The influence of cone adaptation upon rod mediated flicker

Extrinsic cone-mediated post-receptoral noise inhibits the rod temporal impulse response function

We determined how extrinsic white noise correlating with cone inputs to the three primary visual pathways affects both rod-pathway temporal contrast sensitivity and the impulse response function. A four-primary photostimulator provided independent control of rod and cone photoreceptor excitations under mesopic illumination (20 photopic Td). We show that rod-pathway temporal contrast sensitivity uniformly decreases across all temporal frequencies in the presence of cone noise correlating with the inferred magnocellular, parvocellular, or koniocellular pathways. The rod-pathway temporal impulse response functions derived using the Stork-Falk procedure (with a minimum phase assumption) had lower amplitudes in the pathway-specific cone noise. Therefore, cone noise impairs rod-pathway temporal contrast sensitivity without delaying rod-pathway signal transmission.

The morphology of human rod ERGs obtained by silent substitution stimulation

Purpose

To record transient ERGs from the light-adapted human retina using silent substitution stimuli which selectively reflect the activity of rod photoreceptors. We aim to describe the morphology of these waveforms and examine how they are affected by the use of less selective stimuli and by retinal pathology.

Methods

Rod-isolating stimuli with square-wave temporal profiles (250/250 ms onset/offset) were presented using a 4 primary LED ganzfeld stimulator. Experiment 1: ERGs were recorded using a rod-isolating stimulus (63 ph Td, rod contrast, C_rod = 0.25) from a group (n = 20) of normal trichromatic observers. Experiment 2: Rod ERGs were recorded from a group (n = 5) using a rod-isolating stimulus (C_rod = 0.25) which varied in retinal illuminance from 40 to 10,000 ph Td. Experiment 3: ERGs were elicited using 2 kinds of non-isolating stimuli; (1) broadband and (2) rod-isolating stimuli which contained varying degrees of L- and M-cone excitation. Experiment 4: Rod ERGs were recorded from two patient groups with rod monochromacy (n = 3) and CSNB (type 1; n = 2).

Results

The rod-isolated ERGs elicited from normal subjects had a waveform with a positive onset component followed by a negative offset. Response amplitude was maximal at retinal illuminances <100 ph Td and was virtually abolished at 400 ph Td. The use of non-selective stimuli altered the ERG waveform eliciting more photopic-like ERG responses. Rod ERGs recorded from rod monochromats had similar features to those recorded from normal trichromats, in contrast to those recorded from participants with CSNB which had an electronegative appearance.

Conclusions

Our results demonstrate that ERGs elicited by silent substitution stimuli can selectively reflect the operation of rod photoreceptors in the normal, light-adapted human retina.

Anomalous trichromats' judgments of surface color in natural scenes under different daylights

Deuteranomalous trichromacy, which affects medium-wavelength-sensitive cones, is more common than protanomalous trichromacy, which affects long-wavelength-sensitive cones. The aim of the present work was to test the extent to which these two kinds of anomalous trichromacy affect surface-color judgments in the natural world. Simulations of 18 natural scenes under different daylight illuminants were presented on a high-resolution color monitor to 7 deuteranomalous, 7 protanomalous, and 12 normal trichromatic observers, who had to discriminate between reflectance and illuminant changes in the images. Observers' ability to judge surface color was quantified by a standard color-constancy index. Deuteranomalous trichromats performed as well as normal trichromats, but protanomalous trichromats performed more poorly than both. The results are considered in relation to the spectral coverage of cones, rod intrusion, and the characterization of anomalous trichromacy by the Rayleigh match.

