Rod Adaptation Research Articles

Influence of rod adaptation upon cone responses to light offset in humans: I. Results in normal observers.

Dark-adapted rods exert a tonic suppressive influence upon cone-mediated sensitivity to rapid flicker, a phenomenon called suppressive rod-cone interaction (SRCI). However, rod dark adaptation has negligible influence upon cone-mediated thresholds measured with more usual psychophysical procedures. The present study separately examined the influences of rod light and dark adaptation upon cone-mediated sensitivity to transient increases or decreases in illumination using sawtooth flicker with rapid-on (ramp-off) or rapid-off (ramp-on) waveforms. In the parafoveal retina, cones alone were stimulated with flicker by spatially superimposing long- and short-wavelength stimuli presented in counterphase and matched in scotopic illuminance. Several different adaptation procedures were used. For higher (greater than 4 Hz) frequencies, sensitivity of cones to both waveforms is nearly identical under any condition of adaptation; sensitivity decreases as rods progressively dark adapt. A considerably different situation exists for slower frequencies (1-4 Hz). Sensitivity of cones to rapid-off flicker is appreciably greater under light-adapted conditions confirming recent observations by Bowen et al. (1989). But as rods progressively dark adapt, sensitivity of cones to rapid-off waveforms decreases considerably while sensitivity to rapid-on waveforms is much less affected; in the totally dark-adapted eye, sensitivity to both waveforms is identical. These results confirm and extend recent physiological observations in amphibian retina (Frumkes & Wu, 1990) suggesting that SRCI specifically involves responses to transient decreases in illumination.

Influence of rod adaptation upon cone responses to light offset in humans: II. Results in an observer with exaggerated suppressive rod-cone interaction.

In normal observers, sensitivity of cones to rapid sinusoidal flicker decreases by about 0.7 log units as rods progressively dark adapt. However, Arden and Hogg (1985) described a night-vision disorder characterized by normal rod sensitivity but exaggerated suppressive rod-cone interaction (SRCI). We refer to this condition as the exaggerated SRCI syndrome (ESS). The present paper examines the influence of rod-adaptation upon cone-mediated responses to light onset and offset in an observer with ESS. Under all conditions of adaptation examined, sensitivity of cones to rapid-on waveforms is indistinguishable to that of a normal observer tested under identical circumstances; rod sensitivity is also normal. However, the sensitivity of cones to transient decreases in illumination is clearly subnormal under light-adapted conditions. This deficit in cone responsiveness to light offset becomes increasingly subnormal as rods dark adapt and, when completely dark adapted, the ESS observer is nearly blind to 1 Hz rapid-off sawtooth waveforms. These results strongly bolster previous results that suggest that suppressive rod-cone interaction is restricted to the response to transient decreases in illumination.

Retinal Function and Rhodopsin Levels in Autosomal Dominant Retinitis Pigmentosa With Rhodopsin Mutations

We studied rod and cone function in 20 patients from six families with autosomal dominant retinitis pigmentosa, who represented five different point mutations in the gene encoding rhodopsin. In a family with a stop codon mutation at the carboxyl end of the molecule (glutamine-344), young members with the mutation were asymptomatic and clinically unaffected but showed about 1 log unit of rod sensitivity loss across the visual field and decreased rhodopsin levels; at this stage, cone function was essentially normal. In three families with mutations at the border of a transmembrane segment (arginine-135-leucine and arginine-135-tryptophan), there was neither detectable rod function nor measurable rhodopsin; cone function was variably impaired. Two families carrying different mutations (threonine-17-methionine and threonine-58-arginine) had altitudinal visual field defects with less impaired rod and cone function in the inferior than in the superior field. Rod adaptation was abnormal in both families, but the time course of adaptation differed between patients with the two mutations. Differences in the pattern of retinal dysfunction were therefore discernible in patients with different rhodopsin mutations.

Incorporation of chelator into guinea‐pig rods shows that calcium mediates mammalian photoreceptor light adaptation.

