Changes In Ambient Light Research Articles

The electrical response of fruit trees to soil water availability and diurnal light-dark cycles

Recent studies have associated the effect of water stress, irrigation, and light cycles with electrical signaling in fruit tree species including avocado, blueberry, lemon and olive. In those studies, changes in the electrical potential (EP) difference were detected between the base of the stem and leaf in response to drought, irrigation, and diurnal changes in light and dark. In avocado, the changes in EP between the base of the stem and leaf petiole (ΔVL-S) observed in response to decreased soil water content have been associated with a decrease in stomatal conductance, indicating that stomatal closure might be associated with an electrical signal. New experiments were conducted to determine the effects of short- and long-term drought on root to leaf electrical signaling in avocado confirming that in both situations significant changes in EP differences can be detected and that an extra-cellular electrical signal appears to be involved in root to leaf communication initiating stomatal closure. Day-to-night fluctuations in EP observed in fruit trees appeared to correspond to the time of day and were therefore presumably affected by diurnal changes in ambient light and vapor pressure deficit. Addendum to: Gil PM, Gurovich L, Schaffer B, Alcayaga J, Rey S, Iturriaga R. Root to leaf electrical signaling in avocado in response to light and soil water content. J Plant Physiol 2008; 165:1070-8.

画像の平均輝度レベル，観視者の年齢，照明環境を考慮した液晶ディスプレイの輝度制御

We investigated what influences the average luminance level (ALL) of displayed images, screen illuminance, and viewers ages had on the preferred luminance of LCDs. Twenty young subjects (mean age: 21.8) and 24 seniors (mean age: 68.9) adjusted the luminance of a 17-inch 1000: 1 LCD monitor to their preferred levels under different experimental conditions. The results indicate that the preferred luminance of LCDs corresponded to the following formula: Lp=k×ALLα. This is where Lp is the peak white luminance of the LCDs, k is a constant, and ALL is the average luminance level of the displayed images. Here, α is a constant from −0.19 to −0.20 for the seniors and from −0.14 to −0.17 for the young subjects. The influences of age-related changes in vision and ambient lighting on the luminance requirements for LCDs are also discussed. These results can be applied to the design of luminance-control systems for LCDs.

Operational implications of varying ambient light levels and time‐of‐day effects on saccadic velocity and pupillary light reflex

Changes in maximal saccadic velocity (SV), initial pupil diameter (IPD), constriction latency (CL) and constriction amplitude (CA) determined by the pupillary light reflex have been found to be sensitive indicators of impairment as a result of drugs, sleepiness, and/or fatigue. Ambient illuminance and time of day are controlled when these indices are applied as repeated measures in fitness-for-duty determinations. The application of oculometrics in unrestricted operational environments, where ambient illuminance and time-of-day testing are not constant, requires understanding of, and potential compensation for, the effects of, and interactions among, these multiple uncontrolled variables. SV, IPD, CL, and CA were evaluated in the morning and evening on two consecutive days following adequate nightly sleep under one baseline ambient illuminance and seven test ambient illuminances. Sixteen healthy volunteers (21-38 years, eight females/eight males) participated. Within and across days, SV was unaffected by decreasing ambient light or time-of-day effects. With the increase of ambient light from 670 to 3300 lx, CL decreased by 1%, while IPD and CA decreased by 17% and 20%, respectively. IPD increased with time of day by 1-10% (IPD was smaller in the morning). The results show that SV and CL are essentially resistant to changes in ambient light and time-of-day effects, simplifying their application in uncontrolled operational environments.

