A modified silent substitution electroretinography protocol to separate photoreceptor subclass function in lightly sedated dogs

OBJECTIVE

A previously published study successfully isolated photoreceptor responses from canine rods, long/medium-wavelength (L/M) cones, and short-wavelength (S) cones using silent substitution electroretinography (ERG) performed under general anesthesia. We hypothesized that responses would be similar in dogs under sedation and that a curtailed protocol suitable for use in clinical patients could effectively isolate responses from all three photoreceptor subtypes.

ANIMALS STUDIED

Three normal adult purpose-bred beagles (2 females and 1 male).

METHODS

Dogs were dark-adapted for 1 hour. Sine wave color stimuli were delivered using LEDs in a Ganzfeld dome. The ERG protocol under anesthesia was performed as previously published; sedated ERG protocols were performed after a 3-day washout period. Intravenous sedation (dexmedetomidine 1.25 mcg/kg, butorphanol 0.1 mg/kg) was administered for sedation. Statistical analysis was performed using two-way repeated-measures ANOVA and linear regression.

RESULTS

In both anesthetized and sedated animals, rod-derived responses peaked at low frequency (4-12 Hz), L/M-cone responses peaked at high frequency (32-38 Hz), and S-cone responses peaked at low frequency (4-12 Hz). The frequencies eliciting maximal responses were similar in sedated and anesthetized protocols, although rod amplitudes were significantly higher in the sedated protocols compared with anesthetized (P < .001).

CONCLUSION

We present a clinically applicable method to consistently isolate rod and cone subclass function in sedated dogs. This may allow detailed evaluation of photoreceptor function in clinical patients with rod or cone subclass deficits without the need for general anesthesia or protracted adaptation times.

Impact of network topology on the stability of DC microgrids

We probe the stability of Watts-Strogatz DC microgrids, in which droop-controlled producers and constant power load consumers are homogeneously distributed and obey Kirchhoff's circuit laws. The concept of survivability is employed to evaluate the system's response to Dirac-delta voltage perturbations at single nodes. A fixed point analysis of the power grid model yields that there is only one relevant attractor. Using a set of simulations with random networks, we investigate correlations between survivability and three topological network measures: the share of producers in the network and the degree and the average neighbor degree of the perturbed node. Depending on the imposed voltage and current limits, the stability is optimized for low node degrees or a specific share of producers. Based on our findings, we provide an insight into the local dynamics of the perturbed system and derive explicit guidelines for the design of resilient DC power grids.

Human S-cone electroretinograms obtained by silent substitution stimulation

We used triple silent substitution stimuli to characterize human S-cone electroretinograms (ERGs) in normal trichromats. Short-wavelength-cone (S-cone) ERGs were found to have different morphological features and temporal frequency response characteristics compared to ERGs derived from L-cones, M-cones, and rod photoreceptors in normal participants. Furthermore, in two cases of retinal pathology, blue cone monochromatism (BCM) and enhanced S-cone syndrome (ESCS), S-cone ERGs elicited by our stimuli were preserved and enhanced, respectively. The results from both normal and pathological retinae demonstrate that triple silent substitution stimuli can be used to generate ERGs that provide an assay of human S-cone function.

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.

[Recovery time as a potential new progression parameter for patients with advanced glaucomatous optic atrophy]

BACKGROUND

Functional tests in glaucoma diagnosis can monitor a potential perimetric progression. However white-on-white perimetry is limited in advanced glaucoma, whereas contrasts were detected reproducibly. Especially when stressing (i. e. adapting) the visual system, subsequent measurements yielded different results-the visual system needs a "recovery time". In the present study the recovery time was investigated in patients with advanced glaucoma. Additionally correlation analysis was done with standard perimetric parameters.

MATERIAL AND METHODS

Temporal contrast sensitivity (TCS) and recovery time (RT) were measured using the Erlanger Flicker Test in 61 probands (15 normals, 25 primary open-angle glaucomas, 17 secondary open-angle glaucomas, 4 narrow-angle glaucoma). Additionally, ophthalmological examinations and perimetry (Octopus G1) was done.

RESULTS

(1) TCS was significantly reduced in patients with advanced glaucoma (p < 0.001). (2) Test stimuli with 3 and 5 % contrast showed a large variability in contrast to higher contrasts (12 %, 25 %, 35 %). (3) RT_12%, RT_25% and RT_35% were significantly prolonged in advanced glaucoma (p < 0.001). (4) RT_25% correlated significantly with mean defect (p = 0.015).

CONCLUSION

Recovery time seems to be a potential parameter in advanced glaucoma follow-up, as it is reproducible, independently of cataract and fixation.

Retinal disorders in northern Brazilian patients treated with chloroquine assessed by multifocal ERG

The effects of chloroquine intake on the retinal function in a Brazilian population of patients were assessed by multifocal electroretinography. Twenty-four randomly chosen eyes of patients treated with chloroquine for rheumatoid arthritis and systemic lupus erythematosus were examined using multifocal electroretinography (mfERG). Control measurements were acquired from 21 randomly chosen eyes of age-matched healthy subjects. None of the study participants had an inherited retinal disease or a Snellen visual acuity reduced to less than 20/40. In patients and control subjects, cumulative chloroquine dose, total daily dose, duration of treatment, retinal examination, visual field defects, visual acuity, and the mfERG were assessed. The average amplitudes and implicit times of the N1, P1, and P2 components of the mfERGs were measured in the central hexagon (R1) and in five rings (R2-R6). The values measured in patients and normal subjects were compared. The P1 amplitudes in R2 were significantly decreased in the patients. In addition, the amplitudes of N1 and N2 in R1 were significantly smaller in the patients. The implicit times of none of the components were significantly different between patients and controls. The response amplitude was not significantly correlated with cumulative dose and duration of intake. There was no correlation with retinal appearance, visual field, and visual acuity. In agreement with earlier data, the central mfERG amplitudes were decreased in chloroquine patients indicating functional alterations in the retina. These changes are also present in a Brazilian population suggesting that the effects of chloroquine are general and that genetic background and life circumstances probably have, if at all, only little effect.