1. The effects of steady light on the sensitivity and kinetics of the photocurrent response were studied in the rod photoreceptors of the guinea-pig, using suction pipette recordings of circulating current. 2. The sensitivity of the flash response decreased with increasing background intensity according to Weber's law. Ultimately for the brightest backgrounds saturation ensued. The recovery phase of the flash response was accelerated by steady light, while the early rising phase was little affected. 3. These results indicate that guinea-pig rods adapt to light in much the same way as do the rods and cones of lower vertebrates. 4. The role of cytoplasmic calcium concentration in this adaptation was studied by incorporation of the calcium chelator bis(o-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid (BAPTA) into the rod cytoplasm. Superfusion with a solution containing the membrane-permeant acetoxymethyl ester resulted in progressive changes in the response to light. 5. BAPTA incorporation retarded the falling phase of the flash response, thereby increasing receptor sensitivity, but did not affect the early rising phase of the response. BAPTA also slowed the adaptation of the response to steady illumination. 6. These results indicate that cytoplasmic calcium concentration plays a similar role in the light adaptation of guinea-pig rods to that in the adaptation of the rods and cones of lower vertebrates. Calcium therefore appears to act as the messenger of light adaptation in mammalian rods.

Light adaptation in retinal rods of the rabbit and two other nonprimate mammals.

The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.

The influence of short‐term adaptation of human rods and cones on cone‐mediated grating visibility.

1. The influences of short-term visual adaptation of either rods or cones upon cone-mediated grating visibility were compared with their influences upon detection threshold in both the fovea and parafoveal retina. Short-term visual adaptation was induced by 20 deg diameter adapting fields (AFs) generally of 500 ms duration. The AF was either -0.5 log td in illuminance and too dim to influence cones, or 2.5 log td and bright enough to stimulate cones as well as rods. 2. In control experiments, we replicated previous results of Crawford (1947), Baker (1963), and other investigators and determined the influence of these AFs upon the detection threshold of a homogeneous test flash (TF) of 54 min of arc diameter and 10 ms duration. If the AF was 2.5 log td in illuminance and stimuli were presented foveally, TF threshold began to rise several hundred milliseconds before AF onset, was maximal when AF and TF were simultaneous in onset but was less elevated during the remaining presentation of the AF. TF threshold decreased to control value within several seconds after AF offset. These data represent a cone adaptation function since action spectra for both the test and adapting flashes adhere to the spectral sensitivity of the CIE standard luminous efficiency function V lambda. 3. If the AF was -0.5 log td in illuminance and if stimuli were presented parafoveally, the time course of TF detection threshold changes were similar to those described in paragraph 2 above. But these data represent a rod adaptation function since action spectra for both the test and adapting flashes adhered to the spectral sensitivity of the CIE standard luminous efficiency function V' lambda. In the fovea, the -0.5 log td AF had no influence upon the detection threshold of the TF suggesting complete rod-cone independence. 4. The influence of short-term adaptation upon spatial visibility was studied using a vertically oriented, 18 cycle/deg grating which was also 54 min of arc in diameter and 10 ms in duration. We determined the illuminance just necessary to see the bars of the grating (i.e. threshold grating illuminance or TGI) at various time intervals in respect to the onset of an AF. 5. Rod-stimulating (-0.5 log td) AFs, whether 0.5 or 2 s in duration, only influenced TGI after AF onset. TGI gradually decreased (i.e. an increase in sensitivity) during the first 250 ms of AF presentation and then remained stable until AF offset.(ABSTRACT TRUNCATED AT 400 WORDS)

Independent influences of rod adaptation on cone-mediated responses to light onset and offset in distal retinal neurons

1. The influence of rod adaptation on cone-mediated intracellular responses of distal retinal neurons was examined in the larval tiger salamander. Rods were selectively stimulated by the use of 450-520 nm adapting stimuli too dim to appreciably influence cones. Cones were primarily stimulated with the use of deep-red stimuli (maximally sensitive to wavelengths greater than 650 nm). The qualitative properties of rod-cone interaction were assessed with the use of several different photic-stimulus paradigms. 2. Confirming results of prior studies, we showed that rod adaptation changed the time course of cone-mediated responses to the onset of square-wave light flashes in horizontal cells (HCs); rod adaptation had no similar influence in other distal retinal neurons. Rod adaptation also enhanced cone-mediated responses to rapid flicker in cones, hyperpolarizing (HPBCs) and depolarizing (DPBCs) bipolar cells, as well as HCs. 3. We also examined the influence of rod-stimulating background fields on cone-mediated responses to slow (approximately 1-Hz) flicker composed of sawteeth with a rapid onset (ramp offset) or with a rapid offset (ramp onset). Such stimulation maintained a constant state of long-term adaptation while providing cones with transient-ON or transient-OFF stimulation. 4. Rod adaptation speeds up the response of HCs to rapid onset and increases response amplitude. Rod adaptation had no reliable influence on response to rapid onset in cones or bipolar cells. 5. Rod adaptation enhanced the amplitude of responses of HCs to rapid offset without altering response time course. 6. Rod adaptation greatly enhanced the amplitude of DPBC responses to rapid offset having no reliable influence on the time course of the response. 7. Rod adaptation caused responses of HPBCs to rapid offset to become much more transient. Rod backgrounds had a similar but smaller and less reliable influence on the response of cones to rapid offset. 8. The foregoing results indicate that rod adaptation exerts a minimum of two separate influences on cone-mediated responses in distal amphibian retina. Changes at light onset must reflect the operation of a mechanism that alters response kinetics of the HC membrane. Changes at light offset reflect the operation of a separate mechanism or set of mechanisms that must act in part presynaptically to the HCs.

