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.

Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain

Physiological data have revealed characteristic contrast gain and temporal integration signatures of the magnocellular (MC) and the parvocellular (PC) pathways. The goal in this study was to find psychophysical correlates of these signatures. Psychophysical forced-choice, luminance pedestal discrimination data were collected with a stimulus-surround display. A 2.05 degrees four-square stimulus array was varied from 73 to 182 trolands (Td) in a larger 115-Td surround. When the stimulus array was pulsed briefly, discrimination thresholds showed a minimum at the surround retinal illuminance, increasing in a V shape when the stimulus array was incremental or decremental to the surround. When the stimulus array was presented continuously as a steady pedestal within the constant 115-Td surround, discrimination thresholds increased monotonically with stimulus array retinal illuminance, obeying a slope of unity. Exposure duration variation showed temporal summation to extend to longer durations for the pulse increments and decrements than for the steady pedestal condition. Discrimination thresholds for pulsed medium-sized contrast steps showed the contrast pedestal paradigm showed the temporal signature of the MC pathway. Discrimination thresholds for small pedestal steps of the stimulus array from a steady pedestal showed the contrast gain signature of the MC pathway. The data suggested a difference in the spatiotemporal control of adaptation of the two pathways: The MC pathway adapted locally to the stimulus array, while the PC pathways showed little evidence of local adaptation. The experiments show that characteristic signatures of MC- and PC-pathway processing can be demonstrated by use of psychophysical procedures.

Optical density of the human lens

Optic disk reflectance was measured from 27 normal observers with their physiological lenses (aged 21-74 yr) and from two pseudophakic observers (aged 69 and 70 yr) with use of a Utrecht fundus reflection densitometer. Psychophysical heterochromatic flicker photometric luminance matches (10 degrees field) were obtained on the same group of the observers. A four-parameter model incorporating lens density, hemoglobin absorption, optic disk reflectance, and superficial stray light was used to fit the reflectometric data. A model incorporating lens density and the Judd revised spectral luminous-efficiency function was used to fit the psychophysical data. The lens-density spectrum used the two-factor aging model of Pokorny, et al. [Appl. Opt. 26, 1437 (1987)]. The lens density for each normal observer was estimated through a least-squares fitting procedure yielding an estimated lens age. For the reflectometric data the observer's chronological age agreed with estimated lens age with a correlation coefficient of 0.92. The reflectometric regression line underestimated chronological age by approximately 5 yr. The mean reflectance of the optic disk was 0.047 with standard error of the mean of 0.0044. Data from the pseudophakic observers were well described when corneal density was used to replace lens density. The lens density was also estimated from the psychophysical data. The observer's chronological age agreed with psychophysically estimated lens age with a correlation coefficient of 0.92. It was concluded that the in vivo lens density can be estimated from the reflectance spectrum measured off the optic disk. The reflectance spectrum of the optic disk was inferred to be close to spectrally neutral.

Cone-rod receptor spaces with illustrations that use CRT phosphor and light-emitting-diode spectra

The purpose of the study is to characterize the excitation of the three cone types and the rods in a colorimetric system. Two representations of photoreceptor activity are developed. In the first, rod activity is characterized within a cone colorimetric system that is based on three known physical primaries. Examples are given that use color CRT phosphor spectra. We illustrate how this representation can be used to evaluate the range of chromaticities over which rod signals may intrude into color-monitor-based investigations of cone function. In the second representation, mixtures of four physical primary lights are used to manipulate the four receptor excitations independently. This method allows specification of sets of lights that isolate or silence up to three receptor classes or any combination of receptor classes. Examples are given that use spectra from four light-emitting diodes. This approach opens a field of research in which rod input to various retinal pathways can be evaluated.

Color contrast under controlled chromatic adaptation reveals opponent rectification

Color contrast was assessed in the equiluminant plane using asymmetric matching. Test and surround stimuli lay on cardinal axes of a cone opponent chromaticity space, (l-lw, s-sw). Matches were made as a function of both test and surround chromaticity. Some matches showed constant maximal induction consistent with retinal adaptation to the surround; others showed constant minimal induction. These matches were separated by a hiatus in which color appearance did not vary greatly with test chromaticity. The results suggest that rectified retinal spectral opponent pathways do not form a unitary chromatic opponent pathway but are subject to pathway-specific interactions.

