Combination of rod and cone inputs in parasol ganglion cells of the magnocellular pathway

Temporal integration of rod signals in luminance and chromatic pathways

We assessed how rod excitation (R) affects luminance (L + M + S) and chromatic [L/(L + M)] reaction times (RTs). A four-primary display based on the overlapped images of two spectrally modified monitors, which allowed specific or combined [L + M + S + R, L/(L + M) + R] photoreceptor stimulation, was used to present a C-target stimulus differing from the background only by the selected stimulation. For the luminance pathway, rod input increased RTs, suggesting a suppressive rod-cone interaction. The responses of the chromatic pathway were faster when rods were involved, suggesting a major role of rods in mesopic color perception.

Early-stage visual perception impairment in schizophrenia, bottom-up and back again

Visual perception is one of the basic tools for exploring the world. However, in schizophrenia, this modality is disrupted. So far, there has been no clear answer as to whether the disruption occurs primarily within the brain or in the precortical areas of visual perception (the retina, visual pathways, and lateral geniculate nucleus [LGN]). A web-based comprehensive search of peer-reviewed journals was conducted based on various keyword combinations including schizophrenia, saliency, visual cognition, visual pathways, retina, and LGN. Articles were chosen with respect to topic relevance. Searched databases included Google Scholar, PubMed, and Web of Science. This review describes the precortical circuit and the key changes in biochemistry and pathophysiology that affect the creation and characteristics of the retinal signal as well as its subsequent modulation and processing in other parts of this circuit. Changes in the characteristics of the signal and the misinterpretation of visual stimuli associated with them may, as a result, contribute to the development of schizophrenic disease.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

Rod Photoresponse Kinetics Limit Temporal Contrast Sensitivity in Mesopic Vision

The mammalian visual system operates over an extended range of ambient light levels by switching between rod and cone photoreceptors. Rod-driven vision is sluggish, highly sensitive, and operates in dim or scotopic lights, whereas cone-driven vision is brisk, less sensitive, and operates in bright or photopic lights. At intermediate or mesopic lights, vision transitions seamlessly from rod-driven to cone-driven, despite the profound differences in rod and cone response dynamics. The neural mechanisms underlying such a smooth handoff are not understood. Using an operant behavior assay, electrophysiological recordings, and mathematical modeling we examined the neural underpinnings of the mesopic visual transition in mice of either sex. We found that rods, but not cones, drive visual sensitivity to temporal light variations over much of the mesopic range. Surprisingly, speeding up rod photoresponse recovery kinetics in transgenic mice improved visual sensitivity to slow temporal variations, in the range where perceptual sensitivity is governed by Weber's law of sensation. In contrast, physiological processes acting downstream from phototransduction limit sensitivity to high frequencies and temporal resolution. We traced the paradoxical control of visual temporal sensitivity to rod photoresponses themselves. A scenario emerges where perceptual sensitivity is limited by: (1) the kinetics of neural processes acting downstream from phototransduction in scotopic lights, (2) rod response kinetics in mesopic lights, and (3) cone response kinetics as light levels rise into the photopic range.SIGNIFICANCE STATEMENT Our ability to detect flickering lights is constrained by the dynamics of the slowest step in the visual pathway. Cone photoresponse kinetics limit visual temporal sensitivity in bright (photopic) lights, whereas mechanisms in the inner retina limit sensitivity in dim (scotopic) lights. The neural mechanisms underlying the transition between scotopic and photopic vision in mesopic lights, when both rods are cones are active, are unknown. This study provides a missing link in this mechanism by establishing that rod photoresponse kinetics limit temporal sensitivity during the mesopic transition. Surprisingly, this range is where Weber's Law of Sensation governs temporal contrast sensitivity in mouse. Our results will help guide future studies of complex and dynamic interactions between rod-cone signals in the mesopic retina.

Range, routing and kinetics of rod signaling in primate retina

Stimulus- or context-dependent routing of neural signals through parallel pathways can permit flexible processing of diverse inputs. For example, work in mouse shows that rod photoreceptor signals are routed through several retinal pathways, each specialized for different light levels. This light-level-dependent routing of rod signals has been invoked to explain several human perceptual results, but it has not been tested in primate retina. Here, we show, surprisingly, that rod signals traverse the primate retina almost exclusively through a single pathway – the dedicated rod bipolar pathway. Identical experiments in mouse and primate reveal substantial differences in how rod signals traverse the retina. These results require reevaluating human perceptual results in terms of flexible computation within this single pathway. This includes a prominent speeding of rod signals with light level – which we show is inherited directly from the rod photoreceptors themselves rather than from different pathways with distinct kinetics.

