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AIMS: To examine whether the addition of dorzolamide to timolol monotherapy influences oxygen saturation in the human retina. METHODS: Non-invasive spectrophotometric retinal oximetry was used to measure oxygen saturation in retinal vessels. Twenty patients with open-angle glaucoma (11) and ocular hypertension (9) were recruited. The patients were randomised into receiving timolol monotherapy or dorzolamide-timolol combination for an 8-month test period, followed by a second test period, before which the patients switched treatments. Oximetry measurements were performed at 2-month intervals during each period. Of the 20 patients, 13 followed the study protocol into the second test period, and 10 managed all study visits. RESULTS: The oxygen saturation in retinal vessels was stable within the test periods. The mean arteriolar saturation was 96 (2)% (mean (SD)) during timolol monotherapy and 97 (2)% during dorzolamide-timolol combination therapy (p = 0.17, all patients pooled, n = 13). Corresponding values in venules were 66 (5)% during timolol monotherapy and 65 (6)% during dorzolamide-timolol therapy (p = 0.13). Patients who started on dorzolamide-timolol combination showed a significant reduction in arteriolar (98 (2)% to 95 (2)%, p<0.01) and venular saturation (69 (5)% to 66 (6)%, p<0.05) when changing to timolol monotherapy. CONCLUSION: Adding dorzolamide to timolol monotherapy has a minimal effect, but going from dorzolamide-timolol combination to timolol alone lowered arteriolar and venular oxygen saturation. The retinal oxygen saturation measurements show a high degree of stability over an extended period of time. Previous studies have suggested increased retinal and optic nerve blood flow with dorzolamide. Unchanged oxygen saturation and increased blood flow would indicate increased oxygen delivery to the retina.

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AIM: To establish the epidemiology of the grey crescent in a white population within the age range most susceptible to glaucoma. METHODS: Bruce Shields was first to use this term to describe a localised, physiological pigmentation of the optic nerve neuroretinal rim tissue that is distinct from peripapillary pigmentation. An experienced glaucomatologist (KFD) evaluated stereofundus photographs of the participants of the Reykjavik Eye Study (RES)-a random sample from the national population census including people 50 years and older. 1012 right eyes could be evaluated for grey crescent. RESULTS: The prevalence of grey crescent in the right eyes was 22.0% (95% CI 10 to 25). It was more commonly found in women (27.0%: 95% CI 23 to 30) than in men (17.0%: 95% CI 14 to 21), and was most often located temporally (36.9%), 360 degrees (15.9%), or nasally (15.4%). The spherical equivalent was +1.30 dioptres (D) for those with and +0.80 D for those without grey crescent (p = 0.002), respectively. Vertical optic disc diameters were 0.203 v 0.195 units (p<0.001). There was no difference in the prevalence of grey crescent in glaucomatous or non-glaucomatous eyes (OR = 1.05, 95% CI 0.49 to 2.26). The prevalence of a grey crescent was inversely related to the prevalence of peripapillary atrophy (p = 0.001). CONCLUSIONS: The grey crescent needs to be recognised as a physiological variant in order to avoid falsely labelling eyes as having glaucomatous optic nerve damage.

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BACKGROUND/AIMS: Prostaglandins are important in blood flow regulation. Carbon dioxide (CO(2)) breathing and carbonic anhydrase inhibition increase the oxygen tension in the retina and optic nerve. To study the mechanism of this effect and the role of cyclo-oxygenase in the regulation of optic nerve oxygen tension (ONPO(2)), the authors investigated how indomethacin affects ONPO(2) and the ONPO(2) increases caused by CO(2) breathing and carbonic anhydrase inhibition in the pig. METHODS: Optic nerve oxygen tension was measured in 11 pigs with a polarographic oxygen electrode. The tip of the electrode was placed 0.5 mm above the optic disc. The effects of indomethacin, CO(2) breathing (3%) before and after indomethacin treatment, and carbonic anhydrase inhibition with or without indomethacin treatment were investigated. RESULTS: Administration of 300 mg indomethacin decreased optic nerve oxygen tension significantly. Carbonic anhydrase inhibition and CO(2) breathing increased ONPO(2) significantly. After indomethacin had been given, the rise in ONPO(2) caused by CO(2) breathing and carbonic anhydrase inhibition was significantly reduced. CONCLUSION: Systemic administration of indomethacin decreases the optic nerve oxygen tension; this is probably the result of decreased blood flow through vasoconstriction of vessels in the optic nerve. Additionally, indomethacin diminishes the ONPO(2) increasing effect of CO(2) breathing and carbonic anhydrase inhibition, thus affecting the reactivity of vessels in the optic nerve.

