Spectral sensitivity by constant CFF: effect of chromatic adaptation

Regional tuning of photoreceptor adaptation in the primate retina

Adaptation in cone photoreceptors allows our visual system to effectively operate over an enormous range of light intensities. However, little is known about the properties of cone adaptation in the specialized region of the primate central retina called the fovea, which is densely packed with cones and mediates high-acuity central vision. Here we show that macaque foveal cones exhibit weaker and slower luminance adaptation compared to cones in the peripheral retina. We find that this difference in adaptive properties between foveal and peripheral cones is due to differences in the magnitude of a hyperpolarization-activated current, Ih. This Ih current regulates the strength and time course of luminance adaptation in peripheral cones where it is more prominent than in foveal cones. A weaker and slower adaptation in foveal cones helps maintain a higher sensitivity for a longer duration which may be well-suited for maximizing the collection of high-acuity information at the fovea during gaze fixation between rapid eye movements.

Measurements of chromatic adaptation and luminous efficiency while wearing colored filters

The visual system adapts dynamically to stabilize perception over widely varying illuminations. Such adaptation allows the colors of objects to appear constant despite changes in spectral illumination. Similarly, the wearing of colored filters also alters spectral content, but this alteration can be more extreme than typically encountered in nature, presenting a unique challenge to color constancy mechanisms. While it is known that chromatic adaptation is affected by surrounding spatial context, a recent study reported a gradual temporal adaptation effect to colored filters such that colors initially appear strongly shifted but over hours of wear are perceived as closer to an unfiltered appearance. Presently, it is not clear whether the luminance system adapts spatially and temporally like the chromatic system. To address this, spatial and temporal adaptation effects to a colored filter were measured using tasks that assess chromatic and luminance adaptation separately. Prior to and for 1 hour after putting on a pair of colored filters, participants made achromatic and heterochromatic flicker photometry (HFP) settings to measure chromatic and luminance adaptation, respectively. Results showed significant chromatic adaptation with achromatic settings moving closer to baseline settings over 1 hour of wearing the filters and greater adaptation with spatial context. Conversely, there was no significant luminance adaptation and HFP matches fell close to what was predicted photometrically. The results are discussed in the context of prior studies of chromatic and luminance adaptation.

S-cone photoreceptors in the primate retina are functionally distinct from L and M cones

Daylight vision starts with signals in three classes of cone photoreceptors sensitive to short (S), middle (M), and long (L) wavelengths. Psychophysical studies show that perceptual sensitivity to rapidly varying inputs differs for signals originating in S cones versus L and M cones; notably, S-cone signals appear perceptually delayed relative to L- and M-cone signals. These differences could originate in the cones themselves or in the post-cone circuitry. To determine if the cones could contribute to these and related perceptual phenomena, we compared the light responses of primate S, M, and L cones. We found that S cones generate slower light responses than L and M cones, show much smaller changes in response kinetics as background-light levels increase, and are noisier than L and M cones. It will be important to incorporate these differences into descriptions of how cone signaling shapes human visual perception.

Visual sensitivity and cortical response to the temporal envelope of amplitude-modulated flicker

To investigate the perception of a temporal envelope of flickering light is important for understanding nonlinear temporal processing in the visual system. The influence of the frequency components of a flickering light on the perception of the envelope remains unclear, with few studies having investigated the cortical activities for the envelope. We investigated the detection thresholds, brightness, and magnetoencephalographic responses related to amplitude-modulated (AM) flickering lights. The results showed that the sensitivity to flicker at the envelope periodicity of the AM flickering light was lower for a high-frequency carrier (40 Hz) than for a lower-frequency one (10, 20, or 30 Hz). Also, the primary visual cortex responded at the frequency corresponding to the envelope periodicity of the AM flickering light, and the strength of the cortical response reflected the brightness of the flicker at the envelope periodicity.

