Noise and light adaptation in rods of the macaque monkey.

Membrane voltage was recorded in rod photoreceptors in retina isolated from macaque monkey. The size of the single photon response and the magnitude of membrane voltage fluctuations were assessed in dark- and light-adapted retina. The "dark light" rate I(D), defined as the rate of spontaneous photopigment isomerizations that would produce a variance equivalent to that of the noise measured in the dark, was calculated after matched filtering. The average value of 0.08 s(-1) fell at the higher end of psychophysical estimates of dark light in human observers. In light-adapted rods the photon response decreased in amplitude and duration, and the magnitude of the voltage fluctuations increased with increasing background light intensity. The signal-to-noise ratio (SNR) for single rods was defined as the ratio of the peak amplitude of the photon response to the standard deviation of the noise fluctuations. The signal-to-noise ratio for dark-adapted rods SNR(D) was about 7. With increasing background intensity I, the SNR fell as SNR(D)(1 + I/I(D))(-1/2). This function may account for the increment thresholds measured with small brief test flashes in human psychophysical experiments.

The photovoltage of macaque cone photoreceptors: adaptation, noise, and kinetics.

Whole-cell voltage and current recordings were obtained from red and green cone photoreceptors in isolated retina from macaque monkey. It was demonstrated previously that the cone photovoltage is generated from two sources, phototransduction current in the cone outer segment and photocurrent from neighboring rods. Rod signals are likely transmitted to cones across the gap junctions between rods and cones. In this study, the "pure" cone and rod components of the response were extracted with rod-adapting backgrounds or by subtracting the responses to flashes of different wavelength equated in their excitation of either rods or cones. For dim flashes, the pure cone component was similar in waveform to the cone outer segment current, and the rod component was similar to the photovoltage measured directly in rods. With bright flashes, the high frequencies of the rod signal were filtered out by the rod/cone network. The two components of the cone photovoltage adapted separately to background illumination. The amplitude of the rod component was halved by backgrounds eliciting approximately 100 photoisomerizations sec-1 per rod; the cone component was halved by backgrounds of 8700 photoisomerizations sec-1 per cone. Coupling between rods and cones was not modulated by either dim backgrounds or dopamine. Voltage noise in dark-adapted cones was dominated by elementary events other than photopigment isomerizations. The dark noise was equivalent in magnitude to a steady light eliciting approximately 3800 photoisomerizations sec-1 per cone, a value significantly higher than the psychophysical estimates of cone "dark light."

Aberrant photon responses in rods of the macaque monkey.

Recovery from bright light was studied in macaque rods by measuring the membrane current of single outer segments. The recovery phase of some responses displayed a plateau current of about one picoampere lasting for several seconds. The following evidence suggests these "steps" are single photon responses of abnormally long duration. (1) Over a limited range of intensities, step amplitude remained constant and summed linearly with intensity. The collecting area for step generation was about 2.6 x 10(-3) microns2. (2) Step duration varied exponentially with a mean duration of about 6.5 s. (3) Fluctuation analysis of the tail currents was consistent with the idea that a step is evoked by isomerization of a single rhodopsin molecule, and that only 1 in 400 isomerizations leads to a responses with a step-like waveform. (4) With only the distal portion of the outer segment in the electrode, the polarity of the step response reversed when the proximal portion of the outer segment was illuminated, indicating that step generation results from a local change in outer segment conductance near the site of photon absorption. (5) The probability of eliciting a step varied with the wavelength of light in the manner expected from the absorption spectrum of rhodopsin.

Photovoltage of rods and cones in the macaque retina.

The kinetics, gain, and reliability of light responses of rod and cone photoreceptors are important determinants of overall visual sensitivity. In voltage recordings from photoreceptors in an intact primate retina, rods were found to be functionally isolated from each other, unlike the tightly coupled rods of cold-blooded vertebrates. Cones were observed to receive excitatory input from rods, which indicates that the cone pathway also processes rod signals. This input might be expected to degrade the spatial resolution of mesopic vision.

Visual transduction in human rod photoreceptors.