Development of a Trigger-On Indicator for a Weed Sensing Spray Unit

Intermittent spray systems that automatically spray only when weeds are present have been commercially available since 1992 and can significantly reduce herbicide usage by 50 to 90% as compared to broadcast applications (2). The Weedseeker (NTech Industries, Ukiah, Calif.) is one such system that utilizes optics to detect the presence of weeds. The system works by exploiting the fact that chlorophyll selectively absorbs red wavebands of light and reflects near-infrared light (1). Each sensor unit has its own built-in light source and a sensor that detects the light reflected back to the unit over a 12-inch-wide field of view. When the sensor detects an increase in the ratio of near-infrared to red light above the base threshold level, a solenoid valve is activated to spray the weed. Although the sensor unit reliably detects the presence of green plants (2), a major drawback with the current design is that it doesn't provide any feedback to the user when one of the spray units is actively spraying. This is problematic, particularly on wide-boom, multi-sensor-unit sprayers since each sensing unit is calibrated independently in the field prior to spraying from the reflectance of the ground surface beneath it whenever the calibration button is pushed. If one of the sensing units is calibrated when the sensor is over a weed, the unit will not detect weeds of similar size or smaller when spraying. Also, because the sensing unit is sensitive to background reflectance, if one of the sensors is calibrated when it is positioned over a spot of ground that is not characteristic of the rest of the field, the unit will ghost fire and spray when weeds are not present. Either scenario is undesirable, but likely to occur since recalibration is necessary several times during the day due to changes in ambient light and field conditions. To overcome this problem, a trigger-on indicating device was developed to help the operator determine that the sensor units are operating correctly. The Weedseeker unit has a small LED on the back of the unit that illuminates whenever the unit is spraying. The device developed utilizes a phototransistor to detect when the LED is illuminated and activate a super-bright LED that is visible from the tractor cab (Fig. 1). The circuit designed includes a resistor and a NPN transistor that function to prevent low levels of light from activating the super-bright LED (Fig. 2). All electronic components are encased in a waterproof BUD box and attached to a Weedseeker sensor via Velcro and zip ties (Figs. 1 and 3). An opaque, nontranslucent flexible rubber washer is used to seal the phototransistor from ambient light and dust (Fig. 1). Power is supplied by two 1.5-V, AA batteries. Fig. 1. Trigger-on indicator assembly for Weedseeker sensor unit.

環境光の影響を抑制した物体検出用イメージセンサ

Object detection while suppressing the effect of the change of the ambient light is performed by using different values of images with the modulated light on and off.In the actual environment,the changes of ambient light affect conventional object detection.To overcome the effects of changes in ambient light and to achieve wide dynamic range imaging,a new way of detecting objects is introduced.We also describe a pixel circuit specifically designed for use with this method and present the results of evaluations of the fabricated prototype chip.

Rapid light-response curves of chlorophyll fluorescence in microalgae: relationship to steady-state light curves and non-photochemical quenching in benthic diatom-dominated assemblages

Rapid light-response curves (RLC) of variable chlorophyll fluorescence were measured on estuarine benthic microalgae with the purpose of characterising its response to changes in ambient light, and of investigating the relationship to steady-state light-response curves (LC). The response of RLCs to changes in ambient light (E, defined as the irradiance level to which a sample is acclimated to prior to the start of the RLC) was characterised by constructing light-response curves for the RLC parameters alpha (RLC), the initial slope, ETR(m,RLC), the maximum relative electron transport rate, and E (k,RLC), the light-saturation parameter. Measurements were carried out on diatom-dominated suspensions of benthic microalgae and RLC and LC parameters were compared for a wide range of ambient light conditions, time of day, season and sample taxonomic composition. The photoresponse of RLC parameters was typically bi-phasic, consisting of an initial increase of all parameters under low ambient light (E < 21-181 micromol m(-2) s(-1)), and of a phase during which alpha (RLC) decreased significantly with E, and the increase of ETR(m,RLC) and E (k,RLC) was attenuated. The relationship between RLC and LC parameters was dependent on ambient irradiance, with significant correlations being found between alpha (RLC) and alpha, and between ETR(m,RLC) and ETR(m), for samples acclimated to low and to high ambient irradiances, respectively. The decline of alpha (RLC) under high light (Deltaalpha (RLC)) was strongly correlated (P < 0.001 in all cases) with the level of non-photochemical quenching (NPQ) measured before each RLC. These results indicate the possibility of using RLCs to characterise the steady-state photoacclimation status of a sample, by estimating the LC parameter E (k), and to trace short-term changes in NPQ levels without dark incubation.

