Log Units Of Intensity Research Articles

Mice with disrupted canonical phototransductive pathways retain robust VEP responses under light-adapted conditions

The Gnat1−/−, Gnat2−/−, Opn4−/− triple knockout (TKO) mouse line, lacking alpha transducins in rods and cones and melanopsin in ip-RGCs, is generally considered to be blind. Previous studies using electroretinograms have found that these animals retain residual light sensitivity only under dark-adapted conditions. Here, we show cortical responses to light (visual evoked potentials (VEPs)) recorded from TKO mice under light-adapted conditions. VEPs were recorded from C57BL/6J mice (n=6) and TKO mice (n=5) via 50ms flashes over 4 log units of intensity, with maximal illuminance of 44.3 mW/cm∧2 at the surface of the contralateral eye (λcenter = 530nm).

Channelrhodopsin-1 Phosphorylation Changes with Phototactic Behavior and Responds to Physiological Stimuli in Chlamydomonas.

The unicellular alga Chlamydomonas (Chlamydomonas reinhardtii) exhibits oriented movement responses (phototaxis) to light over more than three log units of intensity. Phototaxis thus depends on the cell's ability to adjust the sensitivity of its photoreceptors to ambient light conditions. In Chlamydomonas, the photoreceptors for phototaxis are the channelrhodopsins (ChR)1 and ChR2; these light-gated cation channels are located in the plasma membrane. Although ChRs are widely used in optogenetic studies, little is known about ChR signaling in algae. We characterized the in vivo phosphorylation of ChR1. Its reversible phosphorylation occurred within seconds as a graded response to changes in the light intensity and ionic composition of the medium and depended on an elevated cytosolic Ca2+ concentration. Changes in the phototactic sign were accompanied by alterations in the phosphorylation status of ChR1. Furthermore, compared with the wild type, a permanently negative phototactic mutant required higher light intensities to evoke ChR1 phosphorylation. C-terminal truncation of ChR1 disturbed its reversible phosphorylation, whereas it was normal in ChR2-knockout and eyespot-assembly mutants. The identification of phosphosites in regions important for ChR1 function points to their potential regulatory role(s). We propose that multiple ChR1 phosphorylation, regulated via a Ca2+-based feedback loop, is an important component in the adaptation of phototactic sensitivity in Chlamydomonas.

A Population Representation of Absolute Light Intensity in the Mammalian Retina

Environmental illumination spans many log units of intensity and is tracked for essential functions that include regulation of the circadian clock, arousal state, and hormone levels. Little is known about the neural representation of light intensity and how it covers the necessary range. This question became accessible with the discovery of mammalian photoreceptors that are required for intensity-driven functions, the M1 ipRGCs. The spike outputs of M1s are thought to uniformly track intensity over a wide range. We provide a different understanding: individual cells operate over a narrow range, but the population covers irradiances from moonlight to full daylight. The range of most M1s is limited by depolarization block, which is generally considered pathological but is produced intrinsically by these cells. The dynamics of block allow the population to code stimulus intensity with flexibility and efficiency. Moreover, although spikes are distorted by block, they are regularized during axonal propagation.

Standardized full-field electroretinography in the Green Monkey (Chlorocebus sabaeus).

Full-field electroretinography is an objective measure of retinal function, serving as an important diagnostic clinical tool in ophthalmology for evaluating the integrity of the retina. Given the similarity between the anatomy and physiology of the human and Green Monkey eyes, this species has increasingly become a favorable non-human primate model for assessing ocular defects in humans. To test this model, we obtained full-field electroretinographic recordings (ERG) and normal values for standard responses required by the International Society for Clinical Electrophysiology of Vision (ISCEV). Photopic and scotopic ERG recordings were obtained by full-field stimulation over a range of 6 log units of intensity in dark-adapted or light-adapted eyes of adult Green Monkeys (Chlorocebus sabaeus). Intensity, duration, and interval of light stimuli were varied separately. Reproducible values of amplitude and latency were obtained for the a- and b-waves, under well-controlled adaptation and stimulus conditions; the i-wave was also easily identifiable and separated from the a-b-wave complex in the photopic ERG. The recordings obtained in the healthy Green Monkey matched very well with those in humans and other non-human primate species (Macaca mulatta and Macaca fascicularis). These results validate the Green Monkey as an excellent non-human primate model, with potential to serve for testing retinal function following various manipulations such as visual deprivation or drug evaluation.

