no apparent dead zone; 2) there is no evidence of transients in constant routine assessments of the fitted temperature minimum 1-2 days after completion of the resetting stimulus; and 3) the timing of background room light modulates the resetting response to bright light. These data indicate that the human circadian pacemaker is sensitive to light at virtually all circadian phases, implying that the entire 24-h pattern of light exposure contributes to entrainment.

Adult↗

PubMed1996

Phase-shifting human circadian rhythms: influence of sleep timing, social contact and light exposure.

1. Both the timing of behavioural events (activity, sleep and social interactions) and the environmental light-dark cycle have been reported to contribute to entrainment of human circadian rhythms to the 24 h day. Yet, the relative contribution of those putative behavioural synchronizers to that of light exposure remains unclear. 2. To investigate this, we inverted the schedule of rest, sedentary activity and social contact of thirty-two young men either with or without exposure to bright light. 3. On this inverted schedule, the endogenous component of the core temperature rhythm of subjects who were exposed to bright light showed a significant phase shift, demonstrating that they were adapting to the new schedule. In contrast, the core temperature rhythm of subjects who were not exposed to bright light moved on average 0.2 h later per day and after 10 days had not significantly adapted to the new schedule. 4. The direction of phase shift in the groups exposed to bright light was dependent on the time of bright light exposure, while control subjects drifted to a later hour regardless of the timing of their schedule of sleep timing, social contact and meals. 5. These results support the concept that the light-dark cycle is the most important synchronizer of the human circadian system. They suggest that inversion of the sleep-wake, rest-activity and social contact cycles provides relatively minimal drive for resetting the human circadian pacemaker. 6. These data indicate that interventions designed to phase shift human circadian rhythms for adjustment to time zone changes or altered work schedules should focus on properly timed light exposure.

Adult↗

PubMed1996

Dose-response relationships for resetting of human circadian clock by light.

Since the first report in unicells, studies across diverse species have demonstrated that light is a powerful synchronizer which resets, in an intensity-dependent manner, endogenous circadian pacemakers. Although it is recognized that bright light (approximately 7,000 to 13,000 lux) is an effective circadian synchronizer in humans, it is widely believed that the human circadian pacemaker is insensitive to ordinary indoor illumination (approximately 50-300 lux). It has been proposed that the relationship between the resetting effect of light and its intensity follows a compressive nonlinear function, such that exposure to lower illuminances still exerts a robust effect. We therefore undertook a series of experiments which support this hypothesis and report here that light of even relatively low intensity (approximately 180 lux) significantly phase-shifts the human circadian pacemaker. Our results clearly demonstrate that humans are much more sensitive to light than initially suspected and support the conclusion that they are not qualitatively different from other mammals in their mechanism of circadian entrainment.

Adult↗

PubMed1996

Simulations of light effects on the human circadian pacemaker: implications for assessment of intrinsic period.

The sensitivity of the human circadian system to light has been the subject of considerable debate. Using computer simulations of a recent quantitative model for the effects of light on the human circadian system, we investigated these effects of light during different experimental protocols. The results of the simulations indicate that the nonuniform distribution over the circadian cycle of exposure to ordinary room light seen in classical free-run studies, in which subjects select their exposure to light and darkness, can result in an observed period of approximately 25 h, even when the intrinsic period of the subject's endogenous circadian pacemaker is much closer to 24 h. Other simulation results suggest that accurate assessment of the true intrinsic period of the human circadian pacemaker requires low ambient light intensities (approximately 10-15 lx) during scheduled wake episodes, desynchrony of the imposed light-dark cycle from the endogenous circadian oscillator, and a study length of at least 20 days. Although these simulations await further experimental substantiation, they highlight the sensitivity to light of the human circadian system and the potential confounding influence of light on the assessment of the intrinsic period of the circadian pacemaker.

Biological Clocks↗

PubMed1995

Contributions of human long-wave and middle-wave cones to motion detection.