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BACKGROUND/AIMS: The authors have previously reported that carbonic anhydrase inhibitors such as acetazolamide and dorzolamide raise optic nerve oxygen tension (ONPO(2)) in pigs. The purpose of the present study was to investigate whether timolol, which belongs to another group of glaucoma drugs called beta blockers, has a similar effect. In addition, the effect of dorzolamide and timolol in combination was studied. METHODS: Polarographic oxygen electrodes were placed transvitreally over the optic disc in anaesthetised pigs and ONPO(2) was recorded continually. Drugs were administered intravenously either as 100 mg timolol followed by 500 mg dorzolamide (n = 5), 500 mg dorzolamide followed by 100 mg timolol (n = 5), or 100 mg timolol and 500 mg dorzolamide given simultaneously (n = 5). Arterial blood pressure, blood gasses, and heart rate were recorded. RESULTS: ONPO(2) was unaffected by administration of 100 mg timolol as an intravenous injection (n = 5). Administration of 500 mg dorzolamide by itself significantly increased ONPO(2) from 2.96 (SD 0.62) kPa to 3.69 (SD 0.88) kPa (n = 4, p = 0.035). The dorzolamide induced ONPO(2) increase was not significantly different from the ONPO(2) increases were seen when dorzolamide was administered simultaneous with (n = 5) or 35 minutes (n = 5) after 100 mg timolol. CONCLUSION: Systemic administration of timolol does not affect the optic nerve oxygen tension despite its lowering effect on the intraocular pressure. Additionally, timolol does not affect the ONPO(2) increasing effect of dorzolamide.

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OBJECTIVE: During light adaptation of the retina, cone electroretinograms (ERGs) can be obtained. It is known that during light adaptation considerable changes occur in the cone ERGs of man, monkeys and mice. All these species have vascular retinae. In the present study we examined whether the same applies to mammalian species with a limited retinal vasculature (rabbits) or avascular retinae (guinea pigs), and which both have two types of cones but scotopic ERGs with completely different morphology. MATERIAL AND METHODS: ERGs were recorded from anaesthetized rabbits and guinea pigs with corneal electrodes made from steal wire. Copper wire placed in the mouth of the animal served as reference electrode, and a subcutaneous needle as ground. Recordings were amplified 1000-fold, with bandwidth settings at 1-1000 Hz, and fed into a computer via an A/D converter. Corneas were anaesthetized with a topical application of proparacaine, and pupils dilated with topical application of tropicamide. ERGs were elicited with brief (10 msec) light flashes, and the retina light adapted with a steady white background light. RESULTS: The scotopic b-wave is more than twice the amplitude of the a-wave in rabbits, while the scotopic b-wave in guinea pigs is only slightly larger than the a-wave. The b-wave of the cone ERG is twice the amplitude of the cone a-wave in both species. Once a background light has been turned on, the amplitude increases in both species and the process of light adaptation reaches a peak about 10 minutes thereafter. The b-wave implicit time is shortened by light adaptation in rabbits, but not in guinea pigs. Oscillatory potentials are present in guinea pig ERGs when recorded in dark but not when recorded in light. CONCLUSIONS: Mammals that have avascular retinae and which are without long-wavelength cones show evidence of light adaptation of the cone ERG. In guinea pigs the cone ERG increases in amplitude during light adaptation without concomitant shortening of the implicit time. These changes occur at similar rate in rabbits and guinea pigs. The oscillatory potentials in rabbits increase in amplitude but not in guinea pigs. These results suggest that different mechanisms determine the light adaptation of the cone ERG in guinea pigs than in rabbits.

