Temperature-dependence of rod photoresponses from the aspartate-treated retina of the frog(Rana temporaria)

Temporal vision: measures, mechanisms and meaning

Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.

Summary: Simple measures of temporal vision such as the critical flicker frequency can be useful for modelling natural vision only if their relationship to photoreceptor responses and retinal processing is understood.

A p97/Valosin-Containing Protein Inhibitor Drug CB-5083 Has a Potent but Reversible Off-Target Effect on Phosphodiesterase-6

CB-5083 is an inhibitor of p97/valosin-containing protein (VCP), for which phase I trials for cancer were terminated because of adverse effects on vision, such as photophobia and dyschromatopsia. Lower dose CB-5083 could combat inclusion body myopathy with early-onset Paget disease and frontotemporal dementia or multisystem proteinopathy caused by gain-of-function mutations in VCP. We hypothesized that the visual impairment in the cancer trial was due to CB-5083's inhibition of phosphodiesterase (PDE)-6, which mediates signal transduction in photoreceptors. To test our hypothesis, we used in vivo and ex vivo electroretinography (ERG) in mice and a PDE6 activity assay of bovine rod outer segment (ROS) extracts. Additionally, histology and optical coherence tomography were used to assess CB-5083's long-term ocular toxicity. A single administration of CB-5083 led to robust ERG signal deterioration, specifically in photoresponse kinetics. Similar recordings with known PDE inhibitors sildenafil, tadalafil, vardenafil, and zaprinast showed that only vardenafil had as strong an effect on the ERG signal in vivo as did CB-5083. In the biochemical assay, CB-5083 inhibited PDE6 activity with a potency higher than sildenafil but lower than that of vardenafil. Ex vivo ERG revealed a PDE6 inhibition constant of 80 nM for CB-5083, which is 7-fold smaller than that for sildenafil. Finally, we showed that the inhibitory effect of CB-5083 on visual function is reversible, and its chronic administration does not cause permanent retinal anomalies in aged VCP-disease model mice. Our results warrant re-evaluation of CB-5083 as a clinical therapeutic agent. We recommend preclinical ERG recordings as a routine drug safety screen. SIGNIFICANCE STATEMENT: This report supports the use of a valosin-containing protein (VCP) inhibitor drug, CB-5083, for the treatment of neuromuscular VCP disease despite CB-5083's initial clinical failure for cancer treatment due to side effects on vision. The data show that CB-5083 displays a dose-dependent but reversible inhibitory action on phosphodiesterase-6, an essential enzyme in retinal photoreceptor function, but no long-term consequences on retinal function or structure.

Versatile bipolar temperature controller for custom in vitro applications

Effective temperature control is crucial in many studies of isolated biological tissues, with preparations often requiring specialized holding chambers. In these situations, the design flexibility and optimizations offered by a custom made temperature controller may be preferable over a commercial model. We present a versatile controller for heating and cooling applications, providing simple step-by-step instructions to mathematically model your specific system and optimize controller parameters. The apparatus uses analog components and linear stages to simplify circuit comprehension and customization, achieving fast transitions with small static errors and overshoots over a wide range of temperatures without readjustment. A fully featured rackable enclosure is complemented by two temperature probes based on the LMT70A linear microchip sensor (for the control loop and for bath monitoring). BNC outputs provide scaled probe signals for continuous temperature data acquisition. The maximum achievable power output of the controller is -23.5 W/+22.0 W (-4.7 V/+4.4 V, ±5.0 A), sufficient to bring a well designed holder for standard 35 mm chambers from 23 °C up to 37 °C in ~1 min and down to 3 °C in ~4 min. Any biologist with some technical prowess should be able to follow our instructions from modeling to assembly and calibration.

