Light adaptation and contrast in the outer retina.
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Adaptation, Ocular↗
Biomedical subjects
D A Burkhardt
Publications and source records attributed to D A Burkhardt.
Contrast rectification and distributed encoding By ON-OFF amacrine cells in the retina.
The encoding of luminance contrast by ON-OFF amacrine cells was investigated by intracellular recording in the retina of the tiger salamander (Ambystoma tigrinum). Contrast flashes of positive and negative polarity were applied at the center of the receptive field while the entire retina was light adapted to a background field of 20 cd/m(2). Many amacrine cells showed remarkably high contrast gain: Up to 20-35% of the maximum response was evoked by a contrast step of only 1%. In the larger signal domain, C50, the contrast required to evoke a response 50% of the maximum, was often remarkably low: 24 of 25 cells had a C50 value of < or =10% for at least one contrast polarity. Across cells and contrast polarity, the dynamic ranges varied from extremely narrow to broad, thereby blanketing the range of reflectances associated with objects in natural environments. Although some cells resembled "contrast rectifiers," by showing similar responses to contrasts of opposite polarity, many did not. Thus for contrast gain and C50, individual cells could show a strong preference for either negative or positive contrast. In the time domain, the preference was strong and unidirectional: for equal contrast steps, the latency of the response to negative contrast was 20-45 ms shorter than that for positive contrast. The present results, when compared with those for bipolar cells, suggest that, on average, amacrine cells add some amplification, particularly for negative contrast, to the high contrast gain already established by bipolar cells. In the time domain, our data reveal a striking transformation from bipolar to amacrine cells in favor of negative contrast. These and further observations have implications for the input and output of amacrine cell circuits. The present finding of substantial differences between cells reveals a potential substrate for distributed encoding of luminance contrast within the ON-OFF amacrine cell population.
Ambystoma↗
Contrast enhancement and distributed encoding by bipolar cells in the retina.
Responses of bipolar cells, cone photoreceptors, and horizontal cells were recorded intracellularly in superfused eyecup preparations of the tiger salamander (Ambystoma tigrinum). Contrast flashes of positive and negative polarity were applied at the center of the receptive field while the entire retina was light adapted to a background field of 20 cd/m2. For small contrasts, many bipolar cells showed remarkably high contrast gain: up to 15-20% of the bipolar response was evoked by a contrast step of 1%. There was considerable variation from cell to cell but, on average, no striking differences in contrast gain were found between the depolarizing (Bd) and hyperpolarizing (Bh) bipolar cells. Quantitative comparisons of contrast/response measurements for cone photoreceptors and cone-driven bipolars suggest that the high contrast gain of bipolars is the consequence of a 5-10 x amplification of small signals across the cone-->bipolar synapse. Bipolar cells had a very restricted linear range of response and tended to saturate at stimulus levels that were within the linear range of the cone response. The contrast/response of horizontal cells was similar to that of cones and differed markedly from that of Bh cells. For steps of equal contrast, the latency of the Bh cells was approximately 20 ms shorter than that of the Bd cells regardless of the contrast magnitude. For both bipolar cells and cones, the effect of contrast polarity on latency seems largely due to the absolute value of the light step, delta L. In the large signal domain, properties of the contrast responses of bipolar cells varied appreciably, both within and between the Bd and Bh classes. Cells of either class could be positive- or negative-contrast dominant. These and additional results show that in the light-adapted retina, the bipolar population is functionally diverse and has the potential to provide a rich substrate for distributed encoding of visual images.
Animals↗
Light adaptation and photopigment bleaching in cone photoreceptors in situ in the retina of the turtle.
Light adaptation and photopigment bleaching in cone photoreceptors were studied in the intact, superfused retina of the turtle (Pseudemys scripta elegans). A new method for measuring changes in the photopigment of cones is described. Action spectrum measurements indicate that the signals arise from the red-sensitive cones. Measurements of steady-state bleaching are well described by the monomolecular bleaching equation with a half-bleaching constant of about 5.5 log photons sec-1 microns-2. Quantitative data on light adaptation were obtained by intracellular recording from 15 red-sensitive cones over nearly 8 decades of background illumination obtained from a helium-neon laser (632.8 nm). The steady-state membrane potential, Rs, and the rate of photoisomerization of the photopigment, Pi, rose in parallel with background illumination and then stabilized over the upper 4 decades of illumination. These results are described by the relation Rs = k Pi0.27, and suggest that about 5 x 10(6) photoisomerizations sec-1 lead to the closure of half the cone's light-sensitive channels in the steady state. A full range of decremental and incremental flashes was used to investigate stimulus-response relations. Cones tended to generate responses of approximately constant amplitude to flashes of constant contrast over a substantial range of contrast (< or = 3 x) and background illumination (approximately 3-4 decades). This suggests that a substantial component of contrast constancy in vertebrate vision may originate in cones. Over nearly 7 decades, the small-signal step sensitivity was found to conform closely to Weber's law (sensitivity is inversely proportional to background illumination). Thus, Weber's law extends into the ultra-high-intensity realm, some 3 decades higher than previously known for vertebrate cones. Over the upper 3-4 decades of illumination, Weber's law behavior can be explained by the depletion of photopigment (reduced probability of the photon catch). There remains a substantial low-intensity domain for which light adaptation and Weber's law behavior are presumably mediated by other mechanisms within the cone. These might be the calcium- and/or cGMP-dependent mechanisms recently suggested by others.