Into the twilight zone: the complexities of mesopic vision and luminous efficiency

Of all the functions that define visual performance, the mesopic luminous efficiency function is probably the most complex and hardest to standardise or model. Complexities arise because of the substantial and often rapid visual changes that accompany the transition from scotopic to photopic vision. These are caused not only by the switch from rod to cone photoreceptors, but also by switches between different post-receptoral pathways through which the rod and cone signals are transmitted. In this review, we list several of the complexities of mesopic vision, such as rod-cone interactions, rod saturation, mixed photoreceptor spectral sensitivities, different rod and cone retinal distributions, and the changes in the spatial properties of the visual system as it changes from rod- to cone-mediated. Our main focus, however, is the enormous and often neglected temporal changes that occur in the mesopic range and their effect on luminous efficiency. Even before the transition from rod to cone vision is complete, a transition occurs within the rod system itself from a sluggish, sensitive post-receptoral pathway to a faster, less sensitive pathway. As a consequence of these complexities, any measure of mesopic performance will depend not only on the illumination level, but also on the spectral content of the stimuli used to probe performance, their retinal location, their spatial frequency content, and their temporal frequency content. All these should be considered when attempting to derive (or to apply) a luminous efficiency function for mesopic vision.

Suppressive rod-cone interactions: evidence for separate retinal (temporal) and extraretinal (spatial) mechanisms in achromatic vision

We investigated the influence of selective rod light and dark adaptation on cone-mediated sensitivity to monocular displays modulated sinusoidally in both spatial and temporal domains. Rod light adaptation (1) increased sensitivity to high spatial frequencies [> or = 8 cycles per degree (cpd)] flickered slowly (< or = 2 Hz), an effect that we refer to as grating suppressive rod-cone interaction (gSRCI); (2) increased sensitivity to low spatial frequencies (< or = 2 cpd) flickered rapidly (> or = 8 Hz), an effect that we refer to as flicker suppressive rod-cone interaction (fSRCI); and (3) had relatively little influence on intermediate temporal-spatial-frequency combinations. The magnitudes of both gSRCI and fSRCI increased as the retinal position of the test display was increasingly displaced parafoveally. In parafoveal retina, both forms of suppressive rod-cone interaction increased as the overall dimension of the test stimulus decreased. However, sensitivity to high spatial frequencies is equally well influenced by adaptation of the viewing and the contralateral eye, while the adapted state of the nonviewing eye negligibly influences sensitivity to rapid flicker. Moreover, gSRCI cannot be observed with a small (30-arcmin) grating restricted to the fovea, while fSRCI is a prominent effect with small foveal test stimuli. Collectively, these results and neurobiological evidence suggest that fSRCI reflects a mechanism restricted to distal retinal, while gSRCI involves extraretinal neural circuitry.

Two signals in the human rod visual system: a model based on electrophysiological data

In the human rod visual system, self-cancellation of flicker signals is observed at high rod intensity levels near 15 Hz, both perceptually and in the electroetinogram (ERG). This and other evidence suggests that two rod signals are transmitted through the human retina with different speeds of transmission. Here we report a series of flicker ERG recordings from a normal observer and an observer who lacks cone vision. From these results, we propose a quantitative model of the two rod signals, which assumes (1) that the amplitude of the slow signal grows linearly with log intensity but then saturates at approximately 1 scot. td; (2) that the amplitude of the fast signal grows linearly with intensity; (3) that there is a difference in time delay of approximately 33 ms between two rod signals of the same polarity (or of approximately 67 ms if the signals are of inverted polarity); and (4) that the time delay of both signals declines linearly with log intensity (by approximately 10 ms per log scot. td). These simple assumptions provide a remarkably good account of the experimental data. Our results and model are relevant to current anatomical theories of the mammalian rod visual system. We speculate that the slower signal in the human ERG may reflect the transmission of the rod response via the rod bipolars and the AII amacrine cells, while the faster signal may reflect its transmission via the rod-cone gap junctions and the cone bipolars. There are, however, several objections to this simple correspondence.

