Torsten Nils Wiesel (born 1924) is a Swedish neurophysiologist who, in a partnership with David Hubel that ran for roughly two decades, transformed how scientists understand the visual brain. The two met at Johns Hopkins in the late 1950s and continued their collaboration at Harvard, recording the electrical responses of single neurons in the visual cortex of cats and monkeys.
Their central finding was that cortical cells are tuned: a given neuron fires most when a bar or edge of light appears at one particular orientation, and the cortex is organized so that nearby cells prefer nearby orientations. This hierarchy of simple and complex cells showed that the brain builds up its picture of the world from elementary features detected early in the visual pathway. Wiesel also did influential work on the developing visual system, showing that normal vision in early life is necessary for the cortex to wire itself correctly.
Hubel and Wiesel shared half of the 1981 Nobel Prize in Physiology or Medicine for their discoveries concerning information processing in the visual system. Their model of vision as a layered cascade of feature detectors became one of the clearest bridges between neuroscience and artificial intelligence, echoed decades later in the design of deep convolutional networks. Wiesel went on to lead Rockefeller University and to champion international scientific cooperation.