Wereport the results offour experiments that were conducted to examine both the representations that provide candidate entities available for object-based attentional selection and the influence of bottomup factors (i.e., geometric and surface characteristics of objects) and top-down factors (i.e. context and expectancies) on the selection process, Subjects performed the same task in each of the experiments. They were asked to determine whether two target properties, a bent end and an open end of a wrench, appeared in a brief display of two wrenches. In each experiment, the target properties could occur on a single wrench or one property could occur on each of two wrenches. The question of central interest was whether a same-object effect (faster and/or more accurate performance when the target properties appeared on one vs. two wrenches) would be observed in different experimental conditions, Several interesting results were obtained. First, depending on the geometric (i.e., concave discontinuities on object contours) and surface characteristics (i.e., homogeneous regions of color and texture) of the stimuli, attention was preferentially directed to one of three representational levels, as indicated by the presence or absence of the same-object effect. Second, although geometric and surface characteristics defmed the candidate objects available for attentional selection, top-down factors were quite influential in determining which representational level would be selected, Third, the results suggest that uniform connectedness plays an important role in defining the entities available for attention selection. These results are discussed in terms of the marmer in which attention selects objects in the visual environment.

S. P. Vecera and M. J. Farah (1994) have addressed the issue of whether visual attention selects objects or locations. They obtained data that they interpreted as evidence for attentional selection of objects from an internal spatially invariant representation. A. F. Kramer, T. A. Weber, and S. E. Watson question this interpretation on both theoretical and empirical grounds. First, the authors suggest that there are other interpretations of the Vecera and Farah data that are consistent with location-mediated selection of objects. Second, they provide data, using the displays employed by Vecera and Farah in conjunction with a postdisplay probe technique, that suggests that attention is directed to the locations of the target objects. The implications of the results for space and object-based attentional selection are discussed.

n the past 2 decades, two different classes of models, space-based and I object-based models, have been proposed to account for the distribution of attention in the visual field. Space-based models have suggested that spotlights, zoom lenses, and gradients provide apt analogies for the allocation of attention. For example, models based on the notion of a spotlight argue that attention is distributed in contiguous regions of the visual field (Broadbent, 1982;Posner, Snyder, & Davidson, 1980;Shulman, Remington, & McLean, 1979;Tsal & Lavie, 1988). Stimuli that fall within the spotlight are extensively processed, whereas events that occur outside this area are ignored. The requirement to process information in noncontiguous areas of the visual field necessitates movement of the spotlight.Evidence that has been taken to support the spotlight model has been obtained in response competition, spatial priming, and divided attention paradigms. C. W. Eriksen and colleagues (B. A. Eriksen & Eriksen, 1974; C. W. Eriksen h Hoffman, 1973) have found that responseincompatible distractors produce large performance costs when they are This research was supported by Grant N00014-92-J-1792 from the Office of Naval Research. We thank Michael Coles, Dave Irwin, and Gordon Logan for helpful comments on a previous version of this chapter.

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