Dichotomies of the Visual Brain
Paper by Prof. Melvyn Goodale
"Zombie vision: Action without perceptual awareness in the human visual system."
Abstract:
Almost all of our direct knowledge of the world beyond our bodies comes from vision. But vision does not simply provide information about objects and events in the world; in humans, at least, it provides a conscious percept of that world so compelling that it is sometimes difficult to comprehend that this experience arises entirely from the activity of ensembles of neurons in the central nervous system. Moreover, for most people, including many philosophers and scientists, it seems self-evident that the actions we perform on visible objects make use of the same visual representation that allows us to perceive those objects. According to this view, the visual system creates a single “general-purpose” representation of the external world that provides a platform for both cognitive operations as well as the real time control of goal-directed actions. There are good reasons to believe, however, that such a monolithic account is incorrect.
I will argue that the contents of our visual consciousness are created by complex interaction between bottom-up input and stored knowledge of the world in the ventral stream of visual processing, whereas the moment-to-moment control of visually guided actions, such as skilled prehension, is mediated almost entirely by bottom-up input to the dorsal stream. This might sound rather like Cartesian dualism—the existence of a conscious mind separate from a reflexive machine. But the division of labour between the two streams has nothing to do with the kind of dualism that Descartes proposed. Although the two kinds of visual processing are separate, both are embodied in the hardware of the brain. Moreover, there is a complex but seamless interaction between the ventral and the dorsal streams in the production of adaptive behavior. The selection of appropriate goal objects depends on the perceptual machinery of the ventral stream, while the execution of a goal-directed action is mediated by dedicated on-line control systems in the dorsal stream and associated motor areas. Moreover, as I will argue, the integration of processing in the two streams goes well beyond this. The dorsal stream may allow us to reach out and grasp objects with exquisite ease, but it is trapped in the present. Evidence from the behaviour of both neurological patients and normal observers shows that, by itself, the dorsal stream can deal only with objects that are visible when the action is being programmed. The ventral stream, however, allows us to escape the present and bring to bear information from the past – including information about the function of objects, their intrinsic properties, and their location with reference to other objects in the world. Ultimately then, both streams contribute to the production of goal-directed actions.
Prof. Goodale will present this paper at 2.00 pm on Tuesday, June 17th, 2008.
With any queries, please contact Dr Jakob Hohwy, Ph: (03) 9905 3208; Email: jakob.hohwy@arts.monash.edu.au.
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