1. Photocurrents were recorded with suction electrodes from rod photoreceptors of seven humans. 2. Brief flashes of light evoked transient outward currents of up to 20 pA. With increasing light intensity the peak response amplitude increased along an exponential saturation function. A half-saturating peak response was evoked by approximately sixty-five photoisomerizations. 3. Responses to brief dim flashes rose to a peak in about 200 ms. The waveform was roughly like the impulse response of a series of four to five low-pass filters. 4. The rising phases of the responses to flashes of increasing strength were found to fit with a biochemical model of phototransduction with an 'effective delay time' and 'characteristic time' of about 2 and 800 ms, respectively. 5. Spectral sensitivities were obtained over a wavelength range from 380 to 760 nm. The action spectrum, which peaked at 495 nm, followed the template described for photoreceptors in the macaque retina. Variation between rods in the position of the spectrum on the wavelength axis was small. 6. The scotopic luminosity function derived from human psychophysical experiments was found to agree well with the measured rod action spectrum after adjustments were made for lens absorption and photopigment self-screening in the intact eye. 7. Responses to steps of light rose monotonically to a maintained level, showing little or no relaxation. Nevertheless, the relationship between light intensity and steady-state response amplitude was shallower than that expected from simple response saturation. This is consistent with an adaptation mechanism acting on a rapid time scale. 8. Flash sensitivity fell with increasing intensities of background light according to Weber's law. Sensitivity was reduced twofold by lights evoking about 120 photoisomerizations per second. Background lights decreased the time to peak and the integration time of the flash response by up to 20%.

Visual transduction in cones of the monkey Macaca fascicularis.

1. Visual transduction in macaque cones was studied by measuring the membrane current of single outer segments projecting from small pieces of retina. 2. The response to a brief flash of light was diphasic and resembled the output of a bandpass filter with a peak frequency near 5 Hz. After the initial reduction in dark current there was a rebound increase which resulted from an increase in the number of open light-sensitive channels. The response to a step of light consisted of a prominent initial peak followed by a steady phase of smaller amplitude. 3. Responses to dim light were linear and time-invariant, suggesting that responses to single photons were linearly additive. From the flash sensitivity and the effective collecting area the peak amplitude of the single photon response was estimated as about 30 fA. 4. With flashes of increasing strength the photocurrent amplitude usually saturated along a curve that was gentler than an exponential but steeper than a Michaelis relation. The response reached the half-saturating amplitude at roughly 650 photoisomerizations. 5. The response-intensity relation was flatter in the steady state than shortly after a light step was turned on, indicating that bright light desensitized the transduction with a delay. This desensitization was not due to a reduction in pigment content. In the steady state, a background of intensity I lowered the sensitivity to a weak incremental test flash by a factor 1/(1 + I/IO), where IO was about 2.6 x 10(4) photoisomerizations s-1, or about 3.3 log trolands for the red- and green-sensitive cones. 6. Bleaching exposures produced permanent reductions in flash sensitivity but had little effect on the kinetics or saturating amplitude of subsequent flash responses. The sensitivity reductions were consistent with the expected reductions in visual pigment content and gave photosensitivities of about 8 x 10(-9) microns2 (free solution value) for the red- and green-sensitive pigments. During a steady bleaching exposure the final exponential decline of the photocurrent had a rate constant given by the product of the light intensity and the photosensitivity. 7. In some cells it was possible to measure a light-induced increase in current noise. The power spectrum of the noise resembled the spectrum of the dim flash response and the magnitude of the noise was consistent with a single photon response roughly 20 fA in size. 8. The membrane current recorded in darkness was noisy, with a variance near 0.12 pA2 in the band 0-20 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectral sensitivity of primate photoreceptors.

