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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.
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Increment and decrement threshold vs. intensity curves for achromatic and L-cone tests.
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.
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- PAR ID:
- 10505575
- Publisher / Repository:
- Journal of Vision
- Date Published:
- Journal Name:
- Journal of Vision
- Volume:
- 23
- Issue:
- 9
- ISSN:
- 1534-7362
- Page Range / eLocation ID:
- 5729
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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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.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1167/jov.23.9.5729
