Comparison of red-green equiluminance points in humans and macaques: evidence for different L:M cone ratios between species

The human spectral luminosity function (V(lambda)) can be modeled as the linear sum of signals from long-wavelength-selective (L) and middle-wavelength-selective (M) cones, with L cones being weighted by a factor of approximately 2. This factor of approximately 2 is thought to reflect an approximate 2:1 ratio of L:M cones in the human retina, which has been supported by studies that allow for more direct counting of different cone types in the retina. In contrast to humans, several lines of retinally based evidence in macaques suggest an L:M ratio closer to 1:1. To investigate the consequences of differences in L:M cone ratios between humans and macaques, red-green equiluminance matches obtained psychophysically in humans (n = 11) were compared with those obtained electrophysiologically from single neurons in the extrastriate middle temporal visual area of macaques (M. mulatta, n = 5). Neurons in the middle temporal visual area were tested with sinusoidal red-green moving gratings across a range of luminance contrasts, with equiluminance being defined as the red-green contrast yielding a response minimum. Human subjects were tested under analogous conditions, by a minimally distinct motion technique, to establish psychophysical equiluminance. Although red-green equiluminance points in both humans and macaques were found to vary across individuals, the means across species differed significantly; compared with humans, macaque equiluminance points reflected relatively greater sensitivity to green. By means of a simple model based on equating the weighted sum of L and M cone signals, the observed red-green equiluminance points were found to be consistent with L:M cone ratios of approximately 2:1 in humans and 1:1 in macaques. These data thus support retinally based estimates of L:M cone ratios and further demonstrate that the information carried in the cone mosaic has functional consequences for red-green spectral sensitivity revealed perceptually and in the dorsal stream of visual cortex.     

Cost of cone coupling to trichromacy in primate fovea

Cone synaptic terminals couple electrically to their neighbors. This reduces the amplitude of temporally uncorrelated voltage differences between neighbors. For an achromatic stimulus coarser than the cone mosaic, the uncorrelated voltage difference between neighbors represents mostly noise; so noise is reduced more than the signal. Here coupling improves signal-to-noise ratio and enhances contrast sensitivity. But for a chromatic stimulus the uncorrelated voltage difference between neighbors of different spectral type represents mostly signal; so signal would be reduced more than the noise. This cost of cone coupling to encoding chromatic signals was evaluated using a compartmental model of the foveal cone array. When cones sensitive to middle (M) and long (L) wavelengths alternated regularly, and the conductance between a cone and all of its immediate neighbors was 1,000 pS (approximately 2 connexons/cone pair), coupling reduced the difference between the L and M action spectra by nearly fivefold, from about 38% to 8%. However, L and M cones distribute randomly in the mosaic, forming small patches of like type, and within a patch the responses to a chromatic stimulus are correlated. In such a mosaic, coupling still reduced the difference between the L and M action spectra, but only by 2.4-fold, to about 18%. This result is independent of the L/M ratio. Thus "patchiness" of the L/M mosaic allows cone coupling to improve achromatic contrast sensitivity while minimizing the cost to chromatic sensitivity.     

Spatial summation in human cone mechanisms from 0 degrees to 20 degrees in the superior retina

The maximum area of complete spatial summation (i.e., Ricco's area) for human short-wavelength-sensitive-(S-) and long-wavelength-sensitive- (L-) cone mechanisms was measured psychophysically at the fovea and at 1.5 degrees , 4 degrees , 8 degrees , and 20 degrees along the vertical meridian in the superior retina. Increment thresholds were measured for three observers by a temporal two-alternative forced-choice procedure. Test stimuli ranging from -0.36 to 4.61 log area (min2) were presented on concentric 12.3 degrees adapting and auxiliary fields, which isolated either an S- or an L-cone mechanism on the plateau of its respective threshold versus intensity function. Test flash durations were 50 and 10 ms for the S- and L-cone mechanisms, respectively. The data indicate that, from 0 degrees to 20 degrees, Ricco's area increases monotonically for the L-cone mechanism, is variable for the S-cone mechanism, and is larger for the S-cone mechanism than for the L-cone mechanism for essentially all retinal locations. This pattern of results most likely reflects differences in ganglion cell density and changes in neural convergence with retinal eccentricity.     