Biofouling and the continuous monitoring of underwater light from a seagrass perspective

For more than a decade, inexpensive electronic instruments have made continuous underwater light monitoring an integral part of many seagrass studies. Although biofouling, if not controlled, compromises the utility of the record. A year-long assessment of the time course of sensor fouling, in the Laguna Madre of Texas established that light transmitted through the fouling layer after 2 wk of exposure exceeded 90% except for a 6–8 wk period in May and June. On that basis, a 2-wk interval was chosen for routine servicing. Subsequent monitoring proved this choice to be grossly in error. The period of sub-90% transmittance after 2 wk extended to 4–6 mo annually over the next 3 yr. Fouling was strongly correlated with temperature, ambient light, and year. Since an algal bloom of 7-yr duration finally waned during this study, increased ambient light seemed most likely to explain increased fouling later in the study. The explanatory value of light was less than temperature or year in multiple regression, requiring some other explanation of the date effect than change in ambient light. Allelopathic and suspension-feeding depressant effects of the brown tide are offered as the most likely cause of unusually low fouling in the first year. Biofouling was so unpredictable and rapid in this study that at least weekly maintenance would be required to assure reliability of the light monitoring record.

New generation machine vision: coping with changes in light and surface quality

Bad experiences from the past - where changes in ambient light adversely affected system performance - have made engineers sceptical about adopting machine vision into their production processes. The latest generation of cameras and system software removes many of their issues. During the development of the system the versatility of machine vision software is important because changes invariably require reconfiguration. In relation to the installed system the down-times from subsequent processes could be decreased to zero due to the correct identification that the vision system provides; and as a result of the productivity has increased accordingly.

NAB1 Is an RNA Binding Protein Involved in the Light-Regulated Differential Expression of the Light-Harvesting Antenna of Chlamydomonas reinhardtii

Photosynthetic organisms respond to changes in ambient light by modulating the size and composition of their light-harvesting complexes, which in the case of the green alga Chlamydomonas reinhardtii consists of >15 members of a large extended family of chlorophyll binding subunits. How their expression is coordinated is unclear. Here, we describe the analysis of an insertion mutant, state transitions mutant3 (stm3), which we show has increased levels of LHCBM subunits associated with the light-harvesting antenna of photosystem II. The mutated nuclear gene in stm3 encodes the RNA binding protein NAB1 (for putative nucleic acid binding protein). In vitro and in vivo RNA binding and protein expression studies have confirmed that NAB1 differentially binds to LHCBM mRNA in a subpolysomal high molecular weight RNA-protein complex. Binding of NAB1 stabilizes LHCBM mRNA at the preinitiation level via sequestration and thereby represses translation. The specificity and affinity of binding are determined by an RNA sequence motif similar to that used by the Xenopus laevis translation repressor FRGY2, which is conserved to varying degrees in the LHCBM gene family. We conclude from our results that NAB1 plays an important role in controlling the expression of the light-harvesting antenna of photosystem II at the posttranscriptional level. The similarity of NAB1 and FRGY2 of Xenopus implies the existence of similar RNA-masking systems in animals and plants.

Expression of UV-, blue-, long-wavelength-sensitive opsins and melatonin in extraretinal photoreceptors of the optic lobes of hawkmoths

Lepidopterans display biological rhythms associated with egg laying, eclosion and flight activity but the photoreceptors that mediate these behavioural patterns are largely unknown. To further our progress in identifying candidate light-input channels for the lepidopteran circadian system, we have developed polyclonal antibodies against ultraviolet (UV)-, blue- and extraretinal long-wavelength (LW)-sensitive opsins and examined opsin immunoreactivity in the adult optic lobes of four hawk moths, Manduca sexta, Acherontia atropos, Agrius convolvuli and Hippotion celerio. Outside the retina, UV and blue opsin protein expression is restricted to the adult stemmata, with no apparent expression elsewhere in the brain. Melatonin, which is known to have a seasonal influence on reproduction and behaviour, is expressed with opsins in adult stemmata together with visual arrestin and chaoptin. By contrast, the LW opsin protein is not expressed in the retina or stemmata but rather exhibits a distinct and widespread distribution in dorsal and ventral neurons of the optic lobes. The lamina, medulla, lobula and lobula plate, accessory medulla and adjacent neurons innervating this structure also exhibit strong LW opsin immunoreactivity. Together with the adult stemmata, these neurons appear to be functional photoreceptors, as visual arrestin, chaoptin and melatonin are also co-expressed with LW opsin. These findings are the first to suggest a role for three spectrally distinct classes of opsin in the extraretinal detection of changes in ambient light and to show melatonin-mediated neuroendocrine output in the entrainment of sphingid moth circadian and/or photoperiodic rhythms.