The spectral properties of the two rod pathways

Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. Accordingly, we made similar measurements in dichromats, who lack one of the two longer wavelength cone types. These measurements revealed an L-cone influence on pi'0 in the deuteranope and an M-cone influence in the protanope. This suggests that both cone types can affect the sensitivity of pi'0. The finding that the steady-state cone signals reduce the sensitivity of pi'0 but have little effect on pi 0 could suggest that pi'0 signals travel through a faster cone pathway (with its own gain control at which both rod and cone signals can reduce rod threshold), while pi 0 signals travel through a separate rod pathway. However, it could simply reflect the fact that pi'0 predominates at higher luminances than pi 0 where the cone excitation level is inevitably greater. To examine the influence of the cones on pi 0 more closely, we: (i) produced transient cone excitation by alternating rod-equated 480 and 679 nm fields; and (ii) extended our steady-state measurements to include deep-red backgrounds of 650 and 680 nm. Both experiments revealed a small, but measurable influence of the cones on pi 0.

Cone pathways and the pi 0 and pi 0' rod mechanisms

The field-adaptation properties of two scotopic (rod) mechanisms, pi 0 and pi 0', were measured to test a two-pathway model that associates the fast temporal properties of pi 0' with the processing of rod signals by early cone pathways, possibly including cone photoreceptors, and the sluggish temporal properties of pi 0 with processing of rod signals by classical rod pathways. This model predicts that cone stimulation will differentially affect the flicker sensitivity of pi 0' compared to pi 0. Both rod mechanisms are seen in double-branched flicker-threshold-vs-intensity (FTVI) curves measured with a 15-Hz, square-wave-modulated, rod-detected test stimulus. We show that the position of the upper branch (pi 0') shifts relative to the lower branch in response to changes of background wavelength, indicating that different receptor types regulate sensitivity of pi 0 and pi 0'. Field spectral sensitivity (FSS) functions for pi 0 closely match the scotopic spectral sensitivity function, indicating that only rods adapt pi 0 under these conditions. In contrast, fitting of FSS functions for pi 0' required a combination of cone and rod spectral sensitivity functions. The relative adaptational effect of cone stimulation compared to rod stimulation increases with background light level: at highest levels, cone stimulation has more influence than rod stimulation. Test additivity experiments assessed the degree of additivity between cones and rods to ensure that the pi 0' branch did not result from sub-threshold summation between receptor mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutual rod-cone suppression within the central visual field

Under mesopic conditions the contrast sensitivity of the central visual field is reduced as the result of a non-linear interaction between rod- and cone-mediated signals, each of which is capable of higher sensitivity in isolation. The interaction is produced only when the rod-mediated system is driven at flicker rates above 6 Hz. This finding bears upon how rod and cone signals are combined and therefore affects our interpretation of the significance of the relationship between retinal illuminance and both contrast sensitivity and temporal resolution.

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.

Clinical and electroretinographic comparison between Aland Island eye disease and a newly found related disease with X-chromosomal inheritance

Two subjects representing AIED (Aland Island Eye Disease) and a family with 5 males affected with an AIED related X-linked hereditary eye disease were studied clinically and electrophysiologically. The clinical picture of AIED includes myopia and astigmatism, reduced visual acuity, nystagmus, ocular albinism, hemeralopia and dyschromatopsia (No. 300600, McKusick 1990). The subjects with the related disease showed astigmatism with or without myopia, reduced visual acuity, slight hemeralopia, normal color vision in 3/5 subjects, no ocular albinism and nystagmus only in one case. In both diseases the ERG was abnormal showing defective a- and b-waves, but there were also differences. The most notable was the greater reduction of the b-wave amplitude in the mixed (rod and cone) responses for the white stimulus in the ERG of the AIED related disease. With regard to the pathogenesis we propose that in both diseases rod and cone functions are defective but in an AIED related disease a defective cone function inhibits the transmission of the rod signals to the rod bipolars, causing greatly reduced mixed responses. The clinical and ERG findings of this study suggest that the 5 subjects of our family do not represent AIED but another X-linked hereditary eye disease. The investigation to find out the gene locus of this disease is going on.

