Ioannides, A.; (2007) Scale-space and the implicit coding of luminance in V1. Doctoral thesis , University of London.

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Abstract

This thesis pursues a single line of enquiry: lightness, brightness, and visual illusions. In particular, it focuses on White's effect, simultaneous brightness contrast, and low-level theories that can account for both phenomenon. In the first part (Chapters 1-2), the problem-space is defined before a review of lightness and brightness theories from both low- and high-level vision. In the second part (Chapter 3), the only two low-level VI models of brightness, capable of accounting for both White's effect and simultaneous brightness contrast, are shown to be reliant on the amplification of low spatial frequency information derived for large-scale RFs, to accurately reconstruct images and account for the illusory brightness apparent in both effects. It is argued that these large-scale RFs do not exist in VI. and that the global re-weighting and re-normalisation schemes employed by these models are not constrained by the known local nature of intra-cortical connections. Hence, it was concluded that these models are not biologically plausible. In the third part (Chapter 4), the issue of recovering low spatial frequency and local mean luminance information without explicitly sampling it, is considered. The problem is formally defined in the Scale-Space framework and solved analytically. That is, an algorithm for recovering local mean-luminance (and low spatial frequencies), from the information implicit in contrast coding cells typically found in VI, is constructed, and is referred to as the Implicit Luminance Coding (ILC) model. It is argued that the ILC model is not biologically-plausible, by virtue of its global optimisation framework being unconstrained by the known local nature of intra-cortical connections. Subsequently, a new algorithm is proposed, based on a numerical approximation to the analytical solution. The biologically-plausible ILC algorithm is developed into a complete low-level model of brightness, which makes use of the information present in multiple scale channels. The model is shown to be capable of accounting for both White's effect and simultaneous brightness contrast, by means of an interplay between two independent assimilation and contrast mechanisms. The final part (Chapter 5). is concerned with the application of the model to visual phenomenon synonymous with lightness and brightness, including all known variants of White's effect and simultaneous brightness contrast, and some effects that are traditionally accounted for by appealing to mechanisms from high-level vision, thus facilitating the delineation of low-level from higher-level phenomena. The biologically-plausible ILC model is shown to be in good accordance with this experimental data. Furthermore, qualitative accounts for the temporal evolution of the filling-in process were provided and shown to be in agreement with experiment, and novel predictions as to the temporal evolution of White's effect relative to simultaneous brightness contrast are described.

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