The spectral sensitivities of rods and cones in macaque and human retinas were determined by recording the membrane current from single outer segments. In the macaque retina, the wavelengths of maximum sensitivity were at about 430, 530, and 561 nm for the blue, green, and red cones, respectively, and at 491 nm for the rods. The shapes of the spectra of the three cones were similar when plotted on a log wavenumber scale; the rod spectrum was slightly broader. Spectral sensitivities of the red and green cones from a human retina were virtually identical to those of macaque cones. For comparison with human psychophysical measurements, the rod and cone spectra were adjusted to give the sensitivities expected for light incident on the cornea of the human eye. These functions satisfactorily predicted the scotopic and photopic luminosity functions as well as results from human color-matching experiments. The adjusted spectra of the red and green cones also agreed well with the pi-mechanism of Stiles (1953, 1959).

Spectral sensitivity of cones of the monkey Macaca fascicularis.

1. Spectral sensitivities of cones in the retina of cynomolgus monkeys were determined by recording photocurrents from single outer segments with a suction electrode. 2. The amplitude and shape of the response to a flash depended upon the number of photons absorbed but not the wave-length, so that the 'Principle of Univariance' was obeyed. 3. Spectra were obtained from five 'blue', twenty 'green', and sixteen 'red' cones. The wave-lengths of maximum sensitivity were approximately 430, 531 and 561 nm, respectively. 4. The spectra of the three types of cones had similar shapes when plotted on a log wave number scale, and were fitted by an empirical expression. 5. There was no evidence for the existence of subclasses of cones with different spectral sensitivities. Within a class, the positions of the individual spectra on the wave-length axis showed a standard deviation of less than 1.5 nm. 6. Psychophysical results on human colour matching (Stiles & Burch, 1955; Stiles & Burch, 1959) were well predicted from the spectral sensitivities of the monkey cones. After correction for pre-retinal absorption and pigment self-screening, the spectra of the red and green cones matched the respective pi 5 and pi 4 mechanisms of Stiles (1953, 1959).

Spectral sensitivity of human cone photoreceptors.

The brain computes visual colour by analysing the relative excitations of three types of retinal cones. Each cone excitation is governed by a spectral sensitivity function which relates the amplitude of the neural response to wavelength at constant light intensity. The spectral sensitivities of human cones are not well characterized. We report measuring the sensitivities by recording electrical responses of human cones to stimuli of different wavelengths. Spectral sensitivities of 'green' and 'red' cones, determined over the entire visible region, show peaks near 530 and 560 nm respectively, and are remarkably similar to those of the old-world monkey Macaca fascicularis. They satisfactorily predict the photopic luminosity function, a measure of the sensitivity of cone-mediated human vision to light of different wavelengths. The kinetics of the light responses of human cones also appeared similar to those of macaque cones: the time to peak response to a dim flash was 50-100 ms and there was a characteristic undershoot during recovery.

The photocurrent, noise and spectral sensitivity of rods of the monkey Macaca fascicularis.

Visual transduction in rods of the cynomolgus monkey, Macaca fascicularis, was studied by recording membrane current from single outer segments projecting from small pieces of retina. Light flashes evoked transient outward-going photocurrents with saturating amplitudes of up to 34 pA. A flash causing twenty to fifty photoisomerizations gave a response of half the saturating amplitude. The response-stimulus relation was of the form 1-e-x where x is flash strength. The response to a dim flash usually had a time to peak of 150-250 ms and resembled the impulse response of a series of six low-pass filters. From the average spectral sensitivity of ten rods the rhodopsin was estimated to have a peak absorption near 491 nm. The spectral sensitivity of the rods was in good agreement with the average human scotopic visibility curve determined by Crawford (1949), when the human curve was corrected for lens absorption and self-screening of rhodopsin. Fluctuations in the photocurrent evoked by dim lights were consistent with a quantal event about 0.7 pA in peak amplitude. A steady light causing about 100 photoisomerizations s-1 reduced the flash sensitivity to half the dark-adapted value. At higher background levels the rod rapidly saturated. These results support the idea that dim background light desensitizes human scotopic vision by a mechanism central to the rod outer segments while scotopic saturation may occur within the outer segments. Recovery of the photocurrent after bright flashes was marked by quantized step-like events. The events had the properties expected if bleached rhodopsin in the disks occasionally caused an abrupt blockage of the dark current over about one-twentieth of the length of the outer segment. It is suggested that superposition of these events after bleaching may contribute to the threshold elevation measured psychophysically. The current in darkness showed random fluctuations which disappeared in bright light. The continuous component of the noise had a variance of about 0.03 pA2 and a power spectrum that fell to half near 3 Hz. A second component, consisting of discrete events resembling single-photon responses, was estimated to occur at a rate of 0.006 s-1. It is suggested that the continuous component of the noise may be removed from scotopic vision by a thresholding operation near the rod output.