Increment threshold and purity discrimination spectral sensitivities of X-chromosome-linked color-defective observers

The goal of the study was to evaluate spectral opponency in nine X-chromosome-linked color-defective observers. The tasks included increment threshold spectral sensitivity on an achromatic background, heterochromatic flicker photometry, and colorimetric purity discrimination. With a task of heterochromatic flicker photometry, the anomalous trichromatic observers showed spectral sensitivity of the corresponding dichromat. The increment threshold spectral sensitivity and colorimetric purity discrimination data were analyzed using the concept of standard cone photopigment spectral sensitivities for normal and defective vision, and a model that postulates one cone-additive and two cone-antagonistic systems. The model incorporated a shift of the peak spectral sensitivity of the long-wavelength-sensitive (LWS) pigment (for protan observers) or of the middle-wavelength-sensitive (MWS) pigment (for deutan observers). Two dichromats and two anomalous trichromats did not show clear evidence of LWS vs MWS cone antagonism. Five anomalous trichromats showed such cone antagonism. Molecular genetic analysis of the opsin genes is presented for eight of the observers.

Horizontal cells of the primate retina: cone specificity without spectral opponency

The chromatic dimensions of human color vision have a neural basis in the retina. Ganglion cells, the output neurons of the retina, exhibit spectral opponency; they are excited by some wavelengths and inhibited by others. The hypothesis that the opponent circuitry emerges from selective connections between horizontal cell interneurons and cone photoreceptors sensitive to long, middle, and short wavelengths (L-, M-, and S-cones) was tested by physiologically and anatomically characterizing cone connections of horizontal cell mosaics in macaque monkeys. H1 horizontal cells received input only from L- and M-cones, whereas H2 horizontal cells received a strong input from S-cones and a weaker input from L- and M-cones. All cone inputs were the same sign, and both horizontal cell types lacked opponency. Despite cone type selectivity, the horizontal cell cannot be the locus of an opponent transformation in primates, including humans.

The change of microtubule length caused by endogenous AC fields in cell

In this paper we investigate the change of microtubule cylinder's length in the presence of endogenous AC fields generated by the cell itself. The dynamics of microtubule is described on the basis of classical u4 model. The average stretching is calculated by using Kubo's formalism for linear response of system.

Physiologic and morphologic characteristics of granule cell circuitry in human epileptic hippocampus

Morphological and electrophysiological techniques were used to examine granule cells and their mossy fiber axons in nine surgically resected hippocampal specimens from temporal lobe epilepsy (TLE) patients. Timm histochemistry showed mossy fiber sprouting into the inner molecular layer (IML) of the dentate in a subset of tissue samples. In slices from five tissue samples, stimulus-induced bursting activity could be induced with a low concentration (2.5 microM) of bicuculline; bursts were sensitive to the N-methyl-D-aspartate (NMDA) blocker, APV. There was a general correlation between such sprouting and experimentally induced hyperexcitability. Fourteen granule cells from five tissue samples were intracellularly stained [with lucifer yellow (LY) or neurobiotin]. Axons from a subset of these neurons showed axon collaterals reaching into the IML, but this axon projection pattern for single cells was not directly correlated with degree of mossy fiber sprouting shown grossly by Timm staining. Electron microscopic examination of intracellularly stained elements showed mossy fiber axon terminals making asymmetric synaptic contacts (including autapses on the granule cell dendrite) with dendritic shafts and spines in both apical and basal domains. These data are consistent with the hypothesis that mossy fiber sprouting provides a structural basis for recurrent excitation of granule cells, but does not provide direct support of the hypothesis that mossy fiber sprouting causes hyperexcitability. The data suggest that granule cell bursting activity is at least in part a function of compromised synaptic inhibition, since levels of gamma-aminobutyric acid (GABA) blockade that are generally subthreshold for burst induction were epileptogenic in some tissue samples from human epileptic hippocampus.