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AIM: To investigate the influence of acute changes in intraocular pressure on the oxygen tension in the vicinity of the optic nerve head under control conditions and after intravenous administration of 500 mg of the carbonic anhydrase inhibitor dorzolamide. METHODS: Domestic pigs were used as experimental animals. Oxygen tension was measured by means of a polarographic electrode in the vitreous 0.5 mm anterior to the optic disc. This entity is called the optic nerve oxygen tension. Intraocular pressure was controlled by a hypodermic needle inserted into the anterior chamber and connected to a saline reservoir. RESULTS: When the intraocular pressure was clamped at 20 cm H2O optic nerve oxygen tension was 20 (5) mm Hg (n=8). Intravenous administration of dorzolamide caused an increase in optic nerve oxygen tension of 43 (8)% (n=6). Both before and after administration of dorzolamide optic nerve oxygen tension was unaffected by changes in intraocular pressure, as long as this pressure remained below 60 cm H2O. At intraocular pressures of 60 cm H(2)O and below, dorzolamide significantly increased optic nerve oxygen tension. CONCLUSION: Intravenous administration of 500 mg dorzolamide increases the oxygen tension at the optic nerve head during acute increases in intraocular pressure.

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The electroretinogram (ERG) was recorded from the Xenopus retina, to examine the effects of glycine and strychnine on these responses and to determine the origins of these changes. Glycine at concentrations between 0.1 and 10 mM reduced the b- and d-waves of the ERG in a dose-dependent manner, while strychnine increased their amplitude. 2-Amino-4-phosphonobutyric acid (APB) reduced the b-wave and blocked the effect of glycine, but not strychnine, on the d-wave. When the d-wave had first been blocked by kynurenic acid (KYN) or reduced by (+/-)cis-2,3-piperidine dicarboxylic acid (PDA) the b-wave was enhanced by glycine, but not by strychnine. N-methyl-DL-aspartate (NMDLA), which alters responses in the proximal retina only, blocked the effects of glycine and strychnine on the ERG. This suggests that the glycinergic effects on the ERG are at least partly mediated by processes in the proximal retina. The results further support the suggestion that inhibitory neurotransmitters in the proximal retina may modulate both the b- and d-waves of the Xenopus ERG.

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PURPOSE: To evaluate how the oxygen tension of the optic nerve (ONP(O)2) is affected by the administration of the carbonic anhydrase inhibitors dorzolamide and acetazolamide and by alterations in oxygen and carbon dioxide in the breathing mixture. METHODS: Polarographic oxygen electrodes were placed in the vitreous humor immediately over the optic disc in 20 anesthetized pigs. Blood gasses and cardiovascular physiology were monitored. ONP(O)2 was recorded continuously with breathing gasses of 21% O2-79% N2, 100% O2, 20% O2-80% N2, and 5.19% CO2-19.9%, O2-74.9% N2. Acetazolamide (15-1000 mg) and dorzolamide (6-1000 mg) were administered intravenously. RESULTS: The mean (+/- SD) ONP(O)2 was found to be 24.1+/-11.6 mm Hg when the pigs were breathing room air and 50.7+/-29.3 mm Hg when they were breathing 100% O2 (n = 15; P < 0.001). In response to breathing 5.19% CO2, ONP(O)2 changed from 20.8+/-5.6 mm Hg (with 20.0% O2) to 28.9+/-3.6 mm Hg (n = 4; P < 0.001). Intravenous injections of 500 mg dorzolamide increased ONP(O)2 from 16.4+/-6.1 mm Hg to 26.9+/-12.2 mm Hg, or 52.5%+/-21.2% (n = 5; P = 0.017). A dose-dependent effect on ONP(O)2 was seen with intravenous dorzolamide doses of 1000, 500, 250, 125, 63, 27, 15, and 6 mg. Intravenous injections of 500 mg acetazolamide increased ONP(O)2 from 23.6+/-9.5 mm Hg to 30.9+/-10.0 mm Hg (n = 6; P < 0.001), and a dose-dependent effect was seen with doses of 1000, 500, 250, 125, 31, and 15 mg. CONCLUSIONS: ONP(O)2 is significantly increased by the carbonic anhydrase inhibition of dorzolamide and acetazolamide, and the effect is dose dependent. These data demonstrate for the first time a direct effect of carbonic anhydrase inhibitors on ONP(O)2.