Spectrally opponent inputs to the human luminance pathway: slow +M and -L cone inputs revealed by intense long-wavelength adaptation

The nature of the inputs to achromatic luminance flicker perception was explored psychophysically by measuring middle- (M-) and long-wavelength-sensitive (L-) cone modulation sensitivities, M- and L-cone phase delays, and spectral sensitivities as a function of temporal frequency. Under intense long-wavelength adaptation, the existence of multiple luminance inputs was revealed by substantial frequency-dependent changes in all three types of measure. Fast (f) and slow (s) M-cone input signals of the same polarity (+sM and +fM) sum at low frequencies, but then destructively interfere near 16 Hz because of the delay between them. In contrast, fast and slow L-cone input signals of opposite polarity (-sL and +fL) cancel at low frequencies, but then constructively interfere near 16 Hz. Although these slow, spectrally opponent luminance inputs (+sM and -sL) would usually be characterized as chromatic, and the fast, non-opponent inputs (+fM and +fL) as achromatic, both contribute to flicker photometric nulls without producing visible colour variation. Although its output produces an achromatic percept, the luminance channel has slow, spectrally opponent inputs in addition to the expected non-opponent ones. Consequently, it is not possible in general to silence this channel with pairs of 'equiluminant' alternating stimuli, since stimuli equated for the non-opponent luminance mechanism (+fM and +fL) may still generate spectrally opponent signals (+sM and +sL).

Sunset science. III. Visual adaptation and green flashes

Photographs of green flashes do not preclude a role for physiological effects in these phenomena. While green flashes are certainly not after-images, there is compelling evidence that adaptation in the visual system strongly affects the perceived color of most sunset green flashes. Furthermore, the retinal image of the setting Sun is usually bright enough to bleach most of the red-sensitive photopigment in a few seconds, making the yellow stage of a sunset flash appear green. Even in air so hazy that no green light reaches the eye, a yellow flash may occur and appear green. Many, but not all, visual observations of sunset green flashes are of this yellow flash. The yellow portion of sunset green flashes helps explain their reported durations, which exceed those expected for the appearance of green light alone.

Large-field S-cone flicker test

BACKGROUND

Disturbances of blue color vision and of temporal contrast sensitivity can indicate early damage in glaucoma. For the present study a quick and easy test was devised which examines both functions at one time by testing the temporal contrast sensitivity of a blue flickering light on an intense yellow background.

METHODS

Large coextensive background and test fields (85 degrees) are used, making fixation uncritical. Detailed experiments were made in two normal subjects to derive spectral sensitivity curves from flicker-fusion frequency (FFF) versus intensity functions and to obtain complete temporal contrast-sensitivity (De Lange) curves under different levels of adaptation and test lights. After selection of appropriate luminances and one stimulation frequency from these experiments, test-retest variability was studied in four subjects in five repetitions. In addition, normal values were collected from 22 subjects.

RESULTS

Spectral sensitivities for two levels of FFF (15 Hz and 44 Hz) agree with Stiles' pi 1 at the low and with pi 4 at the high FFF. Temporal contrast-sensitivity curves show a low-frequency section with peak sensitivity at 1 Hz and a high-frequency section with a peak at around 4 Hz. From the basic experiments the following conditions for the clinical examination were selected: Background luminance 2600 cd/m2, test luminance at 451 nm 0.8 cd/m2, stimulation frequency 4 Hz. The test-retest variability showed an acceptable intraclass correlation co-efficient (0.6).

CONCLUSIONS

The present experiments carried out with a very large stimulus led to meaningful results which are in rather good agreement with results reported in the literature on small-field stimuli. The blue-on-yellow flicker test carried out under the conditions mentioned above is a quick and easy test which could be helpful in improving early glaucoma diagnosis.

Faster than the eye can see: blue cones respond to rapid flicker

Flickering lights that are detected by the blue cones of the human visual system fuse to yield a steady sensation at much lower rates of flicker than do lights that are detected by the red or green cones. Yet, although blue-cone-detected lights flickering at 30-40 Hz appear to be steady, they are still able to interact with red- or green-cone-detected flickering lights to produce clearly detectable beats in the form of an amplitude modulation of the red- or green-cone flicker. Thus the blue cones produce a viable high-frequency flicker signal, as do the red and green cones, but one that is normally lost before it reaches sensation. The temporal-frequency response for the blue-cone beat interaction is similar in shape to the temporal-frequency response for directly detected red- or green-cone flicker. When measured through the same pathway (which we identify as the luminance pathway, since it is able to transmit high-frequency flicker), the response of the blue cones seems to be as fast as that of the other cones.

