Understanding contrast and assimilation: the two modes of human brightness perception and the conditions for their mutual transition through an experimental and modelling approach

Article Type

Research Article

Publication Title

Journal of Optics India

Abstract

Brightness in a specific (target) region in the visual field is influenced by the brightness of its neighbouring regions. Depending on the nature of the adjacent regions, this brightness induction is often manifested as brightness contrast, wherein the contrast of the target region with the adjacent region is enhanced. However, a totally reverse effect may also be observed, known as brightness assimilation, wherein the brightness of the target region is averaged over the target and its neighbourhood. There are examples of visual illusions of both these types. For almost the last half a century, people tried to understand whether contrast and assimilation are the manifestation of a single phenomenon, i.e. whether both can be understood under the umbrella of a single unified model, or whether the two illusions are perceived through entirely different methods of computation by the human brain. Experiments have been performed in the past to examine intriguing transition from one of these two types of brightness induction to the other. However, whether it is a low or higher level visual experience is still not clear, though brightness perception is apparently a low-level vision phenomenon. Moreover, to the best of our knowledge, till date there exist neither a step-by-step quantitative systematic data on such transitional processes, nor a classical receptive field (the isotropic difference of Gaussian) based low-level visual modelling of such steps. The main purpose of the present paper is to meet this gap. We have studied with careful psychophysics experiment two such cases of transitions. In the first psychophysics experiment, we took a White illusion (WI) stimulus and converted it to a simultaneous brightness contrast illusion through different stages of semi-White illusion (SeWI) using twenty different comparator arrangements, and quantifying the illusory effect. In the second experiment, we used the shifted White illusion as the experimental stimuli and manipulated the aspect ratio (AR), of the target gray patch and found the transitional/threshold AR where the brightness assimilation illusion gets reversed to an brightness contrast illusion, while quantifying twelve different comparator arrangements. Finally, a linear centre surround receptive field model or, a difference of Gaussian (DoG) filter model is simulated to explain the variations of illusory effect produced under the different conditions of such stimuli. The isotropic DoG filter which is known to work well only for the brightness contrast based illusions, is interestingly found to fit well with brightness assimilation illusions as well, by suppression of the inhibitory surround of the classical DoG filter.

DOI

10.1007/s12596-024-02058-8

Publication Date

1-1-2024

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