Cones block signals from rods

Increase in cortisol concentration due to standardized bright and blue light exposure on saliva cortisol in the morning following sleep laboratory

Research studies on LED light exposure and cortisol are inconsistent and not comparable due to different types of light, exposure times, and sample sizes. Therefore, one hour of standardized exposure LED light at different intensities and the spectral composition during the post-awakening phase at 7:30 were compared. A sample of 23 (Study 1) and 26 (Study 2) healthy males were randomly assigned to: 1) bright white light (414 lux) and 2) dim darkened light (<2 lux) as well as 3) red light (235 lux) and 4) blue light (201 lux) exposure conditions. Results from repeated measures ANOVA confirm that light exposure affects the cortisol concentration. Study 1 revealed an increase in the saliva cortisol concentration after bright light exposure compared to dim light. An increase in the cortisol concentration of blue light compared to red light (Study 2) and dim light was found. This study shows that bright light and blue light affect the cortisol response in contrast to dim light and red light conditions. The HPA axis showed a stimulatory effect by bright versus dim light and different wavelengths of light exposure.Lay summaryThe effects of LED light exposure on the stress hormone cortisol were investigated. The light exposure took place during the hours people would start working at the office. The results showed that after one hour of exposure to bright light or blue light the stress hormones increase in contrast to dim light and red light conditions. Thus, stress hormones can be altered by the types of light people are exposed to.

Rapid Adaptation of Night Vision

Apart from the well-known loss of color vision and of foveal acuity that characterizes human rod-mediated vision, it has also been thought that night vision is very slow (taking up to 40 min) to adapt to changes in light levels. Even cone-mediated, daylight, vision has been thought to take 2 min to recover from light adaptation. Here, we show that most, though not all adaptation is rapid, taking less than 0.6 s. Thus, monochrome (black-white-gray) images can be presented at mesopic light levels and be visible within a few 10th of a second, even if the overall light level, or level of glare (as with passing headlamps while driving), changes abruptly.

Mesopic luminance assessed with minimally distinct border perception

In photopic vision, the border between two fields is minimally distinct when the two fields are isoluminant; that is, when the achromatic luminance of the two fields is equal. The distinctness of a border between extrafoveal reference and comparison fields was used here as an isoluminance criterion under a variety of adaptation conditions ranging from photopic to scotopic. The adjustment was done by trading off the amount of blue against the amount of red in the comparison field. Results show that isoluminant border settings are linear under all constant adaptation conditions, though varying with state of adaptation. The relative contribution of rods and cones to luminance was modeled such that the linear sum of the suitably weighted scotopic and photopic luminance is constant for the mesopic isoluminant conditions. The relative weights change with adapting intensity in a sigmoid fashion and also depend strongly on the position of the border in the visual field.

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

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

Effects of rod activity on color perception with light adaptation

To investigate the effect of rod activity on color perception with light adaptation, chromaticity shifts of monochromatic test lights were measured as a function of background field intensity at 17 deg in the nasal field of view. The measurements were performed both after complete dark adaptation and during the cone-plateau period at a mesopic test intensity level of 15 photopic trolands. To clarify the mechanisms underlying the chromaticity shifts obtained, six supplementary experiments were performed. The results of the experiments strongly suggest that at scotopic background intensities, light adaptation of rods, both within and adjacent to the test area, may reduce rod signals triggered by the test light and thereby produce marked chromaticity shifts with light adaptation. At mesopic background intensities, cones in the background field become activated and may influence the chromaticity shift with light adaptation both by suppressing signals from rods elicited by the test light and by producing a selective chromatic adaptation.

Spatial and temporal properties of light adaptation in the rod system

Little is known about the mechanism that regulates the sensitivity of rod system at its normal operating light levels. Two experiments are reported in this paper. First, we searched for nonlinear distortion products in rod vision that could be generated from any local adaptation process, using a sensitive experimental procedure that has demonstrated local adaptation in cone vision. No local adaptation was evident in the rod system, even at near saturating light levels. Second, to investigate the dynamics of light adaptation in the rod system we presented a uniform flickering background, sinusoidally modulated in time, and measured increment thresholds for brief test flashes that were superimposed on this background at different times during the sinusoidal flicker cycle. At frequencies less than 5-6 Hz, the rod increment threshold follows the background modulation, with a slight phase advance. When the background is modulated faster than 5-6 Hz, the increment threshold remains the same regardless of when the test flash occurred during the background cycle. Thus the rod system sensitivity, unlike that of the cone system, can only change slowly, and is set by a space-integrated signal rather than independently for different rods.

