Building Up Objects

Once the visual image has been parsed into its constituent features, the visual system must put different features together to form representations of individual objects. Scientists working in the Gestaltist tradition, a school of psychology that began in mid-twentieth century Germany, have proposed a number of principles that appear to guide this process of perceptual organization. For example, the principle of good continuation states that, other things being equal, lines or edges that form straight or smoothly curving contours should be grouped together. Thus, in Fig. 2, the lines forming the back edge of the table are combined into a group even though they are separated by the occluding salt and pepper shakers. Other Gestalt principles include proximity (elements close to each other should be grouped together), common fate (elements moving in the same direction should be grouped together), and connectedness (elements connected by other elements should be grouped together).

Whereas Gestalt psychologists have worked to provide intuitive heuristics for which elements should be grouped together, another group of cognitive scientists have worked on the problem of how elements are combined. The key concept in this tradition is that of visual attention, and the metaphor of a spotlight is often used to describe the role of attention in feature combination. The attentional spotlight illuminates a circumscribed portion of the visual field and combines the various features present in that portion of the image into a multidimensional representation, which can then be identified by the object perception processes discussed later. Because attentional capacity is limited, we are only able to analyze in detail a relatively small portion of an image at a time. Therefore, the attentional spotlight must move from place to place in a serial fashion in order to completely analyze an image, even though all individual features are detected in parallel.

Object perception theories generally make the simplifying assumption that all of the features of a to-be-perceived object are eventually (possibly after several shifts of the attentional spotlight) combined together into a single representation, which can then be compared to representations in memory to determine the object's identity. However, research on a phenomenon termed "change blindness'' indicates that this assumption may not be valid. Consider the image in Fig. 3. Is the scene depicted here exactly the same as the one in Fig. 2 (examine the two figures before reading on)? Not quite: the handle of the pepper shaker is missing from the second figure. Behavioral studies using a number of different paradigms have converged on the conclusion that subtle changes such as this one often go unnoticed, especially when visual attention is not focused on the region of the visual field where the change occurs. Even changes to a single, isolated object that is clearly the focus of attention can be missed if the object is obscured from view for a short period of time. These findings imply that our initial

Figure 3 This image is similar, but not exactly the same, as the one in Fig. 2 (see the text for a description of the change). The fact that experimental participants are poor at detecting such changes indicates that our representations of perceived objects are not as complete as we intuitively believe them to be.

Figure 3 This image is similar, but not exactly the same, as the one in Fig. 2 (see the text for a description of the change). The fact that experimental participants are poor at detecting such changes indicates that our representations of perceived objects are not as complete as we intuitively believe them to be.

representations of objects may not be as complete as we intuitively believe them to be. Object perception theories may need to be altered to take this imprecision into account.

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