Weber and noise adaptation in the retina of the toad Bufo marinus.

Responses to flashes and steps of light were recorded intracellularly from rods and horizontal cells, and extracellularly from ganglion cells, in toad eyecups which were either dark adapted or exposed to various levels of background light. The average background intensities needed to depress the dark-adapted flash sensitivity by half in the three cell types, determined under identical conditions, were 0.9 Rh*s-1 (rods), 0.8 Rh*s-1 (horizontal cells), and 0.17 Rh*s-1 (ganglion cells), where Rh* denotes one isomerization per rod. Thus, there is a range (approximately 0.7 log units) of weak backgrounds where the sensitivity (response amplitude/Rh*) of rods is not significantly affected, but where that of ganglion cells (1/threshold) is substantially reduced, which implies that the gain of the transmission from rods to the ganglion cell output is decreased. In this range, the ganglion cell threshold rises approximately as the square root of background intensity (i.e. in proportion to the quantal noise from the background), while the maintained rate of discharge stays constant. The threshold response of the cell will then signal light deviations (from a mean level) of constant statistical significance. We propose that this type of ganglion cell desensitization under dim backgrounds is due to a post-receptoral gain control driven by quantal fluctuations, and term it noise adaptation in contrast to the Weber adaptation (desensitization proportional to the mean background intensity) of rods, horizontal cells, and ganglion cells at higher background intensities.

Spatial properties of rod-cone interactions in flicker and hue detection

Rod-cone interactions in flicker and hue detection were compared to examine the hypothesis that they are mediated by mechanisms with different spatial properties. Flicker and hue thresholds for a 1 deg test stimulus (TS) were measured as a function of background luminance and diameter. Flicker thresholds were reduced from their dark-adapted value by an 11 deg diameter background, but not by a 1 deg background. These results, in agreement with previous work, demonstrate that light adaptation of rods surrounding the TS is necessary to eliminate their effect on cone flicker thresholds. In contrast, hue thresholds were reduced from their dark-adapted value to a comparable degree by 1 and 11 deg backgrounds, indicating that light adaptation of the rods stimulated by the TS is sufficient to abolish the rod-cone hue interaction. Our results support the contention that the rod-cone flicker and hue interactions are mediated by different mechanisms. We also demonstrated that changing the detection task while keeping stimulus parameters constant is sufficient to shift between these two types of rod-cone interactions.

Cytoplasmic calcium as the messenger for light adaptation in salamander rods.

1. In order to study the role of cytoplasmic calcium concentration (Ca2+i) in rod photoreceptor light adaptation, we have attempted to prevent light-induced changes in Ca2+i by minimizing calcium fluxes across the outer segment plasma membrane. This was achieved by exposing the outer segment to a low-Ca2+, 0-Na+ solution, in which sodium was replaced with either guanidinium or lithium and the external calcium concentration (Ca2+o) was reduced to micromolar levels. 2. With guanidinium and 1-3 microM-Ca2+o, the circulating current in darkness was maintained for a period of at least 15 s, consistent with approximate stability of Ca2+i. With Li+ rather than guanidinium most of the initial current was suppressed, but the residual current was again relatively stable. 3. During prolonged exposures (greater than 30 s) to low-Ca2+, 0-Na+ solution followed by dim illumination, the circulating current did not remain constant but slowly increased. Incorporation of calcium buffer into the cytoplasm greatly reduced the rate of change of current, consistent with the idea that the increase arose from a gradual decrease in Ca2+i. 4. Light responses of rods exposed to low-Ca2+, 0-Na+ solution in darkness were altered in a characteristic manner. Although the initial rising phase of the light response was little changed, the peak amplitude of the response was larger and occurred later, and the response decayed more slowly than in control. The response-intensity relation was steepened and was shifted towards lower intensities both for flashes and for steps of light. The normal sag in the response to steps disappeared, and the waveform of the step response could be predicted to a close approximation from the integral of the dim flash response. 5. Presentation of background illumination in Ringer solution produced a marked acceleration of the response to a subsequent bright flash. No such acceleration was observed if the background was given in low-Ca2+, 0-Na+ solution. 6. The results described in paragraphs 4 and 5 indicate that, under conditions expected to minimize changes in Ca2+i, all manifestations of light adaptation disappear, and the rod simply sums the effects of incident photons with an invariant integration time. 7. Exposure of a light-adapted rod to low-Ca2+, 0-Na+ solution altered the responses to superimposed test flashes in much the same way as for rods in darkness. The initial rising phases in low-Ca2+, 0-Na+ solution were unchanged, but the responses were larger, reached peak later and decayed more slowly.(ABSTRACT TRUNCATED AT 400 WORDS)