Parallel Processing of Rod and Cone Signals: Retinal Function and Human Perception

We know a good deal about the operation of the retina when either rod or cone photoreceptors provide the dominant input (i.e., under very dim or very bright conditions). However, we know much less about how the retina operates when rods and cones are coactive (i.e., under intermediate lighting conditions, such as dusk). Such mesopic conditions span 20-30% of the light levels over which vision operates and encompass many situations in which vision is essential (e.g., driving at night). These lighting conditions are challenging because rod and cone signals differ substantially: Rod responses are nearing saturation, while cone responses are weak and noisy. A rich history of perceptual studies guides our investigation of how the retina operates under mesopic conditions and in doing so provides a powerful opportunity to link general issues about parallel processing in neural circuits with computation and perception. We review some of the successes and challenges in understanding the retinal basis of perceptual rod-cone interactions.

Effect of eccentricity and light level on the timing of light adaptation mechanisms

We explored the complexity of the light adaptation process, assessing adaptation recovery (Ar) at different eccentricities and light levels. Luminance thresholds were obtained with transient background fields at mesopic and photopic light levels for temporal retinal eccentricities (0°-15°) with test/background stimulus size of 0.5°/1° using a staircase procedure in a two-channel Maxwellian view optical system. Ar was obtained in comparison with steady data [Vis. Res.125, 12 (2016)VISRAM0042-698910.1016/j.visres.2016.04.008]. Light level proportionally affects Ar only at fovea. Photopic extrafoveal thresholds were one log unit higher for transient conditions. Adaptation was equally fast at low light levels for different retinal locations with variations mainly affected by noise. These results evidence different timing in the mechanisms of adaptation involved.

Correlated cone noise decreases rod signal contributions to the post-receptoral pathways

This study investigated how invisible extrinsic temporal white noise that correlates with the activity of one of the three [magnocellular (MC), parvocellular (PC), or koniocellular (KC)] post-receptoral pathways alters mesopic rod signaling. A four-primary photostimulator provided independent control of the rod and three cone photoreceptor excitations. The rod contributions to the three post-receptoral pathways were estimated by perceptually matching a 20% contrast rod pulse by independently varying the LMS (MC pathway), +L-M (PC pathway), and S-cone (KC pathway) excitations. We show that extrinsic cone noise caused a predominant decrease in the overall magnitude and ratio of the rod contributions to each pathway. Thus, the relative cone activity in the post-receptoral pathways determines the relative mesopic rod inputs to each pathway.

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

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

The Peripheral Flicker Illusion

A new illusion is reported. A visual object suddenly appearing on a red background sometimes causes an impression of flicker or double flash. In Experiment 1, a red, green, or blue object was presented on a red, green, blue, or gray background. Participants evaluated the illusion strength in reference to the physical flicker of a gray object presented in central vision. The results show that the green or blue object presented on the red background caused the illusion. In Experiment 2, the effect of retinal eccentricity on the illusion was tested. The results showed that the illusion was weak in central vision but became stronger as the retinal eccentricity of the objects' presentation increased. In Experiment 3, optimal luminance conditions for the illusion were explored with the green and blue objects. The illusion was strong when object luminance was lower than background luminance and the optimal luminance for the blue object was lower than that for the green object. We propose a tentative theory for the illusion and discuss its cause.