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AIMS: To characterise retinal function using electrophysiological and psychophysical tests in 17 patients with helicoidal peripapillary chorioretinal degeneration. METHODS: The electroretinogram (ERG) was recorded using gold foil corneal electrodes. The electro-oculogram (EOG) was recorded using a standard protocol. Dark adaptometry was recorded with an SST-1 dark adaptometer and colour vision assessed with Ishihara plates and Farnsworth D-15. RESULTS: All subjects had a recordable ERG. The amplitudes and implicit times of the a- and b-waves were within normal limits at all luminances in five subjects (age 21-70 years, mean 40 years). The ERG of six (age 26-55 years, mean 40.7 years) had subnormal amplitudes at all luminances, but normal implicit times, and six (age 38-81 years, mean 60.7 years) had abnormal ERGs with marked reduction of a- and b-waves, and delayed implicit times of the b-wave. The implicit times of the a-wave were normal in all subjects. A reduction in the b/a wave ratios was not found, nor was there selective loss of scotopic, mixed rod/cone, or cone responses. The light/dark ratio of the EOG was subnormal (150-185%) or abnormal (below 150%) in all but three subjects. Two patients with normal EOG showed normal ERGs in both eyes, but one had subnormal ERGs in both eyes. The scotopic sensitivity was normal in all subjects and dark adaptation showed a normal time course. Colour vision was normal in all patients. CONCLUSION: The results suggest that in most cases the function of the retinal pigment epithelium is affected by this disease before any changes in the function of the sensory retina are detectable by our methods, and that retinal dysfunction is focal rather than diffuse.

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We have recorded the electroretinogram (ERG) from the superfused eyecup of the Xenopus retina in order to assess the effects of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and its agonists and antagonists, on individual ERG components. We found that GABA (0.5-10 mM) reduced the amplitudes of both the b- and d-waves of the Xenopus ERG. The GABA uptake blocker nipecotic acid (1 mM) had similar effects on b- and d-waves. GABA at 5 mM and 10 mM also caused an increase in the a-wave. The GABA antagonist picrotoxin (0.1-2 mM) and the GABA/a antagonist bicuculline (0.2 mM) both increased the amplitude of the b- and d-waves of the ERG. The GABA/b agonist baclofen (0.3 mM) reduced the amplitude of the ERG b-wave, enhanced the amplitude of the a-wave, and slightly reduced the amplitude and increased the peak time of the d-wave. The GABA/b antagonists phaclofen and saclofen had no reliable effects on the Xenopus ERG. Glutamate analogs known to affect specific types of retinal neurons were applied to modify the retinal circuitry and then the effects of GABA and its antagonists were examined under these modified conditions. 2-amino-4-phosphonobutyric acid (APB) increased the d-wave, and blocked the b-wave and the effect of GABA on the ERG, but not the antagonist-induced increase in the d-wave. KYN blocked the antagonist-induced increase in the b-wave, while GABA increases the amplitude of the b-wave if the d-wave has been removed by prior superfusion with kynurenic acid (KYN). N-methyl-DL-aspartate (NMDLA), which acts only in the proximal retina, reduced the amplitude of the ERG and blocked the effect of GABA and the antagonist-induced increase in ERG b- and d-waves amplitude. These results suggest that GABAergic mechanisms related to both A and B receptor types can influence the amplitude and light sensitivity of all the components of the Xenopus ERG. Since GABA is found in greatest abundance in the proximal retina, and B type of receptors are present almost exclusively there, the data suggests that most of the effects of GABA agonists and antagonists observed are dependent on proximal retinal mechanisms, and that there are separate mechanisms in the proximal retina related to the b- and the d-waves.

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We have recorded the electroretinogram from 19 superfused eyecups of the Xeiiopus retina in order to assess the effects of agonists of the inhibitory neurotransmitter gamma-aminobutyric acid (GA notBA), on both oscillatory potentials and the b-wave. We found that in seven eyecups the GABA uptake blocker nipecotic acid (0.1-5 mM) reduced the amplitudes of the oscillatory potentials, without having an effect on the b-wave unless it was applied in larger doses. The GABAB agonist baclofen (0.05-3 mM) reduced the amplitude of the ERG b-wave selectively in seven eyecups tested, without any effect on the amplitude of the oscillatory potentials. The GABAA agonist aminovaleric acid (0.05-3 mM) on the other hand, selectively reduced the oscillatory potentials in five, but had no reliable effects on the Xenopus b-wave. These results suggest that GABAergic mechanisms related to both A and B receptor types induce different influence on the amplitude of the oscillatory potentials and the b-wave.