Ex vivo electroretinograms made easy: performing ERGs using 3D printed components

KEY POINTS

Rod and cone photoreceptors convert light into electrochemical signals that are transferred to second order cells, initiating image-forming visual processing. Electroretinograms (ERGs) can detect the associated light-induced extracellular transretinal events, allowing for physiological assessment of cellular activity from morphologically intact retinas. We outline a method for economically configuring a traditional patch-clamp rig for performing high signal-to-noise ex vivo ERGs. We accomplish this by incorporating various 3D printed components and by modifying existing light pathways in a typical patch-clamp rig. This methodology provides an additional set of tools to labs interested in studying the physiological function of neuronal populations in isolated retinal tissue.

ABSTRACT

Rod and cone photoreceptors of the retina are responsible for the initial stages in vision and convey sensory information regarding our visual world across a wide range of lighting conditions. These photoreceptors hyperpolarize in the presence of light and subsequently transmit signals to second-order bipolar and horizontal cells. The electrical components of these events are experimentally detectable, and in conjunction with pharmacological agents, can be further separated into their respective cellular contributions using electroretinograms (ERGs). Extracellular activity from populations of rods and cones generate the negative-going a-wave, while ON-bipolar cells generate positive-going b-waves. ERGs can be performed in vivo or alternatively using an ex vivo configuration, where retinas are isolated and transretinal photovoltages are recorded at high signal-to-noise ratios. However, most ERG set-ups require their own unique set of tools. We demonstrate how, at low cost, to reconfigure a typical patch-clamp rig for ERG recordings. The bulk of these modifications require implementation of various 3D printed components, which can alternatively aid in generating a stand-alone ERG set-up without a patch-rig. Further, we discuss how to configure an ERG system without a patch-clamp rig. Compared to in vivo ERGs, these are superior when measuring small responses, such as those that are cone-evoked or those from immature mouse retinae. This recording configuration provides high signal-to-noise detection of a-waves (300-600 µV) and b-waves (1-3 mV), and is ultimately capable of discerning small (1-2 µV) photovoltages from noise. These quick and economical modifications allow researchers to equip their technical arsenal with an interchangeable patch-clamp/ERG system.

Searching for magnetic compass mechanism in pigeon retinal photoreceptors

Migratory birds can detect the direction of the Earth’s magnetic field using the magnetic compass sense. However, the sensory basis of the magnetic compass still remains a puzzle. A large body of indirect evidence suggests that magnetic compass in birds is localized in the retina. To confirm this point, an evidence of visual signals modulation by magnetic field (MF) should be obtained. In a previous study we showed that MF inclination impacts the amplitude of ex vivo electroretinogram (ERG) recorded from isolated pigeon retina. Here we present the results of an analysis of putative MF effect on one component of ERG, the photoreceptor’s response, isolated from the total ERG by adding sodium aspartate and barium chloride to the perfusion solution. Photoresponses were recorded from isolated retinae of domestic pigeons Columba livia. The retinal samples were placed in MF that was modulated by three pairs of orthogonal Helmholtz coils. Light stimuli (blue and red) were applied under two inclinations of MF, 0° and 90°. In all the experiments, preparations from two parts of retina were used, red field (with dominant red-sensitive cones) and yellow field (with relatively uniform distribution of cone color types). In contrast to the whole retinal ERG, we did not observe any effect of MF inclination on either amplitude or kinetics of pharmacologically isolated photoreceptor responses to blue or red half-saturating flashes. A possible explanations of these results could be that magnetic compass sense is localized in retinal cells other than photoreceptors, or that photoreceptors do participate in magnetoreception, but require some processing of compass information in other retinal layers, so that only whole retina signal can reflect the response to changing MF.