An Assessment of the Contrast Sensitivity in Patients with Ametropic and Anisometropic Amblyopia in Achieving the Corrected Visual Acuity of 1.0

Both visual acuity (VA) and contrast sensitivity (CS) are important parameters for measuring visual function. In this research, we investigated the CS of patients with ametropic or anisometropic amblyopia, whose corrected visual acuity (CVA) recovered to 1.0. Fifty-five cases with amblyopia and 22 control cases with a normal visual acuity of 1.0 were enrolled. The CS of the patients whose ametropic amblyopia had recovered to a CVA of 1.0 at 18 cpd spatial frequency was still lower than that of the normal control group under both photopic and scotopic conditions (P = 0.001, 0.025), but there were no significant differences at low- and middle-spatial frequencies. The CS of amblyopic eyes of the patients with anisometropic amblyopia was lower than that of the normal control group at the 18 cpd spatial frequency under photopic conditions (P = 0.005), and at the 6 cpd, 12 cpd, and 18 cpd spatial frequencies under scotopic conditions (P = 0.008, <0.001, 0.004, respectively). The CS between the amblyopic eyes and the sound eyes of patients with anisometropic amblyopia presented significant differences at the 6 cpd, 12 cpd, and 18 cpd spatial frequencies under scotopic conditions (P = 0.025, 0.045, 0.019, respectively). We suggest that amblyopia treatment should involve not only the correction of VA but also the improvement of CS.

Perifoveal S-cone and rod-driven temporal contrast sensitivities at different retinal illuminances

We evaluated a technique for measuring temporal contrast sensitivities to sine-wave modulation driven by S-cones and rods in the perifovea using triple silent substitution. Isolating stimuli for S-cones and rods were created using an eight-channel, four-primary LED stimulator that has been validated before. Sensitivities were measured at 10 different temporal frequencies between 1 and 28 Hz in three normal observers at 14 different retinal illuminances between 0.07 and 587 photopic troland (phot Td) and at three different retinal illuminances over the same range in one S-cone monochromat. The technique was further validated by measuring bleaching adaptation in two normal subjects, demonstrating sufficient isolation in rods. Good isolation was apparent from the differences in the temporal contrast sensitivity functions and the sensitivity-versus-retinal illuminance functions between S-cones and rods, and also from the results in the S-cone monochromats and the delayed recovery of rod sensitivities after bleaching. The results will help to determine optimal stimulus conditions in future studies. The results in the S-cone monochromat demonstrate the potential clinical value of our protocol.

Luminance and chromatic signals interact differently with melanopsin activation to control the pupil light response

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin. These cells receive afferent inputs from rods and cones, which provide inputs to the postreceptoral visual pathways. It is unknown, however, how melanopsin activation is integrated with postreceptoral signals to control the pupillary light reflex. This study reports human flicker pupillary responses measured using stimuli generated with a five-primary photostimulator that selectively modulated melanopsin, rod, S-, M-, and L-cone excitations in isolation, or in combination to produce postreceptoral signals. We first analyzed the light adaptation behavior of melanopsin activation and rod and cones signals. Second, we determined how melanopsin is integrated with postreceptoral signals by testing with cone luminance, chromatic blue-yellow, and chromatic red-green stimuli that were processed by magnocellular (MC), koniocellular (KC), and parvocellular (PC) pathways, respectively. A combined rod and melanopsin response was also measured. The relative phase of the postreceptoral signals was varied with respect to the melanopsin phase. The results showed that light adaptation behavior for all conditions was weaker than typical Weber adaptation. Melanopsin activation combined linearly with luminance, S-cone, and rod inputs, suggesting the locus of integration with MC and KC signals was retinal. The melanopsin contribution to phasic pupil responses was lower than luminance contributions, but much higher than S-cone contributions. Chromatic red-green modulation interacted with melanopsin activation nonlinearly as described by a “winner-takes-all” process, suggesting the integration with PC signals might be mediated by a postretinal site.

Correlated and uncorrelated invisible temporal white noise alters mesopic rod signaling

We determined how rod signaling at mesopic light levels is altered by extrinsic temporal white noise that is correlated or uncorrelated with the activity of one (magnocellular, parvocellular, or koniocellular) postreceptoral pathway. Rod and cone photoreceptor excitations were independently controlled using a four-primary photostimulator. Psychometric (Weibull) functions were measured for incremental rod pulses (50 to 250 ms) in the presence (or absence; control) of perceptually invisible subthreshold extrinsic noise. Uncorrelated (rod) noise facilitates rod detection. Correlated postreceptoral pathway noise produces differential changes in rod detection thresholds and decreases the slope of the psychometric functions. We demonstrate that invisible extrinsic noise changes rod-signaling characteristics within the three retinogeniculate pathways at mesopic illumination depending on the temporal profile of the rod stimulus and the extrinsic noise type.

