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Purpose: To record transient ERGs from the light-adapted human retina using silent substitution stimuli which selectively reflect the activity of rod photoreceptors. We aim to describe the morphology of these waveforms and examine how they are affected by the use of less selective stimuli and by retinal pathology., Methods: Rod-isolating stimuli with square-wave temporal profiles (250/250 ms onset/offset) were presented using a 4 primary LED ganzfeld stimulator. Experiment 1: ERGs were recorded using a rod-isolating stimulus (63 ph Td, rod contrast, C rod  = 0.25) from a group (n = 20) of normal trichromatic observers. Experiment 2: Rod ERGs were recorded from a group (n = 5) using a rod-isolating stimulus (C rod  = 0.25) which varied in retinal illuminance from 40 to 10,000 ph Td. Experiment 3: ERGs were elicited using 2 kinds of non-isolating stimuli; (1) broadband and (2) rod-isolating stimuli which contained varying degrees of L- and M-cone excitation. Experiment 4: Rod ERGs were recorded from two patient groups with rod monochromacy (n = 3) and CSNB (type 1; n = 2)., Results: The rod-isolated ERGs elicited from normal subjects had a waveform with a positive onset component followed by a negative offset. Response amplitude was maximal at retinal illuminances <100 ph Td and was virtually abolished at 400 ph Td. The use of non-selective stimuli altered the ERG waveform eliciting more photopic-like ERG responses. Rod ERGs recorded from rod monochromats had similar features to those recorded from normal trichromats, in contrast to those recorded from participants with CSNB which had an electronegative appearance., Conclusions: Our results demonstrate that ERGs elicited by silent substitution stimuli can selectively reflect the operation of rod photoreceptors in the normal, light-adapted human retina., Competing Interests: Statement on Human Rights Ethical approval All procedures performed in this study were carried out in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Informed consent Informed consent was obtained from all individual participants included in the study.

Electroretinograms (ERGs) elicited by transient, square-wave L- and M-cone isolating stimuli were recorded from human trichromatic (n=19) and dichromatic (n=4) observers. The stimuli were generated on a four primary LED stimulator and were equated in terms of cone modulation (cone contrast=0.11) and retinal illuminance (12,000 trolands). L- and M-cone isolated ERGs had waveforms similar to those observed for luminance responses. However, M-cone ERGs exhibited a phase reversal in their responses to onset and offset stimuli relative to the L-cone responses. This on-off response reversal was observed in trichromats but not dichromats. Simultaneous counterphase and inphase combinations of L- and M-cone isolating stimuli generated responses that reflected chromatic and luminance processing, respectively. We conclude that L- and M-cone specific ERGs provide a measure of how photoreceptors contribute to postreceptoral mechanisms.

Large, nominally isoluminant chromatic gratings containing a short-wavelength component are prone to luminance contrast intrusions due to retinal inhomogeneity, especially as a result of the uneven distribution of macular pigment. Isoluminance is usually determined for a relatively small, central area, but a significantly larger stimulus cannot be isoluminant across the whole field, largely due to macular pigment absorption of short-wavelength light. This confounds attempts to maintain high selectivity, particularly in suprathreshold electrophysiological and brain-imaging studies that require large stimulus fields. Here we introduce the concept of a panisoluminant grating (PIG), which comprises a series of concentric annular regions, each adjusted to location-specific isoluminance for the observer. Gratings were modulated along subject-specific tritanopic confusion lines and the selectivity of responses to the PIG was tested according to both psychophysical and electrophysiological criteria. The psychophysically-determined temporal tuning function obtained using the PIG showed lower sensitivity and lower resolution than with a conventional tritan grating of equal diameter (18°). Chromatic onset visual evoked potentials (VEPs) to the PIG were dominated by a chromatic-specific negative wave and reduced achromatic response components that were prominent in VEPs to the conventional grating. These data demonstrate that a large tritan PIG is capable of eliciting selective responses of the S-cone-driven pathway at threshold and at suprathreshold levels. The PIG stimulus may prove beneficial in investigations that require large fields such as electrophysiological and brain imaging studies of chromatic processing.

