Entropy reduction and decorrelation in visual coding by oriented neural receptive fields

In biological visual systems, it is not obvious whether coding efficiency as measured by mutual information among the neurons is a factor that explains any of their properties. The center/surround receptive field profiles of neurons in the retina and geniculate are far from an orthogonal set, but a given neuron can still be regarded as a decorrelator of the incoming signal in the sense that it responds primarily to changes in the image. At the level of the brain´s visual cortex, the introduction of the new variable of orientation selectivity can be regarded not only as a means for providing orientation labels for image structure, but also more basically as an effective decorrelator of the neural representation. The present image coding simulations, based on quantitative neurobiological data about the code primitives, provide measures of the bit-rate efficiency of such oriented, quadrature, neural codes. Demonstrations of data compression to below 1 bit/pixel in cortically-based, quadrature self-similar wavelet image codes are also provided.