Summation of rod and S cone signals at threshold in human observers

We examined whether signals from rods and S cones can combine to produce a threshold response. Test flashes of specific wavelengths superposed on a long wavelength adapting field were used to isolate threshold responses from the two receptor systems, simultaneously and at the same retinal location. Dark adaptation experiments and spectral sensitivity determinations indicated that, in the adaptational range from about 1.6 to 2.8 log scot td, 530 nm and 440 nm flashes were detected by rod and S cone photoreceptors, respectively. The intensities of the 530 nm and 440 nm flashes were mixed in various ratios and the increment threshold was then measured with these mixture flashes using the method of constant stimuli. The effects of rod and S cone excitation were found to summate linearly at threshold, under these experimental conditions. Summation occurred presumably at an early stage of the visual process.

Partial additivity of rod signals with M- and L-cone signals in increment detection

Test additivity experiments revealed the combination rules for increment detection by rods and either M- or L-cone-dominated mechanisms isolated by means of chromatic adaptation (Stiles' pi 4 and pi 5, respectively). Increment thresholds were measured for single test wavelengths detected by each mechanism. Pairs of test wavelengths were then superimposed, and increment thresholds were measured for simultaneous detection by both rod and cone mechanisms. The observed degree of additivity was corrected (reduced) to compensate for the partial detection by each mechanism of both test wavelengths in the combined stimuli. We find that subthreshold rod signals are partially additive with subthreshold signals from both M- and L-cones. The degree of additivity is high and similar for both M- and L-cones: less than the ideal prediction of linear addition, but greater than that predicted by either probability summation of independent mechanisms or orthogonal vector addition.

Surface color naming in dichromats

In previous experiments, Montag and Boynton [(1987) Vision Research, 27, 2153-2162] found that many dichromats can categorize colors using color naming in fair agreement with color-normal subjects. The contribution of rods to color vision was suspected as underlying this ability. Here we follow up on these experiments by having dichromats name colors under various conditions. When the stimuli are limited to a brief presentation time (60 msec) the dichromats' categorization in the three dimensions of the OSA color space is impaired. Using high light levels so that the rods are saturated does not impair performance. The dichromats named colors during the period of the cone plateau following a rod bleach. Contrary to Montag and Boynton (1987) there was no deficit. These results suggest that an anomalous third cone pigment is responsible for the categorization in three dimensions. It is concluded that the receptors containing the anomalous pigment require greater temporal and spatial summation in order to contribute to the dichromats' color categorization.

The spectral properties of the two rod pathways

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

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

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

Temporal and spatial summation in the human rod visual system

1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod-isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone-mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod-mediated vision changes with light adaptation. 2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not. 3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries-Rose square-root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co-ordinates is about 0.56 +/- 0.04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0.77 +/- 0.03 is attained. 4. The steeper slopes of the rod-detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity. 5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long-wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions.

Rod-cone interaction in form detection

Using a Wright colorimeter, absolute threshold, absolute form threshold and specific form threshold were measured during long-term dark adaptation in the extrafoveal retina. The specific form threshold was found to fall markedly at about the cone-rod break but thereafter rose steeply. Furthermore, during the rod phase of the dark adaptation the form percept of the small, slender rectangular test field changed qualitatively from a line or rectangle to a circular field at all mesopic intensities. The results indicate that light signals from rods may both facilitate and suppress cone-mediated information about form, and that the rod system may completely dominate the perception of form several log units above the absolute dark-adapted cone threshold when the eye is dark adapted.

