S-cone Responses Research Articles

Stability of the color-opponent signals under changes of illuminant in natural scenes

Illumination varies greatly both across parts of a natural scene and as a function of time, whereas the spectral reflectance function of surfaces remains more stable and is of much greater relevance when searching for specific targets. This study investigates the functional properties of postreceptoral opponent-channel responses, in particular regarding their stability against spatial and temporal variation in illumination. We studied images of natural scenes obtained in UK and Uganda with digital cameras calibrated to produce estimated L-, M-, and S-cone responses of trichromatic primates (human) and birds (starling). For both primates and birds we calculated luminance and red-green opponent (RG) responses. We also calculated a primate blue-yellow-opponent (BY) response. The BY response varies with changes in illumination, both across time and across the image, rendering this factor less invariant. The RG response is much more stable than the BY response across such changes in illumination for primates, less so for birds. These differences between species are due to the greater separation of bird L and M cones in wavelength and the narrower bandwidth of the cone action spectra. This greater separation also produces a larger chromatic signal for a given change in spectral reflectance. Thus bird vision seems to suffer a greater degree of spatiotemporal "clutter" than primate vision, but also enhances differences between targets and background. Therefore, there may be a trade-off between the degree of chromatic clutter in a visual system versus the degree of chromatic difference between a target and its background. Primate and bird visual systems have found different solutions to this trade-off.

Clinical S-cone ERG recording with a commercial hand-held full-field stimulator

Our purpose was to explore S-cone ERG protocols for a commercial full-field hand-held stimulator that contains colored LEDs, and to see whether the test would be useful as a part of routine ERG testing. S-cone responses were elicited by blue flashes over a longer-wavelength background. With the standard stimulator containing blue (461 nm), green (513 nm) and red (652 nm) LEDs, we were unable to obtain satisfactory responses. Reproducible S-cone ERGs were obtained with a stimulator that had been custom-fitted with shorter-wavelength blue (440 nm) LEDs for stimulation, and orange (590 nm) LEDs for background adaptation. S-cone responses took only a few minutes to record, and the typical waveform showed a slow peak at 45-50 ms with amplitude 3-9 microV, but ranging from 0 microV to more than 10 microV. Larger waves appeared in a patient with enhanced S-cone syndrome. S-cone responses could also be obtained with an alternating blue-orange flicker protocol. We added the S-cone response to our regular ERG protocol for a number of months. Although most normal subjects and patients showed recognizable S-cone responses with this stimulator, the amplitudes were small and there was too much variability to make the technique effective for routine clinical testing. In general, the S-cone responses followed the standard cone ERG responses in disease.

The role of short-wavelength sensitive cones and chromatic aberration in the response to stationary and step accommodation stimuli

The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives “static” accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.

Selective reduction of S-cone response and on-response in the cone electroretinograms of patients with X-linked retinoschisis.

To examine the electroretinograms (ERGs) of the short-wavelength-sensitive (S-) and the mixed long- and middle-wavelength-sensitive (L,M-) cones, and the ON- and OFF-responses of the cone ERGs in three patients with X-linked juvenile retinoschisis (XLRS). Cone ERGs elicited by different color flashes and those elicited by long duration stimuli under Ganzfeld conditions were recorded from three patients with XLRS. The S-cone b-waves were undetectable to short-wavelength stimuli in all three XLRS patients, while the L,M-cone ERG b-waves were within the normal range. To long-duration white stimuli, the ON-response (b-wave) was reduced and delayed in all patients compared with that of the normal subjects, while the d-wave or OFF-response appeared normal in amplitude and implicit time. These results support the hypothesis that the normal S-cone ERG arises primarily from the ON-pathway of the cone ERGs and the hypothesis that ON-bipolar cells are predominant in the S-cone system.

The Independent Yellow Channel For Tetrachromatic Imaging

Imaging systems tend at first to simulate the opponent yellow and blue vision channels (YB) even if that perceptibly violates trichromatic simulation of opponent red and green channels (RG). The preference is biologically traced: 8% male population are deficient in RG vision and only 0.003% in YB vision. The YB pair is not the usually assumed alternating combination of M, L, and S-cone responses (say, L+M-S): it is independent of RG sensations (L-M). The complementary yellow (570) and blue (465 nm) are seen after the RG pair has disappeared with decreasing illumination. The yellow is not seen after the 650-nm sensitivity limit of rhodopsin even if the red is over 50 nm more. The ‘blue-cone monochromats’ and some protanopes, having no M and L-cones, see the YB pair. Yellow adaptation eliminates the YB sensations but hardly influences the RG pair. Reflectometry of living retinas shows the ‘M and S-cone pigments’ rather to be long-living products of iodopsin (L-pigment) and rhodopsin.