Light adaptation in cat retinal rods.

It has long been an open question whether individual rod receptors in the mammalian retina show any light adaptation. The prevailing evidence so far has suggested that these cells, unlike those in lower vertebrates, adapt little if at all. The experiments on cat rods reported here, however, indicate that this is not really true. Since the cone system in the cat retina has a fairly high light threshold, the rods also need to adapt so that they do not saturate with light before the cones fully take over vision at higher light intensities. In similar experiments, adaptation was found in rods of other mammalian species, including primates.

Calcium and light adaptation in retinal rods and cones

Retinal rods and cones respond to light with a membrane hyperpolarization. This hyperpolarization is mediated by an ionic conductance (the light-regulated conductance) that is kept open in darkness by cyclic GMP acting as a ligand, and which closes in the light as a result of an increase in cGMP hydrolysis triggered by illumination. Calcium ions appear to have a role in this phototransduction process: they provide negative feedback between the conductance, which is permeable to Ca2+ (refs 4, 5), and the concentration of cGMP, which is sensitive to Ca2+ (refs 6-8). This feedback down-regulates the sensitivity to light of a photoreceptor and probably contributes to the important phenomenon of light adaptation in vision. It is still not clear, however, how much of the light adaptation is actually attributable to this Ca2+ feedback. We have examined the responses of amphibian rods and cones to light with the Ca2+ feedback removed. Normally, the response of a cell to a step of light rises transiently to a peak, but rapidly relaxes to a lower level, indicative of light adaptation. When the feedback is removed, however, the relaxation of the response is completely absent; furthermore, the steady response levels at different light-step intensities are well predicted by a statistical superposition of invariant single-photon responses. We therefore conclude that the Ca2+ feedback underlies essentially all light adaptation in rods and cones.

Visual function and rhodopsin levels in humans with vitamin A deficiency

Details of rod and cone dysfunction in vitamin A deficiency have been studied in two subjects with primary biliary cirrhosis and one with Crohn's disease, all of whom presented with symptoms of night blindness. Visual function in the mid-peripheral retina was monitored with two-color adaptometry and rhodopsin levels were measured by fundus reflectometry. Initially all three subjects had no measurable rod function and delayed cone adaptation. In one case the dark-adapted cone threshold was also elevated. Oral supplementation with vitamin A restored visual function to normal within 8 days in all subjects. During supplementation, cone function was restored more rapidly than that of rods, though the pattern of recovery was similar for each receptor type. Final thresholds improved first, though the rates at which they were reached were abnormally slow. As recovery continued, adaptation kinetics returned to normal. When rod adaptation was delayed, the regeneration of rhodopsin was also abnormally slow. When rod final threshold was 2 log units higher than normal, rhodopsin regeneration was incomplete, reaching about 70% of the normal level. The initial stages of visual dysfunction during onset of vitamin A deficiency were studied in one subject, and were found to mirror the pattern seen during recovery: rod adaptation was initially slower than normal, but reached completion. Cone adaptation remained normal until rod function was almost absent.