Adaptation, Physiological↗
Prolonged depolarization in rods in situ.
Intracellular recordings were made from rods in the superfused retina of the marine toad (Bufo marinus). It was found that injection of a brief depolarizing current pulse (0.04-1 nA) evoked a distinctive, long-lasting response, here called "the prolonged depolarization." The response appears to be regenerative, has a stereotypical waveform, is typically about 6 mV in amplitude and 3 s in duration, and has a relatively long recovery period (10-60 s). As a rule, the response cannot be directly evoked by light but the current-evoked response is significantly enhanced in the presence of steady illumination. The light-evoked hyperpolarization and the depolarizing spikes of the rod are both attenuated in the presence of the prolonged depolarization. The prolonged depolarization is not an altered manifestation of the depolarizing spikes of toad rods since both can be recorded simultaneously and steady illumination suppresses the spikes while enhancing the prolonged depolarization. The response is enhanced in chloride-free superfusate and also appears to be enhanced by the use of electrodes containing chloride. The response is markedly shortened in superfusates that lack calcium or contain 1-5 mM cobalt. On this and other evidence, it is suggested that the response may be generated by the sequential action of calcium channels and calcium-activated chloride channels. Although rarely evoked by light, the prolonged depolarization of toad rods is otherwise remarkably similar to the prolonged depolarization of turtle cones. It is proposed that the prolonged depolarization, in contrast to the feedback depolarization of cones, arises from mechanisms common to both rods and cones.
Animals↗
Ionic influences on the prolonged depolarization of turtle cones in situ.
1. The effects of ion channel blockers and ion substitutions on the prolonged depolarization of cones in the retina of the turtle (Pseudemys scripta elegans) were studied by intracellular recording. 2. The results of current injection experiments indicate that the prolonged depolarization is regenerative and accompanied by a reduction in the cone's input resistance. 3. The addition of cobalt (5-10 mM) or the removal of extracellular calcium suppressed the prolonged depolarization. Raising extracellular calcium or adding strontium (10 mM) lowered the threshold and increased the duration of the response. 4. Unlike the feedback spikes of turtle cones studied by Piccolino and Gerschenfeld, the prolonged depolarization was not blocked by the organic calcium channel blocker, D600. 5. Adding a calcium chelator, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), to the electrolyte caused a progressive shortening of the prolonged depolarization until it was ultimately abolished. 6. Lowering extracellular sodium or use of the potassium channel blockers tetraethylammonium (TEA) and 4-aminopyridine (4-AP) had little effect on the prolonged depolarization. 7. Removing chloride from the superfusate induced a significant enhancement of the prolonged depolarization. In normal superfusate, the response tended to be of larger amplitude when recorded with electrodes containing chloride [1.5 M KCl + 1.5 M potassium acetate (KA)] rather than KA or potassium methylsulfate (KM) alone. 8. The results suggest that the prolonged depolarization is initiated by the regenerative activation of voltage-sensitive calcium channels and sustained by a calcium-dependent chloride efflux. The present findings are also discussed in relation to the functional significance of the prolonged depolarization and mechanisms for the surround antagonism of cones in situ.
Action Potentials↗
Effects of synaptic blocking agents on the depolarizing responses of turtle cones evoked by surround illumination.