Alcohol Intoxication Impairs Mesopic Rod and Cone Temporal Processing in Social Drinkers

BACKGROUND

Alcohol-related driving accidents and fatalities occur most frequently at nighttime and at dawn, that is, a mesopic lighting condition in which visual processing depends on both rod and cone photoreceptors. The temporal functions of the rod and cone pathways are critical for driving in this lighting condition. However, how alcohol influences the temporal functions in the rod and cone pathways at mesopic light levels is inconclusive. To address this, this study investigated whether an acute intoxicating dose of alcohol impairs rod- and/or cone-mediated critical fusion frequency (CFF; the lowest frequency of which an intermittent or flickering light stimulus is perceived as steady).

METHODS

In Experiment I, we measured the CFFs for 3 types of visual stimuli (rod stimulus alone, cone stimulus alone, and the mixture of both stimuli types), under 3 illuminant light levels (dim illuminance: 2 Td; low illuminance: 20 Td; and medium illuminance: 80 Td) in moderate-heavy social drinkers before and after they consumed an intoxicating dose of alcohol (0.8 g/kg) compared with a placebo beverage. In Experiment II, we examined whether the illuminance level (dark vs. light) of the visual area surrounding the test stimuli alters alcohol's effect on the temporal processing of rods and cones.

RESULTS

The results showed that compared with placebo, alcohol significantly reduced CFFs of all stimulus types at all illuminance levels. Furthermore, alcohol intoxication produced a larger impairment on rod-pathway-mediated CFFs under light versus dark surround.

CONCLUSIONS

These results indicate that alcohol intake slows down rod and cone-pathway-mediated temporal processing. Further research may elucidate whether this effect may play a role in alcohol-related injury and accidents, which often occur under low-light conditions.

Rod- and cone-isolated flicker electroretinograms and their response summation characteristics

This study defined the amplitude and phase characteristics of rod- and cone-isolated flicker electroretinograms (ERGs) and determined how these responses summate to generate the nonreceptor-specific ERG. Full-field ERGs were obtained from six normally sighted subjects (age 26 to 44 years) using a four-primary LED-based photostimulator and standard recording techniques. The four primaries were either modulated sinusoidally in phase to achieve simultaneous rod and cone activation (ERGR+C; nonreceptor-specific) or in different phases to achieve rod-isolated (ERGR) and cone-isolated (ERGC) responses by means of triple silent substitution. ERGs were measured at two mean luminance levels (2.4 and 24 cd/m2), two contrasts (20 and 40%), and four temporal frequencies (2-15 Hz). Fundamental amplitude and phase for each condition were derived by Fourier analysis. Response amplitude and phase depended on the stimulus conditions (frequency, mean luminance, and contrast), however, for all conditions: 1) response phase decreased monotonically as stimulus frequency increased; 2) response amplitude tended to decrease monotonically as stimulus frequency increased, with the exception of the 24 cd/m2, 40% contrast ERGR+C that was sharply V-shaped; 3) ERGR phase was delayed (32 to 210 deg) relative to the ERGC phase; 4) ERGR amplitude was typically equal to or lower than the ERGC amplitude, with the exception of the 2.4 cd/m2, 40% contrast condition; and 5) the pattern of ERGR+C responses could be accounted for by a vector summation model of the rod and cone pathway signals. The results show that the ERGR+C amplitude and phase can be predicted from ERGR and ERGC amplitude and phase. For conditions that elicit ERGR and ERGC responses that have approximately equal amplitude and opposite phase, there is strong destructive interference between the rod and cone responses that attenuates the ERGR+C. Conditions that elicit equal amplitude and opposite phase rod and cone responses may be particularly useful for evaluating rod-cone interactions.

A simple retinal mechanism contributes to perceptual interactions between rod- and cone-mediated responses in primates

Visual perception across a broad range of light levels is shaped by interactions between rod- and cone-mediated signals. Because responses of retinal ganglion cells, the output cells of the retina, depend on signals from both rod and cone photoreceptors, interactions occurring in retinal circuits provide an opportunity to link the mechanistic operation of parallel pathways and perception. Here we show that rod- and cone-mediated responses interact nonlinearly to control the responses of primate retinal ganglion cells; these nonlinear interactions, surprisingly, were asymmetric, with rod responses strongly suppressing subsequent cone responses but not vice-versa. Human psychophysical experiments revealed a similar perceptual asymmetry. Nonlinear interactions in the retinal output cells were well-predicted by linear summation of kinetically-distinct rod- and cone-mediated signals followed by a synaptic nonlinearity. These experiments thus reveal how a simple mechanism controlling interactions between parallel pathways shapes circuit output and perception.

