Biological and Computer Vision

Gabriel Kreiman

Cambridge University Press. 2021. ISBN 9781108649995 

Additional Materials

Chapter II: The travels of a photon. Natural image statistics and the retina

The visual system extracts and represents information from the world around us. The types of images reaching the eyes are not random but follow a natural distribution full of regularities, including strong spatial and temporal correlations. Natural images are mostly smooth, except for the presence of sharp edges that tend to delimit objects, and natural dynamics tend to show slow fluctuations, interrupted by rapid eye movements. Photons impinging on the eye are converted into electrical signals in an exquisitely beautiful structure called the retina. Darwin referred to the eye as an organ of extreme perfection and was mesmerized by the seemingly absurd idea that it could arise by evolution. The retina comprises three main layers: photoreceptors, bipolar cells, and retinal ganglion cells, which carry the output to the rest of the brain. The retina's neural circuitry can adaptively process visual signals over changes of many orders of magnitude in illumination and is well-tuned to represent the natural statistics of the world. Exciting neuroscientific advances have characterized the variety of different cell types in the retina, the anatomical map of how these cells connect to each other, and the physiological mechanisms involved in processing visual signals.