The effects of synaptic blocking agents on the antagonistic surround of the receptive field of cone photoreceptors were studied by intracellular recording in the retina of the turtle (Pseudemys scripta elegans). Illumination of a cone's receptive-field surround typically evoked a hybrid depolarizing response composed of two components: (1) the graded synaptic feedback depolarization and (2) the prolonged depolarization, a distinctive, intrinsic response of the cone. The locus of action of synaptic blocking agents was analyzed by comparing their effects on the light-evoked response of horizontal cells, the hybrid cone depolarization evoked by surround illumination, and the pure prolonged depolarization evoked by intracellular current injection. The excitatory amino-acid antagonists, d-O-phosphoserine (DOS) and kynurenic acid (KynA), suppressed the light responses of horizontal cells and eliminated the surround-evoked, hybrid cone depolarization without affecting the prolonged depolarization evoked by current injection. Cobalt at 5-10 mM suppressed horizontal cell responses and thereby eliminated surround-evoked cone depolarizations. Cobalt (5-10 mM) also blocked the current-evoked prolonged depolarization, suggesting that the intrinsic cone mechanisms responsible for the prolonged depolarization are likely to be calcium-dependent. Various GABA agonists and antagonists were found to have no effect on the surround-evoked depolarizations of cones. In contrast, a very low concentration of cobalt (0.5 mM) selectively suppressed the light-evoked feedback depolarization of cones without affecting horizontal cell responses or the current-evoked prolonged depolarization. Cobalt at 0.5 mM thus blocks the light-evoked action of the cone feedback synapse while sparing feedforward synaptic transmission from cones to horizontal cells. The implications of the present work for the possible neurotransmitters used at these synapses is discussed.
Animals↗
An eyecup slice preparation for intracellular recording in vertebrate retinas.
This report describes a new preparation for intracellular recording from the vertebrate retina, the eyecup slice preparation. It consists of a small (2 X 5 mm) strip cut from the posterior wall of the eye and thereby keeps the sclera, pigment epithelium and neural retina in place. Initial results are presented here for two vertebrates: the turtle, Pseudemys scripta elegans, and the toad, Bufo marinus. With conventional microscopy, the histological layers of the retina can be resolved, as well as individual photoreceptors, to provide landmarks for intracellular recording. When superfused, the eyecup slice remains in good condition for many hours and yields intracellular recordings of good quality and stability. Recordings of the light-evoked responses of cones and horizontal cells show that the slice is large enough to preserve the characteristic spatial interactions mediated by the laterally coursing neural networks of the distal retina. Recordings from rods show that full dark adaptation is achieved. Thus, photochemical dark adaptation as well as other normal cellular interactions between the neural retina and pigment epithelium can be preserved in this preparation, in contrast to isolated retinal slice preparations. The eyecup slice preparation might be particularly useful for work on mammalian retinas.
Adaptation, Physiological↗
Prolonged depolarization in turtle cones evoked by current injection and stimulation of the receptive field surround.
1. Responses evoked by stimulation of the receptive field surround were recorded intracellularly from cone photoreceptors in the retina of the turtle (Pseudemys scripta elegans). 2. A distinctive depolarizing response was evoked by flashing an annulus of light while steadily illuminating the centre of the receptive field. The response, here called 'the prolonged depolarization', was found in 67% of a sample of 125 cones and could reach some 20 mV in amplitude. 3. The prolonged depolarization is characterized by a set of properties which include: the capacity to persist up to 17 s after the flash, a stereotypical waveform, a long period of temporal facilitation, a very narrow dynamic range, and a long refractory period (30-45 s). 4. Depolarizing current pulses (0.01-0.1 nA) evoke a prolonged depolarization which is similar to and functionally interchangeable with that evoked by light. The prolonged depolarization is thus apparently generated by a voltage-sensitive mechanism intrinsic to the cone. 5. Brief depolarizing spikes were recorded in a small fraction of cones. The spikes appear to be dissociable from the prolonged depolarization although both might arise for similar regenerative mechanisms. 6. The prolonged depolarization is typically preceded by a graded, stimulus-locked depolarization which can also be recorded in isolation by flashing annuli of low intensity. The graded depolarization is probably a manifestation of the depolarizing influence arising from synaptic feed-back from horizontal cells first described by Baylor, Fuortes & O'Bryan (1971). 7. It is suggested that the graded depolarization triggers the prolonged depolarization and that complex responses arise from the interaction of these disparate components.
Action Potentials↗
The ultrastructure of cones in the walleye retina.
The ultrastructure of single and twin cone photoreceptors in the retina of the walleye was analyzed by scanning and transmission electronmicroscopy. The outer segment disks resemble those of other vertebrates. An accessory outer segment arises from the inner segment, makes frequent contacts with the outer segment proper, and may thus provide a bridge for signal transmission from outer to inner segment. A palisade of some 30 calycal processes surrounds and may provide structural support for the outer segment. The region of apposition between the inner segments of twin cones consists of a space of some 28 nm with no indication of gap junctions. The proximal quarter of the inner segment displays a profusion of some 50 lateral fins which increase the inner segment surface by 3-4 X and do not contact fins of neighboring cones. The fins surround a profusion of Müller cell microvilli and probably promote metabolic exchange between cones and Müller cells. Apart from differences in size and the presence of apposed inner segments, twin and single cones appear to be morphologically similar.