L- and M-cone input to 12Hz and 30Hz flicker ERGs across the human retina

We recorded L- and M-cone isolating ERGs from human subjects using a silent substitution technique at temporal rates of 12 and 30 Hz. These frequencies isolate the activity of cone-opponent and non-opponent post-receptoral mechanisms, respectively. ERGs were obtained using a sequence of stimuli with different spatial configurations comprising; (1) circular stimuli of different sizes which increased in 10° steps up to 70°diameter, or (2) annular stimuli with a 70° outer diameter but with different sized central ablations from 10° up to 60°. L- and M-cone isolating ERGs were obtained from five colour normal subjects using a DTL fibre electrode. Fourier analysis of the ERGs was performed and we measured the amplitude of the first harmonic of the response. For 12 Hz ERGs the L:M cone response amplitude ratio (L:M(ERG)) was close to unity and remained stable irrespective of the spatial configuration of the stimulus. The maintenance of this balanced ratio points to the existence of cone selective input across the human retina for the L-M cone opponent mechanism. For 30 Hz the L:M(ERG) ratio was greater than unity but varied depending upon which region of the retina was being stimulated. This variation we consider to be a consequence of the global response properties of M-cone ERGs rather than representing a real variation in L:M cone ratios across the retina.

Spatial receptive field properties of lateral geniculate cells in the owl monkey (Aotus azarae) at different contrasts: a comparative study

Several physiological properties of owl monkey lateral geniculate nucleus (LGN) cells were studied to verify whether its nocturnal habit has an influence on the organization of its subcortical visual system. Receptive field (RF) dimensions were measured using drifting gratings and bipartite field stimuli. We found that owl monkey cells LGN have larger RFs and were on average more non-linear than those of diurnal monkeys. But, as in other anthropoids, there is an increase in RF centre size with increasing eccentricity, and there is a limited correlation between these centre sizes and retinal ganglion cell dendritic tree sizes. The influence of contrast on sizes and peak sensitivities of RF centres and surrounds and on the response phases was studied. Both the sizes and peak sensitivities of the RF centres and surrounds decrease as contrast increases. As a result, the responses to low spatial frequency stimuli saturate with increasing contrast. Estimates of contrasts at half-maximal responses confirm the presence of saturation. It was found that the magnocellular cells saturate more strongly than parvocellular cells. The response phase increases with increasing contrast. These data resemble those obtained in the common marmoset, indicating that these are basic features of the primate visual system. We conclude that during evolution and as an adaptation to a nocturnal lifestyle, cells in the owl monkey LGN display an increased spatial integration in comparison with diurnal primate species, without a change in the basic organization common to the primate subcortical visual system.

Dendritic thickness: a morphometric parameter to classify mouse retinal ganglion cells

To study the dendritic morphology of retinal ganglion cells in wild-type mice we intracellularly injected these cells with Lucifer yellow in an in vitro preparation of the retina. Subsequently, quantified values of dendritic thickness, number of branching points and level of stratification of 73 Lucifer yellow-filled ganglion cells were analyzed by statistical methods, resulting in a classification into 9 groups. The variables dendritic thickness, number of branching points per cell and level of stratification were independent of each other. Number of branching points and level of stratification were independent of eccentricity, whereas dendritic thickness was positively dependent (r = 0.37) on it. The frequency distribution of dendritic thickness tended to be multimodal, indicating the presence of at least two cell populations composed of neurons with dendritic diameters either smaller or larger than 1.8 microm ("thin" or "thick" dendrites, respectively). Three cells (4.5%) were bistratified, having thick dendrites, and the others (95.5%) were monostratified. Using k-means cluster analysis, monostratified cells with either thin or thick dendrites were further subdivided according to level of stratification and number of branching points: cells with thin dendrites were divided into 2 groups with outer stratification (0-40%) and 2 groups with inner (50-100%) stratification, whereas cells with thick dendrites were divided into one group with outer and 3 groups with inner stratification. We postulate, that one group of cells with thin dendrites resembles cat beta-cells, whereas one group of cells with thick dendrites includes cells that resemble cat alpha-cells.

Photoreceptor topography and cone-specific electroretinograms

It is implicit in many cone-specific ERG studies that the amplitude is proportional to the numbers of cones stimulated. The objective of these experiments was to test this idea by comparing ERGs obtained from different areas of the retina with histological data on cone-density distributions. The histology (Curcio et al., 1990) shows that the cumulative number of cones in the human retina increases exponentially with stimulus diameter between 0- and 40-deg eccentricity. L-, M-, and (L+M) cone-driven 30-Hz ERGs were obtained from a series of stimuli with one of the following configurations: (1) Circular stimuli of different angular subtense up to 70-deg diameter. (2) Annuli with 70-deg outer diameter but variable inner diameter. (3) Annuli of constant area but increasing eccentricity. Cone contrasts were equalized for each stimulus condition. The modulated and nonmodulated regions of the screen had the same mean hue and luminance. The data suggest that the L+M cone ERG amplitude increases with stimulus diameter in direct proportion to the estimated number of cones stimulated. Furthermore, the total L+M responses appear to be predicted from individual L and M responses by simple linear summation for both the disc and annular stimuli.

M-cone opsin gene number does not correlate with variation in L/M-cone sensitivity

Molecular genetic studies demonstrate that the human cone opsin gene array on the q-arm of the X-chromosome typically consists of one long-wave-sensitive (L) cone opsin gene and from one to several middle-wave-sensitive (M) cone opsin genes. Although the presence of the single L-cone opsin gene and at least one M-cone opsin gene is essential for normal red-green colour discrimination, the function of the additional M-cone opsin genes is still unclear. To investigate whether any variations in phenotype correlate with differences in the number of M-cone opsin genes, we selected 13 normal trichromat males, for whom four independent molecular techniques have exactly determined their number of M-cone opsin genes, ranging from one to four. Their phenotype was characterized by estimating their foveal L- to M-cone ratio from heterochromatic flicker photometric (HFP) thresholds, by measuring the wavelength corresponding to their 'unique yellow', and by determining their L- and M-cone modulation thresholds (CMTs). No correlation was found between these psychophysical measures and the number of M-cone opsin genes. Although, we found a reasonably good correlation between the L/M-cone ratios based on HFP and on CMT, we did not find any correlation between the estimated L/M-cone ratios and the settings of 'unique yellow'. Our results accord with previous molecular genetic studies that suggest that only the first two genes in the X-linked opsin gene array are expressed.