Mechanisms subserving temporal modulation sensitivity in silent-cone substitution

Temporal contrast sensitivity data were collected with sine-wave-modulated lights for achromatic, chromatic, and silent-cone-substitution stimuli. Achromatic (556- and 642-nm lights in phase) and chromatic (556- and 642-nm lights in counterphase) modulation sensitivities were measured at a constant time-average retinal illuminance of 1256 trolands (Td) and chromaticity of 595 nm. These data were considered to represent isolated temporal responses of luminance and red-green chromatic channels, respectively. Silent cone substitution was achieved with counterphase modulation of the 556- and the 642-nm lights and by suitable adjustment of the modulations or the radiances of the two lights. (1) The peak modulation depth of the 642-nm light was reduced to silence the long-wavelength-sensitive (LWS) cone, and the peak modulation depth of the 556-nm light was reduced to silence the middle-wavelength-sensitive (MWS) cone. These protocols maintained the time-average retinal illuminance and chromaticity as for the control conditions. (2) The luminance of the 642-nm light was decreased to silence the LWS cone and was increased to silence the MWS cone. In this procedure the time-average retinal illuminance and chromaticity differ for the silenced-LWS-cone (1047 Td and 589.5 nm) and the silenced-MWS-cone (4358 Td and 622 nm) conditions. The response modulation of the achromatic and the chromatic channels was calculated for the silent-substitution conditions. The chromatic channel is more sensitive at low frequencies, with a transition to greater achromatic channel sensitivity near 13 Hz for the silenced-LWS-cone condition and near 6 Hz for the silenced-MWS-cone condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal retinal development as measured by the electroretinogram in premature infants

Light-adapted and dark-adapted electroretinograms were obtained in 27 premature infants who were screened for retinopathy of prematurity shortly after birth. Thirteen showed no retinopathy and 13 had mild to moderate retinopathy, which ranged from stage I, zone III (International Classification) to stage III, zone II. Measurements were made during the first 16 months of life. The configuration of the waveforms under both photopic and scotopic conditions changed during this period showing increased amplitudes of both the a- and the b-waves. A scotopic intensity series was performed and the b-wave amplitudes and implicit times were measured. The b-wave amplitude data were averaged for three adult control subjects, for infants without retinopathy of prematurity measured at 3-4 and at 6-7 months and for infants with retinopathy of prematurity measured at 3-4 and at 6-7 months. The Naka-Rushton function was fitted to the average data. The Rmax increased from 3 to 6 months and from 6 months to adulthood, and the Isat values decreased over this age range. The b-wave implicit times were within normal range in the 6-month data. There was no difference in Rmax or Isat for infants with and without ROP.

Chromatic-discrimination axes, CRT phosphor spectra, and individual variation in color vision

There is a growing use of color monitor systems in visual research and a parallel growth in the use of cone-excitation space to define stimuli and to report data. Color specification in monitor systems is accomplished by combination of the phosphor chromaticities. The effect of interobserver variation on color specification is highly dependent on the spectroradiometric properties of the primaries. We review potential sources of biologic variability and its effect on the nominal axes in a cone-excitation diagram for a color monitor system. Variation in preretinal pigment (lens and macular pigment), in the effective optical density and the spectral sensitivity of the visual photopigments, and in the cone weighting used to derive the spectral luminosity function are considered. The consequences of such biological variability are rotation and translation of the axes for a given observer relative to the nominal axes that the observer used for color specification. The importance of such rotations can be viewed within the framework of a particular experimental paradigm.

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.

Effect of sawtooth polarity on chromatic and luminance detection

Psychophysical studies have documented that many observers show lower thresholds for rapid-off than for rapid-on sawtooth luminance modulation. This finding, together with physiological findings from chromatically opponent ganglion cells of the macaque monkey, prompted a search for a similar bias in psychophysical detection of chromatic increments and decrements of light. Using a luminance pedestal in conjunction with a luminance background to favor detection by chromatic mechanisms, we measured spectral sensitivity for rapid-on and rapid-off sawtooth stimuli presented spatially coextensive with the pedestal. There were two different pedestal chromaticities: one broadband, and the second composed only of long-wavelength light to enhance short-wavelength-sensitive, cone-mediated detection. Spectral-sensitivity measurements for different wavelength stimuli revealed no systematic differences across the visible spectrum as a function of sawtooth waveform polarity or pedestal chromaticity. Similarly, temporal contrast-sensitivity functions for hetero-chromatically modulated red-green sawtooth stimuli did not reveal an asymmetry in sensitivity for rapid-red and rapid-green chromatic change. Some of the observers showed a higher sensitivity for luminance modulated rapid-off sawtooth stimuli, as also noted in previous studies. This asymmetry was not found when a white luminance pedestal and background was used. These results suggest that the cone inputs to chromatically opponent ON- and OFF-center cells are sufficiently balanced to provide equivalent psychophysical thresholds for chromatic increments and decrements of light.