Typical daylight extends from blue (morning sky) to orangey red (evening sky) and is represented mathematically as the Daylight Locus in color space. In this study, we investigate the impact of this daylight variation on human color vision. Thirty-eight color normal human observers performed an asymmetric color match in the near peripheral visual field. Unique hues were identified using a naming paradigm. The observers' performance for matching was almost perfectly coincident with the Daylight Locus but declined markedly in other regions. Interobserver variability reached a conspicuous minimum adjacent to the Daylight Locus and was maximal in the red and yellowish-green regions. In the naming task, unique blue and yellow were virtually coincident with the Daylight Locus. The results suggest that the mechanisms of color perception mediated by the phylogenetically older (blue-yellow) color pathway have been strongly influenced by the different phases of daylight.

Certain hues undergo shifts in their appearance when they are viewed by the peripheral retina. This has often been shown on a 3-primary color CRT monitor. To investigate the possible role of metamerism, we replicated our peripheral color matching experiments using Munsell paper stimuli viewed under real and simulated daylight (using a 3-primary projection system). Using stimuli of constant value and chroma (7/4), observers adjusted the hue of a 3 deg target presented 18 deg nasally, until it matched a 1 deg target presented 1 deg nasally. The magnitude and pattern of measured hue shifts were similar to those measured using CRT stimuli. We conclude that the perceived hue shifts that have previously been reported in the peripheral retina are independent of the nature of the stimulus and of the illuminant., (© 2012 Optical Society of America)

There has been much controversy as to whether there are sex-related differences in human color vision. While previous work has concentrated on testing the central visual field, this study compares male versus female color vision in the near peripheral retina. Male (n = 19) and female (n = 19) color normal observers who exhibited no significant differences either in the midpoints or the ranges of their Rayleigh matches were tested with a color matching paradigm. They adjusted hue and saturation of a 3° test spot (18° eccentricity) until it matched a 1° probe (1° eccentricity). Both groups demonstrated measurable shifts in the appearance of the peripheral color stimuli similar to those that have been previously reported. However, females showed substantially less saturation loss than males (p < 0.003) in the green-yellow region of color space. No significant differences were found in other regions of color space. This difference in the perceived saturation of color stimuli was minimally affected either by the inclusion or exclusion in the analysis of potential heterozygous female carriers of deutan color vision deficiencies. We speculate that this advantage of female over male color vision is conferred by M-cone polymorphism.

It is known that there is a distortion of hue and saturation in the peripheral visual field. In a previous study, when an asymmetric matching paradigm was used, four hues in the blue, red, yellow and green regions of colour space were unchanged and these were referred to as peripherally invariant (Parry et al., J Opt Soc Am A, 23, 2006, 1586). Three of these invariant hues were similar to unique blue, red and yellow. However, for most observers there was a marked difference between unique and invariant green. To investigate this apparent paradox, we have measured unique hues using a range of eccentricities and colourimetric purities. An asymmetric matching and a 4-AFC paradigm were used to establish peripherally invariant and unique hues, respectively. In the asymmetric matching task the observer matched a peripheral spot with a para-foveal spot, for 24 different hues at 18° eccentricity. In the 4-AFC paradigm, 41 hues were presented 20 times at three purities (0.5, 0.75 and 1.0) and three eccentricities (18°, 10° and 1°). The observer had to name the hues as red, blue, green or yellow. Unique hues were found to be constant with eccentricity and purity. The unique green, established with 4-AFC, was found to differ from the invariant green, determined using the matching task. However, red, blue and yellow invariant hues correspond well with unique hues. The data suggest that different mechanisms mediate the matching of green compared with the identification of unique hues. This is similar to the difference between detection and discrimination of spectral stimuli: the detection process is dominated by the cone opponent mechanisms and is most sensitive, whereas more central processes, serving unique hues, influence discrimination., (© 2010 The Authors, Ophthalmic and Physiological Optics © 2010 The College of Optometrists.)

We examined visual short term memory (VSTM) for colour using a delayed-match-to-sample paradigm. In these experiments we measured the effects of increasing inter-stimulus interval (ISI), varying between 0 and 10 s, on the ability of five colour normal human observers to make colour matches between a reference and subsequently presented test stimuli. The coloured stimuli used were defined by different chromatic axes on the isoluminant plane of DKL colour space. In preliminary experiments we used a hue scaling procedure to identify a total of 12 colour stimuli which served as reference hues in the colour memory experiments: four stimuli were exemplars of red, green, blue and yellow colour appearance categories, four were located between these categories and a further four were located on the cardinal axes that isolated the activity of the cone-opponent mechanisms. Our results demonstrate that there is a reduction in the ability of observers to make accurate colour matches with increasing ISIs and that this reduced performance was similar for all colour stimuli. However, the shifts in hue that were measured between the reference and matched test stimuli were significantly greater for the cardinal stimuli compared to those measured for the stimuli defined by the hue scaling procedure. This deterioration in the retention of hue in VSTM for stimuli that isolate cone-opponent mechanisms may be a reflection of the reorganisation of colour processing that occurs in the cortex where colour appearance mechanisms become more prominent., (© 2010 The Authors, Ophthalmic and Physiological Optics © 2010 The College of Optometrists.)