L- and M-cone-driven electroretinograms in Stargardt's macular dystrophy-fundus flavimaculatus

PURPOSE

To study the dynamics of the long (L)- and middle (M)-wavelength-sensitive cone-driven pathways and their interactions in patients with Stargardt's macular dystrophy-fundus flavimaculatus (SMD-FF) and to correlate them with other clinical parameters and individual genotypes.

METHODS

Forty-seven patients with SMD-FF participated in the study. In addition to standard 30-Hz flicker electroretinograms (30-Hz fERG), ERG responses were measured to stimuli that modulated exclusively the L or the M cones (L/M cones) or the two simultaneously. Blood samples were screened for mutations in the 50 exons of the ABCA4 gene.

RESULTS

Patients with SMD-FF did not show a decrease in the mean L/M-cone-driven ERG sensitivity, but there was a significant increase in the interindividual variability. The mean L-/M-cone weighting ratio was normal. However, the L-cone-driven ERG was significantly phase delayed, whereas the M-cone-driven ERG was significantly phase advanced. These phase changes were significantly correlated with disease duration. The amplitude and implicit time of the standard 30-Hz fERG both correlated significantly with the L/M-cone-driven ERG sensitivity and with the phase difference between the L/M-cone-driven ERGs, indicating the complex origin of the standard 30-Hz fERG. Probable disease-associated mutations in the ABCA4 gene were found in 40 of 45 patients, suggesting that they form a genetically fairly uniform SMD-FF study group. There was no correlation between the genotype and the L/M-cone-driven ERGS:

CONCLUSIONS

The changes in L/M-cone-driven ERG sensitivity and phase possibly represent two independent disease processes. The phase changes are similar to those found in patients with retinitis pigmentosa and possibly are a general feature of retinal dystrophies.

Rod-/L-cone and rod-/M-cone interactions in electroretinograms at different temporal frequencies

We recorded electroretinograms (ERGs) under stimulus conditions that only modulated one cone type: either the L- or the M-cones. In these conditions the rods were also modulated. We measured the ERG responses at different temporal frequencies. A simple model that assumes that the first harmonic components of the responses are the result of a vector addition of rod- and cone-driven ERG responses can explain the data satisfactorily for temporal frequencies at and above 6 Hz. From fits of the model to the data, estimates of the gains and phases of the rod- and cone-driven responses can be obtained. At 6 Hz, the fundamental responses originate exclusively in the rods. The gains of the cone-driven responses are substantial at the other temporal frequencies, are maximal at 12 or 18 Hz, and then decrease with increasing temporal frequencies. The gains of the rod-driven responses decrease more steeply with increasing temporal frequencies than the cone gains. Furthermore, the rod and cone phases decrease approximately linearly with temporal frequency, suggesting that they are mainly determined by a response delay. The response delay in the rod-driven ERGs is larger than in the cone-driven ERGs.

Macular dystrophy with protan genotype and phenotype studied with cone type specific ERGs

PURPOSE

To determine the L- and M-cone driven ERG responses in a male patient with macular dystrophy and a protan phenotype.

METHODS

We measured large field ERG thresholds to stimuli which modulated exclusively the L- or the M-cones or the two in various combinations (both in-phase and in counterphase). In none of the stimuli, the S-cones were modulated. Additionally, standard and multifocal ERGs were measured. Analysis of the L- and M-cone pigment genes was performed by means of PCR, RFLP analysis and DNA sequencing techniques.

RESULTS

Macular dystrophy was revealed by the markedly abnormal multifocal ERGs in presence of near normal standard ERGs. The large field ERG responses were exclusively driven by the M-cones with enlarged thresholds when compared with otherwise normal protanopes. In addition, the M-cone driven ERG response phases were abnormal. Pigment gene analysis confirmed a protan genotype with the presence of a single 5'red/3'green hybrid pigment gene.

CONCLUSIONS

Our novel stimulus technique allows a reliable analysis of the separate cone pathways even in cases with macular dysfunction. The increased thresholds and the abnormal phase behavior of the M-cone driven ERGs reflect altered mechanisms of the retinal physiology in this patient. The data strongly suggest that the macular dystrophy and the protanopia have independent origins.

L- and M-cone driven ERGs are differently altered in Best's macular dystrophy

To study the L- and M-cone pathways and their interactions in patients with Best's macular dystrophy (BMD), ERG response thresholds were measured to stimuli which modulated exclusively the L- or the M-cones, or both in various combinations. The ERG threshold data could be described with a vector addition model. Compared with normals, BMD patients showed generally larger amplitudes of the L-cone driven ERGs. However, the M-cone driven ERGs were similar in amplitude but significantly phase advanced. The data confirm our previous observations that L- and M-cone pathways can be affected differently by retinal degeneration, despite their large physiological and biochemical similarities.