Adaptation to Background Light Enables Contrast Coding at Rod Bipolar Cell Synapses

Rod photoreceptors contribute to vision over an ∼ 6-log-unit range of light intensities. The wide dynamic range of rod vision is thought to depend upon light intensity-dependent switching between two parallel pathways linking rods to ganglion cells: a rod → rod bipolar (RB) cell pathway that operates at dim backgrounds and a rod → cone → cone bipolar cell pathway that operates at brighter backgrounds. We evaluated this conventional model of rod vision by recording rod-mediated light responses from ganglion and AII amacrine cells and by recording RB-mediated synaptic currents from AII amacrine cells in mouse retina. Contrary to the conventional model, we found that the RB pathway functioned at backgrounds sufficient to activate the rod → cone pathway. As background light intensity increased, the RB's role changed from encoding the absorption of single photons to encoding contrast modulations around mean luminance. This transition is explained by the intrinsic dynamics of transmission from RB synapses.

Encoding Light Intensity by the Cone Photoreceptor Synapse

How cone synapses encode light intensity determines the precision of information transmission at the first synapse on the visual pathway. Although it is known that cone photoreceptors hyperpolarize to light over 4-5 log units of intensity, the relationship between light intensity and transmitter release at the cone synapse has not been determined. Here, we use two-photon microscopy to visualize release of the synaptic vesicle dye FM1-43 from cone terminals in the intact lizard retina, in response to different stimulus light intensities. We then employ electron microscopy to translate these measurements into vesicle release rates. We find that from darkness to bright light, release decreases from 49 to approximately 2 vesicles per 200 ms; therefore, cones compress their 10,000-fold operating range for phototransduction into a 25-fold range for synaptic vesicle release. Tonic release encodes ten distinguishable intensity levels, skewed to most finely represent bright light, assuming release obeys Poisson statistics.

Behavioral and electrophysiological estimates of visual thresholds in awake rats treated with 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126)

Visual thresholds for luminance increments were obtained behaviorally and electrophysiologically from rats exposed to a polychlorinated biphenyl (PCB) during development. Male Long-Evans rats exposed to 0, 0.25, or 1.0 μg/kg/day of 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126) through gestation and weaning were trained as adults to perform a signal detection task. Estimates of threshold were derived from psychometric functions for each animal relating the proportion of hits to signal intensity. Thresholds derived under three luminance conditions did not differ significantly among the PCB-treated groups. After behavioral testing was completed, flash-evoked potentials were recorded from dark-adapted awake animals. Peak amplitudes increased linearly over approximately 3 log units of intensity. Extrapolations to 0 amplitude along the linear portion of the amplitude-log intensity functions produced estimates of absolute threshold of −5.44 to −5.53 log cd/m 2-s. Waveforms recorded from awake animals had a large late negative component that was absent in previously reported anesthetized preparations. Developmental exposure to PCB 126 had no significant effect on absolute threshold or peak amplitudes and latencies.

The electroretinogram (ERG) of the mouse: normal values, optimal stimulation and recording