Rod light and dark adaptation influence cone-mediated spatial acuity

The influence of rod light and dark adaptation upon cone mediated spatial acuity was studied in the near parafoveal retina of normal human observers. The luminance just necessary to detect squarewave test gratings of variable frequency provided an index of spatial acuity. Such thresholds were determined in the presence of background fields which were varied in luminance, shape, and size, or throughout the time period of dark adaptation. Spectral controls determined the type of photoreceptors influenced by all stimuli. Cone mediated spatial acuity is improved by presenting background fields too dim to directly affect cones, and is increasingly suppressed during the rod recovery stage of dark adaptation. These effects are small with spatial frequencies <4 c/deg but increase with spatial frequency to >1 log 10 unit with the highest spatial frequency examined, 21 c/deg. These influences upon cone mediated spatial vision reflect the state of long-term adaptation of rods in a large annular area surrounding the locus to which the test grating is presented. Our results emphasize the differing influences of long-term dark adaptation and prevailing luminance level upon visual acuity. Ironically, spatial acuity is optimized under dim light conditions by selectively light adapting the receptors most sensitive to feeble stimuli, the rods.

Background and bleaching equivalence in steady-state adaptation of vertebrate rods

We have investigated background and bleaching adaptation in vertebrate rods by intracellular recording in the retina of Bufo marinus. Backgrounds and bleaching produce adaptation in photoreceptors and lead to a shift and a compression of the response operating range. Threshold elevation due to backgrounds follows the Rose-DeVries rule at low intensities and the Weber-Fechner rule at high intensities. Threshold elevation due to bleaching is linear almost up to 17% bleached pigment and exponential thereafter. An equivalence can be established between bleaching and backgrounds with respect to threshold elevation, on the one hand, and with respect to response compression, on the other. These equivalences are the same within experimental error. The equivalence, moreover, appears to extend to the complete response curve. These results have implications for psychophysics as well as for photoreceptor transduction.

The effect of lead on photoreceptor response amplitude — Influence of the light stimulus

The mass receptor potential of the excised, superfused retina of the bullfrog was studied. Photoreceptor responses were isolated by addition of sodium aspartate to the Ringer solutions. Responses of the cones were monitored independently from responses of the rods by employing a two-flash method of stimulation which took advantage of the very different rates of rapid dark adaptation of rods and of cones. Stimulation with paired flashes of white light at regular intervals caused enhancement of rod response amplitude in that the response grew larger with subsequent flashes until reaching a stable plateau. The degree of enhancement was directly proportional to the amount of light exposure and increased with either increasing stimulus intensity or decreasing stimulus interval. Only the rod response was affected; the cone response was not enhanced by continued stimulation. The effects of 12·5 μ m PbCl 2 on rod response amplitude were complex. There was a small (less than 10%) but consistent depression of rod response amplitude even when the rods were in the unenhanced state. However, the most striking effect of lead was on the enhanced response, where treatment with 12·5 μ m PbCl 2 led to a depression of about 33%. When added prior to light stimulation, lead significantly decreased the degree to which the rod response could be enhanced, but never prevented enhancement entirely. Removal of lead resulted in a very large increase in the degree to which the rod response was enhanced by light, even when compared to the first, lead-free control. The cone response was unaffected by 12·5 μ m PbCl 2.

Rhodopsin bleaching and rod adaptation.

Conference Article| December 01 1983 Rhodopsin bleaching and rod adaptation M. CATT; M. CATT 1Department of Visual Science, Institute of Ophthalmology, University of London, Judd St., London WC1 H9QS, U.K. Search for other works by this author on: This Site PubMed Google Scholar W. ERNST; W. ERNST 1Department of Visual Science, Institute of Ophthalmology, University of London, Judd St., London WC1 H9QS, U.K. Search for other works by this author on: This Site PubMed Google Scholar C. M. KEMP; C. M. KEMP 1Department of Visual Science, Institute of Ophthalmology, University of London, Judd St., London WC1 H9QS, U.K. Search for other works by this author on: This Site PubMed Google Scholar P. M. O'BRYAN P. M. O'BRYAN 1Department of Visual Science, Institute of Ophthalmology, University of London, Judd St., London WC1 H9QS, U.K. Search for other works by this author on: This Site PubMed Google Scholar Biochem Soc Trans (1983) 11 (6): 676–678. https://doi.org/10.1042/bst0110676 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn MailTo Cite Icon Cite Get Permissions Citation M. CATT, W. ERNST, C. M. KEMP, P. M. O'BRYAN; Rhodopsin bleaching and rod adaptation. Biochem Soc Trans 1 December 1983; 11 (6): 676–678. doi: https://doi.org/10.1042/bst0110676 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search This content is only available as a PDF. © 1983 Biochemical Society1983 Article PDF first page preview Close Modal You do not currently have access to this content.