DOI:http://dx.doi.org/10.7554/eLife.08033.001

The inner surface at the back of the eye is called the retina and contains two types of light-sensitive cells: rod cells and cone cells. Rods outnumber cones by roughly twenty to one and are responsible for vision under low light levels. Cone cells, by contrast, provide detailed vision in bright light, as well as the ability to see in color.

Rods and cones provide input to two distinct networks of cells that convey information in parallel to cells called ganglion cells, which then relay this information out of the retina. However, the signals from activated rods can feed into the cone pathway at several points, meaning that the responses of rods and cones are not independent. At dawn and dusk—and indeed under street lighting at night—rods and cones are both active and interactions between rod and cone responses influence many aspects of vision, including sensitivity to color and contrast.

Grimes et al. have now identified a neural mechanism behind these interactions by combining measurements of human vision with recordings of electrical activity in retinas from non-human primates. The experiments confirmed that activating either type of photoreceptor briefly suppresses the responses of the other, although unexpectedly rods inhibit cones more than cones inhibit rods. The site of this interaction is the connection—or synapse—between the very last cell in the cone pathway and the retinal output cells. Prior to this ‘gateway’ synapse, rod and cone-mediated responses are largely independent.

Vision at dawn and dusk is shaped by a complex set of interactions between rod and cone signals—such as the ability of activated rods to change color perception at dusk. These findings show that these seemingly complex behaviors can arise from simple interactions at the level of neural circuits.

DOI:http://dx.doi.org/10.7554/eLife.08033.002

Vision under mesopic and scotopic illumination

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.

A synaptic signature for ON- and OFF-center parasol ganglion cells of the primate retina

In the primate retina, parasol ganglion cells contribute to the primary visual pathway via the magnocellular division of the lateral geniculate nucleus, display ON and OFF concentric receptive field structure, nonlinear spatial summation, and high achromatic temporal-contrast sensitivity. Parasol cells may be homologous to the alpha-Y cells of nonprimate mammals where evidence suggests that N-methyl-D-aspartate (NMDA) receptor-mediated synaptic excitation as well as glycinergic disinhibition play critical roles in contrast sensitivity, acting asymmetrically in OFF- but not ON-pathways. Here, light-evoked synaptic currents were recorded in the macaque monkey retina in vitro to examine the circuitry underlying parasol cell receptive field properties. Synaptic excitation in both ON and OFF types was mediated by NMDA as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptors. The NMDA-mediated current-voltage relationship suggested high Mg2+ affinity such that at physiological potentials, NMDA receptors contributed ∼20% of the total excitatory conductance evoked by moderate stimulus contrasts and temporal frequencies. Postsynaptic inhibition in both ON and OFF cells was dominated by a large glycinergic "crossover" conductance, with a relatively small contribution from GABAergic feedforward inhibition. However, crossover inhibition was largely rectified, greatly diminished at low stimulus contrasts, and did not contribute, via disinhibition, to contrast sensitivity. In addition, attenuation of GABAergic and glycinergic synaptic inhibition left center-surround and Y-type receptive field structure and high temporal sensitivity fundamentally intact and clearly derived from modulation of excitatory bipolar cell output. Thus, the characteristic spatial and temporal-contrast sensitivity of the primate parasol cell arises presynaptically and is governed primarily by modulation of the large AMPA/kainate receptor-mediated excitatory conductance. Moreover, the negative feedback responsible for the receptive field surround must derive from a nonGABAergic mechanism.

Effect of rod-cone interactions on mesopic visual performance mediated by chromatic and luminance pathways

We studied the effect of rod-cone interactions on mesopic visual reaction time (RT). Rod and cone photoreceptor excitations were independently controlled using a four-primary photostimulator. It was observed that (1) lateral rod-cone interactions increase the cone-mediated RTs; (2) the rod-cone interactions are strongest when rod sensitivity is maximal in a dark surround, but weaker with increased rod activity in a light surround; and (3) the presence of a dark surround nonselectively increased the mean and variability of chromatic (+L-M, S-cone) and luminance (L+M+S) RTs independent of the level of rod activity. The results demonstrate that lateral rod-cone interactions must be considered when deriving mesopic luminous efficiency using RT.