Influence of contrast on the responses of marmoset lateral geniculate cells to drifting gratings

The responses of lateral geniculate nucleus (LGN) cells in the common marmoset (Callithrix jacchus) to drifting luminance or cone isolating gratings of different spatial frequencies and contrasts were measured. The response noise, defined as the variability of the responses to single sweeps in the complex plane, was independent of stimulus contrast and spatial frequency but increased with increasing overall responsiveness of the cell. The signal-to-noise ratio of parvocellular (PC) cells was smaller than of magnocellular (MC) cells. At each contrast, the response amplitude as a function of spatial frequency could be described with a difference of Gaussians model. With this model, the sizes and the peak sensitivities of the receptive field centers and surrounds were estimated. It was found that receptive field center and surround sizes of LGN cells decrease slightly with increasing contrast. Further, the peak sensitivity decreases with increasing contrast. The two factors are involved in a decrease in responsivity (the response per unit contrast) with increasing contrast which is compatible to response saturation for low spatial frequency stimuli. PC cells did not saturate as much to luminance stimuli although some saturation was found with cone isolating gratings. We found that the response phase lag of both PC and MC cells decreased with increasing contrast, which cannot be explained on the basis of linear response behavior. Apparently the phase of LGN cell responses to drifting gratings is altered in comparison with the retinal inputs by additional nonlinearities.

Visual responses of ganglion cells of a New-World primate, the capuchin monkey, Cebus apella

1. The genetic basis of colour vision in New-World primates differs from that in humans and other Old-World primates. Most New-World primate species show a polymorphism; all males are dichromats and most females trichromats. 2. In the retina of Old-World primates such as the macaque, the physiological correlates of trichromacy are well established. Comparison of the retinae in New- and Old-World species may help constrain hypotheses as to the evolution of colour vision and the pathways associated with it. 3. Ganglion cell behaviour was recorded from trichromatic and dichromatic members of a New-World species (the capuchin monkey, Cebus apella) and compared with macaque data. Despite some differences in quantitative detail (such as a temporal response extended to higher frequencies), results from trichromatic animals strongly resembled those from the macaque. 4. In particular, cells of the parvocellular (PC) pathway showed characteristic frequency-dependent changes in responsivity to luminance and chromatic modulation, cells of the magnocellular (MC) pathway showed frequency-doubled responses to chromatic modulation, and the surround of MC cells received a chromatic input revealed on changing the phase of heterochromatically modulated lights. 5. Ganglion cells of dichromats were colour-blind versions of those of trichromats. 6. This strong physiological homology is consistent with a common origin of trichromacy in New- and Old-World monkeys; in the New-World primate the presence of two pigments in the middle-to-long wavelength range permits full expression of the retinal mechanisms of trichromatic vision.

Large phase differences between L-cone- and M-cone-driven electroretinograms in retinitis pigmentosa

PURPOSE

To study the dynamics and interactions of the signals originating in the long- (L-) and middle (M)-wavelength-sensitive cone pathways in patients with retinitis pigmentosa (RP).

METHODS

Twenty-six patients with RP and 29 normal subjects participated in the study. Electroretinographic (ERG) responses were measured to stimuli that modulated exclusively the L- or the M-cones or the two simultaneously (both in-phase and in counter-phase) with varying ratios of L- to M-cone contrasts. S-cones were not modulated.

RESULTS

The data of the normal subjects and of the patients can be described by a model in which the amplitudes and the phases of the signals originating in the L- and M-cones are vector summed. In the RP patients, there was a general reduction in ERG sensitivity. The L-cone-driven ERG response was significantly delayed, whereas the M-cone-driven ERG response was phase advanced.

CONCLUSIONS

Large dynamic differences between L- and M-cone-driven ERGs can be detected in RP. As a result, the interaction between the L- and M-cone systems, when modulated simultaneously at 30 Hz, is subtractive in RP patients and additive in normal subjects. Our data show that the use of only a standard white flicker ERG might lead to a misinterpretation of the mechanisms involved in retinal disorders, because the phases of different cone-driven responses are not considered.

L/M cone ratios in human trichromats assessed by psychophysics, electroretinography, and retinal densitometry

Estimates of the relative numbers of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones vary considerably among normal trichromats and depend significantly on the nature of the experimental method employed. Here we estimate L/M cone ratios in a population of normal observers, using three psychophysical tasks-detection thresholds for cone-isolating stimuli at different temporal frequencies, heterochromatic flicker photometry, and cone contrast ratios at minimal flicker perception--as well as flicker electroretinography and retinal densitometry. The psychophysical tasks involving high temporal frequencies, specifically designed to tap into the luminance channel, provide average L/M cone ratios that significantly differ from unity with large interindividual variation. In contrast, the psychophysical tasks involving low temporal frequencies, chosen to tap into the red-green chromatic channel, provide L/M cone ratios that are always close to unity. L/M cone ratios determined from electroretinographic recordings or from retinal densitometry correlate with those determined from the high-temporal-frequency tasks. These findings suggest that the sensitivity of the luminance channel is directly related to the relative densities of the L and the M cones and that the red-green chromatic channel introduces a gain adjustment to compensate for differences in L and M cone signal strength.

Spatial and temporal response properties of the major retino-geniculate pathways of Old and New World monkeys

Old World monkeys, apes and humans all enjoy trichromatic colour vision, and the absorption spectra of the photopigments are very similar in all species and all individuals. Colour vision in New World monkeys however, is very heterogeneous. In many species, the majority of individuals is dichromatic. Recently, anatomical and electrophysiological studies revealed that the retinal organisation in Old World monkeys and New World monkeys is very similar, although the cells belonging to the parvocellular pathway do not show any colour opponency and their spectral sensitivity is similar to that of the magnocellular cells. Apparently, the magnocellular and parvocellular pathways in the retina and the LGN have not developed as an adaptation to luminance and chromatic processing. It is more likely that the two pathways originally evolved to cover different ranges in the spatio-temporal domain. In the present paper, several spatial and temporal properties of parvo- and magnocellular cells (which are identical for dichromatic and trichromatic animals) are compared.

Rod-cone-interactions in deuteranopic observers: models and dynamics

We studied the interactions between rods and L-cones in deuteranopic human observers by stimulating the photoreceptors independently. Thresholds were determined using a PEST procedure for different ratios of rod to L-cone modulation without modulating the S-cones. Modulation frequency was either 2 or 10 Hz and the retinal illuminance ranged from 4.7 to 470 td (10.9-1090 scot td). We measured at 2, 7.5 and 20 degrees retinal eccentricity. The threshold data could be described by a model based on a vector addition of responses originating in the rods and the L-cones. The relative strength of rod signals relative to the L-cone signals increased with increasing retinal eccentricity and decreasing retinal illuminance. At 20 degrees eccentricity, rod and cone signals were of about equal magnitude at retinal illuminances as high as 470 td. Temporal frequency did not have a large effect on the ratio of rod to L-cone signal strength.