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PURPOSE: Psychophysical studies have shown that a dark-adapted eye exerts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstrate TIS by means of visual evoked potential (VEP) procedures. METHODS: Evoked cortical potentials were obtained in response to reversing checkerboard patterns with fundamental Fourier frequencies between 3 and 12 cycles per degree. Responses were obtained under monocular viewing conditions when the contralateral "adapting" eye was dark adapted, under monocular viewing conditions when the adapted state of the adapting eye was experimentally manipulated, or under binocular viewing conditions. Data were collected from three healthy young men, two native regarding purpose of experimentation. RESULTS: Regardless of spatial frequency, monocular responses evoked by stimulating a "test eye" were always smaller in amplitude when the contralateral adapting eye was dark adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting field was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of the monocular evoked response decreased: the time course of this decline follows that of psychophysically measured rod thresholds in the directly adapted eye. CONCLUSIONS: TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in VEP amplitude resulting from binocular viewing may be attributable to the removal of TIS rather than to "physiologic, binocular summation."

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1. Spatial sensitivity of human foveal vision was examined using sinusoidally modulated gratings. Our primary concern was the influence of interocular light adaptation upon monocular visibility. 2. Interocular adapting influences depend upon spatial frequency and adapting luminance. Interocular adaptation has a negligible influence upon the sensitivity to 1 cycle/deg gratings. Any visible interocular adapting field improves the sensitivity to intermediate spatial frequencies (2-5 cycles/deg). 3. Brighter interocular backgrounds (greater than 0.1 cd/m2) improve sensitivity to higher spatial frequencies (10-20 cycles/deg). 4. The interocular adapting influences summarized in (2) and (3) above cannot be duplicated by monocular or binocular adaptation. Similarly, monocular or binocular adaptation have negligible influences upon binocular visibility. 5. The interocular adapting effect summarized in (3) above can be duplicated by pressure blinding the contralateral eye. We conclude that monocular spatial sensitivity is subject to a tonic interocular suppression (TIS) from the dark-adapted eye. 6. The spatial sensitivity resulting from binocular viewing is nearly identical to that observed by combining monocular viewing with interocular light adaptation. We suggest that the improvement in sensitivity resulting from two-eyed viewing may be attributable to the removal of TIS instead of to binocular physiological summation.

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The potential difference (PD), short circuit current (SCC) and resistance (R) of rabbit retina-retinal pigment epithelium-choroid-sclera preparations were measured in a modified Ussing chamber. The rabbits were kept in 14 hour light/10 hour dark cycle conditions for 14 days: tissues were taken for measurement at 1.5, 6, 13, 20 and 23 hours after the start of the fifteenth light period. Significant variations in PD and SCC were observed 6 hours into the light cycle while the resistance remained constant during the test period. When in a second experiment the fifteenth light period was replaced by darkness, the highest PD and SCC values recorded occurred 12 hours after the time when the light period should have started in the normal light/dark cycle. In a third experiment the fifteenth light period was replaced by darkness and the retina was removed. There were variations in the PD and SCC as in the second experiment but the SCC peak was reduced in amplitude. From these preliminary studies it is suggested that these variations may be circadian in origin and may be related to variations in retinal function.

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Recordings were obtained from rods and horizontal cells of Xenopus, using an eyecup preparation. The enhancement of cone signals produced by rod backgrounds was measured using flickering red spots of varying intensity and diameter, and the experiments were repeated with cone stimuli consisting of alteration of wavelengths of 660 and 605 nm, adjusted for equal effects on rods or cones ("silent substitution"). Rods responded to red flicker with discrete wavelets up to 5 Hz. The characteristics of suppressive rod-cone interaction (SRCI) depend on the precise stimulus parameters. In particular, the reported low-pass attenuation of SRCI is absent with silent substitution. An analysis of the responses to backgrounds in horizontal cells, and the effects of the red light flashes in rods, led to the conclusion that the characteristics of SRCI are determined partially by the fact that "cone" stimuli excite rods and vice versa. This result simplifies the mechanism of SRCI and permits a comparison between the studies of SRCI using electroretinograms and horizontal cells.