The field adaptation of the human rod visual system

1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a target of 200 ms in duration and 6 deg in diameter superimposed on background fields of various intensities and wavelengths. Measurements were made under rod-isolation conditions in five normal observers and in a typical, complete achromat observer who had no cone function. 2. The rise in threshold with background intensity changes with background wavelength in the normal trichromat observers. On 450, 520 and 560 nm backgrounds the average slope in logarithmic co-ordinates (0.78 +/- 0.04, S.D.) is similar to that found for the achromat--whose slope is independent of background wavelength (0.79 +/- 0.03)--but on a 640 nm background it more nearly approaches Weber's law (0.91 +/- 0.02). This indicates that the sensitivity of the rods to an incremental target is not determined by quantal absorptions in the rods alone but by quantal absorptions in both the rods and the cones. 3. Rod incremental thresholds were also measured in various colour-blind observers lacking one or more of the cone classes: a blue-cone monochromat, four deuteranopes and a protanope. For the blue-cone monochromat, like the achromat, the slope of the increment threshold curve is constant with background wavelength. For the deuteranopes and the protanope, like the normal, the slope increases with wavelength. The protanope, however, shows a smaller increase in slope, consistent with the lower sensitivity of his cones to long-wavelength light. 4. The dependence of the field adaptation of the rods on the cones was confirmed by field-mixture experiments, in which the incremental threshold was measured against bichromatic backgrounds, and in silent substitution experiments, in which backgrounds equated for their effects on either the cones or the rods but not both were instantaneously substituted for one another.

LWS cone effects on rod threshold and saturation in achromats with residual cone function

Rod saturation on flashed and steady red backgrounds was investigated in normals and three achromats, two of whom were found to have some residual cone function. LWS cones selectively reduce the background level at which rod saturation occurs and elevate rod thresholds at flashed background levels well below saturation. Both of these LWS cone actions are also present in eyes with greatly reduced LWS cone function. In normal eyes LWS cones also elevate rod thresholds on steady backgrounds. We thus conclude that LWS cones influence rods through different mechanisms under transient (flashed) and steady-state background stimulation and that the increase in rod visual sensitivity observed during prolonged presentation of a background is due to a time-dependent reduction of LWS cone influence on rods. Finally, the finding that rod-cone interactions of the same magnitude found in normals can be seen in individuals where the cones' ability to mediate vision is severely reduced suggests the rod saturation paradigm as a sensitive technique for revealing residual LWS cone function.

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

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

Rod flicker perception: scotopic duality, phase lags and destructive interference

Rod vision has a duality of organization: at mesopic luminances rod signals have access to a slow, sensitive pathway (which we refer to, following Stiles, as pi 0) and a fast, insensitive pathway (pi' 0). The phase lag between the two rod signals increases with frequency until at 15-Hz the rod signals transmitted through the two pathways emerge out-of-phase, so that destructive interference produces a nulling of the apparent flicker. Relative to the cones, the phase lag of pi' 0 is roughly half that of pi 0. Thus at 15-Hz pi' 0 signals can be out-of-phase with cone signals, so that the signals from the slower pathway, pi 0, are actually in phase with cone signals. We have investigated the frequency response, adaptation behavior and phase characteristics of the two rod processes. The slower process, pi 0 is more sensitive than pi' 0, and dominates from absolute threshold up to low mesopic levels. The adaptation of pi 0 seems not to be associated with a change in time constant, but rather with simple response compression or sensitivity scaling. The time constant of pi' 0, however, does change with adaptation. There are large differences in the way that light adaptation changes the sensitivity of the two processes: signals from pi'0 may evade part of the postreceptoral sensitivity regulating mechanism normally associated with rod vision. The ability of signals from pi 0 and pi' 0 to reinforce or cancel each other, however, suggests that they are later reunited in a common pathway.

The role of spatial filtering in sensitivity regulation

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

Rod-cone interaction in monocular but not binocular pathways

Photopic background stimulation elevates scotopic increment thresholds (rod-cone interaction) at moderate background levels when both test and concentric disk-background stimuli enter the same eye (monocular condition) but not when they enter different eyes (dichoptic condition). Only when background levels are made extremely high is there any measurable dichoptic interaction, and this interaction does not resemble that observed monocularly. Rod-cone interaction, as usually studied, is a property of monocular pathways in human vision.