Diurnal and circadian retinomotor movements in zebrafish

Key indicators of circadian regulation include the persistence of physiological rhythmicity in the absence of environmental time cues and entrainment of this rhythmicity by the ambient light cycle. In some teleosts, the inner segments of rod and cone photoreceptors contract and elongate according to changes in ambient lighting and the circadian cycle. Pigment granules in the retinal pigment epithelium (RPE) disperse and aggregate in a similar manner. Collectively, these movements are known as retinomotor movements. We report the histological characterization of diurnal and circadian retinomotor movements in zebrafish, Danio rerio. Adult fish subjected to a 14:10 light:dark (LD) cycle, constant darkness (DD), or constant light (LL) were sacrificed at 1-13 h intervals and processed for semithin sectioning of the retina. Using bright-field microscopy, 15 measurements of pigment granule position and the inner segment lengths of 30 rods and 30-45 cones were collected from the central third of the dorso-optic retina per time point. In LD, rods and cones followed a clear diurnal rhythm in their inner segment movements. Short-single, UV-sensitive cones were found to contract significantly 1 h before light onset in LD conditions. In DD conditions, the inner segments movements of short-single and double cones displayed statistically significant rhythms. RPE pigment granule movements are rhythmically regulated in both LD and DD although fluctuations are damped in the absence of photic cues. No significant retinomotor movements were observed in LL. These findings indicate retinomotor movements in zebrafish are differentially regulated by an endogenous oscillator and by light-dependent mechanisms.

Diving behavior of immature, feeding Pacific bluefin tuna (Thunnus thynnus orientalis) in relation to season and area: the East China Sea and the Kuroshio–Oyashio transition region

AbstractTwenty‐four archival tags were recovered from Pacific bluefin tuna previously released off Tsushima Island in the East China Sea. By analysis of the time‐series data of the pressure and the ambient and internal temperature from the 24 tags, we examined the relationship between the tuna's pattern of diving and the thermocline depth. In the East China Sea, diving and feeding events occurred throughout almost the entire day in both winter and summer, suggesting that the purpose of diving is for feeding. In summer, the feeding frequency was greater than that in winter, which corresponds to the fact that growth is more rapid in summer than in winter. During summer in the Kuroshio–Oyashio transition region, on the other hand, feeding events were much more frequent than those in the East China Sea, in spite of a lower diving frequency. The mean horizontal distance traveled was also significantly higher and it seems that in this area they may move horizontally to feed on prey accumulated at the surface. We conclude that, in addition to the ambient temperature structure, the vertical and horizontal distribution of prey species plays an important role in the feeding behavior of Pacific bluefin tuna. One bluefin tuna migrated to the Oyashio frontal area, where both the horizontal and the vertical thermal gradients are much steeper. The fish spent most of the time on the warmer side of the front and often traveled horizontally to the colder side during the day, perhaps to feed. This implies that there is a thermal barrier effect, in this case from the Oyashio front, on their behavior. The frequency of feeding events was low, although all the monitored fish dived every dawn and dusk, irrespective of the seasons or location. It is possible that these twice‐daily diving patterns occurred in response to the change in ambient light at sunrise and sunset.

The frequency gene is required for temperature-dependent regulation of many clock-controlled genes in Neurospora crassa.