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.     

Representation of cone signals in the primate retina

Vision begins with specialized retinal circuits that encode diverse types of information. For Old World primates, these circuits sample three submosaics formed by cone photoreceptors sensitive to short, middle, and long wavelengths. For spatial acuity, the photon catch between any two cones is compared for discrimination of patterns as fine as the cone mosaic. For color vision, the photon catch between different cone types is compared for discrimination of fine spectral differences on the basis of hue. The retinal circuits for these two tasks differ at the synaptic level to form distinct representations of signals from the cone mosaic.     

Functional consequences of the relative numbers of L and M cones

Direct imaging of the retina by adaptive optics allows assessment of the relative number of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in living human eyes. We examine the functional consequences of variation in the relative numbers of L and M cones (L/M cone ratio) for two observers whose ratios were measured by direct imaging. The L/M cone ratio for the two observers varied considerably, taking on values of 1.15 and 3.79. Two sets of functional data were collected: spectral sensitivity measured with the flicker electroretinogram (ERG) and the wavelength of unique yellow. A genetic analysis was used to determine L and M cone spectra appropriate for each observer. Rayleigh matches confirmed the use of these spectra. We determined the relative strength of L and M cone contributions to ERG spectral sensitivity by fitting the data with a weighted sum of L and M cone spectra. The relative strengths so determined (1.06 and 3.38) were close to the cone ratios established by direct imaging. Thus variation in L/M cone ratio is preserved at the sites tapped by the flicker ERG. The wavelength of unique yellow varied only slightly between the two observers (576.8 and 574.7 nm). This small variation indicates that neural factors play an important role in stabilizing unique yellow against variation in the L/M cone ratio.     

Interindividual and topographical variation of L:M cone ratios in monkey retinas

We analyzed the ratio of L:M cone photopigment mRNA in the retinas of Old World monkeys, using the method of rapid polymerase chain reaction-single-strand conformation polymorphism. The L:M cone pigment mRNA ratio in whole retina ranged from 0.6 to 7.0, with a mean of approximately 1.6 (standard deviation, +/- 0.56; n = 26). There was no change in this ratio with eccentricity up to 9 mm (approximately 45 degrees), though the ratio was approximately 30% greater in temporal than in nasal retina. The mRNA ratios are in good agreement with the L:M cone ratio in these same retinas, inferred from electrophysiological recordings of cone signal gain in horizontal cell interneurons. The correlation between mRNA ratios and physiological cone gain ratio supports the conclusion that both measures reflect the relative number of L and M cones.     

Relative number of long- and middle-wavelength-sensitive cones in the human fovea

Flicker photometric measurements yield spectral sensitivity curves that are well fitted by sums of the spectral sensitivity curves of long-wavelength-sensitive (L) cones and middle-wavelength-sensitive (M) cones if the L cones are given twice the weight of the M cones. This result has been interpreted as implying that L cones are more numerous than M cones but is also consistent with a different numerical ratio, say, 1:1, and with the assignment of greater weight to the L cone input than to the M cone input by the mechanism subserving flicker photometry. Measurements of temporal sensitivity are presented for lights that modulate the inputs of either only the L cones or only the M cones. Sensitivity to modulation of the L cones is approximately twice that of modulation of the M cones at approximately 30 Hz, but that advantage disappears at approximately 2 Hz. Thus flicker sensitivity is equivocal with regard to cone numerosity. Electrophysiological, anatomical, and psychophysical evidence is reviewed, with particular weight placed on the statistics of color appearance of small, brief, monochromatic lights and on increment thresholds measured on the same observers. It is concluded that, in the central fovea, the ratio of L:M cone numbers is close to unity and may not be so variable as is usually supposed.     

