Toward reliable measurements of perceptual scales in multiple contexts

Abstract

A central question in psychophysical research is how perceptual differences between stimuli translate into physical differences and vice versa. Characterizing such a psychophysical scale would reveal how a stimulus is converted into a perceptual event, particularly under changes in viewing conditions (e.g., illumination). Various methods exist to derive perceptual scales, but in practice, scale estimation is often bypassed by assessing appearance matches. Matches, however, only reflect the underlying perceptual scales but do not reveal them directly. Two recently developed methods, MLDS (Maximum Likelihood Difference Scaling) and MLCM (Maximum Likelihood Conjoint Measurement), promise to reliably estimate perceptual scales. Here we compared both methods in their ability to estimate perceptual scales across context changes in the domain of lightness perception. In simulations, we adopted a lightness constant, a contrast, and a luminance-based observer model to generate differential patterns of perceptual scales. MLCM correctly recovered all models. MLDS correctly recovered only the lightness constant observer model. We also empirically probed both methods with two types of stimuli: (a) variegated checkerboards that support lightness constancy and (b) center-surround stimuli that do not support lightness constancy. Consistent with the simulations, MLDS and MLCM provided similar scale estimates in the first case and divergent estimates in the second. In addition, scales from MLCM-and not from MLDS-accurately predicted asymmetric matches for both types of stimuli. Taking experimental and simulation results together, MLCM seems more apt to provide a valid estimate of the perceptual scales underlying judgments of lightness across viewing conditions.

What Fechner could not do: Separating perceptual encoding and decoding with difference scaling

“…We simulated noise with σ values of 0.03, 0.06, and 0.15. These values correspond with the minimum, average, and maximum noise observed in a previous experiment ( Aguilar & Maertens, 2020 ).…”

Conjoint measurement of perceived transparency and perceived contrast in variegated checkerboards

“…By design, MLCM anchors the scale with the smallest value to zero and the scale with the highest value to a maximum value, which represents the inverse of the estimated judgment noise. Thus, the individual scales from the two experiments cannot be directly compared as they do not share a common metric (see Aguilar & Maertens, 2020 , for a more detailed treatment of the issue of comparing scales across different contexts).…”

“… Robilotto et al (2002) used an asymmetric matching task to measure perceived contrast and perceived transparency as a function of physical transparency parameters. While adjustment procedures are very efficient and thus widely used in the assessment of appearance, they probe the perceptual dimension of interest only indirectly ( Kingdom & Prins, 2010 ; Wiebel et al, 2017 ; Aguilar & Maertens, 2020 ). Hence, matching procedures are insufficient to differentiate between potentially different transducer functions relating sensory to perceived variables (we discuss the linking assumptions in matching procedures in detail below).…”

Scaling and discriminability of perceived gloss

“…Simulated violations of this equal variance assumption do not affect the shape of the perceptual scale produced by MLDS [16,30], but may affect discriminability estimates derived from the signal detection theory formulation of MLDS (Aguilar et al, 2017). The model assumptions underlying MLDS may also interact with stimulus complexity and dynamic range (Aguilar & Maertens, 2020;Protonotarios et al, 2016), both of which have been shown to affect the perception of gloss (Adams, Kucukoglu, Landy, & Mantiuk, 2018;Doerschner, Maloney, & Boyaci, 2010;Obein et al, 2004;Phillips, Ferwerda, & Luka, 2009). It is also plausible that scaling and discrimination tasks induce-or draw on-non-trivial differences in stimulus representation.…”