The circadian clock of Neurospora broadly regulates gene expression and is synchronized with the environment through molecular responses to changes in ambient light and temperature. It is generally understood that light entrainment of the clock depends on a functional circadian oscillator comprising the products of the wc-1 and wc-2 genes as well as those of the frq gene (the FRQ/WCC oscillator). However, various models have been advanced to explain temperature regulation. In nature, light and temperature cues reinforce one another such that transitions from dark to light and/or cold to warm set the clock to subjective morning. In some models, the FRQ/WCC circadian oscillator is seen as essential for temperature-entrained clock-controlled output; alternatively, this oscillator is seen exclusively as part of the light pathway mediating entrainment of a cryptic "driving oscillator" that mediates all temperature-entrained rhythmicity, in addition to providing the impetus for circadian oscillations in general. To identify novel clock-controlled genes and to examine these models, we have analyzed gene expression on a broad scale using cDNA microarrays. Between 2.7 and 5.9% of genes were rhythmically expressed with peak expression in the subjective morning. A total of 1.4-1.8% of genes responded consistently to temperature entrainment; all are clock controlled and all required the frq gene for this clock-regulated expression even under temperature-entrainment conditions. These data are consistent with a role for frq in the control of temperature-regulated gene expression in N. crassa and suggest that the circadian feedback loop may also serve as a sensor for small changes in ambient temperature.

Interacting effects of ambient light and plumage color patterns in displaying Wire-tailed Manakins (Aves, Pipridae)

Sending color signals to conspecifics may attract predators, leading to opposing selection pressures on the evolution of signal expression and display behavior in animals. The costs of signaling can be reduced, however, because conspicuousness is the combined result of the reflectance spectra of the displayer's color pattern and the spectra of ambient light illuminating the animal. Changes in ambient light can alter conspicuousness, even when chemical and structural color-generating mechanisms remain constant, potentially allowing animals to display their colors most fully in light environments where the benefits are greatest relative to the costs. Using spectroradiometric methods, we determined how light habitat use affects conspicuousness in adult males of the Wire-tailed Manakin Pipra filicauda, a lekking bird species with vivid plumage colors. We studied three aspects of visibility, including properties of the entire color pattern, visual contrast within an individual's plumage and a bird's contrast relative to its visual background. Wire-tailed Manakins usually displayed in forest shade environments, which reduced their conspicuousness at larger viewing distances, while maximizing visual contrast within the plumage color pattern at close viewing distances. Compared to sunspots, ambient light in forest shade reduces the contrast of individual bird colors with the background at close viewing distance. However, background contrast of individual bird colors in the shade was still relatively higher during sunny than during cloudy weather which may explain why males were more active when the sun was not blocked by clouds. Assuming that the visual perceptions of predators and other manakins do not differ from the reflectance patterns we measured, Wire-tailed manakins tend to display in light environments that reduce the conflicts between avoiding long distance detection by predators and displaying conspicuous color signals to visiting females.

Photoresponse of the conductivity in functionalized pentacene compounds

We report the investigation of the photoresponse of the conductivity of a recently synthesized class of organic semiconductors based on functionalized pentacene. These materials form high quality single crystals that exhibit a thermally activated resistivity. Unlike pure pentacene, the functionalized derivatives are readily soluble in acetone, and can be evaporated or spincast as thin films for potential device applications. The electrical conductivity of the single crystal materials is noticeably sensitive to ambient light changes. The purpose, therefore, of the present study, is to determine the nature of the photoresponse in terms of carrier activation versus heating effects, and also to measure the dependence of the photoresponse on photon energy. We describe a method, involving the temperature dependent photoresponse, which allows an unambiguous identification of the signature of heating effects in materials with a thermally activated conductivity. We find strong evidence that the photoresponse in the materials investigated is predominantly a highly localized heating mechanism. Wavelength dependent studies of the photoresponse reveal resonant features and cutoffs that indicate the photon energy absorption is related to the electronic structure of the material.

The photophores of Meganyctiphanes norvegica (M. Sars) (Euphausiacea): mode of operation

The photophores of Meganyctiphanes were investigated with regard to the control of light production and with respect to their role in a hitherto unknown communication system using light flashes which became evident from observation of specialised signalling behaviour. To that purpose the light production was recorded during presentation of a range of stimuli delivered to the intact, tethered shrimp. Stimuli used were changes in ambient light, water turbulence, simulated predator approach and light flashes, as well as electric shocks and serotonin injections. Strong negative light gradients, exaggerating the natural sunset signal, reliably elicited light production, the peak of which lasted on average 2 min. In the late phase of this light production, low frequency water oscillations and turbulent flow (assumed intraspecific communication signals at close range) elicited transient increases in light production. Artificial light flashes presented to a group of shrimp evoked a signalling behaviour in which the animal points the light of its photophore beamers (positioned at the ventral side and normally directed downwards) for a fraction of a second at observers within the same depth level. The responses produced by the signalling behaviour indicate a fixed delay with respect to the triggering flash. Electric stimulation of the ventral nerve cord via implanted electrodes resulted in a strong light production with a latency of 160 ms. Injection of serotonin, resulting in haemolymph concentrations of 10–5 M and higher, initiated increasingly strong and increasingly long-lasting continuous light production. Implications for the control of the photophores are discussed.