The electroretinogram (ERG) is an appropriate method to evaluate the retinal function in a variety of animal models. In this study we present suitable conditions of stimulation and recording in the dark-adapted mouse. Mice (n = 15) were dark-adapted during 14 hours and anesthetized with a single intraperitoneal injection of xylazine/ketamine. Pupils were dilated and a d.c.-silk-silver electrode or a AgCl-contact-lens electrode was placed on the cornea. The electroretinogram (ERG) was obtained by Ganzfeld stimulation over a range of 6 log units of intensity (8 x 10(-2) - 8 x 104 cd/m2). Intensity, duration and the interval of the light stimuli were varied separately. Reproducible values of the intensity-response functions are obtained for the a-, b- and c-waves of the ERG under well controlled adaptation- and stimulus-conditions. C-wave amplitudes are best evaluated using d.c.-recording and a stimulus duration of 4 seconds. The position of the d.c.-silk-silver electrode on the cornea can affect the ERG-amplitudes. Using a contact-lens electrode, the recorded b-wave amplitudes are on average 20% below those recorded with a centrally positioned d.c.-silk-silver electrode. Stimulus-intervals of at least 60 seconds are recommended at high intensities. An unequivocal assessment of retinal function requires reproducible ERG-values over a wide range of intensities. To obtain these, well controlled and standardized experimental conditions are required.

Square root intensity coding in turtle cones: Physiological mechanisms

The effect of coupling on the intensity-response functions for full field and for slit stimuli was studied by comparing the shapes of the curves obtained from several strongly coupled red cones of turtle with that of one very weakly coupled cone. The full field V-log I curves could be fitted by a Michaelis-Menten relationship, regardless of the strength of the coupling. For the weakly coupled cone the slit V-log I could also be fitted by a Michaelis-Menten curve. For strongly coupled cones a major portion of the curve (1.2–2 log units of intensity) was better fitted by V ∝ I m. For centered slits “ m” was 0.5. With increased distance between the slit and the center of the receptive field “ m” was found to increase slightly. The results were analyzed in terms of a theory in which the shape of the slit intensity-response curves arises from scattered light progressively recruiting neighboring cone responses. An analytical formulation of this idea is presented and a plausible light distribution function which supports the recruitment hypothesis is derived. A numerical model of the cone network, which includes the effect of scattering and transduction saturation, accounts well for all of the experimental data obtained with single and paired slits.

Regulation of cyclic GMP metabolism in toad photoreceptors. Definition of the metabolic events subserving photoexcited and attenuated states.

Photoreceptor metabolism of cGMP and its regulation were characterized in isolated toad retinas by determining the intensity and time dependence of light-induced changes in the following metabolic parameters: cGMP hydrolytic flux determined by the rate of 18O incorporation from 18O-water into retinal guanine nucleotide alpha-phosphoryls; changes in the total (protein-bound and unbound) concentrations of the guanine nucleotide metabolic intermediates; and changes in the concentration of metabolic (unbound) GDP calculated from the fraction of the alpha-GDP that undergoes labeling with 18O. The latter is interpreted to reflect the state of the equilibrium between GDP- and GTP-complexed forms of G-protein. With narrow band 500 nm light that preferentially stimulates red rod photoreceptors, a range of intensities covering approximately 5 log units produced increases of over 10-fold in cGMP metabolic flux. However, the characteristics of the cGMP metabolic response over the first 2.5 log units of intensity are readily distinguishable from those at higher intensities which exhibit progressive attenuation by an intensity- and time-dependent process. Over the range of low intensities (0.6-3 log photons.micron-2.s-1) the metabolic response is characterized by 1) increases in cGMP hydrolytic flux of up to 8-fold as a logarithmic function of intensity of photic stimulation that are sustained for at least 200 s; 2) small increases or no change in the concentration of total cGMP; 3) large increases of up to 10-fold in the concentration of metabolically active GDP as a linear function of intensity with no significant change in the tissue concentrations of total GDP or GTP; and 4) amplification of the photosignal by the metabolism of approximately 10,000 molecules of cGMP per photoisomerization with the major site of amplification at the level of the interaction of bleached rhodopsin with G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian rhythms in the electroretinogram of the cockroach.