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1. Intracellular recordings were obtained from retinal neurons of the mudpuppy, Necturus, while superfusing the eyecup with various pharmacologic agents. In most experiments, the retina was continuously stimulated with a small spot of red light that was centered over the recording electrode and flickering at rates too fast for amphibian rods to follow. The retina was additionally stimulated intermittently with a dim, spatially diffuse background field of 520 nm wavelength. 2. In general, the dim background greatly enhanced flicker responsiveness. We (16) previously called this effect suppressive rod-cone interaction (SRCI) and showed it reflects a tonic suppressive influence on cone pathways that is removed by selective rod-light adaptation. 3. Lead chloride has been claimed to selectively block rod-related retinal responses (13, 35). While recording from horizontal cells lead chloride decreases responses to the dim, diffuse light flashes, enhances the frequency entrained response attributable to cones, and eliminates a background influence on flicker responses. 4. O-phospho-D-serine (DOP), kynurenic acid (KyA), and piperidine dicarboxylic acid are known to act on horizontal cells as antagonists of the photoreceptor neurotransmitter (26, 32, 33). In both depolarizing and hyperpolarizing bipolar cells, these agents enhance flicker responsiveness with no background present and prevent background enhancement of flicker. 5. Mudpuppy cones were found to have a receptive-field surround, which under our stimulus conditions is attributable to rod input. KyA, which is unknown to have any direct influence on photoreceptors, totally blocks this surround mechanism. This indicates that the cone-surround mechanism is attributable to horizontal cell feedback. The influence of KyA on SRCI in cones is similar to that observed in recordings from depolarizing bipolar cells. 6. Most sustained third-order neurons demonstrate SRCI. In these cells, SRCI is blocked by DOP or KyA. Most ON-OFF neurons fail to demonstrate SRCI under control circumstances. The ON-response of these cells is blocked by 2-amino-4-phosphonobutyric acid (31) which leaves the OFF-response intact. While their ON-response is blocked, ON-OFF neurons demonstrate SRCI. 7. The foregoing results indicate that SRCI reflects a tonic, inhibitory influence of horizontal cells on cone pathways that is removed by light-adapting rods. In part, SRCI must involve horizontal cell feedback onto cones. SRCI in third-order neurons appears to largely reflect distal retinal processing.

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The response to spatially focal flicker is enhanced by dim, spatially diffuse, rod-stimulating backgrounds. This effect is called suppressive rod-cone interaction (SRCI) as it reflects a tonic, suppressive influence of dark-adapted rods upon cone pathways which is removed by selective rod-light adaptation. SRCI is observed in amphibian retina with intracellular recordings from most cone-driven cells including the cones themselves, and is most obvious using stimuli flickering at frequencies too rapid for rods to follow. SRCI is blocked by glutamate analogs which selectively block the photic response of horizontal cells (HCs). In the presence of these agents, flicker responses from bipolar cells and cones are enhanced to levels normally seen only with selective rod-light adaptation. In the HCs themselves, SRCI is similarly blocked by lead chloride which blocks rod-, but not cone-related activity. In amphibian and cat HCs and in human observers, SRCI is limited by a space constant of very similar value (between 100 and 150 microns). We suggest that SRCI in all three species is mediated by HCs: in amphibians, SRCI must at least partially reflect rod-modulation of HC feedback onto cones.

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The influence of dim diffuse adapting fields upon the sensitivity to focal photic stimulation was studied by means of intracellular recording in retinal neurons of the south african clawed frog, Xenopus and the mudpuppy, Necturus. In cones and in most horizontal and bipolar cells lacking color opponency, dim diffuse backgrounds have little influence upon the response to diffuse flicker of low (less than 2 Hz) temporal frequencies; however, with small diameter test probes of higher temporal frequencies, presentation of dim backgrounds enhance the peak-to-peak amplitude of responses to sinusoidal flicker by as much as 800%. This background enhancement effect adheres to the spectral sensitivity of the green-absorbing rod photopigment, and appears to be largely independent of the influence of the adapting field upon cone photopigment or ambient membrane potential in the recorded neuron. This effect cannot be obtained with rod-driven flicker responses. We designate this background influence on flicker, suppressive rod-cone interaction (SRCI) and attribute it to a tonic suppressive (probably inhibitory) influence of rods upon cone pathways that is removed by rod light adaptation. SRCI is also observed in the response of most sustained ON and OFF ganglion cells. However, no corresponding effect occurs in rods, color-opponent second-order neurons, ON-OFF amacrine cells, or most ON-OFF ganglion cells. The spatial and temporal limitations of SRCI observed by means of intracellular recording in amphibians are very similar to those documented by means of psychophysical or electroretinogram (ERG) procedures in a wide variety of species including humans (2, 4, 11, 22, 23, 29). SRCI most probably reflects a process that is mediated by horizontal cells. The specifics of the underlying mechanism remain unclear.

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