The influence of rod light and dark adaptation upon rod-cone interaction.

The influence of a rod-detected mask of illuminance IR on the threshold illuminance of a cone-detected test flash (ICth) was assessed while rods were recovering from the effects of a bleach, and when rods were selectively light adapted. Providing that IR was restricted to within 2 log10 units of rod mask threshold (IRth), results show that ICth/IC0 = K (square root (IR/IRth) + D), where IC0 is cone absolute threshold, D is a dark noise term and K is a proportionality constant. These data were used to obtain 'equivalent background functions' or 'Crawford (1947) transforms' (illuminance of a background field plotted against time in the dark). The same Crawford transform was obtained when either IRth or ICth (in the presence of a fixed illuminance IR) were used as equating variables. All of the foregoing results could be predicted by considering both the influence of light adaptation on rods (see Fain, 1976) and the model developed by Bauer, Frumkes & Nygaard (1982). Under dark-adapted conditions, 40' and 60' diameter rod masks of equal illuminance have very similar influences on ICth. When rods are selectively light adapted 60' masks have smaller influences on ICth. The foregoing results were used to extend the model developed in the previous paper. We suggest that rod adaptation has a distinct influence on neural loci designated E (the excitatory spatial summator) and I (the inhibitory spatial summator), and that E represents a site for both adaptation and spatial summation.

The signal-to-noise characteristics of rod-cone interaction.

1. The influence of rods on cone-mediated vision was assessed in eight human observers. To this end, increment threshold functions were obtained by determining thresholds of a cone-detected test flash (25 ms duration, 655 nm wave-length, 13' diameter) as a function of the illuminance of larger, 500 ms duration, rod-detected masking flashes. The type of photoreceptor influenced by each stimulus was carefully checked by means of a series of control procedures involving action spectra and selective rod adaptation.2. When the rod mask was 512 nm in wave-length, 40' in diameter, and less than one scotopic td in illuminance, increment threshold functions show that [Formula: see text], where I(Cth) is cone test threshold, I(R) is rod mask illuminance, and D is a dark noise term similar to that used by Barlow (1956). Further increases in I(R) have no additional influences on cone test threshold until threshold is influenced by the combined action of the mask on both rods and cones. If I(R) is expressed in terms of scotopic flux rather than illuminance, the functional relationship obtained with all rod masks </= 40' diameter and </= 580 nm wave-length is identical.3. Over the range of illuminance where a square-root relationship is obtained, probability of seeing functions show that a signal-to-noise mechanism limits the detectability of the cone test flash. These findings suggests a quantitative model in which cones produce a signal in a detector which is proportional to the illuminance of the cone test flash. Within a neural locus designated E (excitatory spatial summator), a response is produced which over at least a 40' diameter area, is proportional to the scotopic flux of the rod mask. E, however, feeds into a gain box, S, which saturates at illuminance levels at least 3 log(10) units less than usual estimates of rod saturation. Other than saturation, S behaves in a linear fashion.4. As diameter increases beyond 60', rod masks of equal scotopic illuminance have progressively less influence on cone test threshold; rod masks > 2 degrees have negligible influence on cone test threshold. We propose that I (inhibitory spatial summator), a neural locus which responds to scotopic flux provided over a very large area, attenuates the activity of E. The combined action of E and I is designated a rod channel. The response of cones and the rod channel summate at a detector. Within the detector, cone signals are distinguished from rod-related activity and intrinsic dark noise on the basis of signal-to-noise discriminations.5. The neural substrate for this rod channel most probably involves the combined action of several neurones which synapse within the inner plexiform layer of the retina. The relationship of this rod channel to other perceptual phenomena is discussed.

Light-dependent effects of a hydrolysis-resistant analog of GTP on rod photoresponses in the toad retina.

Responses to 100-ms flashes were recorded intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the toad, Bufo marinus. Properties of photoresponses were analyzed under each condition of adaptation when retinas were superfused with 1.0 mM guanosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppG), a hydrolysis-resistant analog od GTP. When applied to retinas that previously had been subjected to intense light (approximately 30% bleach), p[CH2]ppG increased both the amplitude and duration of photoresponses. By contrast, treatment of dark-adapted retinas with p[CH2]ppG did not alter these response parameters. When similarly applied to either dark- or light-adapted retinas, GTP had no effects on amplitude or duration of photoresponses. These results are discussed in terms of GTP-dependent mechanisms for rod adaptation.