Circadian regulation of the amplitude of the electroretinogram (ERG) of the cockroach Leucophaea maderae was investigated. Two components of the ERG exhibited circadian rhythms in amplitude. Interestingly, the peak amplitudes for the two rhythms were approximately 12 hr out of phase. The dominant corneal negative potential (the "sustained component") exhibited maximum amplitude during the subjective night. A second corneal negative potential (the "off-transient") was at a maximum during the subjective day. Intensity-response curves of the sustained component were measured at both the peak and trough of the rhythm. The results showed that the circadian rhythm in amplitude reflected a sensitivity change equivalent to 0.2-0.6 log unit of intensity. An effort was also made to identify the anatomical locus of the pacemaking oscillator for the ERG rhythm in a series of lesion experiments. Neural isolation of the optic lobe from the midbrain by bisection of the optic lobe proximal to the distal edge of the lobula had no effect on the circadian rhythm of ERG amplitude. Bisection of the optic lobe distal to the lobula abolished the ERG amplitude rhythm. These results suggest that the pacemaker is located in the optic lobe near the lobula; that its motion continues in the absence of neural connections with the rest of the nervous system; and that its regulation of ERG amplitude depends on neural pathways in the optic lobe.

Physiological characterization of electroreceptors in the lampreysIchthyomyzon unicuspis andPetromyzon marinus

1. We examined the physiological properties and distribution of electroreceptors in the skin of adult lampreys (vertebrates, class: Agnatha) by recording electroreceptor afferent fiber activity in the anterior lateral line nerve. Stimulation was with uniform or local electric fields in the water around the fish. 2. Lampreys possess ampullary electroreceptors widely distributed over the head and trunk (Fig. 1) that are sensitive to weak, low-frequency electric fields. The response thresholds to uniform fields are 1–10 μV/cm and the dynamic range of the receptors includes 4 log units of intensity (Fig. 3). Maximum sensitivity with sinusoidal currents is to frequencies ≦ 1 Hz (Fig. 4). 3. Like the ampullary receptors of elasmobranchs and other non-teleost fishes, lamprey electroreceptors are excited by weak cathodal fields (i.e. negative at the receptor opening relative to a distant reference) and inhibited by anodal fields. With very intense stimuli (1–10 mV/cm) the responses reverse so that strong cathodal fields are inhibitory and anodal fields are excitatory (Fig. 3). These results indicate that similar transduction mechanisms exist in the electroreceptors of lampreys and non-teleost jawed fishes. Likewise, as in other non-teleosts, all electroreceptors in lampreys including those on the trunk are innervated by the anterior lateral line nerve. 4. The similarities in receptor physiology and innervation taken in conjunction with known similarities in medullary organization indicate that electrosensory systems of lampreys and non-teleost gnathostome fishes are homologous.

The effects of background illumination on the photoresponses of red and green cones.

1. The photoresponses of light- and dark-adapted red and green cone photoreceptors were recorded intracellularly in the retina of the turtle, Pseduemys scripta elegans. Background illumination produced similar effects on both types of cones. 2. In response to the onset of a prolonged, steady background illumination the cone initially hyperpolarized to a peak which then sagged back to a steady-state polarization that was typically about one half the initial peak amplitude. This sag was observed for all backgrounds studied (dim as well as bright). 3. A resensitization was observed concomitantly with this sag; both the maximum increment and decrement responses grew in amplitude as light-adaptation proceeded. After about 2--3 min of background illumination, the amplitudes of these responses stabilized. 4. The dark-adapted cone produced graded responses to test pulses over a range of intensities spanning about 3.5 log units. The amplitudes of these responses were well fit by the relationship V = I.Vm/(I + sigma). 5. After 2--3 min of background illumination, 500 msec test pulses either brighter or dimmer than the background intensity were substituted for the background. The light-adapted intensity-response curves constructed from this data were similar to the dark-adapted curve but were shifted horizontally and slightly vertically, so that they still spanned about 3.5 log units of intensity. Thus, in the light-adapted cone, graded responses were elicited by a range of bright test pulses which would have produced saturated responses when delivered to the dark-adapted cone. 6. The 'off response' observed at the offset of the background became faster as the background intensity was increased. It also became faster with time following the onset of any particular background intensity. 7. It was concluded that cone sensitivity during any state of light-adaptation is determined by two mechanisms; response compression resulting from the instantaneous non-linearity between 'internal transmitter' concentration and membrane potential and a more active 'cellular adaptation' mechanism which is manifest as a shift in the intensity-response curve. In the steady-state condition of light-adaptation, most of the sensitivity changes are a result of the cellular adaptation mechanism. 8. Photopigment bleaching caused by the backgrounds, negative feed-back from horizontal cells and voltage dependent mechanisms in the cones could not account for this cellular adaptation. These effects of background illumination were interpreted in terms of the 'internal transmitter' hypothesis of phototransduction.