Color coding in the primate visual pathway: a historical view

The physiology and anatomy of the primate visual pathway are reviewed from a historical perspective, especially in relation to color vision. From the work of the last decades, certain issues have been selected which remain unresolved and still pose a challenge for neurobiologists and psychophysicists. It is suggested that the structure of the primate visual pathway has been colored by the evolution of trichromacy and that many features of the parvocellular pathway represent adaptations to this end.

Contributions of rhodopsin, cone opsins, and melanopsin to postreceptoral pathways inferred from natural image statistics

Visual neural representation is constrained by the statistical properties of the environment. Prior analysis of cone pigment excitations for natural images revealed three principal components corresponding to the major retinogeniculate pathways identified by anatomical and physiological studies in primates. Here, principal component analyses were conducted on the excitations of rhodopsin, cone opsins, and melanopsin for nine hyperspectral images under 21 natural illuminants. The results suggested that rhodopsin and melanopsin may contribute to the three major retinogeniculate pathways. Rhodopsin and melanopsin may provide additional constraints in natural scene statistics, leading to new components that cannot be revealed by analysis based on cone opsin excitations only.

Assessing rod, cone, and melanopsin contributions to human pupil flicker responses

PURPOSE

We determined the relative contributions of rods, cones, and melanopsin to pupil responses in humans using temporal sinusoidal stimulation for light levels spanning the low mesopic to photopic range.

METHODS

A four-primary Ganzfeld photostimulator controlled flicker stimulations at seven light levels (-2.7 to 2 log cd/m(2)) and five frequencies (0.5-8 Hz). Pupil diameter was measured using a high-resolution eye tracker. Three kinds of sinusoidal photoreceptor modulations were generated using silent substitution: rod modulation, cone modulation, and combined rod and cone modulation in phase (experiment 1) or cone phase shifted (experiment 2) from a fixed rod phase. The melanopsin excitation was computed for each condition. A vector sum model was used to estimate the relative contribution of rods, cones, and melanopsin to the pupil response.

RESULTS

From experiment 1, the pupil frequency response peaked at 1 Hz at two mesopic light levels for the three modulation conditions. Analyzing the rod-cone phase difference for the combined modulations (experiment 2) identified a V-shaped response amplitude with a minimum between 135° and 180°. The pupil response phases increased as cone modulation phase increased. The pupil amplitude increased with increasing light level for cone, and combined (in-phase rod and cone) modulation, but not for the rod modulation.

CONCLUSIONS

These results demonstrate that cone- and rod-pathway contributions are more predominant than melanopsin contribution to the phasic pupil response. The combined rod, cone, and melanopsin inputs to the phasic state of the pupil light reflex follow linear summation.

Retinal mesopic adaptation model for brightness perception under transient glare

A glare source in the visual field modifies the brightness of a test patch surrounded by a mesopic background. In this study, we investigated the effect of two levels of transient glare on brightness perception for several combinations of mesopic reference test luminances (Lts) and background luminances (Lbs). While brightness perception was affected by Lb, there were no appreciable effects for changes in the Lt. The highest brightness reduction was found for Lbs in the low mesopic range. Considering the main proposal that brightness can be inferred from contrast and the Lb sets the mesopic luminance adaptation, we hypothesized that contrast gain and retinal adaptation mechanisms would act when a transient glare source was present in the visual field. A physiology-based model that adequately fitted the present and previous results was developed.

Mesopic rod and S-cone interactions revealed by modulation thresholds

We analyzed mesopic rod and S-cone interactions in terms of their contributions to the blue-yellow opponent pathway. Stimuli were generated using a four-primary colorimeter. Mixed rod and S-cone modulation thresholds (constant L-, M-cone excitation) were measured as a function of their phase difference. Modulation amplitude was equated using threshold units and contrast ratios. This study identified three interaction types: (1) a linear and antagonistic rod:S-cone interaction, (2) probability summation, and (3) a previously unidentified mutual nonlinear reinforcement. Linear rod:S-cone interactions occur within the blue-yellow opponent pathway. Probability summation involves signaling by different postreceptoral pathways. The origin of the nonlinear reinforcement is possibly at the photoreceptors.