Flicker-photometric electroretinogram estimates of L:M cone photoreceptor ratio in men with photopigment spectra derived from genetics

Relative proportions of long-wavelength-sensitive (L) to middle-wavelength-sensitive (M) cones were estimated by use of the flicker-photometric electroretinogram (ERG). It has been demonstrated that a major source of error in estimates of cone proportions from spectral luminosity functions is the known variation in the lambda(max) of the photopigments [Vision Res. 38, 1961 (1998)]. To correct for these errors, estimates of cone proportions were derived by use of individualized L-cone spectral sensitivity curves deduced from photopigment gene sequences from each subject. For some individuals this correction made a large difference in the estimated cone proportions compared with the value obtained when a fixed standard L cone was assumed. The largest discrepancy occurred in a man estimated to have 62% L cones (L:M ratio 1.6:1) when a standard L pigment was assumed but a value of 80% L cones (L:M ratio 4:1) when his individualized L-cone spectrum was used. From repeated measurements made with the ERG, it was determined that individual estimates of the relative L-to-M cone contributions, expressed as %L cones, are usually reliable within approximately 2%. The average L:M ratio for 15 male subjects was estimated at 2:1 (67% L cones). Previously, a large range of individual variability was reported for L:M ratios obtained from photometry. An unresolved issue concerns how much of the range might be attributed to error. Here efforts have been taken to markedly reduce measurement error. Nonetheless, a large range of individual differences persists. Estimated L:M ratios for individuals ranged from 0.6:1 to 12:1 (40% L to 92% L).     

Spatial order in short-wavelength-sensitive cone photoreceptors: a comparative study of the primate retina

We compared the spatial distribution of short-wavelength-sensitive (SWS or blue) cone photoreceptors in the retinas of eight primate species. The regularity of the SWS cone array was quantified with a statistic (packing factor) that varies between a random distribution (0) and a triangular array (1). We find wide variability among species, with packing factors varying between 0.06 and 0.3. The SWS cone array in at least two New World monkey species is indistinguishable from a random array. The SWS cone density gradient across the retina was measured in the capuchin monkey Cebus apella and the squirrel monkey Saimiri sciureus. Both species show a peak density of 5,000-8,000 cells/mm2 at the fovea and a 50-fold central-peripheral density gradient. In contrast to the wide variation in local regularity, the spatial density and the topography of SWS cones are well preserved across primates.     

Analysis of the optical field on the human retina from wavefront aberration data

Wave aberrations in the human eye are usually known with respect to the ideal spherical wavefront in the exit pupil. Using Kirchhoff's diffraction theory, we have derived a diffraction integral to compute the optical field on the retina from the wave aberration data. We have proposed a numerical algorithm based on the Stamnes-Spjelkavik-Pedersen (SSP) method to solve that integral. We have shown which approximations are admissible to reduce the complexity of the diffraction integral. In addition, we have compared our results with those of the conventional procedure used to compute intensities on the retina. We have found significant differences between our results and the conventional ones.     

Evidence for the contribution of S cones to the detection of flicker brightness and red-green

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection thresholds were determined for small patches in the isoluminant plane defined by flicker brightness. These stimuli were presented at an eccentricity of 40 arc min. By using color naming at the detection threshold, one can attribute different segments of the resulting detection ellipses to different chromatic mechanisms. Linear approximation of these segments provided an estimate for the contribution of the different cone types to the detection of red-green and blue-yellow. The results are consistent with the hypothesis that S cones contribute to the red-green mechanism with the same sign as that of the contribution from L cones. The blue-yellow mechanism very probably subtracts S-cone contrast from luminance contrast. The detection ellipse can be mapped into a circle in cone difference space. The base of this canonical transformation is a set of three cone fundamentals that differs from previously published estimates. Projecting the circle onto the three cone difference axes produces sinusoidal changes within the respective excitations. We propose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency.     