The intensity-dependent changes of the visual evoked responses in man to patterned and blank stimulation.

The visual evoked responses (VERs) to blank and checkerboard-patterned stimuli of 6 degrees angular subtense were recorded referentially (OZ--A2) in 10 dark-adapted subjects using a range of 5.27 log units of intensity (maximum luminance = 6420 cd/m2). The mean peak latencies of the prominent positive-negative deflection of the VERs to the blank stimuli exhibit an exponential shortening with the rise of log stimulation intensity. The peak latencies of the positive-negative deflection of the VERs to the checkerboard-pattern stimulation at low and moderate intensities are by about 50 ms shorter, with the latency of the negative peak of the response to the patterned stimulation corresponding to the positive peak latency of the response to the blank stimuli. Accordingly, the VERs to both types of stimulation are of opposite polarity. With a further increase of the stimulation intensity, the latency of the negative peak of the VER to patterned stimulation gradually lengthens due to the glaring effects of the patterned stimuli. Therefore, at high stimulation intensities the VERs to both types of stimuli are of the same polarity. The mean peak-to-peak amplitudes of the VERs to both types of stimulation differ significantly at high-intensity end of the photostimulation range only, the amplitude of the VER to the patterned stimulation being higher.

E-vector sensitivity patterns in the goldfish optic tectum

1. Spike responses of single optic tectal units to plane polarized light have been recorded with extracellular electrodes in the goldfish. Responses to a series of eight 500 msec flashes were summed and the quantitative effects ofe-vector direction studied over 4–5 log units of intensity with white light and narrow spectral bands of equal quantal content at 460, 540 and 620 nm. 2. As previously reported all or nearly all tectal cells tested (115) were sensitive to the direction of the stimulus' polarization plane. Control experiments with a thin high order birefringent retarder next to the cornea and acting as a pseudo depolarizer provided further proof that thee-vector discrimination found in the optic tectum (as well as the previously reported oriented behavior to polarized light in fish) depends on an intraocular analyzer. 3. Systematic study of tectal cells has been made at various depths and in all directly accessible areas. No localized or differential effects of recording depth one-vector discrimination have been found. However, both directions of maximum response and degree of polarized light sensitivity (PLS) show distinctive patterns over the tectal area. 4. On the basis of established tectal projection maps these data show that preferred retinale-vector directions are tangentially arranged around the eye axis when the stimulus is axial. Distribution of this angular sensitivity seems continuous without discrete channels favoring particular polarization planes. 5. Sensitivity (determined from intensity response curves) is minimal in the center of the retina for an axial stimulus and increases peripherally out to 50–60° or more off axis. Sensitivity ratios of 4–6 are common and much larger ones have been occasionally recorded. 6. Shifting the direction of stimulus from axial to 45° and 60° upward from the axis proved that retinal patterns of preferrede-vector directions and sensitivities are symmetrical relative to the beam axis rather than the anatomical eye axis. Therefore individual tectal cells and before them their retinal receptor elements show different directions ofe-vector preference and different PLS ratios depending on the stimulus configuration. 7. Intensity response curves determined with the red, green and blue narrow spectral bands were essentially superimposable. Therefore, we have no evidence that there is any special interaction of λ and PLS. 8. The effects of stimulus intensity and retinal adaptation on the IR curves do not indicate any particular selective effects. Substantial polarized light sensitivity was present in both the light adapted and dark adapted state. PLS ratios in various cases were (1) about the same at different intensity levels or (2) greater at low than high intensities or (3) greater at high than low intensities. 9. We conclude that linearly polarized light evokes a large entoptic image in the goldfish eye. Two opposite light sectors perpendicular to thee-vector alternate with two dark sectors parallel to thee-vector. Both Haidinger's brushes and Boehm's brushes in human vision show some similarities but also some important differences. The goldfish PL image is achromatic, weak or absent axially and strong peripherally. Also the contrast between the sectors does not depend on movement of the stimulus on the retina to prevent fading as it does in both of the other phenomena. 10. The most likely mechanism for the observed PLS in the goldfish eye is either differential scattering intraocularly or oblique entry of light into the receptor outer segments. Yet present data prevent a firm choice between them.