In vivo monitoring of mouse retinal temperature by ERG photoresponses

Non-damaging heating of the retina and RPE provides a promising treatment for retinal diseases. However, the lack of proper control over the temperature hinders the development of safe and repeatable procedures. Here, we demonstrate with mice a non-invasive method for estimating the temperature changes in the retina and the RPE during a heating procedure. The method is based on monitoring the temperature dependent properties of retinal photoresponses recorded by electroretinography (ERG). In this study, our aim was to investigate the feasibility of ERG signal for retinal temperature estimation, utilizing a-wave and b-wave kinetics as the source of temperature information. We quantified the temperature dependencies of photoresponse kinetics and developed two linear regression models between the temperature and the photoresponse features, enabling temperature estimation. With the first model, based on the a-wave of a single photoresponse, the RMS error obtained for retinal temperature estimation was <0.9 °C. The second model, applying the b-waves of five dim flash responses, an RMS error of <0.7 °C was achieved. In addition, we tested the sensitivity of the method to small changes in light stimulus strength and investigated suitable stimulus intervals for continuous retinal temperature monitoring. The proposed method provides a convenient technique for monitoring mouse retinal and RPE temperature with ERG recording when studying controlled retinal heating. Similar temperature dependencies exist in human ERG suggesting that this approach could also be applicable in clinical heating treatments.

A Novel Method for Mouse Retinal Temperature Determination Based on ERG Photoresponses

This study introduces a novel retinal temperature determination method based on the temperature dependent properties of photoresponses recorded by electroretinography (ERG). The kinetics and amplitudes of ERG photoresponses depend on retinal temperature. Additionally, raising retinal temperature increases the probability of long-wavelength photon absorption, which manifests as temperature dependence of photoreceptor sensitivity. In this study we extract a number of features that represent these properties from the a- and b-waves of mouse ex vivo ERG flash responses and construct three multivariable regression models between temperature and the selected features. The performance of these models was evaluated against a separate test dataset and for two of the models, an RMS temperature determination error of less than 0.50 °C could be reached. Our results demonstrate that the method can be successfully used for reliable retinal temperature determination ex vivo. The method, reflecting the temperature of distal retina, can be applied also in the estimation of retinal pigment epithelium temperature.

A novel Ca2+-feedback mechanism extends the operating range of mammalian rods to brighter light

A previously unidentified calcium-dependent mechanism contributes to light adaptation in mammalian rods.

Sensory cells adjust their sensitivity to incoming signals, such as odor or light, in response to changes in background stimulation, thereby extending the range over which they operate. For instance, rod photoreceptors are extremely sensitive in darkness, so that they are able to detect individual photons, but remain responsive to visual stimuli under conditions of bright ambient light, which would be expected to saturate their response given the high gain of the rod transduction cascade in darkness. These photoreceptors regulate their sensitivity to light rapidly and reversibly in response to changes in ambient illumination, thereby avoiding saturation. Calcium ions (Ca²⁺) play a major role in mediating the rapid, subsecond adaptation to light, and the Ca²⁺-binding proteins GCAP1 and GCAP2 (or guanylyl cyclase–activating proteins [GCAPs]) have been identified as important mediators of the photoreceptor response to changes in intracellular Ca²⁺. However, mouse rods lacking both GCAP1 and GCAP2 (GCAP^−/−) still show substantial light adaptation. Here, we determined the Ca²⁺ dependency of this residual light adaptation and, by combining pharmacological, genetic, and electrophysiological tools, showed that an unknown Ca²⁺-dependent mechanism contributes to light adaptation in GCAP^−/− mouse rods. We found that mimicking the light-induced decrease in intracellular [Ca²⁺] accelerated recovery of the response to visual stimuli and caused a fourfold decrease of sensitivity in GCAP^−/− rods. About half of this Ca²⁺-dependent regulation of sensitivity could be attributed to the recoverin-mediated pathway, whereas half of it was caused by the unknown mechanism. Furthermore, our data demonstrate that the feedback mechanisms regulating the sensitivity of mammalian rods on the second and subsecond time scales are all Ca²⁺ dependent and that, unlike salamander rods, Ca²⁺-independent background-induced acceleration of flash response kinetics is rather weak in mouse rods.

