Transient desensitization of a red-green opponent site

Effects of ageing on postreceptoral short-wavelength gain control: transient tritanopia increases with age

We investigated the effect of ageing on the neural gain control in the short-wavelength opponent channel. In order to tackle specifically postreceptoral changes, we determined the effect of ageing on transient tritanopia, a paradoxical and transient reduction of short-wavelength sensitivity after the presentation of a long-wavelength adapting light. The results demonstrate an unexpected and significant increase of transient tritanopia with age, which cannot be explained by a general decline of short-wave sensitivity or the selective reduction of retinal illumination. Instead, our data imply that ageing affects also short-wavelength gain control at the site of chromatic opponency or beyond. Age-related changes of adaptation processes should therefore be considered an important factor influencing the visual performances of the elderly.

Chromatic discrimination with variation in chromaticity and luminance: data and theory

Boynton and Kambe developed a model of chromatic discrimination in which thresholds are mediated by two independent mechanisms: the short-wavelength sensitive (S-) cones (S-cone axis), and the middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones (M/L-cone axis). In this study, we used a Maxwellian view optical system to investigate fundamental properties of the model as a function of chromaticity and luminance. We confirmed that discriminations along the S-cone axis were dependent on S-cone excitation level. However, changes in chromaticity and changes in mean luminance were not described by a single threshold-vs-radiance (TVR) template. We developed a model to account for the different effects of changing S-cone excitation by varying mean chromaticity and by varying mean luminance. M/L-cone discriminations showed a minimum at the L-cone excitation to white, indicating strong opponency. The thresholds increased with luminance approaching a Weber region and showing parallel functions for differing chromaticities. These data are fit by a model allowing retinal gain controls and spectral opponency.

Red-green color discrimination as a function of stimulus field size in peripheral vision

Red-green color-discrimination thresholds were measured at eccentricities of 10 and 25 deg in the nasal retina. Thresholds were measured as a function of stimulus field size both during the cone plateau and after dark adaptation. During the cone plateau, threshold decreased with increasing field size, but the effect of field size was dependent on the color of the test stimulus. The decrease in threshold was greater for yellow and orange test stimuli than for red and green tests. Two factors, summation and opponent-mechanism adaptation, appear to affect the relation between threshold and field size. An equation suggested by Boynton and Kambe in 1980 [Color Res. Appl. 5, 13 (1980)] provides a good description of the variation in thresholds with field size and eccentricity. After dark adaptation, thresholds increased for all test colors, suggesting that rod signals reduce discrimination. The dark-adapted thresholds could be described well by the addition of a rod term to the Boynton-Kambe equation.

Temporal phase response of the short-wave cone signal for color and luminance

A chromatic discrimination paradigm was used to measure the temporal phase of the S (short-wave cone) signal relative to the L--M (long-wave cone minus middle-wave cone) signal. Suprathreshold equiluminant red-green flicker that stimulates the L--M mechanism was presented on a steady, intense yellow-green adapting field. Violet flicker that stimulates the S cones was added to the red-green flicker at different temporal phase angles, and the violet modulation depth was varied to achieve a chromatic discrimination threshold. A template was fitted to the data relating thresholds to phase: the location of the template symmetry axis showed that the S signal lagged L--M by about 75-90 degrees at 10 Hz. This is about one half the phase lag obtained for luminance or motion discrimination. The phase discrepancy shows that there are separate luminance and chromatic mechanisms receiving S cone inputs. The hue of the flicker in the present study varied strongly with phase angle, with the positive and negative excursions of the S cone signal producing a reddish-blue and greenish-yellow, respectively, and these colors combined with the reddish and greenish hues produced by the L--M signal. The observed phase shift, and measured color appearance of the combined flicker, account for the colors seen on a radially segmented disk of Munsell hues when rotated: the colors differ strikingly depending on the direction of rotation.

Estimating middle-wavelength and long-wavelength cone sensitivity with large, long-duration targets and small, brief targets

Incremental threshold versus field-intensity curves (tvi) and spectral sensitivity functions were measured for 500 nm and 667 nm test flashes, either under the standard two-colour threshold conditions of Stiles (target: 1.0 deg, 200 ms)--which are known to favour detection by the chromatically-opponent pathways--or with a smaller (0.1 deg), shorter-duration (10 ms) target--chosen to favour detection by the non-opponent (achromatic) pathway. The data reveal differences between the two conditions: for the small, brief target, the tvi curves were shallower and less dependent upon wavelength, and the spectral sensitivity functions were narrower than for the standard target.

Transient tritanopia of a second kind

Using a psychophysical method that allows the tracking of very rapid changes in sensitivity, we demonstrate an anomaly in the time-course of light adaptation for the short-wave mechanism: after the onset of a yellow (581 nm) field of approximately 10(5.3) td the threshold for short-wave targets does not recover monotonically but continues to rise for several seconds before falling to its equilibrium value. The phenomenon is absent when the adapting field has a wavelength of 511 nm and has been adjusted to give a similar equilibrium value for the short-wave threshold.