Bacteriophytochrome controls photosystem synthesis in anoxygenic bacteria.

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr). Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria--but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein-protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant-bacteria interaction.

Seasonal changes in chlorophyll fluorescence quenching and the induction and capacity of the photoprotective xanthophyll cycle in Lobaria pulmonaria

Lobaria pulmonaria (L.) Hoffm. survives large changes in ambient light and temperature between winter and summer in temperate deciduous forests. Potential photosystem II (PSII) electron transport, measured at 20°C using chlorophyll fluorescence analysis, was consistent in thallus samples taken in March and August from a temperate deciduous forest, while the potential for nonphotochemical quenching (NPQ) was higher in March than in August. NPQ was, however, similar in March and August in a population from a coniferous site with a permanently closed canopy. Thalli measured at a typical March field temperature showed a depression of PSII electron transport, qp and gross CO2 uptake and a rise in realized NPQ. Xanthophyll cycle pigments were more abundant in the March than in the August samples in the deciduous forest populations but did not change significantly in the permanently closed-canopy population. In August, relatively low NPQ correlated with xanthophyll pool size in the deciduous forest samples. The more intense NPQ derived from low temperature and low CO2-electron demand in the March samples, however, was not correlated with xanthophyll pool size. Thus, most of the NPQ observed in the high light exposure March samples was not explained by variation in xanthophyll pool size.Key words: carotenoids, excitation quenching, lichen, nonphotochemical quenching, PSII electron transport.

Machine vision assisted characterization of machined surfaces

This paper demonstrates the feasibility of using an inexpensive machine vision system to compute non-contact, optical parameters for the characterization of surface roughness of machined surfaces. Two parameters were selected for online analysis, where surface roughness is measured during the rotation of a specimen on a lathe. The sensitivity of the vision-based optical parameters to differences in surface roughness, ambient light and spindle speed of a lathe during measurement was evaluated. Statistical analysis of data collected through experimentation revealed that the vision parameters can discriminate different surface roughness heights and are insensitive to changes in ambient lighting and speed of rotation during measurement. The results of the experimental analysis were used to conclude the feasibility of using machine vision for the evaluation of surfaces.

Regulation of the On Bipolar Cell mGluR6 Pathway by Ca2+

Glutamate produces a hyperpolarizing synaptic potential in On bipolar cells by binding to the metabotropic glutamate receptor mGluR6, leading to closure of a cation channel. Here it is demonstrated that this cation channel is regulated by intracellular Ca(2+). Glutamate-evoked currents were recorded from On bipolar cells in light-adapted salamander retinal slices in the presence of 2 mm external Ca(2+). When glutamate was applied almost continuously, interrupted only briefly to measure the size of the response, the glutamate response remained robust. However, currents elicited by intermittent and brief applications of glutamate exhibited time-dependent run down. Run down of the glutamate response was also voltage dependent, because it was accelerated by membrane hyperpolarization. Run down was triggered, at least in part, by a rise in intracellular Ca(2+); measured as a function of time or voltage, it was attenuated by intracellular buffering of Ca(2+) with BAPTA or by omitting Ca(2+) from the bathing solution. Current-voltage measurements demonstrated that Ca(2+) induced run down of the glutamate response by downregulating cation channel function, rather than by preventing closure of the channel by glutamate and mGluR6. A major source of the Ca(2+) that mediated this inhibition is the cation channel itself, which was found to be permeable to Ca(2+), accounting for the use dependence of the run down. These results suggest that Ca(2+) influx through the cation channel during background illumination could provide a signal to close the cation channel and repolarize the membrane toward its dark potential, an adaptive mechanism for coping with changes in ambient light.