Control of retinal sensitivity. I. Light and dark adaptation of vertebrate rods and cones.

Rods and cones in Necturus respond with graded hyperpolarization to test flashes spanning about 3.5 log units of intensity. Steady background levels hyperpolarize the rods, and the rod responses become progressively smaller as background level is increased. In cones, higher background levels reduce the effectiveness of test flashes, so higher ranges of test intensities are required to elicit the full range of graded responses. When backgrounds are terminated, cones return rapidly, but rods return slowly to the dark potential level. The effects of backgrounds on both rods and cones can be observed at intensities that cause negligible bleaching as determined by retinal densitometry. During dark adaptation, changes are observed in the rods and cones that are similar to those produced by backgrounds. Receptor sensitivities, derived from these results, show that rods saturate, cones obey Weber's law, and sensitization during dark adaptation follows a two-phase time-course.

Control of retinal sensitivity. II. Lateral interactions at the outer plexi form layer.

Test stimuli, presented at the center of the bipolar cell receptive field, spanning less than 2 log units of intensity, elicit the full range of graded response. The intensity range of test stimuli that elicits the graded response depends upon the background conditions. A higher range of log test intensities is required to elicit the graded bipolar response in the presence of surround backgrounds. But surround backgrounds can also serve to unsaturate the bipolar response and thereby increase sensitivity under certain conditions. The results suggest that a second stage of sensitivity-control is mediated by the horizontal cell system at the outer plexiform layer, concatenated with the effects of adaptation in the photoreceptors.

Intensity coding in the frog retina. Quantitative relations between impulse and graded activity.

The impulse discharge of single on-off neurons and a graded field potential, the proximal negative response (PNR), were simultaneously recorded with an extracellular microelectrode in the inner frog retina. Normalized amplitude-intensity functions for the on-response of the PNR and the neuron's post-stimulus time histogram (PSTH) were nearly coincident and typically showed a dynamic range spanning approximately 2 log units of intensity. Thus a nearly linear relation is found between the amplitude of the PNR and the neuron's PSTH. A neuron's PSTH amplitude and maximum instantaneous frequency of discharge were usually highly correlated, but occasional marked disparities indicate that temporal jitter of the first spike latency is an additional, relatively independent variable influencing PSTH amplitude. It typically changes by a factor of 20-30 over the intensity range. These and other findings have implications for the functional significance of the PNR and the PSTH, for a possible linear link between amacrine and on-off ganglion cells, and for a mechanism of intensity coding in which temporal jitter of latency exerts a major role.

A search for color encoding in the responses of a class of fly interneurons

Extracellular recordings were obtained from single units in the lobula region of the optic lobe of the blowfly Calliphora phaenicia and housefly Musca domestica. These units responded selectively to the direction of movement of objects in the visual field. Previously it was suggested that these units are part of the neural system that controls flying behavior. The responses of these units were examined with monochromatic light flashes over four log units of intensity. No “opponent” cells were found. The spectral sensitivity was determined directly and found to be similar for all units studied. Responses patterns were investigated with test flashes of equal physiological brightness. It was concluded that color is not encoded by this class of interneurons. It is suggested that the optomotor response is determined by the activities of retinula cells 1–6.

Critical Fusion Frequency in Rhesus Monkeys

By using a conditioned suppression technique, critical fusion frequencies for three rhesus monkeys were determined across 8.0 log units of intensity. Threshold values ranged from 10 cps for —5.9 log foot-lamberts to 90 cps for 4.1 log foot-lamberts. Although systematic individual differences were observed at each intensity value, the intrasubject variability did not exceed 5 cps upon replication.