Cone signal contributions to electroretinograms [correction of electrograms] in dichromats and trichromats

PURPOSE

To find out how the different cone types contribute to the electroretinogram (ERG) by quantifying the contribution of the signal pathways originating in the long (L-) and the middle (M-) wavelength-sensitive cones to the total ERG response amplitude and phase.

METHODS

ERG response amplitudes and phases were measured to cone-isolating stimuli and to different combinations of L- and M-cone modulation. Conditions were chosen to exclude any contribution of the short wavelength-sensitive (S-) cones. The sensitivity of the ERG to the L and the M cones was defined as the cone contrast gain.

RESULTS

In the present paper, a model is provided that describes the ERG contrast gains and ERG thresholds in dichromats and color normal trichromats. For the X-chromosome-linked dichromats, the contrast gains of only one cone type (either the L or the M cones) sufficed to describe the ERG thresholds for all stimulus conditions. Data suggest that the M-cone contrast gains of protanopes are larger than the L-cone contrast gains of deuteranopes. The response thresholds of the trichromats are modeled by assuming a vector summation of signals originating in the L and the M cones. Their L- and M-cone contrast gains are close to a linear interpolation of the data obtained from the dichromats. Nearly all trichromats had larger L- than M-cone contrast gains. Data from a large population of trichromats were examined to study the individual variations in cone weightings and in the phases of the cone pathway responses.

CONCLUSIONS

The data strongly suggest that the missing cone type in dichromats is replaced by the remaining cone type. The mean L-cone to M-cone weighting ratio in trichromats was found to be approximately 4:1. But there is a substantial interindividual variability between trichromats. The response phases of the L- and the M-cone pathways can be reliably quantified using the response phases to the cone-isolating stimuli or using a vector addition of L- and M-cone signals.

Ganglion cells of a short-wavelength-sensitive cone pathway in New World monkeys: morphology and physiology

We have studied the morphology and physiology of retinal ganglion cells of a short-wavelength-sensitive cone (SWS-cone) pathway in dichromatic and trichromatic New World anthropoids, the capuchin monkey (Cebus apella) and tufted-ear marmoset (Callithrix jacchus). In Old World anthropoids, in which males and females are both trichromats, blue-ON/yellow-OFF retinal ganglion cells have excitatory SWS-cone and inhibitory middle- and long-wavelength-sensitive (MWS- and LWS-) cone inputs, and have been anatomically identified as small-field bistratified ganglion cells (SB-cells) (Dacey & Lee, 1994). Among retinal ganglion cells of New World monkeys, we find SB-cells which have very similar morphology to such cells in macaque and human; for example, the inner dendritic tree is larger and denser than the outer dendritic tree. We also find blue-on retinal ganglion cells of the capuchin to have physiological responses strongly resembling such cells of the macaque monkey retina; for example, responses were more sustained, with a gentler low frequency roll-off than MC-cells, and no evidence of contrast gain control. There was no difference between dichromatic and trichromatic individuals. The results support the view that SWS-cone pathways are similarly organized in New and Old World primates, consistent with the hypothesis that these pathways form a phylogenetically ancient color system.

Response phase of the flicker electroretinogram (ERG) is influenced by cone excitation strength

We measured electroretinogram (ERG) response phases at different cone contrasts in trichromats and dichromats to investigate the dynamics of the long-wavelength-sensitive (L-) and middle-wavelength-sensitive (M-) cone pathways. ERG responses to stimuli, temporally modulated at 30 Hz, were recorded. The stimuli were generated on a computer controlled colour monitor. Thirty-two different combinations of L- and M-cone excitation strength, expressed as cone contrasts, were presented. The short-wavelength-sensitive (S-) cones were not stimulated (S-cone contrast = 0%). The response phase of the fundamental stimulus component was obtained from Fourier analysis. The ERG response phase lags decreased with increasing cone contrast. This was observed in all subjects with a normal appearing fundus. In dichromats and trichromats at low and intermediate contrasts, the phase lags to M-cone isolating conditions were smaller than those to L-cone isolating stimuli. In one dichromat with extreme myopia and cupping of the optic disc, the ERG phase lags increased with increasing cone contrast. The ERG response phase may be potentially useful for detecting retinal abnormalities.

Comparative retinal physiology in anthropoids

During the last decade it has become clear that colour vision in platyrrhines (New World monkeys) differs from the uniform trichromatic pattern normally found in catarrhines (Old World monkeys, apes and human). Colour vision in most platyrrhine species is polymorphic, with many dichromatic individuals. The comparison of response properties in retinal ganglion cells and lateral geniculate cells between catarrhines and playrrhines elucidates how the evolution of trichromatic colour vision influenced the post-receptoral processing. We find that spatial and temporal processing is very similar in the platyrrhine and catarrhine retina, strongly suggesting that the retinal structure and function, found in living anthropoids, was already present in their common ancestor.

Flicker cone electroretinogram in dichromats and trichromats

To measure cone signal strengths in the flicker electroretinogram (ERG) of dichromats and trichromats, we developed a set of flickering stimuli (30 Hz), which excite the middle-wavelength-sensitive (M-) and long-wavelength-sensitive (L-) cones independently. ERG responses to eight different ratios of L- to M-cone contrasts were recorded from each subject. The short-wavelength-sensitive (S-) cone contrast was 0% in all measurements. The recordings were Fourier analyzed to determine the amplitude of the fundamental component. ERG threshold values for each subject resulted in ellipses when plotted in an L-/M-cone contrast space. As expected, the orientations of the threshold ellipses of the protanopes (N = 2) were parallel to the L-cone axis, whereas those of the deuteranopes (N = 2) were parallel to the M-cone axis. For the trichromats (N = 5), there was considerable interindividual variation in ellipse orientation.