A note on the action spectrum of human rod vision

A template representing the spectral distribution of the absorption coefficients of human rhodopsin was fitted to each of 59 individual action spectra of human rod vision (from one of three populations) by an optimization routine. Curve-fitting parameters included peak wavenumber, optical density at this wavenumber and (for those from the population neither aphakic nor constrained to ages where the standard lens transmissivity curve is supposed valid), density of the latter at the wavenumber of peak lens absorption. The average peak wavenumber of each population differed significantly from that of the other two. Either the standard curve of lens absorption (even with peak lens density as a curve-fitting parameter) is inappropriate for correcting the normal spectrum or the rhodopsins in the retinas of these populations do not all have identical wave-numbers of peak absorbance.

Shared pathways for rod and cone vision

We have used heterochromatic gratings falling on 10 deg temporal retina to measure the spatial contrast sensitivities of the isolated rod and cone systems in the mesopic range. As the level of illumination was raised within this range, the contrast sensitivity of the rod system increased, reaching a peak of about 50 (and providing an acuity of 6 c/deg) at 20 scot. td, whereupon the rod system began to saturate. Over most of the mesopic range the sensitivity of the cone system was lower than that of the rod system, although it provided better acuity (up to 15 c/deg). Within the range of spatial frequencies capable of exciting both rod and cone systems, a grating that excited only rods was indistinguishable from a grating of the same spatial frequency that excited only cones. Moreover, contrast adaptation to gratings that excited either rods or cones raised threshold for gratings that excited rods or cones. From these results we conclude that signals from rods and cones travel together in pathways subserving the detection of low spatial frequencies, while only signals from cones travel in pathways subserving the detection of high spatial frequencies.

The effect of light on the spread of signals through the rod network of the salamander retina

Adjacent rods in the amphibian retina are electrically coupled to each other by gap junctions. By injecting current pulses into one rod and recording the voltage change produced in nearby rods, we have studied the extent to which signals spread between rods in the presence and absence of illumination. Light has little effect on the steady potentials produced in nearby rods by the injection of a hyperpolarizing current, but does affect the propagation of transient signals through the rod network. The responses to injection of depolarizing current are increased by light. These effects of light were mimicked by hyperpolarizing the rod network (non-uniformly) by injecting continuous current (on top of which current pulses were superimposed to monitor signal spread). This suggests that the effects of light are due solely to the rod hyperpolarization produced by light. The effects of light are not completely predicted from computer simulations based on a previous characterization of the properties of isolated rods; these experiments thus reveal an inadequacy in the description of the rod membrane currents in that model. Light-induced hyperpolarization of cones has no effect on signal spread between rods. The functional significance of these results is discussed.

The development of photoreceptors in the zebrafish, brachydanio rerio. II. Function

The functional development of rod and cone photoreceptors in the zebrafish (Brachydanio rerio) was studied by electroretinographically measuring flicker fusion frequencies. Two to 3 days after fertilization, fish gave little or no response to high intensity stimuli. Increases in sensitivity and flicker resolution were observed after this time. Biphasic response curves typical of adults could be elicited from fish as young as 2 weeks postfertilization, thus indicating a functional divergence of rods and cones. The results are consistent with behavioral and anatomical analyses of zebrafish photoreceptor development.

The influence of cones on rod saturation with flashed backgrounds

The increment threshold for a middle-wavelength test flash was measured at the onset of a concentric long-wavelength background flash under conditions that have previously been shown to result in rod system saturation. The influence of the cone system on rod saturation under these conditions was assessed using the Stiles-Crawford effect in normal subjects and by measuring rod thresholds in protanopes, who are deficient in long-wavelength cones. When the background flash is made less effective for cones through the Stiles-Crawford effect, the onset of rod saturation occurs at a higher luminance of background flash than normal. Similarly, protanopes do not show the characteristics of rod saturation until a much higher-than-normal luminance of background flash. The results suggest that rod system saturation with flashed backgrounds is strongly influenced by cones.

