The indirect and direct theories are very different, because the theorists concerned have been pursuing very different goals. This can be seen if we consider the distinction between perception for recognition and perception for action (Milner & Goodale, 1995, 1998; discussed more fully later in the chapter). Evidence from cognitive neuroscience and from cognitive neuropsychology has supported the distinction. This evidence has suggested that there is a ventral stream of processing more involved in perception for recognition and a dorsal stream more involved in perception for action (see later in this chapter), although perception for whatever purpose is typically based on both streams of processing. Most perception theorists (including Gregory, Marr, and Biederman) have focused on perception for recognition, whereas Gibson emphasised perception for action.

A simplified account of Milner and Goodale's theoretical view based on dorsal and ventral streams of visual processing

Behaviourist approach Reconstructive approach

• Visually guided actions are central. • Internal representations are central.

• Governed by the dorsal stream. • Governed by the ventral stream.

• Perception for action. • Perception for recognition.

• There is substantial communication and co-operation between the two systems._

There have been several demonstrations of the partial separateness of these two visual systems. For example, as we saw earlier, visual illusions clearly present when the task involves the perception-for-recognition (or ventral) system are much reduced when the task involves the perception-for-action (or dorsal) system (e.g., Aglioti et al., 1995; Gentilucci et al., 1996).

Goodale and Humphrey (1998, pp. 201-202) provided a detailed account of the relevance of the distinction between dorsal and ventral steams of visual processing to major theoretical positions:

The preoccupation with visually guided actions that characterises behaviourist approaches to vision [and Gibson's approach] has meant that most of the visual mechanisms that are being studied are those found in the dorsal stream. In contrast, the reconstructive approach (e.g., Marr, 1982) .is a

"passive" approach in which the representation is central and the external behaviour of the external world is largely ignored. Reconstruction of the external world is exactly the kind of activity which we believe is carried out by the ventral stream.

Where does that leave the relationship between these major approaches? According to Goodale and Humphrey (1998, p. 181), "Marrian or 'reconstructive' approaches and Gibsonian or 'purposive-animate-behaviourist' approaches need not be seen as mutually exclusive, but rather as complementary in their emphasis on different aspects of visual function."

There are two important final points. First, the fact that there are two separate processing systems does not mean that they always function independently of each other. In fact, the two systems are interconnected and there is generally extensive communication and cooperation between them. Second, the descriptions by Milner and Goodale (1995, 1998, see later in this chapter) of the processing occurring within the dorsal and ventral streams are oversimplified, and will be subject to revision.

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