Rod and cone contribution to adaptation processes in cat retinal ganglion cells

Extracellular ganglion cell responses were recorded to investigate mechanisms of light adaptation. Monochromatic test spots (575 nm) were projected onto the receptive field center of off-center cells and superimposed on a steady blue-green Ganzfeld background (Schott Filter BG 28), the strength of which was increased in steps of 0.5 log units to adapt rods. Response vs. log intensity functions were determined over a range of 7 log units of test light irradiance at each background level. At higher adaptation levels response thresholds followed the typical Weber function. Surprisingly at lower adaptation levels the sensitivity of the cell increased by about 0.7 log units, most markedly in a range of 1 log unit of moderate light adaptation when the background was changed from dark to the dimmest detectable background (10(-5) lm/m2). In the dark-adapted state a small off-response of long latency (40-100 ms at 10(2) quanta.s-1.microns-2) is observed at low rod stimulating test light irradiances. A transition to a cone-dominated transient response of 2 to 5 ms duration occurred at high intensities (10(5) quanta.s-1.microns-2). At mesopic levels the two responses seem to cancel each other, rendering a delayed off-response that is probably the result of rod-cone interaction. As in psychophysics, saturation can be observed at very high background intensities (10(6) quanta.s-1 microns-2). These data suggest interactions between rods and cones that determine the sensitivity of cat retinal ganglion cells at low levels of adaptation for suprathreshold stimuli.

Rod suppression of cone-mediated information about colour and form during dark adaptation

Following substantial bleaches, the specific form and hue thresholds were measured during dark adaptation with a test stimulus of 1 x 2 degrees at 40 degrees extrafoveally. The wavelength of the test field was varied between runs. The results show that both thresholds started to rise at about the cone-rod break of the dark-adaptation curve, irrespective of wavelength used in the test. Furthermore, the specific threshold for form was found to rise when a scotopic stimulus was superimposed on a photopic test flash. On the other hand, both thresholds remained at the cone-plateau level when the test flash was confined within the rod-free fovea. In order to explain the rise in the specific thresholds, it is suggested that signals from rods generated directly in response to the test stimulus may suppress both cone-mediated form and colour. It is also suggested that this type of rod-cone interaction represents a general characteristic involved in several kinds of visual information processing.

The incremental threshold of the rod visual system and Weber's law

The incremental threshold of the isolated rod visual system is believed, under certain conditions, to obey Weber's law (that is, to increase in direct proportion to the intensity of the background). This relation was tested at several background wavelengths, over an intensity range for which the target was seen only by the rods. Although the slope on long-wavelength background approximates unity (that is, Weber's law on log-log coordinates), it averages less than 0.8 on short- and middle-wavelength backgrounds. This is the same value as that found for the thresholds of a typical, complete achromat--who lacks cone vision--regardless of background wavelength. These results force the conclusion that Weber's law for incremental threshold detection is achieved not by the rods alone but only by the rods acting together with the cones.

The role of spatial filtering in sensitivity regulation

The role of spatial filtering in controlling sensitivity to increments is hard to evaluate under normal viewing conditions because eye movements lead to a confounding of spatial and temporal transients. We measured sensitivity to increments on different sized backgrounds in photopic and scotopic vision when the backgrounds were stabilized on the retina, thus eliminating temporal transients. The saturating effect of small fields on photopic thresholds was preserved under these conditions indicating that spatial filtering by retinal cells is critical in maintaining photopic sensitivity. Some effect of spatial pattern on sensitivity in stabilized vision was also observed in scotopic vision, although it was much smaller than was observed in photopic vision. The interaction effects between rod and cone systems that are observed with small backgrounds were also preserved in stabilized vision, implicating a very peripheral site for the generation of these interactions.

Detection of the carrier state of X-linked retinoschisis

We determined the extent of suppressive rod-cone interaction in 11 obligate carriers and eight potential carriers of X-linked retinoschisis from eight families. Despite otherwise normal ophthalmoscopic and functional testing, all of the obligate heterozygous carriers demonstrated a complete absence of normal rod-cone interaction. Of the potential heterozygous carriers, three had normal rod-cone interactions, two had no detectable interaction, and two yielded technically unsatisfactory results. This lack of rod-cone interactions allows heterozygous individuals to be identified clinically and has implications concerning the origin of this inherited disorder.

The cellular basis for suppressive rod-cone interaction

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.