The time-course of rod-cone interaction

The time-course of rod-cone interaction (change of scotopic sensitivity caused by photopic background stimulation) was measured in the presence of briskly exchanged, scotopically matched, 490- and 630-nm background disks. In all conditions, interaction rose and fell quickly with changes of photopic stimulation. When the background was a small 0.6 degree-diameter disk, photopic stimulation produced relatively constant maintained interaction of about 0.6 log units. When the background was a large 7.8 degree-dia disk, photopic stimulation produced larger initial (0.6-1.0 log unit) than maintained (0.2 log unit) interaction. When a 0.6 degree by 7.8 degree annulus was used instead of a background, photopic stimulation produced substantial interaction only at offset, a transitory interaction. Thus, the spatial dependence of transitory interactions differs from that of maintained interaction: transitory interactions can be large even when maintained interaction is small or absent. The results are discussed in terms of a simple center-surround model of rod-cone interaction that unifies both maintained and transient interaction.

Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker

In the parafoveal retina of human observers, cone-mediated sensitivity to flicker decreases as rods become progressively more dark-adapted. This effect is greatest when a rod response to flicker is precluded. These results indicate that rods tonically inhibit cone pathways in the dark.

The signal-to-noise characteristics of rod-cone interaction

1. The influence of rods on cone-mediated vision was assessed in eight human observers. To this end, increment threshold functions were obtained by determining thresholds of a cone-detected test flash (25 ms duration, 655 nm wave-length, 13' diameter) as a function of the illuminance of larger, 500 ms duration, rod-detected masking flashes. The type of photoreceptor influenced by each stimulus was carefully checked by means of a series of control procedures involving action spectra and selective rod adaptation.2. When the rod mask was 512 nm in wave-length, 40' in diameter, and less than one scotopic td in illuminance, increment threshold functions show that [Formula: see text], where I(Cth) is cone test threshold, I(R) is rod mask illuminance, and D is a dark noise term similar to that used by Barlow (1956). Further increases in I(R) have no additional influences on cone test threshold until threshold is influenced by the combined action of the mask on both rods and cones. If I(R) is expressed in terms of scotopic flux rather than illuminance, the functional relationship obtained with all rod masks </= 40' diameter and </= 580 nm wave-length is identical.3. Over the range of illuminance where a square-root relationship is obtained, probability of seeing functions show that a signal-to-noise mechanism limits the detectability of the cone test flash. These findings suggests a quantitative model in which cones produce a signal in a detector which is proportional to the illuminance of the cone test flash. Within a neural locus designated E (excitatory spatial summator), a response is produced which over at least a 40' diameter area, is proportional to the scotopic flux of the rod mask. E, however, feeds into a gain box, S, which saturates at illuminance levels at least 3 log(10) units less than usual estimates of rod saturation. Other than saturation, S behaves in a linear fashion.4. As diameter increases beyond 60', rod masks of equal scotopic illuminance have progressively less influence on cone test threshold; rod masks > 2 degrees have negligible influence on cone test threshold. We propose that I (inhibitory spatial summator), a neural locus which responds to scotopic flux provided over a very large area, attenuates the activity of E. The combined action of E and I is designated a rod channel. The response of cones and the rod channel summate at a detector. Within the detector, cone signals are distinguished from rod-related activity and intrinsic dark noise on the basis of signal-to-noise discriminations.5. The neural substrate for this rod channel most probably involves the combined action of several neurones which synapse within the inner plexiform layer of the retina. The relationship of this rod channel to other perceptual phenomena is discussed.

The visual cells of the skate retina: structure, histochemistry, and disc-shedding properties

Earlier studies have shown that visual function in skate is subserved solely by the rod mechanism and that the retina of this elasmobranch contains only rod photoreceptors. Nevertheless, the skate retina is capable of responding to levels of illumination that extend well into the photopic range, and we have detected in histological sections (usually from younger animals) small, proximally displaced, conelike photoreceptors which possibly represent another class of visual cell. However, ultrastructural and histochemical studies showed that the membranous discs of the outer segments of these cells were isolated from the plasma membrane, and that their synaptic terminals appeared immature and unlike those usually associated with cone receptors. In addition, the pattern of incorporation of 3H-fucose, as revealed by radioautography, was similar for both the rods and the smaller visual cells; i.e., the label was concentrated along the basal discs of the outer segment. When we examined the disc-shedding behavior of the visual cells in skates entrained for 2 weeks or longer to a 12-hour light:12-hour dark cycle, enhanced phagocytic activity was seen only following light onset; there was no significant increase following light offset. On the available evidence, it seems reasonable to conclude that the small visual cells are rods that have recently differentiated, and are growing and being incorporated into the photoreceptor layer of the retina.