Functional loss in the magnocellular and parvocellular pathways in patients with optic neuritis

PURPOSE

To evaluate contrast threshold and contrast gain in patients with optic neuritis under conditions designed to favor mediation by either the inferred magnocellular (MC) or parvocellular (PC) pathway.

METHODS

Achromatic and chromatic contrast discrimination was measured in 11 patients with unilateral or bilateral optic neuritis and in 18 age-matched controls with normal vision, using achromatic steady- and pulsed-pedestal paradigms to bias performance toward the MC or PC pathway, respectively. In addition, L-M chromatic discrimination at equiluminance was evaluated using the steady-pedestal paradigm. A physiologically plausible model could describe the data with parameters accounting for contrast gain and contrast sensitivity in the inferred MC or PC pathway. The fitted parameters from the eye affected by optic neuritis were compared with those from the normal eye using generalized estimation equation (GEE) models that can account for within-subject correlations.

RESULTS

Compared with normal eyes, the affected eyes had significantly higher saturation parameters when measured with both the achromatic pulsed-pedestal paradigm (GEE: β [SE] = 0.35 [0.06]; P < 0.001) and the chromatic discrimination paradigm (β [SE] = 0.18 [0.08]; P = 0.015), suggesting that contrast gain in the inferred PC pathway is reduced; the affected eyes also had reduced absolute sensitivity in the inferred MC pathway measured with the achromatic steady-pedestal paradigm (β [SE] = 0.12 [0.04]; P = 0.005).

CONCLUSIONS

Optic neuritis produced large sensitivity losses mediated by the MC pathway and contrast gain losses in the inferred PC pathway. A clinical framework is presented for interpreting contrast sensitivity and gain loss to chromatic and achromatic stimuli in terms of retinal and postretinogeniculate loci contributions to detection and discrimination.

Isolated mesopic rod and cone electroretinograms realized with a four-primary method

The purpose of this study was to evaluate the feasibility of measuring rod and cone electroretinograms (ERGs) at a single mesopic adaptation level. To accomplish this, a four-primary photostimulator was implemented using a commercially available ERG system (Diagnosys ColorDome) to generate three types of stimuli that temporally modulated rods alone, cones alone, and rods and cones simultaneously. For each stimulus type, ERGs were recorded as a function of temporal frequency (2, 4, 8, or 16 Hz) and mesopic light levels (0.02, 0.16, or 1.26 cd/m(2)) in normal observers and patients with retinitis pigmentosa (RP) or cone-rod degeneration. The normal observers ERG waveforms showed a clear periodic pattern, mirroring the sinusoidal stimuli. At all light levels, rod responses were always higher than cone responses for temporal frequencies between 2 and 8 Hz, suggesting that rods dominated the responses. Cone responses were minimal at the lowest light level and increased with increases in light level. The amplitude of the response to the combined stimuli was intermediate between that of the isolated cone and the isolated rod stimuli for all light levels. Good receptoral isolation was confirmed by the results showing (1) minimal or no rod ERGs but recordable cone ERGs in the patients and (2) high correlation between the ERG amplitudes obtained from the four-primary method and those from the ISCEV standard clinical protocol in normal observers.

Rod and cone contrast gains derived from reaction time distribution modeling

Contrast gain reflects the rapidity of response amplitude increase with increase in stimulus contrast. In physiology, contrast gain can be measured directly as the initial slope of cell contrast response function. In psychophysics, contrast gain estimation is not straightforward. Further, rod and cone contrast gains have not been measured psychophysically at mesopic light levels where both rods and cones are active, due to the difficulty in producing stimuli that excite rods and cones separately at the same adaptation level. Here, we estimated rod and contrast gains by fitting reaction time distributions measured at a light level in which rods alone (scotopic), rods and cones (mesopic), or cones alone (photopic) mediate vision. The reaction time distributions were modeled by two different strategies, a simplified diffusion model that assumes a stochastic accumulation process and a model we developed that begins with sensory input based on early visual processing impulse response functions and assumes the reaction time variability originates in the response criterion. Estimates of contrast gain from both models were comparable and consistent with primate physiology measurements.