Spectral sensitivity of single cones in the retina of Macaca fascicularis.

Colour vision depends on the wavelength-dependent absorptions of three different photolabile pigments each located in a particular type of retinal cone. The spectral absorption of primate cones has been examined by microspectrophotometry, but this method gives information only at wavelengths where the absorption is relatively strong. Here we have analysed the absorption of two of the cones over a wider range of wavelengths by recording their electrical responses to monochromatic light. The observations were made on the retina of the monkey Macaca fascicularis, an animal thought to have cone pigments like those of man. The measured spectral sensitivities of the red-sensitive ('red') and green-sensitive ('green') cones agreed well with estimates of the cone pigment absorptions derived from colour-matching experiments in humans. At long wavelengths the sensitivity of the red cones was found to decline more rapidly than that of the green. This behaviour, attributable to the cone pigment molecules themselves, explains the paradoxical hue shift, in which a light of very long wavelength is perceived to be identical to a light of shorter wavelength.

Dependence of the single photon response on longitudinal position of absorption in toad rod outer segments.

Light responses were recorded from toad rods in order to study the dependence of the kinetics and amplitude of the single photon response on the longitudinal position of excitation within the rod outer segment. Membrane current was recorded from the inner segment of an isolated rod with a suction electrode while stimulating the outer segment with a dim transverse slit of light. Flashes at the tip of the outer segment gave smaller average responses than flashes at the base. Comparison of amplitude histograms from the two positions revealed that the fraction of incident photons eliciting an electrical response was the same at tip and base. Characteristic differences in flash sensitivity are therefore attributed to differences in the amplitude of the single photon response. Flash responses from the tip were slower than those from the base. For most cells, the tip response could be fitted by the same multistage filter equation that fitted the base response when only one of the filter time constants was increased. For both tip and base responses, the ensemble variance as a function of time was proportional to the square of the ensemble average. This indicates that single photon responses had the same wave form as their respective averages, and that there was no significant contribution of fluctuations in response latency to the wave form of the average. Background light reduced flash sensitivity at the tip more than at the base. The calculated cable attenuation of rod outer segments is not sufficient to explain the observed differences between the responses of the tip and base. The differences might instead be associated with a longitudinal gradient of internal sodium concentration, or with ageing of the outer segment discs. Calculations suggest that in the intact eye, the amplitude and time course of the average electrical response to absorption of a photon should depend slightly on the wave-length of the photon.

Differences in the kinetics of rod and cone synaptic transmission.

Photoreceptors of the vertebrate retina hyperpolarize in response to light. The hyperpolarization elicited by a brief flash is approximately ten times slower in rods than in cones of the same retina. We have examined the amplification and temporal properties of synaptic transfer of rod and cone signals to a common postsynaptic element, the horizontal cell. We find that the kinetics of signal transfer at these chemical synapses parallels the speed of the light-evoked signals themselves.

Light-induced changes in membrane current in cone outer segments of tiger salamander and turtle.

The peak change in membrane conductance of vertebrate photoreceptors in response to the isomerization of a single photopigment has been estimated from recordings of membrane potential to be about 8 pS in rods and 6 pS in cones. For rods the estimate has been largely confirmed by Yau et al. by directly recording membrane current responses of rod outer segments to single photoisomerizations. However, no similar measurements have been reported for cone outer segments. Here, we report on direct recordings of membrane currents of single cone outer segments using an extracellular patch electrode. The technique was similar to that described by Yau et al. for toad rods. We have measured dark currents of up to 40 pA, and calculate that the conductance change resulting from a single photoisomerization is less than 1 pS.