1. It has been suggested that motion may be best detected by the luminance mechanism. If this is the most sensitive mechanism, motion thresholds may be used to isolate the luminance mechanism and study its properties. 2. A moving (1 cycle deg-1), vertical, heterochromatic (red-plus-green), foveal grating was presented on a bright yellow (577 nm wavelength) field. Detection and motion (direction identification: left versus right) thresholds were measured for different amplitude ratios of the red and green components spatially summed in phase or in antiphase. Threshold contours plotted in cone-contrast co-ordinates (L',M') for the long-wave (L) and middle-wave (M) cones, revealed two motion mechanisms: a luminance mechanism that responds to a weighted sum of L and M contrasts, and a spectrally opponent mechanism that responds to a weighted difference. 3. Detection and motion thresholds, measured at 1-4 Hz, were identical for luminance gratings, having equal cone contrasts, L' and M', of the same sign. For chromatic gratings, with L' and M' of opposite sign, motion thresholds were higher than detection thresholds. A red-green hue mechanism may mediate chromatic detection, and a separate spectrally opponent motion mechanism may mediate motion. 4. The red-green hue mechanism was assessed from 1 to 15 Hz with an explicit hue criterion. The detection contour had a constant slope of one, implying equal L' and M' contributions of opposite sign. For motion identification, L' and M' contributed equally at 1 Hz, but the M' contribution was attenuated at higher velocities. 5. The cone-contrast metric provides a physiologically relevant comparison of sensitivities of the two motion mechanisms. At 1 Hz, the spectrally opponent motion mechanism is approximately 4 times more sensitive than the luminance mechanism. As temporal frequency is increased, the relative sensitivities change so that the luminance mechanism is more sensitive above 9 Hz. 6. The less sensitive motion mechanism was isolated with a quadrature phase protocol, using a pair of heterochromatic red-plus-green gratings, counterphase flickering in spatial and temporal quadrature phase with respect to each other. One grating was set slightly suprathreshold and oriented in cone contrast (L',M') so as to potentiate a single motion mechanism, the sensitivity of which was probed with the second grating, which was varied in (L',M'). This allowed us to measure the motion detection contour of the less sensitive luminance mechanism at low velocities.(ABSTRACT TRUNCATED AT 400 WORDS)

Color↗

PubMed1995

Human cones appear to adapt at low light levels: measurements on the red-green detection mechanism.

Recent physiological evidence suggests that cones do not light adapt at low light levels. To assess whether adaptation is cone-selective at low light levels, the red-green detection mechanism was isolated. Thresholds were measured with a large test flash, which stimulated the L and M cones in different fixed amplitude ratios, on different colored adapting fields. Thresholds were plotted in L and M cone contrast coordinates. The red-green mechanism responded to an equally-weighted difference of L and M cone contrast on each colored field, demonstrating equivalent, Weberian adaptation of the L and M cone signals. The L and M cone signals independently adapted for illuminance levels as low as 60 effective trolands (e.g. M-cone trolands). Since this adaptation is entirely selective to cone type, it suggests that the cones themselves light-adapt. The red-green detection contour on reddish fields was displaced further out from the origin of the cone contrast coordinates, revealing an additional sensitivity loss at a subsequent, spectrally-opponent site. This second-site effect may arise from a net "red" or "green" signal that represents the degree to which the L and M cones are differently hyperpolarized by the steady, colored adapting field. Such differential hyperpolarization is compatible with equivalent, Weberian adaptation of the L and M cones.

Adaptation, Ocular↗

PubMed1994

Temporal properties of the red-green chromatic mechanism.

The temporal properties of the red-green chromatic mechanism were studied with red and green equiluminant flashes of 1 deg diameter presented in the center of a bright (800-3000 td) yellow adapting field. A subthreshold 200 msec red or green flash makes an immediately subsequent, suprathreshold yellow luminance flash appear tinged with the complementary color. The chromatic flash also makes a subsequent chromatic flash of the same hue harder to detect and identify, and makes a flash of the opposite hue easier to detect and identify. These results indicate that the response of the red-green mechanism changes polarity during its time-course, suggesting that the chromatic temporal impulse-response function has a negative lobe. Pairs of chromatic pulses were used to estimate the shape of the chromatic impulse-response. The estimated impulse-response function has a zero crossing near 90 msec, followed by a long, shallow negative lobe. We also measured threshold-duration functions; the critical duration for the chromatic and luminance flashes is about 95 and 45 msec, respectively. Chromatic sensitivity (measured in cone contrast units) is 10 times greater than luminance sensitivity for long durations, and is 3 times greater for all durations less than 45 msec.

Adaptation, Ocular↗

PubMed1994

The time-course of chromatic facilitation by luminance contours.

The threshold for detecting an equiluminant chromatic spot is approximately halved by the presentation of a coincident, suprathreshold luminance pedestal flash. The dynamics of this facilitation were studied by varying the duration and temporal asynchrony of the chromatic test flash and luminance pedestal. Facilitation occurs in a narrow temporal window near the chromatic test presentation; masking may occur when the pedestal is temporally displaced from the test by longer times. The mechanism producing facilitation lags behind the chromatic signal by at least 20 msec.

Color Perception↗

PubMed1994

Separable red-green and luminance detectors for small flashes.