The temporal properties of the human short-wave photoreceptors and their associated pathways

Flicker modulation sensitivity measurements made on high intensity orange steady backgrounds indicate that signals from short-wavelength sensitive cones (S-cones) have access to two pathways. At low S-cone adaptation levels the frequency response falls quickly with increasing frequency, but at higher adaptation levels it extends to much higher frequencies. At these higher S-cone adaptation levels, the following procedures can selectively expose either a process sensitive to low frequencies or one more sensitive to higher frequencies: (1) at high flicker frequencies, the S-cone signal can be nulled by a long-wavelength sensitive cone (L-cone) signal of suitable amplitude and phase, but at low frequencies a residual flicker persists; the modulation sensitivity for the residual flicker is lowpass in shape with a rapid decline in sensitivity with increasing flicker frequency; (2) sensitivity to flicker in the presence of a 17 Hz S- or L-cone mask is also lowpass with a similarly steep loss of high frequency sensitivity; yet (3) sensitivity to flicker during transient stimulation of the S-cones at 0.5 Hz is comparatively wideband (and slightly bandpass) in shape. The S-cone signal produced by the high frequency process is almost as well-maintained towards high frequencies as M- and L-cone signals. Furthermore, it is capable of participating in flicker photometric nulls with M- and L-cone signals. At low frequencies, however, when the low frequency S-cone signal is also present, satisfactory nulls can not be found. From these and phenomenological considerations, we identify the low and high frequency S-cone processes as S-cone inputs to the chromatic and luminance pathways, respectively. The phase adjustments needed to optimize flicker photometric nulls reveal that the S-cone input to the luminance pathway is actually inverted, but this is demonstrable only at relatively low frequencies: at medium or high frequencies the S-cone influence can be synergistic with that of the other cone types because of a delay in the transmission of S-cone signals.

Chromatic suppression of cone inputs to the luminance flicker mechanism

Eisner and MacLeod [J. opt. Soc. Am. 71, 705-718 (1981)] showed that intense green and red chromatic adapting fields may suppress respectively the M and L cone input to the luminance mechanism by a factor considerably greater than Weber's law. We obtained evidence for such chromatic suppression by measuring complete detection contours for different ratios of red and green test lights presented in rapid flicker in the center of a uniform field. The detection contours represent thresholds as the quantal modulation of the M and L cones normalized by the quantal catch owing to the field. Luminance flicker mechanisms were identified by sections of the contours where detection was controlled by a linear sum of the M and L cone test signals. The slope of these sections indicated that intense red fields selectively suppressed the L cone input to the luminance mechanism by a factor greater than Weber's law; evidence was much less firm for an analogous suppression of the M cone input by intense green fields. The shape of the detection contours also suggests that intense red fields, which differentially light-adapt the M and L cones, may produce a moderate temporal phase-shift between the M and L cone signals. The shape of the temporal MTF of the M cone and the L cone input to the luminance mechanism may be determined at the cone stage, with the absolute sensitivity (vertical scaling) being partially dependent on selective chromatic suppression of the cone inputs owing to the intense chromatic field. Luminance and red-green chromatic temporal sensitivity functions are presented in terms of the M and L cone quantal modulations. Chromatic sensitivity progressively rises above luminance sensitivity as temporal frequency is gradually lowered below 15 Hz, with the consequence that 'contrast sensitivity' may be much higher for color than for luminance.

Rod-cone interaction in flicker detection

There is considerable evidence in the literature that rod-cone interaction occurs when both rods and cones simultaneously detect a test target. More recent evidence, however, has shown a parafoveal rod-cone interaction during dark adaptation for a purely cone-detected flickering test stimulus; this influence on cone threshold appears to be mediated by surrounding rods. In this study, we demonstrate a similar rod-mediated influence on parafoveal cone-detected flicker threshold. More surprisingly, foveal cone-detected thresholds are also influenced by rods. This effect occurs over at least a 2 log unit intensity range of mesopic background level; cone-detected 25 Hz flicker sensitivity is enhanced by increasing the radiance of the background. The action spectrum of this effect fits the scotopic spectral sensitivity curve. At higher background levels, this rod-cone interaction disappears and surrounding cone activity then influences the cone flicker threshold. The results suggest that, as rods recover sensitivity, they reduce cone-detected flicker sensitivity, even at the fovea. The rod influence on cone flicker is most apparent for long wavelength test stimuli. Our results, in agreement with recent reports, suggest that the rod-cone interaction is laterally-mediated and may be specific for the long wavelength-sensitive cone type.

Luminance and opponent color contributions to visual detection and to temporal and spatial integration: comment

King-Smith and Carden have postulated a "luminance system" or achromatic channel in the visual system which has a temporal response better than the other channels, and which also responds to high spatial frequencies better than the other channels. All evidence, both psychophysical and electrophysiological, indicates that these properties are contradictory.