Chromatic discrimination with variation in chromaticity and luminance: data and theory

Boynton and Kambe developed a model of chromatic discrimination in which thresholds are mediated by two independent mechanisms: the short-wavelength sensitive (S-) cones (S-cone axis), and the middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones (M/L-cone axis). In this study, we used a Maxwellian view optical system to investigate fundamental properties of the model as a function of chromaticity and luminance. We confirmed that discriminations along the S-cone axis were dependent on S-cone excitation level. However, changes in chromaticity and changes in mean luminance were not described by a single threshold-vs-radiance (TVR) template. We developed a model to account for the different effects of changing S-cone excitation by varying mean chromaticity and by varying mean luminance. M/L-cone discriminations showed a minimum at the L-cone excitation to white, indicating strong opponency. The thresholds increased with luminance approaching a Weber region and showing parallel functions for differing chromaticities. These data are fit by a model allowing retinal gain controls and spectral opponency.

Colorimetric purity discrimination: data and theory

Colorimetric purity, measured as the first step from white toward the spectrum has a V-shaped function. Purity discrimination is best near 400 nm, least at 570 nm and intermediate at mid-spectrum and long wavelengths. A much flatter function occurs when colorimetric purity is measured as the first step from the spectrum toward white. In this study, we applied the formulation of chromatic discrimination thresholds measured along the S-cone and M/L-cone axis to account for chromatic discrimination in the equiluminant plane. The modeling results show that the purity step from white has a 1.6 log unit calculated range, similar to the classical data. The purity step from the spectrum is much flatter. The predicted range is dependent on the individual variance in chromatic discrimination thresholds and the luminance level. We then used psychophysical procedures to test the model's predictions. The resulting purity discrimination functions were generally in agreement with the model. Our modeling indicates that discrepant data of colorimetric purity can be explained with the context of discrimination models.

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.

Pigment tests evaluated by a model of chromatic discrimination

Clinical color-vision tests are evaluated within the framework of a model of chromatic discrimination in terms of cone excitation. The motivation for this study was to derive a method for evaluation of test design, test sensitivity, and observer performance. The discrimination model is based on the assumption that chromatic discrimination is mediated in two independent channels, one for short-wavelength cones and one for long- and middle-wavelength cones. Luminance-dependent templates are derived for each channel, and they describe chromatic-discrimination behavior of the young color-normal observer. The templates incorporate receptor- and opponent-level gain controls. We show how the chromaticities of clinical tests can be calculated in cone-excitation units and how discrimination behavior on the tests can be plotted on the templates. The tests include the Farnsworth-Munsell 100-hue, the Farnsworth Panel D-15, the Farnsworth Panel D-15 desaturated, the American Optical Hardy-Rand-Rittler, the Farnsworth F2 plate, the Standard Pseudoisochromatic Plates, Part II, the Ishihara, and the Minimalist tests. Clinical-test data collected on young color-normal observers at different illumination levels show the validity of the techniques.

Luminance

Luminance was introduced by the CIE as a photometric analog of radiance. This implies that an additive spectral-luminosity function characterizes the human observer. In practice, many different spectral-sensitivity functions characterize human vision, although few produce the additive spectral-luminosity function V (lambda), which is suitable for use in practical photometry. Methods that give rise to additive spectral-sensitivity functions that most resemble V (lambda) tend to have in common the use of spatial or temporal frequencies that will discriminate against signals from the short-wavelength-sensitive cone pathways or against signals in other chromatic pathways. Some of the difference among results obtained with different techniques seems to reflect the extent to which the methods can bring about changes in the state of chromatic adaptation, but it also seems likely that not all tasks tap the same postreceptoral mechanisms. Psychophysical evidence is equivocal regarding the nature of the postreceptoral mechanisms: some evidence suggests just three mechanisms, one of which has a spectral sensitivity that is like V (lambda); other evidence suggests the existence of multiple mechanisms with different spectral sensitivities. Physiological recordings from neurons in the macaque's visual pathway suggest that the properties of the magnocellular system may be sufficient to account for spectral-sensitivity functions measured with the techniques of heterochromatic flicker photometry, minimally distinct border, and critical flicker fusion. These are the psychophysical methods that yield spectral sensitivities that are most like V (lambda). Other methods of measuring spectral sensitivity seem more likely to depend on signals that travel through the parvocellular system.