A new concept of retinal colour coding

A theory of retinal colour coding based closely on recent anatomical and physiological results is presented. Opponent colour channels are shown to be an inevitable result of any randomly distributed retinal cone mosaic, the structure of red-green opponent colour channels remaining uninfluenced by a predominance of "red" or "green" cones. These findings circumvent the conflict between anatomical results with more "green" than "red" cones and psychophysical estimations with more "red" than "green" cones. The effect of receptor compression and opponent colour transformation on colour perception is investigated. Non-opponency of pure green and pure red could be attributed to receptor compression, the Bezold-Brücke phenomenon, however, to the antagonism of "red" and "green" cones within the receptive field surround of red-green opponent cells. The fundamental colours are estimated to be supersaturated violet, yellow-green and yellow-red.

The temporal properties of rod vision

1. Profiles which represent rod thresholds for flickering fields seen against backgrounds of various intensity have shapes which depend on flicker frequency. Low frequency profiles rise smoothly as background intensity is increased. High frequency profiles are only affected by bright backgrounds, which cause them to rise steeply. Intermediate frequency profiles contain two distinct branches which resemble separate increment threshold functions. 2. The high intensity branches of two-branched threshold profiles cannot be attributed to cone intrusion. Instead, both branches of such profiles are mediated by visual mechanisms which have the spectral properties, the dark adaptation properties and the directional insensitivity of rod vision. Thus, the stimuli are detected by rods. 3. Plots of critical flicker frequency (c.f.f.) as a function of intensity contain two rising branches which are separated by a plateau (when modulation depth is large), or they form two enclosed lobes so that only intermediate frequencies, but neither high nor low ones, are seen (when modulation depth is small). C.f.f. is profoundly depressed by very bright light (above 100 scotopic trolands) which saturates rod vision. 4. In dim light rod modulation sensitivity functions resemble those of low-pass filters, but in bright light they resemble those of band-pass filters. 5. Several forms of rod mediated interference occur at moderate intensities, where rod vision's temporal properties ordinarily improve abruptly. With certain stimuli, rod signals conveying temporal information disrupt one another so completely that suprathreshold flicker cannot be seen within a ten-fold intensity range. 6. Many of these observations can be explained by the hypothesis that rod vision comprises two temporal channels which have different properties.

Interactions between rod and cone channels above threshold: a test of various models

Brightness-matching and magnitude estimation data were obtained to test alternative explanations of the rod-cone interaction above threshold. Steady adapting fields were selected such that 500 and 640 nm test flashes presented in the parafovea stimulated largely the rod and cone systems, respectively. In the brightness-matching experiment stimuli of combined 500 and 640 nm light were matched to a white foveal standard flash set at various levels above threshold. Similarly, in the magnitude estimation experiment, brightness estimates were obtained for single 500 and 640 nm flashes as well as for super-imposed combination flashes. Combination stimuli appeared brighter than their individual components. Overall, the results suggest that rod and cone signals elicited by a single flash combine in an excitatory fashion; there is no evidence for rod-cone inhibition in this study. Further, the results for the suprathreshold and threshold conditions are qualitatively similar. The signals from the rod and cone systems combine less well than signals within a system but to a greater degree than predicted by probability summations or vector addition models.

Receptor interactions and visual persistence

Two experiments are described that provide evidence for an interaction between the cone and rod systems following the offset of a stimulus. Experiment 1 employs a visual persistence task in which the observer is required to integrate two brief flashes separated by a variable ISI. Threshold luminance as a function of ISI is determined for several conditions of target wavelength. Striking differences between short and long wavelength targets are found which indicate the possible exclusion of rod participation at the brief ISIs. Experiment 2 employs a forward-masking task to demonstrate that the stimulus conditions of the first experiment result in a transient reduction in rod system activity following stimulus offset while the cone system is still active. Results support this proposal. Implications for the locus of persistence within the visual system are discussed.