Ex vivo ERG analysis of photoreceptors using an in vivo ERG system

The Function of the retina and effects of drugs on it can be assessed by recording transretinal voltage across isolated retina that is perfused with physiological medium. However, building ex vivo ERG apparatus requires substantial amount of time, resources and expertise. Here we adapted a commercial in vivo ERG system for transretinal ERG recordings from rod and cone photoreceptors and compared rod and cone signaling between ex vivo and in vivo environments. We found that the rod and cone a- and b-waves recorded with the transretinal ERG adapter and a standard in vivo ERG system are comparable to those obtained from live anesthetized animals. However, ex vivo responses are somewhat slower and their oscillatory potentials are suppressed as compared to those recorded in vivo. We found that rod amplification constant (A) was comparable between ex vivo and in vivo conditions, ∼10-30s(-2) depending on the choice of response normalization. We estimate that the A in cones is between 3 and 6s(-2) in ex vivo conditions and by assuming equal A in vivo we arrive to light funnelling factor of 3 for cones in the mouse retina. The ex vivo ERG adapter provides a simple and affordable alternative to designing a custom-built transretinal recordings setup for the study of photoreceptors. Our results provide a roadmap to the rigorous quantitative analysis of rod and cone responses made possible with such a system.

Mechanisms and distribution of ion channels in retinal ganglion cells: using temperature as an independent variable

Trains of action potentials of rat and cat retinal ganglion cells (RGCs) were recorded intracellularly across a temperature range of 7-37 degrees C. Phase plots of the experimental impulse trains were precision fit using multicompartment simulations of anatomically reconstructed rat and cat RGCs. Action potential excitation was simulated with a "Five-channel model" [Na, K(delayed rectifier), Ca, K(A), and K(Ca-activated) channels] and the nonspace-clamped condition of the whole cell recording was exploited to determine the channels' distribution on the dendrites, soma, and proximal axon. At each temperature, optimal phase-plot fits for RGCs occurred with the same unique channel distribution. The "waveform" of the electrotonic current was found to be temperature dependent, which reflected the shape changes in the experimental action potentials and confirmed the channel distributions. The distributions are cell-type specific and adequate for soma and dendritic excitation with a safety margin. The highest Na-channel density was found on an axonal segment some 50-130 microm distal to the soma, as determined from the temperature-dependent "initial segment-somadendritic (IS-SD) break." The voltage dependence of the gating rate constants remains invariant between 7 and 23 degrees C and between 30 and 37 degrees C, but undergoes a transition between 23 and 30 degrees C. Both gating-kinetic and ion-permeability Q10s remain virtually constant between 23 and 37 degrees C (kinetic Q10s = 1.9-1.95; permeability Q10s = 1.49-1.64). The Q10s systematically increase for T <23 degrees C (kinetic Q10 = 8 at T = 8 degrees C). The Na channels were consistently "sleepy" (non-Arrhenius) for T <8 degrees C, with a loss of spiking for T <7 degrees C.

Temperature dependence of dark-adapted sensitivity and light-adaptation in photoreceptors with A1 visual pigments: a comparison of frog L-cones and rods

Flash responses of L-cones and rods were recorded as ERG mass potentials in the frog retina at different temperatures (2-25 degrees C). The purpose was to elucidate factors that make cones faster and less sensitive than rods, particularly the possible role of thermal activation of L-cone visual pigment in maintaining a "light-adapted" state even in darkness. Up to ca. 15 degrees C, cones and rods were desensitized roughly equally by warming (Q(10) approximately 2.2-2.7), retaining a 5-fold sensitivity difference. In this range, the cone/rod difference must depend on factors other than thermal activation of the visual pigment. Above 15 degrees C, cones showed an additional component of desensitization compared with rods, coupled to accelerated response shut-off. This behavior is consistent with light-adaptation from temperature-dependent intrinsic activity (dark light). The apparent dark light as measured by the minimum background intensities needed to affect sensitivity and/or kinetics increased by ca. 10-fold between 15 and 25 degrees C, whereas reported increases in visual-pigment activation rates over this range are less than 5-fold. We conclude that the dark state of frog L-cones above 15 degrees C may be largely set by thermal activation of the phototransduction machinery, but only part of the experimentally determined dark light can be ascribed to the visual pigment.