Animals↗
Light adaptation and responses to contrast flashes in cones of the walleye retina.
The effects of light adaptation on the intracellular responses of cones to incremental and decremental light flashes were investigated in the superfused walleye retina. The time course of light adaptation was relatively slow, some 10-20 min being required to reach steady-state conditions. Under steady-state conditions: (1) both dark-adapted and light-adapted amplitude/intensity data were well described by the Michaelis-Menten relation, (2) the incremental response did not saturate on intense backgrounds and (3) the cone sensitivity conformed closely to Weber's law. Effects of the polarity and magnitude of the flash contrast were analyzed by deriving "equivalent contrast" relations for cones and comparing the results to those of recent psychophysical experiments. Results based on the response amplitude suggest that responses to negative contrast flashes may be enhanced by post-receptor mechanisms. Results based on cone latency suggest that equivalent contrast relations for visual latency may be largely shaped in the very early, quasi-linear phase of the cone response.
Action Potentials↗
Response properties of C-type horizontal cells in the retina of the bowfin.
Intracellular recordings were obtained from biphasic- and triphasic-type horizontal cells (C cells) in the retina of the bowfin. For steady-state responses, both cell types displayed a linear stimulus-response function for responses up to at least 20% of maximum. In the linear range, responses to red/green mixtures were well predicted from the assumption that opposed inputs combine by simple summation. Action spectra were measured in the linear range for 30 biphasic and 12 triphasic cells. Biphasic cells showed their peak hyperpolarization near 530 nm and peak depolarization near 680 nm. Triphasic cells showed peak hyperpolarization near 450 nm, peak depolarization near 570 nm and small hyperpolarizing responses to deep red flashes (greater than 670 nm). The response to deep red test flashes was reduced by chromatic backgrounds which either depolarized or hyperpolarized the cell, in contrast to past findings in carp triphasic cells. In both classes of cells, the depolarizing input mechanism had a shorter latency than the hyperpolarizing mechanism, a result not previously observed in other fish retinas. Color opponency was maintained in both classes of C cells for stimuli of small diameter. The findings in bowfin and other species suggest that both feedback and direct pathways shape the depolarizing response of C cells.
Animals↗
Sensory latency and reaction time: dependence on contrast polarity and early linearity in human vision.
Relations between luminance contrast and reaction time were studied for foveal vision over a three-decade range of background luminance. On each background, the contrast equivalence relation between negative and positive contrast flashes conformed almost exactly to the result expected if equal luminance steps of opposite sign produce equal visual effects. The same result held for threshold detection for flashes of variable duration. Analysis of these data suggests that reaction time is triggered by the early, rising phase of an internal response and that the effective stimulus energy that triggers the response is only moderately suprathreshold. On all backgrounds the sensory latency for reaction time (L) was described reasonably well by the relation L = bS-0.67, where b is constant and S is the absolute value of the luminance step. This implies that reaction time is largely independent of contrast polarity and the background luminance. Parallels between the present results and recent intracellular work suggest that the contrast equivalence relation for reaction time is largely shaped by early linear mechanisms in cones.
Humans↗
Telodendrites of cone photoreceptors: structure and probable function.
Intracellular dye injection and compartmental modeling were used to analyze the structure and function of telodendrites of cones in the retina of the walleye. After identifying the spectral type of an impaled cone on the basis of its response to red and green light, horseradish peroxidase and/or Lucifer Yellow were injected for 1 to 25 minutes. In 38 of 58 recovered cells, dye spread into the telodendrites; so in many cases, the detailed pattern of the telodendritic arbor could be reconstructed from serial sections. Typically, five telodendritic processes, about 1 micron in diameter and 18 micron in length, radiated from the cone pedicle. A majority of the processes terminated at pedicles of neighboring cones. Some of the Lucifer Yellow injections provided evidence for electrical coupling between cones via telodendrites. Calculations from a compartmental model, based on the measured dimensions of cones and telodendrites, indicate that the signal arising in the inner segment spreads with little loss to the end of a telodendrite, whereas about half of the signal is lost in transmission from telodendrite to inner segment. Assuming that each contact point within the telodendritic network is an electrical synapse of 2,500 M omega, the model shows spatial interaction over a field of some 80 micron, which is comparable to that measured experimentally. Although our anatomical data indicate that orange- and green-sensitive cones may be interconnected via telodendrites, model calculations indicate that such connections do not appreciably distort the intrinsic spectral sensitivity of walleye cones. This outcome agrees with previous experimental results.
