Abstract
White's effect describes a brightness difference between two gray targets embedded in the black and white phases of a high contrast (square-wave) grating. Lacking an agreed explanation of this effect, it is often used as a target for computational models of brightness perception. Such models provide transfer functions linking input luminance to model output; if interpreting the latter as perceptual magnitude, these transfer functions are analogous to perceptual scales. However, whether the model transfer functions correspond to the perceptual scales has not yet been investigated. Here we estimate perceptual scales for White’s stimulus. On each trial, observers judged which of two targets appeared lighter. Each target varied in luminance across trials, and could appear either on the black or white phase of the grating. Using Maximum Likelihood Conjoint Measurement, perceptual scales were estimated from these judgments. Scales were compressive non-linear functions. Overall, the scale for targets on the black phase is shifted up relative to the white-phase targets (in agreement with the direction of White’s effect). This shift may be larger for intermediate luminance values than when target luminance approaches the luminance of the white phase, which has also been reported using more common matching tasks. Computational models failed to predict the shapes of the scales, suggesting that perceptual scales in addition to matching data could better constrain brightness models.
Funding: This work has been supported by research grants of the German Research Foundation DFG MA5127/3-1 and MA5127/4-1