Mouse cone photoresponses obtained with electroretinogram from the isolated retina

We characterize the dark-adapted photoresponses from mouse cones intact in the isolated retina, their virtually natural environment, by isolating pharmacologically the photoreceptor light responses from the electroretinogram (ERG). Due to the different photoresponse kinetics and sensitivity of rods and cones, the cone responses were readily attained by using a rod-saturating preflash. The stimulus wavelength (544 nm) was chosen to selectively stimulate the green sensitive ("M"-)pigment. Obtained responses were monophasic, showing fast kinetics (mean t(p)=51 ms) and low sensitivity (fractional single-photon response ca. 0.23%). The amplification coefficient of cones (4.6 s(-2)) was very close to that of rods (5.6 s(-2)), while the dominant time constant of recovery was clearly smaller for cones (33 ms) than for rods (160 ms).

Light responses and light adaptation in rat retinal rods at different temperatures

Rod responses to brief pulses of light were recorded as electroretinogram (ERG) mass potentials across isolated, aspartate-superfused rat retinas at different temperatures and intensities of steady background light. The objective was to clarify to what extent differences in sensitivity, response kinetics and light adaptation between mammalian and amphibian rods can be explained by temperature and outer-segment size without assuming functional differences in the phototransduction molecules. Corresponding information for amphibian rods from the literature was supplemented by new recordings from toad retina. All light intensities were expressed as photoisomerizations per rod (Rh*). In the rat retina, an estimated 34% of incident photons at the wavelength of peak sensitivity caused isomerizations in rods, as the (hexagonally packed) outer segments measured 1.7 microm x 22 microm and had specific absorbance of 0.016 microm(-1) on average. Fractional sensitivity (S) in darkness increased with cooling in a similar manner in rat and toad rods, but the rat function as a whole was displaced to a ca 0.7 log unit higher sensitivity level. This difference can be fully explained by the smaller dimensions of rat rod outer segments, since the same rate of phosphodiesterase (PDE) activation by activated rhodopsin will produce a faster drop in cGMP concentration, hence a larger response in rat than in toad. In the range 15-25 degrees C, the waveform and absolute time scale of dark-adapted dim-flash photoresponses at any given temperature were similar in rat and toad, although the overall temperature dependence of the time to peak (t(p)) was somewhat steeper in rat (Q(10) approximately 4 versus 2-3). Light adaptation was similar in rat and amphibian rods when measured at the same temperature. The mean background intensity that depressed S by 1 log unit at 12 degrees C was in the range 20-50 Rh* s(-1) in both, compared with ca 4500 Rh* s(-1) in rat rods at 36 degrees C. We conclude that it is not necessary to assume major differences in the functional properties of the phototransduction molecules to account for the differences in response properties of mammalian and amphibian rods.

The thermal contribution to photoactivation in A2 visual pigments studied by temperature effects on spectral properties

Effects of temperature on the spectral properties of visual pigments were measured in the physiological range (5-28 degrees C) in photoreceptor cells of bullfrog (Rana catesbeiana) and crucian carp (Carassius carassius). Absorbance spectra recorded by microspectrophotometry (MSP) in single cells and sensitivity spectra recorded by electroretinography (ERG) across the isolated retina were combined to yield accurate composite spectra from ca. 400 nm to 800 nm. The four photoreceptor types selected for study allowed three comparisons illuminating the properties of pigments using the dehydroretinal (A2) chromophore: (1) the two members of an A1/A2 pigment pair with the same opsin (porphyropsin vs. rhodopsin in bullfrog "red" rods); (2) two A2 pigments with similar spectra (porphyropsin rods of bullfrog and crucian carp); and (3) two A2 pigments with different spectra (rods vs. long-wavelength-sensitive (L-) cones of crucian carp). Qualitatively, the temperature effects on A2 pigments were similar to those described previously for the A1 pigment of toad "red" rods. Warming caused an increase in relative sensitivities at very long wavelengths but additionally a small shift of lambdamax toward shorter wavelengths. The former effect was used for estimating the minimum energy required for photoactivation (Ea) of the pigment. Bullfrog rod opsin with A2 chromophore had Ea = 44.2 +/- 0.9 kcal/mol, significantly lower (one-tailed P < 0.05) than the value Ea = 46.5 +/- 0.8 kcal/mol for the same opsin coupled to A1. The A2 rod pigment of crucian carp had Ea = 42.3 +/- 0.6 kcal/mol, which is significantly higher (one-tailed P < 0.01) than that of the L-cones in the same retina (Ea = 38.3 +/- 0.4 kcal/mol), whereas the difference compared with the bullfrog A2 rod pigment is not statistically significant (two-tailed P = 0.13). No strict connection between lambdamax and Ea appears to exist among A2 pigments any more than among A1 pigments. Still, the A1 --> A2 chromophore substitution in bullfrog opsin causes three changes correlated as originally hypothesized by Barlow (1957): a red-shift of lambdamax, a decrease in Ea, and an increase in thermal noise.