The purpose of this study is to establish whether nasal-temporal differences in cone photoreceptor distributions are linked to differences in colour matching performance in the two hemi-fields. Perceived shifts in chromaticity were measured using an asymmetric matching paradigm. They were expressed in terms of hue rotations and relative saturation changes and also in terms of activation levels of L-M or S-(L+M) cone-opponent channels. Up to 19 degrees eccentricity there was little difference in chromaticity shifts between nasal and temporal retina for either channel. For matches beyond 19 degrees L-M activation is significantly lower in the nasal field and the S-(L+M) channel was equally activated in both fields. The data are consistent with the asymmetric distribution of L- and M-cones in the nasal and temporal retinae.

Using double silent substitution, it is possible to generate L-cone and M-cone isolating electroretinograms (ERGs) on a CRT. A major limitation of the technique is that the depth of modulation of cone classes is limited by the restricted luminance of the phosphors and their spectral overlap. To address this problem we have ported the technique to a four-color LED Ganzfeld stimulus (Diagnosis ColorDome). This allows higher retinal illuminances, higher contrasts, and triple silent substitution. With careful control over the retinal area stimulated, we show that the same data can be recorded from both CRT and LED stimuli when luminance, size and cone contrast are kept constant. Importantly, the different temporal profiles of the two devices do not influence the ERG amplitude and phase plots. We present data over a much wider range of luminances (up to about 10,000 trolands) and contrasts with the LED stimulator than previously reported with CRT screens. We conclude that the close resemblance between data obtained with an LED stimulator and with a CRT screen indicate that the differences have a purely physiological origin.

Simple reaction times (RTs) were measured to brief temporally blurred (total onset 570 ms) Gaussian isoluminant chromatic patches (s.d. 0.5 degrees) whose chromaticities lay along the cardinal chromatic axes (0 degrees, 90 degrees, 180 degrees, and 270 degrees in MBDKL color space). Bipolar adapting stimuli were employed (0 degrees versus 180 degrees or 90 degrees versus 270 degrees). These were larger Gaussian blobs (s.d. 1 degree), modulating sinusoidally between the two hues at 1 Hz. Throughout, the background was illuminant "C" (x = 0.31, y = 0.316, L = 12.5). In a single run, a series of 64 or 32 stimuli were presented without adaptation, followed by 64 or 32 stimuli each of which was preceded by 3 s of adaptation, either along the same or the orthogonal chromatic axis. Finally, 192 or 128 RTs were recorded to measure the time course of recovery from adaptation. Both adapting and test stimuli were presented at fixed supra-threshold contrasts. The effect of adaptation was seen as a lengthening of the RT, which occurred in the first few seconds of the adaptation period. After cessation of adaptation, there was a similarly rapid shortening of RT, although full recovery took 60-90 s. Adaptation gain functions suggested that the S-(L + M) system was less prone to adaptation than L-M.

Using an asymmetric color matching technique, we measured the perceived changes that occur in the saturation and hue of colored stimuli at different eccentricities within the central 25 degrees of the human retina in nine color-normal subjects. A cone-opponent-based vector model was used to compute the activity of the L-M and S-(L+M) channels. The results show that a large proportion of the shifts in saturation and hue that occur with increasing retinal eccentricity are mirrored by decreased activity of the L-M channel. In comparison, the contribution of the S cone-opponent system undergoes relatively little change within the central 20 degrees . In addition, we also found that changes in saturation and hue are different from each other in terms of their variation across color space and their variation with stimulus size. Our findings suggest that perceived shifts in saturation and hue are mediated largely via the reduction in activation of the L-M cone-opponent channel but that saturation and hue might be subject to different retinal and/or cortical influences that contribute to their differing size dependencies in the peripheral retina.