Post-receptoral mechanisms of colour vision in New World primates

Diurnal platyrrhines, both di- and trichromats, have magnocellular (M-) and parvocellular (P-) retinal ganglion cells which are morphologically very similar to those found in catarrhines. Catarrhine central P ganglion cells contact single midget bipolar cells, which contact single cones. Physiological recordings of retinal ganglion cells of dichromatic Cebus monkeys showed very similar cell properties to the catarrhine macaque, except that P ganglion cells lacked colour-opponency. We describe the presence of single-headed midget bipolar cells in the Cebus retina. These midget bipolar cells have axon terminal sizes in the same range as the dendritic tree sizes of P ganglion cells as far as 2 mm of retinal eccentricity. This result supports the view that, as in catarrhines, central P ganglion cells of platyrrhines receive input from single midget bipolar cells which in turn, receive input from single cones. This finding is consistent with the idea that a P pathway with one-to-one connectivity was present in the anthropoid ancestor before the divergence between catarrhines and platyrrhines.

Interaction between rod and cone signals in responses of lateral geniculate neurons in dichromatic marmosets (Callithrix jacchus)

Parvocellular (P-) and magnocellular (M-) cells in the marmoset LGN can receive prominent rod input up to relatively high illuminance levels (Kremers et al., 1997b). In the present paper, we quantify rod and cone input strengths under different retinal illuminance levels. The stimulus was based on the so-called "silent substitution" method. The activities of P- and M-cells of dichromatic animals were recorded extracellularly. We were able to adequately describe the response amplitudes and phases by a vector summation of rod and cone signals. At low retinal illuminance levels, the cells' responses were determined by rod and cone inputs. With increasing illuminances the strength of the cone input increased relative to the rod strength. But, we often found significant rod inputs up to illuminances equivalent to 700 td in the human eye or more. Rod input strength was more pronounced in cells with receptive fields at large retinal eccentricities. The phase differences between rod and cone inputs suggest that the rod signals lag about 45 ms behind the cone signals.

Receptive fields of primate retinal ganglion cells studied with a novel technique

We have reinvestigated receptive-field structure of ganglion cells of the macaque parafovea using counterphase modulation of a bipartite field. Receptive fields were mapped with luminance, chromatic, and cone-isolating stimuli. Center sizes of middle (M) and long (L) wavelength cone opponent cells of the parvocellular (PC) pathway were consistent with previous estimates (Gaussian radii of 2-4 min of arc, corresponding to center diameters of 6-12 min of arc). We calculate that a large factor of the enlargement relative to cone radius could be blur due to the eye's natural optics. Maps were consistent with cone selectivity in surround mechanisms, which had radii of 5-8 min of arc. For magnocellular (MC) cells, center size estimates were also consistent with grating measurements from the literature (also Gaussian radii of 2-4 min of arc). The surround mechanism contributing the MC-cell frequency-doubled response to chromatic modulation appears to possess a subunit structure, and we speculate it derives from nonlinear summation of signals from M,L-cone opponent subunits, such as midget bipolar cells.

Rod inputs to macaque ganglion cells

The strength of rod inputs to ganglion cells was assessed in the macaque retina at retinal positions within 3-15 deg eccentricity. The experimental paradigm used temporally modulated heterochromatic lights whose relative phase was varied. This paradigm provided a sensitive test to detect rod input. In parvocellular (PC) pathway cells, the gain of the cone-driven signal decreased with decrease in luminance. At 2 td a weak rod response, of a few impulses per second for 100% rod modulation, was revealed in about 60% of cells. For blue-on cells, the cone-driven response also decreased with retinal illuminance, but no rod response could be found. In magnocellular (MC) pathway cells, rod input was much more apparent. Responses became rod dominated at and below 20 td; we cannot exclude rod intrusion at higher retinal illuminances. Responsivity was maintained even at low retinal illuminances. Temporal-frequency dependent rod-cone interactions were observed in MC-pathway cells. Rod responses were of longer latency than cone responses, but there was no evidence of any difference in rod latency between parvocellular and magnocellular pathways.

Temporal properties of marmoset lateral geniculate cells

We measured the temporal modulation transfer functions (TMTFs) of cells in the marmoset lateral geniculate nucleus (LGN) at three different luminance levels, and described the responses with a linear model. It was found that qualitatively there are many similarities with the temporal response properties of macaque and marmoset retinal ganglion cells. M-cells displayed stronger attenuation at lower temporal frequencies, and showed more nonlinearities (such as saturation and a contrast gain control) than P-cells. We therefore propose that the temporal properties of the visual system of New and Old World monkeys are similar at least up to the LGN. However, there are some quantitative differences, indicating that response alterations take place at the stage of synaptic transmission in the LGN. The most important are an attenuation of the responses to higher temporal frequencies and the smaller differences between parvo- and magnocellular cell responsivities. Cell responses to square-wave modulation were also measured and compared with predictions from a linear systems analysis. The linear systems analysis gave reasonable predicted responses to square-wave modulation, but these predictions were poor than those for retinal ganglion cells, indicating that additional nonlinearities are introduced at the synaptic transition in the LGN.

A New Look at Primate Ganglion Cell Receptive Field Structure

We have reinvestigated receptive field structure of macaque ganglion cells using a novel stimulus—counterphase modulation of a bipartite field—with luminance, chromatic, or cone-selective stimulation. We previously used this stimulus (Kremers et al, 1995 Colour Vision DeficienciesXII 399 – 406) to show that surrounds of middle (M) and long (L) wavelength cone opponent cells of the parvocellular (PC) pathway are cone specific, measuring on a finer scale than in previous experiments [Reid and Shapley, 1992 Nature (London)356 716 – 718]. Modelling of response amplitude and phase now confirms this conclusion. Second, centre sizes measured were consistent with those from the literature, with PC and magnocellular pathway (MC) cell centres having similar size. Modelling on the basis of single cone centres for PC-cells plus optical blur provided a partial description of the data, but some inconsistencies were present. Last, the chromatic nonlinearity of magnocellular pathway ganglion cells appears to result from a subunit structure within the receptive-field surround.