Detection contours were measured in L and M cone contrast coordinates for foveal flashes of 200 msec duration and 2.3, 5, 10 and 15 min arc diameter on a bright yellow field. The test flash consisted of simultaneous incremental and decremental red and green lights in various amplitude ratios. At all sizes, the most sensitive detection mechanism was not a luminance mechanism, but rather a red-green mechanism that responds to the linear difference of equally weighted L and M cone contrasts, and signals red or green sensations at the detection threshold. Both temporal and spatial integration were greater for red-green detection than luminance detection. A coincident, subthreshold, yellow flash (a luminance pedestal) did not affect the threshold of the red-green mechanism. Such a pedestal is a sum of equal L and M cone contrast--it represents a vector parallel to the red-green detection contour and thus is expected not to stimulate directly the red-green mechanism. When suprathreshold, the coincident pedestal facilitated chromatic detection by approximately 2x at all tested sizes; intense pedestals did not mask chromatic detection. This insensitivity to intense luminance pedestals further indicates that the red-green mechanism has fixed spectral tuning with balanced opponent L and M contrast inputs. This view of fixed spectral weights contrasts with the "variable tuning hypothesis", which postulates that the weights change with spatial-temporal variations in the test stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Color Perception↗

PubMed1994

Phase-amplitude resetting of the human circadian pacemaker via bright light: a further analysis.

We present here an analysis of strong, weak, and critical bright-light resetting trials in humans, and report not only phase but also amplitude data for the first time. For this analysis, an appropriate iterative smoothing procedure for phase transition curves is introduced, in which the data are sequenced so as to minimize the perpendicular distance from the data to the smoothed fit. From these smoothed data, we create polar phase-amplitude resetting maps (PARMs) in order to fully illustrate the effects of the resetting stimuli on both circadian amplitude and phase, and thereby to determine whether these resetting results can be described by a phase-only model or whether a phase-amplitude model is required. Our results indicate that a single 5-hr episode of bright light induces weak type 1 resetting of the human circadian pacemaker. Two cycles of exposure to the same stimulus on consecutive days induce critical resetting, in which significant amplitude reduction may be observed. A three-cycle stimulus induces strong type 0 resetting with different effects on circadian amplitude, depending on the initial phase of the stimulus application. When a three-cycle stimulus is centered near the nadir of the temperature cycle, large phase shifts are achieved via amplitude suppression. However, when this stimulus is centered away from the temperature nadir, smaller phase shifts are achieved in which both small increases and small decreases in circadian amplitude are observed. These data indicate that the human circadian pacemaker is not a simple, phase-only oscillator. Instead, a full description of human circadian resetting responses to light requires analysis of both phase and amplitude data--a finding that is consistent with a phase-amplitude model of the circadian resetting mechanism.

Adolescent↗

PubMed1994

Sensitivity of the human circadian pacemaker to moderately bright light.

The aim of the present study was to evaluate the sensitivity of the human circadian pacemaker to the resetting effect of moderately bright light (approximately 1260 lux), and to assess the direct effect of such light exposure by comparison to a control group of subjects undergoing the same behavioral manipulations but with a similarly timed exposure to darkness instead of light. Endogenous circadian phase and amplitude were assessed in dim light (approximately 10-15 lux) before and after two consecutive series of three 5-hr exposures to approximately 1260 lux or to darkness (approximately 0.03 lux) in two different groups of young healthy men, using the constant-routine technique. The light or darkness exposure was centered 1.5 hr after the initial fitted endogenous temperature minimum and 12 hr opposite the newly scheduled midpoint of the sleep episode, in order to induce a phase advance in the light-exposed subjects. The phase of the endogenous circadian pacemaker was assessed by a dual-harmonic regression model from core body temperature recorded every minute during constant routines. Urinary volume was measured at each micturition, subjective alertness every 20 min, and cognitive performance hourly. The endogenous circadian phase shifted to a significantly earlier time after each series of light exposures in the treatment group than it did in the control group (two-way analysis of variance for repeated measures: F = 67.91, p = 0.0001). The analysis of circadian curves of urine production, subjective alertness, and cognitive performance scores revealed that all variables maintained stable temporal relationships with the endogenous circadian temperature minimum--an indication that these rhythms shifted in the same direction and by an equivalent amount. Despite comparable behavioral schedules, including the timing of bedrest/sleep and social contacts, circadian temperature rhythm of control subjects free-ran under dim light conditions, indicating that moderately bright light exerted a direct biological effect on the human circadian pacemaker in the treatment group. The present study also demonstrated that light of approximately 1260 lux (which is of substantially lower intensity that the approximately 7000-12,000 lux used in prior experiments) produces robust phase advances of the endogenous circadian temperature rhythm, which has been shown to be an accurate marker of the output of the circadian pacemaker (Czeisler et al., 1989). These results support the hypothesis that the phase-shifting effect of light on the human circadian pacemaker has a strongly nonlinear relationship to illuminance levels, such that it is preserved despite marked reductions in light intensity.

Adolescent↗