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Wednesday, December 15, 2010

Vision as an active process

Of course, our eyes are able to move in their sockets, and this allows the visual system to choose new parts of the image to look at. These rapid eye movements, called saccades, [saccades rapid eye movements in which the fovea is directed at a new point in the visual world] occur several times per second. We are mostly unaware of them and, during a saccade, vision is largely ‘switched off’ so that we do not see the world moving rapidly in the opposite direction to the saccade.

One is a classic study by the Russian psychologist Yarbus (1967), which shows how we move our eyes when looking at a visual object. The other is a study by Gilchrist, Brown, and Findlay (1997), which investigated similar scan patterns by a young woman (a female university undergraduate aged 21) who had no ability to move her eyes due to a condition called extraocular muscular fibrosis. Instead, she moved her whole head using the neck muscles. There are strong similarities between the two sets of scan patterns, indicating that, even if the eye cannot move, the image scan sequence needs to be broadly similar. All of this raises the question of exactly why the eye needs to be moved to so many locations. Presumably it is to resolve fine detail by using the fovea with its small receptive fields. Moving the eyes in this manner is usually associated with a shift of attention. When we move our eye to a given location, we are more likely to be processing the information from that location in greater detail than information from elsewhere. This is the concept of selective attention (see chapter 8). The process is intimately related to physical action, in this case movement of the eyes (or head). This implies that vision is not just a passive process, but an active one. In fact, there appear to be two streams of visual information in the cortex – ventral and dorsal. The former processes information about the nature of objects; the latter allows you to interact with ‘stuff out there’ – i.e. to plan actions – without a detailed representation of objects (see Milner & Goodale, 1995).

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