Light adaptation of cone photoresponses studied at the photoreceptor and ganglion cell levels in the frog retina

The sensitivity and time scale of the dominant (562 nm) cone system of the frog, Rana temporaria, were studied as functions of steady adapting illuminance (IB). Photoreceptor responses to brief flashes of light were recorded as aspartate-isolated ERG mass potentials from the isolated retina. The characteristics of the cone signal after transmission through the retina were derived from response thresholds and stimulus--intensity-response--latency functions for extracellularly recorded spike discharges of single ganglion cells in the eyecup. At 14 degrees C, the single-photon response of dark-adapted cones, extrapolated from ERG intensity-response functions, had an amplitude of 0.5% of the saturated response (Umax) and peaked at tp approximately 0.4 sec. Steady background illumination decreased both tp and flash sensitivity (SF), starting from apparent "dark lights" of, respectively, less than 10 (for time scale) and about 100 (for sensitivity) photoisomerisations per cone per second [P*sec-1]. From there upwards, two distinct ranges of background adaptation were apparent. Under moderate backgrounds (up to IB approximately 10(4) - 10(5) P*sec-1), sensitivity fell according to the relation SF alpha IB-0.64 and time scale shortened according to tp alpha IB-0.16. Under brighter backgrounds, from approx. 10(5) P*sec-1 up to the limit of our light source at 10(7) P*sec-1, the decrease in SF was significantly stronger than predicted by the Weber relation (SF alpha IB-1), while the decrease in tp levelled out and even tended to reverse. All these changes were virtually identical at the photoreceptor and ganglion cell levels, although the absolute time scale of cone signals apparent at the latter level was 2-fold longer. Our general conclusion is that photoreceptors have several distinct regimes for light adaptation, and traditional descriptions of functional changes (in sensitivity and kinetics) relevant to vision need to be restated with higher resolution, in view also of recent insights into the diversity of underlying mechanisms.

Changes in retinal time scale under background light: observations on rods and ganglion cells in the frog retina

The kinetics of rod responses to flashes and steps of light was studied as a function of background intensity (IB) at the photoreceptor and ganglion cell levels in the frog retina. Responses of the rod photoreceptors were recorded intracellularly in the eyecup and as ERG mass potentials across the isolated, aspartate-superfused retina. The kinetics of the retinally transmitted signal was derived from the latencies of ganglion cell spike discharges recorded extracellularly in the eyecup. In all states of adaptation the linear-range rod response to dim flashes could be modelled as the impulse response of a chain of low-pass filters with the same number of stages: 4 (ERG) or 4-6 (intracellular). Dark-adapted time-to-peak (tp, mean +/- SD) at 12 degrees C was 2.4 +/- 0.6 sec (ERG) or 1.7 +/- 0.4 sec (intracellular). Under background light, the time scale shortened as a power function of background intensity, I-bB with b = 0.19 +/- 0.03 (ERG) or 0.14 +/- 0.04 (intracellular). The latency-derived time scale of the rod-driven signal at the ganglion cell agreed well with that of the photoreceptor responses. The apparent underlying impulse response had tp = 2.0 +/- 0.7 sec in darkness and accelerated as I-bB with b = 0.17 +/- 0.03. The photoreceptor-to-ganglion-cell transmission delay shortened by 30% between darkness and a background delivering ca 10(4) photoisomerizations per rod per second. Data from the literature suggest that all vertebrate photoreceptors may accelerate according to similar power functions of adapting intensity, with exponents in the range 0.1-0.2. It is noteworthy that the time scale of human (foveal) vision in experiments on flicker sensitivity and temporal summation shortens as a power function of mean luminance with b approximately 0.15.