Postreceptoral chromatic-adaptation mechanisms in the red-green and blue-yellow systems using simple reaction times

Simple visual-reaction times (VRT) were measured for a variety of stimuli selected along red-green (L-M axis) and blue-yellow [S-(L + M) axis] directions in the isoluminant plane under different adaptation stimuli. Data were plotted in terms of the RMS cone contrast in contrast-threshold units. For each opponent system, a modified Piéron function was fitted in each experimental configuration and on all adaptation stimuli. A single function did not account for all the data, confirming the existence of separate postreceptoral adaptation mechanisms in each opponent system under suprathreshold conditions. The analysis of the VRT-hazard functions suggested that both color-opponent mechanisms present a well-defined, transient-sustained structure at marked suprathreshold conditions. The influence of signal polarity and chromatic adaptation on each color axis proves the existence of asymmetries in the integrated hazard functions, suggesting separate detection mechanisms for each pole (red, green, blue, and yellow detectors).     

Effect of wavelength on in vivo images of the human cone mosaic

In images of the human fundus, the fraction of the total returning light that comes from the choroidal layers behind the retina increases with wavelength [Appl. Opt. 28, 1061 (1989); Vision Res. 36, 2229 (1996)]. There is also evidence that light originating behind the receptors is not coupled into the receptor waveguides en route to the pupil [S. A. Burns et al., Noninvasive Assessment of the Visual System, Vol. 11 of 1997 Trends in Optics and Photonics Series, D. Yager, ed. (Optical Society of America, 1997), p. al; Invest. Ophthalmol. Visual Sci. 38, 1657 (1997)]. These observations imply that the contrast of images of the cone mosaic should be greatly reduced with increasing wavelength. This hypothesis was tested by imaging the light distributions in both the planes of the photoreceptors and the pupil at three wavelengths, 550, 650, and 750 nm, with the Rochester adaptive optics ophthalmoscope. Surprisingly, the contrast of the retinal images varied only slightly with wavelength. Furthermore, the ratio of the receptorally guided component to the total reflected light measured in the pupil plane was found to be similar at each wavelength, suggesting that, throughout this wavelength range, the scattered light from the deeper layers in the retina is guided through the receptors on its return path to the pupil.     

Resonance Raman imaging of macular pigment distributions in the human retina

We describe resonance Raman imaging (RRI) of macular pigment (MP) distributions in the living human eye. MP consists of the antioxidant carotenoid compounds lutein and zeaxanthin, is typically present in high concentrations in the healthy human macula relative to the peripheral retina, and is thought to protect this important central region from age-related macular degeneration. We demonstrate that RRI is capable of quantifying and imaging the spatially strongly varying MP distribution in the human retina. Using laser excitation of the MP molecules at 488nm, and sequential camera detection of light emitted back from the retina at the MP's strongest Raman peak position and at an off-peak position, RRI maps of MP are obtained at a resolution below 50microm within a fraction of a second per exposure. RRI imaging can be carried out with undilated pupils and provides a highly molecule-specific diagnostic imaging approach for MP distributions in human subjects.     

In vivo resonant Raman measurement of macular carotenoid pigments in the young and the aging human retina

We have used resonant Raman scattering spectroscopy as a novel, noninvasive, in vivo optical technique to measure the concentration of the macular carotenoid pigments lutein and zeaxanthin in the living human retina of young and elderly adults. Using a backscattering geometry and resonant molecular excitation in the visible wavelength range, we measure the Raman signals originating from the single- and double-bond stretch vibrations of the pi-conjugated molecule's carbon backbone. The Raman signals scale linearly with carotenoid content, and the required laser excitation is well below safety limits for macular exposure. Furthermore, the signals decline significantly with increasing age in normal eyes. The Raman technique is objective and quantitative and may lead to a new method for rapid screening of carotenoid pigment levels in large populations at risk for vision loss from age-related macular degeneration, the leading cause of blindness in the elderly in the United States.