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Two psychophysical experiments investigated perceptual differences between increases and decreases in stimulation of the short-wavelength (S) cone photoreceptors. In Experiment 1, observers’ suprathreshold perceptual scale responses to S cone stimulation were estimated using the Maximum Likelihood Difference Scaling (MLDS) procedure. In Experiment 2, observers’ pedestal discrimination thresholds were measured with a two alternative forced choice (2AFC) method. Both experiments were performed using incremental (S+) and decremental (S− ) contrasts separately. Substantial asymmetry between S+and S− was found in pedestal discrimination thresholds, but not in S+and S− perceptual scales: perceived S cone contrast was nearly linear with S cone contrast for both polarities. To reconcile perceptual scales and thresholds, a model is proposed in which the noise in the S cone pathway is assumed to be proportional to the square root of stimulus contrast. The model works well for both the perceptual scales and forced-choice discrimination, indicating that S+ and S− signals are processed in an asymmetrical way, likely due to the physiological differences between S ON and S OFF pathways. 

In the Boynton Illusion, the perceived location of a low-contrast chromatic edge is altered by a nearby high-contrast luminance contour. Our study explores this color spreading effect across different chromatic directions using a position judgment task. We used the gap effect stimulus, which consists of a box evenly divided by a central contour, in half of the conditions. The suprathreshold chromatic test area embedded in the box provided a horizontal chromatic edge parallel to the central, high-contrast luminance contour that varied in its distance from the contour. An attraction effect of the nearest high-contrast contour on low-contrast chromatic and achromatic edges was observed. Specifically, when the test area is smaller than the region defined by the outer and middle contours, the edge is perceived to be closer to the middle contour (the colored area is perceived to be larger), a filling-in effect; conversely, when the test area extends beyond the middle contour, the edge is perceived to be closer to the middle contour (the colored area is perceived to be smaller), indicating a filling-out of color. Achromatic directions exhibit a relatively smaller effect than chromatic directions, whereas S-cone and equiluminant red and green edges show the same magnitude of positional displacement. The results can be interpreted as the visual system attempting to assign a single hue or brightness to a demarcated region. 

Neon color spreading (NCS) is an illusory color phenomenon that provides a dramatic example of surface completion and filling-in. Numerous studies have varied both spatial and temporal aspects of the neon- generating stimulus to explore variations in the strength of the effect. Here, we take a novel, parametric, low- level psychophysical approach to studying NCS in two experiments. In Experiment 1, we test the ability of both cone-isolating and equiluminant stimuli to generate neon color spreading for both increments and decre- ments in cone modulations. As expected, sensitivity was low to S(hort-wavelength) cone stimuli due to their poor spatial resolution, but sensitivity was similar for the other color directions. We show that when these differences in detection sensitivity are accounted for, the particular cone type, and the polarity (increment or decrement), make little difference in generating neon color spreading, with NCS visible at about twice detection threshold level in all cases. In Experiment 2, we use L-cone flicker modulations (reddish and greenish excursions around grey) to study sensitivity to NCS as a function of temporal frequency from 0.5 to 8 Hz. After accounting for detectability, the temporal contrast sensitivity functions for NCS are approximately constant or even increase over the studied frequency range. Therefore there is no evidence in this study that the processes underlying NCS are slower than the low-level processes of simple flicker detection. These results point to relatively fast mech- anisms, not slow diffusion processes, as the substrate for NCS. 

The perceptual response to achromatic incremental (A+) and decremental (A–) visual stimuli is known to be asymmetrical, due most likely to differences between ON and OFF channels. In the current study, we further investigated this asymmetry psychophysically. In Experiment 1, maximum likelihood difference scaling (MLDS) was used to estimate separately observers’ perceptual scales for A+ and A–. In Experiment 2, observers performed two spatial alternative forced choice (2SAFC) pedestal discrimination on multiple pedestal contrast levels, using all combinations of A+ and A– pedestals and tests. Both experiments showed the well-known asymmetry. The perceptual scale curves of A+ follow a modified Naka–Rushton equation, whereas those of A– follow a cubic function. Correspondingly, the discrimination thresholds for the A+ pedestal increased monotonically with pedestal contrast, whereas the thresholds of the A– pedestal first increased as the pedestal contrast increased, then decreased as the contrast became higher. We propose a model that links the results of the two experiments, in which the pedestal discrimination threshold is inversely related to the derivative of the perceptual scale curve. Our findings generally agree with Whittle’s previous findings (Whittle, 1986, 1992), which also included strong asymmetry between A+ and A–. We suggest that the perception of achromatic balanced incremental and decremental (bipolar) stimuli, such as gratings or flicker, might be dominated by one polarity due to this asymmetry under some conditions. 

Maximum Likelihood Difference Scaling (MLDS) is an efficient method of estimating perceptual representations of suprathreshold physical quantities (Maloney & Yang, 2003), such as luminance contrast. In MLDS, observers can be instructed to judge which of two stimulus pairs are more similar to one another, or which of the two pairs are more different from one another. If the same physical attributes are used for both the similar and dissimilar tasks, the two criteria should produce the same perceptual scales. We estimated perceptual scales for suprathreshold achromatic square patches. Increments and decrements on the mid-gray background were estimated separately. Observers judged which pair of stimuli were more similar in half of the sessions, and more different in the other half sessions. For most observers, the two tasks produced the same perceptual scales: a decelerating curve for increment contrasts and a cubic curve for decremental contrasts (cf. Whittle, 1992). These scales predicted forced-choice contrast discrimination thresholds for both increments and decrements. However, for a subset of observers, the ‘more different’ judgments produced scales that accelerated with contrast for both increments and decrements; these scale shapes do not predict their discrimination thresholds. Our results suggest that, even with these simple stimuli, observers in an MLDS experiment may attend to different aspects of the stimulus depending on the assigned task. 

A basic problem in psychophysics is to relate the internal representation of a stimulus to its physical intensity. In this study, we measured perceptual scales for achromatic contrast with Maximum Likelihood Difference Scaling (MLDS), using squares against a mid-grey background. Observers compared two stimulus pairs and chose the more different pair. All four squares were either achromatic increments (A+), or achromatic decrements (A-). The MLDS result was then compared with 2AFC achromatic pedestal discrimination, with pedestals and tests that were all combinations of A+ and A-. The main result is not novel: A+ and A- obey different rules. A Naka-Rushton saturating function describes the A+ MLDS result well, and the derivative of that function is proportional to the A+ pedestal discrimination for some (but not all) observers. A- MLDS and discrimination results are more complicated and are reminiscent of the classic findings of Whittle (1986, 1992). The sensitivity of A- is a cubic polynomial function of pedestal contrast. These findings will be compared with a similar study of S-cone contrast (reported at VSS 2022), which found a different type of asymmetry between S+ and S-. Presumably these increment/decrement asymmetries are due to underlying differences between ON and OFF neural pathways. One implication is that using stimuli that include both contrast signs, such as gratings and flicker, may obscure important asymmetries in the processing of contrast.