Rod-cone interrelationships at light onset and offset

The spatial constraints needed to obtain rod-cone interaction under steady adaptation levels apply to transient conditions of adaptation. The influence of cones upon rod threshold, however, is qualitatively different at light onset as opposed to light offset.

Interactions between rod and cone systems in the goldfish retina

Signals from both the rod and the cone receptor systems converge upon the same retinal ganglion cell, but only one or the other of these systems appears to be effective at any particular level of adaptation. In this report we provide evidence that the change from one receptor system to the other is not simply due to the two systems having nonoverlapping dynamic ranges; rather, there is a distance-dependent interaction between the two systems.

Interaction between rod and cone signals studied with temporal sine wave stimulation

If a temporal stimulus is presented through the cones, the threshold for a rod stimulus may be influenced even if the cone stimulus remains subthreshold. We have used sinusoidal stimulation to study the process underlying this rod-cone interaction. A first-order model is proposed consisting of a single threshold mechanism preceded by linear summation of rod and cone signals with a pure latency difference. Two deviations from this model are described. First, occasional additivity failures appear; second, if the phase difference characteristic is corrected for latency, the remaining part cannot be explained in terms of the de Lange curves of the rod and cone systems.

Rod-cone interaction in light adaptation

1. The increment-threshold for a small test spot in the peripheral visual field was measured against backgrounds that were red or blue.2. When the background was a large uniform field, threshold over most of the scotopic range depended exactly upon the background's effect upon rods. This confirms Flamant & Stiles (1948). But when the background was small, threshold was elevated more by a long wave-length than a short wave-length background equated for its effect on rods.3. The influence of cones was explored in a further experiment. The scotopic increment-threshold was established for a short wave-length test spot on a large, short wave-length background. Then a steady red circular patch, conspicuous to cones, but below the increment-threshold for rod vision, was added to the background. When it was small, but not when it was large, this patch substantially raised the threshold for the test.4. When a similar experiment was made using, instead of a red patch, a short wave-length one that was conspicuous in rod vision, threshold varied similarly with patch size. These results support the notion that the influence of small backgrounds arises in some size-selective mechanism that is indifferent to the receptor system in which visual signals originate. Two corollaries of this hypothesis were tested in further experiments.5. A small patch was chosen so as to lift scotopic threshold substantially above its level on a uniform field. This threshold elevation persisted for minutes after extinction of the patch, but only when the patch was small. A large patch made bright enough to elevate threshold by as much as the small one gave rise to no corresponding after-effect.6. Increment-thresholds for a small red test spot, detected through cones, followed the same course whether a large uniform background was long- or short wave-length. When the background was small, threshold upon the short wave-length one began to rise for much lower levels of background illumination, suggesting the influence of rods. This was confirmed by repeating the experiment after a strong bleach when the cones, but not rods, had fully recovered their sensitivity. Increment-thresholds upon small backgrounds of long or short wave-lengths then followed the same course.

Convergence of rod and cone signals in the cat's retina

1. In an attempt to understand the convergence of rod and cone signals in the cat's retina, ganglion cells that received inputs from both rods and cones were stimulated using lights chosen to excite one or other receptor system or both together.2. If a mesopic background was chosen to allow the ganglion cell to be excited by a blue-green test flash primarily through rods and a deep red flash primarily through cones, one light could not be alternated with the other without eliciting a response from the cell.3. This appears to be a result of the different temporal properties of the scotopic and photopic systems. On the mesopic background responses to blue-green test flashes were transient. Responses to red test flashes arose with similar latency, but were more sustained.4. Rod and cone systems responded with similar latencies in the presence of the mesopic background that substantially light-adapted the rod system but left the full sensitivity of the cone system undiminished. When equivalently light-adapted, the cone system was faster.5. When brief flashes that acted through rods were presented with flashes that acted through cones the ganglion cell's response was the sum of the responses to the two flashes presented separately, as long as the flashes were weak. This linear relation ceased to hold when flashes were strong, but the breakdown appears not to be the result of mutual inhibition between rod and cone signals.6. When a background light excited both rod and cone systems it appeared to reduce sensitivity independently in each.7. The scotopic and photopic receptive fields of a given ganglion cell always were of the same type, on- or off-centre, and, within the limits of measurement, the central regions of the receptive fields were concentric and both the same size.