The pi mechanisms of W S Stiles: an historical review

The origins, development, and status of the pi mechanism theory are reviewed. The paper is divided into four sections. In the first section Stiles's general ideas about 'color mechanisms' are examined, and it is concluded that foremost amongst these is a mathematical theory that specifies certain formal rules or laws that should govern a certain class of observations. In the case of pi mechanisms, the class of observations is that of two-color thresholds, and the defining laws are the two well-known displacement laws. Five other laws that two-color increment-threshold observations should obey, if the latter are governed by ideal pi mechanisms, are abstracted from Stiles's writings. In the second section literature pertinent to the testing of the seven Stilesian laws is reviewed, and it is asked whether or not the seven pi mechanisms of Stiles do in fact obey the laws. In the third section the relation of the pi mechanism concept to physiological concepts is examined, and its relation to the 'cone fundamental' is discussed; the evidence pertinent to the question: "Are any of the pi mechanisms of the single-fundamental type?" is then reviewed. The last section is devoted to the evolution of Stiles's ideas in the period after 1959 when Stiles's own investigations and those of others propelled him to reject the initial (1953) pi mechanism theory as an adequate characterization of the data of the two-color threshold.

Spatial and chromatic antagonism between the long- and middle-wavelength cones in the detection of long-wavelength flashes

Field additivity was tested for small red increments on small and large adapting fields. The results for a large adapting field showed the failure of field additivity of the cancellation type, providing strong evidence for cone-antagonistic coding, while the results for a small adapting field showed field additivity. The cancellation effect took place concurrently with the spatial sensitization effect, thus suggesting that the spatial and chromatic antagonism occur concomitantly at some stage in the detection pathway.

The early phase of dark adaptation in human infants

Two types of thresholds with 8 degrees circular test fields were measured in 7- and 13-week-old human infants and in adults. Increment thresholds were measured against adapting fields of 0.50 and 50 cd/m2. All ages showed Weber's Law for incremental sensitivity over this 2 log unit range of luminance. Thresholds during early dark adaptation were also measured for the 5 sec immediately following the offsets of these adapting fields. The reductions in threshold during the early phase of dark adaptation were quantitatively similar in all age groups at both adapting luminances, despite substantial developmental differences in the absolute values of these thresholds. These data do not reject the hypothesis that the neural processes underlying early dark adaptation are adult-like in early infancy.

Second-site adaptation in the red-green chromatic pathways

On different chromatic adapting fields, thresholds were measured with a 1.2 deg flash consisting of simultaneous incremental and decremental red and green components that stimulate the M and L cones in any desired ratio. Thresholds were plotted in normalized coordinates in which the quantal change in the M and L cones due to the flash was divided by the quantal catch due to the field. Detection contours for a wide range of test flashes provide evidence for luminance and chromatic mechanisms that respectively respond to the sum and difference of the M and L cone signals. Field color has little influence on the luminance mechanism but strongly affects chromatic detection, with sensitivity being maximal on yellow fields and declining slightly on green fields and declining strongly on red fields. Similar effects were obtained for long (200 msec) and very brief flashes, although the shape of the contours differed considerably. The results provide evidence for a second adaptation site within the red-green chromatic pathways, similar to the second-site in the S cone pathways. Chromatic fields (green and red) polarize the site and reduce sensitivity to chromatic flashes.

Distinguishing opponent and non-opponent detection pathways in early dark adaptation

Desensitization of the red-green opponent pathway was demonstrated in early dark adaptation with the aid of a test chosen to isolate that pathway. Isolation was achieved by requiring the observer to adjust the intensity of a foveal test light to the threshold for flicker, when the test alternated slowly between luminance-matched red and yellow fields. The luminance matches were precise enough that only an opponent pathway could mediate the flicker thresholds. Desensitization occurred after continuous light adaptation to 626 nm fields, but did not occur after adaptation to yellow fields, or if the 626 nm field was turned on and off at 2 Hz throughout adaptation. The properties of the red-green pathway measured with the flicker thresholds resemble those of the yellow-blue pathway as shown in transient tritanopla.

Exchange thresholds for long-wavelength incremental flashes

Thresholds of 1-deg, 200-msec, 641-nm foveal test flashes rise after an exchange of II5-equated 536- and 626-nm fields, taking about 30 sec to recover. Silent substitution (no rise of threshold after the exchange) occurs, however, if these fields are alternated during adaptation. Thresholds for a 1-deg, 20-msec test rise similarly after an exchange but recover in only 0.5 sec and are not influenced by alternation of the 536- and 626-nm fields. These results can be accounted for if the 641-nm tests are detected not through pathways controlled by long-wavelength cones alone but through nonopponent (20-msc) and opponent (200-msec) pathways whose sensitivities may be reduced by transient inputs from other cones.

Exchange thresholds for green tests

Thresholds for detection of foveal pi 4-detected 522 nm, 1 deg, 200 msec test flashes rise by up to 0.4 log units just after a pi 4-equated "exchange" field has been substituted for the adaptation field, and take 60-90 sec to recover. This effect is not changed by an intense blue auxiliary field, and varies with the difference between adaptation and exchange field wavelengths as if only inputs from long-wavelength (LW) cones are involved. The effect can be virtually eliminated by adaptation to a background which alternates at 15 Hz between selected pi 4-equated long- and short-wavelength fields, which suggests that the LW cones exert their effect at a red-green opponent site.

Adaptation in the long-wavelength pathways

We describe and test predictions of a model of long-wavelength test sensitivity upon large, uniform backgrounds. The model explains changes in sensitivity in the red-green detection pathways strictly based upon losses of sensitivity in the receptors. We derive the prediction that field mixture data for field-mixtures of mu1 (fixed) and an addend, mu 2, must follow the same shape on different intensities of the fixed background, mu 1. This prediction is not in good agreement with the measurements.