David Milner & Melvyn Goodale:
THE vISUAL BRAIN IN ACTION (Oxford University Press, 1995)

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A new paradigm for the study of human vision was introduced by the Canadian physiologist Melvyn Goodale and the British psychologist David Milner. They analyzed the visual pathways in the cerebral cortex (the "visual cortex") and realized that vision is actually a binary operation made of dual processes: on one hand is the conscious visual experience of the world, on the other hand is the visual control of unconscious (instinctive) action. They both require the eye as the organ, but they are functionally and structurally different processes.

Sensory information received from the eye diverges into two streams (two anatomically different pathways) when it leaves the visual cortex: a "ventral" stream flows from the primary visual cortex towards the inferior temporal lobe, while the "dorsal" stream flows from the primary visual cortex towards the posterior parietal lobe. This means that the ventral stream analyzes what object the eye is seeing, whereas the dorsal stream analyzes the spatial location of the object. In other words, the ventral stream is about recognition (e.g. of faces or objects) and conscious perception, whereas the dorsal stream is about automatic, unconscious action in space directed towards the object (typically action by the hand).

In a sense, there exist two kinds of vision: conscious perception and unconscious action. They are physically handled by two separate systems in the brain. There isn't a visual system: there are two visual systems that work in parallel.

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