Spectral sensitivities of short- and long-wavelength sensitive cone mechanisms in the frog retina

ERG mass photoreceptor responses were recorded across the isolated, aspartate-perfused retina of the frog, Rana temporaria, in order to determine spectral sensitivities of cones. Cone responses were distinguished from rod responses by their faster kinetics, and responses from different cone types were isolated by selective background adaptation. Our main finding is that of a novel short-wavelength sensitive cone population peaking at about 431 nm. Further, we find that the sensitivity spectrum of the dominant long-wavelength sensitive cone population fully accounts for the most common type of photopic ganglion cell spectrum. Both can be described by a nomogram with lambda max = 562 nm. This resolves a long-standing apparent conflict between cone absorbance spectra and ganglion cell sensitivities. Including the 502 nm cones previously described by microspectrophotometry, the frog possesses a collection of cones that could support trichromatic photopic vision.

Human cone receptor activity: the leading edge of the a-wave and models of receptor activity

The leading edge of the a-wave of the electroretinogram (ERG) was evaluated as a measure of human cone photoreceptor activity. The amplitude of the cone a-wave elicited by flashes of different energy was compared to the predictions of a class of models from in vitro studies of cone photoreceptors. These models successfully describe the leading edge of the a-wave. Thus, the human cone a-wave can be used to test hypotheses about normal and abnormal cone receptors. The ability of the human cone to adjust its sensitivity in the presence of steady adapting lights was assessed by recording cone a-waves to flashes on adapting fields up to 3.9 log td in intensity and by comparing these responses to quantitative models of adaptation. The first 10 ms of the cone's response is little affected by field intensities up to 2.9 log td. The 3.9 log td field reduced the response to weak flashes by about a factor of 2.5 (0.4 log unit). This relatively small reduction in sensitivity can be attributed to a combination of response compression, pigment bleaching, and an adaptation mechanism that changes the gain without changing the time course. We conclude that either the human cones show relatively little adaptation or that they have an adaptation mechanism that involves a time-course change. That is, as we are limited with the a-wave to the first 10 ms or so of the cone's response, we cannot rule out a gain mechanism linked to a time-course change.

Visual latency and brightness: an interpretation based on the responses of rods and ganglion cells in the frog retina

Rod and cone photoresponses in a variety of species have been accurately described with linear multistage filter models. In this study, the response latency and initial coding of intensity at two higher levels of visual processing are related to such photoreceptor responses. One level is the retinal output (spiking discharges from frog ganglion cells, based on experimental data reported here), the other is the perceptual level in humans (psychophysical latency and brightness functions, based on data from the literature). Photoreceptor responses are described with the "independent activation" model of Baylor et al. (1974). The intensity dependence of the early ganglion cell discharge, its latency and initial impulse frequency, is shown to follow from such a waveform, assuming that 1) latency L = l + D, where l is the time it takes for the rod response linearly summed over the ganglion cell's receptive field to reach a criterion amplitude, and D is a constant delay; and 2) the initial frequency (below saturation) is proportional to the steepness of rise of the summed rod response at time l. It is shown that the intensity dependences of 1) human visual latency and 2) brightness sensation, including effects of stimulus area and duration, are accounted for by the same model. The predicted functions are not power functions of intensity, but approximate such over wide ranges. Thus, a large body of psychophysical data is explained simply by the waveform of photoreceptor responses.