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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. 

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. 

The desensitization of the visual system as a function of the increasing luminance of a background field yields threshold vs. intensity (tvi) curves, classically measured using increment tests. Here we use a new, high-brightness display system to measure both increment and decrement thresholds. Our display system is based upon a PROPixx three-chip DLP LED color projector (VPixx Technologies, Saint- Bruno, Canada), with light from the projector collected into a field lens and focused onto a high gain rear projection screen. This display combines the brightness of traditional optical systems with the flexibility of control provided by modern displays; in particular, it is simple to use the silent substitution method to isolate single cone types. Here we report tvi curves for achromatic and (L-)ong wavelength sensitive cone isolating tests, measured using method of adjustment. Selected thresholds were verified with a spatial, two-alternative forced-choice procedure. The adapting background was white, with luminances ranging from 0.6 to 4.0 log Trolands (a maximum near 3200 cd/m2, bleaching about 1/3 of the L and M cone pigment). Our observers are slightly more sensitive to decrements than increments (about 0.1 log units), for both achromatic and L-cone tests, and to L-cone tests than to achromatic tests (about 0.6 log cone contrast units), over the entire adapting range. Both increment and decrement thresholds follow the Stiles template, approximating Weber’s law except at the lowest adapting levels. The achromatic tvi’s, for both increment and decrement tests, are, on average, slightly steeper than the L-cone tvi’s. In addition, decrement tvi’s are steeper than the increment tvi’s, indicating greater effects of light adaptation for the decrements, which may be due to differences in the effects of light adaptation in ON and OFF pathways.