Receptive field dimensions of lateral geniculate cells in the common marmoset (Callithrix jacchus)

We measured the spatial receptive field dimensions of cells in the lateral geniculate nucleus (LGN) of the common marmoset (Callithrix jacchus) using a bipartite field stimulus in which the two halves of the field were modulated identically but in counterphase. Horizontal and vertical edges between the two fields were positioned at different locations in the receptive field. By assuming that centers and surrounds have gaussian profiles, we were able to obtain a satisfactory mathematical description of the data. Receptive field centers were about a factor 1.6 larger than those of macaque LGN cells, in accordance with the smaller marmoset eye. There was a limited correspondence with dendritic tree dimensions of marmoset retinal ganglion cells. We further found that center and surround gaussians were not always concentric, and that the centers of some cells were elongated. This might allow some direction or orientation biases in LGN cells.

Parallel pathways in the retina of Old and New World primates

Old-world simians are all trichromats, but in most new-world primates there is a polymorphism; males are dichromats but most females are trichromats. In the old world simian, luminance and red-green chromatic channels defined by psychophysical experiments have as a basis parasol ganglion cells of the magnocellular (MC) pathway and midget ganglion cells of the parvocellular (PC) pathway respectively. Small bistratified ganglion cells provide a basis for a blue-yellow chromatic channel, which should probably be considered a separate entity. In both dichromatic and trichromatic new-world animals, the MC pathway and the small bistratified, blue-yellow system seem anatomically and physiologically similar to those in their old-world relatives. The midget ganglion cells of the parvocellular pathway in trichromats are anatomically and physiologically similar to the old-world pattern. In dichromatic animals, they are anatomically similar and physiologically resemble those of trichromatic animals, except for the lack of chromatic opponency. We conclude that these three systems may from a basic pattern for the visual pathway of primates. However, the results from dichromats indicate that the evolution of trichromacy may be found to be more complex than presently supposed.

The time course of adaptation in macaque retinal ganglion cells

The time course of adaptation of cells of the parvocellular (PC) and magnocellular (MC) pathways has been characterized following changes in retinal illuminance or chromaticity. Adaptation state was cycled between high and low luminance levels or between backgrounds with wavelengths metameric to 630 and 570 nm. Cell responsivity was probed with brief bursts of luminance or chromatic modulation. After a change in luminance, adaptation of both MC-cells (tested with a luminance probe) and red-green PC-cells (tested with a chromatic probe) was relatively rapid and largely complete within 100 msec or less. After a change in chromaticity, recovery of responsivity in red-green PC-cells was dependent on cell type. Recovery of responsivity with backgrounds elevating maintained firing was complete within a few seconds, but with backgrounds suppressing cell firing, recovery took many tens of seconds. This very slow time course may be due to a threshold effect. In experiments with backgrounds which selectively adapted one cone type, use of cone-isolating probes indicated that the time course of PC-cell chromatic adaptation may be determined at a site after the subtraction of cone signals. Recovery of responsivity of MC-cells was also prolonged over several seconds following a chromatic change. Our data suggest that adaptation in macaque ganglion cells depends on mechanisms both before and after the site of cone interaction, and that these mechanisms may differ in time course between MC- and PC-cells. The results indicate that it may be important in psychophysical adaptation experiments to consider the presence of multiple postreceptoral mechanisms with different adaptation characteristics.

Visual responses in the lateral geniculate nucleus of dichromatic and trichromatic marmosets (Callithrix jacchus)

New-world primates such as the marmoset (Callithrix jacchus) show polymorphism for the middle- to long-wavelength sensitive cone pigments. Each X-chromosome carries a gene for only one of three possible pigments. All males are thus dichromats, but some females will be trichromats. We have investigated the responses of cells of the parvocellular (PC) and magnocellular (MC) systems within animals from a single marmoset family. The middle- to long-wavelength pigment of dichromats was identified physiologically. Trichromats could readily be distinguished from dichromats by the presence of a high proportion of red-green opponent PC-cells. The physiological classification of phenotypes was confirmed with genetic analysis. The pattern of inheritance was consistent with current genetic models. In trichromatic females, the properties of cells resembled in detail those of cells from the PC- and MC-pathways of the macaque. In dichromats, cell responses resembled those of trichromats (except for the lack of opponency in PC-cells); PC-cells showed sustained and MC-cells transient responses, with a lower contrast gain for the former type. One difference was that a proportion of PC-cells in dichromats showed strong rod input even at high levels of retinal illuminance. Thus, in trichromatic marmosets the presence of two middle- to long-wave pigments appears to permit the elaboration of all the physiological properties associated with trichromacy.

The spatial precision of macaque ganglion cell responses in relation to vernier acuity of human observers

Responses of parafoveal macaque ganglion cells were measured as a function of the contrast and position of an edge flashed within their receptive fields. The goal was to determine the ability of different cell types to signal edge location. For comparison, parafoveal vernier thresholds of human observers were measured with pairs of flashed edges. Cells of the magnocellular (MC-) pathway gave larger responses than cells of the parvocellular (PC-) pathway. Neurometric analyses comparing a cell's response at different edge positions were performed. The positional signal from single MC-pathway cells was more precise than from PC-pathway cells, especially at lower contrasts. In a second analysis, based on the neurophysiological results, responses from a matrix of ganglion cells were generated. Using a simple model, vernier performance expected from such a matrix was predicted as a function of edge length and contrast. Again, the MC-pathway gave a more precise positional signal than the PC-pathway despite the latter's numerical advantage. At contrasts of 20% and below, only the MC-pathway would appear capable of supporting vernier performance with our stimuli. At higher contrasts either the MC- or PC-pathway could provide an adequate signal.