Spectral-luminosity functions, scalar linearity, and chromatic adaptation

We report data for three experiments that assess the effect on the luminosity function of chromatic adaptation arising from the measurement stimuli. First, we report spectral-sensitivity functions (wavelength range, 510-640 nm) measured by heterochromatic flicker photometry for a luminance range of 25-5000 Td. The data were fitted to a linear combination of cone fundamentals. The data narrowed and the fits deteriorated with an increase in luminance level, which indicates that at high luminances chromatic adaptation that is dependent on the spectral composition of the standard and test lights is a factor in spectral-luminosity determination. Second, we report heterochromatic modulation photometry as measured with two spectral lights at constant time-averaged chromaticity and luminance for luminances from 1.6 to 1300 Td. For a time-averaged chromaticity of 570 nm, the red-green ratio of the photometric match was invariant with luminance. For a time-averaged chromaticity of 605 nm, the red-green ratio increased by almost 0.3 log unit for a 2-log-unit increase in luminance, which is indicative of chromatic adaptation to the 605-nm chromaticity. Third, we measured flicker increment detection (wavelength range, 510-640 nm) on 570- and 605-nm backgrounds of 25-5000 Td. The data were fitted to a linear combination of cone fundamentals and showed good fits at all luminances. Fits to the 570-nm-background data set showed little variation in the proportions of the cone fundamentals with luminance. Fits to the 605-nm-background data set required an increased weighting of the middle-wavelength-sensitive cone with luminance. These three experiments indicate that luminance-dependent variation in the spectral-luminosity function as assessed by flicker techniques is caused primarily by chromatic adaptation to the measurement stimuli.

Color perception profiles in central achromatopsia

Central achromatopsia is an impairment of color perception caused by damage to the visual association cortex. Its psychophysical underpinnings remain poorly defined. We report our attempt to characterize the defect along critical dimensions of color space, taking advantage of the same standardized tasks that allow detailed profiles in patients with retinal cone defects. We studied two patients. The results in patient 1 showed that perceptual color space was collapsed along the red-green (R-G) and short-wavelength-sensitive cone (S-cone) dimensions but that discriminations along achromatic dimensions were relatively preserved. Additional observations showed that the defect was dependent on target size, and that processing of surface and light source effects that differ from color (eg, transparency) was intact. Patient 2 showed a less severe color processing defect involving signals arising from the S-cones of the retina, although an R-G defect was also present. The profiles in these two patients demonstrate that central achromatopsia encompasses a range of color processing impairments with varied psychophysical characteristics.

How surrounds affect chromaticity discrimination

Chromatic discrimination thresholds were measured with and without surrounds along two cardinal axes of chromaticity space. On one axis the level of short-wavelength-sensitive (SWS)-cone excitation was varied for constant long-wavelength-sensitive (LWS)-cone and medium-wavelength-sensitive (MWS)-cone excitations, and on the other axis there were equal and opposite changes in LWS-cone and MWS-cone excitations for constant levels of SWS-cone excitation. Results for two of three observers showed that with a dark surround, discrimination mediated by SWS cones was regulated by the level of SWS-cone excitation of the starting chromaticity, showing a function with the form of a threshold-versus-radiance function. For an equiluminant white or yellow surround, the discrimination for all three observers showed a minimum at the level of SWS-cone excitation of the surround, giving a V-shaped function for the white surround. An additional experiment with dimmer white surrounds indicated that while the minimum remained at the white point, the function gradually changed toward the shape with a dark surround. Discrimination thresholds mediated by LWS and MWS cones with a dark surround showed a minimum near the LWS-cone excitation of equal-energy white, giving a V-shaped function. The effect of yellow and white surrounds was to deepen the V. The data can be described by a model of chromatic discrimination incorporating a threshold term, a cone gain control, and an opponent gain control into two equations, one for SWS-cone discrimination and one for LWS-cone and MWS-cone discrimination.