Figure

Object recognition as a function of stimulus type (edge drawings vs. colour photographs) and presence vs. absence of context. Data from Sanocki et al. (1998).

usually idealised versions of the original edge information (e.g., edges that are irrelevant to the object are often omitted). Sanocki et al. (1998) also pointed out that edge-extraction processes are more likely to lead to accurate object recognition when objects are presented on their own rather than in the context of other objects. The reason is that it can be hard to decide which edges belong to which objects when several objects are presented together.

Sanocki et al. (1998) obtained strong support for the view that edge information is often insufficient to allow object recognition. Their participants were presented for 1 second each with objects shown in the form of edge drawings or full-colour photographs, and these objects were presented in isolation or in context. Object recognition was much worse with the edge drawings than with the colour photographs, and this was especially the case when objects were presented in context (see Figure 4.7). Sanocki et al. (1998, p. 346) concluded as follows:

Edge information is far from being sufficient for object recognition. The results call into question psychological and computer vision models [e.g., Biederman, 1987] that use local edge extractors as their only low-level process.

Third, Marr and Nishihara (1978), Biederman (1987, 1990), and others have emphasised the notion that object recognition involves matching an object-centred representation that is independent of the observer's viewpoint with object information stored in long-term memory. This theoretical view was explored by Biederman and Gerhardstein (1993). They argued that object naming would be primed as well by two different views of an object as by two identical views, provided that the same object-centred structural description could be constructed from both views. Their findings supported the prediction, but other findings (e.g., Tarr & B├╝lthoff, 1995, 1998) do not (see later discussion).

Fourth, the theories put forward by Marr and Nishihara (1978), Biederman (1987), and others only account for fairly unsubtle perceptual discriminations, such as deciding whether the animal in front of us is a dog or cow. These theories have little to say about subtle perceptual discriminations within classes of objects. For example, the same geons are used to describe almost any cup, but we can readily identify the cup we normally use.

Fifth, the theories have de-emphasised the important role played by context in object recognition. For example, Palmer (1975) presented a picture of a scene (e.g., a kitchen), followed by the very brief presentation of the picture of an object. This object was sometimes appropriate to the context (e.g., a loaf), or it was inappropriate (e.g., mailbox or drum). There was also a further condition in which no contextual scene was presented. The context had a systematic effect on the probability of identifying the object correctly, with the probability being greater when the object was appropriate to the context, intermediate when there was no context, and lowest when the object was inappropriate to the context.

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