TRANSITION FROM ROD TO CONE VISION:III. Successive contrast anew

S tabell , U. & S tabell , B. Transition from rod to cone vision. III. Successive contrast anew. Scand. J. Psychol ., 1969, 10 , 140–144—The relation between the specific threshold level and the upper limit of the scotopic contrast color was investigated. The achromatic interval between the scotopic and the photopic component increased when time in darkness increased, and when pre-stimulation was reduced as regards intensity, duration, or cone/rod ratio. The results are interpreted on the basis of the opponent theory of color vision.     

CHROMATIC ROD AND CONE ACTIVITIES AS A FUNCTION OF THE PHOTOCHROMATIC INTERVAL

S tabbll , B. & S tabell , U. Chromatic rod and cone activities as a function of the photochromatic interval. Scand. J. Psychol ., 1969, 10,215– 219.—The Sco topic contrast color dominated for longer periods and at higher intensities the larger the magnitude of the photochromatic interval, indicating that the relative responsiveness of the chromatic rod to the chromatic cone activity increases as a function of the photochromatic interval.     

TRANSITION FROM ROD TO CONE VISION I. Simultaneous contrast

S tabell , B. Transition from rod to cone vision. I. Simultaneous contrast. Scand. J. Psychol ., 1969, 10 , 61–64.—Above the specific threshold, color quality depended on the test filter, while at lower intensities the same color was observed irrespectively of the test filter used, confirming the assumption that colors within the photochromatic interval are triggered by rod activity. The lawful relation between rise of specific threshold and increase of rod sensitivity was not found under the condition of simultaneous contrast.     

COLOR THRESHOLD MEASUREMENTS IN SCOTOPIC VISION: Test-stimulation varied

S tabell , U. & S tabell , B. Color threshold measurements in scotopic vision. II. Test-stimulation varied. Scand. J. Psychol ., 1968, 9, 129–132.—The color threshold curve generally coincides with the dark adaptation curve of the rods, irrespectively of test-stimulation variation, confirming the assumption that a threshold response of rods may initiate a color-related process. Variation of color threshold intensity is thus assumed to reflect variation of rod threshold intensity.     

CHROMATIC ROD VISION III. Duration of pre-stimulation varied

S tabell B. & S tabell U. Chromatic rod vision. III. Duration of pre-stimulation varied. Scand. J. Psychol ., 1972, 13 , 136–140.—The scotopic contrast hue was determined as a function of duration of pre-stimulation, together with the additive opponent hue. It is concluded that the red/blue ratio which has to be added to the additive opponent hue to produce the scotopic contrast hue, may change as a function of both wavelength and duration of pre-stimulation.     

CHROMATIC ROD VISION

S tabell B. & S tabell U. Chromatic rod vision. IV. Time between pre-and test-stimulation varied. Scand. J. Psychol ., 1972, 13 , 141–144.—The scotopic contrast hue was determined as a function of time between pre-and test-stimulation. As the interval increased, the scotopic hue was found to change from orange toward yellow in the 440–475 nm region of pre-stimulation and from violet-blue toward violet in the 550–595 nm region. It is concluded that (1) depending on wavelength of pre-stimulation, the processes in the red-green substance may revert to the mid-valued state faster or slower than the processes in the yellow-blue substance, and (2) the red/blue ratio of the violet hue which has to be added to the additive opponent hue to produce the scotopic contrast hue, may change as a function of time between pre- and test-stimulation.     

Rod involvement in peripheral color processing

Abstract.— It has previously been suggested that rods act as blue receptors in peripheral color vision. Two experiments examining this issue were conducted. Experiment 1 investigated the perceived hue of a test light presented at a luminance level above chromatic threshold. At 8° in the periphery, the 500 nm test light was perceived as more blue under conditions of dark adaptation than after light adaptation. Similar differences were not found for foveal presentation. The increased blue in the periphery after dark adaptation was attributed to a rod contribution. In Experiment 2 an attempt was made to mix a rod contribution obtained with a 470 nm light below chromatic threshold, with a cone color obtained from a 670 nm light presented above chromatic threshold. No evidence was obtained to support the idea that a blue produced by rods stimulated below chromatic threshold could mix with a red produced by cones stimulated above chromatic threshold. The results are discussed in terms of a rod contribution to hue which is dependent on the luminance level of short wavelength stimulation.     

Brightness interactions between rods and cones

Two parafoveal test targets with different spectral compositions were matched in brightness to a fixed-luminance foveal reference target under scotopic adaptation conditions. The idea of the experiment was to find a reference luminance for which one of the matching test targets stimulated only rods while the other stimulated both rods and cones. If brightness was proportional to the linear sum of rod and cone responses, then the luminance of the matching rod+cone target would be predictably closer to rod threshold than would that of the rod target. The results were complicated by evidence that rod responses to the test targets selectively enhanced weak chromatic signals. Nevertheless, it was possible to show that cone activity never reduced the matching luminance as much as predicted by the additivity hypothesis, and sometimes even increased it. These findings suggest that cone activity can suppress brightness signals from rods.     

