David Marr (neuroscientist)

David Courtnay Marr (January 19, 1945 - November 17, 1980) was a British neuroscientist and psychologist. Marr integrated results from psychology, artificial intelligence, and neurophysiology into new models of visual processing. He is acknowledged as a founder of the discipline of Computational Neuroscience.

Born in Essex, and educated at Rugby School, he completed his B.A. in mathematics at Trinity College of the University of Cambridge, staying on to do a Ph.D. in neuroscience under Professor G.F. Brindley. His interest turned from general brain theory to visual processing. His doctoral dissertation was submitted in 1969 and described his model of the function of the cerebellum based mainly on anatomical and physiological data garnered from a book by J.C. Eccles. A similar model was later independently proposed by James S. Albus. The Marr-Albus theory still stands today as the standard model of cerebellar function. Subsequently he worked at the Massachusetts Institute of Technology, where he took on a faculty appointment in the Department of Psychology in 1977 and was subsequently made a tenured full professor in 1980. Marr proposed that understanding the brain requires an understanding of the problems it faces and the solutions it finds. He emphasized the need to avoid general theoretical debates and instead focus on understanding specific problems.

Marr died of leukemia in Cambridge, Massachusetts. Marr's findings are collected in the book Vision: A computational investigation into the human representation and processing of visual information (ISBN 0-7167-1567-8), which was published after his death.

Levels of analysis
In order to focus on the understanding of specific problems, he identified (in concert with Tomaso Poggio) three levels of analysis:
 * the problems vision must overcome (computational level)
 * the strategy that may be used (algorithmic level)
 * how it is actually done in the neural activity (implementational level)

Stages of vision
Marr described vision as proceeding from a two-dimensional visual array (on the retina) to a three-dimensional description of the world as output. His stages of vision include
 * a primal sketch of the scene, based on feature extraction of fundamental components of the scene, including edges, regions, etc. Note the similarity in concept to a pencil sketch drawn quickly by an artist as an impression.
 * a 2.5D sketch of the scene, where textures are acknowledged, etc. Note the similarity in concept to the stage in drawing where an artist highlights or shades areas of a scene, to provide depth.
 * a 3 D model, where the scene is visualized in a continuous, 3-dimensional map.

Francis Crick noted that this insight although seminal, has been somewhat modified.

2.5D sketch is related to stereopsis, optic flow, and motion parallax. The 2.5D sketch represents that in reality we do not see all of our surroundings but construct the viewer-centered three dimensional view of our environment. 2.5D Sketch is a paraline drawing and often referred to by its generic term "axonometric" or "isometric" drawing and are often used by modern architects and designers.

Reference
1. Uddin, Saleh. "Conventions and Construction of Paralines." In Axonometric and Oblique Drawing: A 3-D Construction, Rendering, and Design Guide, 1-14. New York: McGraw-Hill, 1997.