S-cone ERGs elicited by a simple technique in normals and in tritanopes

Purpose: to measure changes in the relative spectral sensitivities of the dark adapted and light adapted ERG and thus to establish the possible contribution of rods to the ‘blue cone’ ERG elicited by flashes of blue light. Background: short wavelength stimuli in the light-adapted eye evoke small rounded b-waves which have been considered to be S-cone responses. We have recorded such responses from tritanopes, which called the assumptions into question. Methods: small ERGs were recorded to blue and green flashes. The stimulus was a Ganzfeld which employed light emitting diodes. ERGs were obtained in both the dark-adapted eye and after light adaptation to intense orange light (peak wavelength 610 nm). The change in sensitivity with light adaptation and the relative spectral sensitivity was determined from the voltage/log light intensity functions, using a 10 μV criterion. Results: (1) peak times and changes in sensitivity did not help distinguish light-adapted rod from possible S-cone responses; (2) analysis of the change in the ratio of blue:green sensitivity from darkness to 4.4 log Td. 610 nm background suggests that in seven normal subjects, 90% or more of the ERG evoked by 440 nm flashes is generated by S-cones; (3) three tritanopes have insignificantly reduced S-cone responses. Conclusions: (1) clinical techniques used to isolate S-cone ERGs are appropriate; (2) there are at least two types of tritanope and in those we investigated, functional S-cones are probably displaced into the retinal periphery.

Colour tuning in human visual cortex measured with functional magnetic resonance imaging.

The primate retina contains three classes of cones, the L, M and S cones, which respond preferentially to long-, middle- and short-wavelength visible light, respectively. Colour appearance results from neural processing of these cone signals within the retina and the brain. Perceptual experiments have identified three types of neural pathways that represent colour: a red-green pathway that signals differences between L- and M-cone responses; a blue-yellow pathway that signals differences between S-cone responses and a sum of L- and M-cone responses; and a luminance pathway that signals a sum of L- and M-cone responses. It might be expected that there are neurons in the primary visual cortex with response properties that resemble these three perceptual pathways, but attempts to find them have led to inconsistent results. We have therefore used functional magnetic resonance imaging (fMRI) to examine responses in the human brain to a large number of colours. In visual cortical areas V1 and V2, the strongest response is to red-green stimuli, and much of this activity is from neurons receiving opposing inputs from L and M cones. A strong response is also seen with blue-yellow stimuli, and this response declines rapidly as the temporal frequency of the stimulus is increased. These responses resemble psychophysical measurements, suggesting that colour signals relevant for perception are encoded in a large population of neurons in areas V1 and V2.

Reappraisal of a short-wavelength-sensitive (S-cone) recording technique in routine clinical electroretinography.

The recording of blue cone (S-cone) responses as described by Gouras and MacKay was slightly modified and incorporated into our routine ganzfeld electroretinogram protocol. We found a mean S-cone amplitude of 5.0 microV (range 2.9-6.9 microV) and a mean S-cone implicit time of 41.5 msec (range 40-46 msec). Separation between the combined red and green cone (L-M-cone) response and the S-cone response was obtained with blue flash stimuli on a yellow adapting background.

Contributions of short-wavelength cones to goldfish ganglion cells.

There are conflicting reports about the existence and nature of a short-wavelength cone (S-cone) contribution to ganglion cells in the goldfish retina. The present study sought to resolve these discrepancies by examining the S-cone contribution while recording from single ganglion cells in the excised, isolated goldfish retina. The effect of variations in the retinal preparation (gas content and type of background lighting during recording) on the S-cone input was also examined. Cells were classified into one of three types based on the responses at light onset and offset, when responses were driven only by the long-wavelength cone system (L-cones) of the receptive field's center (L+/-(on-excitation/off-inhibition), L-/+, and L+/+). With rare exceptions, the threshold spectral sensitivities of the centers and surrounds of cells that possessed opposite on and off responses (L+/- and L-/+) exhibited S-cone contributions, either prior to and/or during chromatic adaptation of the middle- and long-wavelength cones; the S-cone response was antagonistic to the L-cone input. The L+/+ center cells also contained a S-cone input, but it was synergistic to the L-cone input at suprathreshold intensities. These findings were robust across all of the retinal preparations employed. The discrepancies in the previous work were probably due to the incomplete classification of cells because of the use of threshold responses only.