ROD VISION AS CHROMATIC VISION

S tabell , B. Rod vision as chromatic vision. Scand. J. Psychol ., 1968, 9, 282–288.—It was found (I) that the smallest quantity of light of pre-stimula-tion which produces color upon test-stimulation, stands in unique relation to the intensity of the specific threshold, and (2) that the size of the pre-and test-stimulation fields may affect the duration of the after-image. The results are judged to indicate that pre-stimulation of cones creates the disposition for the color-related response, and that the color-related response is generated centrally to the photochemical systems of the receptors.     

FACILITATION OF CHROMATIC CONE ACTIVITY BY ROD ACTIVITY

STABELL, U. & STABELL, B. Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity. Scand. J. Psychol. , 1971, 12, 168–174.–At the cone-rod break of the dark adaptation curve, the specific threshold was found to drop to lower intensity levels, while the threshold curves of fovea proceeded in one step only, confirming the suggestion that rods may facilitate chromatic-related cone activity.     

Stereoscopic acuity for photometrically matched background wavelengths at scotopic and photopic levels

Two Ss made equidistance settings in a two-rod apparatus using white and four chromatic (red, yellow, Feen, and blue) backgrounds, photometrically matched at each of eight or nine teat levels within a total retinal-illuminance range of 4 log units. The binocular depth setting were analyzed in terms of the angular magnitude of both the Yariable error, η_AD, and the constant error, η_ΔR When η_AD is plotted as a function of retinal illuminance, the curves for each of the four chromatic IIIId white backlfound coditions show that at low retinal illuminances, η_AD, is initially large, and with increasing background illumination, η_AD progressively decreases to approach a final low asymptotic value. As predicted by the duplicity theory of vision, each experimental curve shows a dircontinulty at about ?1.0 lot td (corresponding to a background luminance value of about 0.0069 fL with the 2.5-mm artificial pupil used) reflecting the transition from rod to cone functioning. The curves representing the different wavelengths essentially overlap throughout the total illumination range, indicating that, for both rod and cone vision, wavelength has no differential effect on the variability of depth settings. The data for η_ΔR are less regular than those for η_AD and the rod-cone discontinuities appear less pronounced. The data are analyzed in terms of the relative contribution of the rod and cone mechanisms to performance level.     

RODS AS COLOR RECEPTORS IN PHOTOPIC VISION

Comparing a foveal and an extra-foveal field during dark adaptation, transition from chromatic to achromatic vision at intensity levels above the cone plateau started around the break of the dark adaptation curve. Pre-stimulating the two fields in a dark-adapted state with deep red, and test-stimulating when returning sensitivity had reached absolute threshold of the dark-adapted eye, with green filters at intensities above the specific threshold, the fields matched as to hue and saturation. It appears that rod and cone activities are integrated and function as a synchronized unit during the initial recovery phase of dark adaptation.     

COLOR THRESHOLD MEASUREMENTS IN SCOTOPIC VISION: Simultaneous color contrast

S tabell , U. & S tabell , B. Color threshold measurements in scotopic vision. III. Simultaneous color contrast. Scand J. Psychol ., 1968, 9, 133–137.—Color may be observed well below the break-point level of the dark adaptation curve, suggesting that the impulse pattern initiated in the rods may trigger a color-related response. Color is induced when the intensity of the inducing field reaches a certain level above the specific threshold, provided the stimulation of the test field is observable.     

The fluttering-heart illusion: a new hypothesis

The fluttering-heart illusion is a perceived lagging behind of a colour target on a background of a different colour when the two are oscillated together. It has been proposed that the illusion is caused by a differential in the perceptual latencies of different colours (Helmholtz 1867/1962), a differential in rod-cone latencies (von Kries 1896) and rod-cone interactions (von Grünau 1975, 1976 Vision Research 15 431-436, 437-440; 16 397-401; see list of references there). The purpose of this experiment was to assess the hypothesis that the fluttering-heart illusion is caused by a differential in the perceived velocities of chromatic and achromatic motion. To evaluate this hypothesis, we tested observers possessing normal colour vision and deuteranopes. The perceived delay of a chromatic target relative to an achromatic target was measured as a function of background cone contrast and target colour. For observers with normal colour vision, the perceived delay of the chromatic target is greater in the L-S than the L-M testing conditions. The reverse is observed in deuteranope observers. We suggest that this is caused by the absence of an L-M opponent mechanism contributing to chromatic motion in deuteranopes. Greater background cone contrasts tended to yield smaller perceived delays in both normal and deuteranope observers, indicating that greater chromatic modulation decreases the perceived delay of the colour target. These results support the hypothesis that the fluttering-heart illusion can be explained by a differential in the perceived velocities of chromatic and achromatic motion.     