Animals↗
Functional properties of twin and single cones.
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Animals↗
Symmetry and constancy in the perception of negative and positive luminance contrast.
The perception of suprathreshold luminance contrast was investigated by forced-choice psychophysical procedures that were designed to define contrast equivalence relations. Observers compared the perceived contrast of rectangular bars that were presented for 500 msec at 3.9 deg on opposite sides of the fovea. The results show a nearly symmetrical relation between the perception of negative and positive contrast that is largely invariant over four decades of background luminance. Thus, for any fixed background luminance, equal absolute contrasts evoke approximately equal perceived contrasts. Symmetry also held with variations in the width, the eccentricity, and the focus of the bars. Symmetry was investigated further by determining equivalent contrast relations for negative contrasts as a function of background luminance and by contrast scaling. These results show evidence for nearly perfect contrast constancy for targets of low to moderate contrast and departures form constancy for high-contrast targets. These new findings on negative contrast, symmetry, and contrast constancy are discussed in relation to underlying mechanisms for contrast perception and classic experiments on brightness and lightness constancy.
Dark Adaptation↗
Cellular mechanisms for color-coding in holostean retinas and the evolution of color vision.
Electrophysiological recording and microspectrophotometry were used to analyze retinal function in representatives of the two surviving genera of holostean grade fish--the bowfin (Amia calva) and gars (Lepisosteus sp.). The properties of the cone photopigments, horizontal cells and ganglion cells show that these holostean retinas have cellular mechanisms for color vision which are fundamentally similar to those previously described for teleosts, turtle and mammals. These findings suggest that trichromatic receptor systems and opponent color-coding mechanisms may have evolved in primitive Neopterygii or more ancient fish, before the advent of teleosts. In conjunction with other recent data on living representatives of primitive fishes, these findings also add renewed plausibility for the view that vertebrate color vision could have taken a common origin some 400 million years ago from an ancestral aquatic jawed vertebrate.
Action Potentials↗
Quantitative relations between color-opponent response of horizontal cells and action spectra of cones.
1. Relations between cones and chromatic-type horizontal cells (C-cells) were investigated by intracellular recording in the retina of the walleye (Stizostedion vitreum). 2. The retina contains two classes of cones, midwave (M) and long wave (L). Their action spectra have maximum sensitivity at 533 and 605 nm, respectively, and have been measured with good precision from 400 to 750 nm. 3. C-cells generate relatively sustained depolarizing and hyperpolarizing responses to deep red (680 nm) and green (530 nm) test flashes, respectively, but prominent on and off transients are evoked by intermediate wavelengths. To minimize these temporal interactions, quantitative analysis was restricted to measurements of the apparent steady-state response amplitude. 4. Response amplitude was linearly related to flash intensity for responses that did not exceed 20% of their respective maxima. This is called the linear range. At higher levels, the nonlinear region of the Naka-Rushton relation holds approximately if input is largely confined to the hyperpolarizing or depolarizing mechanism. 5. Tests with red/green mixture flashes show that the hyperpolarizing and depolarizing inputs interact by simple summation for responses in the linear range. At higher levels, the interaction is complex. 6. Spectral-response curves (response amplitude versus wavelength for flashes of equal photon level) were determined for 16 C-cells in the linear range. The exact form of these curves varied considerably from cell to cell, reflecting differences in the relative strength of the hyperpolarizing and depolarizing mechanisms. 7. The spectral-response curves were analyzed by a simple linear model based on the action spectra of the L and M cones. Measured and predicted spectral-response curves agree closely. The only free parameter in the analysis is a scaling factor that specifies the strength of the L cone input relative to the M cone input. 8. Triphasic spectral-response curves, as predicted by the model, were found in the sauger (Stizostedion canadense). These C-cells have short-wavelength responses due to input from long-wave cones, resemble certain triphasic cells in primate retina, and differ sharply from other triphasic C-cells found in other fish retinas. 9. Although the precise synaptic mechanisms must still be identified, the present results show that the functional information transfer from cones to C-cells in the linear range can be closely approximated by simple operations of scaling, sign inversion, and simple summation. The C-cell thus effectively subtracts one cone action spectrum from the other and displays the difference in the form of the spectral-response curve.
Animals↗