Temporal response of ganglion cells of the macaque retina to cone-specific modulation

The temporal response of cone inputs to macaque retinal ganglion cells were compared with cone-specific sinusoidal modulation used to isolate each cone type. For all cell types of the parvocellular (PC) pathway, temporal responsivity was similar for short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone inputs, apart from small latency differences between inputs to center and surround. The temporal response resembled that expected from receptor physiology. Responses of cells of the magnocellular pathway to M- or L-cone modulation showed more complex properties indicative of postreceptoral processing. Human psychophysical temporal-sensitivity functions were acquired with S-cone modulation under conditions similar to those for the physiological measurements. Ratios of psychophysical to physiological data from S-cone cells (the only cells that respond to this stimulus) yielded an estimate of the central filter acting upon PC-pathway signals. The filter characteristic could be described by a four-stage low-pass filter with corner frequency 3-5 Hz.

Responses to pulses and sinusoids in macaque ganglion cells

The goal of the study was to compare pulse responses with sinusoidal temporal responsivity. The response of macaque ganglion cells was measured to brief luminance and chromatic pulses and to luminance or chromatic sinusoidal modulation. To make both positive and negative lobes of the pulse response visible, responses to pulses of opposite polarity were combined to yield a linearized pulse response. Tests of superposition were used to evaluate the linearized pulse response to different combinations of pulse duration and Weber contrast. A prediction of the pulse response was derived using sinusoidal responsivity functions and Fourier synthesis. For ganglion cells of the parvocellular (PC) pathway, shape and absolute amplitude of linearized pulse responses corresponded well to the predicted responses over a range of pulse durations at 0.5 and 1.0 Weber contrast for both luminance and chromatic modulation. For ganglion cells of the magnocellular (MC) pathway, shape and amplitude of the linearized pulse responses and the predicted responses corresponded when the contrast-duration product was low. This correspondence held for luminance modulation over a thousand-fold range of retinal illuminance. For contrast-duration combinations that produced a more vigorous response, over 100 imp/sec, the linearized pulse responses of MC-pathway cells became larger and time-advanced relative to the linear prediction until saturation became apparent. Incorporation of high Michelson contrast responses in the Fourier synthesis captured the timing but not the amplitude of the linearized pulse response. The data suggest that a mechanism similar to a contrast gain control acts upon MC- but not PC-pathway-cells. The data confirm that use of linear modelling to describe temporal behaviour of retinal ganglion cells is appropriate for small signals.

The response of macaque ganglion cells and human observers to heterochromatically modulated lights: the effect of stimulus size

Psychophysical sensitivity of human observers closely resembles responsivity of retinal ganglion cells of the magnocellular (MC-) pathway as a function of the relative phase of heterochromatically modulated lights. The MC-pathway phase effect is absent if the receptive field centre alone is stimulated. Here we confirm this physiological result, and show that the psychophysical phase shift is also abolished with small stimuli. The space constant of the psychophysical effect is consistent with a surround diameter for MC-pathway cells in the fovea of about 50 min arc, about 10 times estimated centre diameter. On changing retinal illuminance, the amplitude of the physiological and psychophysical phase shifts also changed in a parallel manner. These experiments support the hypothesis that the physiological origin of psychophysical phase shifts is in the MC-pathway, and indicate the spatial frequency (c. 2c/deg) below which the psychophysical phase shift should become apparent.

Responses of macaque ganglion cells and human observers to compound periodic waveforms

We measured responses of macaque retinal ganglion cells to different periodic waveforms (sinusoidal, square, rapid-on and rapid-off sawtooth waveforms) for both luminance and equiluminant chromatic modulation. We analyzed the responses with a peak-to-trough detector. At low frequencies, on-center and off-center magnocellular (MC-) pathway cells showed a ten-fold higher responsivity to the rapid-on and rapid-off sawtooth respectively. Red-on (+L-M) and green-on (+M-L) parvocellular (PC-) pathway cells showed a four-fold greater responsivity to rapid red-on and rapid green-on equiluminant chromatic sawtooth waveforms respectively. At an equivalent retinal eccentricity, we measured psychophysical thresholds for luminance stimuli and chromatic stimuli. We concluded that luminance sawtooth sensitivities from psychophysics are consistent with selective detection through MC-pathway on- and off-center channels in the visual system. The differences between the compound periodic waveforms seen in the PC-pathway cell data did not occur in the psychophysics. In a second analysis, cell responses to sinusoidal modulation were used to predict the linear response to square-wave and sawtooth waveforms. PC-pathway cells showed linear temporal behavior over a wide range of contrasts, but MC-pathway cells displayed linear behavior only for low-contrast luminance modulation. Using these linear fits, we implemented a model incorporating central low-pass filtering in the MC- and PC-pathways before the peak-to-trough detector. This model captured better the time scale and relative sensitivity to periodic waveforms found in the psychophysical data.

Physiological mechanisms underlying psychophysical sensitivity to combined luminance and chromatic modulation

If psychophysical detection thresholds are plotted in a middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cone coordinate system, the shape of the contour can be used to infer underlying detection mechanisms. We measured responses of macaque ganglion cells to combine chromatic and luminance modulation and expressed our results in such an M,L-cone space. Our aim was to test whether, with the use of this space, readily separable luminance and chromatic psychophysical mechanisms might be expected from physiological data. For parvocellular pathway cells, detection contours approximated elongated ellipses with maximum responsivity to chromatic modulation. The degree of elongation decreased as temporal frequency increased. Responses could be well described by linear subtraction of M- and L-cone signals, with a phase delay of 1-3 deg/Hz. For cells of the magnocellular pathway, detection contours were more complex. Orientation was variable between cells and temporal frequency dependent, and a frequency-doubled component was evoked by chromatic modulation. In relation to psychophysical detection thresholds plotted in such a space, the properties of parvocellular-pathway cells were sufficiently linear and homogeneous to make it plausible that this pathway might form the substrate for a linear chromatic mechanism. The properties of magnocellular-pathway cells, however, indicate that, insofar as a psychophysical luminance mechanism is based on their activity, its signature in the M,L-cone contrast space would be more difficult to identify.