Perceived shifts in hue that occur with increasing retinal eccentricity were measured by using an asymmetric color matching paradigm for a range of chromatic stimuli. Across nine observers a consistent pattern of hue shift was found; certain hues underwent large perceived shifts in appearance with increasing eccentricity, while for others little or no perceived shift was measured. In separate color naming experiments, red, blue, and yellow unique hues were found to be correlated with those hues that exhibited little or no perceptual shift with retinal eccentricity. Unique green, however, did not exhibit such a strong correlation. Hues that exhibited the largest perceptual shifts in the peripheral retina were found to correlate with intermediate hues that were equally likely to be identified by adjacent color naming mechanisms. However, once again the correlation was found to be weakest for the green mechanism. These data raise the possibility that perceptually unique hues are linked to color signals that represent the most reliable (minimally variant) chromatic information coming from the retina.

Changes of color perception in the peripheral field are measured using an asymmetric simultaneous matching paradigm. The data confirm previous observations in that saturation changes can be neutralized if the test target is increased in size. However, this compensation does not apply to hue shifts. We show that some hues remain unchanged with eccentricity whereas others exhibit substantial changes. Here the color shifts are plotted in terms of a second-stage cone opponent model. The data suggest that the S-L+M channel is more robust to increasing eccentricity than the L-M channel. Observations are interpreted in terms of the known underlying morphological and physiological differences in these channels.

To investigate the effect of foveal inhomogeneities on sensitivity to chromatic stimuli, we measured simple reaction times (RTs) and detection thresholds to temporally and spatially blurred isoluminant stimuli at retinal eccentricities from 0 deg to 8 deg. Three color-normal subjects participated. Contrast gain was derived from the slope of the RT versus contrast function. With a Gaussian spatial distribution (S.D. = 0.5 deg) and modulation between white (CIE x, y, L = 0.31, 0.316, 12.5 cd x m(-2)) and blue (MBDKL 90 deg), gain was maximal at about 2-deg eccentricity and declined by approximately 1 log unit towards the center and the periphery. The red (0 deg) and green (180 deg) cardinal axes showed maximum gain in the center, whilst the yellow (270 deg) data were intermediate. Although the spatial extent of the Gaussian spot was much larger than the S-cone free zone, we wished to determine whether foveal tritanopia was responsible for the marked drop in sensitivity to the 90-deg stimulus. To align the color vector along a tritan line, we used a smaller disk (0.3 deg) with a blurred edge and measured detection threshold, rotating the vector until minimum central sensitivity was obtained. Other workers have used transient tritanopia or minimally distinct border to similar effect. By repeating this at different locations in color space, a group of vectors were obtained. These converged near to the S-cone co-punctal point, evidence that they lay along tritan confusion lines. These threshold findings were then confirmed using the RT-derived contrast gain function. The tritan vectors were less pronounced as stimulus size increased. With the vector optimized to produce foveal tritanopia, the RT gain versus eccentricity functions for the 90-deg and 270-deg stimuli both fell markedly in the center and periphery, and sensitivity peaked at about 3-deg eccentricity. There are some similarities between these findings and the underlying photoreceptor distributions. As a result, there is a greater difference in gain between red-green and blue-yellow systems in the center than in the near periphery. We conclude that the RT versus contrast function is a sensitive index of foveal opponency.

Purpose: This study examined the influence of stimulus chromaticity on simple reaction times (RTs) to determine the stage of chromatic processing that is most influential in their generation., Methods: Simple RTs were measured in response to the cosinusoidally ramped onset of small, equiluminant, colored Gaussian spots. The chromaticity of these stimuli was varied, to modulate along a series of vectors in color space that included red-green (L-M) and blue-yellow (S-[L+M]) opponent axes., Results: RTs are highly sensitive to small departures from subjective equiluminance. They are also dependent on stimulus chromaticity. The longest RTs are generated in response to equiluminant stimuli that isolate S-cone activity, whereas the shortest are generated by stimuli that modulate the L-M opponent axis. However, temporal processing differences are highly dependent on how the chromatic stimuli are scaled in relation to one another. The differences are reduced when scaling is based on detection threshold. The relationship between chromatic contrast and RT can be described by the modified Piéron equation RT = RT(0) + k x C(-1)., Conclusions: Simple RTs generated in this study conform to the idea that they are largely determined by cone-opponency mechanisms. The use of cone contrast as a metric for scaling chromatic stimuli exaggerates differences between the temporal responsiveness of L-M and S-(L+M) opponency mechanisms.