Purely chromatic perception of motion in depth: two eyes as sensitive as one

Motion hyperacuity (phase) thresholds were measured for both lateral and stereoscopic oscillatory motion in both luminance and equiluminant red/green gratings of 2 cycles per degree. Thresholds for lateral chromatic motion did not exhibit the inhibitory fall-off at low temporal frequencies that was found for luminance motion. Phase thresholds for purely chromatic motion were substantially higher than those for luminance gratings, in proportion to the ratio of cone signal modulation, but they could be predicted from the corresponding contrast sensitivities for both types of stimulus. Stereomovement thresholds in luminance gratings showed the stereomovement suppression effect relative to monocular motion sensitivity previously reported for line stimuli, but purely chromatic gratings did not. Together with the lack of an inhibitory fall-off, these results imply that chromatic and luminance motion are processed by different neural pathways, and that the chrominance pathway is capable of supporting a strong percept of stereoscopic motion from purely chromatic gratings.     

Hue discrimination in peripheral vision under conditions of dark and light adaptation

Rod interference is a possible factor contributing to the elevation of chromatic threshold in peripheral vision. It was found that light adaptation lowered peripheral chromatic thresholds. This result was interpreted as being due to the lowering of rod sensitivity. It was also found that light in the photochromatic interval appeared blue, indicating that rods may add a blue component to peripheral color vision.     

Dark-adaptation mechanisms of the long-wave foveal cones

The ordinary long-term rod and cone dark-adaptation curves have generally been assumed to follow a single exponential rate of recovery. However, in two previous papers on rod dark-adaptation (Stabell et al., 1986a, b), the recovery curve was found to consist of three different sections. The results of the present paper show the same type of recovery function with three different sections for the long-term dark-adaptation curve of the long-wave cone system. During the major, middle section log cone threshold, like log rod threshold, is linearly related to the logarithm of the concentration of bleached photopigment. Presupposing that the bleached cone photopigment acts as a ligand, the change in threshold level obtained during the middle section of the dark-adaptation curve is well described by the change in activity rate of an allosteric, postively cooperative enzyme built as a dimer.     

Pathways in type-B (U-shaped) metacontrast

Rod and cone targets were crossed, in every combination, with rod and cone masks in flanking-bars metacontrast. Strong type-B (U-shaped) metacontrast was obtained in each condition, contrary to the claim that rod and cone masking are independent. In each condition, visibility declined steadily with stimulus-onset asynchrony (SOA) in trials in which target and mask appeared to be simultaneous, and increased with SOA in trials in which they appeared to be successive. The 'U' results from collapsing across these different types of trials, which may reflect distinct monotonic processes in masking. Under the light adaptation conditions used the time, Tmax, at which metacontrast was at a maximum was delayed by about 25 ms if rods, rather than cones, detected the target. Whether rods or cones detected the mask hardly altered Tmax.     

Cone excitation ratios correlate with color discrimination performance in the horse (Equus caballus)

Six horses (Equus caballus) were trained to discriminate color from grays in a counterbalanced sequence in which lightness cues were irrelevant. Subsequently, the pretrained colors were presented in a different sequence. Two sets of novel colors paired with novel grays were also tested. Performance was just as good in these transfer tests. Once the horse had learned to select the chromatic from the achromatic stimulus, regardless of the specific color, they were immediately able to apply this rule to novel stimuli. In terms of the underlying visual mechanisms, the present study showed for the first time that the spectral sensitivity of horse cone photopigments, measured as cone excitation ratios, was correlated with color discrimination performance, measured as accuracy, repeated errors, and latency of approach.     

Spare the rod and spoil the icon

Short-term visual storage was investigated with a successive field paradigm, so that correct performance depended upon combining visual information from two targets that were never on simultaneously. In the first two experiments, the stimuli consisted of two slides, each containing a 10' red dot on a gray surround, and separated by an interstimulus interval (ISI) from 20 to 400 msec. Subjects had to determine if the dots were vertically or horizontally aligned. In Experiment 1, the stimuli had either no contrast for the rods or no luminance contrast for the cones or high contrast. At short ISIs the cone contrast determined performance, whereas at long ISIs the rod contrast determined performance. In fact, when the dots were invisible to the rods, the task was impossible for long ISIs. In Experiment 2, performance was compared for zero log rod contrast and for small departures from zero. Even a small departure from zero log rod contrast resulted in above-chance performance. In Experiment 3, the stimuli were luminous rectangles and the task was to decide whether or not a 4' spatial gap was present between the two successively presented rectangles. Wavelength, luminance, and ISI were varied under both photopic and scotopic adapting conditions. The result was that the rods performed the task for ISIs of 150 msec or longer under scotopic conditions and under the photopic conditions that we were able to test. The results of the experiments taken together are consistent with the hypothesis that the cone icon is short, whereas the rod icon is robust and long lasting.